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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Electra wrote ...
Ok remember , volts x second product have to be the same for positive and negative. The negative part as you call it, is the on time of the mosfet the rectangular pulse (which comes first). Next you see the tall but narrower positive pulse, which is determined by how fast the flux can decay, so it chooses it own level depending on what's limiting it.
Yes, I assume this is seen to trail off in the secondary pics as an RL time product...
Electra wrote ...
By the looks of your latest scope shots you've clamped this completely with a diode. I guess you are getting away with this because the off time is so long, ordinarily it would cause a d.c bias in the core, but hay if it works.
Normally I wouldn't do this, as "flux walking" inevitably leads saturation and blown components, or poor power supplies that don't live long, but I really needed to verify some things with out the full push-pull complications. After all, this whole project was a wildly psychotic idea I wasn't even sure would work.
Saturation due to flux walking wont be a problem in the final push-pull configuration.
Electra wrote ...
I agree with Uspring, that the positive spike, combined with the low voltage rating is probably what's killing them Fets. Yes think you have it right, push pull is in a way like two forward converters in a mirror image of each other operating alternately.
In push pull your mosfets will see 2x Vin, plus maybe a bit of a spike from any leakage inductance on top, I would have thought choosing 60v or more mosfets if you can, would be reasonable.
I will be looking into the increased Vds of a MOSFET for the reasons you state, but I mainly wanted very low Rds for very high current applications. As we all know Rds creeps up with voltage.
Electra wrote ...
Have also seen diodes placed in parallel to the mosfets, then these can be 'better' diodes than the internal ones, and take some of the heat dissipation off the mosfets. Perhaps look at trying to get big variable power supply for testing the scaled up push pull version, so you can turn it down if it gets too hot, too fast.
Yep, I was hoping to put TVS diodes in parallel both from gate to source and from drain to source. that move heat out of the MOSFET. And as you said the internal diodes are a fabrication artifact and not great.
YEEEEEEEEEEEEEEEOOOOOOOOOOOOOWWWWWWW!!!!!!!!!!! 74.8 V / 3 turns = 25 volts per turn, that's just 2 cores! wait till I get to 5!
Should the diode from Drain to V+ be arranged as in the forward circuit, but with out the tertiary winding?
What I meant was a diode across the primary so that it doesn't conduct when the fet turns on. But that is a terrible idea once you add the second fet. A better idea is to use a diode together with the other half of the primary.
Your primary is center tapped, so the voltages on the side are mirror symmetrical to the center. A positive voltage spike on the one side implies a negative one on the other. So once you put in the second fet, its reverse diode will do the clamping and it will also clamp the positive spike on the other fet. For testing I'd leave out the second fet and just install a reverse diode. You do need a complete primary for this, i.e. 2 turns in total, center tapped.
Where did you put the diode now? What is the tertiary winding? I thought you just had a primary and a secondary.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I had to make a forward converter, thought this is what you meant, by adding a diode?
Otherwise where did you mean it to be?
Im just so relieved I can get to high volts per turn rates, as the math predicted. So, with these 2 cores, a common secondary and two uncommon primaries, im getting an effective ratio of 0.5 to 3 which means 13.4 x 6 = 80, very close to 75 actual.
Ive never seen anyone else use this type of transformer or this mode of operation. So im not sure if I should be so bold as to claim a whole new type of transformer, but this is my chosen schematic diagram.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
this thread is nearing its end, so those of you who want to speak up should. Uspring, im already drafting the full push-pull circuit. so the diodes will be different, and I want others to see my schematic before I etch it. I have to admit, I thought id blow out 20 plus MOSFETs getting this to work, but I only had 4 give up the smoke.
The rough push-pull will be: UC28023DW --> FAN7382 --> MOSFETs --> Transformers
Ill have to get all new Gate drivers, MOSFETs and TVSs from mouser Im looking at TVS's here: MOSFETs Im thinking the max input voltage should be 24 volts or so, plus some noise? so that means at least 48 Vds.
New planar ferrite push-pull thread will start soon.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Hello Patrick,
I think your idea is what's generally known as a "voltage adder". The basic concept has been around for a while. There are particle accelerator designs based on this concept - both with and without ferrite cores - and I've seen various HV power supplies using transformers where the "primaries" are connected in parallel and the "secondaries" are connected in series.
An example is here:
The same kind of thing's also been done (for a pulsed output) using coax cable, e.g. as described in the paper: "High-voltage transmission line transformer based on modern cable technology", available here:
Patrick wrote ...
"...Ive never seen anyone else use this type of transformer or this mode of operation. So im not sure if I should be so bold as to claim a whole new type of transformer, but this is my chosen schematic diagram."
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hey JP ! its been so long.
jpsmith123 wrote ...
Hello Patrick,
I think your idea is what's generally known as a "voltage adder". The basic concept has been around for a while. There are particle accelerator designs based on this concept - both with and without ferrite cores - and I've seen various HV power supplies using transformers where the "primaries" are connected in parallel and the "secondaries" are connected in series.
An example is here:
The same kind of thing's also been done (for a pulsed output) using coax cable, e.g. as described in the paper: "High-voltage transmission line transformer based on modern cable technology", available here:
Patrick wrote ...
"...Ive never seen anyone else use this type of transformer or this mode of operation. So im not sure if I should be so bold as to claim a whole new type of transformer, but this is my chosen schematic diagram."
i looked and looked but couldnt find anything like what i was trying out. so this is a relief others have done it, and it works.
so i think the "voltage adder" does have enormous potential though. (especially knowing what its called now.) load sharing between paralleled FETs, large V/t, small ferrite volumes, cooling efficiencies among other favorable factors. and possibly using this SMPS type with fewer fuel cells, solar cells or battery cells, which may have better packing efficiency, im looking at solid oxide fuel cells specifically (SOFC).
NOTE: paralleling MOSFETs to feed current into one transformer has always worried me. You just solder them together, and hope for current sharing. If one fails, you get load shedding, which ends in failure.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Patrick wrote ...
Hey JP ! its been so long.
I know. I've been busy with some other things, but every once in a while I check the forum to see what's going on and who's doing what, etc.
wrote ...
i looked and looked but couldnt find anything like what i was trying out. so this is a relief others have done it, and it works.
To your credit, you came up with the idea on your own, and then you ran with it.
wrote ...
so i think the "voltage adder" does have enormous potential though. (especially knowing what its called now.) load sharing between paralleled FETs, large V/t, small ferrite volumes, cooling efficiencies among other favorable factors. and possibly using this SMPS type with fewer fuel cells, solar cells or battery cells, which may have better packing efficiency, im looking at solid oxide fuel cells specifically (SOFC).
NOTE: paralleling MOSFETs to feed current into one transformer has always worried me. You just solder them together, and hope for current sharing. If one fails, you get load shedding, which ends in failure.
I see other people coming up with novel applications of the voltage adder concept, so maybe you can too .
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
jpsmith123 wrote ...
To your credit, you came up with the idea on your own, and then you ran with it. I see other people coming up with novel applications of the voltage adder concept, so maybe you can too .
yeah but I needed help from others here on the forum, which im grateful for. in any case im planning to use this tech to avoid the forward and flyback converters and transformers.
the forward and flyback transformers, for their simpilicty are good for a start, but there large size per unit of power and huge inefficiencies have always bothered me. As have the excess heat and killed switches. NOTE: Fiddy does deserve credit for finding a custom maker of flybacks to be re-sold to the rest of us, without magic diodes and pots inside.
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High Voltage Planar Ferrite transformers. ( Intial Experiments )
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