POWER SUPPLY

Abstract: An inexpensive highly reliable power supply for small-sized electronic devices, in which any high-withstand-voltage diode which has been used conventionally is not used and reverse leak currents are prevented. A series circuit of a primary-side coil (34) of a switching transformer and a switching element (35) is connected between both ends of an input power source (31) and a series circuit composed of a resistor (32) and a capacitor (33) is also connected between both ends of the power source (31). The connecting point between the resistor (32) and capacitor (33) is connected to one end of the control winding (37) of the switching transformer and the other end of the winding (37) of the transformer is connected to the control terminal of the switching element (35). A discharge circuit (40) which is driven by signals from the control winding (37) of the transformer is used as a means for discharging the capacitor (33).

Description: TECHNICAL FIELD The present invention relates to a power supply device used in various electronic appliances, and more particularly to a power supply device useful as a non-contact power supply device used in small portable appliances, for example, cordless telephone, cellular telephone, PHS, video with camera, and personal computer. BACKGROUND ART In a power supply device generally known, an output is obtained at a secondary side by resonating the voltage of a primary side coil of a switching transformer and a capacitor connected at its both ends. As means for obtaining a stabilized output at the secondary side, a circuit composition for controlling the primary side, or a circuit composition for controlling the secondary side are used, among others. First, as the means for controlling the primary side, Fig. 8 shows a circuit diagram of a conventional power supply device for controlling and stabilizing the on/off period of switching by installing a control circuit at the primary side, and feeding back the gate signal of the switching element as means for stabilizing and oscillating, to the output of the switching element by an impedance circuit composed of a series circuit of resistance and diode. According to the diagram, an input power source 1 is a DC voltage rectified and smoothed from a commercial power source, and a series circuit of starting circuit composed of a resistance 2 and a capacitor 3 is connected to both ends of the input power source 1, and a series circuit of a primary side coil 4 of switching transformer and switching element 5 is connected, and a capacitor 6 is connected to both ends of the primary side coil 4 of the switching transformer. Moreover, the junction of the resistance 2 and capacitor 3 is connected to the drain of the switching element 5 through a series circuit of a resistance 7 and diode 8, and is further connected to the gate of the switching element 5 through a control winding 9 of the switching transformer. A capacitor 11 is connected to both ends of a secondary side coil 10 of the switching transformer, and a capacitor 13 is connected through a diode 12,thereby obtaining an output at both ends of the capacitor 13. Incidentally, the load side after the secondary side coil 10 of the switching transformer is separable, and an output can be obtained as required. The operation of the conventional power supply device is described below. First, when the input power source 1 is applied, a voltage starts to be charged into the capacitor 3 through the resistance 2. The voltage of the capacitor 3 is fed into the gate of the switching element 5 through the control winding 9 of the switching transformer, and when reaching the threshold voltage of the gate, the switching element 5 begins to conduct. As a result, a voltage is induced in the control winding 9 of the switching transformer and the secondary side coil 10 of the switching transformer, and the voltage of the control winding 9 of the switching transformer elevates, and the gate voltage of the switching element 5 is further increased, so that the switching element 5 is completely turned on instantly by the positive feedback action. Therefore, the current of the primary side coil 4 of the switching transformer, that is, the drain current of the switching element 5 increases linearly, and the energy is accumulated in the primary side coil 4 of the switching transformer. As the switching element 5 is completely turned on, an impedance circuit 14 of resistance 7 and diode 8 (or, an impedance circuit 15 shown in Fig. 8, instead of this impedance circuit 14) begins to discharge the voltage of the capacitor 3, that is, the gate voltage of the switching element 5. By such feedback action, when the gate voltage of the switching element 5 becomes lower than the threshold voltage, the switching element 5 is suddenly turned off. As the switching element 5 is turned off, the voltage induced in the primary side coil 4 of the switching transformer is inverted, and resonance with the capacitor 6 occurs at the same time. When this resonance voltage is inverted again, it is driven to turn on the switching element 5 again through the control winding 9 of the switching transformer. At the same time, at the secondary side, too, resonance of the secondary side coil 10 of the switching transformer and the capacitor 11 occurs, and a DC output is supplied into the secondary side load 16 by a rectifying and smoothing circuit of diode 12 and capacitor 13. Next, a prior art of controlling the secondary side output is described in a circuit diagram in Fig. 9. According to the diagram, reference numeral 20 denotes a primary side power supply unit, being composed of a DC input power source 21, a high frequency current generating circuit 22 connected thereto, a primary side resonance capacitor 23, and a primary side coil 24, and reference numeral 25 is a secondary side power source, which is provided in a separate housing from the primary side power supply unit 20, being composed of a secondary side coil 26, a secondary side resonance capacitor 27 connected to both ends