Simple two-way thyristor positive power drive solution

The supply voltage is considered a positive or negative voltage in some cases. For those who don't often deal with two-way thyristor switches, "negative power" sounds weird, after all, integrated circuits never use negative voltage.

In some cases, the triac drive circuit prefers a negative voltage. This article describes several simple two-way thyristor positive power drive solutions.

If the power semiconductor control circuit requires a power supply and the drive reference terminal is connected to the mains (phase or neutral terminal), a non-isolated power supply is required.

This is the case with the trigger circuit of an AC switch such as a triac, an ACST, an ACS, or an SCR (Silicon Controlled Rectifier). These switching devices are all controlled by the gate current. The gate current must be applied to the gate pin and through the gate and reference terminals. The reference terminals include the cathode (K) of the SCR, the A1 terminal of the triac or the COM terminal of the ACST and ACS switches.

Because the AC switch control circuit and its power supply must be referenced to the reference terminal (connected to the phase line voltage), a non-isolated power supply is required.

There are two options for connecting the drive reference terminal of the switch to a non-isolated power supply:

• Option 1: Connect the control circuit ground terminal (VSS) to the drive reference terminal.

• Option 2: Connect the control circuit supply voltage terminal (VDD) to the drive reference terminal.

Figure 1: Power Polarity Definition

Option 1 is the most common solution. The drive reference terminal of the switch is zero voltage point (VSS), as shown in Figure 1a. The power supply voltage (VDD) is higher than the potential of the mains terminal (phase or neutral), and the mains terminal is connected to the drive reference terminal (VSS), so this topology is also called the positive power drive circuit. If the supply voltage is 5V, then VDD is 5V above the mains reference voltage (eg, the neutral terminal in Figure 1a).

This topology is only applicable to standard triacs or SCRs and cannot be used with non-standard triacs, ACS and ACS, as explained below. But with a few simple modifications, you can control all of these switches with a positive power supply, which is explained at the end of this article.

Option 2 is a negative power supply, as shown in Figure 1b. The power reference terminal voltage (VSS) is lower than the voltage of the A1 or COM terminal connected to the mains reference terminal. If the supply voltage is 5V, VSS is 5V below the mains reference voltage, that is, the phase line voltage is referenced to -5V.

This topology can be used for all triacs, ACS and ACST, but not for thyristors, as explained below.

Closing an AC switch, like other bipolar devices, must apply a gate current between the gate (G) of the switch and the drive reference terminal (see STMicroelectronics AN3168 Application Note).

This will happen in several situations.

• If it is an SCR, the gate current must be a positive current (flow from G to K).

• In the case of triacs and ACST, the gate current is positive and negative (depending on the voltage applied to the switch).

• If it is an ACS, the gate current must be a negative current (flow from COM to G).

It is easy to drive the SCR with a positive current. If the cathode of the SCR is connected to the VSS terminal, as shown in Figure 1a, when the output pin of the control circuit (usually the microcontroller) is asserted high, the control circuit outputs current to the SCR gate.

On the other hand, a direct power supply is required to drive the ACS switch directly, as shown in Figure 1b. When the control circuit output pin is asserted low, the control circuit sinks current from the SCR gate.

Depending on the polarity of the gate current and the polarity of the voltage applied before the switch is turned on, we can divide the trigger conditions of the triac, ACS, and ACST into four quadrants. When the current is flowing to the gate, the gate current is a positive current. Taking the driving reference terminal as a reference point, the voltage of the pulling current is a positive voltage. The four quadrants are

• Quadrant 1: Positive gate current and positive gate voltage

• Quadrant 2: Negative gate current and positive gate voltage

• Quadrant 3: Negative gate current and negative gate voltage

• Quadrant 4: Positive gate current and negative gate voltage

Two-way silicon, ACS, and ACST can be activated in each quadrant or only in some quadrants, depending on the semiconductor technology used in the switch.

Because the SCR switch has only a positive gate current to close, a positive voltage is applied to the cathode and anode terminals to turn it on, so the trigger quadrant condition is usually not considered when using SCR.

The table below lists the trigger quadrants for the different switches and the compatibility of the different switches with the power supply polarity of the direct drive circuit of Figure 1. It is not difficult to see that the negative power supply is compatible with all AC switching technologies except SCR. Negative power drive circuit replacement components are more flexible and not subject to technical limitations, so negative output is preferred.

Table 1. Trigger quadrants of switches and switches compatible with direct drive positive and negative supplies

Problems can arise if a positive power control microcontroller is used to trigger a three-quadrant triac, ACST, or ACS. As shown in Table 1, direct control cannot be achieved in this case.

In addition, switching power supplies (SMPS) are often used to meet standby power requirements in accordance with energy efficiency standards. The choice of positive output switching power supply depends primarily on the choice of buck converter, which is the most common topology for low output current offline converters.

