Core Winding Trouble

[ctcorp]

I am starting to build my very first PWM PSU, and found an nice
big toroid core in an old PC PSU and I am having some trouble to achieve the correct number of turns to make it work correctly.
Tryed 7 turns, but with 5 amps running on it, it almost catches on fire (both core and mosfets(irf3205)).

I'm using 25khz frequency with an SG3525 + 2x TC4422 + 2x IRF3205.

Efficiency is alwais under 70% at any load.

So my doubt is: should I wind more turns? how much more?
I believe that this core should work nice and cool with 300w+ power, am I wrong?

plz help.

tkz in advance and sorry for my bad english.

[MartyM]

Hi ctcorp-

I noticed there is some information missing in your post:

1. Desired input & output voltages & number of secondary turns on your transformer
2. Wire you are using on the transformer
3. The load you are using when you have that problem (excessive heat)
4. Waveforms on the MOSFET gates, output voltage (under load), etc.
5. New or used parts? (transistors)

Have you viewed the MOSFETs with an oscilloscope? An oscilloscope is pretty much essential for building your own SMPS.

You are probably best off to begin with ensuring the the SMPS is stable, runs cool with no load and has clean gate drive waveforms, then load testing it. Also a schematic would be helpful.

When I built my first SMPS prototype (car amplifier), I verified full stability before load testing to near 300W, then I was ok.

Ciao

Note: I currently use a toroid close to the size of yours, and turns ratio of 4:11 for 12V:32V+/- inverting SMPS

[wrenchone]

CT corp. If you found the core you're trying to use in the secondary circuit of a PC power supply it is almost certai to be a powdered iron core and totally unsuitable for use as a transformer core due to low permeability and hig losses in your application.

[ctcorp]

well, tkz for the help, the core is actually from the filter stage of an old PC PSU.

the input voltage was 10V, the out was 19V

the turn ratio was 1:2, 7+7 on the primary and 14+14 on the sec
I used 22 awg wires ( 4x on the prim and 2x on the sec)

I was testing it with no more than 20w, and the core and mosfets start to heat a lot and very fast also.
the parts are all brand new.

I think it has an tremendous overshoot but I have no Oscilloscope available now to test it to be shure.

[Eva]

Wrong core material. Must be power ferrite.

[ctcorp]

tks a lot everyone, I will try to use some EI-33 core, and see what happens. seeya