of the secondary side coil 26, a secondary side rectifier 28, and an output capacitor 29 having one end connected to the secondary side rectifier 28, and other end connected to the secondary side coil 26, and moreover an output stabilizing circuit 30 and output capacitor 29 are connected, and a secondary side load (not shown) is connected to this output stabilizing circuit. As described herein, since the prior art is intended to control either the primary side or the secondary side, it is used not only as a power supply device of a general electronic appliance, but also as a non-contact type power supply device having the primary side and secondary side provided in different housings. However, in the circuit composition in Fig. 8, among the above conventional constitutions, the diode 8 used for feedback is required to have a high withstand voltage because a voltage in reverse direction is applied due to resonance of the primary side coil 4 of the switching transformer when the switching element 5 is turned off. Moreover, since the impedance of the control circuit is very high, a significant effect may be applied to the switching action of turn-on and turn-off of the switching element 5 due to reverse leak current of the diode 8 , and the diode 8 is required to have a very small reverse leak current. Yet, for operation at high frequency of several hundreds of kHz, high frequency switching is required at the same time. The diode satisfying such characteristic is very hard to manufacture and very high in cost. In the circuit composition in Fig. 9, incidentally, in order to obtain an output of high precision, a larger power loss occurs in the output stabilizing circuit 30. It is hence an aim of the invention to solve such problems and present a power supply device capable of obtaining a stable secondary output effectively. Reference may be made to EP-A-0690555 (published 3 January 1996), relative to which the present invention is characterised. DISCLOSURE OF THE INVENTION The present invention is defined in the claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit block diagram of an embodiment of a power supply device of the invention, Fig. 2 is a circuit block diagram of other embodiment, Fig. 3 is a circuit block diagram of other embodiment, Fig. 4 is its output characteristic diagram, Fig. 5 is its input voltage-output power characteristic diagram, Fig. 6 is a circuit block diagram of other embodiment, Fig. 7 is a circuit block diagram of essential parts of other embodiment, Fig. 8 is a circuit block diagram of a conventional power supply device, and Fig. 9 is a circuit diagram of other conventional power supply device. BEST MODE FOR CARRYING OUT THE INVENTION (Embodiment 1) An embodiment of a power supply device of the invention is described below while referring to Fig. 1. In the diagram, a control circuit is provided at the primary side, and according to the diagram, an input power source 31 is a DC voltage rectified and smoothed from a commercial power source, a series circuit of a resistance 32 and a capacitor 33 is connected at both ends of this input power source 31, and a series circuit of primary side coil 34 of switching transformer and switching element 35 is connected, and a capacitor 36 is connected at both ends of the primary side coil 34 of the switching transformer. The junction of the resistance 32 and capacitor 33 is connected to one end of a control winding 37 of the switching transformer, and other end is connected to the gate of the switching element 35. A discharge circuit 40 composed of a control transistor 38 and resistances 39a, 39b is driven by a signal of the control winding 37 of the switching transformer, thereby discharging the electric charge of the capacitor 33. A capacitor 42 is connected to both ends of a secondary side coil 41 of the switching transformer, and a capacitor 44 is connected through a diode 43, so that an output is obtained at both ends of the capacitor 44. The load side after the secondary side coil 41 of the switching transformer is separable, so that an output may be obtained as required. The operation is described below. When the input power source 31 is applied, a voltage begins to be charged into the capacitor 33 through the resistance 32. The voltage of the capacitor 33 is fed into the gate of the switching element 35 through the control winding 37 of the switching transformer, and when this voltage reaches the threshold voltage of the gate, the switching element 35 begins to conduct. As a result, a voltage is induced in the control winding 37 of the switching transformer and the secondary side coil 41 of the switching transformer, and by voltage elevation of the control winding 37 of the switching transformer, the gate voltage of the switching element 35 is further increased, and the switching element35 is instantly turned on completely by the positive feedback action. Accordingly, the current of the primary side coil 34 of the switching transformer, that is, the drain current of the switching element 35 increases linearly, and energy is accumulated in the primary side coil 34 of the switching transformer. At this time, when the voltage of the control winding 37 of the switching transformer reaches the threshold voltage of the control transistor 38, the control transistor 38 is turned on instantly, and the capacitor 33 begins to be discharged through the resistance 39b. By such feedback action, when the gate voltage of the switching element35 becomes lower than the threshold voltage, the switching element 35 is suddenly turned off (in the embodiment, since a field effect transistor (FET) is used in the switching element 35, the threshold voltage is the gate cut-off voltage, and when transistor is used as this switching element, the base voltage of the transistor is the threshold voltage). When the switching element 35 is turned off, the voltage induced in the primary side coil 34 of the switching transformer is inverted, and at the same time, resonance with the capacitor 36 occurs. When this resonance voltage is