Help with Vultech pure sine wave 1500va repair attemps with distorted output - Page 1
Forum section: electronics / repair

author: giovy87
author_rank: Newbie
timestamp: January 26, 2025, 10:18:33 am
content: Hello everyone,I have been trying for a while to repair this UPS that stopped working after the battery replacement. The owner replaced the batteries but did not follow the manufacturer's guide, which includes steps like disconnecting the transformer. Upon first power-up, there were fireworks!Here's what I have done so far and the issues I have encountered.Initial Analysis:24V line fuses blown (3 fuses of 30A each)6 faulty MOSFETs in the H-bridge (model TTB115N08A)Gate bias resistors open (22 ohms)Reset IC shorted (KIC73B263T), replaced with an equivalent (R3130N26AC3-TR-FE), but not pin-to-pin compatible, so I mounted it upside down.Suspect 2W 2.2-ohm resistor replacedOpen diode (1N4001) replacedI replaced the 6 MOSFETs with 2 identical ones, one per branch of the H-bridge, to see if the circuit works correctly without damaging the MOSFETs if the controller is faulty. I also replaced the SMD gate resistors with new ones.Testing:I powered the UPS with a 25V bench power supply. The display powers up, and when I try to turn it on, it starts for a couple of seconds, the transformer vibrates, and then the UPS shuts down because the bench power supply goes into protection mode. I tried to provide maximum current (around 5A), but the display shows 2.80A before it goes into protection mode.Oscilloscope Measurements:With an oscilloscope, I measured the output waveforms:Positive Half-Wave: Essentially a triangular wave.Negative Half-Wave: Not distorted and seems acceptable.Further Investigation:I focused on the PWM circuit that drives the power MOSFETs. PWM+ and PWM- are generated by a microcontroller, and I verified that the signals are present and do not overlap in pairs as they should for proper control. However, at the input of a comparator used to drive the transistor gates, two signals matched the corresponding microcontroller channel waveform, while the other two had less steep rising edges, clearly showing a capacitor charging curve.Troubleshooting:I hypothesized that the RC constant of the input stage was out of spec. I identified the circuit: each stage consists of a resistor, a 1N4001 diode, and a capacitor. I replaced two diodes but saw no change. I measured two SMD capacitors, one from the distorted branch and one from the non-distorted branch. The functioning branch capacitor measured 52pF, while the distorted branch measured 502pF (a factor of 10 difference). I replaced the two capacitors, and the waveforms at the comparator input and output were no longer distorted, but the problem persisted.I expected the inverter to work without a load even with a bench power supply. This is my first time repairing a pure sine wave UPS, and I have no schematic, so I'm relying on experience and isolating the fault step by step.Waveform Links:Output Waveformhttps://ibb.co/fQbsQmkThe period is 20ms, indicating a 50Hz sine wave, but as you can see, the positive half-wave is very distorted, causing the controller to go into protection mode (at least, that's my expectation).MOSFET Waveforms - Pair 1https://ibb.co/Y7vRXhcMOSFET Waveforms - Pair 2https://ibb.co/pzHd8GtUpdate 26/1:After replacing the two input capacitors to the comparator (acting as a gate driver), I noticed the UPS did not start cold. The buzzer sounded, but no generation occurred, which usually happens when protection is triggered. Since the behavior had worsened, I reinstalled the original 502pF capacitors. The UPS now functions as before, with a distorted positive half-wave but still starting.Based on my understanding, the issue is 90% likely in the PWM signal generation, as the negative half-wave is not distorted, implying a possible software issue with the PWM signals (PWM+ and PWM-). At the microcontroller output, these signals are identical, while I expect them to be complementary. I don't think the complement is handled by the conditioning network but should be done via software.Could anyone indicate what the PWM signals generated by sampling the sine wave should look like? I have PWM+ and PWM- signals, which are identical at the microcontroller output. Shouldn't they be complementary?Thank you for your suggestions on tests to perform, measurements to share, or approaches to isolate the fault.

