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SS Variac (Low Pass Filter)

Original poster: Sparktron01@xxxxxxxxxxx


I have successfully demonstrated Jim Lux's concept (hi Jim) for a SS Variac.
I'm running at VERY low power (24VAC/~25W) so I can probe stuff without
worrying about detonations. The basic topology is a full wave DC bridge in
series with hot leg, with a 600V, 40A IGBT across +/- of bridge as a commutating
switch. I'm using a 555 manually adjustable PWM (1-99% DC). Right now I've
got about a 10:1 control range voltage wise, but alot of HF noise in LF AC
output. What is correct procedure to design a low pass filter knowing:
1. Full power output (up to 1kVA)
2. PWM carrier frequency (20-50kHz)

One design I read was size inductor for 5% PU reactance at 60Hz, size capacitor
for 1/1000 of 60hz reactance at PWM frequency.  Got to be a simple way to
design (resonance at 2X operational Fo was also mentioned in several IEEE
Power Electronics references I reviewed).  Thanks!

Best Regards

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA