DC-DC converter issue

[JJMI]

A few days ago, the MOSFET (an IRF1407) blew. I saw its temperature was up near 260 F. Oops. It was driving a home-made transformer (a toroid inductor with another wind for the secondary) that has a resistance of .005 ohms, while the IRF1407 has a RDS(on) of .0078. The question I have is, did the MOSFET overheat because its RDS(on) is higher than that of the coil? And if so, if another device is used (probably 4 IRF2804's paralelled) would the low RDS(on) of the MOSFETS improve efficiency and halt future MOSFET burn-ups?

 

[zeitghost]

What frequency were you driving it at?

Did you give it enough drive to turn it on fully?

Remember that the gate has considerable capacitance and the miller effect will make it even more considerable during switching.

Dedicated gate drivers can source/sink up to an amp to give speedy turn on/turn off.

What's your heatsinking like?

You can parallel as many fets as you like but it probly won't get any better unless you address the gate charge/discharge.

 

[cbarn24050]

Hi, "home made transformer" big clue there. Toroids are very easy to saturate. Make sure your pwm control monitors the primary current. You could also have voltage problems as well, scope your fet drain. During the off period, the transformer flux has to be removed from the core, this is usually done with a demagnatising winding for a forward converter.

 

[JJMI]

I am using a NJM2360D, switching at around 98kHz, with a 1.5a internal transistor to drive the MOSFETs. When it was working with a 5-watt 680 ohm resistor, the MOSFET was down at 80 F with only the 6x4 inch case as the heatsink. A 200 ohm resistor is between the gate and ground (a resistor is required for operation). Is 200 ohms too less for this value (since the MOSFET may not be turned on all of the way)?

Thank You,
JJMI

 

[JJMI]

Oh, I seem to have answered my own question. I have just tested the transformer with a wire of .0078 ohms, pulse shorting it by hand, causing an AC voltage to be produced on the secondary; the wire became extremely hot, while the transformer remained cool. I thank all whom have E-mailed me back for their support.

Thanks again,
JJMI