Electronics > Repair

Snubber circuit in TTi PSU overheating

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g0rsq:
I have been given a nice TTI EX354 PSU which will make a nice addition to my test equipment if I can repair it.
It is a 300W SMPSU and effectively working, but the snubber resistors burn out after only a few seconds (30) even when no load applied. The snubber resistor is 680 Ohms 4 watt.

To test the PSU I run it through a 60 watt light bulb (in series), to limit the current, and the bulb is quite bright, so the circuit is drawing too much current also. The snubber resistors get very hot also!

I have tested all the circuit around the snubbers, and the switching mosfets, and everything seems normal.

I would like to eliminate the secondary components (they contain MagAmps) to ensure they are not loading the primary.

Is it safe to disconnect the secondary windings in this SMPSU? There is no feedback control from the secondary to the switching control.

Thanks

Andy Watson:
If anything, I would expect un-loading the secondary would leave more energy for the snubbers to dissipate. Have you checked that C17 and C101 are blocking D.C. ? 

T3sl4co1l:
C17, C101 maybe are cooked, and thus delivering much more snubber power?  (Or DC, as Andy suggests?)

Not really any obvious operational failure that should cause that, otherwise.  Slower switching speed or partially failed switches/diodes should cause greater dissipation in those components, and less if anything in the snubber.

If you need a replacement for C17/C101, I would suggest a 1n, >= 400V, C0G ceramic (disc or MLCC) or snubber-type polypropylene film capacitor.  Avoid X7R and other type II ceramics, and polyester and other cheaper film or other types that have higher losses.

Tim

wraper:
Unlikely that anything other than faulty D8 or C17/C101 could cause this. But if they are ok, check D2-D5 too.
BTW, If you do not desolder D8 (or resistors in parallel), obviously you cannot check if it is open.

TheBay:
TTI will send you via email more in depth schematics if you ask them nicely  :-+

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