Capacitors in a GM34063 controlled switching power supply

[carpenter]

I had a D'Link DL-1008 switch on the wall for over 10 years. It stopped working over the weekend. The fault was clear at first sight, electrolytic capacitors.
Two input 220uf/25V inflated and had around 10uF Replaced 2x 470uF/25V ESR 350mOhm
Switch source output capacitors 2x100uF 25V looked OK but were also around 10uF and were replaced with 2x220uV 16V ESP under 500mOhm. The output voltage is only 2.5V
In the switch there are two more electrolytic capacitors 100uF 25V. , also 15uF were defective , replaced with 220uF ESR under 500mOhm. The voltage on the capacitors is around 1.85V 2,5 minus the loss on the PNP transistor UTC 2SB772L.
Stepdond converter is drived by GM34063 1,5A input 3-40V 100kHz.
Where is the problem?
Switch works again but freezes
Two or three times a day it just freezes and needs to be reset
The adopter is a classic transformer with 4 diodes and a 1000uF 16V capacitor, measured 860uF ESR 380mOhm
The converter (probably the coil) can be heard a little, with the ear at max 5 cm you can hear whistling

A higher capacitor capacity shouldn't be a problem in this case, or am I wrong?

 

[hiphei]

My tips; put some flux on and resolder *all* powersupply smd components (around ic and ic included).. to my eye they look very bad.. you don't need much solder.. plenty of flux and just some solder on a tip of a soldering iron.. at least the resistors, controller ic, and q2 (maybe a switch?) I'd also confirm *all* powersupply electrolytics are replaced.. And all the other values you replaced maybe still alright, but i think replacing 10uF caps with that large may present an issue..i'd go back there but with new ones..!

--- Updated Tuesday at 5:43 PM ---

My tips; put some flux on and resolder *all* powersupply smd components (around ic and ic included).. to my eye they look very bad.. you don't need much solder.. plenty of flux and just some solder on a tip of a soldering iron.. at least the resistors, controller ic, and q2 (maybe a switch?) I'd also confirm *all* powersupply electrolytics are replaced.. And all the other values you replaced maybe still alright, but i think replacing 10uF caps with that large may present an issue..i'd go back there but with new ones..!

Tip for myself..put the glasses on when reading.. (..there were no 10uF caps in the product..)

 

[KlausST]

Hi,

to be honest ... I don´t consider 500mOhms as LowESR. There easily may be switching currents with peaks above 1A ... so 500mV voltage ripple, which is huge.
Even if 2 are in parallel.

ESR is not the only thing you have to care for when selecting a capacitor. It also needs to be designed for the switching frequency (and the ESR should be specified at high frequency) and it needs to widthstand the ripple current.

I´d use a scope to verify the power supply voltages´ ripple and noise.

If you encounter a HF noise/ripple at the electrolytics ... you may solder additional ceramics in parallel .. check again with the scope if they improve the situation.

Klaus

 

[carpenter]

So I looked at it with an oscilloscope and made some adjustments.
The noise on the input capacitors was approx. 400mV
I provisionally added a 1000uF/16V KZH to the unused spot for the diodes
The noise dropped to about 250mV.
the noise on the output capacitors was about 70 mV
replaced one capacitor LowESR 1000uF 6.3V.
The noise dropped to about 25mV.
I don't know if it will help, I'll find out tomorrow during the stress test.
The electrolytic capacitors go away gradually, their capacity decreases and ESR increases, yet it worked without problems and the C parameters were worse than what I put there.I'll see what it does now.
By the way, the switching frequency is 23kHz Datasheet GM34063