Non-contact electrical power transmission system having function of making load voltage constant

Abstract: This system includes a transformer separable/detachable between a primary winding and a secondary winding, and a capacitor connected parallel to the secondary winding of the above-mentioned transformer, and supplies a high-frequency AC voltage to the primary winding of the transformer to generate an induced voltage in the secondary winding of the transformer, and transmits the electrical power to a load in non-contact manner. The voltage supplied to the load is constant and the flowing current varies. The power may be supplied to a different kind of a load with a constant voltage. Taking as a first condition a fact that at the time of a maximum load, the time of the reversal of the voltage polarity of the above-mentioned primary winding substantially coincides with the time when an oscillating voltage of the above-mentioned capacitor reaches a maximum or minimum value, and taking as a second condition a fact that at the time of a minimum load, the time of the reversal of the voltage polarity of the above-mentioned primary winding substantially coincides with the time when the oscillating voltage of the above-mentioned capacitor completes one cycle, the above-mentioned capacitor is set so that the capacity value satisfies simultaneously the above-mentioned first and second conditions. This allows the load voltage to be made constant in a load current range from a minimum to a maximum without requiring a feedback circuit.

Description: BACKGROUND OF THE INVENTION The present invention relate to a non-contact electrical power transmission system which includes a transformer with separable primary and secondary windings and transmits an electric power from the primary side to the secondary side of the transformer in a condition in which a core wrapped with a primary winding and a core wrapped with a secondary winding are not contacted to each other. Such a power transmision system is disclosed in US 5 428 521 having the features of the preamble of claim 1. Heretofore, in many places, a non-contact electrical power transmission system has been commercially applied, which system configures a transformer in which the primary winding and the secondary winding are separable from each other with a core wrapped with a primary winding and a core wrapped with a secondary winding, and which system transmits the power from the primary side to the secondary side by utilization of electromagnetic induction in a condition in which the primary core and the secondary core are not contacted to each other. However, in most of conventional non-contact electrical power transmission systems, a load connected between output terminals at the secondary side of a transformer has been specified. There has not been discovered a commercially applied example of a non-contact electrical power transmission system which is applicable to plural kinds of loads as a load connectable between output terminals, and also even to one kind of load whose current largely changes. Meanwhile, such non-contact electrical power transmission systems transmit an electric power from primary side to secondary side in a condition in which an electrical insulator is provided between the primary side of the above-mentioned transformer as electric power supply side and the secondary side having a load connected between output terminals, whereby the degree of the magnetic coupling of transformer is low, the magnetic flux interlinked to the secondary winding is fewer than that generated in the primary winding, and a leakage inductance develops due to leakage magnetic flux. Although the frequency of a high-frequency AC voltage supplied to the primary winding of such transformers is generally an audible-range frequency or more (about 20 kHz or more), the above-mentioned separable transformer is low in the degree of magnetic coupling and has a leakage inductance, whereby the induced voltage of the secondary winding lowers and a voltage drop in induction reactance due to leakage inductance develops, with the result that the voltage supplied to a load may become smaller than a desired load voltage, or that the current flowing to a load may become smaller than a desired load current. Explaining with a specific example, where various plural kinds of devices with a constant load voltage and various load current are applicable to a load, the larger the load current of a load, the lower the voltage across both ends of the load becomes, whereby a performance inherent in devices cannot be exhibited. By way of example, there will be explained a rectifier circuit 7 provided on the secondary side of a separable transformer T, and a circuit for supplying a load current I to a load 10 consisting of a variable resistance through a choke coil LCH, as shown in Fig. 39 (A). Applied to the primary side of the transformer T is a high-frequency AC voltage by an inverter 3. The rectifier circuit 7, which is a well-known full-wave rectifier circuit, provides a center tap 5e on a secondary winding n2 of the transformer T, connects both ends of the secondary winding n2 to respective anodes of diodes D2, D3, and connects cathodes of both the diodes D2, D3 to each other. Connected between the connection point of the cathodes of both the diodes D2, D3 and the center tap 5e of the secondary winding n2 is a serial circuit consisting of the choke coil LCH and the load 10, a capacitor C3 being connected parallel to the load 10. In the circuit shown in Fig. 39 (A), where a high-frequency AC voltage of the square-wave shape having a maximum amplitude 70 volt and a frequency of about 97 kHz as shown in Fig. 39 (B) is applied to a primary winding n1, with the inductance value of the choke coil LCH taken as 100 µ H, the capacity value of the capacitor C3 connected parallel to the load 10 taken as 100 µF, a gap g between a primary core 5c of the transformer T and a secondary core 5d taken as 2 mm, measuring a load voltage (output voltage) - load current characteristics and a load power - load current characteristics by changing variously the resistance value of the load 10 causes characteristics to be obtained as shown in Fig. 41. In Fig. 41, the axis of abscissa is for a load current I, the axis of ordinate on the left side for a load voltage V0, and the axis of the ordinate on the right side for a load power P, the curve V indicating the load voltage and the curve P indicating the load power. The transformer T has a configuration as shown in Fig. 40, in which the primary winding n1 is separately wrapped at two leg portions of the U-type primary core 5c, the secondary winding n2 is separately wrapped at two leg portions of the U-type secondary core 5d, and the center tap 5e is provided at the middle point of the secondary winding n2. Now, the inductance value when viewed from the primary winding terminals A-A' of the transformer T is 112 µH, the inductance value when viewed from the secondary winding terminals B-B' is 42 µH, and the mutual inductance value between the primary winding n1 and the secondary winding n2 is 91 µ H. It will be understood from Fig. 41 that as the load current I increases, the load voltage V0 substantially decreases monotonously, while the load power P becomes smaller in increased values (becomes saturated) as the load current I becomes larger. In a non-contact electrical power transmission system for charging load 10, a matching capacitor is connected in parallel or series with the secondary winding n2 of the transformer T in order to offset an effect due to the leakage inductance of the transformer T, thereby increasing an effective power taken from the primary side to secondary side of the transformer T (improving a power factor by load matching). Providing such a matching capacitor causes a power transmission efficiency to be significantly improved for a certain load, thereby allowing the system to be miniaturized. Therefore, the matching capacitor is an important component in commercially applying a non-contact electrical power transmission system. However, in a non-contact electrical power transmission system provided with the above-mentioned matching capacitor, a problem exits in that for a load whose load current I varies largely, the load voltage V0 lowers remarkably compared with a case where no matching capacitor is provided. For example, in a system which has substantially the same circuit configuration as shown in the above-mentioned Fig. 39 (A) and in which the secondary winding n2 of the separable transformer T is connected in parallel with a matching capacitor C2 as shown in Fig. 42 (A), supplying a high-frequency AC voltage of the square-wave shape having a maximum amplitude 70 volt and a frequency of about 97 kHz as shown in Fig. 42 (B) to the primary winding n1 of the transformer T and changing variously the resistance value of the load 10 consisting of a variable resistance causes a load voltage-load current characteristics and a load power-load current characteristics as shown in Fig. 44 to be obtained. Now, in Fig. 44, the axis of abscissa is for the load current I, the axis of ordinate on the left side for the load voltage V0, and the axis of the ordinate on the right side for the load power P, the curve V indicating the load voltage and the curve P indicating the load power. Hereinafter, a value obtained with (varying range of load voltage V0)/(varying range of load current) is referred to as the voltage change rate. It will be understood from Fig. 44 that the more the load current I increases, the more the voltage change rate of the load voltage V0 becomes large. It will be also understood that as load current I increases, the load power P exhibits a characteristics having a peak at a certain load current value. Furthermore, it will be understood that in a load current region for a very small load current I, the load voltage V0 becomes large. In the circuit shown in Fig. 42 (A), an equivalent circuit converted to secondary side using a voltage induced in the secondary winding n2 of the transformer T can be expressed as shown in Fig. 43. A portion between an end to which a diode D2 of the secondary winding n2 in Fig. 42 (A) is connected and the center tap 5e is expressed equivalently with a high-frequency AC source 1a and an inductance L03 in Fig. 43, while a portion between the other end of the secondary winding n2 and the center tap 5e is expressed with a high-frequency AC source 1b and an inductance L04 in Fig. 43. In the non-contact electrical power transmission system having characteristics shown in Fig. 44, as a method of making constant (stabilizing) the load voltage for a plural kinds of loads identical in load voltage and different in power, that is, different in load current, there can be devised a method such as that of providing a feedback control circuit which detects a load voltage at the secondary side of the transformer T, compares said detected voltage with a reference value to amplify an error, transmits in non-contact fashion the error-amplified signal to the primary side of the transformer T, and controls the amplitude, frequency and duty of the high-frequency AC voltage supplied to the primary winding n1 of the transformer T, or that of providing an independent stabilizing source circuit at the secondary side of the transformer T and connecting the circuit to the load. However, providing such a feedback control circuit and a stabilizing source circuit causes the number of parts to increase and the cost to become higher. Now, the better the stability of a load voltage, the higher the effect obtained by providing these circuits becomes, thereby allowing a reduced number of additional parts to be expected. Hence, there has been expected an inexpensive non-contact electrical power transmission system having a relatively simple circuit which can make constant a load voltage (output voltage) in a wide load current without adding the feedback control circuit. Furthermore, as a solution to a problem that as mentioned above, the larger the load current in a device, the lower the output terminal voltage becomes, thereby causing a performance inherent to the device not to be exhibited, for example, detecting the output terminal voltage and feedbacking signals from the secondary side to the primary side to control allows the output terminal voltage to be stabilized. However, even in such a manner, the relationship of the output terminal voltage and load power with the load current is such that as shown in Fig. 36, the load power P is