Exploded Single Transistor 60 Amp Forward Converter- Snubber? - Page 1
Forum section: electronics / repair

author: tango17
author_rank: Regular Contributor
timestamp: March 04, 2025, 02:39:30 pm
content: I have a 13.6v 60a smps converter (labeled PD9160-1) intended to supply 12 volts to an RV. It uses a single transistor forward converter design with a UC3845B controller chip.When I received it there were a dozen blown components, some completely blown off the PCB, melted copper traces and vias, and missing silk screen labels.I was able to repair most of this, but I didn't understand one part of the circuit shown in the drawing below. It is based on my reverse engineering, so it is quite possible there are errors there.The components of interest are d22 l12 c24 and r35. All of those components were completely destroyed and missing with the exception of the three watt 100K resistor r35.I guessed they were some type of snubber circuit, and in order to determine the values and components I convinced a friend to let me look at the circuit of their PD4060. that is also a single transistor forward converter design supplying 13.6 volts and 60 amps. it is nearly identical in all of its components, but unsurprisingly the components of interest d22, l12, c24 and r35 were all missing and replaced by the circuit seen in the second drawing below labeled PD4060. It also seems to be some type of snubber circuit.I next tried to look at another friend's PD 9160. It is shown in the circuit at the bottom of the drawing labeled pd9160-2. Although it was nearly identical to the PD9160-1 in every component and label, it was different from mine in the critical area I was interested in.Notice that all three circuits use 3-watt resistor, but mine uses a 100K resistor and the other two use 27 ohm resistors.Can anyone explain the function of these components? Are they a snubber circuit? How can I calculate the values of destroyed l12 and c24 as well as their voltage and current ratings? Would it be easier to substitute one of the other two designs used in the PD4060 or PD9160-2?If it's not clear, the two lower circuits replace Q2, D22 L12 C24 and R35 in the PD9160-1 circuit above.I think I understand the rest of the circuit quite well and didn't bother to draw it up. Transformer T4 is kind of odd. It has two single turn outer windings and a central multi-turn winding. It seems to be a leftover from a earlier design that used two transistors instead of the single transistor forward converter design.There is a patent that shows and describes this design in detail.In the earlier design the two transistors switched to ground through the two single turn outer windings on T4. The output of the center winding of T4 was sent to a full-wave bridge rectifier which then continued on in the same way as shown in the drawing to the current monitoring input of the pwm controller. Instead of a UC3845 the controller was a UC3846.Any comments on the circuit and how to repair or modify it would be gratefully received.

author: tom66
author_rank: Super Contributor
timestamp: March 04, 2025, 04:44:48 pm
content: It looks like some kind of LC snubber. These circuits are rare but sometimes used to improve efficiency.See herehttps://www.researchgate.net/figure/bThe-forward-converter-with-a-RCD-snubber_fig2_44797102(image caption is wrong).The exact values of component will depend upon the ringing frequency and amplitude at the drain of the transistor.It's normal for a forward converter to have two primary windings, if that's what you're indicating. The second winding is used for a diode clamp which is D10. Not sure of the purpose of L10 there. Is that a ferrite bead or a proper inductor? In some forward converters, the diode clamp is replaced with another FET (this is the so-called two-switch forward converter, though usually a shared primary winding is used).The secondary on your diagram looks wrong, there must be a common terminal for that transformer.Now, that aside, I wouldn't generally expect snubber components to be damaged by a transistor failure, which is the most common type of failure you could have happen in a product like this. So, I'd be wondering if something else happened to the unit. For instance, mains over-voltage (I'm assuming it is powered from mains supply) - 240V instead of 120V for instance, or even worse, or an external power surge/lightning damage.I'd be concerned that the controller IC was damaged. When a violent event happens to a semiconductor (the main FET) it's very common for drain-gate to short and destroy the controller IC in the process. This can then cause further downstream damage, such as to the optocoupler, Vcc zener diode, or any other ICs that share that supply voltage (such as PFC circuits if present).

