HIGH FREQUENCY HEATING APPARATUS USING INVERTER-TYPE POWER SUPPLY

Abstract: A high-frequency heating apparatus comprises an inverter (33) including a semiconductor switch (32) and a resonance capacitor (56), a boosting transformer (35) for supplying a high-voltage power and a heater power to a magnetron (39), an inductance device (41) inserted in the heater circuit of the magnetron (39), and an inverter control unit (34) for controlling the semiconductor switch (32). The inverter control unit (34) is controlled by start control means (42) at the start time of the inverter (33) so that the conduction time of the semiconductor switch (32) becomes shorter than that under a normal operating condition and the non-conduction time thereof becomes longer than that under a normal operating condi­ tion and so that the switching period of the semiconductor switch (32) becomes substantially an integral multiple of a resonance period of the resonance circuit (35, 56) formed by the resonance capacitor (56), whereby the operating frequency of the inverter (33) at the time of starting thereof becomes substantially equal to its normal operating frequency.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an improvement of a high-frequency heating apparatus such as a microwave oven for heating foods or liquids by what is called dielectric heating, or more in particular, to an improvement of a high-frequency heating apparatus comprising an inverter using a semiconductor switch such as a transistor for generating high-frequency power thereby to supply high-voltage power and heater power to be supplied to a magnetron. DESCRIPTION OF THE RELATED ART High-frequency heating apparatuses of the above-mentioned type have so far been suggested in various configurations for reducing the size, weight and cost of a power transformer used therewith. Fig. 1 is a circuit diagram of a conventional high-frequency heating apparatus. In Fig. 1, a commercial power supply 1, a diode bridge 2 and a capacitor 3 make up a power supply 5 of an inverter 4. The inverter 4, in turn, includes a reset inductor 6, a thyristor 7, a diode 8 and a resonance capacitor 9. The thyristor 7 is adapted to be triggered at a predetermined frequency f₀ by an inverter control circuit 10, with the result that an inverter of relaxation oscillation type made up of a reset inductor 6 and a series resonance circuit including the primary winding 13 of a boosting transformer 11 and the resonance capacitor 9 is energized at the operating frequency f₀ thereby to generate high-voltage power P₀ and heater power P H respectively in the high-voltage secondary winding 13 of the boosting transformer 11 and the heater winding 14. The high-voltage power P₀ generated in the high-voltage secondary winding 13 is rectified by high-voltage diodes 15, 16 and capacitors 17, 18 and supplied to a magnetron 19. Also, the heater winding 14 makes up a resonance circuit with a capacitor 20, through which the heater power P H is supplied to the cathode heater of the magnetron 19. Numeral 21 designates a start control circuit for controlling the inverter control circuit 10 for a predetermined time during starting of the inverter 4 thereby to reduce the trigger frequency f₀ thereof. This operation is in order to keep low the on-load voltage generated in the high-voltage secondary winding 13 before the cathode of the magnetron 19 is heated up at the start time. Fig. 2 is a diagram showing a change in the high-voltage power P₀, the heater power P H and the anode voltage V AKO of the magnetron 10 under no load at the operating frequency f₀ of the inverter 4. When f₀ is a predetermined steady frequency f₀₁, P₀ and P H assume respective rated values of 1 KW and 40 W. When the inverter 4 is started with f₀ for starting the apparatus, the no-load anode voltage V AKO reaches such a high value as 20 KV or more, thereby making difficult the treatment for dielectric strength both technically and in respect of the production cost. For this reason, the inverter control circuit 10 is controlled by a start control circuit 21 in a manner to reduce f₀ to f 0S for a predetermined length of time during starting. When f₀ is equal to f 0S , it is possible to reduce V AKO to a value lower than 10 KV. The value of P H , on the other hand, is not reduced greatly but to about 30 W due to the resonance effect of the capacitor 20 included in the heater circuit. As a result, although there is a longer time required before complete heating of the cathode than when the rating of P H = 40 W is involved, there is no abnormally high V AKO generated in starting the high frequency heating apparatus. Figs. 3A, 3B and 3C are diagrams showing the manner in which the operating frequency f₀, the anode voltage V AK of the magnetron and the anode current I A of this high-frequency heating apparatus undergo a change during the starting process. As shown in Fig. 3A, the inverter control circuit 10 is controlled by the start control circuit 21 in such a way that f₀ is controlled to f 0S during the period of time from t = 0 to t = t, after which f₀ = f₀₁ holds at time t₂. As a result, as shown in Fig. 3B, the