substantially proportional to the load current I, and the output terminal voltage V0 can be stabilized in a light load through full load region B, but it becomes rapidly larger in a no load through minute load region A. For non-contact charge or power transmission, in order to pick up a more amount of effective power even if a little on the secondary side of the separable/ detachable transformer, a matching capacitor for load matching is often provided on the secondary winding side. It will be assumed that providing the matching capacitor causes the unique above-mentioned voltage rise different from the switching source due to normal contact coupling at the time of no-load/minute load. In order to control the output terminal voltage rise, dummy loads such as dummy resistances are connected parallel to the output terminal to generate a loss at all time to control, but in this method, a power loss at the dummy voltage becomes several watts or more, and the circuit size becomes large or the cost increases in order to control the efficiency decrease and temperature rise. US 5,926,381 describes a DC power supply apparatus including an input-side rectifier which rectifies an input AC voltage and supplies the rectified voltage to a voltage-boosting converter. The voltage-boosting converter converts the supplied voltage to a voltage of a predetermined magnitude. The predetermined magnitude voltage is, then, converted to a high-frequency voltage by an inverter and applied to a primary winding of a transformer. An output-side rectifier rectifies a high-frequency voltage induced in a secondary winding of the transformer. The inverter includes IGBTs and diodes, which are connected in a full-bridge configuration. A capacitor is connected in parallel with each IGBT. When the IGBTs are rendered conductive by the charging and discharging of the capacitors, no current flows through the IGBTs, and when the IGBTs are rendered nonconductive, the voltages applied to the IGBTs are made to become zero. SUMMARY OF THE INVENTION It is an object of the present invention to provide an inexpensive non-contact electrical power transmission system capable of making constant a load voltage in a wide load current range without making complex the circuit configuration. It is another object of the present invention to provide a non-contact electrical power transmission system capable of controlling the output terminal voltage rise even at the time of no load/minute load, and capable of making lower a loss even a case of connecting the dummy load. The non-contact electrical power transmission system according to the present invention comprises a transformer having a primary winding and a secondary winding separable and detachable therebetween, a capacitor connected parallel to the secondary winding of the above-mentioned transformer, and an output terminal provided on the above-mentioned secondary winding side and connected with a load, and supplies a high-frequency AC voltage to the above-mentioned primary winding to flow a high-frequency current to the above-mentioned primary winding, and generates an induced voltage on the secondary winding by the electromagnetic induction action, whereby a power is supplied to the load connected to the above-mentioned output terminal. This system is configured such that a voltage supplied to the above-mentioned load is substantially constant, a flowing current varies, and a power is supplied to a different load, and in the system, taking as a first condition a fact that at the time of maximum load, the time of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the time of the oscillating voltage of the above-mentioned capacitor reaching its maximum or minimum value, and as a second condition a fact that at the time of minimum load, the time of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the time of the oscillating voltage of the above-mentioned capacitor completing one cycle, the above-mentioned capacitor is set so that its capacity value satisfies simultaneously the above-mentioned first and second conditions, thereby making constant the load voltage in a minimum through maximum load current range. The above-mentioned configuration allows the load voltage to be made constant in a wide load current range with an inexpensive circuit configuration without providing the feedback control circuit, that is, without making complex the circuit. Therefore, a substantially-constant load voltage can be supplied to a load whose load current largely varies or to plural kinds of loads whose load voltages are constant and load currents are different from each other. This system may be configured such that in the system, instead of the above-mentioned second condition, taking as a second condition a fact that at the time of minimum load, the timing of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the timing of the oscillating voltage of the above-mentioned capacitor starting oscillation, and at the same time, the timing of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the timing of the oscillating voltage of the above-mentioned capacitor completing one cycle, the above-mentioned capacitor is set so that its capacity value satisfies simultaneously the above-mentioned first and second conditions. In this system, the circuit constant is set so as to satisfy a condition formula of 4 · π · f · (L02 · C2) 1/2 =1 when the leakage inductance converted to the secondary side of the above-mentioned transformer is expressed as L02, the capacity value of the above-mentioned capacitor as C2, and the frequency of the above-mentioned high-frequency AC voltage as f. This allows the output voltage to be made constant for a load current equal to or less than the maximum value in the above-mentioned load current range. This system, in which a dummy load for flowing a current equal to or more than the above-mentioned minimum value even in the load current region smaller than the minimum value in the above-mentioned load current range is connected between the above-mentioned output terminals, can flow a current equal to or more than the above-mentioned minimum value even in the load current region smaller than the minimum value in the above-mentioned load current range. This system includes a drive circuit for supplying the above-mentioned high-frequency AC voltage to the above-mentioned primary winding, and in the above-mentioned drive circuit, the frequency of the above-mentioned high-frequency AC voltage changes automatically so that when the above-mentioned load current is within the above-mentioned load current range, the voltage supplied to a load is made constant. Hence, the load voltage can be made constant in a wide load current range. This system includes a drive circuit for supplying the above mentioned high-frequency AC voltage to the above-mentioned primary winding, the above-mentioned drive circuit consisting of a resonance-type inverter. The above-mentioned drive circuit may be a partial resonance-type inverter having a resonating capacitor which is connected parallel to the above-mentioned primary winding and develops a resonance between the capacitor and the above-mentioned primary winding. This allows the load voltage to be made constant in a wide load current range while maintaining a soft switching. The above-mentioned drive circuit may be the one which includes a voltage resonance circuit by the above-mentioned primary winding and by a resonating capacitor connected parallel to the above-mentioned primary winding, and the voltage waveform of the above-mentioned high-frequency AC voltage exhibits a sinusoidal waveform. The above-mentioned drive circuit should be such that in a period when the on-time of a switching element switched in said drive circuit is constant, and a partial resonance develops, and at the same time, in at least one period of either the rising period or the falling period of the voltage waveform of the above-mentioned high-frequency AC voltage, at least one of either the time of said period or the voltage waveform of said period varies corresponding to the load current. This allows the load voltage to be made constant in a wide load current range while maintaining a soft switching. The above-mentioned inverter can employ a half-bridge type one or a push-pull type one. This causes the utilizing efficiency of the transformer core to be improved. The above-mentioned inverter may be a self-excited one which includes a feedback winding and an auxiliary winding each magnetically coupled to the primary winding of the above-mentioned transformer, a voltage-drive type switching element given an input voltage the control end through the feedback winding, and a charging/discharging circuit connected between both ends of the auxiliary winding for controlling the above-mentioned input voltage, and when a charge voltage due to an induced voltage of the auxiliary winding reaches a specified value, lowers the above-mentioned input voltage to turn off the above-mentioned switching element. This causes the rising period and falling period of, and the waveform of the voltage to vary utilizing a change in the resonance state of the voltage of the primary winding developed in the off-time of the switching element corresponding to the load current, thereby allowing the load voltage to be made constant in a wide load current range. Even in a load current region smaller than the minimum value in the above-mentioned load current range, a resistance for flowing a current equal to or more than the above-mentioned minimum value is sufficient to be connected between output terminals connected with a load. This allows the output voltage to be automatically made constant in all load current ranges. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of the system in accordance with a first embodiment of the present invention. Fig. 2 is a characteristic output graph of the above circuit. Fig. 3 is a characteristic output graph of the above circuit. Fig. 4 is a graph illustrating the operation of the above circuit. Fig. 5 is a graph illustrating the operation of the above circuit. Fig. 6 is a graph illustrating the operation of the above circuit. Fig. 7 (A) is a circuit diagram of the system in accordance with a second embodiment of the present invention; Fig. 7 (B) is a waveform chart of a high-frequency AC voltage. Fig. 8 is a graph illustrating a relationship between a capacity value of capacitors connected parallel to a secondary winding of a transformer and a load current. Fig. 9 is a graph illustrating the relationship. Fig. 10 is a graph illustrating the relationship. Fig. 11 is a graph illustrating the relationship. Fig. 12 (A) is a circuit diagram having a configuration similar to that of the first embodiment; Fig. 12 (B), Fig. 12 (C) are waveform charts of a high-frequency AC voltage. Fig. 13 is a chart illustrating the operation of the above circuit. Fig. 14 is a chart illustrating the operation of the above circuit. Fig. 15 is a circuit diagram including a drive circuit of a system in accordance with an embodiment of the present invention. Fig. 16 is a circuit diagram similar to the above circuit. Fig. 17 is a graph illustrating the operation of the above circuit. Fig. 18 is a circuit diagram of the system in accordance with a third embodiment of the present invention. Fig. 19 is a characteristic output graph of the above circuit. Fig. 20 is a graph illustrating the operation of the above circuit. Fig. 21 is a graph illustrating the operation of the above circuit. Fig. 22 is a circuit diagram of the system in accordance with a fourth embodiment of the present invention. Fig. 23 is a circuit diagram of the system in accordance with a fifth embodiment of the present invention. Fig. 24 is a circuit configuration diagram of the system in accordance with a further embodiment. Fig. 25 is a configuration diagram of a separable/ detachable transformer in the above system. Figs. 26 (a) and (b) are a waveform charts to help explain the operation of the above system. Fig. 27 is a characteristic graph to help explain the operation of the above system. Fig. 28 is a circuit diagram of the system in accordance with a further embodiment. Fig. 29 is a characteristic graph to help explain the operation of the above system. Fig. 30 is a waveform chart to help explain the operation of the above system. Fig. 31 is a circuit diagram of the system in accordance with a further embodiment. Fig. 32 is a circuit diagram of the system in accordance with a sixth embodiment of the present invention. Fig. 33 is a circuit