author: tango17
author_rank: Regular Contributor
timestamp: March 04, 2025, 05:31:03 pm
content: Quote from: tom66 on March 04, 2025, 04:44:48 pmThanks!QuoteThat's part of my problem. I'm not sure how to calculate those values.QuoteYes, I'm showing the normal Reset winding of a single transistor forward converter.QuoteI'm not sure what L10 does, but it's a small surface mount inductor, no windings visible and it looks like a black cap or resistor. No markings on it.QuoteI didn't try to draw the secondary side at all It wasn't damaged and I understand it.QuoteAgreed. I have no idea what happened. There was a melted spot in the middle of a ground plane that made no sense to me. Also that aux winding on T1 I labeled "Fan" is half wave rectified and powers the cooling fan. That whole circuit was also damaged. Lightning? AFAICT, T1 windings are ok.QuoteYes, it died, but I have a spare. R23 also died at the gate, as did the NPN BJT that turns on the fan when the main heatsink is too hot.If you are interested, US Pat. No. 7782002 describes the two transistor circuit design and operation in detail and even has many of the same component numbers in the circuit. It describes the fan circuit, the T4 current monitoring and some other similar circuit parts. But not the single transistor design or snubber.I'm not actually desperate to fix this device. It's more that I want to understand the snubber and circuit operation and how I can figure out component values when the components are totally destroyed.

author: tango17
author_rank: Regular Contributor
timestamp: March 04, 2025, 06:22:38 pm
content: I read the paper linked above. It starts by describing the conventional forward converter that requires an expensive custom transformer with 3 windings - primary, secondary and reset, just like all 3 of the designs I have looked at and described above. Then they go on to describe various snubber circuit designs intended to eliminate the reset winding - a major cost saving when a standard 2 winding transformer can be used. The paper does have some very useful info, like the fact that the mosfet sees a 2.6 times the input voltage. In my case that's above 400 volts for a 120vac rectified dc input. That explains the choice of a 500v Q2 in the designs I've looked at.BTW, I have no idea if these circuits are really intended as snubbers. They do all have 3 watt resistors, so that seemed likely to me, but if others think they perform some other function, please let me know.Thanks!

author: MathWizard
author_rank: Super Contributor
timestamp: March 04, 2025, 09:21:56 pm
content: If you can measure all the values like inductance, you could simulate it, have you used any simulators like LTSpice ?

author: tango17
author_rank: Regular Contributor
timestamp: March 05, 2025, 01:39:50 pm
content: Quote from: MathWizard on March 04, 2025, 09:21:56 pmIt's been a few years since I last did circuit simulation, but the real problem is that the inductors in that snubber circuit are destroyed and can't be measured.I'm thinking about replacing the L12, D22, C24, R35 snubber circuit on mine with the C24 R37 snubber design of the PD4060. That's why I showed those other circuits I found in nearly identical converters.Let me ask this question: I looked more closely at the earlier 2 transistor design using a UC3846 controller shown in the patent. It describes a RC snubber connected between the drains of the two main switching mosfets. The patent states the RC between the drains acts as a snubber. In that design, R=100 ohms 10watts and C=270 pF. The RT CT specified for the switching frequency gives 10kHz for the patent design. I don't remember the switching frequency of mine, but I know it's between about 100-180 kHz. With those numbers, and assuming I use an R=27 ohms 3watts like shown in the PD4060 design (the RC connected from drain to ground/source across Q2) what C value would you use?Does connecting an RC circuit connected source to drain on Q2 to act as a snubber instead of the circuit in mine sound reasonable? I was thinking about 100 pF. Any tips on how to tell if the snubber works correctly using a scope or DVM? Is it worth it to try simulating the circuit to get the answer?

author: tom66
author_rank: Super Contributor
timestamp: March 05, 2025, 03:43:34 pm
content: You can't model this because you don't know the leakage inductance of the transformer, which is what the snubber is dealing with. It is not switch-frequency dependent. Power supplies often have variable switching frequency anyway (pulse-skip or PFM modes), to reduce power dissipation during low load operation.If you had no other reference, the way I would go about determining the ideal component values would be to power the unit up with a low current-limited DC power supply, enough to get the converter switching (you may need to feed Vcc directly) then see what the drain of the transistor looks like. Increase the input voltage until the drain voltage is approaching around half the limit of the transistor. Then experiment with different values to see what works well as a snubber, the ideal turn off waveform will stay within the transistor's maximum rating even with full line voltage applied.I am really struggling to understand how that inductor came to be damaged and think the repair may be more complex than you expect unfortunately.

author: tango17
author_rank: Regular Contributor
timestamp: March 05, 2025, 05:57:15 pm
content: So the idea would be to add a snubber circuit, monitor the drain of Q2 with the scope, once it's switching, increase input voltage slowly, current limited, and adjust snubber circuit design/components until the snubber is keeping spikes at Q2 within its voltage rating?

