SMPS brownout

[theozaurus]

Hi All,

I have a switch mode power supply that has 2 sections. One for standby mode which puts out 5v, and activates a relay when it receives the standby singal, which activates the 2nd section (now fully working). When the PSU is plugged into the equipment and the equipment is turned on the 5v line drops as soon as the power button is released to 3V. This causes the light on the equipment to go from orange (standby) to blue (on) very briefly, to off, then back to standby.

I've meassured the current draw on the 5v line. When the unit is turned on there is an inrush of around 20mA. Then it settles down to less than 1mA. When I press the power button it jumps up to only 20mA (using DMM and min max function). I guess it could be much higher and I'm just not able to capture it. If I attach a 220 Ohm resistor load to the output to give it a tiny load it of course does not fluctuate at all.

Attached is the schematic. A trace of the 5v line and pin 8 of the transformer during the brownout. You can see the frequency change on pin 8 just before the voltage drops. Then there is pin 8 of the transformer at the moment the frequency changes and finally pin 8 of the transformer showing it at a steady 5v. If anyone has any ideas I'd really appreciate it.

[theozaurus]

Here's another with the standby signal in the trace as well

[zvonex66x]

Have you check D903???

[theozaurus]

@zvonex66x - I haven't checked D903 yet, I'll try to later. The 12v circuit last time I checked put out a steady 12v. I also disconnected the relay from the 12v circuit to see if it was a load issue on that side of the powersupply, but it still failed.

I've found the datasheet for the switching regulator here: http://datasheet.octopart.com/TNY274PN-Power-Integrations-datasheet-86828852.pdf.

It looks like I'm hitting a current limit. Page 5 shows the regulator at maximum load and it looks like I'm surpassing that duty cycle. However, I've no idea how to meassure the current into the bypass pin to see if it goes over Isd.

[Armadillo]

I think in the traces, you hit the voltage limit. You should load it more by using 10 ~ 20 ohms resistor instead of 220 ohms resistor or use a DC load bank to test the power supply to its design ampere @5V.
I am thinking that the equipment could be faulty instead of the power supply.

Edit: if I understand correctly, the voltage dipped when you hook up to the equipment?
and if the power supply is unable to supply to its design ampere, you could suspect the 2 X 1,000uf capacitors.

[theozaurus]

D903 meassures 0.491v with the DMM and open in the reverse polarity.

[0xfede]

Do you have a variable PSU?
If so what happens if you deliver 5V from the variable PSU to the equipment?

Best,
0xfede

[theozaurus]

Thanks for the advice everyone. I finally found some time to dig into this a bit more. I got myself a simple DC load tester and hooked it up to the 5V line. It was able to supply 1.5A before the protection mode kicked in. I then hooked it up to the 12v rail before the transistor. That was able to give about ~200mA before protection mode kicked in. I meassured the resistance across the coil of the relay to estimate how much current it would draw and noticed it was far too low. Eventually I established that C903 had failed (no visual clues) and was shorting the 12VP rail out. Once I replaced that with a new capacitor the supply started working properly!

[Armadillo]

Even simple DC Load Bank can be pretty useful.