JPH0695475B2 - High frequency heating device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a high-frequency heating device for performing so-called dielectric heating of a microwave oven or the like, and more specifically, an inverter using a semiconductor switch element such as a bipolar transistor. The present invention relates to an improvement in a high frequency heating device configured to drive a magnetron by generating high frequency electric power by means of a booster transformer.

2. Description of the Related Art Various types of high-frequency heating devices of this type have been proposed to reduce the size, weight, or cost of the power transformer.

FIG. 8 is a circuit diagram of a conventional high frequency heating device. In the figure, the power of the commercial power supply 1 is rectified by the diode bridge 2 to form a unidirectional power supply. Reference numeral 3 is an inductor, and 4 is a capacitor, which serves as a filter for the high frequency switching operation of the inverter.
The inverter is composed of a resonance capacitor 5, a step-up transformer 6, a transistor 7, a diode 8 and a drive circuit 9.

The transistor 7 performs a switching operation by a base current supplied from the drive circuit 9 in a predetermined cycle and duty (that is, a conduction time ratio defined by a ratio of a conduction time to a repeating cycle). As a result, FIG. 9 (a)
Such a current Ic / d, that is, the collector current Ic of the transistor 7 and the current Id of the diode 8 flow. On the other hand, when the transistor 7 is off, the voltage Vce as shown in FIG.
Occurs between C and E. Therefore, high frequency power is generated in the primary winding 10. Therefore, the secondary winding 11 and the tertiary winding 12
High-frequency high-voltage power and high-frequency low-voltage power are generated in each.
This high frequency high voltage power is supplied to the capacitor 13 and the diode 14.
Is rectified by and is supplied between the anode and cathode of the magnetron 15, while high-frequency low-voltage power is supplied to the cathode heater. Therefore, the magnetron 15 oscillates and enables dielectric heating. The magnetron 15 includes a magnetron body 15 and capacitors 16 and 17, which form a filter,
18 and choke coils 19 and 20. Reference numeral 21 is a power transformer of the drive circuit 9.

In such a configuration, the core cross-sectional area of the step-up transformer 6 becomes smaller as the frequency of the electric power supplied to both ends of the primary winding 10 becomes higher. Therefore, for example, when the inverter is operated at a frequency of about 20 KHz-100 KHz, the commercial Compared with the case where the voltage is boosted at the power frequency as it is, the weight and size of the step-up transformer can be reduced to several tenths to tenths, and the cost of the power supply unit can be reduced.

The base current Ib supplied to the base of the transistor 7 is
As shown in FIG. 9C, it is composed of a positive current Ib + and a negative current Ib-. The positive current Ib + is the collector current Ic of the transistor 7.
It is necessary that the current amplification factor (hfe, for example, 30) be larger than 1 / the maximum value Icm. The negative current Ib- is a current flowing by reverse biasing between the base and emitter of the transistor in order to increase the switching speed of the transistor 7 and prevent an increase in switching loss. The positive current Ib + is the collector current during the conduction period of the transistor 7, as is clear from FIGS. 9 (a) and 9 (c).
Value Ibm + determined by the maximum Ic value Icm (eg 60A)
(Eg 2A) was required. Also, negative current
Ibm− is also determined according to the maximum value Icm of the collector current Ic (for example, 15A), and the larger Icm, the larger the power required. Furthermore, the collector current Ic continues to flow for a certain period of time toff after the base current Ib + is cut off due to the so-called minority carrier accumulation effect.
It was changed by the variation of characteristics and temperature. Then, this change in toff resulted in a change in the output of the inverter.

In order to drive the transistor 7 under such a condition, the drive circuit 9 has, for example, a structure as shown in the eighth (b). That is, DC power source 2 obtained from power transformer 21
2, 23, oscillator circuit 24, transistors 25, 26, 27, resistor 2
5-36, and diode 37.

To supply a base current as shown in FIG. 9 (c),
Transistors 25 and 26 that have a fairly large capacity are used.
Moreover, the DC power supplies 22 and 23 also needed to have a considerably large capacity. Therefore, the power transformer 21 also needs to be a large power transformer. For example, a power transformer with a capacity of about 20-50 W must be used.
Cannot be made compact as a whole, and it becomes large and expensive, which impedes the compactness of the entire high-frequency heating device, resulting in large size and high price.

