This question arises from a repair work I'm doing on a friend's plasma TV - a Panasonic TH-P42X30T. The rectifier for the 5V standby supply failed short.
The rectifier is a B3100, a schottky diode rated at 3A & 100V. The service manual I downloaded omits a detailed schematic of the power supply board but at least the load currents of the various voltages are printed on the board. The load on the 5V supply is given as 0.2A.
I did some measurements after replacing the diode. The DC output ia about 7.7V, further regulated down to 5V. My scope shows the negative swing at the diode anode to be slightly less than 40V. The rectifier is used as a half-wave, so the peak reverse voltage on the diode should be around 45V.
At first, I dismissed the defect as one of those repair jobs in which it's difficult/impossible to pinpoint the exact cause of failure. Random part failure, unusual mains spikes, ESD, etc. But then, after poking around on the internet, I came across other cases where the same diode failed.
Now why should a diode keep failing when subjected to less than a tenth of its maximum current rating and half of its PIV rating?