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Registered Member #3926
Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525
Just finished re-building a ZVS, using 25n120's, what would be the highest input voltage for it now?
Will I have to change the tank cap? 1kv .47uF
Thanks.
Also the IGBT's get hot after about a minute. And when my PSU is under load, after stretching the arc it makes a annoyhing screech, not the usual 50hz hum.
(I could put mains through this thing? lol) No need though.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
I think theoretically you could go up to 300vdc in (usual note I see is that the voltage rating of the transistor should be 4x the input voltage) but I wouldn't recommend trying it.
I've also read that IGBTs run into problems when they get warm because they have a negative thermal coefficient, and in addition to the usual RI^2 losses they also dissipate energy every time they switch (0.96mJ for yours.) If I did the math right (might be off by a factor of 2) then that's an extra 48 watts of heating assuming an operating frequency of 25khz. So basically what I'm trying to say is that while your transistors are capable of high power levels, you'll need to make an extra effort to cool them properly.
Also remember that higher input voltages (and especially ripply voltages from rectifying) will require a separate dc supply for the gates section.
The screech is very typical of zvs arcs. The arcing lowers the frequency down into the audible range so you can hear it.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
"audible" sets in below about 20khz, give or take a couple khz depending on the person. Did your last zvs use a smaller value capacitor or less primary turns? That would explain why you couldn't hear it before.
If your goal is mains powered, then I'd probably switch to a half or full bridge inverter design. A lot of the small problems with the zvs get much bigger with a higher voltage supply.
If you do decide to do a mains powered zvs, consider current limiting the input so that it doesn't get out of control.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The reason for the screeching is that the ZVS type circuits can easily get into a chaotic mode of oscillation, caused by non-linearity in the transformer core. The ferroresonant and LC resonant modes of operation compete with each other.
If you make one with a ferrite toroid as the transformer, it's guaranteed.
Try a bigger air gap and/or a bigger tank capacitor.
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Rebuilt my ZVS
