JPS62193086A - Radio frequency heater - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application 2B-7 The present invention relates to a high-frequency heating device that uses a high-power frequency converter as a power source for an induction heater or a power source for driving a magnetron for dielectric heating. .

In particular, the present invention relates to improvements in switching elements for opening and closing resonant circuits, which are often used in this type of circuit.

Prior Art The prior art will be explained using a magnetron drive power source for a microwave oven as an example.

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. 5 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 is composed of a resonant capacitor 5, a step-up transformer 6, a transistor 7, a reverse diode 8, and a drive circuit 9. The transistor 7 performs a switching operation with a predetermined cycle and tutee (ie, on-off time ratio) by a base current supplied from the drive circuit 9. As a result, the current Ic as shown in FIG. 6(a)
/d, that is, the collector current Ic of the transistor 7 and the current Id of the diode 8 flow, while when the transistor 7 is off, the voltage Vce as shown in FIG. 6(b) flows through the transistor due to resonance between the capacitor 5 and the primary winding 10. 7
occurs between C and E. Therefore, high frequency power is generated in the negative 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 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 can perform dielectric heating. The magnetron 15 includes a magnetron main body 15a, capacitors 16, 17, 18, and choke coils 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 becomes smaller as the frequency of the power supplied to both ends of the primary winding 10 increases. Compared to the case of boosting the voltage as is, the weight and size of the step-up transformer can be reduced from a few minutes to a few times as much, and the cost of the power supply section can be reduced.

The base current Ib supplied to the base of the transistor 7 consists of a positive current Ib+ and a negative current Ib- as shown in FIG. 6(c). It is necessary that the positive current Ib-1- is larger than the maximum value Icm of the collector current Ic of the transistor 7 by its current amplification factor (hfe, for example, 30). Further, the negative current Ib- is a current that flows by reverse biasing between the pace emitters 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 FIG. 6, the positive current Ib+ is a value Ibm+ (for example, 2) determined by the maximum value Icm (for example, 60 A) of the collector current Ic during the conduction period of the transistor 7. It was necessary to do so. Further, the negative current Ibm- is also determined according to the maximum value Cm of the collector current Ic (for example, 15
A) The larger the ICm, the more power was 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, and this toff changes depending on the characteristics of the transistor 7, such as irregularities and temperature. . This change in toif results in a change in the output of the input.

In order to drive the transistor 7 under such conditions, the drive circuit 9 has a configuration as shown in FIG. 5(b), for example. That is, a DC power source 22, 23 obtained from the power transformer 21, an oscillation circuit 24, transistors 25, 26,
27, resistors 25-36, and a diode 37.

6 The page oscillation circuit 24 turns on the transistor 2 during a conduction period of a predetermined period.
5.26 is turned on and off alternately, and a base current as shown in FIG. 6(C) 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
had to be large and expensive.

In particular, when the transistor 7 is operated with high frequency and large current, the drive circuit 9 and the power transformer 21 become extremely large and expensive, and require a power of about 201M-50W, for example.

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, a 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 Ibm+ corresponding to the peak value Icm of the collector current Ic to the transistor 7 as shown in FIGS. 6(,) and (c), and this Ibm+ is supplied. The amount of electricity required was quite large. For example, if Icm-60A and the hfe of the transistor 7 is 30, the result will be Ibm+-2A, and the power consumption of the drive circuit 9 will be extremely large, making it difficult to avoid increasing the size and price of the drive circuit 9 and the power transformer 21. Ta.

Furthermore, changes in the storage time of the transistor 7 (the main cause of toff in FIG. 6) due to temperature changes, etc.
In order to cope with changes in the collector current Icm caused by temperature changes and changes over time in the magnetron 15, it is necessary to supply a sufficient base current. Not only does this increase the 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., which reduces reliability and safety. Ta.

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, an inverter that receives power from the unidirectional power source and has a semiconductor switching element, and a power transmission inductance that forms a resonant circuit with a resonant capacitor and transmits the output of the inverter to a load. and a drive circuit for driving the semiconductor switch element, wherein the semiconductor switch element is composed of a semiconductor high-voltage bipolar switch and a semiconductor high-speed bipolar switch, the semiconductor high-speed bipolar switch is driven by the drive circuit, and the drive means is configured to resonate. It consists of an oscillation circuit that generates a pulse generation voltage in synchronization with the circuit.

