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Document Number: 83627
6 Rev. 2.0, 29-Mar-11
This document is subject to change without notice.
THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT
www.vishay.com/doc?91000
IL410, IL4108
Vishay Semiconductors
Optocoupler, Phototriac Output, Zero Crossing,
High dV/dt, Low Input Current
INDUCTIVE AND RESISTIVE LOADS
For inductive loads, there is phase shift between voltage and current, shown in the fig. 12.
Fig. 12 - Waveforms of Resistive and Inductive Loads
The voltage across the triac will rise rapidly at the time the
current through the power handling triac falls below the
holding current and the triac ceases to conduct. The rise
rate of voltage at the current commutation is called
commutating dV/dt. There would be two potential problems
for ZC phototriac control if the commutating dV/dt is too
high. One is lost control to turn off, another is failed to keep
the triac on.
Lost control to turn off
If the commutating dV/dt is too high, more than its critical
rate (dV/dt
crq
), the triac may resume conduction even if the
LED drive current I
F
is off and control is lost.
In order to achieve control with certain inductive loads of
power factors is less than 0.8, the rate of rise in voltage
(dV/dt) must be limited by a series RC network placed in
parallel with the power handling triac. The RC network is
called snubber circuit. Note that the value of the capacitor
increases as a function of the load current as shown in fig. 13.
Failed to keep on
As a zero-crossing phototriac, the commutating dV/dt
spikes can inhibit one half of the TRIAC from keeping on If
the spike potential exceeds the inhibit voltage of the zero
cross detection circuit, even if the LED drive current I
F
is on.
This hold-off condition can be eliminated by using a snubber
and also by providing a higher level of LED drive current. The
higher LED drive provides a larger photocurrent which
causes the triac to turn-on before the commutating spike
has activated the zero cross detection circuit. Fig. 14 shows
the relationship of the LED current for power factors of less
than 1.0. The curve shows that if a device requires 1.5 mA
for a resistive load, then 1.8 times (2.7 mA) that amount
would be required to control an inductive load whose power
factor is less than 0.3 without the snubber to dump the
spike.
Fig. 13 - Shunt Capacitance vs. Load Current
Fig. 14 - Normalized LED Trigger Current vs. Power Factor
21607
Resistive load
Commutating dV/dt
AC line
voltage
AC current
through
triac
Voltage
across triac
I
F(on)
I
F(off)
Inductive load
Commutating dV/dt
AC line
voltage
AC current
through
triac
Voltage
across triac
I
F(on)
I
F(off)
iil410_01
400350300250200150100500
0.001
0.01
0.1
1
I
L
- Load Current (mA
RMS
)
C
s
- Shunt Capacitance (µF)
C
s
(µF) = 0.0032 (µF)*10^0.0066 I
L
(mA)
T
A
= 25 °C, PF = 0.3
I
F
= 2.0 mA
iil410_02
1.21.00.80.60.40.20.0
0.8
1.0
1.2
1.4
1.6
1.8
2.0
PF - Power Factor
NI
Fth
- Normalized LED
Trigger Current
I
Fth
Normalized to I
Fth
at PF = 1.0
T
A
= 25 °C