inverted again, the switching element 35 is driven to be turned on again through the control winding 37 of the switching transformer. Same as in the prior art in Fig. 15, at the secondary side, too, resonance of the secondary side coil 41 of switching transformer and capacitor 42 occurs, and a DC output is supplied into a load 45 through a rectifying and smoothing circuit of diode 43 and capacitor 42. (Embodiment 2) Fig. 2 is a circuit block diagram of other embodiment, and what differs from the embodiment in Fig. 1 is that a clamp circuit 48 composed of a resistance 46 and a diode 47 is added. Unnecessary spike voltage occurring in the control winding 37 of the switching transformer is applied to the gate of the switching element 35, and adverse effects may be caused on the turn-on and turn-off action of switching. Hence, the voltage applied to the gate is clamped by the clamp circuit 48 by forward voltage drop (VF) of the diode 47, thereby removing the unnecessary spike voltage. Herein, the element used in the clamp circuit 48 for correcting the temperature changes of the threshold value of the gate voltage of the switching element 35 may be either one diode or plural diodes , or a zener diode or a circuit combining them. In the foregoing embodiments, the output is determined by the on/off period of the switching action. As mentioned above, the ON period is the time from the switching element 35 being turned on until the voltage of the capacitor 33 is discharged through the resistance 39b to be lower than the threshold voltage of the switching element 35. The OFF period is the time from the switching element 35 being turned off until the voltage of the capacitor 33 is charged from the input power source 31 through the resistance 32 to reach the threshold voltage of the switching element 35. Therefore, it is known that the output is determined by the charge and discharge time of the capacitor 33. It is hence possible to set at an arbitrary output by using a variable resistor in the resistance 32 for charging and discharging the capacitor 33, or the resistance 39b. Thus, according to the embodiments, one end of the control winding of the switching transformer is connected to the junction of the series circuit of the resistance and capacitor connected to the input power source, the other end of the control winding is connected to the control terminal of the switching element, and a discharge circuit for driving by the signal of the control winding of the switching transformer is connected to one end of the control winding, and therefore, when composing the oscillation circuit and control circuit at the primary side, diode of high withstand voltage is not necessary, and there is no effect of reverse leak current, and the operation is stable, so that an almost ideal switching action is effected, and a highly reliable power supply device can be realized by using inexpensive parts. (Embodiment 3) Fig. 3 is a circuit block diagram of a different embodiment, in which an input power source 51 is a DC voltage rectified and smoothed from commercial power source, or a DC power source such as car battery, and at both ends of the input power source 51, a resistance 52 and a capacitor 53 are connected in series, and a resistance 54 is connected parallel to the capacitor 53. Moreover, at both ends of the input power source, a series circuit of a primary side coil 55 of switching transformer and a switching element 56 is connected, and a capacitor 57 is connected at both ends of the primary side coil 55 of the switching transformer. The junction of the resistance 52 and capacitor 53 is connected to one end of the control winding 58 of the switching transformer, and the other end of the control winding 58 is connected to the gate of the switching element 56. A series circuit of resistance 59 and resistance 60is connected between the drain and source of the switching element 56, a peak voltage control circuit 64 is composed by connecting a transistor 62 and a resistance 63 from the junction of the resistance 59 and resistance 60 through a constant voltage diode 61, and the collector of the transistor 62 is connected to the junction of the resistance 52 and capacitor 53. At both ends of the secondary side coil 65 of the switching transformer, a capacitor 66 is connected, and a capacitor 68 is connected through a diode 67, and an output is obtained at both ends of the capacitor 68. Incidentally, the load after the secondary side coil 65 of the switching transformer is separable, and an output may be obtained as required. Describing the operation, first, when the input power source 51 is applied, a voltage begins to be charged in the capacitor 53 through the resistance 52. The voltage of the capacitor 53 is fed into the gate of the switching element 56 through the control winding 58 of the switching transformer, and when this voltage reaches the threshold voltage of the gate, the switching element 56 begins to conduct. As a result, a voltage is induced in the control winding 58 of the switching transformer and the secondary side coil 65 of the switching transformer, and by voltage elevation of the control winding 58 of the switching transformer, the gate voltage of the switching element 56 is further increased, and the switching element 56 is instantly turned on completely by the positive feedback action. Accordingly, the current of the primary side coil 55 of the switching transformer, that is, the drain current of the switching element 56 increases linearly, and energy is accumulated in the primary side coil 55 of the switching transformer. At this time, when the voltage of the capacitor 53 is fixed at a certain voltage by voltage dividing by the resistance 52 and resistance 54, and the gate voltage of the switching element 56 is also limited at this voltage. Therefore, due to the characteristic of FET, by the limitation of the gate voltage, the drain current is also limited, and, as a result, the voltage between the drain and source is raised. Hence, the voltage of the primary side coil 