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

9.B.7 That is, the high frequency heating device of the present invention is configured to include a power transmission inductance, drive the inductance with a resonant inverter, and replace the semiconductor switching elements of the resonant inverter with a semiconductor high speed bipolar switch and a semiconductor high voltage switch. The semiconductor high-speed bipolar switch is configured to operate in synchronization with the above-mentioned resonant circuit, which stabilizes the switching operation of transistors that handle large amounts of power, and reduces the power consumption of the drive circuit and power transformer. This simplifies the drive circuit and power transformer, which had previously been forced to be larger and more expensive, to be made more compact and less expensive. It also has the function of preventing fluctuations in the semiconductor switch element current caused by changes in characteristics, stabilizing the output of the power transfer inductance, and suppressing wasteful power consumption of the semiconductor switch elements, improving reliability. .

The semiconductor high-speed bipolar switch referred to in the present invention has a turn-off time of 1 μSEC or less (withstand voltage of 100 V or less),
Semiconductor high voltage bipolar switch 10 P/゛ means a withstand voltage of 300V or more (turn-off time is several μ5EC)
It refers to the element of The characteristics of bipolar transistors are that the higher the withstand voltage specifications, the lower the speed, and the lower voltage specifications, the higher the speed, and the higher the current amplification rate.

EXAMPLE Hereinafter, an example of the high frequency heating device of the present invention will be described 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. 5 are corresponding components, and the explanation thereof will be omitted.

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. On the other hand, the step-up transformer 6 has a smaller coupling coefficient between the primary and secondary windings (for example, about 0.8) than a normal transformer.
P.

and has a fairly large leakage inductance. This leakage inductance and filter capacitor 1
6.17 acts as a kind of low-pass filter, it is possible to obtain the desired radio wave output while suppressing the magnetron's anode peak current without using the conventionally used high-voltage diode. Even if it is omitted, the magnetron can operate stably.

That is, the leakage interface and filter capacitor 1
6.17, the peak value of the anode current can be suppressed. 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. Further, the first transistor 40 may have a slow switching speed, and the second transistor 41 may be a bipolar transistor with low breakdown voltage, so the power consumption of the drive circuit 9 can be significantly reduced compared to the conventional one. can. The second transistor can be a low-voltage transistor, so it is fast and has a high current increase rate. This eliminates the need for voltage and simplifies the drive circuit. When the second transistor is off, the first transistor momentarily flows into the collector current of the first transistor, but this current is transferred to the first transistor as a slow transistor to charge the capacitor 70. However, by connecting it in series with a high-speed second transistor, it can be used as a high-speed, high-voltage switch element. Therefore, the power transformer 21 may also be compact and inexpensive, and the entire drive circuit 9 can be made compact and inexpensive. Further, the oscillation circuit 43 is configured to detect the voltage of the capacitor 4 and the collector voltage of the first transistor 40 as shown in the figure, and operate in synchronization with the resonance of 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 resistors 45-48,
a zero-cross detection section consisting of a comparator 30, a delay means 50, and a differentiator 51; a longest period timer consisting of resistors 52-54, a capacitor 55, a comparator 56, and a differentiator 57; resistors 57-60, a diode 61, An on-time timer consisting of a capacitor 62, a comparator 63, a variable reference voltage source 64, and the output of the on-time timer of 14 vanes are used as the S input, and the sum output of the longest period timer and the zero-cross detection section is used as the R input. (
-8/FF65.

66 is an AND gate, 67.68 is an inverter gate,
69 is a diode. In addition, the first transistor 4
The DC power supply 42 that energizes the 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, the conventional tof
Characteristic fluctuations due to f can be prevented.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit of FIG.
The voltage Vcd between the collector of the transistor 41 and the emitter of the second transistor 41 is Vcd. Also, the same figure (c) or g)
are operation waveforms of each part of the oscillation circuit 43.

The output A of the zero cross detection section is delayed by a certain time td from the cross point of the voltages Vs and Vcd of the capacitor 4, and reaches zero cross point 15 in the vicinity of the zero cross, as shown in FIG.

Generates Rocross Pulse A. If this cross point is not detected for some reason, the forced zero cross pulse is activated by the longest cycle timer when the pressure B of the capacitor 55 reaches a predetermined value C, as shown by the broken line in (c) and (d) of the same figure. generated.

When the pulse A is input to the S input of the R, -S/FF 65, the Q output becomes H as shown in FIG. At the same time, the capacitor 62 of the on-time timer is charged as shown in the figure (f), and when the voltage E determined by the variable reference voltage source 64 is reached, the on-time timer is connected to the S input of the R-8/FF 65 as shown in the figure (g). pulse F
Enter. Therefore, the output Q becomes L, and the second 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 transistor 41
Since it can be directly driven by a CMO8-IC or the like, it becomes an extremely simple and low-power circuit, making it compact and inexpensive.