diagram of the system in accordance with the sixth embodiment of the present invention. Fig. 34 is a circuit diagram of the system in accordance with the sixth embodiment of the present invention. Fig. 35 is a characteristic graph to help explain the operation of the embodiment shown in Fig. 31. Fig. 36 is a characteristic graph to help explain the operation of a prior art example. Fig. 37 is a perspective view showing a power feeding section to which the system of the present invention is applied. Fig. 38 (A) is a view showing a condition in which a plug and an outlet are not coupled in the above-mentioned application example; Fig. 38 (B) is a view showing a condition in which the plug and the outlet have been coupled. Fig. 39 (A) is a prior art circuit diagram; Fig. 39 (B) is a waveform chart of a high-frequency AC voltage. Fig. 40 is a schematic configuration diagram of a transformer of the above circuit. Fig. 41 is a characteristics illustrating graph of the above circuit. Fig. 42 (A) is another prior art circuit diagram; Fig. 42 (B) is a waveform chart of a high-frequency AC voltage. Fig. 43 is an equivalent circuit diagram of the above circuit. Fig. 44 is a characteristics illustrating graph of the above circuit. It has to be mentioned that the embodiments of Figs. 24 to 31 and 35 as well as the related description parts are not covered by present claims 1 to 20. However, the explanations of these Figures are relevant for the sixth embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS (First Embodiment according to the present invention) Fig. 1 shows a circuit of the non-contact electrical power transmission system in accordance with an embodiment of the present invention. This non-contact electrical power transmission system has a configuration equal to the prior art one of the above-mentioned Fig. 42 (A), except for the setting of circuit constants, such that a rectifier circuit 7 for full-wave rectifying the output of a secondary winding n2 is provided on the secondary side of a separable transformer T, a circuit in series with a choke coil LCH and a load 10 consisting of a resistance is connected between the output ends of the rectifier circuit 7, and a capacitor C3 is connected parallel to the load 10. The secondary winding n2 of the transformer T is connected parallel with a matching capacitor C2. A primary winding n1 of the transformer T is supplied with a high-frequency AC voltage from an inverter 3 as a drive circuit. The rectifier circuit 7, which is a well-known full-wave rectifier circuit, provides a center tap 5e on the secondary winding n2 of the transformer T, connects both ends of the secondary winding n2 to respective anodes of diodes D2, D3, and connects cathodes of both the diodes D2, D3 to each other. The transformer T has the same configuration as the prior art one described in Fig. 40. Meanwhile, the inventor of the present invention discovered a fact that in measuring output characteristics such as a load voltage (output voltage) - load current characteristics and a load power - load current characteristics as shown in the above-mentioned Fig. 44 by changing variously the resistance value of the load 10 in the circuit shown in Fig. 42 (A), there is a region in which changing the capacity value of the capacitor C2 causes the output characteristics to exhibit various shaped, but by selecting appropriately the capacity value of the capacitor C2, a fluctuation of the load voltage (output voltage) with respect to a change of the load current can be made small (That is, there is a region in which the above-mentioned voltage change rate can be made small). Examples of such output characteristics are shown in respective Fig. 2 and Fig. 3. Fig. 2 and Fig. 3 each is a load voltage-load current characteristic graph, and also an output characteristic graph obtained by applying a square-wave-shaped high-frequency AC voltage with a maximum amplitude 70 V and a frequency approximately 97 kHz as shown in the above-mentioned Fig. 39 (B) to the primary winding n1 and by changing variously the resistance value of the load 10 taking the inductance value of the choke coil LCH as 100 µH, the capacity value of the capacitor C3 connected parallel to the load 10 as 100 µF, and the gap g between the primary side core 5c of the transformer T and the secondary side core 5d as 2 mm in the circuit shown in Fig. 42 (A). In Fig. 2 and Fig. 3, the axis of abscissa is for a load current, the axis of ordinate on the left side for load voltage V0, and the axis of the ordinate on the right side for a load power P, the curve V indicating the load voltage and the curve P indicating the load power. It will be understood from Fig. 2 and Fig. 3 that there exists a load current range having a relatively small above-mentioned voltage change rate (changed width of load voltage V0/changed width of load current I) compared to the output characteristics of the above-mentioned Fig. 44. In Fig. 2 and Fig. 3, the maximum value (hereinafter referred to the maximum load current value) of the load current I in the load current range having a relatively small above-mentioned voltage change rate is expressed as an Imax, and the minimum value (hereinafter referred to the minimum load current value) of the load current I in said load current range as an Imin. The voltage change rate in a load current range from the minimum load current value Imin to the maximum load current value Imax is about 0.6/A. It will be understood that on the contrary, in the load voltage-load current characteristics of the above-mentioned Fig. 44, the voltage change rate in the load current range of the load current values 1 through 4 A is about 2.5 V/A, so that the load voltage-load current characteristics shown in Fig. 2 and Fig. 3 becomes sufficiently small in the voltage change rate compared to the load voltage-load current characteristics shown in Fig. 44. That is, the load voltage V0 is substantially stable in the load current range from the minimum load current value Imin to the maximum load current value Imax, thereby allowing the load voltage in the load current range to be made constant. Then, the inventor of the present invention discovered a fact that where the load current range in which the load voltage V0 becomes substantially stable in the load voltage-load current characteristics can be obtained, there are common features as shown below which will be explained with reference to Fig. 4 through Fig. 6. Fig. 4 through Fig. 6 are graphs illustrating the operation waveform of each part in the equivalent circuit converted to the secondary side of the circuit of Fig. 42 (A) shown in Fig. 43, in which (a) indicates a voltage E1S across the primary winding n1; (b), a voltage E3S across a power source part 1a; (c), a voltage VC2 across the capacitor C2; (d), a voltage VL03 across the inductance L03; (e), a voltage VL04 across the inductance L04; (f), a voltage EL across output ends of the rectifier circuit 7; (g), a current IL03 flowing to the inductance L03; (h), a current IL04 flowing to the inductance L04; (i), a current IC2 flowing to the capacitor C2; (j), a current Id2 flowing to the diode D2; (k), a current Id3 flowing to the diode D3; and (1), a load current I flowing to the load 10. First, in a circuit condition in which the magnitude of the load current I is the maximum load current value Imax, the polarity reverse time (for example, times t1, t3) of the voltage E1S across the primary winding n1 as shown in Fig. 4 (a) substantially coincides with the time (times t1, t3) when the voltage VC2 across the capacitor C2 reaches the maximum and minimum values as shown in Fig. 4 (c). The fact that this condition becomes a load matching condition in non-contact electrical power transmission has been already proposed in U.S. Ser. No. 09/407761. On the one hand, in a circuit condition in which the magnitude of the load current I is the minimum load current value Imin, at the time when the voltage VC2 across the capacitor C2 completes charging in its charging/discharging as shown in Fig. 5 (c), that is, at the times of t1, t3 when in the charging/discharging of the capacitor C2, the voltage VC2 across the capacitor C2 starts from an approximate zero volt condition the charging of the capacitor C2, reaches the maximum value or the minimum value, starts the discharging of the capacitor C2, then returns back to the approximate zero volt condition, the polarity of the voltage E1S across the primary winding n1 (and the induced voltage E3S converted to the secondary side) reverses. That is, in a circuit condition in which the magnitude of the load current I is the minimum load current value Imin, the time of the polarity reverse of the voltage E1S across the primary winding n1 substantially coincides with the time when the discharging of the capacitor C2 completes in the charging/discharging of the capacitor C2, in other words, the time when the oscillation waveform of the voltage VC2 across the capacitor C2 completes one cycle. One-cycle completion means that the voltage VC2 across the capacitor C2 starts the oscillation, passes through the maximum value or the minimum value once, then returns again back to the approximate zero volt condition, Also, the operation waveform at the time when the magnitude of the load current I exhibits an arbitrary current value from the above-mentioned minimum load current value Imin to the maximum load current value Imax is an intermediate waveform between the waveform satisfying the condition in which the load current I exhibits the maximum load current value Imax and the waveform satisfying the condition in which the load current I exhibits the minimum load current value Imin. Meanwhile, making constant the load voltage V0 is usually required for a region from a no-load time to a full-load time of a load device, so that it is desirable to have output characteristics as shown in Fig. 3 and to set the load current range at a range from the minimum load current value Imin to the maximum load current value Imax. In this case, in a circuit condition in which the load current value I exhibits a current value close to the minimum load current value Imin, that is, in a condition close to the no-load time, assuming that the waveform of a high-frequency AC voltage supplied to the primary winding n1 is a square wave, it has been discovered that the operation waveform of each part has features as shown in Fig. 6. The features exist in that the time when the oscillation waveform of the voltage VC2 across the capacitor C2 starts oscillation as shown in Fig. 6 (c) substantially coincides with the time (for example, times t1, t3) of the polarity reverse of the voltage E1S across the primary winding n1 as shown in Fig. 6 (a), and that at the time of the polarity reverse of the voltage E1S across the primary winding n1 as shown in Fig. 6 (a), the current IC2 flowing through the capacitor C2 becomes approximately zero as shown in Fig. 6 (i). If the time of starting of the oscillation of the voltage VC2 across the capacitor C2 coincides only with the time of the polarity reverse of the voltage E1S across the primary winding n1, various-shaped output characteristics other than those shown in Fig. 3 are obtained, and however, only a case exists where at the time of the polarity reverse of the voltage E1S across the primary winding n1, the current IC2 of the capacitor C2 also becomes approximately zero, and only the time of having these features, the load voltage V0 can be made constant (stabilized) in a load current range from the minimum load current value Imin at the time of approximately no-load to the maximum load current value Imax as shown in Fig. 3. It will be understood that a circuit condition satisfying such features exists in a case where expressing the leakage inductance converted to the secondary side of the transformer T as L02, the frequency (drive frequency) of the high-frequency AC voltage supplied to the primary winding n1 of the transformer T as f, and the capacity value of the capacitor C2 as C2, the following formula (1) is satisfied: 4 · π · f · (L02 · C2) 1/2 =1 Now, the leakage inductance L02 is a sum of the inductance value of the leakage inductance L03 in the equivalent circuit shown in Fig. 43 and the inductance value of the leakage inductance L04, so that the capacity value of the capacitor C2 to obtain characteristics as shown in Fig. 3 can be determined by the following formula (2): C2 = (1/L02) · {1/(4 · π · f)} 2 Also where the center tap 5e is provided on the secondary winding n2 to perform rectification