JPS62110294A - 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 Application Field The present invention relates to the improvement of high-frequency heating devices for so-called dielectric heating such as microwave ovens. This invention relates to an improvement in a high-frequency heating device configured to generate high-frequency power using an inverter using a high-frequency power converter, and boost the voltage using a step-up transformer to drive a magnetron.

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.

Figure @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.

Impark is composed of a resonant capacitor 5, a step-up transformer 6, a transistor 7, a diode 8, and a drive circuit 9. The transistor 7 performs a switching operation with a predetermined cycle and duty (ie, on-off time ratio) by a base current supplied from the drive circuit 9. As a result, the current 1 c / d of FIG. 5a, that is, the collector current 1 c of the transistor 7 and the current 1 of the diode 8
d flows. On the other hand, when the transistor 7 is off, the capacitor 5 and the primary winding 10. Due to the resonance, a voltage in the 70 range as shown in FIG. 5 is generated between C and E of the transistor 7.

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.
On the other hand, 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 15' and capacitors 16 and 17 that constitute a filter.
．． 18, Chiroku carp l and 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 100 increases.
When operating at a frequency of about 100 KHz, the weight of the step-up transformer is lower than when boosting the voltage at the commercial power frequency.
It has the special advantage of being able to reduce the size from a few minutes to a few thousandths of an inch, and making it possible to reduce the cost of the power supply section.

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. Positive current 1b+ is collector current 1c of transistor 7
It is necessary that the current amplification factor (hfe, for example, 30) is larger than - for the maximum value of 1 cm. In addition, the negative current 1b- is reverse biased 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. This is the current flowing through knee and K. As is clear from FIG.
It was necessary to set the distance determined by A) to 1 bm+ (for example, 22A). Further, the negative current 1 bm- is also determined according to the maximum value 1 cm of the collector current 1c (for example, 1 cm).
5A), the larger the value of 1 am, the more power was required.

Further, 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, and this toff changes depending on variations in characteristics of the transistor 7, temperature, etc. 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. 4, for example. That is, a DC power source 22 obtained from a power transformer 21
．． 2a, oscillation circuit 24, transistor 25.26.27
, resistors 25-36, and a diode 37.

The oscillation circuit 24 turns on the transistor 2 during a conduction period of a predetermined period.
6 and 26 (5 are turned on and off alternately, supplying a base current as shown in FIG. 5C to the transistor 7.

However, does this transistor 25,26 have a fairly large current? It was necessary to be able to handle the 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 transistor 7 with a base current Ibm+ corresponding to the peak width of 1cm of the collector current 1c as shown in FIG. 5a and C, and the power required to supply this lbm+ is quite large. It became. For example, if 1cm=60A, the transistor 70
hfe'i30 is 1bm+-2-A, and the power consumption of the drive circuit 9 is extremely large, making it difficult to avoid increasing the size and price of the drive circuit 9 and the power transformer 21.

Furthermore, changes in the storage time of the transistor 7 (the main cause of toff in FIG. 5) due to temperature changes, etc.
In order to cope with a 1 cm change in the collector current caused by temperature changes or changes over time in the magnetron 16, 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.

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.

In other words, a unidirectional power source obtained from a commercial power source, etc., and a 111
(2) The inverter that receives the staining force from the unidirectional power supply 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 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 N fm semiconductor switch element is configured by a series connection of first and second transistors. , fj
The drive circuit is provided with a direct current source that energizes the iJ-th transistor, an E source, and an oscillation circuit that energizes the second 1-transistor, and 1) the 1J oscillation circuit is synchronized with the resonant circuit. t'11♂i which 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 connected to the secondary winding of a step-up transformer, the step-up transformer is driven by a resonant inverter, and the semiconductor switching elements of the resonant inverter are connected to the first and second windings. It consists of a series connection of second transistors, the first transistor is energized by a DC power supply, and the second transistor is energized by an oscillation circuit, so that the oscillation circuit operates in synchronization with the resonant circuit. , it is possible to drive the magnetron without using an expensive high-voltage diode 2, stabilize the switching operation of transistors that handle large amounts of power, and reduce the power consumption of the drive circuit and power transformer, making the configuration more innovative. This makes it possible to make drive circuits and power transformers, which previously had to be larger and more expensive, to be more compact and less expensive. In addition, it prevents fluctuations in the semiconductor switch element current caused by changes in characteristics, stabilizes the output of the magnetron, and eliminates wasteful power consumption of semiconductor switch elements. This has the effect of suppressing the noise 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. 4 are corresponding components, and the explanation thereof will be omitted.

In FIG. 1, 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 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 bending inductance. This leakage inductance and fill capacitor 16
．． 17 Because it acts as a kind of lo-panuffi/reta force, it is possible to obtain the desired radio wave output while keeping the magnetron's anode peak power supply small without using the conventionally used high-voltage diode, eliminating the need for a high-voltage diode. The magnetron can operate stably even when

That is, leakage inductance 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, resulting in a switch loss of the switch element 44 as a whole. It is possible to realize a stable and efficient magnetron drive inverter with a circuit configuration in which the voltage is kept low and high voltage diodes are omitted. In addition, since the first transistor 40 may have a low speed, the high hfe
) transistor can be used, and the second transistor 41 can be a low voltage field effect transistor, so the power consumption of the drive circuit 9 can be uniformly reduced compared to the conventional one. 21 may also be compact and low-priced, making it possible to make the entire drive circuit 9 compact and low-cost.Furthermore, the oscillation circuit 43 is constructed by connecting the voltage of the capacitor 4 and the first transistor as shown in the figure. This configuration detects the collector voltage of 40 and operates in synchronization with the resonance between the resonant capacitor 5 and the voltage transformer 6.