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2025, 06:33:56 pm
content: Quote from: tango17 on March 05, 2025, 05:57:15 pmOr design the snubber circuit for the worst case and then relax its design, but the whole objective is to clamp the voltage spike from the leakage inductance of the transformer to a level below the transistor's breakdown voltage.The diode based snubber is more efficient because it does not absorb power during the turn on, but it is more expensive and the demands on the diode are considerable. I think the small inductor in series with the diode controls reverse recovery current during turn-off. Sometimes in lower power applications, a big TVS diode is used in place of the power resistor. A power shunt regulator could be used but is more expensive than a power resistor.

author: tango17
author_rank: Regular Contributor
timestamp: March 05, 2025, 07:48:26 pm
content: Quote from: David Hess on March 05, 2025, 06:33:56 pmThat's going to take some research for me to do properly. 😀I'm a bit broken, so I only get 10-15 minutes at a time on the bench. Currently, here is the state of the repair after replacing components and traces:Up to about 30 vac input, the output climbs slowly to 13.6 vdc. Then from there up to about 55 vac input it holds steady and regulates the output correctly at 13.6 vdc. Above that input voltage, the output rises rapidly towards 16-20 vdc or higher. The current draw is low. This is with the entire snubber circuit removed/disconnected. The converter has a jumper that reduces output to 13.1 vdc, and with that jumper on, the output regulates at 13.1 vdc but otherwise acts the same as described above.Does it seem likely that the snubber removal is involved in this odd output regulator behavior or am I looking at some other problem?My choices are to:1. try to design the original snubber using an inductor, then test2. Try to use a simpler RC snubber as in the more recent 4060 and 9160 designs3. Do scope measurements trying to find out why regulation fails at higher input voltage.Comments?

author: tom66
author_rank: Super Contributor
timestamp: March 05, 2025, 08:01:31 pm
content: No, I doubt the snubber has anything to do with that.It sounds like the feedback circuit isn't working properly. I would begin with that. Does the circuit work better with load on the output?

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 05, 2025, 08:43:30 pm
content: Do you know the conditions the unit was operating at when the failure occurred?It may shed some insight into the analysis.

author: tango17
author_rank: Regular Contributor
timestamp: March 05, 2025, 09:36:15 pm
content: Quote from: tom66 on March 05, 2025, 08:01:31 pmI haven't tried a load yet I don't have the heatsink mounted, so I don't want to put much load on it. I've checked most of the feedback circuit. The voltage divider works, the output of the TL431 that feeds an optocoupler works, at least at the voltages I tested. Since it regulates to the correct voltages at two different settings (for a while at lower input voltages) I'm thinking something may be damaged, but not destroyed. The optocoupler and TL 431 in the voltage regulation circuit are on the secondary side and should be seeing the lower voltage of the output even as the input voltage rises, so it seems to me I should look at the high voltage primary side circuitry? But.... there's not much there. Perhaps the 15v zener that supplies power to the controller UC3845 through a high wattage resistor that connects to the rectified AC 160 vdc.

author: tango17
author_rank: Regular Contributor
timestamp: March 05, 2025, 09:37:13 pm
content: Quote from: schmitt trigger on March 05, 2025, 08:43:30 pmUnfortunately, no. It was given to me broken without any info.

