BD234C surge power rating - Page 1
Forum section: electronics / projects

author: pulse
author_rank: Contributor
timestamp: January 21, 2017, 04:02:32 pm
content: Hi!I'm wondering if transistors have any overload rating for power? For example: continuous current is 6A, peak current is 10A - how about peak power - or should I really avoid that? I have a circuit that needs to be shorted for 1 second max (ideally less) and when shorted the power on transistor is about 2-3 times its power rating - is this acceptable for short duration or can I expect the transistor to short itself and blow up?Thanks for your help!

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2017, 04:34:22 pm
content: It's in the datasheet, called "Safe Operating Area" or SOA.Analyzing whether your design stays under this curve under all dynamic conditions is essential for reliability.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2017, 05:34:32 pm
content: Bipolar transistor datasheets are often 'short form', without stuff like pulse SOA curves. I found theOnSemi BD234Gdatasheet had the required curve and it *may* be similar enough to your BD234C to indicate if you have a problem. However a small difference in the collector tab of the leadframe's thickness could result in a relatively large change in the device's pulsed thermal characteristics so there's no substitute for having the actual specific manufacturer's datasheet for the part in front of you.The BD234G's 5ms pulse SOA loadline (the longest pulse shown) represents a max power of only 42W, 68% more than the DC max rating of 25W, and that's at an initial temperature of 25 deg C derated linearly to zero at 150 deg C, so you haven't got a hope in hell of dissipating 50W for 1 second non-destructively at a realistic temperature for a transistor mounted inside equipment.

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2017, 07:10:01 pm
content: QuoteTrue.My only reaction to that: I find a different supplier with proper datasheets. The abbreviated datasheets usually come from "me too"/copy manufacturers anyway.

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2017, 07:18:55 pm
content: BTW: the OP mentions BD234: a much better and more rugged transistor in the same package is 2N5194. Try comparing the SOA curves.(go towww.onsemi.com)

author: SeanB
author_rank: Super Contributor
timestamp: January 21, 2017, 07:41:41 pm
content: 50W for 1 second is effectively steady state. You need either a different transistor, or to put a few in parallel with ballast resistors to get the SOA within limits for DC operation. You will need a heatsink, though if the duty cycle is low enough you can get away with using a thick aluminium block with the transistors screwed to it, so the thermal time constant of the whole unit is more than 1 second, allowing it to absorb the pulse and slowly dissipate it.The cheap UPS manufacturers do this, as they know the battery runtime will be 2-3 minutes at best, so make the heatsink able to handle the load for 3 minutes ( we hope) of operation then have a half hour cooling off time. This makes it smaller, and thus cheaper, and takes up less space than a properly rated unit does.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2017, 10:46:38 pm
content: BJTs can't really draw much more current than their ratings, because there's no more charge carriers left to inject. hFE drops, even while Vce remains high (as opposed to ordinary saturation, where Vce drops, causing hFE to tank as well).You can dissipate tons of power, but you are limited by the hot-spot temperature of the die. If the transistor is "free from 2nd breakdown", then your SOA is square (for as much time as it takes for the die to reach failure temperatures, that is, hence the sloped corner for different pulse durations).If subject to 2nd breakdown, then it's the same thing -- they measured the SOA for a reason -- but you get less power at high voltages, because the high power density (more voltage for the same current = more power per area) causes hot spots.Tim