as mentioned above, the leakage inductance converted to the secondary side is indicated to be divided into two of the leakage inductance L03 and the leakage inductance L04 as shown in Fig. 43, while where the secondary winging n2 of the transformer T is uniformly wrapped on to the center tap 5e, the inductance values of both the inductances L03, L04 are substantially equal to each other, so that the capacity value of the capacitor C2 to obtain the output characteristics as shown in Fig. 3 can be determined by the following formula (3): C2 = (1/ (2 · L03) · {1/(4 · π · f)} 2 Further, the inductance value of the leakage inductance L02 can be obtained by shorting between the winding terminals A-A' of the primary winding n1 and measuring the inductance value when viewed from the winding terminals B-B' of the secondary winding n2 in the configuration of the transformer T shown in Fig. 40. Also, where the voltage waveform of a high-frequency AC voltage supplied to the primary winding n1 cannot be assumed to be a square wave, for example, even in a case of a trapezoidal-shaped voltage waveform, the output characteristics as shown in Fig. 3 can be obtained by setting circuit constants to satisfy the relationship of these formulas (1) through (3). However, in this case, at the time of the polarity reverse of the voltage E1S across the primary winding n1, the current IC2 of the capacitor C2 is not necessarily required to become approximately zero. Concluding the above discussion, the non-contact electrical power transmission system of this embodiment, which is the same as the prior art configuration shown in Fig. 42 (A), has features in that taking as a first condition a fact that the time of the polarity reversal of the voltage E1S across the primary winding n1 of the transformer T substantially coincides with the time when the voltage VC2 across the capacitor C2 reaches the maximum and minimum values to specify the maximum load current value Imax of the maximum value in a load current range supplied to the load 10 in the circuit shown in Fig. 1, and as a second condition a fact that the time of the polarity reversal of the voltage E1S across the primary winding n1 substantially coincides with the time of the completion of the discharging of the capacitor C2 to specify the minimum load current value Imin of the minimum value in a load current range supplied to the load 10, the capacity value of the capacitor C2 is set to satisfy simultaneously the above-mentioned first and second conditions. Then, in the non-contact electrical power transmission system of this embodiment, compared to the configuration of the prior art non-contact electrical power transmission system shown in Fig. 42 (A), without providing the above-mentioned feedback control circuit or stabilizing power source circuit, that is, without making complex the circuit, and with an inexpensive circuit configuration, the load voltage V0 can be made constant in a wide load current I, so that a substantially -constant load voltage V0 can be supplied to a load 10 largely changing in the load current I, or to various kinds of loads 10 in which the load voltages V0 are constant and the load currents I different from each other. Now, expressing the leakage inductance converted to the secondary side of the transformer T as L02, the capacity value of the capacitor C2 as C2, and the frequency of the above-mentioned high-frequency AC voltage as f, the circuit constants are set to satisfy the following condition formula: 4 · π · f · (L02 · C2) 1/2 =1 so that for the load current I equal to or less than the maximum load current value Imax, the load voltage V0 (output voltage) can be made constant. Also, in the circuit shown in Fig. 1, in the no-load region, the current hardly flows to the load 10, so that a larger load voltage V0 region may develop as shown in Fig. 44. To solve this problem, it suffices to provide a dummy resistance between output terminals connected with the load 10 so that even in no-load condition, the current of the minimum load current value Imin can positively flow. In other words, even in a region of a load current smaller than the minimum load current value Imin, connecting a dummy load for flowing a current equal to or larger than the minimum load current value Imin between the output terminals connected with the load 10 allows a current equal to or larger than the minimum load current value Imin to flow even in a region of a load current smaller than the minimum load current value Imin. (Second Embodiment according to the present invention) Fig. 7 (A) shows a circuit of the non-contact electrical power transmission system in accordance with this embodiment. This non-contact electrical power transmission system has the same configuration as that of the first embodiment shown Fig. 1, and however, different from it in that the circuit uses a rechargeable battery as a load 10 connected between output terminals and is utilized as charging device for recharging the rechargeable battery. Components similar to those in the first embodiment are accorded the same references to omit their explanation. In this manner, where a constant-voltage load such as the rechargeable battery as a load 10 is connected, the characteristics expressing the relationship between the capacity value of the matching capacitor C2 connected parallel to the secondary winding n2 and the load current I has a trend as shown in Fig. 8. In the characteristics, the capacity value of the capacitor C2 when the load current I reaches a maximum value by changing the capacity value of the capacitor C2 is referred to as C2max. In a word, when the capacity value of the capacitor C2 is made C2max, the maximum value of the load current (charge current) I can be obtained, while when the capacity value of the capacitor C2 is made shifted from C2max, the charge current I is decreased. Meanwhile, in the circuit shown in Fig. 7 (A), changing the frequency (hereinafter referred to as the drive frequency) of the high-frequency AC voltage with a square wave supplied to the primary winding n1 as shown in Fig. 7 (B) causes the load current characteristics for the capacity value of the capacitor C2 to vary in a fashion, for example, shown with lines 1, 2, 3, 4 in Fig. 9, in which the drive frequency becomes lower in the order of 1, 2, and 3, followed by 4. Changing the drive frequency causes C2max to vary, and the higher the drive frequency is made, the lower the capacity value of C2max becomes. Therefore, as shown in Fig. 10, when the capacity value of the capacitor C2 is selected to become C2max in a condition "1" with higher drive frequency, changing the drive frequency from lower to higher value allows the charge current I to the rechargeable battery of the load 10 to be increased. Also, as shown in Fig. 11, when the capacity value of the capacitor C2 is selected to become C2max in a condition "4" with lower drive frequency, changing the drive frequency from lower to higher value allows the charge current I to the rechargeable battery of the load 10 to be decreased. Although the above-mentioned characteristics are those in case where the load voltage V0 is constant, the characteristics indicate that when such a load that an increase in load current I causes a decrease in load voltage V0 is used such as the resistant load of the load 10 as shown in Fig. 42, or the load provided with a smoothing capacitor, changing the drive frequency corresponding to a change of the load current I allows the load voltage V0 to be kept approximately constant value. More specifically, when the frequency control is performed in a manner such that the drive frequency is made highest in full load (maximum load) condition, and the drive frequency is made lowest in a no-load condition, setting the capacity value of the capacitor C2 at C2max so as to provide a desired load voltage V0 stable in the full load condition, and making lower the drive frequency as the load 10 becomes lighter allows the load voltage V0 to be made approximately constant in a wide load range from no-load to full load. Now, the capacity value C2max of capacitor C2 is set according to the self-inductance of the primary winding n1, the self-inductance of the secondary winding n2, the mutual inductance between the primary winding n1 and the secondary winding n2, and the highest drive frequency. Therefore, the drive frequency is required to be automatically changed so as to correspond to the load current I. Also, generally, when the load voltage V0 is constant, the magnitude of the voltage amplitude of the primary winding n1 is proportional to the load current I. Therefore, as a method of making constant (stabilizing) the load voltage (output voltage), there is devised a method of controlling the voltage amplitude of the input voltage (high-frequency AC voltage) of the primary winding n1 depending on the change of the load voltage V0 due to the change of load current I. As a method embodying it, there is a method with which, in a circuit as shown in Fig. 12 (A), the voltage waveform of the high-frequency AC voltage supplied to the primary winding n1 of the transformer T is made changed from the square-wave voltage waveform as shown in Fig. 12 (C) to the trapezoidal-wave-shaped voltage waveform (or the sinusoidal-wave-shaped voltage waveform not shown) as shown in Fig. 12 (B). Here, changing the voltage waveform means changing the inclination of the high-frequency AC voltage at its rising time and falling time. This utilizes voltage properties in which like the voltage E1S (high-frequency AC voltage) shown in Figs. 13 and 14, even when the maximum amplitude Emax is same, the voltage waveform varies from the square wave to the trapezoidal wave, and the smaller the inclination of rising and falling, the smaller the equivalent voltage average amplitude (equivalent amplitude voltage) becomes. Here, the voltage average amplitude Ee1 of the square wave shown in Fig. 13 is equal to the maximum amplitude Emax, and the voltage average amplitude Ee2 of the trapezoidal wave shown in Fig. 14 is smaller than the maximum amplitude Emax. When the load current I is small, the input voltage of the primary winding n1 is made the trapezoidal wave shape to make smaller the equivalent voltage amplitude applied to the primary winding n1, thereby restraining the rise of the load voltage V0, while as the load current I becomes larger, the input voltage is made close to the square wave shape to make larger the equivalent voltage amplitude applied to the primary winding n1, thereby restraining the drop of the load voltage V0, whereby the load voltage V0 can be made constant (stabilized) in a wide load current I range. Only the equivalent voltage amplitude is required to be changed without changing the maximum amplitude, so that the waveform to be changed from the square wave is not limited to the trapezoidal wave. This commercial application requires means of changing automatically the waveform corresponding to the load current I, thereby controlling automatically the equivalent voltage amplitude of the high-frequency AC voltage applied to the primary winding n1. There will be explained hereinafter a non-contact electrical power transmission system capable of changing automatically the drive frequency and the voltage waveform of the high-frequency AC voltage corresponding to the load current I. Fig. 15 is an example in which a drive circuit 3 for supplying the high-frequency AC voltage to the primary winding n1 of the transformer T is configured by a half-bridge type inverter. Components similar to those in the first embodiment are accorded the same references. This non-contact electrical power transmission system comprises the transformer T whose primary winding n1 and secondary winding n2 are separable therebetween, a DC power source E, a drive circuit 3 consisting of the half-bridge type inverter for converting the voltage of the DC power source E connected between the output terminals of the DC power source E to the high-frequency AC voltage to supply the voltage to the primary winding n1 of the transformer T, the matching capacitor C2 connected parallel to the secondary winding n2 of the transformer T, the rectifier circuit 7 for rectifying the voltage generated in the secondary winding n2 of the transformer T and supplying the rectified voltage to the load 10, the choke coil LCH inserted between the rectifier circuit 7 and the load 10, and the capacitor C3 connected parallel to the load 10. The drive circuit 3, which is a half-bridge type inverter as mentioned above, in