Debugging a suicidal UC3842-based power supply - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: January 11, 2014, 01:22:34 am
content: Hi guys,For a bit of evening fun, I decided to pull a dead power supply from my "maybe I'll repair this later" box. In this case, it's a Haining Deyi Electric Co. AF12V4A -- 240V AC in, 12V DC 4A out.Initial fault diagnosis was a dead short of the chopper transistor (shorted across all three legs) which also took out the UC3842 SMPS controller, a 0R33 fusible resistor and the inlet fuse (which was found in its heatshrink wrapper, exploded into a million pieces). In the process of fixing that, I also replaced a bunch of obviously cooked resistors. By cooked I mean literally cooked: burned black and brown. Thankfully most of them were just discoloured and the measured reading was fairly close to my best-guess estimate of what the colour bands indicated.I also replaced all the obviously damaged filter capacitors (all "low ESR" electrolytics) and the 100uF startup capacitor. The replacement parts are all Panasonic FC and FM series low-ESR electrolytics. These alone are probably worth more than the power supply originally cost...After replacing those components, the PSU worked fine for a while, but blew out the replacement UC3842 a few seconds after switch-on. The failure mode was a dead short between pins 7 and 5 (Vcc and GND), suggesting the 3842's voltage-regulation Zener had blown shorted. I swapped in a second 3842, which promptly went the same way. I eventually figured out that unlike almost every other SMPSU I've seen, this one not only won't start if its run unloaded, it'll actually destroy itself!I've increased the value of the auxiliary winding current limiting resistor (the one between the Aux winding and the diode which feeds the standby capacitor) from 10 to 22 ohms and this has curbed the PSU's suicidal tendencies. Now with no load the PSU sits merrily ticking away (output pulses toat the cost of causing the PSU to occasionally fail to start first time (it starts, gets to ~6V DC on the output, then shuts down as the startup cap drains under load and the 3842's Vcc drops below the undervoltage lockout threshold). I'm not sure if this is down to the bulb presenting an initial very high load to the PSU, before falling as the filament warms up. If it isn't, then I suspect the limiting resistor needs to be reduced a bit...Can anyone think of a better way to resolve the "self-destructs with no-load" issue? I suspect a dummy load resistor would be one way to do it, but how much current do I need to dump? Even 10% of the rated load is 400mA, and at 12V that's nearly five watts. 1% is 40mA, but even that's half a watt...Thanks,Phil.

author: nickm
author_rank: Regular Contributor
timestamp: January 11, 2014, 01:49:53 am
content: What topology? Sounds like forward.

author: Ghydda
author_rank: Regular Contributor
timestamp: January 11, 2014, 01:50:21 am
content: If the feedback circuitry is functioning correctly, there should be no problem with running in a no-load situation.Well unless some really strange design decision has been deployed on your PSU to make behave that way. Who knows. Does the output voltage rise above 12 volt during these no-load episodes?If so and since the '42 goes belly up during no-load I suspect the aux winding is forcing way to much power into aux supply-rail and thus killing of the '42. This could be alleviated by shunting the excess power away from the '42. Just for testing.It's just a thought. Could very well be something else entirely.Post more info for better help (topology, schematic, pictures, measurements, scopeplots, birthdate, favorite color and so fort).More info means better help.Cheers!

