Transistor selection on a two switch forward converter

[neonwarrior]

Hello everyone,

How do you select a switching transistor?



http://www.irf.com/product-info/datasheets/data/irg4pc40w.pdf

The datasheet says 20 Amps continuous collector current but the converter is pulsing aprox 20 Amps constant with duty cycle 50% so is it suitable??
Or if I use figure 1 from datasheet and using the triangular waveform and frequency = 40 kHz it says 20 Amps rms so with Duty cycle 50% is I peak = Irms/sqrt(0.5/3) = 48 Amps peak.
But it is not a pure triangular wave with initial zero current as stated in datasheet. How can I recalculate the values to match the attached figure waveform.
If I use the square waveform gives 12 Amps rms @ f = 40 kHz and a peak current = Irms/sqrt(0.5)= 17 Amps but actually the current is not constant.

Thanks in advance

 

[tgootee]

I did not read the datasheet, but if the transistor is good for 20 Amps continuous, then 20 Amps with 50% duty cycle should be fine.

For switching applications like this, you want low Rdson, so that when it is conducting, the dissipation is low.

But you also need fast switching time, because any time spent in the linear region (i.e. between on and off) will have high dissipation and heating.

So, even with a suitable transistor, you will want to bang the gate pretty hard, to try to make it switch very quickly.