12 Power supply repair - Page 1
Forum section: electronics / repair

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 12:27:47 pm
content: Hi , i'm trying to repair(yet another) cheap chinese power supply with no success , i ran out of ideas and need some help.Things already checked and replaced just to be sure:-Main capacitor(hot side)-Small capacitors(hot side around the IC)-Diode in the hot side(1n4007) , had burn marks around the pcb , measured ok but put another one.-Mosfet transistor(12N60)-12V output filtering capacitor-Output dual diodeSymptoms:Power supply starts(somehow) but the green led pulses , aplying a load pulls the output voltage down , removing the load gets the power supply in "pulsing" mode again.

author: PKTKS
author_rank: Super Contributor
timestamp: May 04, 2019, 12:47:27 pm
content: Those SMPS follow the pudim formula.. nothing too muchfar from the basics.But you lack vital information to the helpers here..You must= put the number of the supervisors and the details= details are from the fb loop like opto coupler and error samplerotherwise a pretty clear photo where those things can be reasonably guessedin general they follow TL494 or UC38xx pretty much to the basic letterwhich leads us to protection mode being entered..like bad soft start or just crappy feedback loop where bad componentsare totally out of spec and the fb loop brings the soft start back downPaul

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 01:00:20 pm
content: The PSU uses UC3842D CI , i attached a photo of the area , just measured the Vcc for the CI , it oscilates from 10V to 13V , trying to figure more (looking for the start resistor).Thx for the involvement.

author: PKTKS
author_rank: Super Contributor
timestamp: May 04, 2019, 01:19:14 pm
content: the UC38xx layouts usually have a companion TL431 ...check if you have one of this in general very close to thesampler resistors on the secondary side or very close tothat opto coupler...If not - you probably have a zener or 2 making referencesthose references are used in the fb loop to put theUC38xx on the stable operation point.It does have a soft start RC which slowly will putthe current "sense" pin in proper operation modeWhen that fails it goes wacky on and off trying to startPaul

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2019, 01:52:35 pm
content: Find and check the value of the bootstrap resistor.It will be like 300-800 kohms, it might be that 470k one i can see.China loves to use low wattage resistors for the bootstraping circuit and they fail high-value from the heat after a while.The PSU then fails to start up.Used to see that a lot on computer psu and on metal cage psu module.

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 02:06:21 pm
content: Indeed it uses the 431 reference , i think i have some around , but i have better news , i managed to blow 2 diodes and the mosfet by tricking the feedback loop (put the dmm probes across the optocoupler (primary side), stoped the pulsing and i put a load , the mentioned parts blew) , now i have no voltage supply at the UC3842D(i replaced the diodes and mosfet)Progress: -3 points.

author: PKTKS
author_rank: Super Contributor
timestamp: May 04, 2019, 02:29:15 pm
content: Unfortunately that shit happens with SMPS...they are very tricky to debug and experience helpsbut does not prevents to do some bad things...testing SMPS requires some of ahead thinking...the dont's arenever cross the sides of the transformer as primary side is high voltagenever probe that crappy side which may have pulses of 1KVa usable advice is to put a "winding" loop on the primary to tryto see the waveform... if required..those parts are cheap. you may replace them easilybut don't blow the transformer you hardly will find equal easilyPaul

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 05:01:24 pm
content: Measured the resistors (startup resistors) that go from + side(hot positive) to the IC vcc pin , they seem ok , so probably something went wrong with the IC and it pulls the voltage down to 1 volt , will try to change it if i find one.Sent from my mobile.

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 05:09:38 pm
content: Quote from: Psi on May 04, 2019, 01:52:35 pmMeasured ok , thx for the sugestion.Sent from my mobile.

author: blackbrayn
author_rank: Contributor
timestamp: May 04, 2019, 10:28:49 pm
content: Started trobleshooting a second , identical supply that has the same symptoms , turns on and starts pulsing if i conect a load (seems more stable than the other one that pulsed the output even if no load was conected.)Just noticed that the mosfet (12N60) gets hot fast with no load).

