1kw active clamp flyback smps with too high idle current - Page 1
Forum section: electronics / repair

author: Vicus
author_rank: Regular Contributor
timestamp: June 09, 2020, 02:23:56 pm
content: Hi, I'm try to repair a power amplifier module that (now) have a ho high idle current that cause to overheath it to fast and without load.The schematic of the power amp can be found here:https://elektrotanya.com/qsc_k8_k10_k12_ksub_k_series_sch.pdf/download.htmlso the amp came in with the main mosfet explosion and a piece of the pcb burn. I clean best as I can removing the burned pcb and restoring the ground plane underneath. I replace all the damage component like both mosfet, ic controller, gate resistor, protecting diode ecc. Now I have that it works, all the voltage at the secondary if fine and the class D power amp work. The problem is that after a couple of minute the alluminum shall that act like a heatsink beacame hot and the corrisponding area that's under the main switching mosfet became impossible to touch even without load (no audio signal). I tested all the component in the primary side and all seems fine, I try also to replace both mosfet and controller ic with no luck. I measure the idle current with my Agilent 34401A in AC current soldering the probe lead in a blown fuse (offcourse with another main fuse in the power lead). I did measere 550mA with the power amp running and 440mA with the poweramp forced in shutdown mode. That mean 126W with 230V voltage. Too high because the service manual say from 100 to 200mA with 230V ad idle. I think that the measure is not so good because without mosfet and power trasnformer it still read 90mA sink current. Another strange thing is that the 100ohm resistor R166 that form a snubber (with a 220p cap) from primary to secondary of the transformer seems to overheat (discoloration in the pcb). Now I think can be a bad power transformer (maybe partially shorted) or a bad driving of the mosfet. Any idea how to measure a flyback transormer? I cannot measer the gate signal because I don't have an isolation transoformer or a differential problem.Some idea ?Thanks.

author: CaptDon
author_rank: Super Contributor
timestamp: June 09, 2020, 03:10:18 pm
content: You will need to decide if the fault is on the power amplifierside, perhaps bias idle current is to high after replacing blownout components, and maybe you haven't found all of the badparts over in the audio amplifier section. What you really needto know is the 'normal' pulse width of the switch mode powersupply at idle and see if your pulse width is much longer. Ifthe power transformer has a shorted turn that would causethe pulse width to increase. Also if any of the bridge rectifierson the output side of the transformer are bad that could causeextra loading or the regulated voltages to be way off also makingthe duty cycle to long. You will surely need an isolation transformerto do this testing. Since most of your troubleshooting will be atlow audio power, or even no audio output you can use a muchsmaller iso transformer, perhaps 200va rated. I have over 25years of SMPS skills repairing the Carver PM-2.0T 1 kilowattaudio amps specifically built for Clair Brothers / ShowCo.Tell us more as you continue to troubleshoot, we may be ableto offer additional ideas.

author: CaptDon
author_rank: Super Contributor
timestamp: June 09, 2020, 03:28:26 pm
content: Are the output voltages on the secondary side of thetransformer extremely high or low? If the snubberis running hot there may be a problem in the clampcircuit not draining energy from the transformer coreand the transformer is saturating even without a loadon it. It looks strange, they rectify the A.C. line input andthen have the audio power amplifier section referencedto the negative side of the line rectifier? That almostseems dangerous? I can't exactly follow their designin that area. I have no experience with QSC products.

author: Vicus
author_rank: Regular Contributor
timestamp: June 09, 2020, 09:39:11 pm
content: The audio parts have no issue. I'll try to remove the output power transistor but I'm quite shure it has no problem. For shure my english is very bad so I explain that the previus mentioned mosfet is only in the psu.itSo if the problem is in the transformer how to measure the fault? I measure the inductance od every winding and it seem ok but I realy don't know what to looking for so I can be wrong. Does the indutance reflect the transformer ration? (if it's possible to talk about ratio in a flyback transformer)About the voltage at the secondary, the +-85V is regulated so it's perfect. The other one I don't know. I'll measure.The snubber (can be called snubber in that position?) from primary to secondary now that the psu "work" no longer heat up.The secondary gnd is linked to the earth via the chassis.

author: xavier60
author_rank: Super Contributor
timestamp: June 10, 2020, 08:22:08 am
content: Can you test C116?

author: Vicus
author_rank: Regular Contributor
timestamp: June 10, 2020, 09:28:52 am
content: Yes, with the peak atlas LCR45 read 684nF.

author: xavier60
author_rank: Super Contributor
timestamp: June 10, 2020, 09:39:20 am
content: There are so many possible problems that are difficult to check for.Maybe still a faulty part in the Gate circuit for Q14 or maybe no drive to Q13.Can you safely measure the voltage across C116?, although I don't know what it should be.I wouldn't try loading the amplifier in case it causes faster heating of Q14.

author: Vicus
author_rank: Regular Contributor
timestamp: June 10, 2020, 09:52:21 am
content: I did check all component from U15 to Q13 and Q14 by removing and testing it. D24, R157, R168 and D23 new. During the troubleshooting procedure I did not replace D23 that was shorted. So no drive to the active clamp section, the result is not enought power transfered to the secondary so the aux winding that power U15 din't reac enought voltage to sustain the gate driving action. By that time I did replace again U15, Q13 and Q14.