which a serial circuit of a pair of capacitors Ca, Cb and a serial circuit of switching elements S1, S2 consisting of a pair of power MOSFETs are connected parallel to each other between the output terminals of the DC power source E, and in which the primary winding n1 of the transformer T is inserted between the connecting point of both the capacitors Ca, Cb and the connecting point of both the switching elements S1, S2. Although circulating diodes DS1, DS2 connected reversely parallel to the switching elements S1, S2 consist of body diodes of the MOSFETs of each switching elements S1, S2, these diodes DS1, DS2 may be provided separately. The DC power source E is obtained , for example, by smoothing a commercial power source. This drive circuit 3 applies the high-frequency AC voltage with a square wave to the primary winding n1 of the transformer T in a manner such that the switching elements S1, S2 are turned alternately on and off by a control circuit (not shown). The rectifier circuit 7 is a well-known full-wave rectifier circuit, in which the center tap 5e is provided on the secondary winding n2 of the transformer T, both ends of the secondary winding n2 are connected to the anodes of the diodes D2, D3, respectively, and the cathodes of both the diodes D2, D3 are connected to each other. The above-mentioned choke coil LCH is inserted between the mutual connecting point of the diodes D2, D3 and one end of the load 10, and the center tap 5e is connected to the other end of the load 10. Although the choke coil LCH is required to make continuous and smooth the load current I, the capacitor C3 is not necessarily required to be provided. Fig. 16 shows a circuit of the non-contact electrical power transmission system having the capacitor C1 connected parallel to the primary winding n1 of the transformer T in the circuit configuration shown in the above-mentioned Fig. 15. In this example, the capacity value of the capacitor C1 is made relatively large to utilize a resonance of an inductance component L1 of the primary winding n1 and the like therewith. In fact, the entire circuit is an object of the resonance when the load 10 side is viewed from the primary winding n1, the resonance condition varies according to the primary winding n1, the secondary winding n2, the mutual inductance, the capacitor C2 on the secondary side, and the load 10. The capacitor C1 is not necessarily required to be connected parallel to the primary winding n1, but may be connected to provide an equivalent circuit. For example, in the circuit shown in Fig. 16, the capacitor may be provided parallel to the switching element S1 and the switching element S2. Fig. 17 shows the operation waveform of each part of the circuit of Fig. 16, in which (a) indicates a voltage VL across the primary winding n1; (b), a current IL flowing through the primary winding n1; (c), a voltage VD2 across the switching element S2; (d), a current (drain current) ID2 flowing through the switching element S2; (e), a voltage VD1 across the switching element S1; (f), a current (drain current) ID1 flowing through the switching element S1; (g), an on/off of the switching element S1; and (h), an on/off of the switching element S2. Meanwhile, in the non-contact electrical power transmission system shown in Fig. 16, providing a time period (referred to as the dead time) in which both the switching elements S1, S2 become off as shown in Fig. 17 (g), (h) during alternately turning on/off of the switching elements S1, S2 cause the voltage VL across the primary winding n 1 together with the inductance component L1 of the primary winding n1 and the like to generate a resonance voltage by the resonance circuit of the capacitor C 1-during the dead time as shown in Fig. 17 (a), whereby the voltage varies until it reaches the power source voltage or GND level, and when the power source voltage or GND level is reached, the voltage is clamped by the circulating diodes DS1, DS2. This is a well-known effective method of eliminating the turn-on loss of each switching elements S1, S2 as a partial resonance technique or a soft switching technique. When the loss reduction is an object, the capacity value of the capacitor C1 is not required to make large, so that utilizing the parasitic capacity or the like of the switching elements S1, S2 consisting of MOSFETs allows the loss reduction to be embodied even omitting the capacitor C1. On the contrary, the present invention has features in that the waveform of the voltage VL across the primary winding n1 is made a trapezoidal wave shape as shown in Fig. 17 (a) by utilizing the capacitor C1 having a relatively large capacity value to make large the resonance frequency by the above-mentioned inductance component L1 and the like with the capacitor C1. Then, selecting a specific capacity value as the capacity value of the capacitor C1 in order to obtain a resonance voltage forming the rising and falling (both in the dead time period) of the trapezoidal wave causes both the rising and falling time to become relatively long in at no-load, and both the rising and falling time to become relatively short at full-load (provided that the on-time period of the switching elements S1, S2 is constant in a load range), whereby the drive frequency becomes low at no-load, and becomes high at full-load, so that it works in a manner to stabilize the load voltage V0. Also, the waveform of the voltage VL across the primary winding n1 is changed from the trapezoidal wave to the square wave, whereby the equivalent voltage amplitude of the voltage applied to the primary winding n1 at no-load becomes lower than for the square wave, thereby restraining the rise of the load voltage V0. The resonance determining the rising time and falling time of the voltage VL across the primary winding n1 is not established only the capacitor C1 and the inductance component L1 as mentioned above, but by the capacitor C1, the primary winding n1 when the load side is viewed from the inductance component L1 side, the secondary winding n2, the mutual inductance, and the circuit including the capacitor C2 on the secondary side and the load 10. Further, the resonance is also affected by rectifying-smoothing system. Therefore, it will be assumed that the condition change of the load 10 (for example, where the load 10 is a resistance, change of resistance value) acts mutually with the circuit system, whereby the change is reflected as change of the resonance frequency, with the result that the rising and falling time and the waveform are automatically changed. (Third Embodiment according to the present invention) Fig. 18 shows a circuit of the non-contact electrical power transmission system in accordance with this embodiment. While the drive circuit 3 in the circuit described in the second embodiment shown in Fig. 17 is a separately-excited half-bridge type inverter, the drive circuit 3 in the non-contact electrical power transmission system of this embodiment consists of a self-excited partial oscillation converter. In this embodiment, there is not required to provide separately a control circuit including an oscillation circuit for turning on/off voltage-drive type switching elements S 1 and S2 such as MOSFETs. The configuration of the transformer T, and the configuration of the rectifier circuit 7 for rectifying the output of the secondary winding n2 thereof are similar to those of the second embodiment. In the drive circuit 3, the serial circuit of a pair of capacitors Ca, Cb, and the serial circuit of the switching elements S1, S2 consisting of a pair of power MOSFETs are connected parallel to each other to the serial circuit of the DC power source E and a switch SW; the primary winding n1 of the transformer T is inserted between the connecting point of both the capacitors Ca, Cb and the connecting point of both the switching elements S1, S2; and the capacitor C1 is connected parallel to the primary winding n1. A circulating diode (not shown) consisting of the body diodes of MOSFETs is connected reversely parallel to each switching elements S1, S2. The DC power source E is obtained, for example, by rectifying/smoothing a commercial power source. This drive circuit 3 includes two feedback windings nf1, nf2 and two auxiliary windings ns1, ns2 magnetically coupled to the primary winding n1 of the transformer T, respectively. The serial circuit of resistances R7, R8 and capacitors C5, C6 is connected across respective switching elements S1, S2; and the serial circuit of the above-mentioned feedback windings nf1, nf2 and resistances R1, R4 is inserted between the connecting point of the resistances R7, R8 with the capacitors C5, C6, and the control terminals of the switching elements S1, S2. Applied to the control terminals of the switching elements S1, S2 is an input voltage through the above-mentioned feedback windings nf1, nf2. Connected across the auxiliary windings ns1, ns2 is a charging/discharging circuit for controlling the input voltage of the switching elements S1, S2. Here, the charging/discharging circuit connected across the auxiliary windings ns1 consists of resistances R2, R3, R10, diodes D11, D12, D16, D21, capacitors C7, C8, and a transistor Tr1, the diode D21 and the transistor Tr1 configuring a discharging circuit. The charging/discharging circuit connected across the auxiliary windings ns2 consists of resistances R5, R6, R11, diodes D13, D14, D17, D22, capacitors C9, C10, and a transistor Tr2, the diode D22 and the transistor Tr2 configuring a discharging circuit. There will be explained hereinafter the operation of the non-contact electrical power transmission system of this embodiment. Closing the switch SW causes the capacitors C5, C6 to be charged through the resistances R7, R8. The voltage of the capacitors C5, C6 is applied to the gate of the switching elements S1, S2, so that when the voltage of either of the capacitors C5, C6 reaches a threshold value of the switching elements S1, S2, for example, when the voltage of the capacitor C5 reaches a threshold value of the switching element S1, the switching elements S1 is turned on to cause the current IL to begin to flow through the primary winding n1. Then, an induced voltage develops in the feedback winding nf1 in the direction in which the switching element S1 continues to be on, whereby the switching element S1 maintains a stable on-condition. This induced voltage is superimposed on the electric potential of the capacitor C6. At this time, although the voltage of the capacitor C5 is discharged to the ground level by the diode D21 connected between the resistance R7 and the switching element S1, the on-condition of the switching element S1 can be sufficiently maintained even only by the induced voltage of the feedback winding nf1. Meanwhile, together with the developing of the induced voltage of the feedback winding nf1, an induced voltage develops also in the auxiliary windings ns1. The auxiliary windings ns1 is connected with the above-mentioned charging/discharging circuit, so that when the induced voltage develops in the auxiliary windings ns1, the capacitor C7 is charged through the diode D11 and the resistance R2, and at the same time, the capacitor C8 connected between the base and emitter of the transistor Tr1 is charged through the diode D15. When the charging of the capacitors C7, C8 proceeds, and in a short time, the transistor Tr1 is turned on, the gate voltage of the switching elements S1 lowers, so that the switching elements S1 is turned off. When the switching elements S1 is turned off, the current IL having flowed through the primary winding n1 is communicated to the capacitor C1 so as to maintain the current, and in such a condition, there begins a free oscillation (for convenience, referred to as the resonance) of the capacitor C1 with a circuit when the load side is viewed from the inductance component LI side of the primary winding n1. When the resonance begins, and in a short time, the voltage VD1 across the switching elements S1 becomes the power source voltage VE, the voltage VD1 is clamped through a circulating diode (not shown) consisting of the body diode of the switching elements S2 to the power source voltage VE. On the one hand, the polarity reversal of the voltage VL and the current direction change of the current IL causes a reverse voltage to be induced in the feedback winding nf1 and the auxiliary winding ns1, thereby maintaining the off-condition of the switching elements S1, and at the same time, the reversely-induced voltage of the auxiliary winding ns1 pulls through