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Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Hi people, I'm new here but I'd like to share my own scheme for fly-back HV source. It took me a while to design it, despite the final scheme is very simple. The biggetst advantage of this circuit is that it needs no tranformer and the transistor needs almost no heat sink. I'm using this HV source for my 500KV Marx generator which I post next time (It's my big baby :) ). Any comments or questoins are welcomed
OK according to the spikes: You can try diodes rated for 400V, after you destroy them you can try 600V than 1200V and you end up with 2000V minimum. That's my experience (I sent a tens of diodes in silicon heaven!!!)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I don't know what you mean by super efficient, how much heat does it dissipate? Over 95% efficiency is not uncommon in a flyback driver. You don't need TVS or varistors, a mosfet will clamp the voltage by itself if it exceeds maximum d-s rating. But nevertheless, it's respectable that you got it working this way, in my experience the flyback spikes in a single-ended flyback driver exceed at least 5x supply voltage (in the case of rectified mains it would be over 1500V.) What is the driver frequency and average input power?
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
I´m using 25KHz 1:1 squere signal and as driving circuit tl494 (555 is crap!) powered by two 9V (18V) bateries. The rectifier diodes are needed at least for 2000V, I'm using 4000V because I'm using as a HV source for my 500KV Marx and I want to be sure. The reason why at least 2KV is that this circuit is woking in "resonant" mode with the primary of the Fly-back where the induction sometimes leads to couple of KV. Mates
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
The TVS are absolutely necessary for protecting the FET...(my collection of death MOSFETs is huge so know what I´m saying). I mean by the super efficecy the fact that there is no single part in this circuit which is becoming hot. But of course the half-bridge do the same job. The power consuption - I have no idea - my quess is around 200W - I'll mesure it...
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
jmartis wrote ...
I don't know what you mean by super efficient, how much heat does it dissipate? Over 95% efficiency is not uncommon in a flyback driver. You don't need TVS or varistors, a mosfet will clamp the voltage by itself if it exceeds maximum d-s rating.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Dago wrote ...
jmartis wrote ...
I don't know what you mean by super efficient, how much heat does it dissipate? Over 95% efficiency is not uncommon in a flyback driver. You don't need TVS or varistors, a mosfet will clamp the voltage by itself if it exceeds maximum d-s rating.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Mates wrote ...
The rectifier diodes are needed at least for 2000V, I'm using 4000V because I'm using as a HV source for my 500KV Marx and I want to be sure. The reason why at least 2KV is that this circuit is working in "resonant" mode with the primary of the Fly-back where the induction sometimes leads to couple of KV. Mates
I was under the impression that the flyback spikes appear at the drain/primary connection and were clamped to ground by the snubber at x volts. This means the voltage at the mosfet drain is 1500V at a spike, but the voltage at ground remains 0, and the voltage at V+ remains V+. Have I misunderstood flyback operation? I'm pretty sure you can get away with a rectifier sized for the input voltage since the spikes are clamped and have relatively little energy, but you may want to hear this from someone else, since I'm actually in doubt here.
Oh btw, instead of double posting just edit your previous post and add the extra content. It keeps the moderators happy.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I don't want to start a new thread, but I had an idea for a snubber that would return the left over energy in the transformer to the filter capacitor. Do you think this will work?
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Super effcient Fly-back driver, over 55KV, single mosfet, direct 220V
I'm pretty sure you can get away with a rectifier sized for the input voltage since the spikes are clamped and have relatively little energy, but you may want to hear this from someone else, since I'm actually in doubt here.