JPS62168382A - 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 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. This invention relates to an improvement in a switching element for opening and closing a resonant type circuit that is often used in circuits.0 Prior Art The prior art will be explained using a magnetron drive power source for a microwave oven as an example.

Various configurations have been proposed for this type of high-frequency heating device in order to make the transformer smaller, lighter, or lower in cost. Figure 6 shows a conventional high-frequency heating device. FIG. In the figure, power from a commercial power source 1 is rectified by a diode dove 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 a resonant capacitor5. Step-up transformer 6, transistor 7, reverse diode 8. and a drive circuit 9. The transistor 7 has a predetermined period and duty depending on the base current supplied from the drive circuit 9.
(ie, on/off time ratio). As a result, the current Io/d as shown in FIG.
flows, while when the transistor 7 is off, the capacitor ′
Due to the resonance between the closure 5 and the primary winding 10, a voltage Vce as shown in FIG. 6 is generated between C and E of the transistor 7. High frequency power is generated in this secondary winding 1o. 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 canards of the magnetron 15, while the high frequency low voltage power is supplied to the cathode heater. Therefore, the magnetron 16 oscillates and dielectric heating becomes possible. The magnetron 16 includes a magnetron main body 15a, capacitors 16 and 17 constituting a filter,
18. It consists of a choke coil 19°2o. 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 1° becomes higher.
When operated at a frequency of about kHz, the weight and size of the step-up transformer can be reduced from a few minutes to a few minutes compared to when boosting the voltage at the commercial power frequency, making it possible to reduce the cost of the power supply section. It has the following features.

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. 6C. Positive current Ib+ is collector current Ic of transistor 7
It is necessary that the current amplification factor (hfe, for example, 30) is greater than the maximum value Icm. Further, the negative 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. Positive current 1b+ is shown in Figure 6a.

As is clear from C, it was necessary to set the value I t)m+ (eg, 2 A) determined by the maximum value Icm (eg, 6° A) of the collector current Ic during the conduction period of the transistor 7. Further, the negative current Ibm- is also determined according to the maximum value Icm of the collector current Ic (for example, 15 A),
The larger 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 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. 6, for example. That is, a DC power source 22 obtained from a power transformer 21
, 23 , oscillation circuit 24 . Transistors 25, 26
, 27, resistor 25-36° and diode 37
It is composed of

The 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. 6C 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 are extremely large and expensive, and require power of about 20W to 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 rbm+ corresponding to the peak value Icm of the collector current Ic to the transistor 7 as shown in FIGS. 6a and 6C, and this I
The power required to supply bm+ was quite large. For example, if I cm = 60 A and hfe of the transistor 7 is 30, then m+ = 2 A, and the power consumption of the drive circuit 9 becomes extremely large, making it difficult to avoid increasing the size and price of the drive circuit 9 and the power transformer 21. Met.

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, and the drive circuit 9. This not only increases the size and price of the power transformer 21, etc., but also increases the output fluctuation of the high-frequency heating device, making it unstable, and causes unnecessary losses in the transistor 7, etc., which impairs reliability and safety. It had the disadvantage of causing a decline in

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 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, the semiconductor switch element comprising a current drive type semiconductor switch and a voltage drive type semiconductor switch, and the voltage drive type semiconductor switch is driven by the drive circuit. , the small movement means is constituted by an oscillation circuit that generates a pulse generation voltage in synchronization with a resonant circuit.

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 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 voltage-driven semiconductor switch and a current-driven semiconductor switch. Since the voltage-driven semiconductor switch is configured to operate in synchronization with the resonant circuit, the switching operation of transistors that handle large amounts of power is stabilized, and the power consumption of the drive circuit and power transformer is reduced. This makes it possible to make the drive circuit and power transformer, which had previously been made larger and more expensive, more compact and less expensive.In addition, it prevents fluctuations in the semiconductor switch element current caused by changes in characteristics. However, it has the effect of stabilizing the output of the power transmission inductance, suppressing wasteful power consumption of semiconductor switching elements, etc., and improving reliability.

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 transmits the generated high-frequency output to the magnetron. The magnetron 16 is connected, 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 has a smaller coupling coefficient between the primary and secondary windings than a normal transformer (for example, 0, S), and has a fairly large leakage inductance.

This leakage inductance and filter capacitor 16.1
7 acts as a kind of low-pass filter, it is possible to suppress the magnetron's anode peak current and obtain the desired radio wave output without using the conventionally used high-voltage diode, and the high-voltage diode can be omitted. The magnetron can operate stably even when i.e. leakage inductance and filter capacitor 16.1
7, 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, the first and second transistors 40 and 41 are connected in series, and the base of the first transistor 40 is connected to the direct temperature power supply 42, and the gate of the second transistor 41 is connected to the oscillation circuit 43. With this configuration, the withstand voltage of the switch element 44 can be shared by the first transistor 4o, and the switching operation can be shared by the second transistor 41, and the switch element 44 as a whole can be It is possible to realize a stable and efficient magnetron drive inverter that suppresses loss and has a circuit configuration that eliminates high-voltage diodes. Furthermore, since the first transistor 4o may 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 excavation 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 of the resonant capacitor 6 and the step-up transformer e.