author: tom66
author_rank: Super Contributor
timestamp: March 05, 2025, 11:24:14 pm
content: Controller chips like the UC3845 have undervoltage lockout, when Vcc is too low they shut down, so it is probably nothing to do with Vcc. Nonetheless it is still worth checking Vcc to see that it is stable but I doubt it is the problem.However, a better idea would be to check what Vfb (pin 2, measured wrt to pin 5) is doing as you increase the input voltage up. When the output voltage is below the set point (13.1V in the lower setting), Vfb should be relatively high, more than the reference voltage which according to the datasheet is 2.5V. (Not related to TL431 reference - just a coincidence.) As the output begins to regulate, Vfb should fall. When regulating, it should be around 2.5V. Now experiment with the higher voltage range which you say just climbs when the input voltage is increased. In this condition, I would expect Vfb to fall much below 2.5V.If it does fall, and the output voltage continues to rise, I would suspect UC3845 - either defective, counterfeit or damaged.If it doesn't fall, then the optocoupler and the feedback circuit are suspect.I have seen optocouplers fail in strange ways in the past. And perhaps an explanation for the failure for the unit was loss of regulation via the optocoupler, causing it to output far more charging current than it would normally support (since lead-acid battery in the RV will act to clamp the voltage, but will soak up a lot of current in charge phase), which then caused the damage on the primary side due to overheating and/or catastrophic failure of the main FET. I -guess- that excessive switching current could potentially damage the snubber circuit, although I admit that is still a stretch of the imagination.

author: tango17
author_rank: Regular Contributor
timestamp: March 06, 2025, 03:57:45 pm
content: Before I first posted here, I made a lot of changes and repairs to the circuit and the last thing on my list of repairs was the snubber circuitry. I really couldn't figure out where to go with that because of the missing and exploded components etc.That's basically why I came here. I couldn't even be sure if this was a snubber circuit. After reading the helpful comments here, I decided that it was actually a snubber circuit and that I it could probably just test the circuit without the snubber installed.That's when I learned for the first time that the circuit was actually operating up until the point where the input voltage reached some critical value and it stopped regulating properly.It was clearly regulating the output for at least a while once the input voltage was sufficiently high and the output voltage reached 13.1 volts DC or 13.6 volts DC depending upon how the jumper was set.About 10 minutes after performing that test and realizing that it was operating somewhat I came back here and posted so people would know what had happened.After reading the helpful posts here I had a plan of action. I decided to monitor the feedback voltage at the UC 3845 to see what was happening when regulation began to fail. As I mentioned I can only go downstairs to the bench to work for about 15 minutes at a time and when I went down the last time with the intention of monitoring the feedback voltage I cranked up the input voltage with the variac feeding through a current limiting lightbulb and it operated as described previously.I decided to quickly check the 15 volt zener Vcc power for the UC3845. To my surprise, Vcc was only 8 volts. As I increased the input voltage it would hold a steady 8 volts.I'm somewhat hesitant to increase the input voltage too much as I don't want the output voltage to jump so high that it damages the output capacitors. Vcc for the UV3845 never reached 15 volts at least not when I did that test. Since it was holding steady at eight volts my thought was perhaps the zener had been damaged.I stopped and came upstairs and thought about it for a while. I did a quick test and the circuitry that runs off the 15 volt supply uses about 28 milliamps of current at 15 volts. That supply comes directly from the rectified 160 volt DC through a 3K 10 watt resistor which feeds the 15 volt zener. After thinking about it for a while I realized that the input voltage, which I had only run up to about 55 volts DC rectified, was not supplying sufficient current through the 3K current limiting resistor into the zener to supply the 28 milliamps needed. That current through the 3K resistor is 84 volts and add 15 for the zener diode and we're up to 99 volts needed just to get the zener to turn on.I know that when I provide help to someone via a forum I'm always interested in knowing what happened and how they're getting along so I thought I'd make this quick post. It takes me forever to do a repair due to the short time I can spend at the bench and I thought while I'm lying here waiting for the pain to subside I'd make a quick update and test out my voice recognition software (which is why this is so long).I pretty much know what I'm going to do next to figure out whether or not the UC 3845 is damaged and why it's not regulating beyond a certain input voltage.I've also got some ideas for adjusting the snubber circuit and how to monitor what's happening to see whether or not I'm overstressing the main switching MOSFET. Anyway, that's all for now. I'm always thankful for any comments anyone has, but I thought I'd provide this update.