Problems to be Solved by the Invention Such a conventional high-frequency heating device has the following drawbacks as described above.

In the conventional high-frequency heating device, the step-up transformer 6 is energized by an inverter composed of a transistor 7 or the like to reduce the size, weight and cost of the power supply device.

However, it is necessary to supply the base current Ibm + corresponding to the peak value Icm of the collector current Ic to the transistor 7 as shown in FIGS. 9 (a) and 9 (c).
The power to supply bm + was quite large. For example, if Icm = 60A and hfe of the transistor 7 is 30, Ibm + = 2A, and the power consumption of the drive circuit 9 becomes extremely large.
1. Therefore, it was difficult to avoid increasing the size and cost of the entire power supply device.

In addition, a base sufficient to cope with changes in the storage time of the transistor 7 (the main cause of toff in FIG. 9) due to temperature changes and changes in the collector current Icm caused by temperature changes and aging changes of the magnetron 15. It is necessary to supply a current, which not only makes the driving circuit 9 and the power supply transformer 21 larger and more expensive, but also makes the output fluctuation of the high-frequency heating device unstable and wasteful. There is a drawback in that the loss of the transistor 7 and the like occurs, and the power supply efficiency and reliability are reduced.

Especially, when trying to further increase the frequency to achieve further miniaturization, the above-mentioned tendency is extremely remarkable, and it is difficult to realize practical performance at a frequency of about 30-40 KHz or more at the current semiconductor technology level. There was no sufficient miniaturization and cost reduction.

Means for Solving the Problems The present invention has been made to solve the drawbacks of the conventional high-frequency heating apparatus, and is a high-frequency heating apparatus constituted by the means described below.

That is, a unidirectional power supply having a full-wave rectification-like waveform obtained by rectifying a commercial power supply, an inverter having a semiconductor switch element that receives power from the unidirectional power supply, a step-up transformer that boosts the output of the inverter, and A drive circuit for driving a semiconductor switching element at a predetermined conduction time ratio, and a magnetron is energized by the output of the step-up transformer, and the semiconductor switching element is a serial connection body of first and second transistors. In addition, the DC power supply for energizing the first transistor and the instantaneous value of the unidirectional power supply voltage are detected, and when the voltage is a predetermined value or less, the conduction time ratio of the second transistor is set to the predetermined value. And a valley circuit that is controlled so as to be smaller than the conduction time ratio of 1. in the drive circuit.

Action The present invention has the following actions with the above configuration.

That is, the high-frequency heating device of the present invention is configured to drive the magnetron by boosting the output of the inverter energized by the unidirectional power supply having the full-wave rectification-like voltage waveform by the step-up transformer to drive the magnetron. The first transistor and the second transistor are connected in series so that the first transistor is energized by a DC power supply, and the second transistor conducts when the instantaneous value of the unidirectional power supply voltage is less than a predetermined value. Since the valley circuit that makes the time ratio smaller than the predetermined value is provided, while completely preventing the adverse effect of the DC power supply on the inverter operation in the valley of the unidirectional power supply voltage,
The switching operations of the first and second transistors can be performed extremely stably. For this reason, the power consumption of the drive circuit has been remarkably reduced, the configuration thereof has been simplified, and the drive circuit and the power supply transformer, which had to be increased in size and price, have been made compact and low-priced. This has the effect of reducing the switching loss power of the switch element and stabilizing the switching operation of the transistor handling a large amount of power, thereby enabling downsizing and cost reduction by further increasing the frequency.

Embodiment An embodiment of the high-frequency heating device of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a high-frequency heating apparatus showing an embodiment of the present invention, and those having the same reference numerals as those in FIG.