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-crossing detection section consisting of a comparator 60, a delay means 60, and a differentiator 61; a longest period coumer consisting of resistors 52-54, a capacitor 66, a comparator 66, and a differentiator 75; resistors 57-60, a diode 61, An on-time timer consisting of a capacitor 62, a comparator 63, and a variable reference voltage source 64, and an R-8/ in which the output of this on-time timer is the S input, and the sum output of the longest period timer and the zero-cross detection section is the R input. Consists of N\FF65. 66 is an anime game)
, 67 and 68 are inverter gates, and 69 is a diode. Also, a capacitor 70 is connected to the DC power supply 42 that energizes the first transistor 40. Resistor 71, diode 7
2 are connected as shown in the figure, and the first transistor 4
0 can perform base grounding operation well and efficiently. Therefore, characteristic fluctuations due to conventional toH can be prevented.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit shown in FIG.
a/d and the collector of the first transistor 4° and the second
The voltage V between the drain of the field effect transistor 41 and
It is a CD. Further, C to q in the same figure show operation waveforms of each part of the oscillation circuit 43.

Zero cross detection section output A is connected to capacitor 4 as shown in figure C.
A certain time t from the cross point of the voltages Vs and Vcd
A zero cross pulse A is generated near the zero cross with a delay of d. If this cross point is not detected for some reason, the voltage B of the capacitor 55 reaches the predetermined value C as shown by the broken line in C and d of the same figure. A forced zero cross/curse is generated by the longest period timer at the point 0
When sills A is input to the R input of the R-8/FF 86, 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 are turned on, and Ic flows. At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG.
3/Input the pulse F shown in q in the figure to the S input of the FF66.

Therefore, the output becomes QHL, and the second field effect transistor 41 and therefore the switch element 44 are turned off, returning to the initial state, and repeating this process.
Since it can be directly driven by S-ICfx, it becomes an extremely simple and low-power circuit, making it compact and inexpensive. Furthermore, the first transistor 4o has a low switching speed.

Therefore, a transistor with extremely high hfe can be used, and the DC power supply 42 can also be made compact and inexpensive.

Particularly in this embodiment, the diode 8 is placed in parallel with the first transistor 4o. When the second transistor is constructed from a power MO8 type field effect transistor, a parasitic diode 8a is formed between the drain and source. This diode is fast and has a reverse recovery time of several 10 On8.
Therefore, this diode is actively used.

FIG. 4 shows an embodiment using an electromagnetic cooker, in which a pie-filler winding 73 is wound around the winding 1o functioning as a zero-heating work coil and a diode is connected in series with the winding 1o. This winding 73 and diode 8 minimize the change in resonance voltage polarity and the reverse voltage to the transistor that occurs during switching. As a cooking load, 73 indicates a pot. Here, the winding 1o functions as an inductance for transmitting power to generate heat by interlinking the generated magnetic lines with the pot.

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.

Effects of the Invention 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, it is also possible to reduce the control power of the inverter to a fraction of that of the conventional example. . Furthermore, it has the following characteristics.

(:) When it comes to individual switching elements, there is a trade-off between increasing the frequency to achieve miniaturization and increasing the breakdown voltage to obtain large amounts of power. It is easy to make a bipolar type with a high withstand voltage, but when it comes to 0toff, which is difficult to make a high-speed element, especially when connected with Darlington, the withstand voltage is about 2olSEC at 1000V.

On the other hand, field effect transistors have high speed but low breakdown voltage. When compared at the same current density, the withstand voltage is approximately one order of magnitude lower than that of the bipolar type. By adopting the composite structure of the present invention, it has become possible for the first time to provide a large power of 1 kW level like a high frequency heater as a device that is miniaturized by increasing the frequency.

(11) 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 yield of semiconductors, it is not necessarily disadvantageous to make it compact and use a large amount of power. For example, when the conventional type is realized as a bipolar type, the breakdown voltage is 1 ooov,
Elements with specifications for a peak current of 60 A did not meet the specifications in either the breakdown voltage characteristics or the switching speed, and it was difficult to increase the yield of the elements even if the operating frequency was lowered to about 25 kHz. -According to Shiki's invention, the current-driven switch only needs to have a high withstand voltage specification and can be a low-speed type.

Also, voltage-driven switches have a withstand voltage of S.

7th place would be enough and my good speed (toff''=.

The performance of 0.1μ5EC) is so fast that there is no need to check it at around 25kHz.

Therefore, in addition to being more compact by setting the operating frequency to around 50 kHz, each switch only needs to have the performance it is good at, whether it is voltage resistance or speed. Since it 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.

[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 6 is a circuit diagram of another embodiment of the present invention, FIG. 6 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...
・・Inductor, 4・・Capacitor), 6・・・・
・Resonance capacitor, 6... Power transfer inductance, 8... Reverse diode, 9...
...Drive circuit, 1o...-Next winding, 11...
... Secondary winding, 16... Magnetron, 4°.
...First transistor (current-driven switch)
, 41... Second transistor (voltage driven switch), 42... DC power supply, 43...
-Oscillation circuit, 44...Semiconductor switch element. Name of agent: Patent attorney Toshio Nakao and 1 other person
3 Figure 4 Figure 4, wo Figure 5 'Z/ (I))

Claims (4)

[Claims] (1) A unidirectional power source obtained from a commercial power source, a semiconductor current-driven switch that controls large current and generates high-frequency power, a semiconductor voltage-driven switch that controls the opening and closing of this current-driven switch, and a semiconductor voltage-driven switch that controls the opening and closing of this current-driven switch. A driving means for supplying a control voltage to the driven switch, 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 the converting means. a high-frequency heating device having a heating means to which an output of is supplied. (2) The semiconductor current-driven switch is configured with a bipolar transistor, the semiconductor voltage-driven switch is configured with a field-effect transistor, and a diode is provided at least in parallel with the current-driven switch with respect to the unidirectional power supply. A high-frequency heating device according to claim 1. (3) 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 a transistor serving as a current-driven switch and the voltage of a unidirectional power source. (4) The high-frequency heating device according to claim 1, wherein at least a current-driven semiconductor switch and a voltage-driven semiconductor switch are constructed of one or more chips and housed in a single package.