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Old 08-09-2021, 09:04 AM   #1
Dannyx
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Question NCP1271 power supply not working under load

Good day folks. Bit of a dilemma here and I'd like some help so as not to spend too much time and resources on something that may be either really easy to fix or a rabbit hole.

I pulled out an old Bosch analog PTZ CCTV camera from one of our sites. The camera was dead - no power and no signal at all. I took it apart and after some tinkering I found it's not getting any power to its "brains". It takes a 24Vac input which is then rectified and dropped down to 12v (I think) by a small power supply based around an NCP1271. When I first got it, there was absolutely no power on the output of this internal PSU at all. I managed to get it going to a degree, but it's still not enough: initially, I found a short on the VCC pin of the IC (D359 - sorry I don't have a pic of the whole board and it's pretty messy on there). Based on how it was installed (anode to GND and cathode to VCC) I figured it's a protection diode and after removing it, I tried to fire it up without it, since I don't have a similar one. I now get 17v on the VCC pin of the IC and 12v on the output caps, which is fine, but when I connect the head of the camera, it shuts down and power cycles every 5s or so. I though the head itself may have some bugs of its own, like a short, so I tried a small 12v fan on that 12v output, but got the same result - the output shuts down and begins to power cycle. It's worth noting the the VCC pin of the IC ALSO drops down to almost nothing with each cycle. It then SLOWLY goes back up again and once it reaches around 12v, the output goes high again, before immediately shutting down. This makes perfect sense, since the datasheet clearly tells us UVLO is 12.6v.

Now I'm pretty stumped, since I already replaced the two caps on that VCC line, but still no change - the lightest load shuts it down. When unloaded, it's perfectly stable at 12v, but what good is that ? Can the chip itself be faulty in some way ? How come it runs normally when unloaded ? :|

It's not so much about the camera since I could just scrap it and tell them to get a replacement, or just wire a power adapter straight to the output of the internal supply and that would keep it going for a few more years. It's mostly about fixing stuff and enjoying the fruits of your labor. Cheers

EDIT: I now realize that diode may've been a zener diode...whether or not that affects the functionality, I can't tell....
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Last edited by Dannyx; 08-09-2021 at 09:47 AM..
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Old 08-09-2021, 12:29 PM   #2
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Default Re: NCP1271 power supply not working under load

If the actual circuit is similar to the sample circuit on the datasheet, one possibility is that the current sense resistor in the MOSFET Source circuit has increased in value and loading the power supply causes a current limit trip. Another possibility is that the output capacitor is so high ESR that the output voltage droops long enough that the controller interprets the consequent FB voltage change as an overload.
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Old 08-09-2021, 12:32 PM   #3
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Default Re: NCP1271 power supply not working under load

I'll check both of those, though the CS resistors are SMD by my recollection and measured fine. I actually pulled one of the output caps (there's 3 in parallel) and it measured somewhere around 430uF instead of 470uF....which probably means nothing on the surface - it could still be good/bad - but the ESR was only nought-point something and had a Vloss of like 1v...
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Old 08-09-2021, 05:17 PM   #4
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Default Re: NCP1271 power supply not working under load

How is the main cap that filters the 24vac? The dc should be around 34vdc, is it steady?
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Old 08-10-2021, 10:34 AM   #5
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Default Re: NCP1271 power supply not working under load

Quote:
Originally Posted by R_J View Post
How is the main cap that filters the 24vac? The dc should be around 34vdc, is it steady?
There's 2 main caps. They read around 38vdc if I remember correctly, which remains fairly stable even when the supply shuts off. I'll provide a better pic of the whole board tomorrow.

My bad: the CS resistor is indeed a regular low value thru-hole resistor and it reads fine (at least as low as my meter can measure), so right now it's either between those 3 output caps or the IC itself. I also got an 18v zener to replace the one I removed, just in case.
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Old 08-11-2021, 08:57 AM   #6
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Default Re: NCP1271 power supply not working under load

Here's a follow-up with some pic of the whole board. There's not too much going on. Power supply is on one end, while the stuff at the opposite end most likely has to do with RS232, as I spot some trimmers which probably set the address.

The CS resistor is the black thru-hole 0.4ohm one and it reads almost a dead-short, so it's probably fine. I'll try swapping the 6 remaining caps. The two on the left are in parallel and are for the HV DC input and the 4 on the right and top are also in parallel and are the output. You also see two wires soldered on there: I used them to place a load on the output and also to test if the PTZ head still works by sending 12v from an external supply straight into the output of the internal supply, so as not to make this whole attempt pointless in the end. Cheers and thanks.
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Old 09-02-2021, 07:41 AM   #7
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Default Re: NCP1271 power supply not working under load

Update: camera is now working. Turns out it was a combination of missing zener diode AND bad caps. I installed the missing 18v zener diode and replaced all the caps, both for the input and output sides. I just replaced them all in one go, so I'm not entirely sure which ones were at fault, especially since they appeared to be within spec on my ESR meter. Cheers and thanks for the input
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