JP2913683B2 - Frequency tuning circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency tuning circuit used in a power converter for supplying alternating-current power to a resonance circuit.

[Conventional technology]

FIG. 3 shows a conventional frequency tuning circuit of this type of capacitor detection system. In the figure, reference numeral 1 denotes a three-phase commercial power supply, and reference numeral 2 denotes a single-phase inverter, which converts a commercial alternating current into a direct current by a forward converter 2A and then converts the commercial alternating current into an alternating current of a required frequency by an inverter 2B. Reference numeral 3 denotes a reactor, 4 denotes a resonance capacitor, and 5 denotes a heating coil (L is an inductance, and R is a resistance) of an induction heating device. The forward converter 2A of the inverter 2 is a converter composed of a thyristor Thy whose phase is controlled, and its output voltage (average value) is variable and supplied to the inverter 2 through a smoothing circuit composed of a reactor 6 and a capacitor 7. You. Inverter section 2B
Has a bridge connection of four transistors Tr, and a flywheel diode D is connected in anti-parallel to each transistor Tr.

8A is a gate control circuit that drives the forward conversion unit 2A, and 8B is a drive circuit that turns on / off the transistor Tr forming the inverter unit 2B. Reference numeral 9 denotes a voltage / frequency converter (V / F converter) which sends a frequency command F ^* to the drive circuit 8B. Reference numeral 10 denotes a voltage detector for detecting the output voltage Vo of the inverter 2, and the detected voltage is shaped by a comparator 10A. Reference numeral 11 denotes a voltage detector for detecting the voltage Vc across the capacitor 4. The detected voltage is shaped into a rectangular wave by a comparator 11A. Reference numeral 12 denotes an exclusive OR circuit (hereinafter, referred to as an exclusive OR), which outputs the output voltage Vo 'of the comparator 10A and the comparator 11A.
Output voltage Vc 'is input. Reference numeral 13 denotes an integrator. The output Vφ of the exclusive OR 12 (for convenience of explanation, the pulse height is also Vφ
To) and integrates the difference voltage V.phi 'of the bias voltage V _B, and sends the integrated value V _IN to V / F converter 9. 14 is a bias circuit, and sends the bias voltage V _B. The bias voltage V _B is set to be V _B = V.phi / 2.

When the load of the power conversion device is an inductance load such as an induction heating device or an induction melting furnace, it is common to insert a resonance capacitor 4 and operate at a frequency (tuning frequency) fo very close to the resonance frequency. The frequency tuning is performed so that the output frequency f of the inverter 2 becomes the tuning frequency fo.

In the configuration of FIG. 3, when the inverter output frequency f is the tuning frequency fo (FIG. 4 (a)), the voltage Vo 'and the voltage Vc' have a phase difference of 90 °, so that the exclusive OR12 Is a signal having a waveform in which the positive period is equal to the negative period of the level L. As long as the output of the integrator 13 at this time (when one cycle average value is V _IN ') is V _IN ' (= 0), the value of the V / F converter 9 is the tuning frequency fo. The frequency command F ^* = fo is sent to the drive circuit 8B. If it goes out of sync,
For example, FIG. 4 (b) (an example when the tuning frequency is high),
As shown in FIG. 4 (c) (example when the tuning frequency is low), the period of the level H of the output Vφ of the exclusive OR 12 and the level L
Is no longer equal to the period of
V _IN ′, and the frequency command F ^* changes in accordance with the fluctuation, and the inverter frequency f is raised or lowered toward the tuning frequency.

[Problems to be solved by the invention]

In this manner, the frequency tuning is performed. However, the maximum level of the voltage Vc of the capacitor 4 (the output of the voltage detector 11) greatly changes between the time of tuning and the time of non-tuning.
Almost Q times the output voltage of the rectifier 2A, where Q = 2
πfoL / R), the maximum output of the forward converter 2A and Vc at the time of tuning
Is the maximum allowable input value of the comparator 11A, when tuning is not performed and the output of the forward conversion unit 2A is small, the characteristics of the comparator 11A using zero potential as a threshold Container 11A
In this case, it is difficult to accurately detect the phase of the voltage Vc by erroneously interpreting the input as a zero level.