voltage V AK is regulated as V AKOmax < 10 KV, and as shown in Fig. 3C, the anode current I A starts and reaches I A1 during the time t between t₁ and t₂ thereby to produce a rated high voltage output P₀ = 1 KW. Specifically, this apparatus is so configured that after the transient period of the region B through a preheating period of the region A, the steady state of the region C is reached. In this way, the frequency f₀ is reduced to f 0S at the time of starting in a manner compatible with the resonance of the capacitor 20 in the heater circuit, thereby preventing an abnormal high voltage from being generated at the time of first starting. It is thus possible to realize a high-frequency heating apparatus that can be started stably. This conventional high-frequency heating apparatus, however, has the disadvantages mentioned below. The heater power P H is supplied from a heating winding 14 wound on the same core as a high-voltage secondary winding 13 for producing a high voltage power P₀. Therefore, as shown in Fig. 2, it is difficult to maintain P H constant against the frequency f₀, and even with the provision of a resonance capacitor 20, what can be expected is not more than preventing the value P H from changing in proportion to P₀, thus attaining the characteristic as shown by a dashes curve at most. Specifically, it is impossible to realize more than attaining P H of 30 W when f₀ is reduced to f 0S . Fig. 4 is a diagram showing an example of the relationship between the heater power P H and the time before start of oscillation of the magnetron after the heater power P H is supplied to heat the cathode sufficiently, that is, the oscillation start time t s . As seen from this diagram, in the prior art, it is possible to prevent generation of an abnormal high voltage but it is difficult to supply a sufficient heater power P H during the starting process, so that the oscillation start time t s is increased to several times longer than when the rated P H (= 40 W) is supplied. Specifically, the region A shown in Fig. 3C is lengthened, with the result that an application of the prior art to a high-frequency heating apparatus such as a microwave oven featuring quick cooking in the order of seconds would unavoidably lead to a reduced material function. In Fig. 5A, the period of time t from t₁ to t₂ is one where the heater power P H is gradually increased while the high-voltage power P₀ to the magnetron (that is, the anode current I A ) is increased in the manner shown in Fig. 5C. Figs. 5A, 5B and 5C are diagrams showing a relationship in which the heater power P H , cathode temperature T C and high-voltage power P₀ increase with the increase in f₀ from f 0S to f₀₁. As obvious from these diagrams, the cathode temperature T C which has a predetermined thermal time constant is delayed by τ behind the increase in P H , and reaches a rated temperature when t is t₃. The power P₀, on the other hand, increases at the same time as P H , and therefore the period involved, that is, from t₁ to t₃ is one when the cathode is liable to be short of emission or the like phenomenon. That such a region as this is long results in a very material disadvantage of reducing the service life of the cathode of the magnetron extremely. Further, to configure a resonance circuit including a capacitor 20 in the heater circuit of the magnetron 19 is very inconvenient in view of the small cathode impedance and the high potential thereof. A prior art circuit of this type is described in JP-B-5914235. The present invention has been developed in order to solve the above-mentioned problems of the prior art. According to the present invention there is provided a high-frequency heating apparatus comprising power supply means, an inverter including at least one semiconductor switch and a resonance capacitor, a boosting transformer forming a resonance circuit with the resonance capacitor for supplying high-voltage power and heater power to a magnetron, inductance means connected in series with the cathode of the magnetron, and inverter control means for controlling the conduction time of the semiconductor switch with a predetermined repetition period, characterised by comprising start control means for supplying a modulation command to the inverter control means at the time of starting the inverter, and wherein the inverter control means is constructed to respond to the modulation command and to control the conduction time of the semiconductor switch so that it is shorter at said time of starting than when the magnetron has started oscillation and the non-conduction time thereof so that it is longer at said time of starting than when the magnetron has started oscillation, where the non-conduction time length is substantially equal to or an integral multiple of a resonance period of the resonance circuit, thereby controlling an operating period of the inverter to become substantially equal to or longer than the operating period thereof after the magnetron (39) has started oscillation. A heating apparatus having a configuration described above has the effects and functions described below. At the time of starting