Snubbers

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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AnGuy
Posts: 208
Joined: Tue Jul 20, 2004 7:49 pm
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Re: Snubbers

Post by AnGuy »

>The antiparallel diode to which you refer is for commutation of the transformer magnetizing current when both switches of the push pull converter are off - it has nothing to do with snubbing

Yes that is true. In my haste, I didn't fully explain the concept. I was trying to offer a method that I used for my switch mode PS. The idea using the combination of Hexfreds was from a IRF Application note article I read a long time ago. By using a set of Hexfreds one can avoid the need for snubber networks. The Hexfred across the Mosfet bypasses the built in intrinsic as the carrier because the Hexfreds have a faster switching time. By using a pair of Hexfreds reverse current is limited which reduces thermal load.

Unfortunately I can't see your circuit diagram, the image link is currently broken. When I experimented using Snubber networks, I found the performance poor and caused additional thermal stress on the Mosfets. You will also need some hefy resistors and caps if your working with a large currents. The amount of current the snubber is going to handle is limited to amount the RC circuit can absorb it. Large R's & C's going to cost a lot more than a pair of Hexfreds (especialy to deal with heat disapation) In RC Snubbers, when the Mosfet turns back on, there is a current loop which just adds thermal load.

>The series diodes for the zener diodes may not be necessary. For zeners try something like 2 X P6KE150 or P6KE200 in series. You could also return the zeners to Vin

This is fine for low current switching. But if we are talking about a switch mode PS for a fusor, thermal run away is guarenteed with these devices.

Finally I would suggest not using Push-Pull circuits for driving high current applications. The higher the frequency the higher the power losses are introduced. In a Flyback circuit, the losses are acceptable but if your building a high current switch mode PS, those losses become significant. Instead,either put more turns on the secondary, reduce the primary turns (if possible), or use a higher input voltage. Avoid using P-Mosfets (all P devices) are inherently less efficient that N-Mosfets. The P-Mosfets are also more expensive than their N-Mosfets counterparts. JMTC.
Richard Hester
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Joined: Thu Jul 05, 2001 12:07 am
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Re: Snubbers

Post by Richard Hester »

Using a Hexfred as an antiparallel diode only gets rid of one problem - the magnetizing current in the MOSFET intrinsic diodes. It will not replace a snubber intended to clamp leakage spikes. The thermal stress caused by turning on the intrinsic diodes has more to do with forcing the recovery of the diodes during the switching time than the actual DC current flow in the diodes.

The snubber schemes shown would be practical for power levels of a hundred watts or so (maybe a couple), provided that the leakage inductance of the transformer are not too large. A properly designed RCD clamp circuit will keep out of the way of the MOSFET drains after the first couple of switching cycles. Adding the diode(s) allows the clamp capacitor to keep some charge between cycles so that the clamp voltage stays above 2X Vin. Simple RC snubbers get in the way and can burn up a lot of power.

A push-pull converter is a much more practical at medium-high power than a flyback, as the switches will run at much lower currents for an equivalent power level. The transformer can also be much smaller, as it is not required to store energy. The best choice for this application would be a resonant half-bridge design, but that would be way, way beyond the scope of this discussion.
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