Good day folks. A fairly particular problem here, so it's more of a generic discussion about a circuit that may be present in other products, not just this one. Over at my new workplace which covers many different fields, the chaps over at the CCTV installation department gave me a small Hikvision dome camera to look at, mostly for fun if anything, since they're rather disposable. The problem is that it runs on DC 12v, but not on PoE (power over ethernet, just in case someone is jut "tuning in" ). Must admit the task felt overwhelming at first since I thought there can be a dozen and one things that can cause such behaviour. I took them up on it nonetheless, not wanting to shy away from any job that could make me shine and I opened it up. I had no idea how a PoE circuit works and how it draws power from the RJ45 jack. I did some research and it's all fairly clear now and I made some progress on my own but the camera still doesn't run. I got it to the point where the power LED tries to come on but doesn't. It cycles on and off very fast.
The way this works is you take the 48VAC present on the line, pass it though a bridge rectifier (two in this case), then through a controller and then send it over to a DC-DC converter which we're all familiar with. You can see the little transformer in there for this purpose and going from the transformer to the right we have a transistor (AO4430), the DC-DC controller (UC2845B) and the PoE controller (TPS2376H). Initially the FET was shorted which of course meant no power at all. I replaced it and got the intermittent power cycling I'm mentioning and the FET seems to get hot, almost as if there's a short down the line, but I can't find it. I replaced the capacitor to the right of the transformer but no change. Could the transformer itself be bad ? That seldom happens in a SMPS. I couldn't find any shorts anywhere, or at least not yet and the camera runs fine on DC, so there's something related to the PoE section. Has anyone had a go with PoE devices like these that could lay a helping hand ? It would be great if I could fix this one - big thumbs up for me Bad SMPS controller is what I'm thinking, but at the same time I'm not getting stable voltage on the "bulk cap" following the PoE IC either....I was thinking of supplying 48v straight to the bulk cap with a bench supply to see if the behaviour is the same, essentially bypassing the PoE IC which I'm not sure what exactly does TBH. If it suddenly get power, it means it's bad. If it does the same thing, the DC-DC converter is at fault somehow....perhaps the IC got damaged by the FET shorting ?
The way this works is you take the 48VAC present on the line, pass it though a bridge rectifier (two in this case), then through a controller and then send it over to a DC-DC converter which we're all familiar with. You can see the little transformer in there for this purpose and going from the transformer to the right we have a transistor (AO4430), the DC-DC controller (UC2845B) and the PoE controller (TPS2376H). Initially the FET was shorted which of course meant no power at all. I replaced it and got the intermittent power cycling I'm mentioning and the FET seems to get hot, almost as if there's a short down the line, but I can't find it. I replaced the capacitor to the right of the transformer but no change. Could the transformer itself be bad ? That seldom happens in a SMPS. I couldn't find any shorts anywhere, or at least not yet and the camera runs fine on DC, so there's something related to the PoE section. Has anyone had a go with PoE devices like these that could lay a helping hand ? It would be great if I could fix this one - big thumbs up for me Bad SMPS controller is what I'm thinking, but at the same time I'm not getting stable voltage on the "bulk cap" following the PoE IC either....I was thinking of supplying 48v straight to the bulk cap with a bench supply to see if the behaviour is the same, essentially bypassing the PoE IC which I'm not sure what exactly does TBH. If it suddenly get power, it means it's bad. If it does the same thing, the DC-DC converter is at fault somehow....perhaps the IC got damaged by the FET shorting ?
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