the inverter, a modulation command signal of start control means is applied to inverter control means, which reduces the conduction time of a semiconductor switch to a length shorter than the conduction time when the magnetron has started oscillation, while at the same time increasing the non-conduction time of the semiconductor switch to a length longer than the non-conduction time after the magnetron has started oscillation, and that, to a value in proximity to an integral multiple of the resonance period of the resonance circuit, thereby rendering the operation period of the inverter equal to or longer than the normal period. Since the conduction time of the semiconductor is reduced, the output voltage of the boosting transformer is kept low so that both the high output voltage and the heater output voltage are controlled at a low level. At the same time, the non-conduction time is prevented from being increased thereby to prevent the operation cycle to be shortened and is controlled at a period equal to or longer than the one for a normal operation. As a result, the impedance of the inductance means arranged in series with the cathode of the magnetron is prevented from increasing, and therefore the current flowing in the cathode is controlled at a proper value equal to or larger than the one for a normal operation. Further, since the non-conduction time is controlled substantially at an integral multiple of the resonance period of the resonance circuit, the terminal voltage for conducting the semiconductor switch takes almost a minimum value. The switching loss of the semiconductor switch is thus reduced greatly while realizing the modulation control for the starting operation mentioned above. As a consequence, the loss of the semiconductor switch is reduced thereby to prevent an abnormal high voltage from being generated at the time of starting on the one hand, and the heater power is controlled at a proper value equal to or larger than the one for a normal operation on the other hand. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a circuit of an example of the prior art. Fig. 2 is a diagram showing a characteristic of an example of the prior art. Fig. 3 is a diagram showing waveforms produced at various parts in operation of the prior art. Fig. 4 is a diagram showing a characteristic of a magnetron according to the prior art. Figs. 5A to 5C are diagram showing waveforms for illustrating the characteristics of the same magnetron. Fig. 6 is a block diagram of a high-frequency heating apparatus according to an embodiment of the present invention. Fig. 7 is a circuit diagram of the same apparatus. Figs. 8A to 8G are diagrams showing waveforms of various parts in operation of the same circuit. Figs. 9A to 9F are diagrams showing waveforms produced at various parts in operation at the time of starting of the same circuit. Figs. 10A to 10F show waveforms illustrating changes in various parameters of the same circuit at the time of starting. Fig. 11 is a circuit diagram of inverter control means and starting control means of the same circuit. Fig. 12 is a diagram showing a part of the circuit of a high-frequency heating apparatus according to another embodiment of the present invention. Figs. 13A to 13D are diagrams showing voltage and current waveforms for explaining the operation of the same circuit. Fig. 14 is a circuit diagram illustrating another embodiment of the starting control means. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be explained below with reference to the accompanying drawings. A block diagram of a high-frequency heating apparatus according to the present invention is shown in Fig. 6. In Fig. 6, a power supply 31 is a unidirectional power supply of a direct current or a pulsating voltage obtained from a battery or a commercial power supply for supplying power to an inverter 33 including a resonant capacitor and one or a plurality of semiconductor switches such as transistors. Inverter control means 34 operates the semiconductor switch 32 with a predetermined conduction time and a non-conduction time substantially equal to the resonance period of the resonant capacitor and a boosting transformer 35 thereby to supply a high-frequency power to the primary winding 36 of the boosting transformer 35. As a result, high-voltage power P₀ and heater power P H are generated in the high-voltage secondary winding 37 of the boosting transformer 35 and the heating winding 38, both of which power are respectively supplied to the anode-cathode circuit of the magnetron 39 and a cathode heater 40. The cathode heater (that is, a cathode) is connected in series with inductance means 41, so that a load of the heater winding 38 is made up of a series circuit including the inductance means 41 and the cathode heater 40. Start control means 42 is for giving a modulation command to the inverter control means 34 at the time of starting the inverter 33. In response to this modulation command, the inverter control means 34 controls the conduction time of the semiconductor switch 32 in starting operation at a value smaller than under a normal condition, while at the