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
www.st.com
AN3168
Contents
Contents
1
2
Triggering quadrants and key parameters . . . . . . . . . . . . . . . . . . . . . . . 3
Triggering circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
2.2
2.3
2.4
2.5
2.6
Two kind of power supply bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Gate resistor value definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
SCR and Triac triggering circuit with a positive power supply . . . . . . . . . . 7
Triac and ACS / ACST triggering circuit with negative power supply . . . . . 8
SCR triggering circuit with negative power supply . . . . . . . . . . . . . . . . . . . 9
Diac control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3
Examples of negative power supplies . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
3.2
3.3
3.4
3.5
3.6
Linear power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Capacitive power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Resistive power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Buck-boost power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Flyback power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Comparison of negative power supplies . . . . . . . . . . . . . . . . . . . . . . . . . 18
4
5
6
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Doc ID 17193 Rev 1
2/20
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
A1 (or COM)
A1 (or COM)
G
I-
V
T
N2
N2
G
N
P1
N1
I+
I
T
A2 (or OUT)
N+
P
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
GATE
N1
OUT
N2
OUT
P
N
V
T
G
I
T
COM
P1
P
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.
Doc ID 17193 Rev 1
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AN3168
Figure 3.
Triggering quadrants and key parameters
Triggering quadrants according to gate current and voltage polarities
VT
+-
++
A
2
(or OUT)
Q2
Q1
IG
Q3
Q4
G
V
T
I
G
A
1
(or COM)
--
-+
The usable triggering quadrants depend on the family and the class of the device used.
Table 1
shows the triggering quadrants available for ST Microelectronics devices.
Table 1.
Family
Standard
Triac
Snubberless and logic level
Snubberless high temperature
ACS
ACS / ACST
ACST
Yes
Yes
Yes
No
Available triggering quadrants according to device family and class
Triggering quadrants
Class
Q1
Yes
Yes
Yes
No
Q2
Yes
Yes
Yes
Yes
Q3
Yes
Yes
Yes
Yes
Q4
Yes
No
No
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
GT
, the triggering gate voltage V
GT
and the
latching current I
L
.
●
I
GT
is the minimum gate current to turn on the device. This current has to be applied
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
GT
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
GT
is recommended.
V
GT
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
GT
current is applied.
I
L
is the latching current. The latching current is the minimum value that the load current
must reach before gate current removal to avoid device switch-off (see Reference
1).
●
●
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AN3168
Triggering quadrants and key parameters
These parameters are specified at 25 °C and vary with the junction temperature as shown in
Figure 4.
The I
GT
, V
GT
and I
L
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
2.5
I
GT
, V
GT
, I
L
[T
j
] / I
GT
, V
GT
, I
L
[T
j
= 25 °C]
2.0
I
L
1.5
I
GT
1.0
V
GT
0.5
0.0
-40
T
C
(°C)
-20
0
20
40
60
80
100
120
140
160
Doc ID 17193 Rev 1
5/20
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