ST AN303 Application note
AN303
Application note
Thyristors and TRIACs: latching current
Introduction
The purpose of this note is to familiarize TRIAC or thyristor (SCR) users with the latching
current I
The importance of this parameter is illustrated with some typical examples. Procedures are
given for measurement of I
sensitivity are described.
This application note presents only the TRIAC case. However, the concepts are valid for
SCRs (except for the various conduction modes).
Definition
The latching current (IL) of a TRIAC is the minimum value of the load current (current flowing
between electrodes A
removed (see Figure 1).
.
L
. The variation of IL with operating conditions and device
L
and A1) that keeps the device conducting when the gate signal is
2
Figure 1 below shows the latching current and the gate current pulse. After the TRIAC
triggering, a current (I
the current I
is lower than the latching current IL, the TRIAC switches off.
T
) flows through the TRIAC. If the gate current (IG) is removed while
T
Figure 1. Blocked TRIAC
I
G
IG > I
GT
I
I
T
Gate current
t
I
G
L
A
A
R
L
2
I
T
1
G
Main current
t
April 2009 Doc ID 3654 Rev 3 1/13
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Application examples AN303
1 Application examples
The importance of the latching current is highlighted by the following application examples.
1.1 Example 1: low power lamp control
In the application circuit shown in Figure 2 a TRIAC is used to control a 10 W signaling light.
For the European mains (V
could be used to control this load current, but the maximum latching current is specified as
50 mA if a Z0110 device is used in quadrant II. The peak load current is then very close to
the maximum latching current given in the datasheet. The device will turn off if the gate
current pulse is too short.
Figure 2. Control of a low-power lamp
Lamp (10 W)
= 230 V), the peak load current is about 61 mA. A Z01 device
rms
I
T
I
L
Main
current
A
230 V
I
T
2
A
1
G
I
G
I
G
Gate
current
t
t
p
Thus, a TRIAC could not remain on if its latching current is higher than the load current at
the moment the gate current is removed (refer to Figure 2). For correct operation, a
continuous gate current should be applied or a longer gate current pulse should be applied.
For example, for a sinusoidal load current (I
the following equation (refer to AN302 for a complete definition of I
tp >
1
ω
· arcsin
I
(
HMAX
I
peak
1
· arcsin
+
)
ω
I
(
LMAX
I
peak
= I
(t)
x sin(ωt)), the pulse width is given by
peak
(a)
):
H
)
To reduce the pulse width duration, a more sensitive TRIAC could be used (for example, for
the Z0103 I
max is 15 mA in QII).
L
t
a. The minimum current that keeps a TRIAC conducting is called the hypostatic or holding current IH.
2/13 Doc ID 3654 Rev 3
AN303 Application examples
1.2 Example 2: inductive load control
When a TRIAC controls an inductive load (L), the rise of the load current IT is slowed down.
The approximate load current slope dI
dI
V
T
A
~
=
dt
L
/dt is given by:
T
where V
is the mains voltage when the gate signal is applied.
A
In Figure 3 the impact of the gate pulse width on the TRIAC conduction is shown.
● In continuous lines: A short gate signal (T1) is applied. The TRIAC doesn’t remain in
the on state because the load current I
doesn’t reach the TRIAC latching current level
T
before the gate current removal.
● In dotted lines: A longer gate signal (T2) is applied. In this case, the TRIAC turns on
and remains in the on state. The TRIAC turns off when the load current reaches zero
after the gate current removal.
Figure 3. Control of an alternating current (AC) motor
M
A
I
T
V
I
G
V
T
V
A
2
I
T
A
1
T
1
G
G
T
2
t
t
I
G
I
L
t
For correct operation a gate current has to be applied until the load current reaches the
TRIAC latching current. The control mode shown in Figure 3 is a square pulse.
Another TRIAC control mode is to apply a gate pulse train. Application note AN308 offers
some TRIAC control circuits specially designed to work with inductive loads.
Doc ID 3654 Rev 3 3/13
Application examples AN303
1.3 Example 3: varying power load control
For most applications, the load power is controlled by the TRIAC conduction time. For arc
welding (see Figure 4), the controlled power can be subjected to considerable variations.
The device current rating is chosen and validated for full-wave and full load operation. The
application operation is then ensured in the worst case but, for low power loads, a TRIAC
triggering issue could occur.
In the case of an open load operation, the load current equals the transformer magnetizing
current, which is much lower than full load current. The load current could even be below the
TRIAC latching current in one triggering quadrant. Thus, the TRIAC could turn on properly in
one quadrant but could not turn on in another quadrant, for which the latching current is
higher. An unbalance then occurs and induces a direct current (DC) through the
transformer, which heats its coils and can cause transformer failure.
For correct circuit design, the TRIAC operation should be validated in full load and also in
open load. (See AN308 for a schematic circuit diagram dedicated to this welding
application.)
Figure 4. Arc welding control
A
G
2
A
1
4/13 Doc ID 3654 Rev 3
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