same time increasing the non-conduction time longer than under normal condition to a length substantially equal to an integral multiple of the resonance period, so that the semiconductor switch is turned on when the terminal voltage thereof is minimum. In this way, the output voltage of the inverter 33 is reduced while reducing the switching loss of the semiconductor switch, and at the same time, the operation period is controlled to a length substantially equal to or longer than under a normal condition, thereby preventing the impedance of the inductance means 41 from increasing. The current flowing in the cathode heater 40 is thus substantially controlled at a proper value equal to or larger than the current under a normal condition. This configuration prevents the voltage generated in the high-voltage secondary winding 37 from increasing abnormally, and is capable of supplying a heater current (that is, heater power P H ) that can assure a stable, superior operation of the cathode heater 40. Further, the loss of the semiconductor switch is kept low. As a consequence, a complicated resonance circuit is not required in the heater circuit, and the oscillation start time of the magnetron 39 is sufficiently reduced, thereby making possible a speedy start of dielectric heating. Also, a condition liable to cause emission shortage of the cathode is prevented from occurring thereby to assure a long service life and high reliability. At the same time, a small loss of the semiconductor switch makes it possible to provide a high-frequency heating apparatus that realizes high reliability and a low cost. Fig. 7 is a circuit diagram showing a high-frequency heating apparatus more in detail according to an embodiment of the present invention. Those component parts corresponding to those in Fig. 6 are designated with the same reference numerals as in Fig. 6 and will not be described any further. In Fig. 7, a commercial power supply 51 is connected through an operation switch 52 to a diode bridge 53 and also to inverter control means 34. When the operation switch 52 is turned on, unidirectional power is supplied to the inverter 33 through the capacitor 55 while at the same time energizing the inverter control means 34 and the start control means 42. The inverter 33 includes a composite semiconductor switch 32 having a resonance capacitor 56, a bipolar MOSFET (hereinafter referred to as MBT) 58 and a diode 59. The conduction time and the non-conduction time of the inverter 33 are controlled by a sync oscillator 61 of the inverter control means 34. The start control means 42 is for giving a modulation command to the operation of the sync oscillator 61 of the inverter control means 34 for a predetermined length of time when the operation switch 52 is turned on. Now, the operation of the embodiment shown in Fig. 7 will be explained with reference to Fig. 8. Figs. 8A, 8B, 8C and 8D are diagrams showing waveforms of a current I c/d flowing in the composite semiconductor switch, a terminal voltage applied thereto, a control voltage V G applied to the gate of the MBT 58, an anode-cathode voltage V AK of the magnetron 39 and an anode current I A . The sync oscillator 61 is so configured as to detect a point P in Fig. 8B, that is, a point where the voltage V CC of the capacitor 55 crosses the terminal voltage V CE of the composite semiconductor switch 32 and, a predetermined time Td later, to apply V G to the MBT 58. The oscillator 61 is thus adapted to turn on the MBT 58 in synchronism with the timing when the voltage V CE generated by the resonance of the resonance capacitor 56 and the primary winding 36 of the boosting transformer 35 is reduced to zero (synchronous control). Since the MBT 58 is turned on when the resonance voltage is substantially zero, the switching loss is greatly saved. A detailed explanation of the timing for controlling the MBT 58, which will be made later with reference to Fig. 11, will be omitted here. An output of the inverter 33 is capable of being regulated by controlling the ratio between the conduction time T on and the non-conduction time T off of the MBT 58. The value T off is actually determined by the circuit constant of the resonance circuit as a result of the above-mentioned sync control (that is to say, the time T off takes a value in proximity to the resonance period of the resonance circuit), and therefore it is possible to regulate the output of the inverter 33 by controlling the time T on . Since the voltage of the capacitor 55 is a pulsating voltage, the current I c/d and voltage V CE in Fig. 8A and Fig. 8B take waveforms having an envelope as shown by dotted lines in Figs. 8F and 8G. In this way, the inverter 33 performs the sync oscillation operation by sync control under a normal condition. The sync oscillator 61, however, performs a modulating operation as described below in response to a modulation command of the start control means 42 for a predetermined length of time (such as one second or two at the time of start of the inverter 33. Figs. 9A, 9B and 9C show waveforms of I c/d , V CE and V G produced at the time of such a modulating operation. Unlike