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Full bridge driver design

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Marko

Sat Mar 07 2009, 09:12AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't think you need a fullbridge to drive any realistic flyback. Just take care to heatsink the mosfets well, and to have enough leakage inductance in your circuit (wind the primary on a core leg opposite to the secondary).

Other thing you absolutely need is more primary turns, a flyback core absolutely won't handle up to 20V/turn well and especially not at just 16kHz. For frequency so low, you might need something like 1V/ turn (lots of winding and reduced output voltage).

I would increase frequency to some 50 to 100kHz in any case. I don't see a point to go so low.

Also, you could include some few-ohm gate resistors to reduce ringing on gate waveforms.

Marko

Mads Barnkob

Sat Mar 07 2009, 09:32AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Hey Marko

I know a full bridge is overkill, but its just as much to build a full bridge before I start making myself a big lab smps.

Its not mains in, but 24VAC, so 15-20 turns should be fine, its running around 16kHz to match the frequency of NTSC as its an american flyback its build for. I would have to build some other driver to experiment to find its other resonant frequencies then.

As I said myself, I forgot to add the gate resistors :)

Myke

Sat Mar 07 2009, 09:01PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Shouldn't the phasing on A and D match? Same with B and C. Otherwise I think that the two positive side MOSFETs will turn on at the same time. Same with the negative side MOSFETs.

uzzors2k

Sun Mar 08 2009, 06:39PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Myke wrote ...

Shouldn't the phasing on A and D match? Same with B and C. Otherwise I think that the two positive side MOSFETs will turn on at the same time. Same with the negative side MOSFETs.

Roger that.

Dr. Dark Current

Sun Mar 08 2009, 07:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

MadsKaizer wrote ...

Its not mains in, but 24VAC, so 15-20 turns should be fine, its running around 16kHz to match the frequency of NTSC as its an american flyback its build for.

Well, you are already driving it in a completely different way than in the TV, why not set optimal running frequency then.

And yes, fullbridge is overkill for a single TV transformer. Myself I prefer to build a half-bridge with two powerful devices over a full-bridge with slightly weaker devices (so the performance is the same in both cases), mainly because driving is easier and the cost of the transistors is often lower.

Mads Barnkob

Mon Mar 09 2009, 05:57PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I will change phasings, add gate resistors and variable frequency so its not only build for one flyback, also I have seemed to misunderstand something about flybacks optimal frequency :)

About how fast can I run with the MAX4420s before they get too weak? They can deliver 2500pF at 30nS rise/fall.

so thats somewhere around 1/0,00000006 ~ 16,6MHz? sounds rather high...

Dr. Dark Current

Mon Mar 09 2009, 07:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

MadsKaizer wrote ...

I will change phasings, add gate resistors and variable frequency so its not only build for one flyback, also I have seemed to misunderstand something about flybacks optimal frequency :)

About how fast can I run with the MAX4420s before they get too weak? They can deliver 2500pF at 30nS rise/fall.

so thats somewhere around 1/0,00000006 ~ 16,6MHz? sounds rather high...

Your limit would probably be thermal dissipation, which I would guess to be in the several hundreds of kHz range...

Anyway, you want to drive that transformer at ca. 30-60kHz, and pay attention not to excite sub-harmonic resonances of the transformer as this would destroy it/or your driver/. Do a test with a light-bulb in series with the power input and set the running frequency so that the bulb is dim or completely goes out with open-circuit secondary...

MRacerxdl

Wed Mar 11 2009, 01:09AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

for a Fullbridge of IRFP460, for the 4420's its arround 400Khz the maximum frequency to the heat dissipation of mosfet drivers, more than that, it will heat up much, and wont take longer to die.
Tested by me =D

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