Datasheet

9/2/04
Page 5 of 11
© 2004 Fairchild Semiconductor Corporation
6-PIN DIP RANDOM-PHASE
OPTOISOLATORS TRIAC DRIVERS
(600 VOLT PEAK)
MOC3051-M MOC3052-M
Minimum LED Off Time in Phase Control
Applications
In Phase control applications one intends to be able to control
each AC sine half wave from 0 to 180 degrees. Turn on at zero
degrees means full power and turn on at 180 degree means
zero power. This is not quite possible in reality because triac
driver and triac have a fixed turn on time when activated at
zero degrees. At a phase control angle close to 180 degrees
the driver’s turn on pulse at the trailing edge of the AC sine
wave must be limited to end 200 ms before AC zero cross as
shown in Figure 5. This assures that the triac driver has time
to switch off. Shorter times may cause loss of control at the
following half cycle.
I
FT
versus dv/dt
Tr iac drivers with good noise immunity (dv/dt static) have inter-
nal noise rejection circuits which prevent false triggering of the
device in the event of fast raising line voltage transients. Induc-
tive loads generate a commutating dv/dt that may activate the
triac drivers noise suppression circuits. This prevents the
device from turning on at its specified trigger current. It will in
this case go into the mode of “half waving” of the load. Half
waving of the load may destroy the power triac and the load.
Figure 8 shows the dependency of the triac drivers I
FT
versus
the reapplied voltage rise with a Vp of 400 V. This dv/dt condi-
tion simulates a worst case commutating dv/dt amplitude.
It can be seen that the I
FT
does not change until a commutat-
ing dv/dt reaches 1000 V/ms. The data sheet specified I
FT
is
therefore applicable for all practical inductive loads and load
factors.
Figure. 7 Leakage Current, I
DRM
vs. Temperature
T
A
, AMBIENT TEMPERATURE (
o
C)
-40 -20 0 20 40 60 80 100
I
DRM
, LEAKAGE CURRENT (nA)
0.1
1
10
100
1000
10000
Figure 5. Minimum Time for LED Turn–Off to Zero
Cross of AC Trailing Edge
AC SINE
0
ϒ 180 °
LED PW
LED CURRENT
LED TURN OFF MIN 200 µ s
Figure. 6 Holding Current, I
H
vs. Temperature
T
A
, AMBIENT TEMPERATURE (
o
C)
I
H
, HOLDING CURRENT (mA)
-40
1
0.9
0
-30 -20 -10 0 10 20 30 4050607080
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
dv/dt (V/ µ s)
0.001
1.5
0.5
10000
NORMALIZED TO:
I
FT
at 3 V
I
FT
, LED TRIGGER CURRENT (NORMALIZED)
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.01 0.1 1 10 100 1000
Figure. 8 LED Trigger Current, I
FT
vs. dv/dt