author: tango17
author_rank: Regular Contributor
timestamp: March 06, 2025, 04:45:23 pm
content: A report after a quick run to the bench:As soon as input AC voltage rises enough to supply current to the 15v zener, the Vcc supply for UC3845 starts to rise above 8vdc and the output immediately rises above the requested 13.1vdc. Prior to that it's steady at 13.1 vdc. The Vref on pin 8 of UC3845 is about 2.4 volts (not the 5vdc it should be) and as soon as it rises, the output also rises. I can see that pin 3 feedback also starts to rise as it calls for less output voltage. None of this looks right. Time to replace UC3845 when next the pain subsides.

author: tango17
author_rank: Regular Contributor
timestamp: March 06, 2025, 06:32:59 pm
content: Replaced UC 3845 - no change. Same behavior.

author: tom66
author_rank: Super Contributor
timestamp: March 06, 2025, 07:24:57 pm
content: I might have the feedback behaviour the wrong way around. I would consider replacing the optocoupler. This is a bit of a parts cannon but I can't see much else that could cause this behaviour. Maybe the TL431? But that shouldn't have failed when the primary components did. Very odd.

author: tango17
author_rank: Regular Contributor
timestamp: March 06, 2025, 07:51:54 pm
content: Quote from: tom66 on March 06, 2025, 07:24:57 pmI'm enjoying the repair process. It's all about learning. I'm going to spend some time hooking up the scope, checking the switching, TL431, optocoupler, etc. Once I've got more info, I'll have a better idea of where to go from here.

author: tango17
author_rank: Regular Contributor
timestamp: March 07, 2025, 01:26:16 pm
content: Quote from: tom66 on March 06, 2025, 07:24:57 pmI have a theory. I'm familiar with these converters and know that they do not need a load to regulate their output correctly. However, I'm testing without the snubber circuit installed. I wonder if that could be causing the behavior I'm seeing. What if there is a switching spike leaking through the main transformer to the output that is normally suppressed by the snubber. As the input voltage climbs that switching spike gains energy and the output would climb rapidly without a load. That would sort of explain the symptoms I'm seeing.I'll try adding a load to see if that helps. If it does, I'll try adding a decent snubber circuit. I'll also connect the scope next time I'm at the bench.

author: tom66
author_rank: Super Contributor
timestamp: March 07, 2025, 06:41:45 pm
content: The energy in the leakage inductance is dissipated - it doesn't accumulate. So I doubt that is the cause myself. The snubber just acts to protect the transistor.

author: tango17
author_rank: Regular Contributor
timestamp: March 08, 2025, 02:46:35 pm
content: Quote from: tom66 on March 07, 2025, 06:41:45 pmI think you are right. Adding a load did not help. AFAICT, the UC3845 is in undervoltage lockout at about 8 volts. It is powered by a nominal 3K 10watt resistor (measured about 3K3) feeding a 15v zener. They are good. At about 95 vdc from the bridge rectifier the Vcc to the controller is still right about 8 volts dc, which is the UVLO value according to the datasheet. At that point the output is about correct, but as the variac turns up, the output rapidly climbs.

author: tango17
author_rank: Regular Contributor
timestamp: March 08, 2025, 03:49:38 pm
content: I'm coming to the conclusion that my UC 3845B purchased from AliExpress is fake. With the correct 15 volts applied to the chip I see the maximum 50% duty cycle at the output to the MOSFET gate. If I monitor the feedback voltage at the UC 3845 and connect a supply to the output of the PD9160 I can increase or decrease the voltage that the TL431 and optocoupler see in the feedback circuit. Below the set voltage of 13.1 volts the voltage at the pin 3 voltage feedback pin of the UC 3845 is close to 2.5 volts. If I increase the output voltage with my supply, as I cross the 13.1 volt set threshold, the voltage at the feedback pin suddenly jumps up to 5 volts. Presumably an increased voltage at the feedback pin should call for a lower duty cycle but the duty cycle doesn't change at all. I am clipped to the gate of the switching mosfet. I can go up and down on the output voltage that the feedback circuitry sees and each time I cross 13.1 volts it switches between 2.5 volts when it is below the set output and 5 volts at the feedback pin 3 when above the set voltage. If I change the set output voltage to 13.6v, the behavior switches to switching at 13.6 vdc. So, it looks like both my UC3845 chips are always at 50% duty cycle unless the input AC voltage is so low that the UC3845 is in UVLO.Comments?