55 of the switching transformer is decreased, and at the same time the voltage of the control winding 58 of the switching transformer is decreased, and thereby the gate voltage of the switching element 56 decreases until becoming lower than the threshold voltage, so that the switching element 56 is suddenly turned off. In this embodiment, meanwhile, since the field effect transistor (FET) is used in the switching element 56, the threshold voltage is a gate cut-off voltage, but when a bipolar transistor is used as the switching element, the base voltage becomes the threshold voltage. When the switching element 56 is turned off, the voltage induced in the primary side coil 55 of the switching transformer is inverted, and resonance is induced with the capacitor 57 at the same time. At this time, the drain-source voltage of the switching element 56is elevated in a sinusoidal form by the resonance phenomenon. Supposing the peak value of the drain-source voltage of the switching element 56 to be Vp, when becoming Vp > [Vz + Vbe] * R13 + R16 R16 where R13 is the resistance value of the resistance 59, and R16 is the resistance value of the resistance 60, a peak voltage control circuit 64 is put in action to effect negative feedback control to lower the voltage of the capacitor 53, thereby controlling so that Vp may be constant in every pulse of switching. Soon this resonance voltage is inverted again, and the switching element 56 is controlled to be turned on again through the control winding 58 of the switching transformer. At this time, at the secondary side, too, resonance of the secondary coil 65 of the switching transformer and capacitor 66 occurs, and a DC output is supplied to the secondary side load from the rectifying and smoothing circuit of the diode 67 and capacitor 66. Hence, the peak value Vp of the resonance voltage is controlled to be always constant in every pulse of switching in spite of fluctuations of the input power source 51. Accordingly, the output voltage generated in the secondary side coil 65 of the switching transformer is always constant as shown in Fig. 4, and an extremely stable voltage may be supplied to the secondary side output. Fig. 5 is a characteristic diagram showing the relation between the input voltage and output voltage, which shows that the output power is always kept constant as being controlled by the peak voltage control circuit 64. (Embodiment 4) Fig. 6 is a circuit block diagram of an embodiment of the invention, being a developed example of the embodiment in Fig. 3, and explaining only the difference from the embodiment in Fig. 3, in constitution, the output of the peak voltage control circuit 64, that is, the collector of the transistor 62 is directly connected to the gate of the switching element 56, and the same effects as in the embodiment in Fig. 3 are obtained. (Embodiment 5) Fig. 7 is a circuit block diagram of a peak voltage control circuit which is an essential component in an embodiment of the invention, and other circuit parts are same as in the circuit composition in Fig. 3. Instead of the transistor 62 and constant voltage diode 61 in Fig. 3, a comparator (or operational amplifier) 69 and a reference voltage 70 are used to compose a peak voltage control circuit 64a, and the same effects as in the embodiment in Fig. 3 are obtained. In the foregoing embodiments, the control circuit is provided at the primary side, and an embodiment of installing the control circuit at the secondary side is described below. INDUSTRIAL APPLICABILITY Thus, the power supply device of the invention is a power supply device capable of obtaining a stable output by controlling the primary side. In particular, (1) In an aspect in which one end of a control winding of a switching transformer is connected to a junction of a series circuit of a resistance and a capacitor connected to an input power source, other end of the control winding is connected to a control terminal of a switching element, and a discharge circuit driven by a signal of the control winding of the switching transformer is connected to the one end of the control winding, when composing an oscillation circuit and a control circuit at the primary side, high withstand voltage diodes are not necessary, there is no effect of reverse leak current and the operation is stabilized, and a nearly ideal switching operation is effected, so that a highly reliable power supply device is realized by using inexpensive parts. (2) In an aspect in which a clamp circuit is provided, by clamping the voltage applied to the gate of the switching element, unnecessary spike voltage can be removed. (3) In an aspect in which a primary side coil of switching transformer and a switching element are connected in series to both ends of an input power source, a first resistance and a capacitor are connected in series to the input power source, a second resistance is connected parallel to the capacitor, the junction of the first resistance and the capacitor is connected to one end of a control winding of the switching transformer, other end of the control winding of the switching transformer is connected to a control terminal of the switching element, and a peak voltage control circuit, as means for discharging said capacitor, composed of a voltage detection unit for detecting the peak voltage by dividing the resistance from the drain of the switching element and a control unit composed of transistor and resistance through a constant voltage diode from the junction divided by the resistance is provided as means for discharging the capacitor, when composing an oscillation circuit and a control circuit at the primary side, each pulse of switching can be controlled, and precise and stable action is realized, so that nearly ideal switching characteristic and output characteristic may be obtained. (8) In an aspect in which a primary side coil and a secondary side coil are provided in different housings, it is useful for portable electronic appliances such as cordless telephone, as a non-contact type power supply device.

