Hi to all my fans,

just completed repairs on one of these:

http://www.sound-power.ru/files/doc/..._datasheet.pdf

Found it fitted inside a mini-size bass instrument amplifier, which the owner had just purchased over the net and failed on him at first power up.

Smelling a rat, I checked the AC switch on the rear panel first - blow me down, it was set to 120VAC and we live in a 240V country.

Reset the switch, then it drew zero AC current - no surprise.

Replaced the blown 5AT fuse in the IcePower module and then the PSU began hiccupping - drawing an amp or two very briefly then shutting off every few seconds.

IME feeding double AC supply voltage to a unit with a SMPS is almost always fatal, sure the AC fuse blows but semis in the HV part of the circuit fail in a fraction of a second. But these must be OK this time because the PSU was hiccupping - ie working but sensing an overload so shutting down.

Soon enough, multimeter testing showed two small power mosfets on the output side of the SMPS tranny were shorted while two others seemed OK. But what the heck are mosfets doing there ?

Well, it's the fist time I have come across seen a *synchronous rectifier* in such an unit. IcePower had certainly gone all out to reduce losses and heat in this module by using a synchronous bridge.

The particular TO-252 fets here are made by ON, number FDD86110 rated at 100V and 8mohms on resistance.

Removing the duds cleared all shorts and after fitting some new ones ( delivered to me by Element14, from their Singapore warehouse ) I gingerly Variaced the unit up from zero.

No hiccupping this time and it began to run normally - after which it passed all my usual bench testing, no problems.

FYI:

AC current draw was 120mA at idle, 2.4A at full power (340W rms/4ohms)

No PFC and inrush surges were about 25A for a few milliseconds.

There was a residual sine wave signal on the audio output of about 1V at 500kHz - no biggie except it makes THD testing a right PITA.

I find it odd the way the SMPS failed, just two mosfets in the secondary rectifier, nothing else. For a brief time, the DC supply to the switching fets must have been nearly double voltage.

Seems the two rectifier fets failed SHORT instantly and protected the rest of the circuitry - remember the power supply is NOT regulated, just a square wave inverter running at 100kHz.

I also feel that supplying an expensive amplifier to a buyer living in a 240V country with the AC inlet set to 120V and giving NO warning is criminal. BTW the amp was fitted with a regular IEC inlet which most of the world uses for 240VAC.

Any comments?



.... Phil