# Cause of shoot-through in H-bridge

I've designed an H-bridge for a 15A DC motor. Power is applied by plugging in a very high-capacity Lithium battery. Unfortunately, when I plug in the battery, one of the transistors goes pop instantly and shorts across itself. This ultimately shorts the battery to itself which makes me unhappy. V_BATT is a 14.8V Lithium battery. The transistors are N-channel MOSFETS. U1 is an H-bridge driver IC.

I've come up with two possible root causes. I'd like some help figuring out which one is more likely.

1) A nasty inductive spike is being generated during the initial current surge due to the natural inductance of the battery cables and copper traces. The spike exceeds the Vds of the FETs, causing them to fail. The Vds of the FETs I'm using is 40V, so I find it likely the inductive spike is exceeding that. I plan to use a snubber and voltage clamp circuit to reduce the spike.

Or,

2) The sudden rise of voltage on the drain of the FETs causes the gate voltage to also rise due to the intrinsic capacitance between the drain and the gate. This effectively turns on all of the FETs and causes shoot-through and burns them out.

I'm leaning towards #1 being the main culprit, but I can't justify why #2 isn't also causing a problem. Especially since I have 50$\Omega$ resistors on each gate pin. Even if the gate drive pins of U1 are trying to hold the pins low, the resistors would isolate the gate pins for a short time.

Is it likely the problem is a combination of both items? Or is #2 not really an issue? If #2 is actually happening, what can be done to suppress it?

• What have you got driving your 4 phase inputs? – WhatRoughBeast May 14 '15 at 0:50
• Directly from a uC. – Dan Laks May 14 '15 at 0:53
• Have you watched on a scope, what happens to those uc outputs when you plug in the battery? – Some Hardware Guy May 14 '15 at 0:57
• No, but I know what you're getting at. In my first attempt to make a "quick fix" on the board, I isolated the h-bridge power from the uC/driver using separate batteries (with common grounds, of course) . I plugged in the uC/driver's battery first and then the h-bridge battery. Therefore, the uC and the driver IC were fully powered on and stable before I applied power to the h-bridge. With that in mind, I don't think it's a power-on artifact of the uC or the driver. – Dan Laks May 14 '15 at 1:02
• Try powering the mcu separately with the phase inputs in a good state then plug in the battery – Some Hardware Guy May 14 '15 at 1:03