UC3895dw - Page 1
Forum section: electronics / repair

author: kraychek
author_rank: Newbie
timestamp: August 30, 2021, 10:44:11 am
content: Hi,I am trying to repair an hp server power supply DPS-460EB. Does anyone have anything close to a circuit diagram or maybe even a good block diagram? Also, has anyone got any experience in troubleshooting the circuitry of UCC3895 ic? This chip is a phase shift pwm controller used in the mentioned power supply. Has anyone played around with this chip and possibly built a test circuit for it on a breadboard?Many thanks - Kraychek

author: fzabkar
author_rank: Super Contributor
timestamp: August 30, 2021, 08:12:46 pm
content: Is this evaluation module circuit diagram of any use?https://www.ti.com/lit/pdf/sluu109

author: TheMG
author_rank: Frequent Contributor
timestamp: August 30, 2021, 11:34:51 pm
content: With power supplies since service information is usually unavailable you have to do a little bit of reverse engineering, or at least figure out the topology and operational blocks, and begin troubleshooting by determining the faulty block.Any type of computer/server power supply will have a small mostly independent "standby" supply, so the very first thing to check is that it is operating properly. The standby supply needs to be functioning in order for the main supply circuit to become active.If the standby supply is working, next thing to check, is the PFC regulator operating? Easy to tell by measuring voltage on the bulk input capacitors with the power supply enabled. Should be in the range of approx 380-420V typically, if it is only measuring rectified mains voltage then it's not operating.If standby and PFC is ok, time to turn your attention to the main part of the power supply. Referring to the datasheet, make sure the PWM controller IC has proper Vdd voltage (refer to datasheet for UVLO turn on voltage) and that it is not "hiccuping". Ensure it's not being sent a disable signal either, if the disable pin is active it could be due to a protection being triggered (over-voltage, under-voltage, over-current, etc), usually computer power supplies have a dedicated IC handling all the protections.Generally speaking if fuse is intact and there is no obvious damage (burnt/blown components), failures are usually due to bad capacitors, so it's worth quickly going over them in-circuit with an ESR meter.It may seem daunting at first but once you've figured out all the building blocks and important voltages/signals in the power supply, they are all very similarly designed so generally repairing them in the absence of diagrams and schematics is not a problem.An isolated high voltage differential probe for an oscilloscope is invaluable in troubleshooting SMPS. Can get by with isolation transformer and a regular probe too but extra caution must be observed in the primary side of supply.

author: kraychek
author_rank: Newbie
timestamp: August 31, 2021, 08:45:37 am
content: Thank you for the elaborate and helpful reply. I'll check the power factor correction circuit as I am getting only about 360V on the bulk capacitor. I'll check that first and move on from there. Stand by circuts seem to be working fine.Once more, many thanks to you.

author: TheMG
author_rank: Frequent Contributor
timestamp: August 31, 2021, 05:47:40 pm
content: 380-420 is only a ballpark figure. Compare it to your mains voltage multiplied by 1.414, in this case if your mains voltage is lower than 255V then I'd say the PFC is working. The PFC stage is a boost regulator so if it's higher than straight rectified mains then it's a safe bet that it's working.It's much easier to tell in countries with a lower mains voltage, for instance here where mains is 120V you'd only see about 170VDC if the PFC is not working.One more thing to keep in mind is due to energy efficiency standards some newer power supplies will actually disable the PFC when in standby mode. So if things don't appear to be working it's worth going through and checking if the PS_ON signal is making it to all the right places. There will usually be several optocouplers, at least one for negative feedback and a second one for enable/disable function. If the main PWM controller is on the primary side of the power supply, there will definitely be an optocoupler who's job is to enable the main power supply, so a good spot to check is to see if you get forward bias on the optocoupler (secondary side of PSU/input LED side of optocoupler) when you bring the PS_ON pin low to turn on the PSU.

author: kraychek
author_rank: Newbie
timestamp: September 07, 2021, 10:03:35 am
content: Sorry for the delay. Thank you again for your answers. The way things stand right now, there is no output from the UC3895. I have power to its Vcc pin, I have the RC ramp signal of aprox 250 kHz, but the chip is not providing any drive output. Acording to the data sheet, the chip seems to be disabled (turned off) for some reason. The peculiar thing is that it doesn't even try to start up and provide the drive to the switching transistors. I've got stand by voltages and the voltages that supply the pfc chip and the UC3895. There are also two microcontrollers on the PSU and I don't have a clue of what they are for. I'll take it easy and play with it whenever I find some spare time.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 07, 2021, 10:11:47 am
content: check on the page the dps-460 linkhttps://www.rcgroups.com/forums/showthread.php?1292514-How-to-convert-Server-Power-Supplies/page4other link for some infoshttps://simonwyss.me/blog/?tag=hp-dps-460eb-a

author: aqibi2000
author_rank: Regular Contributor
timestamp: September 09, 2021, 10:47:41 pm
content: Using a curve tracer I’ve repaired about 200+ boards using the 2895dw/38295dwCan’t tell you any other readings, what I do recall is replacing a lot of them if the output mosfets went short, it would normally take the te drivers out too which in turn would take this controller