Application Note AN-3004 Applications of Zero Voltage Crossing Optically Isolated Triac Drivers www.fairchildsemi.com REV. 4.00 5/7/02 Introduction The zero-cross family of optically isolated triac drivers is an inexpensive, simple and effective solution for interface appli- cations between low current dc control circuits such as logic gates and microprocessors and ac power loads (120, 240 or 380 volt, single or 3-phase). These devices provide suffi cient gate trigger current for high current, high voltage thyristors, while providing a guaran- teed 7.5 kV dielectric withstand voltage between the line and the control circultry. An integrated, zero-crossing switch on the detector chip eliminates current surges and the resulting electromagnetic interference (EMI) and reliability problems for many applications. The high transient immunity of 5000 V/s, combined with the features of low coupling capaci- tance, high isolation resitance and up to 800 volt specifi ed V DRM ratings qualify this triac driver family as the ideal link between sensitive control circuitry and the ac power system environment. Optically isolated triac drivers are not intended for stand alone service as are such devices as solid state relays. They will, however, replace costly and space demanding discrete drive circuitry having high component count consisting of standard transistor optoisolators, support components including a full wave rectifi er bridge, discrete transistor, trig- ger SCRs and various resistor and capacitor combinations. This paper describes the operation of a basic driving circuit and the determination of circuit values needed for proper implementation of the triac driver. Inductive loads are dis- cussed along with the special networks required to use triacs in their presence. Brief examples of typical applications are presented. Construction The zero-cross family consists of a liquid phase EPI, infra- red, light emitting diode which optically triggers a silicon detector chip. A schematic representation of the triac driver is shown in Figure 1. Both chips are housed in a small, 6-pin dual-in-line (DIP) package which provides mechanical integrity and protection for the semiconductor chips from external impurities. The chips are insulated by an infrared transmissive medium which reliably isolates the LED input drive circuits from the environment of the ac power load. This insulation system meets the stringent requirements for isolation set forth by regulatory agencies such as UL and VDE. The Detector Chip The detector chip is a complex monolithic IC which contains two infrared sensitive, inverse parallel, high voltage SCRs which function as a light sensitive triac. Gates of the individ- ual SCRs are connected to high speed zero crossing detec- tion circuits. This insures that with a continuous forward current through the LED, the detector will not switch to the conducting state until the applied ac voltage passes through a point near zero. Such a feature not only insures lower gener- ated noise (EMI) and inrush (Surge) currents into resistive loads and moderate inductive loads but it also provides high noise immunity (several thousand V/s) for the detection circuit. Figure 1. Schematic of Zero Crossing Optically Isolated Triac Driver MT ZERO CROSSING DETECTOR ZERO CROSSING DETECTOR MTLED IF DETECTOR AN-3004APPLICATION NOTE 2 REV. 4.00 5/7/02 Figure 2. Simplified Schematic of Isolator Figure 3. Triac Voltage-Current Characteristic ZERO CROSSING DETECTOR MT MT IF VF IDRM IDRM BLOCKING STATE Q111 ON STATE Q1 BLOCKING STATE ON STATE VDRM VDRM IH IH A2 A2+ VINH VINH I I Electrical Characteristics A simplifi ed schematic of the optically isolated triac driver is shown in Figure 2. This model is suffi cient to describe all important characteristics. A forward current fl ow through the LED generates infrared radiation which triggers the detector. This LED trigger current (I FT ) is the maximum guaranteed current necessary to latch the triac driver and ranges from 5 mA for the MOC3063 to 15 mA for the MOC3061. The LEDs forward voltage drop at I F = 30 mA is 1.5 V maxi- mum. Voltage-current characteristics of the triac are identi- fi ed in Figure 3. Once triggered, the detector stays latched in the “on state“ until the current fl ow through the detector drops below the holding current (I H ) which is typically 100 A. At this time, the detector reverts to the “off“ (non-conducting) state. The detector may be triggered “on“ not only by I FT but also by exceeding the forward blocking voltage between the two main terminals (MT1 and MT2) which is a minimum of 600 volts for all MOC3061 family members. Also, voltage ramps (transients, noise, etc.) which are common in ac power lines may trigger the detector accidentally if they exceed the static dV/dt rating. Since the fast switching, zero-crossing switch provides a minimum dV/dt of 500 V/s even at an ambient temperature of 70 C, accidental triggering of the triac driver is unlikely