author: philpem
author_rank: Frequent Contributor
timestamp: January 11, 2014, 03:21:09 am
content: Quote from: nickm on January 11, 2014, 01:49:53 amOff-line flyback. Three windings on the transformer: primary, secondary and auxiliary (the last one powers the 3842).Quote from: Ghydda on January 11, 2014, 01:50:21 amI'm not keen to repeat the no-load test given that it's essentially guaranteed to blow the 3842 chip. I think you're on the right track - but it's a combination of overvoltage and possibly overcurrent which is killing the 3842. Ideally there should be a resistor between 3842 Vcc to limit the current to less than 30mA (safe limit for the regulator) and a few tweaks to the aux-winding circuitry.I've got a half-done schematic in my lab notebook. I'll upload it when I've redrawn it an understandable form. Apparently the pirated version of DaveCAD changes all your text to chickenscratchIt looks like this thing is a near perfect copy of one of the appnote designs -- a slope compensated flyback converter. Oddly enough, it seems to synchronise its clock to the AC line via a few resistors and capacitors, which is unusual for a circuit like this. I wonder what they were trying to do?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 11, 2014, 04:17:51 am
content: Try a bigger cap on the controller's power rail.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 11, 2014, 11:27:40 pm
content: What is the status of the transformer?I tried to fix a stepdown regulator (24V to 5V) based on some UC**** control circuit with an external FET and current shunt. Replaced almost everything, except for the energy storage choke, but without any luck. The choke was 1.5mm enameled copper wire wound on a 25 mm toroid and the insulation was still ok. The toroid behaved almost like it was made of plastic or some other none magnetic material. No ramp up of the coil current, killing the MOSFET and the shorted MOSFET then fried the controller IC.

author: philpem
author_rank: Frequent Contributor
timestamp: January 12, 2014, 02:10:39 am
content: As far as I can tell, the transformer is fine.Anyway, here's the schematic - reverse engineered by scanning the bottom of the PCB into The GIMP and marking off the tracks as I transcribed them into schematic form... I hope you can all read my handwriting!If a component value is listed as "DNF", that means "Do Not Fit", i.e. there was no component in that location on the PCB.

author: Ghydda
author_rank: Regular Contributor
timestamp: January 12, 2014, 08:21:01 pm
content: Hi philpem,UC3842 pin 2 to GND --> makes no sense at all. Check the layout again.C5 --> makes little sense. More likely the bottom end is connected to gnd instead.R8-R10 --> only makes sense if the designer wanted some spread spectrum, ie. continually change the switching frequency up and down. This would help pass EMI requirements.D6-D7+C2 --> for the life of me I cannot determine what they are supposed to accomplish. Perhaps the circuit is decoded incorrectly here?R7 --> effectively modifies the current sense trip point with respect to the capacitor voltage. Pretty normal procedure.The dotting of the transformer is wrong, the primary dot needs to be at the bottom of the winding.Most of the schematic looks ok, however some scwewy is going on withpins 1-2-3 on the controller chip. Recheck those conections.Regarding being a little uneasy about powering up the PSU with no load:If you have a variac (with galvanic isolation) you could run it from that + have a lab supply (or a suitable battery pack) feed the controller with power. Then you have a fighting chance of seeing what happens at no load without risk of blowing more parts.

author: philpem
author_rank: Frequent Contributor
timestamp: January 12, 2014, 08:55:18 pm
content: Quote from: Ghydda on January 12, 2014, 08:21:01 pmI just did. It's definitely connected to ground. My multimeter's continuity beeper agrees with me... Pins 2 and 5 are connected together, and both go to the negative side of the 33uF/400V filter capacitor.Quote from: Ghydda on January 12, 2014, 08:21:01 pmAgain: C5 definitely sits between pins 3 and 4 of the 3842.Quote from: Ghydda on January 12, 2014, 08:21:01 pmThat might explain it... "win by cheating"Sadly I don't have an AC-rated LISN to do an EMC testQuote from: Ghydda on January 12, 2014, 08:21:01 pmMe either, but that part is definitely correct.Quote from: Ghydda on January 12, 2014, 08:21:01 pmNow that part I guessed. Obviously I guessed wrongQuote from: Ghydda on January 12, 2014, 08:21:01 pmGoing by the datasheet, it looks like they're grounding pin 2 (Vref) to keep the error amp active, then using the opto on pin 1 (COMP) to regulate the output voltage.Quote from: Ghydda on January 12, 2014, 08:21:01 pmIn other words, disable the startup supply (R6 and D8) and inject power straight into the circuit?

