JPH04105590A - Ac voltage controller - Google Patents

Abstract

PURPOSE:To absorb surge voltage sufficiently and to reduce loss in the circuit without increasing the capacity of a capacitor in a snubber circuit by connecting a main switching element in parallel with a switching snubber circuit comprising a series circuit of a capacitor and a switching element incorporating a diode. CONSTITUTION:Upon turn ON of a main switching element TR1 within positive half period of a power supply 10, load current flows through a diode D1, the main switching element TR1, and a diode D2 into a loas 12. Upon turn OFF of the main switching element TR1, a switching element TR2 is turned ON and a capacitor C3 in a switching snubber circuit 30 is charged by the flywheel current of the load 12 with a polarity shown on the drawing. When the main switching element TR1 is turned ON and the switching element TR2 is turned OFF, the capacitor C3 is discharged through the element TR1, a diode d2 built in the element TR2 and stray reactance thus charging the capacitor C3 with reverse polarity.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention provides positive and
The present invention relates to an AC voltage control device that performs switching in the negative direction.

(Background of the Invention) Single-phase sliding transformers (SLIDAC), saturable reactors, induction voltage regulators, etc. are known as devices that steplessly control AC voltage, but all of these are heavy, large, and costly. There is also the problem that it is expensive. A phase control method that uses a triac or the like to control the phase is also widely used because it is low-level and highly reliable, but this method has a large distortion of the current waveform and contains many harmonic components, and these harmonics are usually Since it occurs in the audible band, it has the disadvantage of causing vibration and noise problems with loads such as motors.

Therefore, a method has been proposed in which the voltage is controlled by switching in the positive and negative directions at a sufficiently higher frequency than the AC power supply and changing the on/off time ratio of this switching, that is, the duty ratio. Jitsukō 56
-49277, Special Publication No. 56-33724, etc.).

FIG. 2 shows an example of a circuit using this system, in which reference numeral 10 is an AC power supply and 12 is a load. For example, the power source 10 is 5o,
It is a 60Hz commercial power source. The load 12 is, for example, a single-phase induction motor. 14 is a main switching circuit connected in series with these, and includes four diodes D1 to D4 forming a diode bridge, and a high-speed switching transistor TR,n which is a main switching element connected between the positive and negative terminals of this bridge. has. 16 is a flywheel switching circuit connected in parallel to the load 12, which includes four diodes D forming a diode bridge.
, ~D8, and a high-speed switching transistor TRf connected between the positive and negative terminals of this bridge. Transistors TR and TRt are alternately turned on and off at a frequency sufficiently higher than the power supply 1o, for example, 20 KHz or higher.

Therefore, by changing the time ratio of these transistors TR, TR, it is possible to continuously change the load current.

However, the configuration shown in FIG. 2 requires a flywheel switching circuit 16 consisting of a diode bridge, which requires a large number of parts, leading to an increase in cost.

Therefore, instead of this switching circuit 16, it has been considered to connect a capacitor C1 in parallel to the load 12.

Figure 3 shows an example of this circuit, where this capacitor C1
Instead of or in addition to this, a known snubber circuit 20 is connected to the main switching transistor TRff. This snubber circuit 20 consists of a series circuit of a capacitor C2 and a resistor r, which is connected in parallel to transistors TR, .

According to this circuit, the capacitor C1 is charged to the polarity shown in the figure when the transistors TR, . When TR,n is off, the charge of this capacitor C1 is discharged through the load 12, and when the discharge is finished or charged to the opposite polarity, the transistor TR
m is turned on. Therefore, the capacitor C1 is charged again to the polarity shown, and the above operation is repeated.

However, in this system, when the transistor TR is turned on, a charging current not only flows through the load 12 but also through the capacitor C1. In particular, since the inrush current of the capacitor C1 becomes large, it becomes necessary to use a large capacity transistor TR, and a problem arises in that the loss becomes large.

Furthermore, when the snubber circuit 20 is provided instead of the capacitor C1, it becomes necessary to use a large capacitor C2 to absorb the surge voltage due to the inductance of the load 12. Since the electric charge of the capacitor C2 is absorbed as heat by the resistor r, increasing the capacitance of the capacitor C2 causes an increase in loss in the resistor r.

(Objective of the Invention) The present invention was made in view of the above circumstances, and it is an object of the present invention to reduce the surge voltage based on the flywheel current generated when the main switching circuit is turned off due to the inductance of the load.
If the snubber circuit capacitor absorbs the
Without increasing the capacitance of this snubber circuit,
It is an object of the present invention to provide an AC voltage control device that can sufficiently absorb surge voltage and reduce circuit loss.