the diode D12 and the resistance R3 out the electric charge of the capacitor C7 to make approximately zero. At this time, the electric charge of the capacitor C8 is also gradually discharged through the resistance R10. Simultaneously with the operation, a positive induced voltage develops in the feedback winding nf2 and the auxiliary winding ns2. Even when the induced voltage develops in the feedback winding nf2, a delay time develops by the resistance R4 and the input capacity of the switching elements S2, so that the switching elements S2 is delayed to become on, thereby maintaining on-condition. With the voltage change period due to this resonance and with the above-mentioned delay, there is provided a period during which both the switching element S1 and the switching element S2 become off, that is, the dead time. The voltage induced in the feedback winding nf2 proceeds to charge through the diode D13 and the resistance R5 the capacitors C9, C10. Then, as the charging voltage of the capacitor C10 increases with time, the transistor Tr2 is turned on and the switching elements S2 turned off. Thereafter, a similar operation is repeated to continue the self-oscillation. Even when the dead time changes, this circuit can make substantially constant the time duration of the on-time of the switching elements S1, S2. In also the non-contact electrical power transmission system of this embodiment, the capacity value of the capacitor C2 is set to satisfy the above-mentioned formula (1). That is, the capacity value of the capacitor C2 is set to satisfy the condition of the following formula: 4 · π · f · (L02 · C2) 1/2 =1 In the circuit of Fig. 18, taking the capacity value C2 of the capacitor C2 as 0.062 µF, and the capacity value of the capacitor C1 as 0.022 µF, there is shown in Fig. 19 a result obtained by changing the resistance value of the load 10, and measuring the load voltage-load current characteristics and the load power-load current characteristics. It will be understood in Fig. 19 that the voltage change rate in a load current range from the minimum load current value Imin to the maximum load current value Imax of the load current I is on the order of 0.4 V/A, which is improved further than the voltage change rate (0.6 V/A) in the output characteristics of Fig. 3 described in the first embodiment. An example of the waveform at the time of a load current close to no-load involved in this improvement, and an example of the waveform at the time of a load current close to full-load are shown in Fig, 20 and Fig. 21, respectively. In Fig, 20 and Fig. 21, (a) indicates the voltage VL across the primary winding n1; (b), the voltage VC2 across the capacitor C2; (c), the load current I flowing through the load 10; and (d), the load voltage V0. In the load current I close to no-load shown in Fig. 20, the voltage waveform of the primary winding n1 is a trapezoidal waveform with a frequency approximately 70 kHz, while at full-load shown in Fig. 21, the voltage waveform of the primary winding n1 is a trapezoidal waveform with a frequency approximately 80 kHz. In a word, the frequency varies automatically on the order of 10 kHz, and the waveform also can automatically embody a change of a shape such as an inclination at the rising time and the falling time. The on-time of each switching elements S1, S2 at this time has been approximately 4 µseconds constantly. In this embodiment, the dead time is provided as mentioned above, and there can be discovered in a manner of trial and error a combination of circuit constants which can change automatically the rising time and falling time of the resonance voltage (either of the time being a time period from the resonance starting time to the time when the resonance voltage is clamped to the source voltage or ground potential) utilizing resonance during the dead time, or the voltage waveform according to the load. More specifically, measuring the waveform at a load close to no-load and at full-load can discover a combination of constants appropriately varying with respect to period or waveform. The output characteristics of Fig. 19, the load voltage V0 becomes large in a current region in which the load current I is smaller than the minimum load current value Imin. Since such a state is often found actually, it is sufficient to connect previously a resistance and the like as a dummy load to an output terminal (load connecting terminal) so that a load current equivalent to the minimum load current value Imin can flow. (Fourth Embodiment according to the present invention) The basic configuration of the non-contact electrical power transmission system according to this embodiment is substantially the same as that of the first and second embodiments, and the system has features in that as shown in Fig. 22, it uses a push-pull type inverter as a drive circuit 3 for supplying the high-frequency AC voltage to the primary winding n1 of the separable transformer T. (Fifth Embodiment according to the present invention) The basic configuration of the non-contact electrical power transmission system according to this embodiment is substantially the same as that of the first and second embodiments, and the system has features in that as shown in Fig. 23, there are provided diodes Df1, Df2 connected in series to the switching elements S1, S2 and reversely to circulating diodes DS1, DS2 so that the voltage of the primary winding n1 is not clamped at ground level, thereby operating the resonance voltage as the waveform of sinusoidal wave. Further, although the present invention relates to a non-contact electrical power transmission system using a separable transformer having inevitably a low-degree of magnetic coupling and a large leakage inductance for power transmission, the present invention can also be applied to a case where a fixed transformer used with the degree of magnetic coupling made lowered is used for power transmission, whereby the load voltage can be stabilized in a wide load current change range without providing a feedback control circuit, and even when providing a feedback control circuit, the number of additional parts and an additional cost can be reduced. (Further Embodiment) There will be explained hereinafter another embodiment. Fig. 24 shows a configuration of a non-contact electrical power transmission system. The system comprises a AC power source 1, an input rectifying/smoothing circuit 2 for rectifying/smoothing the AC power source 1, an inverter circuit 3, a first capacitor 6 connected parallel to a secondary winding 5b for matching the load on the secondary winding 5b side, a rectifier circuit 7 for full-wave rectifying an induced voltage of the secondary winding 5b, a current matching reactor 8 for smoothing an output current I7 of the rectifier circuit 7, a variable resistance 10 being a load, and a capacitor 11 connected parallel to the variable resistance 10. The inverter circuit 3 consists of a power converting section 4 for converting the output of the input rectifying/smoothing circuit 2 to a high-frequency power, and a transformer 5 into which the high-frequency power is inputted from the power converting section 4. The transformer 5 consists of a primary winding 5a, and a secondary winding 5b having a center tap 5e and for receiving a power whose voltage is induced by the primary winding 5a. The variable resistance 10 is connected to the output terminal of the current matching reactor 8 and to the center tap 5e of the secondary winding 5b. The primary winding 5a and the secondary winding 5b configure the separable/detachable transformer 5. The rectifier circuit 7 consists of diodes 7a, 7b, and is configured by connecting one ends of the diodes 7a, 7b serially and mutually in an opposite direction with both output ends opposite to the center tap 5e of the secondary winding 5b, and by connecting the other ends of the diodes 7a, 7b to each other. The capacitor 11 not only includes a capacitor for smoothing the output terminal voltage supplied to the variable resistance 10 and a noise controlling capacitor, but also includes a capacity due to elements or substrates and a capacity of an input part inside the variable resistance 10. Fig. 25 shows a configuration of the separable/detachable transformer 5, in which the primary side is formed by wrapping the primary winding 5a on a core 5c consisting of a magnetic substance, while the secondary side is formed by wrapping the secondary winding 5b on a core 5d and by being provided with the center tap 5e, both the primary side and the secondary side being oppositely arranged through a gap G. The characteristics of the output terminal voltage V0 and the load power P with respect to the load current I supplied to the variable resistance 10 when using such a separable/detachable transformer 5 is shown as in Fig. 36 similarly to the above-mentioned prior art example, in which the load power P is substantially proportional to the load current I, and the output terminal voltage V0 is stable in a light load-full load region B, while in a no-load-minute load region A, the output terminal voltage V0 becomes rapidly large. To restrain the output terminal voltage V0 to the voltage at a point D of Fig. 36 in the no-load-minute load region A, it is necessary to connect a dummy load such as a resistance capable of flowing the load current I at a point C at all time parallel to the variable resistance 10. However, when a loss in the dummy load exhibits several watts or more, a dummy load with a large capacity is required, leading to a large circuit size and a increased cost. When a load current I equivalent to the no-load-minute load region A in Fig. 36 is made to flow, and when a load current I equivalent to the light load-full load region B is made to flow, an induced voltage V6 of the secondary winding 5b, a voltage (pre-rectification voltage) V61 between one end of the secondary winding 5b and the center tap 5e, a voltage (pre-rectification voltage) V62 between the other end of the secondary winding 5b and the center tap 5e, a voltage (input part voltage of reactor) V8 between the input terminal of the current smoothing reactor 8 and the center tap 5e, an output terminal voltage V0, and an output current (rectifier circuit output current) I7 of the rectifier circuit 7 are shown in Fig. 26 (a) and Fig. 26 (b), respectively, with respect to their waveform. As shown in Figs. 26 (a), (b), the induced voltage V6 of the secondary winding 5b and the pre-rectification voltages V61, V62 are of a similar AC waveform with sinusoidal wave shape in both the no-load-minute load region A and the light load-full load region B. However, the rectifier circuit output current 17 exhibits a discontinuous condition in which a current flowing time and a no-current flowing time are alternately repeated in rectifier circuit in the no-load-minute load region A, while it exhibits a continuous condition in which a current flows at all time in the light load-full load region B. The reactor input part voltage V8 exhibits a waveform on which a sinusoidal wave-shaped AC voltage is superimposed in the light load-full load region B, while in the no-load-minute load region A, a ringing is superimposed to cause the waveform to fluctuate during zero period in which no rectifier circuit output current I7 flows. The output terminal voltage V0 at this time becomes larger in the no-load-minute load region A than in the light load- full load region B, so that it can be predicted that the rapid increase of the output terminal voltage V0 in the no-load-minute load region A is caused by the rectifier circuit output current I7 and the reactor input part voltage V8. In the light load-full load region B, the rectifier circuit output current I7 exhibits a continuous condition with respect to all load currents I in which the rectified waveform of the pre-rectification voltage V61 (or V62) appears as the reactor input part voltage V8 as it is. The reactor input part voltage V8 exhibits a waveform in which a sinusoidal-wave shaped AC voltage is superimposed as an AC voltage component on an amplitude V0' of the output terminal voltage V0, an amplitude V1' of the sinusoidal-wave shaped AC voltage being equal to the amplitude V0' of the output terminal voltage V0. Therefore, it will be assumed that if each waveform satisfies these features, a rise of the output terminal voltage V0 can be prevented at the load current I at that time. However, in the no-load-minute-load region A, the waveform of the reactor input part voltage V8 does not exhibit the rectified waveform of