in the case of Figs. 8A, 8B and 8C, the sync control in synchronism with one time the resonance period of the resonance circuit is not effected. Specifically, in Fig. 8B, a waveform of the resonance operation that appears as a waveform of V CE is similar to the one one time the resonance period of the resonance circuit, in synchronism with which the MBT 58 is subjected to on-off control. In spite of this, a non-conduction time T off , which is an integral multiple of twice the resonance period T r of the resonance circuit is involved for the modulation operation as shown in Fig. 9B. (In Fig. 9B, T off , is approximately double the value of Tr) As explained above, the sync oscillation control exactly one time the resonance operation may not be effected but as shown in Fig. 9, the value T off , may be controlled to a value substantially equal to an integral multiple of Tr, whereby the MBT 58 is turned on with a small V CE , and the peak current I CS for switching the MBT 58 is kept comparatively small, thus reducing the switching loss. If T off , is displaced from a value proximate to an integral multiple of Tr as shown in Figs. 9D, 9E or 9F, however, the MBT 58 turns on when V CE takes a large value, and therefore the current I CS assumes a very large value as compared with the case of Fig. 9A. As a result, the switching loss of the MBT 58 becomes extremely large, and the reliability of the MBT is unavoidably reduced on the one hand while a large cooling fan is required for radiation on the other, thereby undesirably leading to a high cost. In the case of Figs. 9D, 9E and 9F, T off , is about 1.5 times Tr, and therefore the MBT turns on when V CE is maximum. The conduction time T on , of the MBT 58 is thus controlled smaller than T on for a normal operation, and at the same time, the non-conduction time T off , is kept larger than T off under a normal condition at a value one time or a greater integral multiple of the resonance period Tr of the resonance circuit, with the result that the repetitive period T o , is controlled at a value substantially equal to or larger than T o under a normal operation. As a consequence, the MBT 58 turns on when the terminal voltage V CE thereof is minimum, thereby keeping the switching loss at a small level, and T o may be controlled to a value equal to T o or T o′ which is longer than T o at the time of starting the inverter. Thus the high voltage generated in the secondary winding 37 of the boosting transformer 35 is dampened while at the same time controlling the heater current supplied from the heater winding to the cathode of the magnetron 39 at a value equal to or higher than under a normal condition. The values T on′ , T on , T off′ , T off , T o and T o′ may be determined appropriately depending on the values of the ratio of the impedance of the inductance means 41a and 41b inserted in the heater circuit of the magnetron 39 to the impedance of the cathode heater, the self inductance and mutual inductance of the three windings of the boosting transformer 35 and the resonance capacitor 56. An example is described now. As shown in Fig. 7, the inductance means 41a and 41b of the heater circuit are so constructed as to serve also as a choke coil making up a TV noise-dampening filter of the magnetron. The inductance of the inductance means is thus selected at about 1.8 µH respectively. Also, the impedance of the cathode heater often finds applications in the value of about 0.3 ohm. An experiment conducted by the inventors using a magnetron satisfying such conditions as mentioned above and a boosting transformer of an appropriate constant together with a resonance capacitor shows that if the sync oscillator 61 performs a modulating operation by the start control means 42 in the manner mentioned below, it is possible to maintain the anode-cathode voltage V AKC below 10 KV at the time of starting while at the same time increasing the starting heater current I H , to be larger than the value I H under a normal condition. Specifically, T o = 40 µS, T on = 29 µS and T off = 11 µS are modulated to T o′ = 63 µS, T on′ = 8 µS and T off′ = 55 µS, respectively, thereby to realize I H′ = 12 A for I H = 10.5 A, and hence an extremely stable starting process. At the same time, the average loss of the MBT 55 during modulation is reduced to less than about 50 W. This reduction in average loss is, for example, about 60% of the average loss of about 80 W for 1.5 times the resonance period Tr. The starting heater power P H′ is thus increased by 1.3 times as indicated by P H′ /P H = (12 A/10.5A)² 1.3 as compared with the value P H for a normal operation, thus making possible rapid heating of the heater. In addition, an excessive loss of the MBT is prevented, thereby assuring high reliability without using any large heat radiation fin. Fig. 10 is a diagram showing the above-mentioned conditions for starting, in which Figs. 10A to 10F show the manner in which the operating frequency f₀ (= 1/T o ), T on , T off , I H , V AK and I A of the inverter undergo a change from starting to a normal steady operation. During the period of t S of 1.5 seconds when T