AN439
Application note
Snubberless™ and logic level TRIAC behavior at turn-off
Introduction
The use of TRIACs is limited by their switching behavior. Indeed, there is a risk of spurious
triggering after conduction if the slope of the decreasing current is too high, and/or if the
slope of the reapplied voltage is too high. The designer must then take some precautions:
device over-rating, switching aid network (snubber), and junction temperature margin, and
so on. This generally involves additional costs.
After a brief discussion of commutation when a TRIAC is turned off, this article will describe
the behavior of the logic level and Snubberless TRIACs, which present high commutation
capabilities.
Contents
1
TRIAC turn-off description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1
1.2
1.3
Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
(dI/dt)c versus (dV/dt)c characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Application requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1
1.3.2
TRIAC with resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
TRIAC with inductive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4
TRIAC without snubber network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Logic level and Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1
2.2
Operation in Q1-Q2-Q3 quadrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Performances and specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1
2.2.2
Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3
Typical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1
2.3.2
Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3
4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
March 2008
Rev 3
1/16
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TRIAC turn-off description
AN439
1
1.1
TRIAC turn-off description
Definition
The TRIAC can be compared to two thyristors mounted in back-to-back and coupled with a
control area which allows the triggering of this Alternating Current Switch with only one gate
(see
Figure 1).
Looking at the TRIAC silicon structure (see
Figure 2),
it can be noted that the conduction
areas, corresponding to these two thyristors, narrowly overlap each other on the control
area.
Figure 1.
Simplified equivalent
schematic of TRIAC circuit
A2
G
Figure 2.
A1
Example of TRIAC silicon
structure
G
I
-
N1
N4
P1
P1
N2
I
+
Gates
Ctrl
.
V
T
Gates
Ctrl
N2
P2
N3
I
-
P2
A1
I
+
A2
During the conduction time, a certain quantity of charge is injected into the structure. The
biggest part of this charge disappears by recombination during the current decrease, while
another part is extracted after the turn-off by the reverse recovery current. Nonetheless, an
excess charge remains, particularly in the neighboring regions of the gate, which can induce
the triggering of the other conduction area when the mains voltage is reapplied across the
TRIAC. This is the problem of commutation.
For a given structure at a determined junction temperature, the turn-off behavior depends
on:
1.
The
quantity of charge which remains when the current drops to zero.
The
quantity of the charge is linked to the value of the current which was circulating in the
TRIAC approximately 100 µs, about two or three times the minority carriers’ life time,
before the turn-off. Thus, the parameter to consider is the slope of the decreasing
current, called the turn-off dI/dt or dI/dt
OFF
. (see
Figure 3)
The slope of the reapplied voltage during turn-off.
This parameter is the
commutation dV/dt, called the turn-off dV/dt or dV/dt
OFF
(see
Figure 3).
A capacitive
current, proportional to the dV/dt
OFF
, flows into the structure, and therefore charges are
injected and added to those coming from the previous conduction.
2.
2/16
AN439
Figure 3.
dI/dt and dV/dt at turn-off
I
T
V
Mains
TRIAC turn-off description
OUT
dI/dt
OFF
V
T
t
I
T
V
T
I
G
G
COM
dV/dt
OFF
I
G
t
t
1.2
(dI/dt)c versus (dV/dt)c characterization
To characterize the turn-off TRIAC behavior, we consider a circuit in which the slope of the
decreasing current can be adjusted. In addition, the slope of the reapplied voltage can be
controlled by using, a circuit of resistors and capacitors connected across the TRIAC. For a
determined dV/dt
OFF
((dV/dt)c), we progressively increase the dI/dt
OFF
until a certain level
which induces the spontaneous triggering of the TRIAC. This is the critical dI/dt
OFF
, called
the (dI/dt)c in TRIAC datasheets. This is also the way to trace the curve of the TRIAC
commutation behavior (see TRIAC datasheet curve “Relative variation of critical rate of
decrease of main current (dI/dt)c versus reapplied (dV/dt)c”).
