6/15/05

Page 5 of 11

© 2005 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.

Triac 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.

the reapplied voltage rise with a Vp of 400 V. This dv/dt condi-

tion simulates a worst case commutating dv/dt amplitude.

therefore applicable for all practical inductive loads and load

factors.

Figure. 7 Leakage Current, I

TA , AMBIENT TEMPERATURE (

-40

-20

0

20

40

60

80

100

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

µs

Figure. 6 Holding Current, I

TA, AMBIENT TEMPERATURE (

-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/

µs)

0.001

1.5

0.5

10000

NORMALIZED TO:

IFT at 3 V

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