on and T off are controlled to T on′ and T off′ respectively by the start control means 42, the inverter output is held low, and in spite of the voltage V AKO being limited to 8 KV, the current I H , is controlled to 12 A which is larger than I H of 10.5 A for a normal operation. By this control operation, speedy oscillation start of the magnetron is realized while preventing generation of an abnormally high voltage without configurating any complicated resonance circuit in the heater circuit which takes a high potential. Further, by preventing any case of emission shortage of the cathode, a high-frequency heating apparatus is realized which has very high reliability. Furthermore, the increase in the loss of the MBT 58 which is likely to occur in the process is kept small and thus high reliability is assured without any bulky cooling unit. Fig. 11 is a circuit diagram showing the inverter control means 34 and the start control means 42 of Fig. 7 more in detail according to an embodiment. In Fig. 11, the parts designated by the same reference numerals as in Fig. 7 indicate component parts having corresponding functions and will not be described here. Fig. 11 shows a specific example of a configuration of the sync oscillator 61 of the inverter control means 34 and the start control means 42. In order to produce a sync signal shown in Fig. 8B, a voltage V CC of a capacitor 55 and a collector voltage of the MBT 58 are detected by a comparator 104 as voltages divided by resistors 100, 101 and 102, 103 respectively. The rising output of the comparator 104 is converted into a pulse signal in a delay circuit 105 and a differentiation circuit 106, and resets an RS-FF 108 through an OR circuit 107. The Q output of the RS-FF is used to drive the gate of the MBT 58, while at the same time starting an on-timer for determining the time T on . The on-timer is comprised of resistors 109 to 111, a capacitor 112, a diode 113, a comparator 114 and a reference voltage source 115. Numeral 116 designates an inverter buffer through which an output of the comparator 114 is applied to the S input terminal of the RS-FF. As a result, the FF is set so that Q becomes Lo after the lapse of the time T on determined by the reference voltage source 115. The output Q of the FF is adapted to start an off-timer including resistors 117 to 119, a capacitor 120, a diode 121 and a comparator 122 and determines the maximum value of the time T off . More specifically, an output of the comparator 122 is supplied through an inverter buffer 123 and a differentiation circuit 124 to the OR circuit 107. In the case where a sync signal fails to be detected by the comparator 104 after the lapse of a predetermined time length following the time point when Q becomes Hi (that is, when the MOS FET 58 turns off with Q at Lo), the RS-FF is forcibly reset to cause Q to become Hi. If the value T off determined by the off-timer is set to a value proximate to an integral multiple of the resonance period of the resonance circuit, it is possible to turn on the MBT 58 when V CK is comparatively small as shown in Fig. 9B. Numeral 125 designates a start circuit which is energized by resetting the RS-FF with one pulse applied to the OR circuit 107 when the inverter is started. During a normal operation of the inverter 33, a sync pulse is applied to the RS-FF from the comparator 104, and due to the resultant sync oscillation, the inverter produces operation waveforms shown in Fig. 8. When the inverter is started, the sync oscillation is prevented and controlled at a sync oscillation by the start control means 42 including resistors 125 to 128, a capacitor 129, a comparator 130, an inverter buffer 131, diodes 132, 133 and a resistor 134. At the same time, the time T on is controlled at a value smaller than under a normal operation. Specifically, when the inverter is started, the output of the comparator remains Hi for a predetermined length of time t S (1.5 seconds), and therefore the resistor 103 is substantially shortened, and the comparator 104 is prevented from detecting the sync signal. For this reason, the inverter becomes asynchronous, so that the non-conduction time T off of the MBT 58 is determined by the off-timer including the comparator 122, etc. If this off time is set to 55 µS, for instance, the condition shown in Fig. 10C is realized. Further, at the same time, output of the comparator 130 operates to apply a voltage, which is obtained by dividing the voltage of the reference voltage source 115 by the resistors 110 and 134, to an input to the comparator 114. As a result, the time T on during the period t S is smaller than under a normal condition, since the set time of the on-timer is small, and therefore the condition of Fig. 10B is realized by setting the on-timer to, say, 8 µS. The inverter control means of sync oscillation type having a timer limiting the non-conduction time is constructed as described above in such a way that a sync signal is interrupted for a predetermined length of time t S at the time of starting the inverter while at the same time controlling the time T on to be smaller than that under normal