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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I think the SG3525 should be ok as is for driving a half-bridge, but for a full bridge maybe not. I'm using 2SD882 and 2SB772 complementary BJTs in mine.
Alex1M6 wrote ...
Daedronus wrote ...
A zvs is not controllable. on,off, not much else. To control a resonant SMPS you have to control the input, so you actually have 2 SMPS. A full bridge can be made to adjust the duty cycle on the fly to keep the current within reasonable limits. This is more or less demonstrated in the first video, just before the end.
I have abused this bridge a lot with things like transformers going into saturation, glitched pulses because of EMF, heat etc.. and so far I haven't managed to kill a mosfet.
I got a monster 10kw variac that I'm going to use to gradually test this bridge up to off-line voltage....in a couple of weeks.
Right now the bridge was powered by a 250w trafo with the output at about 70V dc
Edit: The PWM IC is UC3843, not a SG
More edits: Alex, forget about what the PWM IC can output, use gate driver ICs or NPN+PNP buffers (totem poles). Don't drive big mosfets (or gate drive transformers) directly from PWM ICs no mater what they are, the all don't have the drive power to turn on-off a power mosfet fast enough.
Hmm I was going to make a start on this circuit over the week-end so I will keep the idea of using a totem pole and gate drive IC in mind if things start to heat up too much (ie the MOSFET's).
Are there any particular gate drive IC's that you recommend? and are the 2n3094 and 2n3096 complementary NPN and PNP pair good for this sort of thing?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
If you plan to use the SG3525, then a GDT maybe easier to get working than a bjt totem pole. i think fig 11 shows what might be possible with lo gate charge mosfets.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Patrick wrote ...
If you plan to use the SG3525, then a GDT maybe easier to get working than a bjt totem pole. i think fig 11 shows what might be possible with lo gate charge mosfets.
Figure 11 is similar to how it is connected up in the schematic, except in the schematic the resistors go before the gates and there are some 12v zeners protecting the gates.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
the IRFP250 have 2800pf of Ciss. so a properly designed GDT should be able to drive two of these ok, but id use a scope and do the math to be sure. The datasheet suggests .2 amps max average output for the SG3525, but ive often seen large 1.5 amp peak currents coming into/out of my chips for good gate drive.
As long as all these factors are considered, a half or full bridge like Daedronous' work is definatley doable. Also i second his point that the ZVS are limited in many ways. like Vout, and controlability, where as this topology solves those problems.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys
Well SG3525 drives 2 IRFP450's easily, and I'm pretty sure it would drive 4 just as well... though a half bridge is surely more than enough to fry any existing flyback no matter how resillent it is.
I was wondering what can one do after killing all worthy flybacks he had... So I hooked up two PC power supply transformers with their outer 12V secondary ends in parallel (which go up to double at 50kHz so the winding takes about 50V total) and the former primaries in series. I ran this at 100kHz and up to full mains voltage which interestingly didn't make them run as hot as I expected.
They had to be ran with an inductor in series with input to limit the current for the arcs to be drawn. I wasn't satisfied by the size of the arcs with the transformers alone, so I stuck a doubler onto the outputand pulled out some pretty epic arcs.
Ofcourse I had no control of myself after that point so I burned some holes in the table and finally my inductor melted from overcurrent after I pushed the transformers into deep saturation by lowering the frequency and increasing voltage.
If you want to repeat the same but properly make sure to use a sane amount of V/turn on the transformers and use several multiplier stages. An inductor is required on the multiplier output to smooth out the current for the arc to properly form (I tried resistors, but they would cook in seconds at the resistances that seemed to be required). An old fluorescent tube or mercury lamp iron ballast is excelent :)
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Daedronus wrote ...
The internal diode in that flyback was already killed in this video. It outputs AC instead of the normal (doubled?) DC.
The bridge itself is meant for other purposes but it was too fun not to hook it up to a flyback.
I have another video where the same bridge drives a self made secondary with much less turns in the secondary, the output voltage is far less impressive but the damage done to the electrodes is.
As soon as you strike the arc, a siren starts to wail in the background. Nice effect!
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Full bridge flyback transformer driver