author: tango17
author_rank: Regular Contributor
timestamp: March 08, 2025, 04:55:55 pm
content: I incorrectly referred to the feedback voltage input as pin 3. It's really pin 2, which was what I was actually monitoring. It never gets below 2.52 vdc no matter how low the output goes. When the output is above the setpoint voltage (13.1 or 13.6) pin 2 jumps to 4.9+ vdc.

author: tango17
author_rank: Regular Contributor
timestamp: March 09, 2025, 11:01:54 pm
content: For those who are following along, here is a quick update:First, the UC 3845B that I purchased from AliExpress is not a fake. At least it seems to be working properly. I put a known good UC 3845B from another working converter into the one I'm trying to repair and it has the same symptoms.Moreover, I put one of the Aliexpress controller chips into the working unit and it works perfectly.Secondly, I have been testing by supplying 15 volts as VCC to the UC 3845 and then supplying a voltage at the output of the converter to fake out the feedback circuitry. As I increase the voltage at the output of the converter I exceed the set point and I see the voltage at the feedback pin 2 of the 3845 go from below 2.5 volts to 5 volts.That should cause the output to shut off and stop sending pulses to the gate of the MOSFET. (In fact, that is exactly what happens when doing the same tests on the known good unit.) It seemed to me that the 5 volts at the feedback pin 2 should shut off the pulses/output to the gate of the switching transistor. Certainly the feedback circuitry all seems to be working correctly because I've tried it now at four different voltage settings and each time, as I increase the output voltage by faking it and reverse feeding it with another voltage as I pass the set point the voltage at the feedback pin suddenly rises rapidly.That's why I thought that these chips might be fake. I couldn't understand how the controller chip would still be outputting at maximum when the feedback pin was signaling that the output was too high.After thinking about it for a while, however, and knowing that the chip is good, I realized that just about the only way that the duty cycle can remain at 50% when the feedback voltage at pin 2 is telling it to shut off is for there to be a problem at the compensation pin 1.The compensation pin feeds directly into the output of the comparator error amplifier. If that is overriding the error amplifier it wil cause the UC 3845 to continuously output at maximum.I haven't had a chance to track this down yet, but I just can't see any other way to get the symptoms I am seeing. I haven't paid a lot of attention to the compensation circuitry, but I think my reverse engineering sketch (which is really a mess) probably has sufficient information for me to figure out where to go next.Wish me luck and I'll let you know as I learn more.

author: tom66
author_rank: Super Contributor
timestamp: March 10, 2025, 02:35:25 pm
content: I would not think the compensation pin would be the issue. Think about this from the point of view of the failure - what is the path of failure where MOSFET goes 'bang' and destroys UC3845 and also the compensation pin circuitry? They are typically tens of kohm resistor plus capacitor. It would have to be a really bad failure to do damage there. Anyway, bad compensation circuits more often than not just cause the PSU to oscillate (output voltage not stable) rather than cause problems regulating.Have you eliminated the optocoupler as a possibility yet? Have you checked if the PSU does operate well with a load on it? For a 12V power supply a good tool for using as a load is an old car headlamp, get one from a junkyard with good bulbs... or just put the bulbs on your desk. That would be 50W for dip beam, 50W for main beam (may vary in USA but around this range).Have you tried freezer spray on some parts to see if the feedback voltage changes? Bad optocouplers can have temperature dependencies. Many years ago I had a Panasonic plasma TV where the Vscn voltage (~190V) was not well regulated - side effect was TV would shut down with scan fault code SOS7. If I froze the optocoupler that was responsible for this, regulation restored and TV operated until opto warmed up again.

