JPH046789A - Inverter power source for induction heating - Google Patents

Abstract

PURPOSE:To perform a high-speed response with a low loss by operating an inverter at the frequency 20kHz or above, and perform a frequency shift at the frequency band higher than the tuned frequency. CONSTITUTION:An inverter 5 with the drive frequency 20kHz or above is used. A tuning circuit 23 primarily outputs the frequency signal (f) (serves as the frequency command of the inverter 5) so that the output phase of a voltage detector 12 has a phase lag 90 deg. against the output phase of a voltage detector 1 if V2=0, V3=V1, however in order to feed no recovery current to a diode D, the integrating circuit input is adjusted to set the value of the frequency signal (f) to a value slightly higher than the tuning frequency f0, and the voltage advance phase is controlled to attain the power factor 0.9 or above. If V0<VL, V2<0, and V3=V1, the output of an integrating circuit 43 is increased, the value of the signal (f) is increased, and the output IL of the inverter 5 is decreased. If VS>VL on the contrary, the output of an adding circuit 45 is blocked by a diode, thus the circuit 23 performs an action based on outputs of the voltage detectors 11, 12.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an inverter power supply suitable for use in high frequency induction heating.

[Conventional technology]

FIG. 5 shows a conventional inverter power supply for low frequency induction heating. In the figure, 1 is a three-phase AC power supply, 2 is a three-phase full-wave rectifier, 4 is a voltage smoothing capacitor, 5 is a single-phase inverter, 6 is a matching transformer, and 7 is a matching capacitor (
Capacity C), 8 is an induction heating load having a resistance bone (resistance value R) 9 and an inductance portion (inductance value L) 10,
11 is a voltage detector that detects the inverter output voltage Vo;
12 is a voltage detector that detects the voltage Vc of the matching capacitor, and 13 is a tuning control circuit. I4 is a driver, and inverter 5 is a full-bridge circuit of transistors formed by connecting flywheel diodes D in antiparallel. Each transistor Tr receives a drive signal from driver 14 as a base and repeats 0N10FF switching operation. Note that 15 is an inrush current limiting resistor, and 16 is a switch that is turned on after a predetermined time has elapsed after receiving power.

In this inverter power supply, the inverter output frequency f is set to L
The output of the inverter 5 is controlled by shifting the resonant frequency (tuning frequency) fo of the C series resonant circuit to the lower frequency side.

This will be explained below. For example, when heating iron, etc., the load Q=(Lω/R) may be about Q=5, and in this case, the inverter output tilL is reduced to 115.

Since the load impedance Z is, the inverter output current IL is set to 115 (11
If ω〉ωo=2πfO, Lω-=2 6R・ ・ ・ ・ ・ ・ (4)C
ω Here, if Q=(Lω/R)=5, it becomes 0.02. That is, in order to reduce the inverter output current (load current) IL to 115 at resonance, the inverter output frequency f must be made approximately seven times the tuning frequency fo. In addition, in the case of Q-10, ω=1.4ω0・・・・・・・・・・・・・・・(7)
Therefore, the inverter frequency f is changed from the tuning frequency fo by 40
Just shift it by %.

(2) In the case of ω〈ω0, Lω=26R・・・・・・(8) Cω, and in the case of QI5, ω=0.71 ω0 ・ ・ ・ ・ ・ ・ ・ ・
・ ・ ・ ・ ・ ・α0). Also Q=10
In the case of , ω= 0.82ω0・・・・・・・・・・・・＠.

In this way, by controlling the inverter 5 in the frequency range ω<ω0, the load current IL can be sufficiently controlled with a smaller frequency deviation than when controlling in the range ω>ω0. Conventionally, in the inverter power supply having the configuration shown in FIG. 5, as described above, it has been common to control the inverter frequency f in a frequency range lower than the tuning frequency fo to control its output.

However, in high-frequency induction heating using high frequencies, if the inverter output is controlled in the frequency range ω<ω0, the loss due to the recovery current of diode D becomes extremely large. are using.

In FIG. 6, 17 is a phase-controlled thyrisk type rectifier, 3 is a voltage smoothing lever, 18 is a voltage setting device (setting voltage Vs), 19 is a deviation amplifier, 20 is a phase shifter,
21 is a pulse transformer. 22 is a voltage detector that detects the voltage of the load 8.

In this configuration, the tuning control circuit 13 always ensures that the capacitor voltage Vc is 90% of the inverter output voltage VO.
'It functions to generate a frequency command that causes a delay,
Since the transistor Tr constituting the inverter 5 is always turned off when the current is zero, the switching loss is low, which is very advantageous in the frequency band of 10 KHz or higher where the switching loss becomes large. Inverter for high frequency induction heating! It has been used as #.

However, since the inverter power supply with this configuration uses a three-phase phase control method, its control response is limited by the input frequency (50Hz, 60Hz), and the overall circuit configuration is complicated because it requires tuning control and phase control. There was a problem that it became expensive.

The present invention has been made in order to solve the above problems, and aims to provide an inverter power supply for induction heating that is less expensive, has less loss, and can respond faster than conventional ones. .