author: strawberry
author_rank: Super Contributor
timestamp: May 05, 2019, 02:21:26 am
content: Is RCD clamp working properly?UC3842D need above 16V to operatesnuber capacitor across MOSFET DS can make it run hot

author: blackbrayn
author_rank: Contributor
timestamp: May 05, 2019, 05:10:12 pm
content: Quote from: strawberry on May 05, 2019, 02:21:26 amDidn't find the clasical topology of a rcd i just learned about (thanks)but the only 3 diodes on the hot side measured ok , there is no capacitor across drain-source , voltage on vcc on uc3842 is 13.9 V.Sent from my mobile.

author: tppc
author_rank: Regular Contributor
timestamp: May 05, 2019, 05:34:58 pm
content: 13.9V is low (maybe due to protection mode)Did you check the Vcc capacitor? somewhere near the bootstrap resistor...

author: blackbrayn
author_rank: Contributor
timestamp: May 05, 2019, 05:56:29 pm
content: Quote from: tppc on May 05, 2019, 05:34:58 pmCapacitor checked fine , it's a 220uF/35v in my psu , also the start resistors (it uses 3 in a series/paralel configuration , in the above scehamtic it's the 56k one).Sent from my mobile.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 05, 2019, 11:01:35 pm
content: "RCD clamp" or snubber, will be minimalist on cheap designs.You have one on secondary with 4.7resistor and series blue capacitor. Check resistor is not open capacitor not leaky.You may also have a snubber on primary side with C8+diode?MOSFET runs hot but isn't it meant to be heatsinked?Open bootstrap resistors will usually not cause hiccup, they just cause no starting or very random starts.In case of hiccupping it is more the UC3842 steady supply that needs checking R19+series diode.Seeing the quality of the PSU you may also want to check input fuse and thermistor for too high resistance.

author: blackbrayn
author_rank: Contributor
timestamp: May 29, 2019, 09:17:35 pm
content: Got some free time and decided to verify more stuff on the PSU:"RCD clamp" or snubber, will be minimalist on cheap designs.You have one on secondary with 4.7resistor and series blue capacitor. Check resistor is not open capacitor not leaky. --->Checked resistor and replaced capacitor.You may also have a snubber on primary side with C8+diode?--->Replaced capacitor , diode too.MOSFET runs hot but isn't it meant to be heatsinked?--->Solved , put a test heatsink.Open bootstrap resistors will usually not cause hiccup, they just cause no starting or very random starts.In case of hiccupping it is more the UC3842 steady supply that needs checking R19+series diode.-->Checked resistor , replaced diode with another generic fast, low drop-out one.Seeing the quality of the PSU you may also want to check input fuse and thermistor for too high resistance.--> Shorted both of them for testing.I will keep trying , thank a lot for the time and help.Measuring again the UC3842D with the DMM , voltage oscillates between 10 and 16v , no load attached to the output of the power supply , i also made a video with the PSU start and load behaviour.https://youtu.be/nTkPOHXzn5A

author: PKTKS
author_rank: Super Contributor
timestamp: May 30, 2019, 02:15:25 pm
content: Quote from: blackbrayn on May 29, 2019, 09:17:35 pmUC chips are trickyThey were designed to induce the transformer in CCM mode on the secondaryIn other words by sensing the primary current UC should be able todrive an "average" CCM current on the secondary sideBut for that to happen some steps should occur first - like soft startingthe primary inductor to leverage the secondary amperage enoughon the sampling FB loop by the "transcondutance" error amp done via TL431that TL431 is very critical in the UC FB loop because it will inducethe proper CCM mode - or notSo it may help to put some light load at start just in case you havea wild deviated or even "forged" TL431 chip - which does happena LIGHT RESISTIVE LOAD will definitely helps to put stable loopthere are a plethora of forged chips out there which just go wackyfor no reasonthe reason to load the regulator is fairly simple as to force a highercurrent on the whole FB loop - mostly on the transcondutance erroramp which will induce a higher error current ... if not.. it is crappythe fb error current should vary with the load... a nice way to checkthe loop is to measure the error amp current with the load variationSome forged chips can be spotted some dontPaul

author: blackbrayn
author_rank: Contributor
timestamp: May 30, 2019, 06:21:21 pm
content: Quote from: PKTKS on May 30, 2019, 02:15:25 pmThanks for the advice , i will put a power resistor on the output and startup the PSU when i get home.Sent from my mobile.