In FIG. 1, a magnetron 15 is connected to the secondary winding 11 of the step-up transformer 6, and its filter capacitors 16 and 17 are connected in parallel as shown in the figure. On the other hand, the step-up transformer 6 is configured to have a primary-secondary winding coupling coefficient smaller than that of a normal transformer (for example, about 0.6 to 0.8), and has a considerably large leakage inductance. Since this leakage inductance and the filter capacitors 16 and 17 act as a kind of low-pass filter, it is possible to obtain a predetermined radio wave output while suppressing the anode peak current of the magnetron to a small value without using a high voltage diode that has been used conventionally. Therefore, even if the high voltage diode is omitted, the magnetron can be operated stably. That is, the peak value of the anode current can be suppressed by the leakage inductance and the filter capacitors 16 and 17.
In the case of such a circuit configuration, the switching transistor is inevitably subjected to a large current load condition, so that a simple bipolar transistor inevitably causes switching loss.

Therefore, as shown in the figure, the first and second transistors 40,
41 are connected in series, a DC power supply 42 is connected to the base of the first transistor 40, and an oscillation circuit 43 is connected to the gate of the second transistor 41, and a reference voltage source is provided.
A valley circuit 44 composed of 44 ′ and a voltage comparator 44 ″ is provided, and when the voltage of the capacitor 4 is equal to or lower than the reference voltage, the valley circuit 44 suppresses the oscillation circuit 43 so that the conduction time ratio of the second transistor 41 becomes a predetermined value. The configuration is such that it is controlled to be smaller.Reference numeral 42 'is a base capacitor.By this configuration, the withstand voltage of the semiconductor switch element S is shared by the first transistor 40, and the switch operation is performed by the second transistor 41.
Can suppress the switch loss of the switch element S as a whole, and realize a stable and efficient magnetron drive inverter even with a circuit configuration in which a high voltage diode is omitted. Even if the inverter is energized by a full-wave rectification-like waveform power source, stable inverter operation can be realized as described later. Further, since the first transistor 40 may have a slow switching speed, a high hfe transistor can be used, and the second transistor 41 can be a low breakdown voltage field effect transistor. The electric power can be remarkably reduced as compared with the conventional one. In particular, when the first transistor 40 is turned off, the base reverse current of the same value as the collector current flows from its collector to its base.
Since Icbo flows and charges the base capacitor 42 'provided in parallel with the DC power supply 42, the power consumption of the DC power supply 42 can be made almost equal to zero. Because the first
This is because the transistor 40 can be considered as a grounded base transistor in terms of alternating current, and the current amplification factor is 1. Therefore, the power transformer 21 may be compact and low-priced, and the entire drive circuit 9 can be compact and low-priced.

The oscillation circuit 43 of the drive circuit 9 is configured to detect the voltage of the capacitor 4 and the collector voltage of the first transistor 40 and operate in synchronization with the resonance between the resonance capacitor 5 and the step-up transformer 6. FIG. 2 shows a more detailed circuit example of the drive circuit 9. The same reference numerals as those in FIG. In FIG. 2, the oscillator circuit 43 is
Zero-cross detector composed of resistors 45-48, comparator 49, delay means 50, differentiator 51, resistors 52-54, capacitor 55,
The longest cycle timer consisting of the comparator 56 and the differentiator 57, the logical sum circuit 58 for taking the logical sum of the zero-cross detector and the longest cycle timer, the resistors 59-61, the capacitor 62, the comparator 63,
An on-time timer consisting of a variable reference voltage source 64 and an RS flip-flop (RS-FF) which receives the output of the on-time timer and OR circuit 58 at the S and R inputs, respectively.
65, and activates the gate of the field effect transistor which is the second transistor 41. Note that 66 and 67 are diodes, and 68 and 69 are inverter gates. The DC power supply 42 is connected in parallel with the base capacitor 42 ', and is configured to energize the first transistor 40 via a parallel circuit of a diode 70 and a resistor 71. On the other hand, the valley circuit
44 is composed of a comparator 44 ', resistors 72-75, and a diode 76, and its inverting input voltage directly uses the voltage of the DC power supply 42, and the voltage of the capacitor 4 is the voltage of the DC power supply 42. When it becomes lower, the ON time of the output of the oscillation circuit 43, that is, the conduction time ratio is made smaller than the predetermined conduction time ratio until then, and the duty of the conduction signal supplied to the gate of the field effect transistor 41 is made small. ing. That is, by controlling the conduction time ratio as described above, when the voltage of the capacitor 4 becomes smaller than a predetermined value (in this case, the voltage of the DC power supply 42), the on time of the second transistor 41 becomes small. Since the load of the load connected in parallel to the capacitor 4 can be controlled (that is, the impedance can be increased) by control, it is possible to prevent an inconvenience described later due to the voltage of the capacitor 4 further drastically decreasing. You can Also,
If this conduction time ratio changes abruptly depending on the value of the impedance of the inductor 3 (that is, the power source impedance of the inverter), spike voltage may occur, which may cause inconvenience. Therefore, a time constant element is provided in the valley circuit.
It is also possible to gradually increase or decrease the conduction time ratio. That is, the above-mentioned inconvenience can be prevented by increasing or decreasing the reference voltage E of the comparator 63 gradually or stepwise with a predetermined time constant. This is the comparator 44 '
This can be easily realized by providing a time constant element in the output of.