FIG. 2 is a more detailed circuit example of the drive circuit 9,
Components with the same reference numerals 7- as in FIG. 1 are equivalent components, and their explanations will be omitted.

In FIG. 2, the oscillation circuit 43 includes resistors 45-48,
A zero cross detection section consisting of a ratio unit 50, a delay means 50, a differentiator 51, resistors 52-54, and a capacitor 55.
, the longest node 1- consisting of ratio 11 tie j; 56 and differentiator 57
] timer, resistor 5r-ao, diode 61, capacitor 62, comparator 63, variable reference voltage source 6
4, and an RS/FF 65 whose R input is the sum output of the longest period timer and the zero-cross detection section.

66 is an ant gate, 67, 68 is an inverter gate, and 69 is a diode. Also, a capacitor 70 is connected to the DC power supply 42 that energizes the first transistor 40. A resistor 71 and a diode 72 are connected as shown in the figure.
The first transistor 40 can perform base-grounding operation well and efficiently. Therefore, the conventional to
Characteristic fluctuations due to ff can be prevented.

FIG. 3 is a waveform diagram illustrating the operation of the drive circuit of FIG.
c/d and the voltage Vcd between the collector of the first transistor 40 and the drain of the second field effect transistor 41. In addition, C to g in the figure are oscillation circuits 44.
These are the operation waveforms of each part.

Zero cross detection section output A is connected to capacitor 4 as shown in figure C.
A zero-cross pulse A is generated in the vicinity of the zero-cross with a delay of a certain time td from the cross point between the voltage (8) and V c d. 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 tIC, as shown by broken lines in FIGS. When the pulse A is input to the R input of the R-3/F F 65, the Q output becomes H as shown in e of the figure, and the second field effect transistor 41 and therefore the switch element 44 are turned on, and 1C flows. At the same time, the capacitor 62 of the on-time timer is charged as shown in FIG. do. 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 'ih field effect transinutor 41 can be directly driven by an OMO3-IC or the like, resulting in an extremely simple and low-power circuit, making it compact and inexpensive.

Furthermore, since the first transistor 40 only needs to have a low switching speed, a very high hfe transistor can be used, and the DC power supply 42 can also be made compact and inexpensive.

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

The structure is such that the magnetron is driven by energizing the voltage transformer using a semiconductor switch inverter that receives power from a unidirectional power source obtained from a DC power source or a commercial power source, and the capacitor and magnetron are connected in parallel to the secondary winding of the step-up transformer. In addition, the semiconductor switch element is configured with a series connection of a first and a second transistor, 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 be operated stably without an expensive high-frequency high-pressure diode.

(2) By eliminating the high-frequency, high-voltage diode, the load on the semiconductor switch elements can be reduced, and a highly efficient and reliable inverter circuit can be realized.

(3) The power consumption of the drive circuit of the semiconductor switch element can be significantly reduced, 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 downsized and reduced in size.

(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, resulting in high reliability. A high frequency heating device can be provided.

[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 wave heating device of the drive circuit of the same device, and Fig. 5 a, b, and c are a circuit diagram of the same device. FIG. 3 is a diagram of operating current waveforms of each part. 1...Commercial power supply, 2.3.4...Unidirectional power supply 2,...Diode bridge, 3...
...Inductor, 4...Capacitor), 5.
...Resonance capacitor, 6...Step-up transformer, 9...Drive circuit, 10...-Secondary winding, 11...Secondary winding Line, 15... Magnetron, 4o... First transistor, 41
...Second transistor, 42...DC power supply, 4G...Oscillation circuit, 44...
Semiconductor switch element. Agent's name: Toshio Nakao and one other person Figure 3 Figure 4

Claims (5)

[Claims] (1) A unidirectional power source obtained from a commercial power source, an inverter that receives power from the unidirectional power source and has a resonant capacitor and a semiconductor switch element, and a booster that forms a resonant circuit with the resonant capacitor and boosts the output of the inverter. The step-up transformer includes a transformer and a drive circuit that drives the semiconductor switch element, and a capacitor and a magnetron are connected in parallel to the secondary winding of the step-up transformer to energize the magnetron. and a second transistor connected in series, the drive circuit is provided with a DC power source that energizes the first transistor and an oscillation circuit that energizes the second transistor, and the oscillation circuit is A high-frequency heating device configured to oscillate in synchronization with the resonant 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) The first transistor and the second transistor are 1
The high-frequency heating device according to claim 1 or 2, which is constituted by a chip or two chips and is integrated by being housed in a single package. (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 is also used as a capacitor for preventing unnecessary radiation of the magnetron.