FIG. 4 shows an example using an electromagnetic cooker. A bifilar winding 73 is wound around the winding 1o functioning as a heating work coil, and a diode is connected in series with this. The winding 73 and the diode 8 minimize changes in resonance voltage polarity and reverse voltage across the transistor that occurs during switching. 73 indicates a pot as a cooking load. Here, the winding 1o functions as an inductance for transmitting power by interlinking the generated lines of magnetic force with the pot to generate heat.

In addition to the method in which the first and second transistors are connected in series as in the embodiment, the present invention can also be realized by a Darlington connection method in which a field effect transistor is used as the first stage.

As described above, according to the present invention, in addition to being smaller and more compact as a power source for a high-power high-frequency heating device, the control power of the inverter can be reduced to a fraction of that of the conventional example. Furthermore, it has the following characteristics.

As for a single switch element, there is a trade-off between increasing the frequency for miniaturization and increasing the breakdown voltage for obtaining high power. Bipolar types are easy to make with high voltage resistance, but difficult to make high-speed devices. Regarding toff, especially when connected with Darlington, the iooov withstand voltage is about 20μSEC.

However, when the withstand voltage is set to a low voltage specification of 100V or less,
Transistors with high speed and a large current amplification factor can be easily manufactured. Therefore, by employing the composite structure of the present invention, it has become possible to provide a high-frequency electric power such as a high-frequency heater (IKW level) as a miniaturized device by increasing the frequency.

The present invention is disadvantageous in that it uses two elements compared to the conventional implementation using one element. However, since it is a high-frequency heating device, considering the semiconductor yield, it is not necessarily a disadvantage to use a large amount of power and make it compact. For example, if a conventional device is implemented as a bipolar type device with specifications for a withstand voltage of 1000 V and a peak current of 60 A, either the withstand voltage characteristics or the switching speed of 18 pages meet the standard, and the operating frequency is 25 K.
Even if the frequency is reduced to about Hz, it is difficult to increase the yield of devices. On the other hand, according to the present invention, the high-voltage type switch only needs to have a high withstand voltage specification and may be a low-speed type. Also, the high-speed switch has a withstand voltage of S.

7th place would be enough and his good speed (toff #0.
The performance of the 5 μS E C) is so fast that there is no need to check it at around 25KHz.

Therefore, in addition to being more compact by lowering the operating frequency to around 50 KHz, each switch only needs to have its own performance, whether it is voltage resistance or speed, which results in a reduction in the device manufacturing process. Since the characteristic measurement process can be omitted and the yield is high, devices can be manufactured at low cost. Furthermore, it also has the effect of preventing wasteful use of resources, such as being thrown away because one of the characteristics is bad, as in the past.

When the second transistor is formed of a field effect transistor, it is difficult to form two transistors on one chip. In the case of the present invention, both can be manufactured using a bipolar process, making it easy to form one chip.

Effects of the Invention As described above, according to the present invention, it is possible to provide a high-frequency heating device having a compact and power-saving high-frequency power source.

[Brief explanation 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 the drive circuit of the same device, Fig. 3 is a diagram of operating current waveforms of each part of the device, and Fig. 4 5 is a circuit diagram of another embodiment of the present invention, FIG. 5 is a circuit diagram of a conventional high frequency heating device, and FIG. 6 is a diagram of operating current waveforms of various parts of the same device. 1...Commercial power supply, 2,3.4...Unidirectional power supply (2.
・Diode bridge, 3... Interfter, 4...
Capacitor), 5... Resonance capacitor, 6...
・Power transfer inductance, 8... Reverse diode, 9... Drive circuit, 10...-Next winding, 11
...Secondary winding, 15...Magnetron, 40...
First transistor (semiconductor high voltage bipolar switch)
, 41... - Second transistor (semiconductor high-speed bipolar switch), 42... DC power supply, 43... -
Oscillation circuit, 44... semiconductor switch element. Name of agent: Patent attorney Toshio Nakao Haga 1 person No. 5
Figure 6' 76. l' ← 1 j Ic ' 1 欥%゛. (OJ) 1 J〆 11 Sting f-Ll- ゝ"Iris 'ha□

Claims (2)

[Claims] (1) A semiconductor switch element consisting of a unidirectional power source obtained from a commercial power source, a semiconductor high-voltage bipolar switch that controls large current and generates high-frequency power, and a semiconductor high-speed bipolar switch that controls the opening and closing of this switch. , a driving means for supplying a control signal to the semiconductor switch element; a converting means for converting the high frequency power to form a resonant circuit; an oscillation circuit provided in the driving means and oscillating in synchronization with the resonant circuit; and heating means to which the output of the converting means is supplied. (2) A semiconductor high-voltage bipolar switch and a semiconductor high-speed bipolar switch are configured in at least one chip,
The high frequency heating device according to claim 1, which is housed in a single package.