High current transistor switching and sequencing circuit

[cdw3423]

I want a circuit that will sequence through 4 pairs of transistors turning on one pair at a time. It is critical that only one pair be on at a time. Also they must pass 30 amps at about 15 volts. Since they will be rapidly sequenced (a few hundred times a second or faster), each transistor should only see an average of 7.5 amps max.

If someone could suggest a specific transistor and how large of a heat sink I need as well as get me started on a circuit that will sequence the pairs of transistors, I would be grateful.

Chris W

 

[colin55]

Consider FETs

 

[simonbramble]

Firstly the FETs need to be rated to at least 15V and 30A. You cannot get away with a 15A FET. I would use a micro to do the sequencing, but since they only have a low output current, they might not drive the MOSFET on with the speed that you need to get the switching losses down. Therefore consider using a totem pole driver:
http://en.wikipedia.org/wiki/Push%E2%80%93pull_output

If you dont fancy using a micro, a decade counter will work. Look at this link (and play the sequence - it is pretty to look at if nothing else):
**broken link removed**

 

[cdw3423]

Will the 4017 supply enough current to drive the fet? And would an IRL3714ZPBF be suitable?

 

[colin55]

You don't need CURRENT to drive a FET. A 4017 will deliver 10mA.

 

[alec_t]

You don't need CURRENT to drive a FET

Actually you do, to charge/discharge the gate capacitance. That's why driver ICs were invented!

And would an IRL3714ZPBF be suitable?

It's only rated at 20V, 36A so would be marginal. I'd go for a higher rated one to be on the safe side. The price difference isn't huge.

 

[colin55]

You only need "drive" to a FET when ooperating at high frequencies. He is operating at a few Hz. You don't need current in THIS situation.

 

[simonbramble]

I've just done the calcs. With a 2000pF gate source capacitance, driven from a 10mA source this should give a rise time of 5V/us. Passing 30A with 5V across the FET give a rough power instantaneous power dissipation of 75W, but if this power is present for 1us and his period is 1 second, the thermal lag of the FET should absorb this. Therefore you should not need a FET driver

 

[ronv]

Here is a heat-sink I use quite a bit because it's cheap (like me). I think with this one you could use your original FET with like a 20C rise in temperature. That FET has a really low gate charge so it can be switched with low current.
I'm like alec, I hate to switch them slow.

https://www.mouser.com/ProductDetail/Aavid-Thermalloy/504222B00000G/?qs=XF/z8BtdrptQos6B0WOfeg==