How to use a photoelectric bidirectional thyristor to realize an ACS AC switch trigger circuit

There are many different technologies and designs used in solid-state switches sold on the market today. Standard bidirectional thyristors and snubberless bidirectional thyristors, as well as the ACS series introduced in the early 1990s, are the most familiar solid-state switch products. The conduction of these switches is triggered by the gate current, but depending on the technology or design used, the current can be the current poured from the gate or the current sourced to the gate. Therefore, the trigger circuit must consider the AC switch type and then trigger the AC switch correctly. For ACS switches, because of the silicon structure, the gate current can only be poured from the gate.

In some cases, the control circuit must also be isolated from the AC supply voltage, for example, when the microcontroller reference voltage is different from the AC switch reference voltage. When a new appliance uses an inverter to control a 3-phase motor, if the microcontroller is connected to the DC voltage rail and the ACS switches are referenced to the line voltage, the control circuit must be isolated from the AC supply voltage. If the designer wants to isolate all low-voltage circuits from the line, the control circuit must also be isolated from the AC supply voltage. This solution is usually expensive because it is simpler to use a well-insulated user interface and make all electronic circuits referenced to the line voltage, which is the case for appliance designs with only a few buttons on the control panel.

The standard solution for voltage isolation of the bidirectional thyristor trigger circuit is to connect a photo bidirectional thyristor in series with the A2 and G terminals of the bidirectional thyristor. Of course, a resistor is also required in series to reduce the gate current on the photo thyristor. This driving solution is suitable for all bidirectional thyristors. Therefore, when the voltage on the bidirectional thyristor is a positive voltage before turning on, the positive gate current triggers the thyristor to turn on. Conversely, when the voltage on the bidirectional thyristor is a negative voltage before turning on, the negative gate current triggers the thyristor to turn on. Therefore, the bidirectional thyristor is turned on in the Q1 and Q3 quadrants.

As mentioned above, the ACS switch can only be triggered by negative current. If a photothyristor is used to drive the ACS switch, the ACS can only be turned on at negative bias (because the gate current is negative at negative bias), which will cause the ACS switch to be turned on only for half a cycle.

This switching mode is not suitable for most applications, however, some new applications only need half-cycle conduction mode. For example, a coffee machine pump with a built-in diode and a washing machine door lock solenoid, these applications only need a half-cycle conduction operation.

If line voltage is applied to the gate and COM terminals, the internal PN junction of the ACS switch may be burned because the breakdown voltage of the switch is about 10V. This happens when the switch handles transient voltages or when the photothyristor is short-circuited. The solution is to connect a low-voltage or high-voltage diode in parallel to the COM-G junction or to connect a low-voltage or high-voltage diode in series with the photothyristor (Figure 1). Note that in the second case (a diode in series with the photothyristor), the photothyristor can be replaced by a reverse-isolated photothyristor with the anode connected to the ACS gate.

Figure 1 – Half-cycle ACS switch control solution using opto-triac

For home appliances, most loads use full-cycle control mode. To ensure that the ACS switch can be turned on every cycle, we must modify the previous circuit schematic. The solution is to add a low-voltage capacitor to apply a gate current when the positive current starts to conduct. As shown in Figure 2, this solution also uses two low-voltage diodes, and the working principle is shown in Figure 3.

1: The phototriac is turned on and the capacitor C is charged until VGT reaches (~ 0.7 V). Then, ACS is turned on in the 3rd quadrant and the IGT current is smaller than the gate conduction current in the 2nd quadrant.

2: ACS remains on until the next zero current crossing point. The G-COM voltage drops to -0.7 V and capacitor C is charged.

3: The ACS switch current increases, the VG-COM voltage rises, the capacitor C discharges through the G and COM poles, and a maximum peak current of 10 mA is applied to the gate, turning on the ACS switch.

If you want to apply a larger gate current, you must choose a capacitor around 330 µF.

It should be noted that the ACS switch will be turned off every cycle, and capacitor C will use this time to charge. When the voltage on the terminal exceeds about 10V, the ACS switch will be turned on. Because the line current is not cut off, this feature will not cause excessive EMI interference. Because of capacitor C, the waveform of the line current is still close to a sine wave.

These three circuit diagrams can also be modified. Adding a RC buffer circuit between the resistor R and the photoelectric bidirectional thyristor can improve the anti-interference ability of the final switch, expand the gate pulse width, and better trigger the AC switch.

Figure 2 – Full cycle ACS switch control solution using opto-triac

Figure 3 – Operating curve diagram of the circuit in Figure 2