The present invention has been made to solve the above problems,
It is an object of the present invention to provide a frequency tuning circuit capable of reliably and accurately detecting the phase of a voltage even when the voltage of a resonance capacitor is at a low level.

[Means for solving the problem]

In order to achieve the above object, according to the present invention, a second comparator for detecting a phase of a capacitor voltage is supplied with a detected voltage of a capacitor voltage through a clamp circuit, and the clamp circuit includes a parallel connection of a forward series diode and a reverse series diode. This is a circuit configuration.

[Action]

According to the present invention, since the detection voltage of the capacitor is input to the comparator through the clamp circuit, a change in the input of the comparator can be limited to an appropriate level, and a malfunction at the time of low level input can be prevented. Further, since the clamp circuit is composed of a series diode in the forward and reverse directions, the clamp circuit follows the change of the detection voltage at a high speed, and there is no possibility of delaying the phase detection.

〔Example〕

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

In FIG. 1, reference numerals 21 and 22 denote clamp circuits,
It comprises a parallel circuit of a forward series diode 23 formed by connecting a plurality of diodes D in series and a reverse series diode 24 formed by connecting N diodes D in series. Other configurations are 3rd
Since they are the same as those in the figure, the same components are denoted by the same reference numerals.

In this configuration, the voltage passing through the clamp circuit 22
Vc has a maximum value and a minimum value as shown in FIG.
It is clamped to the clamp level corresponding to the voltage of the oscillating layer of the series diodes 23 and 24. As a result, the signal change width of the input signal of the comparator 11A becomes narrow, and this signal change width is selected when the number of diodes of the series diodes 23 and 24 is not tuned and when the output of the forward converter 2A is small. In addition, this can be set to a signal change width that is not mistaken for the 0 level. When the voltage Vo exceeds the clamp level, the clamp circuit 21 also restricts the voltage Vo to the clamp level and sends it to the comparator 10A.

As such a clamp circuit, a Zener diode is usually used. However, the Zener diode is unsuitable for a high-frequency resonance circuit because of its large capacity, which causes a delay. On the other hand, a series diode is particularly suitable for a high-frequency resonance circuit because it conducts and cuts off faster than a Zener diode.

〔The invention's effect〕

As described above, the present invention applies the detection voltage of the capacitor to the means for detecting the phase through the clamp circuit composed of the forward series diode and the reverse series diode. There is an advantage that voltage phase detection can be performed reliably without time delay, and it is particularly suitable for use in a high-frequency tuning circuit.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIGS. 3A and 3B are an input waveform diagram and an output waveform diagram of the clamp circuit in the above embodiment, FIG. 3 is a circuit diagram showing a conventional frequency tuning circuit, and FIGS. 4A to 4C are frequency tuning. 4 is a waveform time chart for explaining the operation of the circuit. 2 ... Inverter as power converter, 2A ... Forward converter,
2B …… Inverter part, 4 …… Resonant capacitor, 5 ……
Load, 8B ... Drive circuit, 9 ... V / F converter, 10, 11 ...
Voltage detector, 10A, 11A …… Comparator, 12 …… Exclusive OR, 13
…… Integrator, 14 …… Bias circuit.

Continuation of the front page (56) References JP-A-60-180478 (JP, A) JP-A-53-129817 (JP, A) JP-A-59-190097 (JP, U) (58) Fields investigated (Int) .Cl. ⁶ , DB name) H02M 7/42-7/98 H05B 6/02-6/12

Claims (1)

(57) [Claims] 1. A first and a second comparator for detecting a phase of an output voltage of a power converter for supplying AC power to a series resonance circuit including a load and a phase of a capacitor voltage of the series resonance circuit. A logic circuit for receiving an output and transmitting a pulse having a pulse width corresponding to the phase difference between the two voltages, an integrator for integrating the output of the logic circuit, and converting the output of the integrator into a frequency to convert the power into the power converter And a voltage / frequency converter for giving a command to the at least one second comparator, at least the second comparator receives a detected value of the voltage of the capacitor through a clamp circuit, and the clamp circuit includes a forward direction series diode and A frequency tuning circuit comprising a parallel circuit of reverse series diodes.