author: tango17
author_rank: Regular Contributor
timestamp: March 10, 2025, 03:52:27 pm
content: Quote from: tom66 on March 10, 2025, 02:35:25 pmI've checked it several ways. The diode voltage tests good on the secondary side. The transistor impedance drops when current passes thru the diode. But think about this: The voltage at the opto output (trannie emitter) goes from about 2.3v when the PSU output voltage is below the PSU setpoint and immediately jumps to 5v (the voltage at the opto trannies collector) when it is raised above the setpoint. That voltage is applied to the controller feedback pin. It should be seeing a too high output voltage and reduce the duty cycle. That's exactly what happens on the good unit, but not the damaged one. How can the controller still be sending at the maximum duty cycle when the feedback pin is telling it the output is way too high? Look at the block diagram on the UC3845 datasheet I posted. The feedback pin goes to the inverting input on a comparator that compares it to 2.5v. When feedback goes high it should cause the error amo/comparator to go low, which should cause the duty cycle to drop to zero. It doesn't.However the compensation pin feeds directly into the output of that error amplifier / comparator.QuoteYes, I've tried but remember, the psu isn't working correctly. It doesn't seem to have much of an effect it still operates as before.I just tried checking the resistance between the feedback pin and the compensation pin. On the good unit that resistance is about 5K. On the bad unit that resistance is about 2.3 k. As best as I can see there is a 5K resistor from the compensation pin to the feedback pin and another 5K resistor from the feedback pin to ground both of those resistors are good as far as I can tell and both of them have a single capacitor in parallel with them. I removed the resistor 5K on the bad unit and checked it and it is good. I'm now back into bed, but if the capacitor is damaged and leaking and is producing the 2.3 k resistance, then when the optocoupler goes high, the feedback pin will go high to 5 volts which should cause the duty cycle to drop to zero. However the compensation pin is connected to the output of the error amplifier, ie, it is not an inverting input. If the two 5K resistors form a voltage divider that is connected to the feedback pin, then the low resistance at the compensation pin will cause it to be higher than it should or at least it could be doing that, and the higher voltage at the output of the error resistor would indicate a low output voltage of the PSU. This would cause the UC 3845 to continue at maximum duty cycle which is what I'm seeing. Bottom line, my next step is to check the capacitor in parallel with the resistor between the compensation pin and the feedback pin to see if it has been damaged.I really appreciate your comments about thinking what might have happened or could have happened during a incident that caused the damage, however this was an extreme accident and we really don't know anything about how it occurred whether it was some lightning strike which is certainly possible given the multiple melted parts of the ground plane and who knows where the voltage went and what components were damaged. I really think I'm on the track of something given that I know the controller chip is good and that it's refusing to stop the duty cycle that is telling the main MOSFET to continue switching even though we know the controller chip is seeing a very high output voltage at the feedback pin which should be causing it to turn off.

author: tango17
author_rank: Regular Contributor
timestamp: March 10, 2025, 04:12:43 pm
content: I quickly checked the resistance between the Vref (5v) on the UC 3845 and the compensation pin. Vref feeds the opto coupler collector and that is the voltage applied to the feedback pin when the output of the psu goes above the set point. On the good unit that resistance is 12K on the bad unit that resistance is 250 ohms. I think that makes it pretty clear that on the bad unit there is a problem causing the Vref voltage to be fed to the compensation pin which is overriding any input on the feedback pin. Both the 5K resistor and the capacitor between the compensation pin and Vref seem to be good so the problem is elsewhere, but wherever it is it is leaking onto the compensation pin through that low resistance.The compensation pin sees a high enough voltage that it thinks the output is low below the set point even though the feedback pin is high.

author: tom66
author_rank: Super Contributor
timestamp: March 10, 2025, 04:41:26 pm
content: No worries, good luck with the repair, it sure is a strange one. Let us know how it goes.

author: tango17
author_rank: Regular Contributor
timestamp: March 10, 2025, 07:25:18 pm
content: It would sure help to have a schematic.My reverse engineered schematic showed nothing connected to the compensation pin, beyond the cap and resistor I tested as good. However, looking closely at the circuit board I eventually found a via under a component which ran off to a far corner of the PC board.That corner of the board seems to be dedicated to controlling the fan and monitoring a thermistor on the heat sink. Possibly the connection to the compensation pin is some kind of backdoor pseudo enable/disable connection that turns off the switching by controlling the voltage at the compensation pin when the heat sink gets too hot. I'm not really sure.Anyway I found a dual Schottky diode marked KL2. One of the diodes was bad and some research identified that component as a BAT54A. I removed that component and the low resistance connection between the feedback pin and the compensation pin disappeared and began to match the resistance measured on the good board.Next up will be to hook up the test system again and this time see if it begins to control the duty cycle as needed to hold the set point voltage. I'll let you know.