Claims: A power supply device, wherein one end of a primary sidewinding (34) of a switching transformer is connected to a first terminal of an input power source (31), other end of this primary side winding is connected to one end of a switching element (35), other end of this switching element is connected to a second terminal of the input power source further one end of a first resistance (32) is connected to the first terminal of said input power source, other end of this first resistance is connected to one end of a capacitor (33), other end of this capacitor (33) is connected to the second terminal of said input power source, the junction of said first resistance (32) and said capacitor (33) is connected to one end of a control winding (37) of said switching transformer, other end of the control winding (37) of said switching transformer is connected to a control terminal of said switching element (35), characterised in that a series circuit of a second resistance (39b) and a control element (38) is connected between the second terminal of said input power source and the control terminal of said switching element (35), and a control terminal of said control element (38) is connected to the junction of said first resistance (32) and said capacitor (33), whereby the primary side winding (34) of said switching transformer and the control winding (37) may be a positive feedback through said switching element (35), and the control element (38) may be conducted by elevation of the voltage of the control winding (37) of said switching transformer over a specified value to control the voltage at both ends of said capacitor (33). A power supply device of claim 1, further comprising a clamp circuit (48) composed of a resistance (46) connected between one end of the control winding (37) of said switching transformer and the control terminal of said switching element (35), and one or a plurality of diodes (47) of same polarity connected in series, connected between the control terminal of said switching element (35) and the other end of the control winding (37) of said switching transformer. A power supply device, wherein one end of a primary side winding (55) of a switching transformer is connected to a first terminal of an input power source (51), other end of this primary side winding is connected to one end of a switching element (56), other end of this switching element is connected to a second terminal of the input power source (51), further one end of a first resistance (52) is connected to the first terminal of said input power source, other end of this first resistance (52) is connected to one end of a capacitor (53), other end of this capacitor is connected to the second terminal of said input power source (51), the junction of said first resistance (52) and said capacitor (53) is connected to one end of a control winding (58) of said switching transformer, other end of the control winding (58) of said switching transformer is connected to a control terminal of said switching element (56), characterised in that a second resistance (54) is connected parallel to said capacitor (53), a peak voltage control circuit (64) is provided as means for controlling the voltage of the capacitor (53) for dividing and detecting the peak voltage between the junction of the end of the primary side winding (55) of the switching transformer and one end of the switching element (56) and the second terminal of the input power source (51), and applying a control output signal to the control terminal of the switching element (56) when exceeding a specified voltage, the primary side winding (55) of the switching transformer and the control winding (58) are connected so as to be a positive feedback through the switching element (56), and the voltage of the capacitor (53) is discharged by the control output signal of the peak voltage control circuit (64). A power supply device of claim 1 or claim 3 wherein a primary side coil (35;55) and a secondary side coil (41;65) are provided in different housings.

Inventors: HASHIMOTO FUMIAKI, OKURA HIDEKI, ONO MAKOTO, TSUJIMOTO ETSUO, HASHIMOTO, FUMIAKI, OKURA, HIDEKI, ONO, MAKOTO, TSUJIMOTO, ETSUO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H02M3
IPC codes: H02M1, H02M3