Dell D550p Not Starting

[mmartell]

This is from a Precision 470 and is my back up supply.

This was dead and had blown the fuse. Found a shorted 24N60C3 in what I think is the PFC circuit and replaced it and its companion for good measure.

The supply starts fine into standby with a good standby voltage present. When asked to power up the secondaries it ticks several times a second until the power is pulled.

I am using a dim bulb tester with a 100w bulb when trying to start it. The lamp lights in unison with the ticking.

Is this indeed the PFC section and should I be looking at the driver ic that is probably on the riser card beside the heatsink ? Can I test the drive signal without pulling that card ?

[mmartell]

Re: Dell D550p Not Starting

Pics....

[mmartell]

Re: Dell D550p Not Starting

Anyone ?

[Per Hansson]

Re: Dell D550p Not Starting

Ticking can be due to a short.
Have you checked all secondary rails to make sure they are not shorted?
Ticking can also be due to bad startup caps, check the small caps on the primary side, or just replace them.
That the light comes on briefly is normal, that is the main capacitor charging up.

[mmartell]

Re: Dell D550p Not Starting

I'm sure something is still shorted. Not likely to be startup caps as the original fault caused the fuse to blow but I will heat them and see if that changes anything.

How do I check the secondary rails exactly ? Don't they always measure a low resistance to ground even when good ?

[Agent24]

Re: Dell D550p Not Starting

Yes, outputs will measure a low resistance to ground but a dead short is not normal, if you find one.

I would be investigating the PFC controller, if the MOSFET blew, the driver circuit could be toast. If that chip integrates the PFC and PWM control in the same IC then it could be damaged and affecting both functions.

[mmartell]

Re: Dell D550p Not Starting

Ok I will try removing that riser card after first checking the outputs. It's giving me a bad vibe because it reminds me of an ipm module lol.

[mmartell]

Re: Dell D550p Not Starting

I removed the riser board. Nothing is visually amiss with it. I can't find any information on the DNA1005A. There are dead shorts with pins 2-3-15-16 to ground but it looks like they're tied together at the back side of the board.

[mmartell]

Re: Dell D550p Not Starting

Oops I should have said pins 2-14-15-16 are tied to ground at pin 1.

I found this at Elektrotanya for a smaller Delta PS is it helpful ?

[Agent24]

Re: Dell D550p Not Starting

Well, at least you can buy that chip at UTSource if it's blown.

Can you post some more photos? Which heatsink has which devices? What are those 3 white things on middle heatsink? I'm guessing sense resistors? Where's the PFC FET and diode?

Have you tried a larger bulb? It might be shutting down from drop in line voltage when the bulb warms up.

[stj]

Re: Dell D550p Not Starting

check the pfc diode, i read that the diode & fet often go as a pair.

[mmartell]

Re: Dell D550p Not Starting

Well, at least you can buy that chip at UTSource if it's blown.

Can you post some more photos? Which heatsink has which devices? What are those 3 white things on middle heatsink? I'm guessing sense resistors? Where's the PFC FET and diode?

Have you tried a larger bulb? It might be shutting down from drop in line voltage when the bulb warms up.

The middle heatsink is where the two 24N60C3's were replaced. The white things seem to be ceramic spacers allowing you to use a smaller package, or conversely by removing them allow you to use a large package of the same device notwithstanding lead spacing issues. I've not seen them before.

Where's the PFC fet and diode ? I was hoping someone could tell me.

I'm already using a 100w bulb, what would be a reasonable wattage to try without putting my silicon in jeopardy ?

[Agent24]

Re: Dell D550p Not Starting

Well if the two MOSFET are on that heatsink, maybe the third device is the diode? I can't see anywhere else it might go, then.

I'm guessing the PSU switching transistors are on the far right heatsink next to the filter capacitor and transformer.

Which means the smallest heatsink on the left next to PFC coil must be for the bridge rectifier, since I can't see it anywhere else.


As for lightbulb, I'm not sure, sorry.

I do notice that the little relay next to the PFC controller board looks broken on one end? It's not got contacts jammed or something?

[mmartell]

Re: Dell D550p Not Starting

Yes, the bridge rectifier is on the left and I would guess you're right about the switching transistor being on the far right heatsink. The diode next to the 24N60C3's on the middle heatsink does not test shorted.

Nice catch on the relay. Ya I was a little rough and busted part of the lid. Shouldn't cause a problem like I'm seeing should it ?

[mmartell]

Re: Dell D550p Not Starting

After some delay I'm cleaning up some stuff and this is back on my bench.

I've reinstalled the riser card and plugged the power supply direct to mains after jumping PSON to ground. No ticking but no secondary voltages. 5vsb is present at all times.

I measure 165v on the main cap and on pins P807 and P808 of the riser card and about 19v on P802 and P803. The rest are 0v.

Would the Relay come into play here or is it not a factor ?

[mmartell]

Re: Dell D550p Not Starting

Wait.

That relay is used to supply voltage to the main circuit enabling the secondary to produce the various voltages, right ? So yes it comes into play...

I measured the relay coil and it comes right in at 320 ohms so that's ok but there seems to be a short or NC condition on the contacts. With no power supplied to the coil the resistance between the relay contacts is 12 ohms. Isn't that far too low ?

So I'm going to try and locate a replacement relay but can someone confirm or not if I should be seeing a boosted voltage about 340v on the main filter cap or does this smps not use it ?

[PeteS in CA]

Re: Dell D550p Not Starting

The 165V across the main input 'lytic probably means the PFC is not operating. And if the secondary-side controller gets status info from the primary side, the secondary-side controller is probably keeping the PWM or phase-shift controller shut down due to the PFC not operating.

I'm not familiar with that particular model, but higher end high-power PSUs often have microcontrollers on both the primary and secondary side. The former often communicates status data to the latter. So the secondary-side controller "knows" when the PFC is not working.

Relays are frequently used to short the inrush limiting thermistor(s) to allow use of higher resistance thermistors and keep them from overheating after limiting the inrush current (typically several AC cycles) and improve efficiency.

[mmartell]

Re: Dell D550p Not Starting

I fed 12v to the relay and it clicks so it's good. With PSON jumped to ground the relay does NOT click when the unit is plugged in.

I presume jumping the relay contacts would just be inviting disaster in the form of exploding silicon ?

How can I troubleshoot this without just throwing parts at it ?

[mmartell]

Re: Dell D550p Not Starting

I know the prevailing wisdom when replacing shorted primary side fets is to replace the driver ic as well. I don't have a problem doing so except these ic's can only be found in China and will take up to two months by slow boat so I would like to prove if possible it has failed before ordering.

To that end what measurements can I take or what tests can I run that will get me closer to a repair ?

The schematic in post #9 uses the same driver ic and has many similarities to what I see on my board.

Picture requests or any help will be appreciated.

[mmartell]

Re: Dell D550p Not Starting

Can a Mosfet only conduct for a short period before blowing up ?