JPH0677474B2 - High frequency heating device - Google Patents

Description

TECHNICAL FIELD The present invention is intended to improve a switching element for a power source for an induction heater or a power source for driving a magnetron for dielectric heating, which uses a high-power frequency converter. The present invention relates to a high frequency heating device.

2. Description of the Related Art As a high-frequency heating apparatus of this type, various configurations have been proposed in order to reduce the size, weight, or cost of the power transformer.

FIG. 5 is a circuit diagram of a conventional high frequency heating device. In the figure, the power of the commercial power source 1 is rectified by the diode bridge 2 to form a unidirectional power source. 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 step-up transformer 6, a transistor 7, a diode 8 and a drive circuit 9, which function as an inductance for power transmission. The transistor 7 performs a switching operation with a base current supplied from the drive circuit 9 at a predetermined cycle and duty (that is, on / off time ratio). As a result, the current Ic as shown in FIG.
/ 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 resonance of the capacitor 5 and the primary winding 10 causes a voltage Vce as shown in FIG. Therefore, high frequency power is generated in the primary winding 10.
Therefore, high frequency high voltage power and high frequency low voltage power are generated in the secondary winding 11 and the tertiary winding 12, respectively. This high frequency high voltage power is rectified by the capacitor 13 and the diode 14 and 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 enables dielectric heating. The magnetron 15 includes a magnetron body 15a and capacitors 16, 17, 18, which form a filter,
It is composed of choke coils 19 and 20. Again 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 becomes higher.
Compared to the case of boosting at the frequency of commercial power supply, operating at a moderate frequency can reduce the weight and size of the step-up transformer to a few tenths to a tenth, and to reduce the cost of the power supply unit. It has features.

The base current Ib supplied to the base of the transistor 7 is composed of a positive current Ib ⁺ and a negative current Ib ⁻ as shown in FIG. 6c. The positive current Ib ⁺ is the maximum value Ic of the collector current Ic of the transistor 7.
It is necessary that the current amplification factor (hfe, for example, 30) be larger than 1 / m. The negative current Ib ^- is a current that flows by reverse biasing the base and emitter of the transistor 7 in order to increase the switching speed of the transistor 7 and prevent an increase in switching loss.
As is clear from FIGS. 6a and 6c, the positive current Ib ⁺ needs to be a value Ibm ⁺ (eg 2 A) determined by the maximum value Icm (eg 40 A) of the collector current Ic during the conduction period of the transistor 7. there were. The negative current Ibm ^-is also determined according to the maximum value Icm of the collector current Ic (for example, 15A), Ic
The larger m was, the more power was required.

Further, the collector current Ic is toff for a fixed time after the base current Ib ⁺ is cut off due to the so-called minority carrier accumulation effect.
However, this toff changes depending on variations in the characteristics of the transistor 7 and temperature. Then, this change in toff resulted in a change in the output of the inverter.

The driving circuit 9 for driving the transistor 7 under such a condition has a structure as shown in FIG. 5b, for example. That is, the DC power source 22,2 obtained from the power transformer 21
3. The oscillator circuit 24, transistors 25, 26, 27, resistors 25-36 and diode 37.

The oscillating circuit 24 turns on the transistors 25, 2 during the conduction period of a predetermined cycle.
6 is alternately turned on and off to supply the base current as shown in FIG. However, the transistors 25 and 26 needed to be able to handle a fairly large current, and the DC power supplies 22 and 23 also needed to supply this large current. Therefore, the drive circuit 9 and the power supply transformer 21 are inevitably large and expensive.

In particular, when the transistor 7 is operated with a high frequency and large current, the drive circuit 9 and the power supply transformer 21 are extremely large and expensive, and require electric power of, for example, about 20H-50H.

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 and 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. 6A and 6C, and the power for supplying this Ibm ⁺ is considerably large. It was a thing. For example, if Icm = 40A and hfe of the transistor 7 is 30, Ibm ⁺ = 1.3A, the power consumption of the drive circuit 9 becomes extremely large, and it is difficult to avoid the drive circuit 9 and the power transformer 21 from becoming large and expensive. Met.

In addition, a base sufficient to cope with changes in the storage time of transistor 7 (the main cause of toff in FIG. 6) due to temperature changes, and changes in the collector current Icm caused by temperature changes and aging 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 that the transistor 7 and the like are lost, and the reliability and safety are reduced.

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

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

That is, a unidirectional power source obtained from a commercial power source, an inverter having a semiconductor switching element that receives power from the unidirectional power source, a power transmission inductance that transmits the output of the inverter to a load, and a drive that drives the semiconductor switching element. And a semiconductor switch element comprising a circuit,
The current drive type semiconductor switch is composed of a voltage drive type semiconductor switch, the voltage drive type semiconductor switch is driven by the drive circuit, and the drive means is composed of an oscillation circuit for generating a pulse generation voltage.

