JPH0574919B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improvements in high-frequency heating devices for so-called dielectric heating such as microwave ovens, and more specifically, inverters using semiconductor switch elements such as bipolar transistors. This invention relates to an improvement in a high-frequency heating device configured to generate high-frequency power using a step-up transformer to drive a magnetron.

BACKGROUND ART Various configurations of high-frequency heating devices of this type have been proposed in order to reduce the size, weight, or cost of the power transformer.

FIG. 6 is a circuit diagram of a conventional high frequency heating device. In the figure, power from a commercial power source 1 is rectified by a diode bridge 2 to form a unidirectional power source. 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 includes a resonant capacitor 5, a step-up transformer 6, a transistor 7, a diode 8, and a driving circuit 9.

The transistor 7 performs a switching operation with a predetermined cycle and duty (ie, on/off time ratio) by a base current supplied from the drive circuit 9.
As a result, the current Ic/d as shown in FIG. 7a, that is,
Collector current Ic of transistor 7 and diode 8
A current Id flows. On the other hand, when the transistor 7 is off, a voltage Vce as shown in FIG. 7b is generated between C and E of the transistor 7 due to resonance between the capacitor 5 and the primary winding 10. Therefore, high frequency power is generated in the primary winding 10. Therefore, the secondary winding 11,
High-frequency high-voltage power and high-frequency low-voltage power are generated in the tertiary winding 12 and the tertiary winding 12, respectively. This high frequency high voltage power is rectified by a capacitor 13 and a diode 14 and is supplied between the anode and cathode of the magnetron 15, while the high frequency low voltage power is supplied to the cathode heater. Therefore, the magnetron 15 oscillates and is capable of dielectric heating. The magnetron 15 is composed of a magnetron body 15a, capacitors 16, 17, 18 and chiyoke coils 19, 20 that constitute a filter. Further, 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 power supplied to both ends of the primary winding 10 increases. This method has the advantage that the weight and size of the step-up transformer can be reduced from one-tenth to one-tenth of that of the case where the voltage is stepped up while maintaining the power supply frequency, and the cost of the power supply section can be reduced.

The base current Ib supplied to the base of the transistor 7 is composed of a positive current Ib+ and a negative current, as shown in Figure 7c.
Consists of Ib−. The positive current Ib+ is the transistor 7
It is necessary that the current amplification factor (hfe, for example, 30) is larger than the maximum value Icm of the collector current Ic. Further, the current Ib- is a current that flows 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. As is clear from FIGS. 7a and 7c, the positive current Ib+ must be set to a value bm+ (for example, 2A) determined by the maximum value Icm (for example, 60A) of the collector current Ic during the conduction period of the transistor 7. It was hot. Further, the negative current Ibm− is also determined according to the maximum value Icm of the collector current Icn (for example, 15A), and Icm
The larger the value, the more power was required. Furthermore, the collector current Ic increases for a certain period of time after the base current Ib+ is cut off due to the so-called minority carrier accumulation effect.
Only toff continues to flow, and this toff changes depending on variations in the characteristics of the transistor 7, temperature, and the like. This change in toff results in a change in the output of the inverter.

In order to drive the transistor 7 under such conditions, the drive circuit 9 had a configuration as shown in FIG. 6b, for example. That is, DC power supplies 22 and 23 obtained from the power transformer 21, an oscillation circuit 24,
Transistors 25, 26, 27, resistors 25 to 3
6 and a diode 37.

Problems to be Solved by the Invention However, in the above configuration, in order to supply the base current as shown in FIG.
5 and 26 must be of considerably large capacity, and the DC power supplies 22 and 23 must also have considerably large capacity. Therefore, the power transformer 21
However, it is necessary to use a large power transformer, for example, one with a capacity of about 20-50W, and the power transformer 21 and drive circuit 9 cannot be made compact as a whole, and are large and expensive. This hinders the compactness of the high-frequency heating device as a whole, making it larger and more expensive.

In the conventional high-frequency heating device, a step-up transformer 6 is energized by an inverter including a transistor 7, etc.
The aim was to make the power supply device smaller, lighter, and lower in cost.

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 FIG. 7a and c, and the power required to supply this Ibm+ is quite large. It was becoming. For example, Icm=60A
If hfe of transistor 7 is 30, then Ibm+=
2A, the power consumption of the drive circuit 9 becomes extremely large, and the power consumption of the drive circuit 9 and the power transformer 2 becomes extremely large.
1. Therefore, it has been difficult to avoid increasing the size and price of the entire power supply device.

