JPH0555996B2 - - 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. 4 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 drive circuit 9. The transistor 7 has a predetermined cycle and duty (i.e., on/off time ratio) by the base current supplied from the drive circuit 9.
Switching works. As a result, a current 1c/d as shown in FIG. 5a, that is, a collector current 1c of the transistor 7 and a current 1d of the diode 8 flow. On the other hand, when the transistor 7 is off, a voltage Vce as shown in FIG. 5b 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 and the tertiary winding 1
2, high-frequency high-voltage power and high-frequency low-voltage power are generated, respectively. This high frequency high voltage power is supplied to the capacitor 13,
and rectified by the diode 14 and 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 is capable of dielectric heating. The magnetron 15 includes a magnetron main body 15', capacitors 16, 17, 18 constituting a filter, a chiyoke coil 19,
20. 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 is equal to that of the primary winding 10.
The higher the frequency of the power supplied to both ends of the
The weight of the step-up transformer is lower when operating at a frequency of
It has the advantage that the size can be reduced from one-tenth to one-tenth, and the cost of the power supply section can be reduced.

The base current 1b supplied to the base of the transistor 7 consists of a positive current 1b+ and a negative current 1b- as shown in FIG. 5c. It is necessary that the positive current 1b+ is larger than the current amplification factor (hfe, for example, 30) of the maximum value of 1 cm of the collector current 1c of the transistor 7. Further, the negative current 1b- is a current that flows by applying a reverse bias 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 1b+ is shown in Figure 5a,
As is clear from c, the maximum value of the collector current 1c during the conduction period of the transistor 7 is 1 cm (for example,
It was necessary to set the value to 1bm + (for example, 22A) determined by 60A). Further, the negative current 1bm- is also determined according to the maximum value 1cm of the collector current 1c (for example, 15A), and the larger 1cm is, the more power is required.

Furthermore, the collector current 1c continues to flow for a certain period of time toff after the base current 1b+ is cut off due to the so-called minority carrier accumulation effect.
This toff varied depending on variations in 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 has a configuration as shown in FIG. 4b, for example. That is, DC power supplies 22, 23 obtained from the power transformer 21, oscillation circuit 24, transistors 25, 26, 27, and resistor 25-3.
6 and a diode 37.

The oscillation circuit 24 alternately turns on and off the transistors 25 and 26 during a conduction period of a predetermined period, as shown in FIG.
A base current such as is supplied to the transistor 7. However, the transistors 25 and 26 were required to be able to handle a considerably large current, and the DC power supplies 22 and 23 were also required to supply this large current. Therefore, the drive circuit 9 and the power transformer 21 have to be large and expensive.

In particular, when operating the transistor 7 with high frequency and large current, the drive circuit 9 and the power transformer 21 become extremely large and expensive; for example, 20W to 50W.
This required a certain amount of electricity.

Problems to be Solved by the Invention As mentioned above, such conventional high-frequency heating devices have the following drawbacks.

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

However, it is necessary to supply the base current 1bm+ corresponding to the peak value 1cm of the collector current 1c to the transistor 7 as shown in Figure 5 a and c, and the power required to supply this 1bm+ is quite large. I was getting used to it. For example, if 1cm = 60A and hfe of transistor 7 is 30, 1bm + = 2A
Therefore, the power consumption of the drive circuit 9 becomes extremely large, and it is difficult to avoid increasing the size and price of the drive circuit 9 and the power transformer 21.

Furthermore, the storage time of transistor 7 due to temperature changes (the main cause of toff in Figure 5)
In order to cope with changes in the collector current of 1 cm caused by changes in the temperature of the magnetron 15 and changes over time, it is necessary to supply a sufficient base current. Not only does this result in a larger size and higher price, but it 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., reducing reliability and safety. There were flaws.

Furthermore, it is difficult to reduce the loss of the high-voltage high-frequency diode 14 and achieve practical performance, and even if it were realized, it would be extremely expensive.

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 by the means described below.

That is, a unidirectional power source obtained from a commercial power source or the like, an inverter that receives power from the unidirectional power source and has a semiconductor switch element, a step-up transformer that forms a resonant circuit with a resonant capacitor and boosts the output of the inverter, and the semiconductor a drive circuit for driving a switch element, a capacitor and a magnetron are connected in parallel to the secondary winding of the step-up transformer to energize the magnetron, and the semiconductor switch element is connected to a first and a second The driving circuit includes a series connection of transistors, and includes a DC power source that energizes the first transistor and an oscillation circuit that energizes the second transistor, and the oscillation circuit is synchronized with the resonant circuit. The structure is such that it oscillates.

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 such that a capacitor and a magnetron are provided in the secondary winding of a step-up transformer, the step-up transformer is driven by a resonant inverter, and the semiconductor switch elements of the resonant inverter are connected to the first and second windings. The device is composed of a series connection of second transistors, the first transistor is energized by a DC power supply, the second transistor is energized by an oscillation circuit, and the oscillation circuit is configured to operate in synchronization with the resonant circuit. Therefore, it is possible to drive the magnetron without using an expensive high-voltage diode, stabilize the switching operation of the transistor that handles high power, and realize the integration of the drive circuit configuration, thereby reducing the size and price of the magnetron. The drive circuit and power transformer that would otherwise be required can be made compact and inexpensive. It also has the effect of preventing fluctuations in the semiconductor switch element current caused by changes in characteristics, stabilizing the output of the magnetron, suppressing wasteful power consumption of the semiconductor switch elements, etc., and improving reliability.

