JPS5911259B2 - frequency converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency converter, and particularly to a frequency converter that serves as a high-frequency power source for an induction heating cooker.

Conventionally, in frequency converters that serve as high-frequency power sources for induction heating cookers, special frequency converter circuits that can operate even under light loads have been used.

However, for this purpose, small metal items such as spoons and forks are also heated, and if they are accidentally heated to high temperatures,
There was a desire for a frequency converter that would not operate with a small load because there is a risk of burns if touched by a person. Furthermore, as a conventional frequency converter, it has been proposed to control oscillation by comparing a signal of either an input current or a voltage or current of a component of the frequency converter with a constant reference value. However, when controlling the output by varying the oscillation frequency, if the output is made smaller, the input current or component voltage or current during normal load and light load become similar, making it difficult to distinguish between light loads. 15 disadvantages of being
It was hot.

The present invention provides a frequency conversion device that can reliably determine a light load even when the output is reduced.One embodiment of the present invention will be described below with reference to the accompanying drawings.

20 In FIG. 1, the frequency conversion device according to the present invention is
It directly converts low-frequency alternating current into high-frequency current, and an alternating current voltage is applied from an alternating current power supply 1 to a frequency conversion circuit 3 and its control circuit 4 via a power switch 2.

The frequency conversion circuit 3 connects the input capacitor 32 in series with the choke coil 2531. Connect the current limiting inductor 33a from one terminal U of the input capacitor 32 to the other terminal V.
Thus, the current limiting inductor 33b is connected to a series connection body of an antiparallel connection block of thyristors 34a and 34b and an antiparallel connection block of 30 thyristors 35a and 35b, which are power switching semiconductors. Also, input capacitor 3
A series connection body of resonance capacitors 36a and 36b is connected in parallel between U and V of 2. The connection point N of the anti-parallel connection block of the thyristors 34a and 34b and the anti-parallel connection block of the thyristors 3535a and 35b, and the resonance capacitor 36
An induction heating coil 37, which also serves as a resonance inductor, is connected between the connection point N' of a and 36b. The control circuit 4 controls the conduction of the thyristor, and is connected to the power transformer 41.
This serves as a power source for the oscillation control circuit 42, and its terminal 43 is connected between the gate and cathode of the thyristors 34a, 34b, 35a, and 35b.

Moreover, the control method is to control the conduction order and conduction interval of the thyristors so that the current of the current transformer 44 that detects the current of the heating coil 37 is set to a predetermined value. That is, when the terminal U of the input capacitor 32 is positive, the thyristors 34a, 34b, 35a, and 35b are made conductive in this order, and when the terminal U is of reverse polarity, the thyristors 35b, 35a, and 34 are made conductive.
34b and 34a are made conductive in this order. Also, as shown in Figure 2,
Control is performed such that the time TD from when the current 1 of the thyristor 34a becomes zero to when the thyristor 35a is next turned on is kept constant. In other words, this is called commutation margin time constant control.
The ΔTD period shown in the figure is between thyristor 34b and thyristor 3.
5a is conductive, and is called the commutation overlap period.
FIG. 3 shows the current waveform of the heating coil 37, where C shows the waveform when loaded and D shows the waveform when no load is applied. If the capacitance of the resonance capacitor 36a or the resonance capacitor 36b is C1, the equivalent inductance of the heating coil 37 is L1, and the equivalent resistance is R, then 1/4 period of the resonance frequency, that is, the commutation margin time TD=7! −VτWd. At this time, assuming that the damping factor is α=R/2V “7F”, α=0.1 is set as the third
In Figure C, α=0.005 is shown in D. As is clear from the figure, when the commutation margin times are equal, the output current increases when there is no load or a light load. Also, TD=5J〒Bottom～
The output current changes within a narrow variable range, allowing for a sufficient power control range for cooking.

FIG. 4 shows an embodiment of the control circuit according to the present invention, and FIG. 5 shows the relationship between heating coil current and commutation margin time.

