SMPS Troubleshooting.  About to give up... - Page 1
Forum section: electronics / repair

author: Milmat1
author_rank: Contributor
timestamp: August 16, 2022, 01:45:55 pm
content: Background, this is a SMPS board from a DRO system on a milling machine. It's an YCL PCB153 board, No schematic available.It is an KA3842 PWM chip. / triple output +5V, +36V, +6V (all outputs dead)At the bridge rectifier, I have 120VAC in and 165.3 VDC across the bulk filter cap. (100uf/400V) So I assume everything ion the primary circuit is OK (??).I have reverse engineered the part of the board dealing with the chip and found it different from any of the sample drawings on this chip. But still a basic SPMS design.I recapped the board, found an "Open" startup resistor (180K) and replaced it. Found the startup cap(86uf) had a very high ESR . I have checked all the silicon, i.e. Diodes, Zeners etc.An R/C circuit between pin 4 and pin 8 is supposed to set the output freq. They are fine.The chip will not start. pin 6 (output) is just dead and nothing going to the gate of the MOSFET. (And voltage across D/S is exactly the same as the DC across the BFC).EDIT: Also pin 8 should be a 5VDC refference voltage, it has no voltage present. Should this be present all the time when Vcc is present ?The Vcc to the chip (pin7) looks like the attached pic. Is this normal for a Vcc startup until the aux winding can take over ?How likely is it this chip itself has failed, is there a way to test the chip to confirm ?Any help or direction appreciated...Thanks, Matt

author: kripton2035
author_rank: Super Contributor
timestamp: August 16, 2022, 03:17:48 pm
content: there is usually a small cap (around 10uF) between pins 7 and 5. often this cap has high esr or too low capacity.it's often worth to try to replace it.

author: Milmat1
author_rank: Contributor
timestamp: August 16, 2022, 03:45:48 pm
content: There is an 82uf between pin 7 and pin 5 (gnd). It did in fact have very high ESR, but I replaced it.

author: Bud
author_rank: Super Contributor
timestamp: August 16, 2022, 04:24:57 pm
content: Check the chip for shutdown conditionQuote

author: Milmat1
author_rank: Contributor
timestamp: August 16, 2022, 05:23:32 pm
content: Quote from: Bud on August 16, 2022, 04:24:57 pmPin 3 = 105mvPin1 = 6mv

author: Bud
author_rank: Super Contributor
timestamp: August 16, 2022, 06:35:32 pm
content: So it looks the chip is in shutdown mode because of pin 1 being pulled to ground.

author: inse
author_rank: Super Contributor
timestamp: August 16, 2022, 07:31:58 pm
content: According to the datasheet, Vcc should be above the startup threshold of 15V.As your oscillograph is taken with AC coupling, we don't know whether the condition is fulfilled.

author: Milmat1
author_rank: Contributor
timestamp: August 16, 2022, 07:41:38 pm
content: Quote from: inse on August 16, 2022, 07:31:58 pmVcc is fluctuating between 13-15 VDC. Steady fluctuations as if "hunting" If that makes sense.Pin 1 is connected to pin 2 via : 10K resistor in parallel with 100nf cap . Pin 2 is Vfb .....CONFUSED..

author: Milmat1
author_rank: Contributor
timestamp: August 16, 2022, 07:53:06 pm
content: Studying the application notes, The chip should output 5vdc reference voltage on pin 8 all the time. I think,,,

author: fzabkar
author_rank: Super Contributor
timestamp: August 16, 2022, 08:30:54 pm
content: The way that these chips work is that they remain in "start mode" until the Vcc capacitor has charged up to a certain threshold level. The chip then switches to run mode and pulses the MOSFET chopper. The secondary winding of the transformer generates an auxiliary voltage which then augments the Vcc supply. If this secondary supply does not appear, such as when the DC output is overloaded, the Vcc supply decays with each successive pulse until it falls below the UVLO threshold. The chip then reverts to start mode and the process repeats. Some "smart" chips will give up after a certain number of attempts.Typical faults with these circuits are the startup resistor and the small auxiliary filter capacitor.

