AN4363
Application note
How to select the Triac, ACS, or ACST that fits your application
Introduction
This document gives basic guidelines to select the AC switch device according to the
targeted application requirements. These guidelines will allow the appropriate Triac, ACS or
ACST to be selected, for most of the applications. Some very specific cases could require a
higher level of expertise to ensure a reliable and efficient operation.
January 2014
DocID025296 Rev 1
1/20
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Contents
AN4363
Contents
1
Current rating selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
Load current and operating temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Turn-off limitation for specific applications . . . . . . . . . . . . . . . . . . . . . . . . . 5
Inrush current and stalled rotor operation . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
Voltage rating selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1
2.2
2.3
Peak line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Motor control applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Over-voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3
Gate current and triggering quadrants . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
3.2
Triac triggering quadrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Operating quadrants versus application . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4
Package selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1
4.2
Surface mount device or through-hole packages . . . . . . . . . . . . . . . . . . . 16
Pin-out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5
6
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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AN4363
Current rating selection
1
1.1
Current rating selection
Load current and operating temperature
The maximum current an AC switch (we call AC switch Triac, ACS or ACST in this
document) can handle is limited only by the maximum allowed operating temperature,
except for specific applications where the turn-off capability of the AC switch can also
become a limiting factor (refer to
Section 1.2: Turn-off limitation for specific applications).
But, for most applications, where the current waveform is sinusoidal, the current of the AC
switch will depend only on the operating temperature and the device power losses.
For an AC switch, as the switching times (around 100 ns) are very low compared to the
switching period (50 Hz or 60 Hz for most cases), the device power losses are considered
as only linked to the conduction losses.
Then, the junction temperature will be (refer to STMicroelectronics Application note AN533
for further information):
Equation 1
T
j
=
T
a
+
P
d
⋅
R
th( j
−
a )
or:
Equation 2
T
j
=
T
c
+
P
d
⋅
R
th( j
−
c )
With respectively:
T
j
: junction temperature
T
a
: ambient temperature
T
c
: case temperature
P
d
: conduction losses
For a sinusoidal current, P
d
is given by:
Equation 3
P
d
=
2
⋅
2
2
⋅
V
t 0
⋅
I
RMS
+
R
d
⋅
I
RMS
π
Where V
t0
and R
d
are respectively the threshold voltage and the dynamic resistance of the
AC switch ON-state voltage drop.
For each device, the datasheet gives the maximum allowed current calculated so that the
associated power losses (with
Equation 2)
will keep the junction temperature below the
maximum operating temperature (with
Equation 1).
This maximum temperature is usually
125 °C or 150 °C for high temperature devices.
Figure 1
and
Figure 2
give the example of such datasheet curves for respectively an
ACS108-6SA and a T1635T-8FP. As the ACS108-6SA package is a TO92, there is no way
to add a heatsink to the device to evacuate its power losses. That is the reason why the
curve is given versus the operating ambient temperature.
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Current rating selection
AN4363
For example, for a 0.4 A rms current, this device has to work with a maximum ambient
temperature lower than approximately 72 °C. In other words, up to 72°C, the ACS108-6SA
can control a maximum rms current of 0.4 A.
Figure 1. Maximum rms current versus ambient temperature for an ACS108-6SA
I
T(RMS)
(A)
0.9
0.8
SOT-223
TO -92
TO-92
0.7
0.6
0.5
RMS load
0.4
current
0.3
0.2
0.1
0.0
0
25
50
75
100
125
Single layer Printed
circuit board FR4
Natural convection
Ta °C
Tamb
For the T1635T-8FP, as the device package is a TO220FPAB, and as this device can handle
up to 16 A for a 150 °C junction temperature, a heatsink is used in most of the cases. The
curve is then given versus the operating case temperature as shown in
Figure 2.
This figure shows that for instance, for a 12 A rms current, the package case temperature
has to be lower than 107 °C. So the used heatsink has to ensure of evacuating the power
conduction losses (P
d
= 13 W for 12 A rms current, as shown in
Figure 3)
while keeping the
case temperature below 107 °C.
Figure 2. Maximum rms current versus ambient temperature for a T1635T-8FP
I
T(RMS)
(A)
TO -220FPAB
18
16
14
RMS load
12
current
10
8
6
4
2
0
0
25
50
75
T
C
(°C)
100
Tc
125
150
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Current rating selection
Figure 3. Maximum power dissipation versus rms on-state current for a T1635T-8FP
20
18
16
P(W)
P
d
14
12
10
8
6
4
2
0
0
2
4
6
8
10
180°
I
T(RMS)
(A)
12
14
16
RMS load
current
1.2
Turn-off limitation for specific applications
One particularity of an AC switch is that it may remain ON when the gate current is
removed, even after that the current has reached zero. This kind of behavior could only
occur if the Triac operates with conditions beyond its datasheet guaranteed operation, and
particularly when the rate of load current decrease, or the rate of reapplied voltage after
turn-off are higher than the specified (dI/dt)c and (dV/dt)c rates.
An AC switch can be compared to two Thyristors mounted back-to-back and coupled with a
single control area. To trigger the two Thyristors, the control area overlaps the two
conduction areas.
During the conduction time, a certain quantity of charge is injected into the structure. This
charge disappears by recombination during the current decrease and by extraction after the
turn-off with the reverse recovery current (refer to
Figure 4.a).
Nonetheless, if an excess of
charge remains, particularly in the neighboring regions of the gate, this can induce the
triggering of the other conduction area when the mains voltage is reapplied across the Triac
(refer to Figure
Figure 4.b).
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