author: Ghydda
author_rank: Regular Contributor
timestamp: January 12, 2014, 10:02:37 pm
content: Quote from: philpem on January 12, 2014, 08:55:18 pmBesides the beeping DMM, can you visually confirm a track connecting pins 2 and 5?Quote from: philpem on January 12, 2014, 08:55:18 pmHmmm... how the hell is the designer going to suppress the leading edge spike then?!? Hmmm....Quote from: philpem on January 12, 2014, 08:55:18 pmIt gets scwewier and scwewier...Quote from: philpem on January 12, 2014, 08:55:18 pmPin 2 --> that's voltage feedback on the datasheets I've checked!?!Vref is on pin 8. Maybe you should attach the datasheet here as well?And if it is Vfb, then that means the designer disabled the internal error-amp by leaving its output in a high-pulling state and, as you point out, being forcefully set to a lower voltage by the optocoupler transistor. I haven't looked into whether this violates the error-amps parameters. But if it is okay to do so, it will certainly work.Quote from: philpem on January 12, 2014, 08:55:18 pmWell yes. And start slowly from the bottom of the variac, keep the stored energy in C1 low until you've figured out what's wrong.

author: philpem
author_rank: Frequent Contributor
timestamp: January 12, 2014, 10:59:30 pm
content: Quote from: Ghydda on January 12, 2014, 10:02:37 pmYes, there is a track connecting pins 2 and 5. I used the multimeter to confirm it.Quote from: Ghydda on January 12, 2014, 10:02:37 pmThe original part was a Unisonic UC3842:http://www.unisonic.com.tw/datasheet/UC3842A_3843A.pdfThe part I replaced it with was the STMicroelectronics UC3842B:http://www.st.com/web/catalog/sense_power/FM142/CL1454/SC352/PF65547And you're quite right, pin 2 is the feedback signal. I was working from a badly scanned datasheet for a while, which shortened it to "VF", which I misread as "VR" and expanded to "VREF"... Oops.Quote from: Ghydda on January 12, 2014, 10:02:37 pmIt's probably been "value engineered" to the point of insanity. I figured I'd repair it for two reasons:1) It doesn't look particularly badly engineered (FR3 PCB, fairly large heatsink, chunky components)2) The initial fault looked fairly simple (dead shorted chopper FET and a bunch of blown electrolytics)3) I had most of the parts (with the exception of the 3842 and line fuse) on hand.Quote from: Ghydda on January 12, 2014, 10:02:37 pmSadly, I don't have a variac -- I've been powering the PSU from a 12V bench supply (Instek PSM2010) via a cheap 230V inverter (an "iCan vertor" I got from maplins ages ago). That fulfils the requirement for isolated power (for safety) and limits the fault current (the 230V inverter shuts down cleanly if overloaded). I didn't test on AC mains until I was sure the PSU worked on the inverter. That said, it does behave itself a little better on "proper" 50Hz 230V sinusoidal AC (the inverter is modified-sine).Cheers,Phil.

author: Christopher
author_rank: Frequent Contributor
timestamp: March 17, 2014, 08:21:41 pm
content: Take a look at Rt pin on the inverter and the mains. Maybe switching frequency of the controller is higher on the "modified sine" AC? Probably unrelated....Other than that I always check the secondary-side output of the opto, just because I can. Bad but I shove volts from a bench supply into the output and look at the output of the opto on the DVM.

author: oldway
author_rank: Super Contributor
timestamp: March 18, 2014, 07:16:18 am
content: I guess you are looking for problems that does'nt exist.Flyback configuration acts as a current source and can't work without any load.With no load, you loose the voltage control, the output voltage increases and so do the voltage feeding the UC3842...Flyback power supplies have a specified minimum load or have a fixed load in the secundary. (power resistor)Probably, this power supply has no secundary load and can't work without a minimum load current.I guess there is nothing wrong with this power supply, simply, you can't let it work without any load.Read the specifications of this power supply....For sure, you'll find something like : Min. load: 0.3A or 0.5A