ST AN302 Application note
AN302
Application note
Thyristors and TRIACs: holding current - an important parameter
Introduction
The purpose of this note is to familiarize the users of TRIACs or thyristors with the
hypostatic current parameter, I
The importance of this parameter is illustrated by some typical examples. Then a description
is given of how to measure it and on its variation with the conditions of use and the
sensitivity of the components.
This application note discusses only the TRIAC; however, the concepts are also valid for
SCRs.
Definition
To keep an electromechanical relay turned on, it is necessary to have a minimum current
circulating in its coil. If the current falls too low the relay would turn off. The same
phenomenon can be observed in a TRIAC. This minimum current which keeps the TRIAC
conducting is called the hypostatic or holding current, I
Figure 1 below shows the holding current and the gate current pulse, I
the basic TRIAC circuit. After the TRIAC is turned on, a current, I
TRIAC current falls below the holding current the TRIAC is blocked and requires another
gate pulse before it can turn on again.
, also known as the holding current parameter.
H
. (see Figure 1).
H
, which is applied to
G
, flows through it.When the
T
Figure 1. Current control
I
I
T
T
I
I
H
H
t
t
March 2008 Rev 3 1/10
www.st.com
Application examples AN302
1 Application examples
The importance of the holding current is highlighted by the following application examples.
1.1 Example 1: light dimmer
Figure 2. Dimmer with interference suppression filter (coil and capacitor)
Load
L
I
T
220V
C
Interference suppression
LC filter
Tr i ac
I
G
RC phase-shift network
Figure 3. Current in the dimmer TRIAC
I
T
I
O
I
H
t
In the dimmer circuit of Figure 2 the interference suppression filter can produce oscillations.
If the minimum current during these oscillations is higher than the holding current, that is, if
I
in Figure 3, the TRIAC remains turned on. But if IO falls below IH, the TRIAC will be
O>IH
blocked.
It is possible, if the coil is the incorrect type or of poor quality, that the oscillation is
insufficiently damped and the TRIAC current falls below the holding current. This results in
untimely blocking of the TRIAC. However, it is turned on again at the next gate current pulse;
but the oscillations again prevent continuous conduction and the lamp flickers. Hence this is
known as the flicker effect.
How to prevent the flicker effect
The flicker effect can be prevented by using an appropriate interference suppression filter
which does not produce extensive oscillations, and then by choosing a TRIAC with a lower
holding current.
2/10
AN302 Application examples
1.2 Example 2: motor control (1)
Figure 4. Control of a small motor by TRIAC
M
Induction motor
The designer wishes to control a small high-impedance motor (2500 Ω, for example) by a
TRIAC. He obtains the parts and an operating manual and carries out some tests. The
circuit, based on that in Figure 4, operates as expected at first. However, after one year of
production, the manufacturer complains of low torque in the motor and blames the TRIAC.
What’s happened?
The circuit was designed with a type of TRIAC whose maximum specified holding current I
H
was 50 mA. But the TRIACs used for the tests were not worst-case, they were more
sensitive, having I
= 13 mA and IH- = 8 mA. The designer based his choice on these
H+
results.
After a year of delivery, the component manufacturer continues to deliver parts which are in
conformity with the specification but less sensitive than before, in fact now I
I
= 20 mA, typically.
H-
= 40 mA and
H+
With these different values the conduction time decreases, the asymmetry is greater as
shown in Figure 5, and the resulting DC component of current causes the motor to gradually
lose torque.
To prevent this kind of difficulty, one must, when designing the circuit, take into account not
the typical value of the sample used but the worst-case value specified by the component
manufacturer.
Figure 5. Voltage across the TRIAC and current for the motor control
V
I
I
H +
I
H –
t
3/10
Loading...