In TRIAC datasheets, the commutation behavior is specified in different way according to
the TRIAC technologies. For standard TRIAC, a minimum (dV/dt)c is specified for a given
(dI/dt)c. For logic level TRIACs, a minimum (dI/dt)c is specified for two given (dV/dt)c (0.1
V/µs and 10 V/µs). For Snubberless TRIACs, a minimum (dI/dt)c is specified without
(dV/dt)c limitation.
Figure 4
represents the curve of the commutation behavior obtained with a standard 4 A
TRIAC. This TRIAC is available with different sensitivities:
●
●
●
●
Z0402: I
GT
= 3 mA;
Z0405: I
GT
= 5 mA;
Z0409: I
GT
= 10 mA;
Z0410: I
GT
= 25 mA.
For lower sensitive gate TRIACs (Z0409 and Z0410), the (dI/dt)c is slightly modified
according to the (dV/dt)c. For sensitive gate TRIACs (Z0402 and Z0405), this parameter
noticeably decreases when the slope of the reapplied voltage increases.
3/16
TRIAC turn-off description
Figure 4.
AN439
Relative variation of (dI/dt)c versus (dV/dt)c for a 4 A standard TRIAC
(typical values)
Area of spurious firing at
commutation
Safe area
In practice, the current waveform, and thus the dI/dt
OFF
, is imposed by the load. Generally
we cannot change it.
So, in TRIAC applications, it is always necessary to know the dI/dt
OFF
of the load to choose
a TRIAC with a suitable (dI/dt)c. This is the most important parameter.
Suppose a circuit in which the dI/dt
OFF
reaches 2 times the specified (dI/dt)c. The standard
4 A TRIACs, characterized by the curves in
Figure 4,
will be not suitable even if the dV/dt
OFF
is equal to 0.1 V/µs.
1.3
1.3.1
Application requirements
TRIAC with resistive load
In this case, the TRIAC current and the mains voltage are in phase (see
Figure 5).
When the
TRIAC switches off (i.e. when the current drops to zero), the mains voltage is equal to zero
at this instant and will increase across the TRIAC according to the sinusoidal law:
Equation 1
V
Mains
= V
Max
· sin(
ω
·t )
For the European mains, i.e. V
RMS
= 220 V at 50 Hz, the slope will be:
Equation 2
dV / dt
OFF ( V /
µ
s )
= V
RMS ( V )
· 2 ·2
π
·f
( Hz )
·10
- 6
≈
0.1 V / µ s
For 110 V, 60 Hz mains, the slope will be: dV/dt
OFF
≈
0.06 V/µs.
These relatively low dV/dt
OFF
correspond to the left points on the curves in
Figure 4.
The
dI/dt
OFF
only depends on the load rms current and the mains frequency. For resistive loads,
as for most other loads, we will have:
Equation 3
dI / dt
OFF ( A
/
ms )
= I
RMS ( A )
· 2 ·2
π
·f
( Hz )
·10
-3
≈
0.5 ·I
RMS ( A )
4/16
AN439
Figure 5.
TRIAC turn-off description
Current and voltage waveforms for resistive loads (phase control)
I
G
t
I
T
dI/dt
OFF
V
Mains
V
T
t
dV/dt
OFF
t
1.3.2
TRIAC with inductive load
An inductive load induces a phase lag between the TRIAC current and the mains voltage
(see
Figure 6).
When the current drops to zero, the TRIAC turns off and the voltage is abruptly applied
across its terminals. To limit the speed of the reapplied voltage, a resistive / capacitive
network mounted in parallel with the TRIAC is generally used (see
Figure 13).
This
“snubber” is calculated to limit the dV/dt
OFF
at a value for which the dI/dt
OFF
is lower than
the (dI/dt)c specified in the datasheet. The dI/dt
OFF
is also determined in this case by the
load impedance (Z) and the mains rms voltage. (see. AN437 for RC snubber circuit design)
Figure 6.
Current and voltage waveforms for inductive loads (phase control)
I
G
t
I
T
dI/dt
OFF
V
Mains
V
T
t
t
dV/dt
OFF
5/16
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