Tv SMPS reboots when backlight turns on. - Page 1
Forum section: electronics / repair

author: Tony.T
author_rank: Regular Contributor
timestamp: March 02, 2024, 06:03:56 pm
content: UE40N7005 Samsung tv.I have a problem. When the backlight turns on the smps reboots. If i feed 5V, 12V and backlight 85V from other powersupplies everything works.The backlight is correct and every segment of the ledstripes uses exactly the same voltage and current draw. That i have tested.The switching ic is ICE1HS01G.i think that the whole ledstripe must consume a little to much current and the switching ic is seensing that and hits the limit and reboots. Is there any suggestion what to do? can i slightly increase the tolerance by adjusting the feeedback circuit that involves the optocoupler. ? Any suggestions?

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 02, 2024, 11:05:25 pm
content: It's a 1/2 bridge topology so check that the film capacitor in series with the transformer's primary hasn't lost capacitance.

author: wraper
author_rank: Unknown
timestamp: March 02, 2024, 11:25:29 pm
content: Check electrolytic capacitors on primary side. Usually caused by a small cap on PWM controller VCC, but bad bulk HV cap can cause this too.

author: Tony.T
author_rank: Regular Contributor
timestamp: March 03, 2024, 11:04:38 am
content: Have checked the PWM Controller VCC capacitor. it is stable....and within range.Also film capacitor in series with transformer is ok. Bort capacitance and ESR.The optocoupler on the diode side is stable...1.1V ...that optocoupler is connected to 12V point on the secondary. There is no optocoupler on the led driver side that somehow sense the voltage...But on the other side of optocoupler it boosts to 5V on the feedback pin when trying to turn on the backlight....EVEN if the diode side of the optocoupler is still at 1.1V. And in the datasheet it says that if the feedback Pin 3 (on 1HS01G) gets over 3,9 then it will stop switching. I get 5V and then the ic reboots. I have checked all the components around the feedback circuit on the primary side connected to the IC and everything seems ok.My only feeling is that something is drawing to much current when the transformer is on and tries to feed the led stripes.IF 5V...12V and 61V are feed from external PS...directly after secondary rectifier diodes.... the power consumptions are...800mA on 5V rail900mA on 12V RailAnd 1A on led rail....BUT i can only feed 61V as i do not have any powersupply that can feed 85V to that rail and where it boosts the voltage to 169V. On the board there is a label saying LedOut 169V (149mA)Which is a lot less than my 1A ...but as i said...i can not feed more than 61V... (hmmm...perhaps put PS in series....) And then everything works as my PS does not stop providing power. So the only conclusion is that the backlight led consumes a lot of current and therefore the SMPS just stop switching.i must test with external PS in series and provide exactly 85V as the original PS supplies that to the Leddriver before boosting to 169VHmm...i just thought about something..... when i measure after the boostconverter...it is 135V only...and not 169 as it says on the label.. could that be the problem?Need to test further...but any suggestions are appreciated...

author: wasedadoc
author_rank: Super Contributor
timestamp: March 03, 2024, 11:44:37 am
content: Check all electrolytics after the rectifiers on the secondary side of the transformer. When they lose capacitance or ESR is much too high that causes the primary side to work harder.

author: Runco990
author_rank: Frequent Contributor
timestamp: March 03, 2024, 05:38:12 pm
content: Samsung and failing capacitors go hand in hand. I cannot tell you HOW many sets I fixed by just replacing caps. EVEN IF they measure ok.Some are spec'd so marginally to begin with that just loosing 10% of their value can cause trouble. Accountants at work.Caps are pretty cheap, just whack some in and see. Often it's much more time and cost effective to do than fart around for many hours.I used to re-cap Samsung sets for a TV station on a regular basis. Good sets.... but always the same problem. Easy fix for me!Anyway, my $.04 adjusted for inflation.

author: MathWizard
author_rank: Super Contributor
timestamp: March 05, 2024, 03:33:20 am
content: If you had a scope or maybe a DMM could see it, a lot of the chips in the PSU section will have some power-good or power-ok pin, or some reset or enable pin, that will change digital state, when a problem is detected, and trigger the reset of some chip closer to the mains input.This reminds me I have a TV to fix, but I think it's the LED strip, and IDK if it's worth it to buy some replacement. I've taken screens completely apart too scrap them, but had to break them to do it.