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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
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Posts: 600
I've been working on a smps for a gate drive system for brick igbts. The smps I designed puts out 25-0-12.5, which then gets regulated via zener shunts to 24-0-12. the problem is that the smps is capable of dumping huge amounts of current out, so I anticipate that the zeners will catch fire with the many watts they'll be dissipating with their 25ohm series resistance and 1 watt rating.
I originally designed it with a capacitive input filter, but now realize that i need some sort of chock filter to ensure that nothing gets blown up. The problem with a choke filter is that the output voltage is (2*sqrt(2)/Pi)*Vrms. my Vrms is 17v assuming a sine wave(which it's not, its more triangular with an irregular ramp down), so that kills my hopes of having 24v: it now drops to ~14v.
As I understand it, this is because the choke filter passes the average current of that wave through. A capacitor just takes the peak. If I placed a capacitor filter before the choke, then I'd be filtering the average dc current of the fully rectified output, ie. doing nothing to the voltage, just keeping current stable. Now if I place a cap after that, CLC, I can be rid of voltage transients generated by fast changes in current from the driver itself. Will this work to both maintain a minimum current through the regulation zeners while still achieving a full dc voltage supply?
Yes, changing the number of secondary windings or the primary voltage would do the same thing, but the transformer was wound carefully and painfully to get high isolation between the windings and have as little parasitics as possible. I dont want to rewind it from scratch. As for the primary voltage, I have a 12v primary rail to run all electronics on that side, and a high power boost converter is just making things more complicated than they should be.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
why not use an off-the-shelf integrated switching regulator? there are many designs which would give you those results with only a couple components...
why don't you use a classic design with a switcher and multiple transformer outputs like an ATX supply? you could also reuse one by rewiring the transformer with what you need...
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Input filters vs supply voltage and shunt current