condition. And the non-conduction time is rendered to coincide substantially with an integral multiple of the resonance period of the resonance circuit, whereby the loss of the semiconductor switch is kept low and thus high reliability is assured without using any bulky cooling configuration. In this way, the inconveniences of the prior art are overcome, and the complicated resonance circuit is eliminated from the heater circuit, thereby realizing a high-frequency heating apparatus that can assure high reliability as well as speedy start of magnetron operation. Fig. 12 is a diagram showing a circuit of a high-frequency heating apparatus according to another embodiment of the present invention. This circuit configuration is a modification of the configuration of the high-voltage secondary circuit of the embodiment shown in Fig. 7. In Fig. 12, a high-voltage secondary winding 37 of a boosting transformer 35 is connected with a high-voltage capacitor 150 and a diode 151 thereby to make up a multiple voltage rectifier circuit. In this configuration, the self inductance and mutual inductance of the primary winding 36 of the boosting transformer 35, the high-voltage secondary winding 27 and heater winding 38 and the resonance capacitor 56 are set to appropriate values respectively in design thereby to attain substantially the same functions and effects as in the aforementioned embodiments. Fig. 13 shows waveforms of I c/d and V CE at the time of a normal steady operation and starting with the circuit of Fig. 12. Figs. 13A and 13B show I c/d and V CE for a normal condition, in which T o , T on and T off take values of about 45 µS, 30 µS and 15 µS respectively. Under this normal condition, the conduction time of the MBT 58 is controlled at T on , as shown in Fig. 13C, and I c/d and V CE assume waveforms shown in Figs. 13C and 13D respectively, thereby performing the repetitive operation at time intervals of T o′ , T on′ and T off′ , which respectively assume values of about 42 µS, 20 µS and 22 µS in the process. As a result of measuring the heater current I H supplied to the magnetron 39 in this case, it was found that the heat current may be regulated to 10 A for a normal operation and 12 A for starting operation on condition that the value V AKO is kept at 7 KV. Specifically, by appropriately selecting the constants of the boosting transformer 35 and the resonant capacitor 56, the resonance waveform of V CE for starting time (that is, the time of non-oscillation of the magnetron) can be made to have a low frequency resonance waveform as compared with a normal condition. In starting, therefore, as shown in Fig. 13D, the non-conduction time T off′ can be controlled at a value about one time the resonance period Tr of the resonance circuit thereby to make the repetitive period T o′ have a length substantially equal to T o . As a result, it is possible to supply a heater current I H larger than under a normal condition to the magnetron without generating an excessively high voltage V AKO at the time of starting, thus providing a high-frequency heating apparatus with a magnetron of which a speedy actuation and high reliability are assured without using any complicated resonance circuit in the heater circuit. In this case, the setting of the off-timer including the comparator 122 shown in Fig. 11 at its center may be T off′ substantially equal to the starting resonance period Tr shown in Fig. 13D, or the diode 132 may be removed for effecting a sync oscillation control using the comparator 104. The start control means 42 shown in Fig. 11 is a simple timer circuit with the starting modulation time thereof determined simply by the time such as 1.5 seconds. This start control means 42, however, may be alternatively so constructed in an improved performance as to detect that the cathode of the magnetron 39 has been sufficiently heated and has started oscillation. For instance, a change in the anode-cathode voltage V AK of the magnetron 39 from V AKO = 7 to 8 KV for non-oscillation to V AK = 4 KV for oscillation or the beginning of a slight flow of the anode current I A as shown in Fig. 10F may be detected. In other words, by constructing the start control means 42 as shown in Fig. 14, it is possible to detect the start of an oscillation of the magnetron 39 as mentioned above from the decrease in the voltage V AK (from 7 KV to 4 KV). In Fig. 14, the boosting transformer 35 has an output voltage detection winding 160 for detecting the magnitude of the voltage V AK , an output signal of which is converted into a DC voltage through a diode 161, a capacitor 162, and resistors 136, 164 and supplied to a comparator 130. When the magnetron 39 oscillates and the voltage V AK drops with the terminal voltage of the resistor 164 lowering from a reference voltage determined by the resistors 126, 127 and 128, the output of the comparator 130 becomes "High". As a result, the positive input voltage of the comparator 114 in Fig. 11 also increases and becomes equal to the reference voltage, so that the conduction time of the MBT 58 becomes as long as a normal conduction time. In this way, the start control