Switched power supply capacitors

Lots of electronic hardware gets problems its psu and more specificly: with the electrolytic capacitors.

How do old capacitors usually affect the capacitance of that electrolytic capacitor?

What effect does a bad electrolytic capacitor have on the input of the PSU?

What effect will have if the bad capacitor is on the output?

Note: I have measured the voltage several times and it is usualy correct but the electronic equipment only starts after replacing the capacitors.

• You have accepted an answer that has errors and offers opinions without hard facts or evidence of failure investigation. Questions that seek opinions are usually closed hence my reason for closing this question. Jul 18 '20 at 17:55
• Close vote retracted. Jul 18 '20 at 18:49
• @Andyaka the other two answers didnt read the questions.. Jul 18 '20 at 20:57
• You asked three questions two of which call for guesswork (opinions) and the first one relies on reading the data sheet to tell you what the end-of-life endurance specification tells you (as I tried to explain). Jul 18 '20 at 21:51

3 Answers

From my experience in industrial electronics repair: pre 2000 caps used to fail after 10-15 years in service post 2000 caps just start to fail n extreme cases. Most failed capacitors INCREASE capacitance and ESR in initial failure stages, if left long enough they:

a) leak electrolyte out

b) swell

c) become open circuit

d) decrease capacitance

d) all of above

On the input of Switchmode PSU Increased ESR capacitors cause ripple voltage increase and negatively affect transient load regulation, can lead to power transistor failure.

On the output they increase output noise negatively affect transient load regulation.

On the switchmode IC supply they usually cause failure to start - applies only to mains PSU's and DC/DC converters where input voltage is above ~80 V

• As i wrote in the "note": i have tested some screens and TVs on the osilloscope - the voltage was ok - may be something about the current is happening ? Jul 18 '20 at 17:27
• I bet if you would measure removed capacitor's ESR it would be well into 10s of Ohms, new ones should be sub 1 ohm for capacitances above ~47uF. Depending on PSU topology it may rely on small(22-100uF) cap on smps IC(ie UC2844) to start. Top2xxx based PSU's rely on main bus caps to start, high ESR can mess them up, and seeing 1us voltage dip might be tricky even on oscilloscope. Jul 18 '20 at 17:39
• You use the term "fail" implying that the electrolytic capacitor failed. In all probability it was a gradual degradation of its capacitance that caused the circuit to operate incorrectly i.e. not a component failure as such but a failure of the designer to take into account the things that can happen. MTBF for capacitors (i.e. failure rate) is still millions of hours or over 100 years to fail. There is an important distinction here so -1 for that. Jul 18 '20 at 17:52
• When you mention historical data on electrolytic capacitors, you might want to also mention the capacitor plague, which will skew any such data. Jul 18 '20 at 17:56
• I do recall plague of poor quality electrolytic capacitors in consumer(mostly PC component) electronics in... 2003-2008? Was there any other i'm not aware of that hit industrial equipment too? Jul 18 '20 at 18:21

Electrolytic capacitors are usually specified as "so many" hours at such and such a temperature. From that and the operating environment you can predict how long it takes for the capacitor to have degraded to the limit values specified in the data sheet. So, if the capacitor doesn't have a data sheet that contains this: -

• Temperature value
• Operating hours at that temperature
• End of life specification

Then they are probably trashy capacitors and should not be used.

The operating hours might be 2,000 h and, quite a few are good for 10,000 h but, 10,000 hours is still only 1.14 years so "what's the trick" you might ask. It's the operating temperature. A capacitor might be rated at 10,000 h at 105 °C but at 95 °C the operating life (or endurance) will be twice as long and, at 85 °C it will be 4 times as long.

$$\boxed{\text{Every 10 °C reduction doubles the endurance period}}$$

Operating at half the rated voltage for its lifetime will also deliver a lifetime extension of 2.

$$\boxed{\text{Halving the applied voltage compared to rated doubles the endurance period}}$$

But electrolytic capacitors will degrade and you have to decide how much your circuit is affected by the degradation and if the circuit can cope with the degradation quoted in the data sheet for the "end of life".

See also this answer for more details.

Second how will affect a bad electrolitic capacitor on the input of the PSU? And how will it affect if the bad capacitor is on the output?

If you can avoid electrolytics then you get a certain improvement but if you design your power supply assuming end-of-life degradation levels then you are also good to go. For an undisclosed generic power supply with no details of its design it's impossible to generalize without doing a full reverse engineering of it.

• Why the down-vote? Jul 18 '20 at 17:16
• "Operating at half the rated voltage for its lifetime will also deliver a lifetime extension of 2. Operating at a quarter will deliver a 4x lifetime extension." is not true. Slight derating is recommended, but derating more than a factor of 0.7 (depending on manufacturer, product line, etc) does not increase lifetime. Jul 18 '20 at 18:19
• @TemeV you might be right - amendment in progress. Jul 18 '20 at 18:28

Underlying the temperature effects on lifetime and failure is ---- the removal of HEAT from INSIDE the capacitor.

The capacitor has only 2 leads. Ensure those 2 leads go directly to WIDE pieces of metal foil. Since usually one of the leads goes to GROUND, you have an excellent opportunity to perform thermal engineering and keep that capacitor much cooler.

Also ensure the ungrounded lead has wide foil, and that foil is immediately passing OVER GROUND, for about a centimeter. A centimeter is a useful distance (in FR-4) to dump heat into an underlying plane. You could include an otherwise useless 1cm by 1cm bulge on the HOT LEAD, right by the lead, to dump heat into the underlying PLANE (Gnd or otherwise).

You might strongly focus on "thermal reliefs" around the lead, if the cap is throughhole mounting. Thermal Reliefs will DOUBLE THE THERMAL RESISTANCE of that region of foil.

Copper foil of standard thickness is 70 ° C per watt per SQUARE OF FOIL. Thus a trace 10mm long and 1mm wide has 700 ° C of thermal resistance.

Its your task to explore, on a quadrille pad, assuming each little square is 70 ohms of resistance, with one amp needing to enter the grid, and realize your PCB layout has the lifetime of that capacitor in your hands.

==========================================

If you have moving air, then the CASE can dump heat into the air. Or --- even directly radiate to the other oomponents and chassis.

I recall old TVs with 2" by 4" aluminum electrolyitcs and 2 or 3 or 4 mounting tabs that twisted to lock, and then might be soldered. Good heat flow. But we don't have that anymore.

What do do? With vertical mount of 2_wire caps? Perhaps BEND over the wires, to provide a large wide heat flow path.

This might work for NASA one-of-a-kind power supplies, but not for automated assembly;

On other hand, automated assembly likely has only some warrantee 5_year expectation, not 100 years in orbit or out past the solar flux limit past Pluto or surviving another 2 minutes in a Venus probe-to-800 ° C satellite.

• Someone down voted this but take a +1 because it contains good information. Jul 18 '20 at 18:53
• "The capacitor has only 2 leads. Ensure those 2 leads go directly to WIDE pieces of metal foil." Aluminum electrolytic caps don't cool that much via the PCB. Rather they cool directly out of the metal can surrounding the thing. Jul 18 '20 at 21:53
• "Thermal Reliefs will DOUBLE THE THERMAL RESISTANCE of that region of foil." It really depends on the shape of the thermal relief. Might double, might triple, might increase by 10%. But, the whole point of a thermal relief is to increase the thermal resistance. This is to increase solderability which is crucial for making sure that the capacitor wiill not detach from the board. And as I said in the other comment, thermal resistance is not such a big deal with aluminum electrolytics. Jul 18 '20 at 21:56