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

That is, the high-frequency heating apparatus of the present invention is configured so that an inductance for power transmission is provided and the above-mentioned inductance is driven by the inverter, and the semiconductor switch element of the inverter is constituted by the voltage-driven semiconductor switch and the current-driven semiconductor switch. A drive circuit and power transformer that had to be upsized and priced by stabilizing the switching operation of transistors that handle power, reducing the power consumption of the drive circuit and power transformer, and simplifying the configuration. It can be made compact and inexpensive.

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, the step-up transformer 6 is a power transmission inductance that transmits the generated high frequency output to the magnetron and functions as an inductance that resonates with the capacitor 5. 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 an ordinary transformer (for example, about 0.8), and has a considerably large leakage inductance. Since the 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 conventionally used high voltage diode. 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 the switching loss inevitably increases in a simple bipolar transistor. Therefore, as shown in the figure, the first and second transistors 40 and 41 are connected in series, the direct temperature power source 42 is connected to the base of the first transistor 40, and the second transistor 41 is connected.
The oscillator circuits 43 are respectively connected to the gates of. With this configuration, the withstand voltage of the switch element 44 can be shared by the first transistor 40, and the switch operation and the second transistor 41 can be shared. It is possible to realize a stable and efficient magnetron drive inverter with a circuit configuration that omits. Further, since the first transistor 40 may have a low 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. Therefore, the power transformer 21 may be compact and low-priced, and the entire drive circuit 9 can be compact and low-priced. Further, the oscillator circuit 43 is configured to detect and operate the voltage of the capacitor 4 and the collector voltage of the first transistor 40 as shown in the figure.

FIG. 2 shows an example of a more detailed implementation circuit of the drive circuit 9,
The same reference numerals as those in FIG. 1 are corresponding components, and the description thereof will be omitted.

In FIG. 2, the oscillator circuit 43 includes resistors 45-48 and a comparator 5
0, a delay unit 50, a zero-cross detector composed of a differentiator 51, resistors 52-54, a capacitor 55, a comparator 56, and a differentiator 75.
And a longest period timer consisting of a resistor 57-60, a diode 61, a capacitor 62, a comparator 63, a variable reference voltage source 64, and an output of this ontime timer as an S input, It is composed of RS-FF65 which receives the sum output with the zero-cross detector as the R input. 66 is an AND gate, 67 and 68 are inverter gates, 69
Is a diode. Further, a capacitor 70, a resistor 71, and a diode 72 are connected as shown in the figure to a DC power supply 42 for energizing the first transistor 40.
The 40 is able to perform the base grounding operation satisfactorily and efficiently. Therefore, it is possible to prevent the characteristic variation due to the conventional toff.

FIG. 3 is a waveform diagram for explaining the operation of the drive circuit of FIG. 2, and FIGS. 3A and 3B show the current Ic / d flowing through the switch element 44.
And Vcd between the collector of the first transistor 40 and the drain of the second field effect transistor 41.
Further, FIGS. 6C to 6G are operation waveforms of each part of the oscillator circuit 43. The output A of the zero-cross detector is td for a certain time from the cross point of the voltages Vs and Vcd of the capacitor 4 as shown in FIG.
A zero cross pulse A is generated in the vicinity of the zero cross with a delay. If this cross point is not detected for some reason, a forced zero-cross pulse is generated by 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.

When the pulse A is input to the R input of RS-FF65, the Q output becomes H as shown in FIG.
1, therefore the switch element 44 is turned on and Ic flows. At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG. 6f, and when the voltage E reached by the variable reference voltage source 64 is reached, the on-time timer applies the pulse F of g in the same figure to the S input of RS-FF65. input. Therefore, the output Q becomes L, the field effect transistor 41 of the power source 2, and thus the switch element 44 is turned off and returns to the initial state, and this is repeated.

In such a circuit operation, the second field effect transistor 41 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. Further, since the first transistor 40 may have a low switching speed, a transistor having an extremely high hfe can be used, and the DC power supply 42 can be made compact and inexpensive.

In particular, in this embodiment the diode 8 is placed in parallel with the first transistor 40. Second transistor for power
When the MOS field effect transistor is used, a parasitic diode 8a is formed between the drain and the source. Since this diode is fast and the reverse recovery time is about several 100 ns, it is actively used.

FIG. 4 shows an embodiment using an electromagnetic cooker. A winding 73 that is a bifilar winding is wound around the winding 10 that functions as a work coil for heating, and a diode is connected in series with the winding 73.
The winding 73 and the diode 8 suppress the change of the resonance voltage polarity and the reverse voltage to the transistor generated at the time of switching to the minimum. 73 as cooking load
Indicates a pot. Here, the winding wire 10 functions as a power transmission inductance that links the generated magnetic field lines with the pan to generate heat. The power supply current detection transistor 80, the rectifying breather 81, and the smoothing capacitor 8I form a current detection circuit.