the pre-rectification voltage V61 (or V62), so that a ringing is superimposed to cause the waveform to fluctuate during the zero period when no rectifier circuit output current I7 flows. The longer the zero period when no rectifier circuit output voltage I7 flows, the larger the resistance value of the variable resistance 10 becomes (the lighter the variable resistance 10 as a load becomes). Then, the longer the zero period when no rectifier circuit output voltage I7 flows, the largely the waveform of the reactor input part voltage V8 varies compared to the waveform in the light-load-full-load region B, thus it will be assumed that the output terminal voltage V0 also increases in association with that fact. The increase in the discontinuity of the rectifier circuit output current I7 (the increase in the zero period when no rectifier circuit output voltage I7 flows) corresponds to a fact that the amplitude V1' of the sinusoidal wave-shaped AC voltage superimposed on the amplitude V0' of the output terminal voltage V0 becomes decreased compared to the amplitude V0'. In a completely no-load condition, the output terminal voltage V0 increases to near the peak voltage of the pre-rectification voltage V61 (or V62), and the amplitude V1' approaches zero. To control the rise of the output terminal voltage V0 in the no-load-minute-load region A and make small the circuit size, it will be assumed to be necessary that the load current I when the rectifier circuit output current I7 begins to vary from the discontinuous condition to the discontinuous condition is made as small as possible, and a dummy load is used combinedly. The dummy load may be substituted by the self-loss of already-used circuit parts in some cases. (Further Embodiment) Then, it has been discovered that first of all, it suffices to make larger the inductance value of the current smoothing reactor 8 as one of methods. Fig. 27 shows characteristics of the load current I and the output terminal voltage V0 when the inductance value L8 of the current smoothing reactor 8 is made changed to a large, medium, and small value. Making large the inductance value L8 of the current smoothing reactor 8 causes the load current I when the rectifier circuit output current I7 begins to vary from the discontinuous condition to the continuous condition to become smaller, thereby allowing the range in which the rise of the output terminal voltage V0 is restrained to be widened. This also allows the loss of the dummy load connected parallel to the output terminal to be made small in order to provide a commercial application. (Further Embodiment) There will be explained hereinafter a second method of making small the load current I when the rectifier circuit output current I7 begins to vary from the discontinuous condition to the continuous condition. Fig. 28 shows the circuit in which a second capacitor 9 is connected parallel to the current smoothing reactor 8 of the circuit shown in Fig. 24. Selecting appropriately the electrostatic capacity of the second capacitor 9 can widen the range in which the rise of the output terminal voltage V0 is restrained even in the no-load-minute-load region A compared to Fig. 36, as shown in Fig. 29. At the point E in the no-load-minute-load region A of Fig. 29, the induced voltage V6 of the secondary winding 5b, the voltage (pre-rectification voltage) V61 between one end of the secondary winding 5b and the center tap 5e, the voltage (pre-rectification voltage) V62 between the other end of the secondary winding 5b and the center tap 5e, the voltage (reactor input part voltage) V8 between the input terminal of the current smoothing reactor 8 and the center tap 5e, and the rectifier circuit output current I7 (the current flowing into the current smoothing reactor 8 and the second capacitor 9) are shown in Fig. 30 with respect to their waveform. Now, the induced voltage V6 of the secondary winding 5b, the pre-rectification voltage V61, and the pre-rectification voltage V62 exhibit a sinusoidal wave-shaped AC voltage waveform. Selecting appropriately the electrostatic capacity of the second capacitor 9 causes the zero period of the rectifier circuit output current I7 to be shortened, the reactor input part voltage V8 to approach the reactor input part voltage V8 shown in Fig. 26 (b), the rectifier circuit output current 17 to exhibit a continuous condition in some cases, and the reactor input part voltage V8 to exhibit a waveform on which a sinusoidal wave-shaped AC voltage waveform is superimposed, thereby allowing a rise of the output terminal voltage V0 to be restrained. The setting of the electrostatic capacity value of the second capacitor 9 to make close the rectifier circuit output current I7 to the continuous condition at the smallest load current I as shown in Fig. 30 and to make close the reactor input part voltage V8 to the rectified waveform of the pre-rectification voltage V61 (or V62) is reliably performed while confirming experimentally each waveform in the trail and error manner, and however, a condition formula to set numeral values as a guide has been discovered. This is performed by setting each value so that the inductance value L8 of the current smoothing reactor 8, the electrostatic capacity value C9 of the second capacitor 9, and the frequency f of the voltage applied to the primary winding 5e satisfy the following relationship: f = 1/{4 × π × √ (L8 × C9)} The concept of the formula (4) will be explained hereinafter. To prevent the rise of the output terminal voltage V0, it is required that the rectifier circuit output current I7 exhibits the continuous condition, and that the reactor input part voltage V8 exhibits the rectified waveform of the pre-rectification voltage V61 (or V62). Thus, it has been devised that when the reactor input part voltage V8 exhibits the waveform of the reactor input part voltage V8 of Fig. 26 (a) in the no-load-minute-load region A, if that waveform can be forcedly shaped into the waveform of the reactor input part voltage V8 shown in Fig. 26 (b), as a result, the rise of the output terminal voltage V0 will be possible to be restrained. The reactor input part voltage V8 shown in Fig. 26 (b) exhibits a waveform in which a sinusoidal wave-shaped AC voltage being an AC voltage component is superimposed on the amplitude V0' of the output terminal voltage V0, the amplitude V1' of the sinusoidal wave-shaped AC voltage becoming equal to the amplitude V0' of the output terminal voltage V0. The induced voltage V6 of the secondary winding 5b is also a sinusoidal wave-shaped AC voltage, and has the same frequency as the frequency f of the voltage applied to the primary winding 5a, while the sinusoidal wave-shaped AC voltage of the reactor input part voltage V8 has a frequency of two times the frequency f. Thus, it has been devised that in order to change forcedly the reactor input part voltage V8 into a sinusoidal wave-shaped AC voltage having a frequency of two times the frequency f of the voltage applied to the primary winding 5a, setting the inductance value L8 of the current smoothing reactor 8, and the electrostatic capacity value C9 of the second capacitor 9 so as to provide a resonance with a frequency of two times the frequency f of the voltage applied to the primary winding 5a as shown in the formula (4) allows the waveform shaping to be embodied due to filter effect. As a result of verification, given an optimum improving condition for the waveform of the reactor input part voltage V8, the formula (4) can be used to predict the electrostatic capacity value C9 of the second capacitor 9 capable of shortening the zero period of the rectifier circuit output current I7 to a largest extent, and restraining the rise of the output terminal voltage V0 down to a load current I smaller than that before improvement in the no-load-minute-load region A. Fig. 31 is an equivalent circuit obtained by converting the circuit shown in Fig. 28 to the secondary side of the transformer 5. The inductance value measured from the two terminals except for the center tap of the secondary winding 5b with the two terminals of the primary winding 5a of the transformer 5 shorted out is the leakage inductance value L5 (not shown) converted to the secondary side of the transformer 5. The leakage inductance value L5 is equivalent to a sum of inductance values L5i of two leakage inductances 5i shown in Fig. 31. Thus, the secondary winding 5b is equivalent to the serial circuit of voltage sources 12a, 12b and the two leakage inductances 5i; the voltage sources 12a, 12b generate the pre-rectification voltages V61, V62; and the outlet of the center tap of the secondary winding 5b becomes the connecting middle point of the voltage sources 12a, 12b. Here, each values are set so that the leakage inductance value L5 converted to the secondary side, the electrostatic capacity value C6 of the first capacitor 6 connected parallel to the secondary winding 5b, and the frequency f of the voltage applied to the primary winding 5a satisfy the following relationship, and the configuration is made to include the capacitor 9. f = 1/{4 × π × √(L5 × C6)} This can stabilize the output terminal voltage V0 in a wide region F from near the no-load region to full-load even without performing feedback control as shown in the characteristics of the output terminal voltage V0 with respect to the load current I of Fig. 35. (Sixth Embodiment according to the present invention) Fig. 32 is an embodiment in which the inverter circuit 3 on the primary side is made a half-bridge configuration, and a resonance capacitor 34 is connected parallel to the primary winding 5a to provide a partial-resonance type inverter. The power source is expressed as a DC power source 31 by omitting the AC power source 1 and the input rectifying/smoothing circuit 2. The inverter circuit 3 comprises the DC power source 31, the serial circuit of capacitors 32, 33 and the serial circuit of switching elements 35, 36 connected parallel to the DC power source 31, diodes 37,38 connected parallel to the switching elements 35, 36, respectively, and the parallel circuit of the primary winding 5a and the resonance capacitor 34 connected to the connecting point of the capacitors 32, 33 and to the connecting point of the switching elements 35, 36. The voltage waveform applied to the primary winding 5a exhibits a trapezoidal wave shape, the switching loss of the switching elements 35, 36 can be reduced, and the noise also be reduced, so that not only the secondary side but also the primary side can be made small in size. Therefore, there can be provided a small-size non-contact electrical power transmission system capable of stabilizing the output terminal voltage V0 in a wide load range from near no-load to full-load. However, even when restraining the output terminal voltage V0 as shown in the above-mentioned embodiment, a rise of the output terminal voltage V0 occurs near the no-load region as shown in Fig. 29. To restrain the rise, it suffices to connect to the output terminal (connect parallel to a load) a dummy load such as a resistance, a constant-voltage element, a light-emitting element and a display element any of which can flow the load current I at all time in the point E of Fig. 29. The load current I becomes considerably small when the output terminal voltage V0 rises, so that common small-size electronic parts can be used, thereby preventing the size from becoming large. Fig. 33 is an example in which the serial circuit of a light-emitting diode 13 and a resistance 14 as a dummy load is connected to the output terminal. In this system, it is desirable that there is a function of informing a user on whether a power can be transmitted to the secondary side, and thus using the light-emitting diode 13 both as the dummy load and power transmission informing parts can prevent the size from becoming large. Fig. 34 is an example in which a constant-voltage element 15 as a dummy load is connected to the output terminal. This not only allows the output terminal voltage V0 to be stabilized, but also constrains the output terminal voltage V0 to a constant value even if a transient abnormal voltage develops on the secondary side due to some cause. (Applied Example) Then, the present invention relates to a non-contact electrical power transmission system utilizing an electromagnetic induction, and when this system is applied to a detachable plug and an outlet, an exposed metallic contact can be made unnecessary, thereby eliminating an electric shock and a poor contact. Indoors, this