colin said:
maybe you dont need to go ac-dc-ac,
maybe you could just put a bidirectional switcher,
ie 2 mosfets back to back or inside a bridge rectifier
and feed them with a fixed duty cycle.
this would give you a fairly close fraction of the AC input.
I havnt given this much thought yet,
coffee not working at all today,
you would also need a second bidirectional switch driven in antiphase to
clamp the voltage,
after the first fet before putting it into an LC filter.
this would be like a flywheel diode wich ofc you cant use a diode becuase
its AC,
so its like a synchronous diode.
usual bootstrapped mosfet drivers might be a challenge,
or you go with opto coupled with seperate power supplies.
Colin =^.^=
I have done some experimenting and simulation for this type of application.
I used two IGBTs or MOSFETs in series, with the emitters or sources
connected together to a common rail. The same PWM signal can drive both at
the same time, but it might be better to alternate the drives at the zero
crossings. I found that a parallel LC circuit at multiples of the PWM
frequency helped reduce the chopped waveform to be fairly smooth.
It is also necessary to provide a current path during the off period of the
PWM. This complicates matters, but I have simulated it by using an
optoisolator, a bridge rectifier, and a MOSFET. The LTSpice ASC file is at
the end of this post. I was simulating a high current AC source, so I have
a transformer with a high ratio and a low impedance output.
There's probably a better way, but this proves the concept.
Good luck,
Paul
=========================================================================
Version 4
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