** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.



..... Phil