PWM Failure Analysis on MuRata PS Converter

Gideon Analytical Labs received pulse width modulators PWM failing in a MuRata Ps DC-DC converter. The PWM was removed from the encapsulation and tested. Several I/O pins were either open or showed no connection to the circuitry of the IC. The PWM CS51022AG is a fixed frequency current mode controller family provides the necessary features for DC−DC primary side control. In addition to low startup current (75A) and high frequency operation capability, the CS51022A includes over-voltage and under-voltage monitoring, externally programmable dual threshold over-current protection, current sense leading edge blanking, current slope compensation, accurate duty cycle control and an externally available 5.0 V reference. The CS51022A has a sleep mode with 100 A maximum IC current consumption. The CS51022A is in a 16 lead narrow body SOIC package.

The thermal crack was caused by the die receiving heat faster (current from pins 14, 15, and 16) than the silicon’s ability to dissipate it. It appears pin 14 (Vcc) received a current surge (a crater in the bond pad) and melted the gold wire. This, in turn, shorted the ground (pin 15 and the carbon ground rail) and the output power stage (pin 16). Because the gate is connected to the pin 16 circuitry, the gate on the MOSFET experienced this overcurrent as well. Overvoltage will not melt the wire. According to the literature, this wire will handle about 1.2 amps. Since the wires are melted, the PWM received a current excess of 1.2 amps. The current was coming from a transformer with the improper gauge wire for the winding.

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