author: tango17
author_rank: Regular Contributor
timestamp: March 11, 2025, 04:52:34 pm
content: Quick update:My test setup involves supplying 15 volts Vcc to the UC3845B controller chip and reverse feeding a voltage at the PSU output. That lets me control the voltage feedback/monitor system by simulating an output that is either too high or too low. I am monitoring the output of the controller chip feeding the gate of the main switching MOSFET with an oscilloscope to see the duty cycle the controller chip is sending.With the Schottky diode removed, the controller reduces the duty cycle to zero when the simulated output goes above the output voltage set point and then turns the duty cycle back up to its maximum of 50% when the simulated output voltage drops below the set point. Hooray! That's exactly what it should do. Next step is to actually supply 120 volt AC to the input and put a load on the converter to see if it survives. I'll start by limiting the input AC current and voltage with the variac and a series light bulb current limit as usual.I didn't have a SOT 23 dual Schottky diode to replace the bad one. In fact, I didn't have any small Schottky diodes. So I replaced it with a pair of small signal diodes. I'm not really sure of what these diodes do. They are related to the control circuitry for the cooling fan. I think they connect to a dual comparator or quad op amp over in that area which adjust fan speed and/or turns it on and off according to the heat sensed by the thermistor. So I'm not really sure if the forward voltage difference between the standard diodes I've installed and the Schottky diode makes a difference. Eventually I will install the correct components if I decide I really want to use this unit after it is repaired. But at this point Michael is just to learn a bit more about switch mode power supplies and repair procedures.I will comment that the circuitry for the cooling fan was badly damaged when I received the damaged unit. The filter capacitor at the fan supply was blown and the main switch transistor that turns the fan on and off was also blown.I'll let you know what happens after another couple of hours of recovery.Please forgive any errors in the typing. I'm using voice dictation software and posting with my Android phone. I try to correct any errors but might have missed some.

author: tango17
author_rank: Regular Contributor
timestamp: March 12, 2025, 04:17:21 pm
content: It works up to the limit I tested of about 5 amps output.However, the control circuitry for the fan is not working. There's a dual comparator connected to that blown dual Schottky diode that isn't working. For some reason the ground pin on that chip isn't connected to ground and there are many blown components in that fan circuitry including a ground plane a short distance away that has a hole melted in it. I suspect another via is gone (I found a blown one elsewhere) because the ground pin on the working unit is connected to ground correctly.I still have to figure out the snubber circuitry (which is why I came here in the first place) and what circuit I'm going to use for that. I'm also going to have to reverse engineer the control circuitry for the cooling fan to get that working. I'm very slow at reverse engineering so it probably will be a while if ever before I post again.Thanks for all your help. I appreciate all your comments.

author: tom66
author_rank: Super Contributor
timestamp: March 12, 2025, 04:51:02 pm
content: If you aren't too bothered about noise, I'd just connect the fan directly to the 13V output - it'll run all the time, of course, but depending on the location/application this may be acceptable. You'll want to make sure the fan is off when the battery isn't charging though, which might be the reason the control circuit is there.

author: tango17
author_rank: Regular Contributor
timestamp: March 15, 2025, 05:13:39 pm
content: Quote from: tom66 on March 12, 2025, 04:51:02 pmI started to try this, just to make sure the fan and fan supply worked. Instead of using the isolated 13.6v output to drive the fan, I tried to connect the normal fan supply derived from an auxiliary winding. However, it didn't seem to work. The fan oscillated, but didn't turn, so I first checked the fan. It worked down to 2 volts and 50 mA. I then checked the voltage to it with the fan disconnected. It was 15 volts. At that point I did something stupid when a probe slipped, which I'm not going to admit further to save my pride.Also, I found a comment in the manual that says it cuts the output completely when it exceeds a certain temp. I think that the compensation pin, which connects through some diodes to a comparator is how that shutoff is done The comparator receives a signal from the thermistor, and I think when the thermistor says its too hot, the comparator switches state which drives the compensation pin to shut off the output until it cools.