[SOLVED] SCR / Thyristor sizing

[andrelux]

Hi All,

I am starting to work on a pulsed power supply using SCR's as controlled rectifiers. Basically, my first intention is a to use a simple design: power transformer with dual secondary, 2 SCR's, filtering output choke and 24 VDC battery bank. A good old TCA785 will drive the SCR's.

Over current protection will be performed using a LEM sensor and a pair of OAmps.

I am looking for a SCR able to support 500 amps current during 200 ms and repeat this (forever) every 3 seconds.

if I have understood the SCR data sheet correctly, I can use the 180 RK1 SCR at 2000 amps during 200 ms, giving me a margin factor of 4.

Any comments ?

 

[RCinFLA]

That chart is for non repetitive surge current, like an initial turn on surge.

It boils down to heat transfer from SCR die to outside heat sink.

During a heavy surge you will still have to deal with the voltage drop across SCR. You can have several volts drop at high surge currents.

 

[andrelux]

OK, in my case I have a 500 Amps repetitive surge current with a 200 ms duration and a 3200 ms period.
So, what is the best approach to size the SCR ?

 

[FvM]

For an exact calculation, you would need the SCR's complete thermal data. But you can try a derating of the non-repetitive surge current data. E.g. if the average SCR power dissipation is 50% of the allowed value, considering voltage drop at 500A and actual cooling condidtions, you would keep 50% of the non-repetitive rating. In addition, you should consider the fact, that continuous pulsed operation of power components causes thermal stress and results in a reduced lifetime, also without violating maximum ratings. So keeping a sufficient margin is always a good idea.

 

[andrelux]

Thanks FvM. This is a good indication for me. Does anybody here has some experience about pulsed charger design for lead-acid banks ?