IL410_12 Datasheet, PDF (6/9 Pages) Vishay Siliconix – Optocoupler, Phototriac Output, Zero Crossing

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.
I
F(on)
I_F(off)
AC line
voltage
I_F(on)
I_F(off)
AC line
voltage
Commutating dV/dt
AC current
through
triac
Voltage
across triac
Commutating dV/dt
AC current
through
triac
Voltage
across triac
21607
Resistive load
Inductive load
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.
1
C_s (µF) = 0.0032 (µF)*10^0.0066 I_L (mA)
0.1
0.01
TA = 25 °C, PF = 0.3
IF = 2.0 mA
0.001
0
iil410_01
50 100 150 200 250 300 350 400
I_L - Load Current (mA_RMS)
Fig. 13 - Shunt Capacitance vs. Load Current
2.0
I_Fth Normalized to I_Fth at PF = 1.0
1.8
T_A = 25 °C
1.6
1.4
1.2
1.0
0.8
0.0 0.2 0.4 0.6 0.8 1.0 1.2
iil410_02
PF - Power Factor
Fig. 14 - Normalized LED Trigger Current vs. Power Factor
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Document Number: 83627
Rev. 2.0, 29-Mar-11
This document is subject to change without notice.
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