means 42 is provided with means for detecting a change in the condition of the magnetron 39, the inverter 33 or the boosting transformer 35 in some form or other, and thus switching the conduction time of the MBT 58. Thus it is possible to control the starting modulation in accordance with the rate of temperature increase of the cathode of the magnetron 39, thereby permitting the operation of the magnetron 39 always at a maximum output with the shortest length of time. It will thus be understood from the foregoing description that according to the present invention, an output of an inverter is supplied to the anode-cathode circuit and a cathode heater of the magnetron through a boosting transformer, inductance means is connected in series with the cathode heater, and start control means is inserted for giving a modulation command at the time of starting the inverter. In response to this modulation command, the inverter control means reduces the conduction time of a semiconductor switch to a value smaller than that under a normal condition, while at the same time increasing the non-conduction time one time or an almost integral multiple of the resonance period of a resonance circuit, whereby the operating period of the inverter becomes substantially equal to or longer than that under a normal condition. As a result, an abnormally high voltage is prevented from being generated at the time of starting without the need of any complicated resonance circuit in the heater circuit producing a high potential on the one hand and keeping the loss of the semiconductor switch to a low level on the other. In addition, a speedy start of oscillation of the magnetron is realized. Further, the cathode is preheated sufficiently at the time of starting, and therefore any phenomenon of emission shortage of the cathode and hence the deterioration of the cathode is prevented, thus realizing a high-frequency heating apparatus with high reliability.

Claims: A high-frequency heating apparatus comprising power supply means (31), an inverter (33) including at least one semiconductor switch (32) and a resonance capacitor (56), a boosting transformer (35) forming a resonance circuit with the resonance capacitor (56) for supplying high-voltage power and heater power to a magnetron (39), inductance means (41) connected in series with the cathode of the magnetron (39), and inverter control means (34) for controlling the conduction time of the semiconductor switch (32) with a predetermined repetition period, characterised by comprising start control means (42) for supplying a modulation command to the inverter control means (34) at the time of starting the inverter, and wherein the inverter control means (34) is constructed to respond to the modulation command and to control the conduction time of the semiconductor switch (32) so that it is shorter at said time of starting than when the magnetron (39) has started oscillation and the non-conduction time thereof so that it is longer at said time of starting than when the magnetron (39) has started oscillation, where the non-conduction time length is substantially equal to or an integral multiple of a resonance period of the resonance circuit (56), thereby controlling an operating period of the inverter (33) to become substantially equal to or longer than the operating period thereof after the magnetron (39) has started oscillation. A high-frequency heating apparatus according to claim 1, wherein the inductance means (41) is formed by a choke coil of a noise filter which is connected in series with the cathode of the magnetron (39). A high-frequency heating apparatus according to claim 1, wherein the impedance of the inductance means (41) is equal to or greater than that of the cathode of the magnetron (39) when the magnetron (39) is oscillating. A high-frequency heating apparatus according to claim 1, wherein a voltage doubler rectifier circuit (150, 151) is inserted between the boosting transformer (35) and the magnetron (39). A high-frequency heating apparatus according to claim 1, wherein the inverter control means (34) comprises detecting means (102, 103, 104) for detecting a resonant operation of the resonance circuit (35, 56), and control the non-conduction time of the semiconductor switch (32) in synchronism with the resonance of the resonance circuit (35, 56). A high-frequency heating apparatus according to claim 1, wherein the start control means (42) also includes timer means (126-135) for supplying the modulation command to the inverter control means (34) during a time period determined by the timer means at the time of starting the inverter. A high-frequency heating apparatus according to claim 1, wherein the start control means (42) also includes start detecting means (161, 162, 163, 164) for detecting the start of oscillation of the magnetron (39) thereby to cut off the supply of the modulation command to the inverter control means (34).

Inventors: BESSYO, DAISUKE, KUSUNOKI, SHIGERU, MAEHARA, NAOYOSHI, MATSUMOTO, TAKAHIRO, NIWA, TAKASHI, SAKAMOTO, KAZUHO, SHITAYA, TAKAO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H05B6
IPC codes: H05B6