Dell Power Supply NPS-250KB B

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Pilm2013

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For the above mentioned power supply, I've had issues where the power supply will not restart after a power outage. Both times I removed the power supply and examined it looking for any damaged parts like a blown fuse, or caps, etc, but found nothing. Then did some live probing, had AC where expected and rectified high voltage (doubled) DC in some places, but w/o a schematic that's about all I could do, nothing thorough. Anyway, despite having some AC and the high voltage DC, it had none of the lower voltages (+12, +/-5, etc). So I'm wondering if there's something that isn't getting RESET properly after an unexpected loss of power that's inhibiting the lower voltages. I see there's a PS_ON signal, I think it comes from the motherboard, to put the PS into Standby mode, wondering if that might not reset to 0V after power failure? Or maybe something in the protection circuit isn't resetting? Anyway, what I've found is if I leave the unit unplugged overnight, it'll restart in the morning, at least it has the 2 times this has happened. Kind of seems like something on the board is still energized and inhibiting turn on. I did by the way discharged the 2 primary caps, but perhaps there's more stored energy elsewhere that's keeping the thing from immediately restarting after a power failure. Any ideas appreciated, thanks.
 

I have something similar experience with my printer power adapter. It suddenly stopped permanently. I opened it, checked all the components. First I thought maybe the power MOSFET is burnt. I changed it, but no success. Thoroughly I checked each and every component one-by-one. They all works fine. So later I concluded that the protection IC is somehow malfunctioning. Because this IC's are precisely calibrated with the input and output voltage. Even, if its calibrating resistors deviates from its value, it will stop the power supply. This IC's will cut the power supply if they detect voltage/current different from the desired value.
As you said that keeping your adapter off for over night makes it work again, it implies there maybe a fault in proper calibration. Either any particular IC in the adapter is malfunctioning, or some resistors & caps (probably smd) are deviating from its marginal value after long operation. This can happen if those components have Q factor of less than 100 (inferior quality).
I don't think any electrolytic cap can hold any unwanted charge for such a long time.
 

I can't be specific without a schematic but in general, when an SMPS fails to start up, the culprit is usually a dried up capacitor on the primary side. Most of this kind of PSU need a 'kick' to start them running and it usually comes from the current flowing through a capacitor as it charges. The lower value when it dries out means less current and sometimes the oscillator or driver can't start running. Look for low value, high voltage electrolytics, maybe in the 1 - 10uF range and rated at 250V or more. Similarly, when a PSU fails to RE-start it's around the same part of the circuit but open circuit (or increased value) resistors that are at fault. For that initial kick to get it started to occur, the internal power rails need to be discharged first and a 'bleed' resistor is placed across them to ensure no residual charge is present after a short time. When the resistor goes high in value it fails to fully discharge the internal (NOT the PSU output) power rail and the start current is insufficient to get things moving again.

Brian.
 


Thanks Brian. Do you know if these kind of PSU's have a power off sequence to discharge themselves when shut down normally? What I'm thinking, if I turn off my PC normally, I'm able to restart it almost immediately without any trouble, so it works fine in normal operation. However when a power outage shuts off my PC so it doesn't have a chance to shut down normally, that's when it won't restart unless I let it sit for a long time (5-10 hours). Also, since it works fine if shut down normally, I'm guessing there *probably* isn't a bad part, but maybe a design issue in that Dell didn't design the power supply to account for an abnormal shutdown like when the power goes out.
 
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It's possible but the only difference between a 'shut down' and complete power failure is that the auxiliary supply is left turned on after shut down. It's the aux power that lets the computer monitor the power button when you turn it back on again. When it happens again, it would be useful to see if the aux supply is present or not. It should be there whenever the AC is present and the main power switch (if you have one) is turned on, even if the computer itself is shut down.

Brian.
 

OVP (over voltage protection) circuit is kicking in. Get the schematic of computer switch mode power supply (do the google search, there are tons of them around) and most are same as far as voltages concern. Also it will give you an idea of how they turn on and how windows shuts them of when you turn off the computer. OVP is the circuit if voltages go over the rating it kicks in and will not let you start again unless you completely disconnect from the power source.


 

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