Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Why is my homebrew controller blowing mosfets? [Solved]

electrowizard · 07-08-2010, 01:03 PM

I'm working on a homebrew controller inspired by P+S's controller.

However my controller keeps blowing mosfets and I do not know why!

I'm currently testing at 12V with a 1999 sunfire starter as the load.

mosfet digikey part # IRLS4030TRL7PPCT-ND
[not allowed to hyperlink yet]

The failure mode for my mosfets is shorted source-drain, which is apparently the common failure. Earlier tests shorted source-gate-drain but that hasn't happened recently.

I know of two reasons why this happens:
1. Vgs is too high and you blow the gate insulation.
2. Id is too high and the transistor overheats

When switching the gate without load (i.e. main battery disconnected) the gate drive looks good, little overshoot and no ringing. Since I had no TVS and long gate drive leads I thought maybe I was breaking down the insulation when the starter was drawing current (Vgs_max on these mosfets is +-16V, and I am driving 0 to +12)

Last night I added a TVS with as short a loop as possible from the source to gate lead. I kept the 10 ohm gate resistor. The gate drive was significantly slower; I used to have 10ns rise/fall times, now I have 600ns rise/fall. I am thinking the TVS must have a large capacitance to drive, though I am not sure why it would be so massive.

Anyways I threw the scope on peak detect and was looking at Vgs directly at the transistor. (using standard scope ground clip, not HF clip - problem?)

I video taped the scope since from experience the mosfet dies after only 2-3 seconds; today was no different.
However I did confirm that the scope doesn't see a single spike over 12.2V.

So I should not be exceeding Vgs_max.

And the current spec: according to the datasheet you can pulse up to 700A with a small (<100us) pulse. My starter draws 380A, and my pulses were only 2us wide at 16kHz. The mosfet was cold to the touch when it died silently 2 seconds later, and the starter drove to full speed until I pulled the battery wire.

Can anyone suggest reasons why my mosfet may be failing? I'm all out of ideas

For more information on what I've done so far, please visit will-ev.blogspot.com

07-08-2010, 01:03 PM	#1 (permalink)
electrowizard EcoModding Lurker Join Date: Jun 2010 Location: Fort Erie, Canada Posts: 19 Thanks: 1 Thanked 1 Time in 1 Post	Why is my homebrew controller blowing mosfets? [Solved] I'm working on a homebrew controller inspired by P+S's controller. However my controller keeps blowing mosfets and I do not know why! I'm currently testing at 12V with a 1999 sunfire starter as the load. mosfet digikey part # IRLS4030TRL7PPCT-ND [not allowed to hyperlink yet] The failure mode for my mosfets is shorted source-drain, which is apparently the common failure. Earlier tests shorted source-gate-drain but that hasn't happened recently. I know of two reasons why this happens: 1. Vgs is too high and you blow the gate insulation. 2. Id is too high and the transistor overheats When switching the gate without load (i.e. main battery disconnected) the gate drive looks good, little overshoot and no ringing. Since I had no TVS and long gate drive leads I thought maybe I was breaking down the insulation when the starter was drawing current (Vgs_max on these mosfets is +-16V, and I am driving 0 to +12) Last night I added a TVS with as short a loop as possible from the source to gate lead. I kept the 10 ohm gate resistor. The gate drive was significantly slower; I used to have 10ns rise/fall times, now I have 600ns rise/fall. I am thinking the TVS must have a large capacitance to drive, though I am not sure why it would be so massive. Anyways I threw the scope on peak detect and was looking at Vgs directly at the transistor. (using standard scope ground clip, not HF clip - problem?) I video taped the scope since from experience the mosfet dies after only 2-3 seconds; today was no different. However I did confirm that the scope doesn't see a single spike over 12.2V. So I should not be exceeding Vgs_max. And the current spec: according to the datasheet you can pulse up to 700A with a small (<100us) pulse. My starter draws 380A, and my pulses were only 2us wide at 16kHz. The mosfet was cold to the touch when it died silently 2 seconds later, and the starter drove to full speed until I pulled the battery wire. Can anyone suggest reasons why my mosfet may be failing? I'm all out of ideas For more information on what I've done so far, please visit will-ev.blogspot.com Last edited by electrowizard; 09-12-2010 at 01:52 PM.. Reason: [Solved]