Electronic

Application Analysis of Random Phase Interleaved Triac Driver

This document comprehensively discusses random phase crossing triac drivers and their most common applications. It will start with the basic electrical description of the triac driver. What follows is a discussion about the use of the driver itself and its input circuit. The last part will be an application example of the device.

This document comprehensively discusses random phase crossing triac drivers and their most common applications. It will start with the basic electrical description of the triac driver. What follows is a discussion about the use of the driver itself and its input circuit. The last part will be an application example of the device.

structure

The MOC30XX series random phase (non-zero crossing) triac driver consists of an aluminum gallium arsenide infrared LED, optically coupled to a silicon detector chip. The two chips are assembled in a 6-pin DIP package to provide 7.5KVAC (PEAK) insulation between the LED and the output detector. These output detector chips are designed to drive triacs that control loads on 115 and 220V AC power lines. The detector chip is a complex device whose function is the same as that of a small triac. It can generate the signals required to drive the gate of a larger triac (such as Fairchild semiconductor‘s FKPF12N80). MOC30XX triacs can control higher power triacs with a minimum number of additional components.

Basic electrical instructions

The nominal forward voltage drop of AlGaAs LED is 1.3 V at 10 mA, and the reverse breakdown voltage is greater than 3 V. The maximum current through the LED is 60 mA.

In the off state, the minimum blocking voltage of the detector in either direction is 250 Vdc. In the on state, the detector will pass 100 mA in either direction,
The voltage drop across the device is less than 3 V. Once triggered into the on (conducting) state, even if there is no current flowing through the LED, the detector will remain there until the terminal current drops below the holding current (usually 100 μA), at which time the detector returns to off (non-conducting) )state. By exceeding the forward blocking voltage, passing the voltage ramp of the detector at a rate exceeding the static dv/dt rating, or photons from the LED, the detector may be triggered into the on state.

Application Analysis of Random Phase Interleaved Triac Driver
Schematic diagram of optically coupled random phase triac driver

When the current through the LED is equal to or greater than the IFT(max) specification, the specification guarantees that the LED triggers the detector to enter the conducting state. For example, MOC3011M needs at least 10mA of LED current to ensure turn-on. The similar device MOC3012M has exactly the same characteristics, but it only needs 5 mA to trigger.

Since these devices essentially look like a small light-triggered triac, we chose to represent it as shown in Figure 1.

Use MOC3011M as a triac driver

Application Analysis of Random Phase Interleaved Triac Driver
Simple triac gate control circuit

Figure 2 shows a simple triac drive circuit using MOC3011M. The maximum inrush current rating of MOC3011M sets the minimum value of R1 by the following formula:

R1 (min) = Vin(pk)/1.2A

If we operate at 115 Vac nominal line voltage, Vin(pk) = 180 V, then:

R1 (min) = Vin(pk)/1.2A = 150 ohms.

In fact, this will be a 150 or 180 ohm resistor. If the IGT of the triac = 100 mA and VGT = 2 V, the voltage Vin required to trigger the triac will be given by the following equation:

VinT = R1•IGT + VGT + VTM = 20 V

Resistive load

When driving a resistive load, the circuit shown in Figure 2 can be used. Incandescent lamps and resistive heating elements are the two main categories that use 115 Vac resistive loads. The main limitation is that the triac must be selected correctly to maintain the proper surge load. Incandescent lamps sometimes produce a peak current called “flashover”, which can be very high, and the thyristor should be protected by a fuse or rated high enough to maintain this current.

Inductive load-commutation dv/dt

Inductive loads (motors, solenoids, magnets, etc.) have problems for both triacs and MOC3011M, because voltage and current are out of phase with each other. Since the triac turns off at zero current, it may try to turn off when the applied current is zero but the applied voltage is high. This is like a sudden rise in applied voltage to a triac. If the rate of rise exceeds the commutation dv/dt of the triac or the static dv/dt of the MOC3011M, the triac will be turned on Thyristor switch.

Input circuit

Resistance input

When the input conditions are well controlled, such as when driving MOC3011M from a logic gate, only one resistor is needed to connect the gate to the input LED of MOC3011M. The resistor should be selected to set the current into the LED to a minimum of 10 mA but not more than 50 mA. 15 mA is a suitable value, which allows the LED to degrade significantly over time and ensures a long working life of the coupler. Currents higher than 15 mA will not improve performance and may accelerate the inherent aging process of LEDs. Assuming that the forward voltage drop is 1.5 V at 15 mA, a simple formula can be used to calculate the input resistance.

RI = (VCC C 1.5)/0.015

Input protection circuit

Application Analysis of Random Phase Interleaved Triac Driver
MOC3011M input protection circuit

In some applications, such as solid state relays, where the input voltage varies greatly, designers may wish to limit the current applied to the LEDs of the MOC3011M. The circuit shown in Figure 3 allows the non-critical range of the input voltage to drive the MOC3011M correctly, while protecting the input LED from inadvertent application of reverse polarity.

Applications

Using MOC3011M on 240 Vac lines The rated voltage of MOC3011M is not sufficient for direct use on 240 Vac lines; however, the designer may connect two of them in series. When used in this way, two resistors are needed to balance the voltage drop across them.

solid state relay

Application Analysis of Random Phase Interleaved Triac Driver

Figure 4 shows a complete general solid state relay for inductive loads with input protection. When the designer has more control over the input and output conditions, he can eliminate those components required for his specific application to make the circuit more cost-effective.

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