Thyristor current ratings?

Hello,

I'm trying to understand the difference between current rating on a thyristor datasheet.
Continuous on-state current = 2950 A AND
Average on-state current (half sine wave) = 1500 A

Seems like a large difference in current rating. Is this because the half sign wave will have a large peak current??

That is because on average half sine wave, under maximum conditions the current would peak up to maximum peak current.

Nb: you misread the datasheet. It says Continuous on-state current = 2580, not 2950A (that's RMS).

[edit]
The uploaded figure should tell it all

But the data sheet states the maximum peak current as 23000A no?
Or is that maybe peak current with a long duty cycle?

The difference in average versus RMS currents is that the average is at 55C heatsink temperature and the RMS is at 25C heatsink temperature.

It plainly states that the peak current of 23000A is for a peak one-cycle surge.

You need to read the whole data sheet, not just bits and pieces.

Hi,

As Carl pointed out the temperature has a lot to do with it too. One rating is at one temperature, and the other rating is at another temperature. That's a little strange but if you make a note of the temperature difference PLUS the fact that one rating is rms and the other is average, you find that the ratings are comparable. We can go into the math behind this if you would like to see how this works. Basically you would convert one rating into the other and then scale for temperature. You should come up with comparable ratings.

Thank you both.

This makes sence now - Carl you were quite right after looking at the graphs it made perfect sence! I spend so much time on here telling people to check the datasheet properly, it was always bound to come back and bite me in the ass!

So now i have another question to which i'm sure there is a obvious answer but i'll ask anyway:
I have an inductor with a current flow of 1500A, the supply suddenly stops producing an overvoltage at the inductor. To prevent this the thyristor is triggered and the inductor current discharges via the thyristor (there is a reason this flyback circuit has to be triggered)
Anyway this gives me a waveform with a peak current of 1500A that decays with a 22ms time constant. So should i compair the average current of this waveform against the sine-wave maximum current or the square wave maximum current. And over what timespan should i avarage my waveform.

Edit - after a bit of thinking, and several "ahh now i get it" moments i belive i should be averaging my current over 3 time constants and compair this against the DC value (which i can now see is the lowest current rating)

Hi again,

You only have to average over one complete cycle period. One period is anywhere the current or voltage starts out at one voltage and ends at the same voltage after going through a shape that is repeated. For example, if it goes from 0 to 1000A and then down to 500A and then back down to 0 and then repeats that same shape over and over, one cycle is from 0A to the next 0A point. You only have to average over any one of those cycles to get the average of the total wave.