JPS62168383A - 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] Industrial Field of Application The present invention aims to improve the switching element of a 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 a high frequency heating device.

BACKGROUND OF THE INVENTION 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 filler for the high frequency switching operation of the inverter.

The inverter includes a step-up transformer 6 that functions as a power transfer inductance, a transistor 7, a diode 8, and a drive circuit e. The transistor 7 performs a switching operation with a predetermined period and duty (ie, on/off time ratio) by a base current supplied from the drive circuit 9. As a result, the electric current work a/d as shown in Fig. 6a.

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, a voltage vce as shown in FIG. 6 is generated between the C- terminal of the transistor 7 due to the resonance between the capacitor 5 and the primary winding 10. Therefore, high frequency power is generated in the negative winding 10. Therefore, the secondary winding 11 and the tertiary winding 12
High-frequency high-voltage power and high-frequency low-voltage power are generated, 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 16, while the high frequency low voltage power is supplied to the cathode heater. Therefore, the magnetron 16 oscillates and can perform dielectric heating. The magnetron 15 includes a magnetron main body 15a and capacitors 16, 17, 18, 16, 17, and 18, which constitute a filter. Choke carp/L/
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 becomes smaller as the frequency of the power supplied to both ends of the primary winding 10 increases.
- Operating at a frequency of about 100 kHz can reduce the weight and size of the step-up transformer from a few to a dozen times lower than when boosting the voltage at the commercial power frequency, making it possible to reduce the cost of the power supply. It has the characteristics of:

The base current rb supplied to the base of the transistor γ consists of a positive current rb+ and a negative current Ib-, as shown in FIG. 6C. From the positive current, the collector current I of transistor 7
It is necessary that the current amplification factor (hfe, for example, 30) is greater than the maximum value Icm of c. Also,
The negative current xb- 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 Ib is shown in Figure 6'a,
'c It is clear that the maximum value Icm of the collector current rc during the conduction period of the transistor 7 (for example, 40)
It was necessary to set the value Ibm (for example, two people) to be determined by the number of people. Further, the negative current Ibm- is also determined according to the maximum value Ice of the collector current Ic (for example, 15 people), and the larger Icm is, the more power is required.

Furthermore, the collector current IC 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, and this cutoff f'f changes depending on variations in the characteristics of the transistor 7, temperature, etc. Ta. 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, the DC power source 2 obtained from the power transformer 21
2, 23, oscillation circuit 24゜l/transistor 25,
26, 27, resistor 26-36 and diode 3
It is composed of 7.

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 26, 26 were required to be able to handle a considerably large current, and the DC power supplies 22, 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 20H to 50H, 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 transistor 7 with a base current Ibm corresponding to the peak value ICl11 of the collector current IC as shown in FIGS. 6a and 6C.
The electric power required to supply this Ib♂ was quite large. For example, if Icm=40 people and hf'e of the transistor 7 is 30, then I bm+=1.3 people, and the power consumption of the drive circuit 9 becomes extremely large, making the drive circuit 9 and the power transformer 21 larger and more expensive. It was difficult to avoid.

Furthermore, changes in the storage time of the transistor 7 (the main cause of torr 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, which further increases the size and cost of the drive circuit 9, power transformer 21, etc. This has the disadvantage that it not only causes the high-frequency heating device to become unstable, but also increases the fluctuation in the output of the high-frequency heating device, causing unnecessary loss in the transistor 7, etc., and reducing reliability and safety. .

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. an inverter that receives power from the unidirectional power source and has a semiconductor switching element, an inductance for transmitting power that transmits the output of the inverter to a load, and a drive circuit that drives the semiconductor switching element. The semiconductor switch element comprises:
It is composed of a current-driven semiconductor switch and a voltage-driven semiconductor switch, the voltage-driven semiconductor switch is driven by the drive circuit, and the drive means is composed of an oscillation circuit that generates a pulse generation voltage.

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

That is, the high-frequency heating device of the present invention is configured such that a power transmission inductance is provided and an inverter is used to drive the inductance, and the semiconductor switch elements of the inverter are configured with a voltage-driven semiconductor switch and a current-driven semiconductor switch. Stabilizes the switching operation of transistors that handle high power, reduces power consumption of drive circuits and power transformers, and simplifies their configurations. Drive circuits and power transformers have no choice but to become larger and more expensive. can be made compact and inexpensive.

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, a step-up transformer 6 is a power transmission inductance that transmits the generated high-frequency output to the Macnetron and functions as an inductance that resonates with the condenser 6. A magnetron 15 is connected to the secondary winding 11 of the step-up transformer 6, and its filter condensers 16 and 17 are connected in parallel as shown.

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 O, a) than a normal transformer, and has a considerably large leakage inductance. This leakage inductance and filter capacitor 1
6.17 and act as a low-pass filter, it is possible to obtain the desired radio wave output while suppressing the anode peak current of the McNetron without using the conventionally used high-voltage diode. Even if the diode is omitted, the magnetron can operate stably.

That is, leakage inductance and filter co/denna 1
8.17, 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 a simple bibolar transistor inevitably increases switching loss. Therefore, as shown in the figure, first and second transistors 40 and 41 are connected in series, a DC power supply 42 is connected to the base of the first transistor 4o, and an oscillation circuit 43 is connected to the gate of the second transistor 41. are connected to each other.

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, since the first transistor 4o can be a transistor with a slow switching speed, a high-hfe transistor can be used, and the second transistor 41 can be a low-voltage field effect transistor. Power consumption 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, the oscillation circuit 43 is connected to the voltage of the co/denna 4 and the first transistor 4o as shown in the figure.
It is configured to operate by detecting the collector voltage of .

FIG. 2 is a more detailed circuit example of the drive circuit 9,
Components with the same reference numerals as those in FIG. 1 are corresponding components, and their explanation will be omitted.

