Hi,
For synchronous rectification of a single phase AC signal, I made a circuit that compares the AC phase voltages to the high rail and ground to detect when to turn on the high and low side fets. The detection circuit works fine in LTspice, here are some waveform pics from it:
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I noticed from the waveforms that the lowside switches are always conducting longer than the highside switches, which only are on during the time required to recharge the highside rail. For best efficiency, I guess turning the FET's on whenever the source voltage is greater than the drain is best, but maybe it is easier to only turn on the low side FET's when the high side are ready to go on, that way only the high side needs to be monitored.
I use a resistor divider on the high rail and also on each of the phase voltage signals, and LTspice's resistors are perfect so it works, but when I change a resistor value by 1%, then the circuit no longer works because of the measured voltage error. I use 1Mohm and 17.8Kohm resistors (57.18X dividers) from the 170Volt(max) rail, and the phase
voltages, and then feed those signals into a picoamp input current opamp, then some comparators. Is there a better way to do this?
The comparator outputs will be sent to a uC which also interfaces to the bridge-driver IC's which drive the fets.
Here is the LTspice circuit so far: "
cheers, Jamie