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LM5005: Damage case near pins 3 & 4 on failure. Free-wheeling diode shorted.

Part Number: LM5005

Industrial application. Feed with 48VDC from switching power supply (90 to 240VAC in), 12v output 0.9A, about 300KHZ. Free-wheeling diode rated 60V at 2A. Circuit output feeds two regulators of 3.3V and 5V. When the circuit fails usually chip case damage by pins 3 & 4 and diode is shorted (B360A-F) attached 5 & 3 volt regulators also damaged. Replacing the damaged regulators and diode, circuit works as normal. I have some twenty boards that have failed. This only happens in China and India running 220VAC and not Europe and elsewhere. No 120VAC models to date. Any suggestions? Thanks Mike

42422U.pdf

  • Hi Mike,

    Comparing with webench design from the schematic i can see the differences is that the output capacitor is 22uF vs 3x10uF. This might be ok. 

    Also in general the value of C110 is normally much smaller than the C109 meaning that the value should be in pF normally. I have attached the webench design here

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=520A7329FCEA689E

    But i think what could happen in your design is somekind of overvoltage condition at the VIN pin. SInce your input is 48V, the worst case transient for ringing normally is 2x48V = 96V. Therefore this might happen and damage the IC. Hence you will see damage in the IC and diode also. 

    To get these type of transient is normally happen when you have some kind of wire (inductance) between the 48V supply and you hot-swap the IC (ex: plug in to a connector). The ringing on this hot-swapping events could reach a damaging voltage of the IC. 

    One way to mitigate this is to add a TVS diode to protect the input of the IC or maybe implement some soft-start function on the 48V supply

    http://www.vishay.com/docs/87606/87606.pdf

    Ideally, you would need a converter with 80V abs max such that it aligns with the TVS max clamping voltage. 

    Thanks and i hope that helps

    -Arief