In many cases, only the AC switch needs to be controlled, so a negative power supply can be considered. The buck boost converter supports a negative voltage output, and the topology is as easy to implement as a buck converter. In addition, buck boost converters save output load resistance or output Zener diodes compared to buck converters. During the turn-on of each MOSFET, the output capacitance of the buck converter is charged, causing excessive output current when no load or load is small.

Compared to buck converters, buck boost converters have lower energy efficiency (and maximum output current) and larger output capacitance. Inside the buck converter, the entire current of the inductor is used to charge the output capacitor, while inside the buck boost converter, the inductor current charges the output capacitor only when the freewheeling diode is turned on. However, the duty cycle of the 230 V AC / 12 V DC converter is very low, so there is little difference in performance between the buck boost converter and the buck converter. Under the same reactance device, the energy efficiency of the two topologies is basically the same.

However, even if the switching power supply has a negative output, it is best to select the switching power supply for the positive output. Positive output reduces standby power consumption. The positive-voltage linear regulator consumes less than 50 μA internal power, while the negative-voltage regulator consumes approximately 2 mA, which has a large impact on switching power supply standby power.

Another reason for choosing a positive voltage output is that current 3.3 V microcontrollers are widely used and it is difficult to find a 3.3 V negative voltage regulator with very low power consumption.

For these reasons, the circuit schematic of Figure 2 combines the dual advantages of a negative supply and a positive regulator. In this diagram, the ST715M33R is a positive regulator with a maximum quiescent current of 5.5 μA. It is connected to a “negative” 15V output to provide a 3.3V supply voltage to the microcontroller. The -15V voltage is based on the VIPer06 buck boost conversion. The output of the converter or flyback converter (see ST's AN4564 application note). The T1635T-8 is a T-Series three-quadrant triac that accepts current from the T1635T-8.

Figure 2: Negative power supply with positive regulator in the triac control circuit

By modifying the gate circuit, a three-quadrant triac can be driven with a positive power supply.

In addition to selecting the power topology, there are other reasons to use a positive power supply.

For example, the sensor uses the mains as a reference voltage to monitor certain electrical parameters. For example, inside a general-purpose motor controller, a shunt is usually connected in series with the AC switch to detect the load current and achieve closed-loop control of the speed or torque. In electricity meter applications, the electrical energy input to the grid is calculated and the mains parameters must be measured.

In the past, the reason why the drive circuit used a positive power supply was that the measured voltage increased as the shunt or phase line voltage increased, so the design seemed logically more reasonable.

These application schematics can also be switched to a negative supply. If you consider the reverse polarity measurement method, the microcontroller firmware logic must also be modified (see application note AN4564 for details).

If it is determined to use a positive power supply, there is a solution to drive the three-quadrant triac, ACS or ACST, which is to connect a capacitor (C1) in series with the gate resistor (R1), as shown in Figure 3a, in order to The gate sinks current.

The schematic diagram of this circuit diagram is as follows:

• When the microcontroller I/O pin is asserted high (VDD), capacitor C1 is charged and the triac current is absorbed through resistor R1. Because the three-quadrant triac cannot be triggered in the fourth quadrant, if the voltage between the two terminals A2 and A1 is negative, the triac will not conduct (but if the voltage is positive, it can be turned on) , that is, the first quadrant trigger condition).

• When the C1 capacitor is fully charged (connecting the microcontroller power supply, here 5 V), the gate current disappears.

• When the microcontroller I/O pin is asserted low (VSS), capacitor C1 is discharged and a negative current is output through resistor R1 to the triac gate. The triac triggers in the second or third quadrant depending on whether the thyristor terminal is positive or negative. The negative current does not disappear until capacitor C1 is discharged.

Figure 3b is a derivative diagram of the schematic of Figure 3a for controlling the special case of the ACS switch (like the ACS 108 in this example). Because the ACS switch has a PN junction between the COM and G terminals, any pull current is prevented from flowing from G to COM. Diode D1 is used to charge capacitor C1 when the microcontroller I/O pin is asserted high.

Figure 3: Positive-powered three-quadrant triac or ACS driver circuit

In both diagrams, the driver circuit applies a different gate current as long as a short voltage pulse is applied to the microcontroller I/O pins. The advantage of this control method is that in the event that the microcontroller terminates operation due to reset or latch-up, the capacitor can block DC current and improve the safety level of the application.

In order to comply with various energy efficiency standards regarding standby power consumption, power supply solutions often use switching power supplies. Positive output power supplies are commonly used. However, negative power supply voltages are compatible with various AC switches, so in some cases, negative output is preferred.

The advantage of a positive voltage output is that it reduces standby power consumption. This article introduces two solutions, one is to modify the driver circuit, so that the positive regulator with the negative power supply, to achieve complementary advantages. Another solution is to add a capacitor to the gate circuit that can sink current from the triac gate even if a positive supply is selected.