Furthermore, the storage time of transistor 7 due to temperature changes (the main cause of toff in Figure 7)
In order to cope with changes in the collector current Icm caused by changes in the temperature of the magnetron 15 and changes over time, it is necessary to supply a sufficient base current. The disadvantage is that it not only increases the size and price, but also increases the fluctuation in the output of the high-frequency heating device, making it unstable, and causes unnecessary loss in the transistor 7, etc., which reduces power supply efficiency and reliability. It was hot.

In particular, when trying to further increase the frequency and achieve further miniaturization, the above-mentioned tendency is extremely noticeable, and with the current level of semiconductor technology, it is practically difficult to achieve practical performance at frequencies above about 30-40kHz. However, it was not possible to achieve sufficient miniaturization and cost reduction.

Means for Solving the Problems The present invention has been made to solve the drawbacks of such conventional high frequency heating devices, and is a high frequency heating device constructed from the means described below.

That is, a resonant circuit is formed by a unidirectional power source obtained from a commercial power source, an inverter that receives power from the unidirectional power source and has a resonant capacitor and a semiconductor switch element, and the resonant capacitor, and boosts the output of the inverter. and a drive circuit that drives the semiconductor switch element, the output of the step-up transformer energizes a magnetron, and the semiconductor switch element is connected to a series connection of first and second transistors. In addition, a DC power supply for energizing the first transistor is provided in the drive circuit, and a base capacitor having a larger capacity than the resonant capacitor is provided in parallel with the DC power supply.

Effects The present invention has the following effects due to the above configuration.

That is, the high-frequency heating device of the present invention is configured so that a step-up transformer is driven by a resonant inverter to energize a magnetron, and a semiconductor switch element of the resonant inverter is configured by a series connection of first and second transistors. Since the first transistor is energized by a DC power supply and a base capacitor with a larger capacity than the resonant capacitor of the resonant inverter is provided in parallel with the DC power supply, the resonant operation of the inverter is not adversely affected. This can be done by storing Icbo during switching of the first transistor in the base capacitor. Therefore, by significantly reducing the power consumption of the drive circuit and simplifying its configuration, the drive circuit and power transformer, which had previously been forced to become larger and more expensive, can be made compact and inexpensive. This has the effect of reducing the switching power loss of the switch element and stabilizing the switching operation of the transistor that handles high power, thereby enabling miniaturization and cost reduction due to higher frequencies.

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 device showing an embodiment of the present invention, and the same reference numerals as in FIG. 6 are corresponding components, and the explanation thereof will be omitted.

In FIG. 1, the secondary winding 11 of the step-up transformer 6
A magnetron 15 is connected to the magnetron 15, and its filter capacitors 16 and 17 are connected in parallel as shown in the figure. The step-up transformer 6 is configured to have a smaller coupling coefficient between the primary and secondary windings (for example, about 0.8) than a normal transformer, and has a considerably large leakage inductance. This leakage inductance and filter capacitor 16,1
7 acts as a kind of low-pass filter,
A predetermined radio wave output can be obtained while suppressing the anode peak current of the magnetron to a small value without using the conventionally used high voltage diode, and the magnetron can be operated stably even if the high voltage diode is omitted. That is, the peak value of the anode current can be suppressed by the leakage inductance and the filter capacitors 16 and 17. In such a circuit configuration, the switching transistor is inevitably subjected to a large current load condition, so that switching loss inevitably increases with a simple bipolar transistor.

Therefore, as shown in the figure, first and second transistors 40 and 41 are connected in series, and 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.
are connected to each other, and a base capacitor 42a is connected in parallel to the DC power supply 42. With this configuration, the withstand voltage of the semiconductor switch element 44 can be shared by the first transistor 40, and the switching operation can be shared by the second transistor 41, and the switching loss of the switch element 44 as a whole can be kept low, and the high voltage Even with a circuit configuration in which diodes are omitted, a stable and efficient magnetron drive inverter can be realized.

In addition, since the first transistor 40 only needs to have a slow switching speed, a high-hfe transistor can be used, and the second transistor 41 can be a low-voltage field effect transistor, which reduces the consumption of the drive circuit 9. Power can be significantly reduced compared to conventional methods. And especially the first
When the transistor 40 is turned off, a base reverse current Icbo having the same value as the collector current flows from its collector to the base, charging the base capacitor 42a provided in parallel with the DC power supply 42, so the power consumption of the DC power supply 42 is almost zero. can be made equal to This is because the first transistor 40 can be considered as a common base transistor in terms of AC, and the current amplification factor is 1. Therefore, the power transformer 21 may also be compact and inexpensive, and the entire drive circuit 9 can be made compact and inexpensive.