FIG. 3 is a waveform diagram for explaining the operation of the drive circuit of FIG. 2, and FIGS. 3 (a) and 3 (b) show the current Ic / d flowing through the switch element S and the collector of the first transistor 40 and the second transistor.
Voltage Vcd between the drain of the field effect transistor 41 of
Is. Further, (c) to (g) of FIG. 3 are operation waveforms of each part of the oscillator circuit 43. The zero-cross detector output A generates a zero-cross pulse A in the vicinity of the zero cross with a delay of a certain time td from the cross point between the voltages Vs and Vcd of the capacitor 4 as shown in FIG. If this cross point is not detected for some reason, a forced zero cross pulse is generated from the longest period timer when the voltage B of the capacitor 55 reaches a predetermined value C, as shown by the broken lines in FIGS. To be done. When the pulse A is input to the R input of the RS-FF65, the output becomes H as shown in FIG. 8E, the second field effect transistor 41, and thus the switch element S is turned on and Ic / d flows. Becomes At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG. 6 (f), and when the voltage E reached by the variable reference voltage source 64 is reached, the on-time timer is input to the S input of RS−FF65 at the input of FIG. The pulse F of is input. Therefore, the output becomes L, the second field effect transistor 41, and hence the switch element S is turned off and returns to the initial state, and this is repeated.

In such a circuit operation, the field effect transistor 41, which is the second transistor, can be directly driven by a CMOS-IC or the like, so that the circuit is extremely simple and has low power, and can be made compact and inexpensive. Furthermore, since the first transistor 40 may have a low switching speed, an extremely high hfe transistor can be used, and
Since the base capacitor 42 'is provided, the energy when the first transistor 40 is off can be stored and used as its drive energy. Therefore, the DC power supply 42
The energy supplied from the device may be extremely small, and the drive circuit such as the DC power supply 42 can be made compact and inexpensive.