The terminals 401a and 401b are for detecting the heating coil current, one of which detects the zero point of the current, and the other of which detects the output current level. After detecting the zero point of the heating coil current, that is, the thyristor current, and commutation margin time TD, a gate trigger signal is obtained from a conduction interval control circuit 402 that obtains a thyristor trigger signal and a gate signal reversal circuit 403 that also serves as an inhibition circuit. A value corresponding to the output current level from the terminal 401b is compared with a set level S of the comparison amplifier circuit 404, and the commutation margin time of the gate control circuit 402 is controlled by the control terminal C. Resistors 405 and 405' perform power control by changing the set level S. A signal corresponding to the output level of the heating coil current is connected to a voltage comparison circuit 407 via a resistor 406.
The output signal of the comparator circuit 404 is input to the other input terminal of the comparator circuit 404 . In FIG. 5, the value of the heating coil current when the commutation margin time TD is changed is shown as Fu when there is no load and when NOl is loaded. Even with the same commutation margin time, the heating coil current is different,
When the heating coil current is larger than TD, oscillation is temporarily stopped. That is, one of the inputs of the voltage comparison circuit 407 is a signal corresponding to the heating coil current, and the other is a signal corresponding to the commutation margin time TD. When the control level, that is, the voltage at the C terminal becomes high, the commutation margin time TD becomes long, and when the output current level becomes high with respect to the control level C, the output level e of the voltage comparison circuit 407 becomes high and becomes a prohibition signal. The gate trigger signal will no longer be output. The signal reversal circuit 403 is inverted by the polarity signal P,
The output becomes zero due to the prohibition signal e. This is a so-called multiplexer circuit. To explain FIG. 5 in more detail, the horizontal axis is the commutation margin time TD, but if it is assumed that the output voltage of the comparison amplifier circuit 404 and the commutation margin time TD are in a proportional relationship, the horizontal axis is the commutation margin time TD. It has the same meaning as the output voltage of the amplifier circuit 404.

In this example, when the output setting is increased, the load curve shifts from the lower left to the upper right, as shown in Figure 5. In other words, it is not possible to determine whether the load is light or the output is small by comparing the heating coil current with a certain predetermined value, but in this embodiment, the current level of the heating coil 37 input to the voltage comparison circuit 407 and this current level are By using the output level of the comparison amplifier circuit 404, which is controlled to be constant, as a variable, it is possible to reliably determine when the load is light. That is, a discrimination level exists between the NO characteristic and the Fu characteristic in FIG. As described above, the present invention controls the output heating coil current level to a predetermined level and stops oscillation when control is impossible or when the output current is large compared to the commutation margin time.

At no load or light load, the output current becomes larger than the commutation margin time control signal or conduction interval control signal level and stops oscillation, so even small loads stop oscillation and do not heat up, so there is no chance of accidentally overheating. It does not generate heat even when placed close to the coil. Furthermore, it does not oscillate in a no-load state, which saves power. In addition, the control width of the commutation margin time that is actually required during load is TD-1J i to -J〒h, and if the design takes the no-load current into consideration, the TD variable range will become wider, but No worries.

The frequency conversion circuit according to the present invention is not limited to a half-bridge type, but may be an input capacitor separated type or a T-bridge type.

Further, it is also possible to control the output current without performing feedback control to set the output current to a predetermined level.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing an embodiment of the frequency conversion device according to the present invention, Figs. 2 and 3 are diagrams showing waveforms of various parts of the frequency conversion circuit, and Fig. 4 is a circuit diagram showing an embodiment of the frequency conversion device according to the present invention. FIG. 5 is a block diagram showing an embodiment of the control circuit according to the present invention, and FIG. 5 is a diagram showing the characteristics of the frequency conversion circuit according to the present invention. 1... AC power supply, 3... Frequency conversion circuit, 4... Control circuit, 34a, 34b, 35a
, 35b... Thyristor, 37... Induction heating coil, 402... Gate control circuit, 4
03...Gate signal reversal circuit.

Claims (1)

[Claims] 1 Consists of a frequency conversion circuit that converts low-frequency alternating current into high-frequency alternating current and its control circuit, the frequency conversion circuit consisting of at least one power switching semiconductor, one resonant inductor, and one resonant capacitor, and the control circuit a conduction interval control circuit that controls the conduction interval of the power switching semiconductor; a comparison circuit that compares the voltage or current of the frequency conversion circuit with the output of the conduction interval control circuit; A frequency conversion device equipped with an oscillation stop circuit that stops oscillation of the conversion circuit.