system is suitable for a power supply source in the places relating to water such as a bathroom, a washroom, a kitchen and a toilet. Fig. 37 shows an example in which this system is applied to a bathroom outlet system. Fig. 38 (A) shows a condition in which the plug and outlet of the system have not been connected; Fig. 38 (B) shows a condition in which the plug and outlet of the system have been connected. In these figures, a wall 54 dividing the room into a bathroom interior 52 including a bathtub 51 and a bathroom exterior 53 is provided with a power feed section 55 to which this system is applied. In the power feed section 55, the primary side as an outlet is arranged to be buried in the wall, and the primary coil 5a is given a high-frequency AC from a commercial power source to generate a high-frequency flux. The primary coil 5a is opened through an insulator to the bathroom interior 52, the opening serving as an outlet 55a. An inverter 3 is contained in the outlet 55a. As an example of a load apparatus 56, a motor-driven brush for cleaning the bathroom is shown. The load apparatus 56 has through a cord 57 a plug 55b containing a waterproof-secondary coil 5b integrally with an electrical insulator, in which apparatus, the plug 55b is attached to or removed from the outlet 55a to receive or stop power transmission. The operation voltage of the load apparatus 56 is set at, for example, about 12 V. Applying the present invention allows a voltage to be maintained at about 12 V in a range from a heavy load when rotating a motor to a no load or a light load (such as for LED lamp display). In prior art, the voltage drop becomes large proportional to the load to cause the performance of an apparatus not to be exhibited, or the apparatus to be damaged. Monitoring the voltage of a load apparatus to perform an active control by PMW control and the like in order to stabilize the output voltage causes the size and cost of an outlet and a plug to be increased, while applying the present invention causes an active control to be eliminated, and even when using combinedly the active control, only a few parts to be added, thereby reducing a burden of size and cost. In a bathroom, it is necessary to provide sufficiently an electric shock prevention, so that it suffices to fill the circuit or coil contained the outlet and plug with a potting material and to insulate the coil and core with an insulating bobbin. The application of the present invention is suitable for a power feed section provided on the garden, park, road and the like subject directly to rain outdoors, for a power fed section for transmitting a power from inside the room or vehicle through a window glass to outdoors, and for a power feed section under water. No problem with a poor contact causes this system to be suitable for the power transmission in a dusty outdoors or a salty place such as seaside. With this system, power transmission can be performed through an insulator, so that a power transmission can be performed from outside to a tightly-closed space containing bacteria, hazardous material and the like. Making the coupling of the primary coil and the secondary coil to exhibit an axially-symmetric shape allows a power transmission to be performed at a rotatable area such as a robot arm. Also, the present invention is not limited to the configurations of the above-mentioned embodiments, and may be embodied in various modifications, so that the present invention includes configurations, which are equivalent in circuit and mechanism and easily replaceable, such as output rectifying systems, waveforms of voltage applied to the primary winding, primary side circuit systems, and kinds of loads.

Claims: A non-contact electrical power transmission system comprising: a transformer (T) separable/detachable between a primary winding (n1) and a secondary winding (n2); a capacitor (C2) connected parallel to the secondary winding (n2) of said transformer (T); and an output terminal provided on said secondary winding side and connected with a load (10); wherein a high-frequency AC voltage is supplied to said primary winding (n1) to flow a high-frequency current to cause an induction voltage to be generated on the secondary winding (n2) by the electromagnetic induction action, whereby an electrical power is supplied to a load (10) connected to said output terminal; characterized in that the system is configured such that the voltage supplied to said load is substantially constant and the flowing current (3) varies, or that the power is supplied to different loads; and in that taking as a first condition a fact that at the time of a maximum load (load current Imax), the time of the reversal of the voltage polarity of said primary winding (41) substantially coincides with the time when an oscillating voltage of said capacitor (C2) reaches a maximum or minimum value, taking as a second condition a fact that at the time of a minimum load (load current Imin), the time of the reversal of the voltage polarity of said primary winding (n1) substantially coincides with the time when the oscillating voltage of said capacitor (C2) completes one cycle, said capacitor (C2) is set so that the capacity value satisfies simultaneously said first and second conditions, thereby making constant the load voltage in a load current range from Imin to Imax. A non-contact electrical power transmission system as set forth in claim 1, characterized in that at the time of a minimum load (load current Imin), instead of the second condition described in claim 1, taking as a second condition a fact that the time of the reversal of the voltage polarity of the primary winding (n1) substantially coincides with the time of starting oscillation of the oscillating voltage of said capacitor (C2), and that the next time of the reversal of the voltage polarity of the primary winding (n1) substantially coincides with the time when the oscillating voltage of said capacitor completes one cycle, said capacitor (C2) is set so that the capacity value satisfies simultaneously said first and second conditions. A non-contact electrical power transmission system as set forth in claim 1, characterized in that when expressing a leakage inductance converted to the secondary side of said transformer (T) as L02, the capacity value of said capacitor as C2, and the frequency of said high-frequency AC voltage as f, circuit constants are set so as to satisfy the condition formula 4 · π · f · (L02 · C2) 1/2 = 1. A non-contact electrical power transmission system as set forth in claim 1, characterized in that in a load current range smaller than a minimum value of said load current, a dummy load for flowing a current equal to or larger than said minimum value is connected between said output terminals. A non-contact electrical power transmission system as set forth in claim 1, characterized in that there is provided a drive circuit for supplying said high-frequency AC voltage to said primary winding (n1), in which drive circuit, the frequency of said high-frequency AC voltage automatically varies so that when said load current is within said load current range, the voltage supplied to the load is made constant. A non-contact electrical power transmission system as set forth in claim 5, characterized in that in said drive circuit, the larger the load current, the higher the frequency of said high-frequency AC voltage automatically becomes. A non-contact electrical power transmission system as set forth in claim 5, characterized in that in said drive circuit, at least one of the rising time and the falling time of said high-frequency AC voltage automatically varies so as to correspond to a change of the load current, whereby the frequency of said high frequency AC voltage varies. A non-contact electrical power transmission system as set forth in claim 7, characterized in that said drive circuit includes a resonating capacitor connected parallel to said primary winding (n1), in which drive circuit, at least one of the rising time and the falling time of said high-frequency AC voltage is determined by utilizing a resonance voltage by said resonating capacitor and a leakage inductance component. A non-contact electrical power transmission system as set forth in claim 1, characterized in that there is provided a drive circuit for supplying said high-frequency AC voltage to said primary winding (n1), in which said drive circuit, the waveform of said high-frequency AC voltage varies so that in said load current range, the voltage supplied to the load is made constant. A non-contact electrical power transmission system as set forth in claim 9, characterized in that said high-frequency AC voltage varies in the voltage waveform so that the equivalent voltage amplitude of the high-frequency AC voltage increases/decreases corresponding to the increase/decrease of the load current. A non-contact electrical power transmission system as set forth in claim 10, characterized in that the voltage waveform of said high-frequency AC voltage is a trapezoidal wave shape, and the inclination of the oblique side of the voltage waveform varies corresponding to the load current, whereby the equivalent voltage amplitude varies. A non-contact electrical power transmission system as set forth in claim 11, characterized in that said drive circuit includes a resonating capacitor connected parallel to said primary winding (n1), in which drive circuit, the waveform of at least one of the rising time and the falling time of said high-frequency AC voltage is determined by utilizing a resonance voltage by said resonating capacitor and a leakage inductance component. A non-contact electrical power transmission system as set forth in claim 12, characterized in that there is provided a drive circuit for supplying said high-frequency AC voltage to said primary winding (n1), and said drive circuit consists of a resonance-type inverter. A non-contact electrical power transmission system as set forth in claim 13, characterized in that there is provided a drive circuit for supplying said high-frequency AC voltage to said primary winding, and said drive circuit is a partial resonance-type inverter having a resonating capacitor connected parallel to said primary winding for generating a resonance with said primary winding. A non-contact electrical power transmission system as set forth in claim 13, characterized in that said drive circuit includes a voltage resonance circuit by said primary winding and the resonating capacitor connected parallel to said primary winding, and the voltage waveform of said high-frequency AC voltage becomes a sinusoidal wave shape. A non-contact electrical power transmission system as set forth in claim 14, characterized in that in the drive circuit, the on-time of a switching element switched in said drive circuit is constant, and during a period when a partial resonance develops, in at least one of the rising time and the falling time of the voltage waveform of said high-frequency AC voltage, at least one of the time of the period and the voltage waveform in the period varies. A non-contact electrical power transmission system as set forth in claim 16, characterized in that said inverter is a half-bridge type inverter. A non-contact electrical power transmission system as set forth in claim 16, characterized in that said inverter is a push-pull type inverter. A non-contact electrical power transmission system as set forth in claim 17, characterized in that said inverter includes a feedback winding and an auxiliary winding each magnetically coupled to the primary winding of said transformer, a voltage-drive type switching element to which an input voltage at the control terminal is given through the feedback winding, and a charging/discharging circuit connected between both ends of the auxiliary winding for controlling said input voltage, and in that said inverter is a self-excited inverter which when a charging voltage due to the induced voltage of the auxiliary winding reaches a predetermined voltage, lowers said input voltage to cause said switching element to be turned off. A non-contact electrical power transmission system as set forth in claim 19, characterized in that even in a load current region smaller than a minimum value of said load current range, a resistance for flowing a current equal to or larger than said minimum value is connected between output terminals connected with a load.

Inventors: ABE HIDEAKI, ABE, HIDEAKI
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 2001
Main classification: H02M3
IPC codes: H02M3