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?
Best Answer
Some driver circuits dont have UVLO [undervoltage-lockout], and they can come on wrong or oscillate at the beginning of the work cycle. Do a transistor Schmitt-trigger undervoltage-lockout circuit with several independent outputs: one output to keep the driver disabled by applying a shut-down on its input, and other outputs to keep the gates of MOSFETs down to source voltage (careful with the upper transistors gates, they have different source potential and therefore the transistors to lockdown them should be kept on by some voltage shifters, like the voltage shifter in the H-bridge driver inside the integrated circuit...)
The trigger should release the gate lockdowns and the driver shutdown simultaneously, or eventually prioritise the lockdown of the top mosfet by the synchronous diodes lockdown (the top mosfet to not be possible to be "activated" without diodes being "activated") Am sorry but my english may be not very well inteligible here... Try use anyway a safety circuit to not release the gate of transistors (to shunt them to source) for a few miliseconds after the battery is first applied.
A possible help (yes, i suspect more the #two of your guesses) can be use of a coil on the supply wire, several tens of microhenries, plus several thousand microfarads parallel on the supply lines. This will slow the voltage rise on the supply and possible avoid Miller charging of the gates. Calculate the number of microhenries and microfarads to slowdown the rising voltage to 0,5-1V/microsecond and should be fine from this point of view.. But from my opinion, i will apply the previous method, the UVLO circuits..