LOW_POWER_ZERO_VOLTAGE_SWITCH_TEMPERATURE_CONTROLLER

Published:2009/7/6 21:38:00 Author:May | From:SeekIC

The zero voltage switching technique is widely used to modulate heating and similartypes of ac loads where the time constant associated with the load(tens of seconds tominutes) is sufficiently long to allow smooth proportional modulation by time ratio control,usmg one complete cycle of the ac input voltage as the mlnlmum switching movementoDespite its attractions,the traditional triac-based ZVS is virtually unusable for the controlof very low power loads,especially from 220 volt ac inputs due to the triac's reluctanceto latch-on into the near-zero instantaneous currents that flow through it and the loadnear the ac voltage zero crossover point so The circuit side-steps the latching problem by employing a par of very sensitive low current reverse blocking thyristors(C106)connected in antiparallel;these are triggered by a simple thermistor modulated differential amplifier(Q1,Q2),with zero voltage logic furnished by an H11AA1 ac input optocoupleroWith the NTC thermistor TH calling for heat,transistor Q1 is cut off and Q2 is on,which would normally provide contmuous base drive to Q3,with consequent triggeing of either SCR,or of SCR 2 vla SCR1,depending on phasing of the ac input.Note that when the ac input voltage is positive with respect to SCR 2,SCR 1 isreverse biased and,in the presence of gate current from Q3,behaves as a remotebase transistor, whose output provides vla blocking diode CR1,positive gate triggercurrent for SCR 2.When the ac input polarity is reversed(SCR 1's anode positive),SCR 1 behaves as a direct fired conventional thyristoro Trigger current to SCR 1,however,is not continuous,even when TH is calling for heat and Q2 is delivering basecurrent to Q3．In this situation,Q3 is inhibited from conduction by the clamping actionof POI,an H11AA photocoupler,except during those brief instants when the ac input voltage is near zero and the coupler input diodes are deprived of current.Triggering of either SCR can occur Only at ac voltage crosslng points,and RFI-less operation resultsoThe proportional control feature is injected via the positive feedback action of capacitor CM,which converts the differential amplifier Q1,Q2 into a simple multivibrator,whose duty cycle vanes from one to 99 percent according to the resistance of TH.Zener diode 21 is operational,being preferred when maxlmum immunity from ac voltage induced temperature drift is desired.



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