SMPS - Output volage drops to near zero under any load - Page 1
Forum section: electronics / repair

author: xyberlink
author_rank: Contributor
timestamp: August 26, 2022, 02:48:38 pm
content: Hi,I have a failed EDac Power EA11003F-195 SMPS. It is (or was) new old stock - made in 2015, never used, till yesterday. Plugged it in (it powers a Targus USB Dock - ACP71 / 77), worked fine for ~8 hours, then dead.Symptoms....When plugged in, I can measure 19.45v on the output pin, with no load. Apply any load (I tried as little as 0.3A) and the voltage drops to almost nothing.I also noted the following...- Plug in, turn on - no load voltage ~19.45v.- Apply load - voltage drops to near zero.- Turn off - then disconnect load - no load voltage back at ~19.45v.So it would appear the supply is now unable to deliver more than a mA or so of output, such that there is enough in the main filter cap after powering off and then removing load, to return the unloaded output to 19.45v (until the bleed resistor / LED drains the filter cap).I've cracked it open - it's a difficult one to see what's what. No obvious signs of anything blown. Photos attached.As the unloaded output is 19.45v I would assume there is at least some regulation going on.Non-standard connector with 3 pins, but looks like 2 pins + shell are ground, then a single +ve pin.Any advice on what might cause this would be very welcome.Thanks.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: August 26, 2022, 03:04:48 pm
content: The SMPS may run just a few cycles, then shut down.So the 19.45v you are measuring may just be the briefly charged output caps keeping their voltage, if there are no bleed resistors or any other circuitry that would discharge them.Any load would empty these caps pretty quickly.

author: SeanB
author_rank: Super Contributor
timestamp: August 26, 2022, 07:00:16 pm
content: Going to take a stab that C408 on the primary side has gone close to open circuit, as this is a typical failure, the capacitor has decreased in value, so the low width drive to run no load is allowing the start up circuitry to barely keep the controller running above the undervoltage trip point, but load requires more stored energy than the capacitor can hold, or D15 has gone open circuit, or the winding that provides the primary side power is open circuit.All fixable, but more easy to simply buy a new laptop power brick, and transplant the lead so you can use it instead.

author: wraper
author_rank: Unknown
timestamp: August 26, 2022, 07:06:40 pm
content: The first thing I would check is small (usually 22-100uF) electrolytic capacitor which is connected to VCC of PWM controller IC, the second thing would be large high voltage electrolytic cap.

author: xyberlink
author_rank: Contributor
timestamp: August 30, 2022, 10:05:45 am
content: Thanks for the replies.D15 is ok, as are all other diodes.C16 (10uF 50v) appears be linked to VCC on the PWM chip - in circuit reading for this look sensible, as do readings for the other electrolytics (which I'd expect to be the case with only 8 service hours).3x film caps also seem ok.I have tested most other components, at least in circuit, a couple out, and so far found nothing amiss.I'm therefore assuming it's either something failing under load / at high frequency, or else one of the 3 chips has failed.Based on the symptoms (see first post) I'm thinking the issue is primary side.