(Structure of the Invention) According to the present invention, this object is to provide an AC voltage source having a main switching circuit that is connected in series between an AC power source and a load and performs switching in both positive and negative directions at a frequency higher than the frequency of the AC power source. In the control device, a switching snubber circuit consisting of a series circuit of a capacitor and a switching element with a built-in diode is connected in parallel to the main switching element of the main switching circuit, and this switching element is turned on and off alternately with the main switching element. This is achieved by an AC voltage control device characterized by:

(Example) FIG. 1 is a circuit diagram of an embodiment of the present invention.

In this embodiment, a main switching circuit 14B consisting of a diode bridge substantially similar to that shown in FIG.
A parallel circuit of a main switching element TR1 made of a MOS-FET and a switching snubber circuit 30 is connected between the positive and negative terminals of this bridge. The switching snubber circuit includes a capacitor Cs and a switching element TR2 made of a MOS-FET. Note that the main switching element TR and the switching element TR2 of the switching snubber circuit 30 are each a diode d +
It has a built-in d2.

40 is a gate control circuit, and the main switching circuit 14
A switching signal G for turning on and off B at a predetermined duty ratio is sent to the gate of the main switching element TR+. The polarity of this switching signal G is inverted in the inverter 42, and this inverted signal is guided to the gate of the switching element TR2. For this reason, the main switching element TR and the switching element TR.

is controlled to be turned on and off alternately.

Therefore, in this embodiment, within the positive half period of the power supply 10,
When the main switching element TR, is on, the load current flows through the diode D1, the main switching element TR, and the diode D.
2 to the load 12. Main switching element TR,
When the switching element TR2 is turned off, the switching element TR2 is turned on, and the flywheel current of the load 12 charges the capacitor C3 of the switching snubber circuit 30 to the polarity shown in the figure.

When the main switching element TR is turned on and the switching element TR2 is turned off, the charge in the capacitor C3 is discharged via the diode d2 built into the main switching element TR and the switching element TR2, and a stray reactance (not shown). and charges this capacitor C3 to the opposite polarity (opposite to the polarity shown). That is, in FIG. 1, capacitor C3 is charged so that its top pole becomes negative. This charged charge is prevented from discharging by the diode d2 and is held in this state.

Next, when the main switching element TR is turned off and the switching element TR2 is turned on, the flywheel current of the load 12 flows to the capacitor C3. Here, since the upper pole of this capacitor C3 is negatively charged, this negative pole is charged in the positive direction. Therefore, the capacitor C3 can absorb a larger amount of energy than when charging from zero potential. This means that a large surge voltage can be absorbed by the capacitor C3 having a small capacity. Further, the discharge of this capacitor C3 does not oscillate in a half wave, and this discharge path does not have a resistor r like the snubber circuit 20 of FIG. 3 described above. Therefore, there is no heat generation due to charging and discharging of the capacitor C3, and there is no loss.

The above operation is repeated within the positive cycle of the power supply 10. Power supply 1
In the negative cycle of 0, the load current only flows through the diodes D3 and D4, and the operation is the same, so the explanation thereof will not be repeated.

In the above embodiment, the surge voltage caused by the flywheel current flowing through the load 12 when the main switching circuit 14B is turned off is configured to be absorbed by the capacitor C3 of the switching snubber circuit 30. A capacitor C1 in parallel with the load 12 as shown in FIG. 3 may be used in combination with TR.

(Effects of the Invention) As described above, the present invention provides a switching snubber circuit for the main switching element, and the surge voltage when the main switching element is off can be absorbed by the capacitor of this snubber circuit. Since this capacitor is charged with the opposite polarity to the charging polarity when it is discharged, a large surge voltage can be absorbed with a small capacitance capacitor. Furthermore, this snubber circuit does not cause vibration during discharging of the capacitor and there is no resistance loss, thereby improving efficiency.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIGS. 2 and 3 are circuit diagrams of a conventional device. DESCRIPTION OF SYMBOLS 10... AC power supply, 12... Load, 14-14B... Main switching circuit, 30... Switching snubber circuit, C3... Capacitor, TR,... Main switching element, TR,... - Switching elements, D1 to D4...diodes, d,, d2... built-in diodes. Patent applicant: Shibaura Seisakusho Co., Ltd.

Claims (1)

[Scope of Claims] An AC voltage control device comprising a main switching circuit connected in series between an AC power source and a load and performing switching in both positive and negative directions at a frequency higher than the AC power frequency, the main switching circuit comprising: An alternating current voltage characterized in that a switching snubber circuit consisting of a series circuit of a capacitor and a switching element with a built-in diode is connected in parallel to the main switching element, and this switching element is turned on and off alternately with the main switching element. Control device.