[Means to solve the problem]

In order to achieve the above object, the present invention includes a rectifier that converts alternating current into direct current, an inverter that inputs the direct current voltage sent out by this rectifier through a smoothing circuit, and the output of this inverter is connected to a capacitor and an inductive load via a transformer. In an inverter power source for induction heating that supplies power to a series resonant circuit consisting of a series resonant circuit and controls the inverter output frequency by shifting the inverter output frequency from a tuned frequency, the tuned frequency is 20 KHz or more, and the frequency shift is The tuning control circuit is configured to perform the tuning in a frequency range higher than the tuning frequency, and in claim 2, the tuning control circuit includes a circuit that shapes the detection signal of the impatter output voltage into a pulse waveform, and a circuit that shapes the detection signal of the capacitor voltage into the pulse waveform. an exclusive OR circuit that receives the output of the circuit, an integration circuit that calculates the average value of the output of the exclusive OR circuit, and a voltage controlled oscillation circuit that sends out a frequency signal proportional to the output of the integration circuit, When the feedback value of the load voltage is larger than the inverter setting voltage, the above-mentioned tuning circuit is added with means for adding the deviation between the two voltages to the input of the above-mentioned integrating circuit in a direction that increases the output of the above-mentioned integrating circuit, The frequency signal is now used as the frequency command for the inverter.

[Effect]

In the present invention, since the inverter is operated at a frequency of 20KH2 or more, the Q of the load is high and output control is possible with a small frequency deviation.

Furthermore, since no phase control rectifier is used, high-speed response is possible.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, 22 is a voltage detector that detects the voltage across the load 8, and 23 is a tuning circuit. The deviation amplifier 19 uses the set voltage Vs sent out by the voltage setter 18 and the voltage detector 22.
The deviation (2) between the feed hack voltage (2) and the feed hack voltage (2) sent out by the two is detected and input to the tuning circuit 23. This tuning circuit 23
It has the circuit configuration shown in the figure.

In FIG. 2, reference numerals 31 and 32 denote waveform shaping circuits, which respectively shape the inverter output voltage Vo and the capacitor voltage Vc into pulse waveforms and input them to the exclusive OR circuit 33. 34 and 35 are switches, 36 to 42 are resistors, 43 is an integrating circuit, and 44 is a voltage-controlled oscillation circuit, and the voltage-controlled oscillation circuit 44 is used to control the H period and H period of the output C of the exclusive OR circuit 33.
A signal (pulse) f having a frequency proportional to the average value of the period is generated. 45 is an adder circuit which adds the output (2) of the deviation amplifier 19 and the voltage (bias voltage) Vl divided by the resistor 41.42. The adder circuit 45 is connected to the voltage dividing point a via a cutoff diode 46 and a resistor 38. V3 indicates the output of the adder circuit 45.

The present invention focuses on the fact that when the output frequency f of the inverter becomes 20 KHz or more, the above-mentioned Q becomes at least about Q-15, and the above-mentioned current control can be realized with a small frequency deviation of about 20%. The inverter 5 in this embodiment has a driving frequency of 20K.
Use an impeller of Hz or higher.

In this embodiment, the tuning circuit 23 has V2=0, V3
= V1, basically, the frequency signal f (which becomes the frequency command of the inverter 5) is such that the output phase of the voltage detector 12 is delayed by 90 degrees with respect to the output phase of the voltage detector 11. However, in order to prevent the recovery current of diode D from flowing, the input of the integrating circuit is adjusted so that the value of this frequency signal f is slightly higher than the tuning frequency fo, and the power factor is 0. It is controlled so that it is 9 or more.

In this state, VS<VL, V2<0, and V3<V
1, the output of the integrating circuit 43 increases, the value of the frequency signal f increases, and the inverter output current IL decreases. Conversely, when ■s>VL, the output of the adder circuit 45 is diode-blocked, so the tuning control circuit 23 operates based only on the outputs of the voltage detectors 11 and 12.

In this embodiment, the load current IL is decreased by increasing the inverter output frequency f, so that the peak current value of the switching current 0 of the transistor Tr decreases as shown in FIGS. 3 and 4. The off-state loss of the transistor Tr is almost the same level as in the case of the simultaneous tuning method shown in FIG. 6.

〔Effect of the invention〕

As explained above, the present invention provides an inverter with a frequency of 20KHz.
By configuring the inverter to operate at a higher frequency than the above, and the frequency deviation is performed in a frequency range higher than the tuning frequency, it is possible to control the inverter's output without significantly changing the output frequency, resulting in low loss. It is possible to obtain an inverter power supply capable of high-speed response at a lower cost than in the past.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a specific circuit diagram of the tuning circuit in the above embodiment, and FIGS. 3 and 4 are for explaining the switching-off loss of the above embodiment. The waveform diagram, FIGS. 5 and 6 are block diagrams showing a conventional inverter power supply for induction heating. 2. Rectifier, 4. Capacitor, 5. Inverter, 1
1, 12, 22...voltage detector, 18/voltage setting device,
23. Tuning control circuit, 31, 32. Waveform shaping circuit, 3
3... Exclusive OR circuit, 43. Integrating circuit, 44. Voltage controlled oscillation circuit, 45. Adding circuit.

Claims (2)

[Claims] (1) Equipped with a rectifier that converts alternating current to direct current, an inverter that inputs the direct current voltage sent by this rectifier through a smoothing circuit, and the output of this inverter is fed to a series resonant circuit consisting of a capacitor and an inductive load via a transformer. In an inverter power source for induction heating that controls inverter output by shifting the inverter output frequency from a tuning frequency, the tuning frequency is 20 KHz or more, and the frequency deviation is in a frequency range higher than the tuning frequency. An inverter power supply for induction heating. (2) An exclusive OR circuit that receives the outputs of the circuit that shapes the inverter output voltage detection signal into a pulse waveform and the circuit that shapes the capacitor voltage detection signal into a pulse waveform, and the output average of the exclusive OR circuit. It is equipped with an integrator circuit that calculates a value, and a voltage controlled oscillator circuit that sends out a frequency signal proportional to the output of the integrator circuit. 2. A tuning control circuit having a tuning control circuit added to the input of the integrating circuit in a direction in which the output of the integrating circuit increases, the frequency signal being used as a frequency command for the inverter.
Inverter power supply for induction heating described.