SMPS repair Astec MP4-1N-1N-4EE

[tonyh88]

Hi ,

I'm repairing an older modular SMPS at the moment from the ASTEC company whic is now known as Artesyn Embedded Technologies.

The SMPS is a modular rack with 2 slot occupied with single output 15V DC  units and 1 slot occupied with a +5V dual output.

I'm not an SMPS expert but I think it is using a kind of half bridge topology. The module itself converts 380VDC that was rectified and boosted from the 120VAC in the main rack to the required DC output (5V in my case).

The +5V module is dead, no output.  I checked the usual ( output diodes, filter caps, switching mosfets, bridge rectifier .....)

I narrowed down the problem to the Pwm controller. The pwm controller is UC2825DW from TI. There is no output waveform on the controller. Vref is good at 5.1V and vcc is good at 17V.

What I don't get is that while troubleshooting, I accidently shorted pin 14 which is OUTB of the smps to VCC and the supply started. I confirmed this multiple time. If I force a voltage on the output pins of the pwm controller the switching action will start and the power supply works just fine.

What I don't understand is why is this so and what could be the problem? A bad component in the start up circuit locking the controller? A bad chip? I have two of these broken SMPS with exactly the same behavior.

Any ideas or help would be welcome. It is a complex PSU and I don't understand all the circuits for now.

Thanks

[voltz]

Hi,
Since pins 11 and 14; AOut and BOut are the actual drive outputs presumably to some switching device, then the power needed to get that output stage started ie. Pin13 VC may be missing. So temporarily pulling BOut high could be starting the process without VC. So.. is Vc missing? If there is voltage at that pin, then the chip itself may be dead and Vc is internally disconnected. A lot of guesswork there..

[tonyh88]

Yes lot of guessing.... There is voltage (17V) on the VC pin 13. The outputs of the controller switch a pulse transformer which then drives the  2 mosfet alternately if I understand the layout correctly.

Maybe I'll try replacing the chip since I can get them fairly cheap from digikey. Looking at it under the microscope there might be a blow hole on one of the capacitor on pin 3 on both of the devices so I will check that first.

I could always replace the PSU but since we have 2 of them with the same problem we would like to find the culprit. For some reason It's not easy to find PSU with 2 15V outputs and 2 5Volt outputs without going in custom expensive PSU.

Thanks for the help.

I do have some photo that I could upload if anyone has an idea.