ST AN3168 APPLICATION NOTE

AN3168

Application note

Non-insulated SCR / Triac control circuits

Introduction

In alternating current applications the direct current power supply for low-voltage electronic devices (MCU, LEDs, optocouplers, Triacs and so on) can be provided using one of several different circuits. There are traditionally two major types of power supplies used in appliances, capacitive power supply and linear power supply using a step-down transformer. Today, designers are using more and more switches mode power supplies (SMPS) to achieve higher output current levels and especially lower standby power consumption. The power supply choice is a trade-off between several parameters. These are the cost, the required power, the output voltage level and polarity, the standby power consumption and the necessity or not of an electrical insulation between the mains and the low output DC voltage.

This application note considers only non-insulated power supplies. After a brief description of the triggering quadrants and key parameters for SCR, Triac, ACS and ACST, the usual control circuits are described according to the output voltage polarity of the power supply. Finally, some examples of negative power supply circuits are introduced.

March 2010 Doc ID 17193 Rev 1 1/20

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AN3168 Contents

1 Triggering quadrants and key parameters . . . . . . . . . . . . . . . . . . . . . . . 3

2 Triggering circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Two kind of power supply bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Gate resistor value definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 SCR and Triac triggering circuit with a positive power supply . . . . . . . . . . 7

2.4 Triac and ACS / ACST triggering circuit with negative power supply . . . . . 8

2.5 SCR triggering circuit with negative power supply . . . . . . . . . . . . . . . . . . . 9

2.6 Diac control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Examples of negative power supplies . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Linear power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Capacitive power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Resistive power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4 Buck-boost power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.5 Flyback power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.6 Comparison of negative power supplies . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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AN3168 Triggering quadrants and key parameters

1 Triggering quadrants and key parameters

To switch-on an SCR, Triac, ACS or ACST, a gate current must be applied on its gate pin (G). The gate current flows between Gate (G) and Cathode (K) for SCR, or between Gate and terminal A1 for Triac, or between Gate and terminal COM for ACS and ACST.

For Triac and ACST, the gate current could be positive or negative. Figure 1 illustrates the simplified schematic of a Triac or an ACST and the associated silicon structure. A Triac or an ACST could be switched on by a positive or a negative gate current through the diodes embedded back-to-back between G and A1. These 2 diodes are implemented at the P1-N1 and P1-N2 junctions.

Figure 1. Simplified schematic and silicon structure of Triac / ACST circuit

N2 N1

A1 (or COM)

A2 (or OUT)

N+

A2 (or OUT)

The silicon structure of an ACS is different from a Triac or an ACST (see Figure 2). Here the gate is the emitter of a NPN bipolar transistor. So there is only one PN junction implemented by P1 and N1. The gate current can then only be sunk from the gate, and not sourced to it.

Figure 2. Simplified schematic and silicon structure of an ACS

OUT

GATE

OUT

COM

N+

COM

N+

Four triggering quadrants can be defined according to the polarity of the gate current and the polarity of the voltage applied across the device, as shown on Figure 3.

For an SCR, only a positive gate current can switch-on the device. Thus, the triggering quadrants are not considered for SCR devices.

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AN3168 Triggering quadrants and key parameters

Figure 3. Triggering quadrants according to gate current and voltage polarities

+ -

- -

+ +

- +

A (or OUT)

A (or COM)

The usable triggering quadrants depend on the family and the class of the device used.

Ta bl e 1 shows the triggering quadrants available for ST Microelectronics devices.

Table 1. Available triggering quadrants according to device family and class

Triggering quadrants

Family Class

Q1 Q2 Q3 Q4

Standard Yes Yes Yes Yes

Tr i ac

ACS / ACST

Snubberless and logic level Yes Yes Yes No

Snubberless high temperature Yes Yes Yes No

ACS No Yes Yes No

ACST Yes Yes Yes No

As the triggering quadrants Q2 and Q3 are common to all Triacs and ACS / ACST devices, the control mode in Q2 and Q3 is recommended. In this way the replacement of one device by another one (for example, if an ACST is used in place of a standard Triac) is possible. Triggering in Q4 is not advised because the triggering gate current is the highest. Also the dI/dt capability of Triacs is lower in Q4 compared to the other quadrants. Working in Q2 / Q3 quadrants is then advised, even for standard Triacs, to decrease the board consumption and increase the board reliability.

To design the control circuit and the power supply, the device triggering parameters must be considered, i.e. the triggering gate current I latching current I

● I

is the minimum gate current to turn on the device. This current has to be applied

, the triggering gate voltage VGT and the

between gate and cathode for an SCR, gate and A1 for a Triac or gate and COM for an ACS / ACST. The applied gate current must be higher than the I

specified at the

lowest expected operating temperature. As a high gate current value provides an efficient triggering, a gate current of twice the specified I

● V

is the voltage measured between gate and cathode for an SCR, gate and A1 for a

Triac or gate and COM for an ACS / ACST when the I

● I

is the latching current. The latching current is the minimum value that the load current

is recommended.

current is applied.

must reach before gate current removal to avoid device switch-off (see Reference 1).

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AN3168 Triggering quadrants and key parameters

0.0

0.5

1.0

1.5

2.0

2.5

-40 -20 0 20 40 60 80 100 120 140 160

These parameters are specified at 25 °C and vary with the junction temperature as shown in

Figure 4. The I

, VGT and IL variations are the same for most devices, except for sensitive

and low current SCRs (P0102BL, P01, X06, X02, X04 and TS420 series) and for ACS / ACST devices.

Figure 4. Typical variations of the triggering gate current, the triggering gate

voltage and the latching current versus the junction temperature

I , V , I [T ] / I , V [T = 25 °C]

GT GT L j GT GT j,IL

T (°C)

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AN3168 Triggering circuits

2 Triggering circuits

2.1 Two kind of power supply bias

To trigger a Triac, ACST, ACS or SCR, a gate current has to be applied on the gate pin and circulates between gate and cathode (K) for SCR, or between gate and terminal A1 for Triac, or between gate and terminal COM for ACST and ACS.

For non-insulated control circuits, this means that the reference of the control circuit has to be related to K, A1 or COM. Then there are two ways to connect this drive reference.

● Solution 1: connect the control circuit ground (V

● Solution 2: connect the control circuit voltage supply (V

Solution 1 is called a positive power supply. The voltage supply V drive reference (V

) which is connected to the mains terminal (line or neutral) as shown in

Figure 5. If the supply is a 5 V power supply, then V

Figure 5. SCR / Triac control with positive power supply

) to K or A1

) to A1 or COM

is indeed above the

is 5 V above the mains reference.

CONTROL

CIRCUIT

Vac

LOAD

Solution 2 is called a negative power supply. The voltage supply reference (V below A1 or COM, which is connected to the mains reference (line or neutral) as shown in

Figure 6. If the supply is a 5 V power supply, then V

This topology can be used with all Triacs, ACS and ACST, but not with SCR.

Figure 6. Triac and ACS / ACST control with negative power supply

Vac

LOAD

2.2 Gate resistor value definition

The minimum gate current (IGT) required to trigger a Triac, SCR or ACS / ACST increases as the junction temperature (T equals the minimum ambient temperature. For appliance systems, the minimum ambient temperature is generally 0 °C.

) decreases (see Figure 4). The worst case appears when Tj

Vac

A1 or COM

LOAD

is 5 V below the line reference.

CONTROL

CIRCUIT

CONTROL

CIRCUIT

) is indeed

For example, the ACS108-6Tx I When T

equals 0 °C, IGT becomes 15 mA.

level is given as lower than 10 mA with Tj equals 25 °C.

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