Could PI device be used for 400VDC output for universal input?
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go to:
http://www.cke.com/pg_catalog/catalog.aspx
You'll find a variety of diodes to 40 nsec, like their UX-FOB, 8 kV, 500 mA, 40 NS.
Hi; I realize that topic is already too old, but could be useful for other readers too.
Simply use 4 diodes in series in the secondary. The diodes must each be paralleled by a resistor of 330k-680k / 500mW, for equalizing the voltage stress, which otherwise will be distributed randomly by the internal inverse impedance of diodes. Also a clamp group of 100pF + 22kohmi should be added on each diode, for the same purpose.
Secondary of the transformer should be wounded carefully, with good insulation between each layer. If possible, to reduce at half the voltage stress on windings, at the end of the first layer a rectangle of insulation is added, the wire is returned to the beginning side of the winding, a layer of insulation is applied and after that the next layer of turns is started...and so on. (This technique i used 20 years ago when i wounded high voltage transformers for TV sets, with perfect results. Also for diode series circuit, the same.).
Hello Prasun,
the limitation with high voltage outputs is the voltage rating of the output diode required. For a 400 V output the diode rating will be in the 1500 V to 2500 V range - you can check this by using PIXls (part of the PI Expert Suite) to do a paper design (I used a TOPSwitch-HX TOP255PN device).
The diode has to withstand the output voltage plus the additional reverse voltage induced when the primary side MOSFET turns on. If the transformer turns ratio (primary: secondary) is 1:3 then for a 375 V bus voltage (265 x sqrt 2) the addition diode stress is 1125 V giving diode minimum diode rating of 1125+400=1525 V.
The issue with a diode of this voltage rating is they have a long reverse recovery time. We normally recommend reverse recovery times (trr) <50 ns. For a 2000 V diode the trr is ~500 ns to 2000 ns.
Long recovery time causes large initial current spikes on the primary (and therefore the internal MOSFET) during start up. These typically exceed the leading edge blanking time and trigger the internal current limit, preventing proper start up.
It's been 10+ years since I tried such a design - anyone out there had success with high voltage outputs and has any tips to share?
Regards
Chekov