In FIG. 2, the oscillation circuit 43 includes a zero cross detection section consisting of resistors 45-48, a comparator 60, a delay means 60, and a differentiator 51, and a zero-cross detection section consisting of resistors 52-54, a capacitor 66, a comparator 66, and a differentiator 67. a longest period timer consisting of resistors 57-60. An on-time timer consisting of a diode 61, a capacitor F62, a comparator 63, and a variable reference voltage source 64; an R input with the output of this on-time timer as the S input, and the sum output of the longest period timer and the zero-cross detection section as the R input. -3/FF65. 66 is an AND gate, 67 and 68 are inverter gates, and 69 is a diode. Further, a capacitor 701, a resistor 71, and a diode 72 are connected as shown in the figure to a DC power source 42 that energizes the first transistor 4o, so that the first transistor 40 can perform base-grounding operation well and efficiently. It looks like this. Therefore, characteristic fluctuations caused by conventional toff can be prevented.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit shown in FIG.
c/d and the collector of the first transistor 40 and the second
The voltage V between the drain of the field effect transistor 41 and
It is cci. In addition, C to C in the same figure indicate operation waveforms of each part of the excavation circuit 43.

The output/input of the zero cross detection section is connected to container 4 as shown in Figure C.
A zero cross pulse A is generated near the zero cross after a certain time ta delay from the cross point between the voltages Vg and Wcd.
occurs. 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 condenser 55 reaches a predetermined value C, as shown by broken lines in C and d of the figure.

When pulse A is input to R input of R-5/FF65, Q
The output becomes H as shown in the figure e, and the second field effect transistor 41 and therefore the switch element 44 are turned on.
Ic flows. Condenser 6 with on time timer at the same time
2 is charged as shown in FIG.
/Input the pulse F shown in the figure g to the S input of the FF65. Therefore, the output Q becomes L, and the electric 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 an OMO5-IC or the like, resulting in an extremely simple and low power circuit, making it compact and inexpensive. Furthermore, the first transistor 4
0 can be a low switch speed, so the hfe is extremely high.
transistors 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 tens of On.
Since it is about 19, this diode is actively used.

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 γ3 and the diode 8 minimize changes in the resonant voltage polarity and reverse voltage to the transistor that occurs during switching. 73 indicates a pot as a cooking load. Here, the winding 10 functions as an inductance for transmitting power by interlinking the generated lines of magnetic force with the pot to generate heat. power supply current detection transformer 80;
The rectifier bleed saw 81 and the smoothing capacitor 82 constitute a current detection circuit.

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 made 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.

(1) Regarding a single switch element, there is a trade-off between increasing the frequency to achieve miniaturization and increasing the voltage resistance to obtain large amounts of power. Bipolar types are easy to make with high voltage resistance, but difficult to make high-speed devices. Regarding to fT, especially when connected with Darlington, the withstand voltage of 1000V is about 20 zA BC. 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 about 1 get lower than that of the bipolar type. By adopting the composite structure of the present invention, for the first time, it has become possible to provide large IKW level power 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, considering the semiconductor yield as a high-frequency heating device, it is necessary to make it compact with high power (2 is not only a disadvantage. Conventional devices, for example, can be realized with bipolar type). In case of pressure resistance 1000
An element with a specification of V 1 peak current of 50 people does not meet the standard in either the withstand voltage characteristics or the switching speed.
Even if the operating frequency was reduced to about 25 KHz, it was difficult to increase the yield of devices. -According to Rikiki's invention, current-driven switches only need to have a high withstand voltage specification.
A low speed type is fine.

In addition, the voltage-driven switch has a withstand voltage of about 50V, which is sufficient, and the speed it is good at (toff-+0.1 asIcG
)'s performance is so fast that there is no need to check it at around 25 kHz.

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, such as voltage resistance or speed. Since the measurement process can be omitted and the yield is high, devices can be manufactured at low cost. Furthermore, it would be even more effective to prevent wasteful use of resources, such as being thrown away because one of the characteristics was bad, as was the case in the past.

(iii) If the drive means is provided with an oscillation circuit that corrects the closing period of the semiconductor switch according to the voltage and current of the power supply, even if there are fluctuations in the power supply situation, a constant current will flow from the power transfer inductance to the load. can be supplied and also prevent overload operation of the switch element.

[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 showing 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...Condenser), 6.
...Power transfer inductance, 8...
Reverse diode, 9...Drive circuit, 10...
...-Secondary wire, 11...Secondary winding, 16.
...Macnetron, 40...First transistor (current-driven switch), 41...Second transistor (voltage-driven switch), 42...
...DC power supply, 43...Oscillation circuit, 44...
...Semiconductor switch element, 8o...Current detection transformer (power information horizontal means). Name of agent: Patent attorney Toshio Nakao and 1 other person No. 3
Figure 4 Figure 5 (Ella (b)

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 high-frequency heating device comprising a drive means for supplying a control voltage to a drive type switch, and a heating means for supplying a current from the unidirectional power supply controlled by the current drive type switch. (2) The semiconductor current-driven switch is configured with a bibolar transistor, the semiconductor voltage-driven switch is configured with a field-effect transistor, and a diode is provided in parallel with at least the current-driven switch with respect to the unidirectional power supply. A high-frequency heating device according to claim 1, having a configuration as follows. (3) The high frequency drive device according to claim 1, wherein the drive means has a power supply information detection means and an oscillation circuit that corrects the closing period of the semiconductor switch according to the voltage or current of the power supply flowing into the inverter. heating device. (4) Claim 1, in which at least a current-driven semiconductor switch and a voltage-driven semiconductor switch are composed of one or more chips and housed in a single package.
The high-frequency heating device described in Section 1.