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FET Switch-in (triac) pre-rectification stage.

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ACharnley

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I've hit a snag with the attached circuit. It's two stage, a switch-in triac which is supposed to switch-out a capacitor.

Screenshot_2018-03-25_20-20-49.png

When AC1 is positive biased rectification flows as expected. All good.

When AC1 is negative the FET's don't work because ground isn't referenced by AC1X. Not good!

Your help very much appreciated!

Regards,

Andrew
 

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crutschow

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ACharnley

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Ok, so M1 & M5 are two P-FET's mimicing a TRIAC. By default they are "on" (I deleted the mcu control for brevity). These pass power from AC1X to AC1, or at least do when AC1 is positive. When it's negative the circuit fails.

The purpose of the FET based TRIAC is to load match to the power input, which has some inductance (not shown). These switch capacitors in and out of the AC line in both series and parallel.
 

unclejed613

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it might be useful to know the purpose of this exercise. is it to switch power on and off by controlling the circuit with a mcu, or to provide some kind of voltage regulation? you can simulate a triac with a pair of transistors in LTSpice. you mentioned you wanted the triacs to default to the "on" state, then it would be better to use a GTO (Gate Turn Off) rather than a standard triac.
 
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