The base capacitor 42a is the first capacitor as described above.
When the transistor 40 is turned off, a state in which it is equivalently connected to the resonant capacitor 5 occurs. Therefore, if the base capacitor 42a is smaller than the resonant capacitor 5, resonance will occur between the step-up transformer 6 and the base capacitor 42a, and the resonant inverter will not be able to perform commutation through stable resonant operation, and the switch elements will This not only causes an increase in loss, but also causes a serious defect in that it induces destruction. Therefore, it is extremely important that the base capacitor 42a has a larger capacitance than the resonant capacitor 5, and preferably has a sufficiently large value (for example, about 5 to 10 times).

Further, 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 of the resonant capacitor 5 and the booster transistor 6.
FIG. 2 shows a more detailed actual circuit example of the drive circuit 9, and the same reference numerals as those in FIG. 1 represent corresponding components, and the explanation thereof will be omitted. In FIG. 2, the oscillation circuit 4
3 is a zero cross detection section consisting of resistors 45 to 48, a comparator 49, a delay means 50, and a differentiator 51;
Resistors 52 to 54, capacitor 55, comparator 56,
A maximum cycle timer consisting of a differentiator 57, an OR circuit 58 that ORs the zero cross detection section and the longest cycle timer, and an ON circuit consisting of resistors 59 to 61, a capacitor 62, a comparator 63, and a variable reference voltage source 64. R-S flip-flop (R-S/FF) which receives the outputs of the time timer, on-time timer, and OR circuit 58 at its S and R inputs, respectively.
65, and the second transistor 41
energizes the gate of a field effect transistor. Note that 66 and 67 are diodes, and 68 and 69 are inverter gates. On the other hand, the DC power supply 42 is connected in parallel with the base capacitor 42a, and the first
The transistor 40 is connected to the diode 70 and the resistor 7.
This configuration is energized through one parallel circuit.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit of FIG.
The current Ic/d flowing in and the first transistor 4
0 collector and the second field effect transistor 41
The voltage between the drain and the drain is Vcd. Further, c to g in the same figure show operation waveforms of each part of the oscillation circuit 44. The output A of the zero cross detection section 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 of 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 by the longest collection timer when the voltage B of the capacitor 55 reaches a predetermined value C, as shown by broken lines in c and d of the figure. R
When pulse A is input to the R input of the -S/FF65, the -Q output becomes H as shown in e in the same figure, leading to the second field effect transistor 41 and therefore the switch element 4.
4 is turned on and Ic flows. At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG. do. Therefore, the output -Q becomes L, and the second field effect transistor 41 and therefore the switch element 44 are turned off, returning to the initial state, and this process is repeated.

With such a circuit operation, the field effect transistor 41, which is the second transistor,
Since it can be directly driven by a CMOS-IC, etc., it becomes an extremely simple and low-power circuit, making it compact and inexpensive. Furthermore, the first transistor 40 can be of low switching speed, so a very high hfe transistor can be used;
Moreover, since the base capacitor 42a is provided, the energy when the first transistor 40 is off can be stored and used as drive energy. Therefore, the energy supplied from the DC power supply 42 only needs to be extremely small, and the drive circuit such as the DC power supply 42 can be made compact and inexpensive.

FIG. 4 is a circuit diagram of a high-frequency heating device showing another embodiment of the present invention, and the same reference numerals as in FIG. 1 are corresponding components, and the explanation thereof will be omitted. In the figure, a semiconductor switch element 44 includes first and second transistors 40, 41, flywheel diodes 8, 7a connected as shown, a resistor 71,
73, diodes 70 and 72, and a Zener diode 74, and also has a capacitor 42b. This capacitor 42b supplements the action of the base capacitor 42a, and when the operating frequency of the inverter is high (for example, about 50 kHz or more), it suppresses the adverse effects caused by the inductance of the wiring between terminals T1 and T1' and stabilizes the operation of the inverter. It plays an important role in ensuring reliability, and its integration into the semiconductor switch element 44 is extremely important.