Next, the operation of the valley circuit 44 will be described with reference to FIGS. 4 to 6. The voltage Vs of the capacitor 4 has a full-wave rectification-like waveform as shown in FIG. 4 (a), and the voltage in the valley period shown by tv.
Vs becomes lower than the voltage VB of the DC power supply 42. Therefore, during this period, current flows from the DC power supply 42 to the capacitor 4 through the primary winding 10 of the step-up transformer 6 as shown by the broken line in FIG. If the first transistor 40 is a Darlington transistor, the transistor at the subsequent stage will act as a reverse transistor with the collector and the emitter connected in reverse. Therefore, during this period, the switching action of the first transistor 40 is substantially lost. Therefore, Ic / d and Vcd shown in FIGS. 4 (b) and 4 (c) do not have normal operating waveforms in the valley of Vs in FIG. 4 (a), that is, in the period tv. ) And (b) result in an abnormal waveform. For this reason, the operation of the inverter is extremely unstable if such operation is left as it is, the reliability is not deteriorated, and it is not possible to realize a high-frequency heating device that can withstand actual use. So the valley circuit
44 is provided to decrease the conduction time ratio of the second field effect transistor 41 when the voltage Vs of the capacitor 4 drops to a predetermined voltage before it drops to a voltage that causes the above inconvenience.
It is configured to prevent a situation where Vs <VB. Specifically, the comparator of the valley circuit 44 shown in FIG.
The output of 44 'provides a valley detection signal through diode 76 and resistor 75. That is, the comparator
44 'indicates that the voltage Vs of the capacitor 4 is the voltage VB of the DC power supply 42.
When it becomes smaller than (or other predetermined voltage), its output becomes L, and as a result, the reference voltage E of the comparator 63 of the on-time timer is lowered and controlled to E'in FIG. 3 (f). It is a thing. As a result, as indicated by the broken line in FIGS. 3 (a), (e) and (f), the second transistor 41
The on-time of is reduced and the step-up transformer 6 to magnetron
The power supply to 15 will be close to zero. That is, since the load capacity of the capacitor 4 is significantly reduced, the capacitor 4 hardly discharges and its voltage
It is possible to prevent the above-mentioned inconvenience caused by Vs being smaller than VB. The description of the waveforms in FIGS. 3B, 3C, 3D, and 3F in this case is omitted. With such a configuration, by reducing the conduction time ratio of the field effect transistor 41 in the valley period tv of Vs in FIG. 4 (a), the load of the inverter is reduced, and further discharge of the capacitor 4 is substantially hindered. Therefore, the operation is extremely stable, and the disadvantages of the case where two transistors are connected in series to form the semiconductor switch element S are eliminated, and only the advantages of the inverter having the above-described configuration are brought out. You can The above inconvenience can be prevented by increasing the capacity of the capacitor 4 so that the voltage in the valley period tv of Vs does not become lower than the voltage VB of the DC power supply 42.
The size and cost of
The input power factor of the inverter deteriorates, and it cannot be put to practical use. Therefore, the configuration in which the valley circuit 44 is provided is extremely useful for preventing the above-mentioned inconvenience.

FIG. 7 (a) is a circuit diagram showing another embodiment of the present invention, and FIG. 7 (b) is an operation explanatory diagram thereof to explain the control state of the conduction time ratio of the second transistor 41. It is a figure. In FIG. 7A, the valley circuit 44 is a reference voltage source.
44 ', a voltage comparator 44 "and a timer 76 activated by its output. The voltage Vs of the capacitor 4
Is reduced to VB, the voltage comparator 44 ″ activates the timer 76 to decrease the output pulse width of the oscillation circuit 43 for a constant time tv and decrease the conduction time ratio of the second transistor 41. This constant time tv is appropriate. It is possible to stabilize the operation of the inverter during the period when Vs is too low and the inverter operation is unstable with the current conduction time ratio as it is by setting the valley circuit 44 of this method.
As a result, it is possible to prevent Vs from rising instantaneously to cause chattering in the voltage comparator 44 ″, or to prevent chattering due to noise in the voltage comparator 44 ″ due to noise when Vs is close to VB. This has the effect of making the operation of the valley circuit 44 more stable and reliable.

Effects of the Invention As described above, according to the present invention, the following effects can be obtained.

An inverter having a semiconductor switching element that obtains power from a unidirectional power supply with a full-wave rectification-like voltage waveform obtained by rectifying a commercial power supply is used to drive a magnetron by energizing a step-up transformer. The first and second transistors are connected in series so that the first transistor is energized by a DC power source provided in a drive circuit, and a valley circuit is provided so that the instantaneous value of the voltage of the unidirectional power source is predetermined. Since the conduction time ratio of the second transistor is controlled to be smaller than the predetermined conduction time ratio when the value is less than or equal to the value, (1) the two transistors function as switches, that is, withstand voltage action and switch action. To reduce the load on the semiconductor switch element, realize a highly efficient and high-frequency operation, and realize a highly reliable inverter circuit. (2) Moreover, even in the case of a unidirectional power supply having a full-wave rectification-like voltage waveform, the capacitor of the unidirectional power supply does not have to be large and costly, and is caused by the configuration of the semiconductor switch element. It is possible to prevent an unstable operation of the inverter in the valley of the voltage and realize a stable and reliable inverter.

