Switch Mode PSU gurus - a little advice needed on a repair - Page 1
Forum section: electronics / repair

author: linker3000
author_rank: Contributor
timestamp: February 20, 2016, 04:18:56 pm
content: I am repairing a small 12V 3A switch mode PSU board from an LED DMX stage light. The PSU vaporized the 1A 20mm fuse in the IEC power socket on the unit with a 'pop'. So far I have replaced a blown rectifier diode (1N4007) and a suspect 10uF 25V electrolytic capacitor. The power MOSFET is also shorted and I will replace the HV capacitor as it's a Nichicon knock-off + the two electrolytics on the low voltage side 'just because'!The board uses a Chip-Rail CR6853T PWM controller IC and I wondered whether anyone can advise from experience on whether this chip is robust, whether it's often taken-out too when things go wrong or whether they are notorious for going wrong!?In any event, the chip is not expensive so I may get in a few anyway. Under normal circumstances, I probably would have just replaced the board, but I am using this repair as an exercise as this is the first time I have come across the CR6853T.Pic shows the board waiting for a MOSFET and HV cap replacement to be fitted. The board supppliers didn't bother fitting any connectors, plus there's room top right for a power indicator LED and resistor.Supplementary question: If anyone can suggest a reliable, drop-in replacement for the whole board, do let me know. The board is 90mm x 45mm.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2016, 09:54:46 pm
content: Transistors typically fail all-ways, so, you likely had a sizable fraction of +V (160 or 320VDC) on the gate pin. Which means that controller is very likely a burnt husk inside, and maybe the gate resistor too.Unless this plugs into a shielded enclosure with filtered mains supply, it's going to be shitting out RF interference to no end. I'd recommend dumping the whole thing (you're practically replacing half the parts, anyway..) and buying a good quality e.g. Meanwell or CUI or TDK or ... power supply brick, with similar ratings. And doing that for all of them, not just this one...Tim

author: PChi
author_rank: Frequent Contributor
timestamp: February 21, 2016, 11:45:30 am
content: I agree with T3sl4co1L it's junk. There doesn't seem to be any EMC filter on the mains input so will spew out interference and any transients on the mains supply have a good chance of blowing up the transistor and associated components. I replaced many parts on a numbers of rubbish switch mode power supplies blown up because they didn't include EMC filters a long time ago.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 21, 2016, 02:59:32 pm
content: On top of the other comments: never replace just one of the rectifier diodes.Look at the diagram of a bridge rectifier and you know why.

author: station240
author_rank: Unknown
timestamp: February 21, 2016, 06:26:32 pm
content: The usual way to drive a mosfet from an IC is a simply low value resistor. If the gate in the mosfet shorts to one of the other pins, then the chip has no chance of survival.

author: linker3000
author_rank: Contributor
timestamp: February 22, 2016, 12:29:29 pm
content: Totally agree with all the comments; it's a rubbish design. The IEC inlet socket is a filtered one and I could stick a MOV across the feed to the board, but as for RF shielding - um yeah, it's a crappy plastic RGB DMX LED PAR.Perhaps it's not worth the fun value of resurrecting it; and the PLL chip either has to be sourced (I'm in the UK) in quantities of 10 from general suppliers in Far East, or I can get 'one for the cost of ten' from a UK supplier that is probably sourcing them from the same Far East resellers.There's enough room to fit a decent 12V power brick or board inside the casing anyway.

author: linker3000
author_rank: Contributor
timestamp: February 22, 2016, 09:46:58 pm
content: Just a footnote on this DMX RGB LED PAR in case someone comes across this post by searching; the unit has IEC in and out (male/female) power connectors so that they can be used in a daisy chain, BUT the internal wiring in my unit does not link the earth/ground pins between connectors, so it breaks the continuity of the safety conductor. One more thing to fix.