5a and 5b show a more detailed embodiment of the drive circuit power supply circuit of the embodiment of the present invention shown in FIG. 1 or 4, and the same reference numerals as in FIG. 1 or 4 have corresponding configurations. This is an element and its explanation will be omitted. In the figure a, the base capacitor 42a is also used as a smoothing capacitor constituting the DC power supply 42, and the DC power supply 42 is composed of a diode 75 and the base capacitor 42a having high frequency characteristics. The diode 76, the capacitor 77, the resistor 78, and the Zener diode 79 constitute a DC power source for the oscillation circuit 43.

Further, FIG. 5b shows a configuration in which the secondary winding of the power transistor 21 is unified into one, and current is charged to the base capacitor 42a from the diode 76 via the resistor 80 together with the smoothing capacitor 77. It's becoming like that. That is, the base capacitor 42a is started with energy charged from the diode 76 before starting the inverter,
Once activated, energy is replenished by the reverse charge energy generated when the first transistor 40 is turned off. Therefore, it is possible to adopt a configuration as shown in the figure in which high impedance is passed through the resistor 80, and the power transformer 21 can be made into an extremely small capacity transformer, making it possible to make the entire drive circuit compact and inexpensive.

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

A step-up transformer is energized by a resonant inverter having a semiconductor switch element that receives power from a unidirectional power source obtained from a DC power source or a commercial power source to drive the magnetron. In addition to the configuration in which the first transistor is energized by the DC power source of the drive circuit, a base capacitor with a larger capacity than the resonant capacitor is installed in parallel with the DC power source. (2) It is possible to share the role of withstand voltage and switch function, reduce the burden on semiconductor switch elements, and realize highly efficient and high frequency operation, as well as highly reliable inverter circuits. Moreover, the charge (energy) at the time of turn-off of the semiconductor switch element can be accumulated in the base capacitor and used as drive power for the first transistor without adversely affecting the resonant operation of the inverter.

(3) Therefore, with an extremely simple configuration, the power consumption of the drive circuit for the semiconductor switch element can be significantly reduced, and the drive circuit and its power transformer can be made significantly more compact and lower in cost.

(4) As a result of the above, the switching performance of the semiconductor switch element has been significantly improved, and the efficiency and reliability of the entire high-frequency heating device have been significantly improved.By making the drive circuit etc. more compact, the entire high-frequency heating device has been significantly reduced in size and cost. It can be reduced to cost.

(5) In particular, by configuring the series connection of the first and second transistors and their peripheral circuits on one chip or multiple chips into a single package, further compactness and higher reliability can be achieved. This makes it possible to provide a highly reliable high-frequency heating device.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a high-frequency heating device showing an embodiment of the present invention, Fig. 2 is a detailed diagram of a drive circuit of the same device, Fig. 3 is an explanatory diagram of operating current waveforms of each part of the device, and Fig. 4 is a circuit diagram of a high-frequency heating device showing another embodiment of the present invention, FIG. 5 is a circuit diagram showing a detailed embodiment of the power supply circuit of the drive circuit of the same device, and FIG. 6 is a circuit diagram of a conventional high-frequency heating device. 7 is a detailed diagram of the drive circuit of the same device, and FIG. 7 is an explanatory diagram of operating current waveforms of each part of the same device. 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 ... Drive circuit, 40 ... First transistor, 41 ... Second transistor, 42 ... DC power supply, 42a ... Base capacitor, 43 ... Oscillation circuit, 44 ... Semiconductor switch .

Claims (1)

[Scope of Claims] 1. A unidirectional power source obtained from a commercial power source, etc., and an inverter that receives power from the unidirectional power source and includes a resonant capacitor and a semiconductor switch element;
A resonant circuit is configured with the resonant capacitor,
A step-up transformer that boosts the output of the inverter and energizes the magnetron; and a drive circuit that drives the semiconductor switch element; The second transistor is connected in series with a second transistor, and the second transistor is controlled on and off by the drive circuit, and the drive circuit is provided with a DC power supply that energizes the first transistor, A high-frequency heating device configured to include a base capacitor with a larger capacity than the resonant capacitor in parallel with a power source. 2. The high frequency heating device 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 heating device according to claim 1 or 2, wherein the first transistor and the second transistor are constructed of one chip or a plurality of chips, and are housed in a single package and integrated. 4. The high-frequency heating device according to claim 1, wherein the base capacitor is configured to double as a smoothing capacitor of the DC power supply. 5. The high-frequency heating device according to claim 1, wherein the base of the first transistor and the base capacitor are connected by a circuit element including a diode and a resistor.