[Solved] Repair / DC 3.3V 2.5A Open Frame Power Supply / Network Switch

[Armadillo]

Measure Resistance ohms across the IC pin 1 and 8, check whether it is shorted or low ohms.

[xan]

Ok Armadillo... why i should measure  across pin 1 and 8 can you explain?? (u know  i'm newbie )
is this my IC datasheet ? https://www.digchip.com/datasheets/download_datasheet.php?id=1913236&part-number=IRIS-A6351

(i will do measurement tomorrow morning.... do i have to take care about some high voltage during measurement??)

Thank's for suppport!

[Bushougoma]

Pin 1 and 8 is the source and drain of the internal switching FET that drives the transformer. If it was open you wouldn't have any output if it was shorted the fuse would have blown but it couldn't hurt to check it.

It sounds like the supply is in hiccup mode. It is starting up, sensing a fault, shutting down for a predetermined time, and attempting to start again.

This can be due to an overload (a short on the output) or it could be that the primary winding dedicated to supplying the PWM controller isn't functioning.

It sounds like the bootstrap circuit is working but the primary dedicated to supplying the IC after the PWM controller starts isn't. Check the 1/2 watt resistor and diode to the left of the transformer it is responsible for rectifying the PWM controllers VCC also check the small capacitor right above the IC.

The bootstrap circuit consists of a rectifier diode connected to the AC mains, a resistor, and a capacitor. The resistor trickle charges the capacitor and it's voltage slowly rises once it hits a certain voltage the PWM IC comes to life and begins switching the transformer. The capacitor will maintain the voltage needed to run the IC for a short time once it starts switching it's supposed to supply power to itself through the dedicated primary shown in the datasheet. If it doesn't the voltage on the capacitor falls the IC shuts down and the process repeats again.

[Armadillo]

Ok Armadillo... why i should measure  across pin 1 and 8 can you explain?? (u know  i'm newbie )
is this my IC datasheet ? https://www.digchip.com/datasheets/download_datasheet.php?id=1913236&part-number=IRIS-A6351

(i will do measurement tomorrow morning.... do i have to take care about some high voltage during measurement??)

Thank's for suppport!

Turn off the AC power, pull out the plug. You MUST discharge C1 for your safety. After the proper discharge, then you can start to measure the ohms.

Please note that this power supply is HOT, even the IC is HOT, meaning it is connected to the AC mains, of high KVA, and can cause serious damage. So be very careful with it.

Because the circuit tried to start up, but there is a over-current situation that forbid it to start up [when pulse voltage at pin 1 exceed 0.7 volt]. Either the mosfet is shorted or the PWM frequency has degraded too low. You will most likely be looking at either the IC or the transformer. Either one will be hard to get. The IC is obsoleted. [ok one other measurement, you can measure the current sense resistor R7 that it is 0.9 ohms and is not open circuited, for double sure its not external circuit causing the problem]

And because the 3.3v power supply off ebay is getting so cheap, it is not worth the efforts to repair it, really.
But if this is just for your learning, I will say, please find yourself a more safer circuit to start off with. 

Edit: you know why I ask you to test it without buying the capacitor? yeah! because there may be some other important and deterministic parts to consider, like the IC or transformer. 

[Bushougoma]

Switching transformers rarely fail and shouldn't even be considered until the usual suspects are ruled out.

And because the 3.3v power supply off ebay is getting so cheap, it is not worth the efforts to repair it, really.

As long as that supply has a proper EMI filter like this one does many cheap Chinese supplies don't and spit noise back onto the A/C line. Along with abysmal separation on the PCB between the primary and secondary don't even get me started on the construction of the transformers.

If you do go that route purchase your power supply from a reputable manufacturer.

[Monkeh]

Elephant in the room: Which capacitor(s) did you replace and what did you replace them with?

[james_s]

At least 90% of the time I work on power supplies like this, replacing the electrolytic capacitors is the only thing they need.

If it goes tick tick tick that strongly suggests a shorted part in the output. Check the diode(s) and also look carefully for solder bridges you may have created while working on it.

[xan]

ok guys so i have to measure resistence across this 2 pins ? right ? (be more specific ...)
how many Ohm on my instruments i need to set ?

Thank's!

[xan]

ok C1 discharged correctly with resistor and i have this reading across pin 1 and pin 8 on IC .632    (scale 2k on my instrument)
(image on previous post is wrong... attached right pin configuration..)
so now, what's next ??

[james_s]

I think what is next is you need to study switchmode power supply design so that you understand how the circuit works, it's very difficult for someone to walk you through the whole diagnosis. If it's not working you have a bad component somewhere.

[xan]

   don't worry let's try!!!!

[Monkeh]

I shall repeat: Which capacitor(s) did you replace and what did you replace them with?

[xan]

i changed this one in attachment! damaged: teapo 1000uF 10v -> new: Rubycon 1000uF 10v

[Monkeh]

And the other half of my question?

[xan]

sorry  refresh and read again up pls..

[Monkeh]

What type of Rubycon? They make quite a few different parts..

[xan]

Rubycon 1000uF 10v ZLH do need more specs?

[Monkeh]

Okay.

What value is the other large capacitor on the output? If it's quite close to 1000uF (it probably is), please replace it as well. If it still does not remain running, replace the small electrolytic next to the IC.

[james_s]

Without access to an ESR meter, the first thing I'd do on a power supply like this is replace *all* electrolytic capacitors on the secondary side. The bulk filter on the primary side is usually ok unless it's bulging or leaking.

[xan]

ahh ok.. i don't have ESR meter...  anyway... yes it is 1000uF 10 v Su'scon... the small one 22uF 50v Do You mean that above iC ?

[Monkeh]

ahh ok.. i don't have ESR meter...  anyway... yes it is 1000uF 10 v Su'scon... the small one 22uF 50v Do You mean that above iC ?

Yes, that little 22uF one. Replace the other output cap first, if it still fails, replace the little one. If after that it fails, frankly throw it in the bin and go buy another.

Without access to an ESR meter, the first thing I'd do on a power supply like this is replace *all* electrolytic capacitors on the secondary side. The bulk filter on the primary side is usually ok unless it's bulging or leaking.

The bulk filter we don't care about, the IC supply we do.

[xan]

ok got it.. can you help me to find on ebay an open frame pSu as a replacement?? or something provide an input like this ?

[Monkeh]

You've got a huge empty case, it's not hard to find something which fits. And frankly, switches are cheap.

[xan]

  ok will look for some on ebay but i still want to repair this... just for understand..

[Armadillo]

If I recommend you the GBP$2 power supply, I know I will receive some comments from few free people here who just seems to enjoy on commenting others. For the cheaper ones, therefore you will have to look for yourself.

So, I will suggest you the UL TUV CB CE safety listed Meanwell made in Taiwan power supply. This is rated at 3A. So you don't need to spend nearly equivalent amount to repair the existing power supply to get quarter life or half life out of it. In my opinion, Meanwell power supply is known brand in my region. So you know you can get full life out of this all new power supply [new capacitors, new transformers, new diodes, new mosfets etc..]

http://www.ebay.com/itm/1pc-Switching-Power-Supply-RS-15-3-3-3-3V-3A-9-9W-62-5x51x28mm-Mean-Well-/141684943081?hash=item20fd14a8e9:g:fUYAAOSwrklVcHa-

I will let others to recommend to you the more perfect and better ones for you, "as usual".

You will have to be imaginative on how to do the mounting and how to hook this up to the existing "power socket" etc. However, please do this at own risks and own responsibility.
We cannot be walking you through every steps but remember this - "Safety First", still you don't know, just stop and ask!.