The present invention can be realized not only by the method of connecting the first and second transistors in series as in the embodiment but also by the method of Darlington connection having the field effect transistor as the first stage.

EFFECTS OF THE INVENTION As described above, according to the present invention, as a power source for a high-power high-frequency heating device, in addition to being compact and compact, the control power of the inverter can be reduced to a fraction of that of the conventional example. It also has the following features.

(I) Regarding a single switching element, there is a trade-off relationship between high frequency for downsizing and high breakdown voltage for obtaining large power. It is easy to make a bipolar type with a high breakdown voltage, but it is difficult to make a high-speed element. Speaking of toff, especially with Darlington connection, 20μSEC with 1000V withstand voltage
Rank On the other hand, a field effect transistor has a high speed but a low breakdown voltage. Compared with the same current density, the breakdown voltage is about one digit lower than that of the bipolar type. Only by using the composite structure of the present invention, 1 KW like a high frequency heater.
It has become possible to provide high-level power as a device that is designed to be downsized by increasing the frequency.

(Ii) The present invention is disadvantageous in that it uses two elements, as compared with the conventional method that uses one element.
However, considering the yield of semiconductors because it is a high-frequency heating device, it is not necessarily the only disadvantage in achieving high power and compactness. In the case of the conventional type, for example, when implementing with an ipolar type, an element with a withstand voltage of 1000 V and a peak current of 50 A does not meet the standard in either withstand voltage characteristics or switching speed characteristics, and the operating frequency is
It was difficult to increase the yield of the device even if it was reduced to about 25 kHz. On the other hand, according to the present invention, the current drive type switch only needs to have a withstand voltage specification, and may be a constant speed type. Also, with a voltage-driven switch, a withstand voltage of about 50 V is sufficient, and the performance of its specialty (toff≈0.1 μSEC) is so fast that it does not need to be checked at about 25 kHz.

Therefore, in addition to being able to make the operating frequency around 50kHz and making it more compact, it is sufficient for each switch to simply secure the withstanding voltage, speed, or one of its strengths. Since the process can be omitted and the yield is increased, the device can be manufactured at low cost. Furthermore, since one characteristic is poor as in the conventional case, it is possible to prevent the waste of resources such as being discarded at the same time.

(Iii) If an oscillating circuit that corrects the closing period of the semiconductor switch according to the voltage and current of the power supply is provided in the driving means, a constant current is applied to the load from the power transfer inductance even if the power supply situation changes. Can be supplied and overload operation of the switch element can be prevented.

[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 detailed circuit diagram of a drive circuit of the device, FIG. 3 is an operation current waveform diagram of each part of the device, and FIG. Is a circuit diagram showing another embodiment of the present invention, FIG. 5 is a circuit diagram of a conventional high-frequency heating device, and FIG. 6 is an operating current waveform diagram of each part of the device. 1 ... Commercial power supply, 2, 3, 4 ... Unidirectional power supply, (2 ... Diode bridge, 3 ... Inductor, 4 ... Capacitor), 6 ... Power transfer inductance, 8 ... Reverse diode, 9 …… Drive circuit, 10 …… Primary electric wire, 11 ……
Secondary winding, 15 ... Magnetron, 40 ... First transistor (current drive type switch), 41 ... Second transistor (voltage drive type switch), 42 ... DC power supply, 43 ... Oscillation circuit, 44 …… Semiconductor switch element, 80 …… Current detection transformer (power supply information detection means).

Claims (4)

[Claims] 1. A unidirectional power supply obtained from a commercial power supply, a semiconductor current drive type switch for controlling a large current to generate high frequency power, and a semiconductor voltage drive type switch for controlling opening / closing of the current drive type switch. A high-frequency heating device comprising a driving means for supplying a control voltage to the voltage driven switch and a heating means for supplying a current from the unidirectional power source controlled by the current driven switch. 2. A semiconductor current drive type switch is constituted by a bipolar transistor, a semiconductor voltage drive type switch is constituted by a field effect transistor, and a diode is arranged at least in parallel with the current drive type switch with respect to a unidirectional power source. The high-frequency heating device according to claim 1, wherein the high-frequency heating device is provided. 3. The drive circuit according to claim 1, further comprising an oscillating circuit for correcting the closing period of the semiconductor switch according to the voltage or current of the power source flowing into the inverter, the drive means having the power source information detecting means. High frequency heating device. 4. The high frequency heating apparatus according to claim 1, wherein at least the current drive type semiconductor switch and the voltage drive type semiconductor switch are configured by one chip or a plurality of chips and housed in a single package.