(3) Therefore, the power consumption of the drive circuit of the semiconductor switch element can be remarkably reduced by the extremely simple structure, and the drive circuit and the power supply transformer thereof can be significantly downsized and the cost can be reduced.

(4) As a result of the above, the switching performance of the semiconductor switching element is remarkably improved, the efficiency and reliability of the entire high-frequency heating device is significantly improved, and the driving circuit and the like are made compact, so that the entire high-frequency heating device is significantly downsized. The cost can be reduced.

(5) Further, in particular, the serial connection of the first and second transistors and the peripheral circuit thereof are configured by one chip or a plurality of chips into a single package, thereby further reducing the size and increasing the reliability. A high-frequency heating device that can be realized and has high reliability can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a high-frequency heating device showing an embodiment of the present invention, FIG. 2 is a circuit diagram of a drive circuit of the device, FIG. 3 is an operating voltage / current waveform diagram of each part of the device, and FIG. The operating voltage and current waveform diagram of each part corresponding to the full-wave rectified unidirectional power supply voltage of the device, Fig. 5 is the operating voltage and current waveform diagram of abnormal operation in the valley of the unidirectional power supply voltage of the device, and Fig. 6 is abnormal FIG. 7 is a circuit diagram of the same device for explaining the operation, FIG. 7 is a circuit diagram showing a structure of a valley circuit of a high-frequency heating device showing another embodiment of the present invention, and a waveform diagram for explaining the operation, FIG. FIG. 9 is a circuit diagram of a conventional high-frequency heating device and a circuit diagram of a drive circuit of the device, and FIG. 1 ... Commercial power supply, 2, 3, 4 ... Unidirectional power supply, (2 ...
Diode bridge, 3 ... Inductor, 4 ... Capacitor), 5 ... Resonant capacitor, 6 ... Step-up transformer,
9 ... Driving circuit, 15 ... Magnetron, S ... Semiconductor switching element, 40 ... First transistor (bipolar transistor), 41 ... Second transistor (field effect transistor), 42 ... DC power supply, 42 '... Base capacitor, 43 ... Oscillation circuit, 44 ... Valley circuit, (44' ... Reference voltage source, 44 "... Voltage comparator).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 62-143390 (JP, A) JP 62-193093 (JP, A) JP 62-110295 (JP, A)

Claims (6)

[Claims] 1. A unidirectional power supply having a full-wave rectification-like waveform obtained by rectifying a commercial power supply, an inverter having a semiconductor switch element that receives power from the unidirectional power supply, and a step-up transformer for boosting the output of the inverter. A driving circuit for driving the semiconductor switching element at a predetermined conduction time ratio, and energizing a magnetron with the output of the step-up transformer, and the semiconductor switching element includes a first transistor and a second transistor. And a DC power supply for energizing the first transistor, and an instantaneous value of the unidirectional power supply voltage is detected, and when this voltage is below a predetermined value, the conduction time of the second transistor A high frequency heating device, wherein a valley circuit for controlling a ratio to be smaller than the predetermined conduction time ratio is provided in the drive circuit. 2. The high frequency heating apparatus according to claim 1, wherein the first transistor is a bipolar transistor and the second transistor is a field effect transistor. 3. The high frequency wave according to claim 1 or 2, wherein the first transistor and the second transistor are formed by one chip or a plurality of chips and housed and integrated in a single package. Heating device. 4. A valley circuit is configured to make a conduction time ratio of a second transistor smaller than a predetermined conduction time ratio when an instantaneous value of the unidirectional power supply voltage is equal to or lower than a DC power supply voltage for energizing the first transistor. The high frequency heating apparatus according to claim 1. 5. The high frequency heating apparatus according to claim 1, wherein the valley circuit is composed of a voltage comparison circuit, and the drive signal of the second transistor is controlled by the output of the comparison circuit. 6. The valley circuit comprises a voltage comparison circuit and a timer circuit which operates by receiving the output of the voltage comparison circuit,
The high frequency heating apparatus according to claim 1, wherein the output of the timer circuit makes the conduction time ratio of the second transistor smaller than a predetermined conduction time ratio.