Switching power supply fry

[D. Schouten]

Hi Christopher,


Repairing switchers can be very frustrating. When something in a SMPS is
wrong it will most likely blowup or oscillate immediately before you can
even find out what went wrong.
When servicing and/or repairing SMPS safely, especially offline types, make
sure you use a variac with isolation transformer. This way you can gently
increase the input (line-) voltage with the SMPS connected to it (with a
small output load). Starting a faulty SMPS immediately with full line
voltage applied, may cost you a small fortune on MOSFETs and other specific
components.

>While testing the setup, I accidently fired up the PS with the drive
>attached, and the AC connections, - but not the ground :o
>
>The PS died, but the drive and the sampler are OK. The PS has no power
>at the +5v output and about 8v at the 12v output. I understand switching
>supplies up to the FWR and the caps after it. The rest I haven't figured
>out yet...

Reading the above I assume you can still power up the SMPS without blowing
any fuses? You lost the +5V output and the +12V output has dropped to +8V
(even with a small load, for example 100 Ohms, applied?).
Looking at the previous application (powering a CDROM drive), I guess the
converter topology must be a flyback type. The most important parts in such
a converter are : input rectifier, input buffer cap, ferrite transformer,
switching element (MOSFET), controller IC, output rectfier(s) and output
capacitor(s). To minimize cost and size there are some nice fully integrated
controller/MOSFET chips outhere too nowadays, like the TOPswitch.

Not having the SMPS here on my desk, makes it not easy to troubleshoot it.
But let me try.
Since you still have +8V on the +12V output, and your fuses won't blow, your
controller IC and switch are most likely not damaged. The place were I would
look at first, is on the output (secondary) side. In these types of multi
output SMPS's, usually some kind of post regulation is used for the non fed
back outputs. My first guess would be that the post regulation part on your
+5V output is fried, and is causing a heavy load (low impedance) for the
SMPS so that your +12V output is out of regulation.
If no post regulation is used on your +5V output, maybe the +5V output cap
is damaged which is not a rare thing in cheap SMPS's. This damaged output
cap can cause a short on the +5V output. Output caps in badly designed
SMPS's are often conducting large switching ripple currents to ground
through the internal ESR. This causes the cap to heat up internally which
will greatly affect the expected life time of that cap. So be sure when
replacing an output cap in a SMPS, it's of a low ESR type.

>To speed the repair process I present this question;

With SMPS's, mostly the fastest way to get your stuff working again is to
get an other unit. But when you really want to repair a SMPS yourself, it's
a very challenging piece of electronics. IMO a SMPS in general, is an
underestimated piece of electronics. There's a lot of 'black magic' involved
in the design of sophisticated SMPS's.

I hope the above helped you a bit.

Good luck and be carefull.

Daniel...