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How does this AC-AC power source design work?

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Matt_Cuino

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Here's the schematic: (please note that pinouts marked as PxN, where x = A,B,C... and N= 1,2,3... are all connected to an MCU controlling how it is working)

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We are only interested in the "single hot wire" configuration, or when the switch is at 1-2 position.

This is what we managed to figure out so far, correct or not:

PD3 is used to detect zero crossing and as such helps decide how PD4 and PD5 should trigger and open up to allow the positive half and the negative half of the AC sine wave to come in. By modifying the time window of PD4 and PD5, this circuit adjusts the output power to the load.

PA1 is some sort of overcurrent detection, which shuts off the right half of the circuit completely if it decides that the current is too much.

We programmed our own MCU to control such a circuit based on our potentially seriously flawed understanding of it, and it turned out we couldn't really control the power output, more over, we weren't able to cut off the circuit almost completely - almost in that the current will be reduced to a very, very small amount, but the original board very much could.

We are out of ideas and need your help. Can somebody provide some insight?
 

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