LT4356 fried by minor overvoltage

Hi, 

we have an issue with a LT4356 getting fried during in-house lab testing. Its voltage clamp is set to about 39 V, and during static overvoltage conditions (tested by providing a continuous 45 V to the input), the IC malfunctions in a way that it stops driving the GATE above the OUT voltage. It is not a temporary condition, since after power cycle the condition stays the same for normal voltages (35 V and below). 

See schematic:

For testing purposes, TVS diode D40 was removed to prevent clamping the input voltage. Output current was minimal, cca 20 mA, during the whole test. The input voltage was increased from 35 V up to 45 V, when it stopped working. Gate voltage on Q4 was cca 12 V above the OUT voltage during the test, until it stopped working at 45 V input. Note that the output current wasn't changed at all during the whole test.

With the fried board, I tried the following: decouple LT4356 gate driver from transistors, and provide a fixed 35 V on its OUT pin (which then propagates to Vcc and SNS). The gate voltage still remains almost the same as OUT, doesn't rise above it. Is it possible that the internal schottky diode that protects the GATE pin got fused? What could cause this?

  • Hi,  Please note that with a 39V output clamp, the gate voltage will be pulled low (to roughly a MOSFET threshold above output) to regulate the output when it reaches ~39V, i.e., don't expect 12V gate drive above ~39V. Here are some possibilities or things to check:

    - Check if the MOSFET is damaged.

    - Maybe there was a positive transient on the drain of Q4 or LT4356 VCC that exceeded its 100V absolute maximum rating since D40 was removed. Depending on inductance back to supply, Q4 drain spikes up whenever Q4 shuts off while current is flowing.

    - Maybe the internal GATE to OUT clamp was damaged while probing GATE pin, especially if GATE was pulled sharply below OUT.

    For further questions specific to your application, please submit a Technical Support Request: https://form.analog.com/form_pages/support/integrated/techsupport.aspx

  • Hi, 

    thanks for the feedback. I will solder a new MOSFET to check that first, and then a new LT4356. Will report test results here.

  • To follow up on this issue, maybe it will be useful to someone else also: LT4356 was replaced with a new one which worked normally (so it was LT4356 that got fried in first test). The cause of issue was likely the combination of removed TVS diodes and supply inductance. This was checked by soldering a TVS with higher breakdown voltage (cca 50 V) and running the test again, by applying a continuous 50 V to the system. Everything survived this time, and the test was repeated multiple times without issues.