author: xavier60
author_rank: Super Contributor
timestamp: August 16, 2022, 10:11:32 pm
content: When VCC reaches the run threshold, it's being discharged very quickly back to UVLO.Maybe a short on the 5V or on the drive output pin 6 or the IC is faulty.Also during run state, some minimum voltage needs to develop on pin 1 either internally from the error amp or externally applied. I'm guessing about 1.5V for now.

author: Milmat1
author_rank: Contributor
timestamp: August 17, 2022, 07:51:20 pm
content: Quote from: fzabkar on August 16, 2022, 08:30:54 pmThank You !Originally the startup resistor was open and the cap (82uf) had high ESR. I replaced both.I'm fairly sure at this point that I have a failed chip. Because I cannot get the reference voltage on pin 8 to come at at all. And the chip appears to have everything it needs, just refuses to bring up the output. .Anyone ever just pull the chip and breadboard it to see if you could get it to run ?

author: lowimpedance
author_rank: Super Contributor
timestamp: August 17, 2022, 11:13:36 pm
content: Why bother, Digikey have the UC3842 in stock for a dollar each. If you're confident all other components are good and the original 3842 has failed, just replace it.Unless it's for educational purposes of course then have a go.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2022, 11:36:22 pm
content: Note that failures generally cascade. A typical failure starts with an overheating MOSFET, which fails as three-way short, dumping a fair fraction of the DC bus cap plus mains fault current into the current sense resistor, and gate circuit. Both flow into the controller, which is subsequently damaged. Resistors may also fail (shunt, or the resistor between it and ISENSE, or gate resistor and whatever else is with it (often a parallel diode for fast turn-off)).The fault current blows the fuse as a final act, which might be tempting for its simplicity / obviousness -- it might be the first thing you check, but replacing it is the last thing you need to do in this case. (Rarely, the fuse blows by itself, perhaps due to fault manufacture, thermal cycling, or mains surge; highly suspicious, but can happen.)Startup resistor going open is relatively inexplicable, but perhaps an overvoltage condition occurred, or it was a thin-film type subject to environmental effects (corrosion).A damaged chip may continue to work, in part. A typical case is the output devices or ESD clamp diodes failing shorted, resulting in excessive current consumption, or excessive input loading, or insufficient output voltage. This could be the case here, where the chip's UVLO/startup circuit is working fine, but when it enables power to the rest of the chip, it tanks the supply. It could also be more direct, such as gate drive dumping into a shorted MOSFET -- you didn't mention if this was replaced or checked so as far as we know, it is still an option.Also, because of ESD clamp diodes, it may be that the aux supply got overvolted, which could damage that capacitor for example.More likely the aux cap just dries out, as it's a fairly stressed part, actually, despite the fairly low output current -- the controller typically needs a couple 10s of mA to run -- but it's also a small capacitor, and ratings in small sizes are rather modest.Secondary side can end up toasted too; such a transient can dump excessive current into the rectifier, or maybe the failure was induced by a partial or complete failure of output filter cap or rectifier, or regulator (TL431?) for that matter. And then the primary side tries to start up into a partial or complete short, and keeps chugging like that forever, or until something else blows up.Have also seen one where the controller was just dead. No idea how. Everything else checked out, replaced it, worked fine. Go figure...In short, replace everything active (transistor, controller, some diodes even), and check everything passive (resistors and capacitors). Replacing electrolytics wholesale isn't a bad idea, but generally isn't necessary as long as they test OK.Tim

author: xavier60
author_rank: Super Contributor
timestamp: August 18, 2022, 01:40:05 am
content: A faulty start up resistor can lead to more serious problems if the PSU design doesn't include a Gate bleed resistor.If VCC doesn't reach the UC3842's threshold needed for it to become active, the output pin remains in a Hi-Z state allowing the Gate to gradually get charged by leakage current.It's possible that this problem has been fixed with current versions.

author: thague4568
author_rank: Newbie
timestamp: June 02, 2024, 10:42:09 pm
content: Have the same power supply. Different problem. HV side working but LV side is resetting. In troubleshooting managed to short something and smoked R24 in HV side. Anyone know what the value of this resistor is? Am in the process of reverse engineering the board. If I can get HV side working again may be able to assist with your problem.