MOC3071M, MOC3072M, MOC3073M

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6

APPLICATIONS INFORMATION

Basic Triac Driver Circuit

The random phase triac drivers MOC3071M, MOC3072M

and MOC3073M can allow snubberless operations in

applications where load is resistive and the external

generated noise in the AC line is below its guaranteed

dv/dt withstand capability. For these applications, a

snubber circuit is not necessary when a noise insensitive

power triac is used. Figure 7 shows the circuit diagram.

The triac driver is directly connected to the triac main

terminal 2 and a series resistor R which limits the current

to the triac driver. Current limiting resistor R must have a

minimum value which restricts the current into the driver

to maximum 1 A.

The power dissipation of this current limiting resistor and

the triac driver is very small because the power triac

carries the load current as soon as the current through

driver and current limiting resistor reaches the trigger

current of the power triac. The switching transition times

for the driver is only one micro second and for power

triacs typical four micro seconds.

Triac Driver Circuit for Noisy Environments

When the transient rate of rise and amplitude are expected

to exceed the power triacs and triac drivers maximum

recommended. Fast transients are slowed by the R-C

snubber and excessive amplitudes are clipped by the Metal

Oxide Varistor MOV.

Triac Driver Circuit for Extremely Noisy Environments

As specified in the noise standards IEEE472 and IEC255-

4.

Industrial control applications do specify a maximum

transient noise dv/dt and peak voltage which is super-

imposed onto the AC line voltage. In order to pass this

environment noise test a modified snubber network as

LED Trigger Current versus Temperature

Figure 3 shows the increase of the trigger current when the

device is expected to operate at an ambient temperature

for LED degradation over time.

Example:

LED Trigger Current vs. Pulse Width

Random phase triac drivers are designed to be phase

controllable. They may be triggered at any phase angle

within the AC sine wave. Phase control may be

accomplished by an AC line zero cross detector and a

variable pulse delay generator which is synchronized to

the zero cross detector. The same task can be

accomplished by a microprocessor which is synchronized

to the AC zero crossing. The phase controlled trigger

current may be a very short pulse which saves energy

delivered to the input LED. LED trigger pulse currents

shorter than 100 µs must have increased amplitude as

shown on Figure 4. This graph shows the dependency of

graph is normalized in respect to the minimum specified

with the devices guaranteed static trigger current.

Example:

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°. Turn on

at 0° means full power and turn on at 180° 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°the

driver’s turn on pulse at the trailing edge of the AC sine

wave must be limited to end 200 µs before AC zero cross

time to switch off. Shorter times may cause loss of control

at the following half cycle.

Static dv/dt

Critical rate of rise of off-state voltage or static dv/dt is a

triac characteristic that rates its ability to prevent false

triggering in the event of fast rising line voltage transients

when it is in the off-state. When driving a discrete power

triac, the triac driver optocoupler switches back to off-

state once the power triac is triggered. However, during

the commutation of the power triac in application where

the load is inductive, both triacs are subjected to fast rising

voltages. The static dv/dt rating of the triac driver

optocoupler and the commutating dv/dt rating of the

power triac must be taken into consideration in snubber

circuit design to prevent false triggering and commutation

failure.