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. 4 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. On the other hand, 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, it is possible to obtain a specified radio wave output while keeping the magnetron's anode peak power low without using the conventionally used high-voltage diode.
Even if the high-voltage diode is omitted, the magnetron can operate 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 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. This is how it was done. With this configuration, the withstand voltage of the 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 diode It is possible to realize a stable and efficient magnetron drive inverter with a circuit configuration that omits the above. Furthermore, 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. Therefore, the power transformer 21 may also be compact and inexpensive, and the entire drive circuit 9 can be made compact and inexpensive.
Further, as shown in the figure, the oscillation circuit 43 detects the voltage of the capacitor 4 and the collector voltage of the first transistor 40 and operates in synchronization with the resonance between the resonant capacitor 5 and the step-up transformer 6.

FIG. 2 shows a more detailed circuit example of the drive circuit 9, and the same reference numerals as in FIG. 1 represent corresponding components, and the explanation thereof will be omitted.

In FIG. 2, the oscillation circuit 43 includes a resistor 45
-48, comparator 50, delay means 50, differentiator 51
A zero cross detection section consisting of a resistor 52-5 and a resistor 52-5.
4. A longest period timer consisting of a capacitor 55, a comparator 56, and a differentiator 57, and resistors 57-60.
Diode 61, capacitor 62, comparator 63,
an on-time timer consisting of a variable reference voltage source 64, and the output of this on-time timer as an S input;
It is composed of an RS/FF65 whose R input is the sum output of the longest cycle timer and the zero-cross detection section. 66 is an AND gate, 67 and 68 are inverter gates, and 69 is a diode. Further, the DC power supply 42 that energizes the first transistor 40 includes a capacitor 70, a resistor 71, and a diode 72.
are connected as shown in the figure, so that the first transistor 40 can perform base-grounding operation well and efficiently. Therefore, characteristic fluctuations caused by conventional toff can be prevented.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit of FIG.
The current 1c/d flowing through the collector of the first transistor 40 and the second field effect transistor 4
This is the voltage Vcd between the drain and the drain of 1. 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 for some reason this cross point is not detected,
When the voltage B of the capacitor 55 reaches a predetermined value C, as shown by the broken line in and d, a forced zero-crossing pulse is generated by the longest period timer. R-
When pulse A is input to the R input of the S/FF 65, the Q output becomes H as shown in e in the same figure, and the second field effect transistor 41 and therefore the switch element 44
turns on and 1c flows. At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG. input. 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.

In such a circuit operation, the second field effect transistor 41 can be directly driven by a CMOS-IC or the like, resulting in an extremely simple and low-power circuit, which can be compact and inexpensive. Furthermore, since the first transistor 40 can be of low switching speed, a very high hfe transistor can be used, and the DC power supply 42 can also be compact and low cost.

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

The configuration is such that the step-up transformer is energized and the magnetron is driven by a semiconductor switch inverter that receives power from a unidirectional power source obtained from a DC power source or a commercial power source, and a capacitor and the magnetron are connected in parallel to the secondary winding of the step-up transformer. At the same time, the semiconductor switch element is constituted by a series connection of a first and a second transistor, and the first transistor is energized by the DC power supply of the drive circuit, and the second transistor is energized by the oscillation circuit of the drive circuit. (1) The magnetron can operate stably without an expensive high-frequency, high-voltage diode.

(2) Eliminating the high-frequency, high-voltage diode reduces the burden on semiconductor switch elements, making it possible to realize a highly efficient and reliable inverter circuit.

(3) The configuration of the drive circuit of the semiconductor switch element can be simplified, 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, by omitting components such as high-voltage diodes and making drive circuits more compact,
The entire high-frequency heating device can be significantly reduced in size and cost.

(5) Furthermore, if the series connection of the first and second transistors and their peripheral circuits are integrated into one chip or multiple chips, further compactness and high reliability can be achieved, and highly reliable high frequency A heating device can be provided.

[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 circuit diagram of a drive circuit of the device, and FIG. 3 is a diagram of operating current waveforms of each part of the device.
Figures 4a and 4b are circuit diagrams of a conventional high-frequency heating device;
FIGS. 5a, 5b, and 5c are diagrams of operating current waveforms of each part of the 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, 10 ... Primary winding, 11 ... Secondary winding, 15 ... Magnetron, 40 ... First transistor, 41 ... Second transistor, 42 ... DC power supply, 43... oscillation circuit, 44... semiconductor switch element.

Claims (1)

[Scope of Claims] 1. A unidirectional power source obtained from a commercial power source, etc.; an inverter receiving power from the unidirectional power source and having a resonant capacitor and a semiconductor switch element; and an inverter configured to form a resonant circuit with the resonant capacitor; comprising a leakage step-up transformer that steps up the voltage, and a drive circuit that drives the semiconductor switch element,
A capacitor and a magnetron are connected in parallel to the secondary winding of the step-up transformer to energize the magnetron, and the switch element is composed of a series connection of a first transistor and a second transistor, The high-frequency heating device has a structure in which the drive circuit is provided with a DC power supply that energizes the first transistor and an oscillation circuit that energizes the second transistor, and the oscillation circuit oscillates in synchronization with the resonance circuit. 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. Claim 1 in which the first transistor and the second transistor are composed of one or two chips and are housed in a single package and integrated.
The high frequency heating device according to item 1 or 2. 4. The high-frequency heating device according to claim 1, wherein the oscillation circuit is synchronized with the resonant circuit by detecting the collector voltage of the first transistor and the voltage of the unidirectional power source. 5. The high-frequency heating device according to claim 1, wherein the capacitor connected to the secondary winding also serves as a capacitor for preventing unnecessary radiation of the magnetron.