Switch Mode Power Supply Voltage Sag - Page 1
Forum section: electronics / repair

author: PetaVolt
author_rank: Contributor
timestamp: March 26, 2018, 03:07:30 pm
content: I'm sure this is a very simple repair but being a "young player" I don't know where to start...I have a SMPS inside a wireless microphone receiver power supply and and antenna distribution panel that has failed. The power supply is rated for 12 volts @ 4.5 amp output. Under no load the supply sits happily 12 volts with no issues. However, when a 2 amp load is connected the output sags to around 9 volts, no good for the receivers that are connected to it.Visually, there doesn't appear to be any catastrophic failures. Any ideas on where to start on this one?Thank You!

author: rx8pilot
author_rank: Super Contributor
timestamp: March 26, 2018, 11:22:36 pm
content: Where are you probing the 9v when the supply is loaded? Directly at the output terminals?

author: PetaVolt
author_rank: Contributor
timestamp: March 26, 2018, 11:43:51 pm
content: Quote from: rx8pilot on March 26, 2018, 11:22:36 pmYes, at the output terminals.Quote from: wilfred on March 26, 2018, 11:18:34 pmNo, I didn't connect an external P/S but I will try that...The date code on the P/S is 1998 so I suppose capacitor failure is possible? I don't have an ESR meter but I can give the function gen/scope method a try.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 27, 2018, 12:01:32 am
content: I would do capacitor change, but the first section would be near your right angled board in the center, maybe somekind of feedback capacitor are due, normally an smps has an feedback line for the controller chip ...

author: tautech
author_rank: Super Contributor
timestamp: March 27, 2018, 12:08:17 am
content: Quote from: PetaVolt on March 26, 2018, 03:07:30 pmIdentify the controller IC and getsomedatasheets. It appears to be on the daughterboard.I saysomeas they often list typical applications and quite often they will match the application closely or even exactly.Having the schematic allows better understanding of the circuit and possible test points.This is how I start with an unknown SMPS every time.

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2018, 12:09:39 am
content: Every time I've seen a problem like this the issue has been worn out electrolytic capacitors.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 27, 2018, 12:15:55 am
content: Quote from: james_s on March 27, 2018, 12:09:39 amMe as well.....

author: PetaVolt
author_rank: Contributor
timestamp: April 03, 2018, 09:02:12 pm
content: Quote from: james_s on March 27, 2018, 12:09:39 amI replaced all of the potentially suspect electrolytic caps, no luck. The voltage still sags to 9 volts or below with a small load.For reference, I replaced the caps with matching nichicon caps that came out. The series of the original was discontinued so I used nichicon's replacement series caps.I circled all of the caps that have been replaced... all of them that have anything to do with the load (I think)

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2018, 09:54:51 pm
content: Quote from: PetaVolt on April 03, 2018, 09:02:12 pmAnd not the IC Vcc one next to the the daughter board ?They're normally 22-50uF and a common SMPS failure point. Flyback diodes too.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 04, 2018, 12:54:28 am
content: as tautech said you forgot the small one near the right angled pcb, normally the sag is caused by a bad feedback loop, the circuit is not able to compensate the load, a bad capacitor is always the first suspect, change it, i never change the big one as you did.One thing i saw in the past, is when you had 2 identical high ohms 2 to 10 megs ohms values resistors on the main ac line and once rectified, they where used as an voltage divider to create a floating grounddo a check for this ??

author: PetaVolt
author_rank: Contributor
timestamp: April 05, 2018, 06:52:06 pm
content: Quote from: tautech on April 03, 2018, 09:54:51 pmI went ahead and replaced those too.Hoping that the caps were the issue I put the power supply back into service and left it powered up. I checked the supply after about 12 hours and was holding a load just fine, however, between 12 and 24 hours it went off. I wasn't in a good spot to check the output voltage but the devices this supply powers with were off. After letting it cool down for a while and re-powering it powers up with no problem.It certainly looks like a heat related issue but it's strange it took upwards of 24 hours to fail under a constant load.

author: tautech
author_rank: Super Contributor
timestamp: April 05, 2018, 07:31:02 pm
content: Quote from: PetaVolt on April 05, 2018, 06:52:06 pmFeedback diode too ?

author: PetaVolt
author_rank: Contributor
timestamp: April 05, 2018, 10:41:45 pm
content: Quote from: tautech on April 05, 2018, 07:31:02 pmI've been looking and I can't find any discrete diodes. Would this would be in the same region as the optoisolator?

author: ArthurDent
author_rank: Super Contributor
timestamp: April 06, 2018, 02:47:25 am
content: It is hard to tell because I can't read the designation on the board but the device screwed to the heatsink near the 2 output capacitors may be a dual schottky diode. If it is and one or both sides are iffy then that may be your problem. If you look at the waveform on the common cathode before and during when the voltage drops you may see the waveform change from full wave to half wave or just distort when the output drops.Keep in mind it is hard to remotely diagnose your problem but this might be worth checking and if it is the problem, not too difficult or expensive to replace. Here's a diode that is similar but you'll want to match the one in the circuit if it is the problem.http://www.galco.com/buy/NTE-Electronics/NTE6087?source=googleshopping

author: tautech
author_rank: Super Contributor
timestamp: April 06, 2018, 04:18:34 am
content: Quote from: PetaVolt on April 05, 2018, 10:41:45 pmSMPS controller IC's normally have 2 VCC supplies, one sourced from the HV DC rail via dropper resistors and another from a feedback/flyback winding on the SMPS transformer. This usually has a series diode.Both supplies are common'ed at the VCC pin or close to to it. Sometimes there's a cap on each supply. You need ensure Vrun/min is met at all times.SMPS diagnosisshouldstart with identifying the controller IC and checking the datasheet for 'typical applications' where it's very common to find the manufacturers circuit example followed quite closely and therefore the necessary componentsandtheir values/part #'s listed too.Then if the example closely resembles your DUT, components can be checked/tested to ensure they meet spec.Further, there's often a paragraph or two about the IC's features like Soft start and the like and with not much gear you can check critical points of the circuit for correct operation.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 06, 2018, 06:53:09 am
content: A long time ago, I had some issues with PS units looking quite exactly like this one:Check the output connector (RHS on you photo, the black and red wire in the Molex connector), its contacts may have gone too high resistance over time, causing voltage drop under load. If this is the problem, solder the wires directly to the pins.If not, buy a new PS module, this is quite a standard size, should neither be too expensive nor too complicated to source.