High current transistor switching and sequencing circuit

C

cdw3423

New Member

• Jan 4, 2012

• #1

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

 

C

colin55

Well-Known Member

• Jan 4, 2012

• #2

Consider FETs

 

S

simonbramble

Active Member

• Jan 4, 2012

• #3

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**

 

C

cdw3423

New Member

• Jan 4, 2012

• #4

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

 

C

colin55

Well-Known Member

• Jan 4, 2012

• #5

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

 

alec_t

Well-Known Member

Most Helpful Member

• Jan 5, 2012

• #6

You don't need CURRENT to drive a FET

Click to expand...

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

And would an IRL3714ZPBF be suitable?

Click to expand...

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.

 

Last edited: Jan 5, 2012

C

colin55

Well-Known Member

• Jan 5, 2012

• #7

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.

 

S

simonbramble

Active Member

• Jan 5, 2012

• #8

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

Well-Known Member

Most Helpful Member

• Jan 6, 2012

• #9

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==