author: giovy87
author_rank: Newbie
timestamp: February 01, 2025, 02:17:18 pm
content: ciao potresti tradurre in ingflese questo testo: Ciao a tutti, ho visto che il thread viene visualizzato ma nessuno ha postato un feedback immagino che comunque possano interessare degli aggiornamenti. Ho continuato la mia analisi e di fatto ho fatto reverse engineering di questo circuito ecco cosa ho verificato: - micro genera i segnali PWM attesi e con la forma d'onda corretta - segnale di riferimento presente e corretto (sinusoide a 50Hz) - lo stadio di driver mosfet funziona e amplifica correttamente i 4 segnali da inviare ai gate Ho notato però che nel ramo che ha bruciato i 3 mosfet che compongo un ramo superiore del ponte ad H i segnali generati non sono "puliti"come per gli altri 3 gate. Mi sono concentrato sul circuito driver di questo canale seguendo il segnale dal comparatore fino a giungere ad un mosfet 2N7002K che in pratica è interposto tra il comparatore ed il mosfet di potenza (questo picco mosfet c'è solo nel ramo superiore del ponte ad H in quello inferiore non è presente) . Facendola breve in parallelo a questo mosfet ho trovato un bjt NPN (2N5551) aperto e l'ho sostituito a questo punto il problema non si è risolto ma come potete veder nell'immagine allegata non ho più un fronte lineare ma una sinusoide distorta, quindi il comportamento è migliorato. Allora ho capito che il problema è sicuramente in questo ramo del ponte e per esclusione sono arrivato a sospettare dell'integrato di reset che si era bruciato e che ho sostituito con un equivalente. Mi sto riferendo al KIC73B263T che ho sostituito con un R3130N26AC3 che sulla carta dovrebbe essere compatibile a livello funzionale, ma non pin2pin (ho risolto saldandolo al contrario). Ai tempi dell'ordine non notai però che l'integrato nuovo assorbe una corrente di circa 3 volte rispetto all'originale quindi non ho escluso che questo potesse creare degli sbilanciamenti. Per provare che fosse questo il problema allora ho deciso di scambiare l'equivalente con un integrato originale del ramo gemello. Effettuato lo scambio ho visto che la situazione si è ribaltata adesso la semionda positiva è corretta quella negativa è distorta!!! (immagine -->https://ibb.co/KYh6Jqq) In conclusione credo di aver trovato la causa del guasto ma la brutta notizia è che non riesco a trovare un sostituto del KIC73B263T perché ormai sembra non più in produzione. Non arrendendomi ho cercato di usare alcuni sistemi di generativeAI (copilot in primis) per trovare un equivalente ma quando ricerco questi nei vari siti non risulta mai disponibile, per esempio ho trovato : TLV809ED26RDBZ ma questo non risulta disponibile. dopo l'aggiornamento ecco la mia richiesta di supporto: qualcuno mi saprebbe indicare dove posso comprare anche usato il ricambio originale(KIC73B263T ) o un equivalente al 100% ?Sure! Here's the translation of your text into English:Hello everyone,I noticed that the thread is being viewed, but no one has posted any feedback. I imagine updates might still be of interest.I continued my analysis and effectively reverse-engineered this circuit. Here's what I verified:The microcontroller generates the expected PWM signals with the correct waveform.The reference signal is present and correct (50Hz sine wave).The MOSFET driver stage works and amplifies the four signals sent to the gates correctly.However, I noticed that in the branch where three MOSFETs burned out (forming the upper branch of the H-bridge), the generated signals are not "clean" like the other three gates.I focused on the driver circuit for this channel, following the signal from the comparator to a MOSFET 2N7002K, which is essentially placed between the comparator and the power MOSFET (this MOSFET is only present in the upper branch of the H-bridge, not in the lower one).In short, in parallel with this MOSFET, I found an open NPN transistor (2N5551) and replaced it. At this point, the problem was not resolved, but as you can see in the attached image, I no longer have a linear front but a distorted sine wave, so the behavior has improved.Therefore, I concluded that the problem is definitely in this branch of the H-bridge, and by exclusion, I suspected the reset IC that had burned out and replaced with an equivalent. I'm referring to the KIC73B263T, which I replaced with an R3130N26AC3, which should be functionally compatible, but not pin-to-pin (I resolved it by mounting it upside down). At the time of ordering, I didn't notice that the new IC absorbs about three times the current compared to the original, so I didn't exclude that this could create imbalances.To verify that this was the problem, I decided to swap the equivalent with an original IC from the twin branch.After the swap, I noticed that the situation reversed: now the positive half-wave is correct, but the negative half-wave is distorted! (image -->https://ibb.co/KYh6Jqq)In conclusion, I believe I have found the cause of the failure, but the bad news is that I can't find a replacement for the KIC73B263T as it seems to be out of production.Not giving up, I tried to use some generative AI systems (Copilot first) to find an equivalent, but when I search for these on various sites, it's never available. For example, I found:TLV809ED26RDBZbut this is not available.After the update, here's my support request:Could anyone indicate where I can buy the original replacement (KIC73B263T) or a 100% equivalent?