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Old 04-29-2021, 02:42 PM   #1
theoregano60
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Default 8.5w Psu

Hi all
i have a 50v 1uf electrolytic cap that i think is a soft start cap for a PWM chip
VIP22a (or smoothing cap for VDD) all the readings on a DER 5000(set @ 120hzand calibrated) are reasonable 0.98uf
tan 85
D 0.04
Q 14
but ESR was 113ohms im guessing the cap is bad so my question why is the ESR is so out of tally with the rest of the readings, is the cap shot (the only other cap i have @ hand is a 50v 10uf is it possible to use that)
Original fault no dc out put but PSU LED lit found 400v cap gone now circuit back up & running with steady 12v output

Last edited by theoregano60; 04-29-2021 at 02:45 PM..
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Old 04-29-2021, 06:23 PM   #2
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Default Re: 8.5w Psu

Did you remove cap to test esr?
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Old 04-30-2021, 12:34 PM   #3
theoregano60
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Default Re: 8.5w Psu

Yes removed & tested for all the above readings
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Old 05-03-2021, 06:33 AM   #4
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Default Re: 8.5w Psu

Those bootstrap caps always go bad in low wattage smps. Not sure if that 10uf cap will work in it's place but it's worth a shot.
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Old 05-03-2021, 12:23 PM   #5
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Default Re: 8.5w Psu

Quote:
Originally Posted by jayjr1105 View Post
Those bootstrap caps always go bad in low wattage smps. Not sure if that 10uf cap will work in it's place but it's worth a shot.
They're basically an output capacitor for a discontinuous flyback power supply output, a relatively stressful application.
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Old 05-03-2021, 02:21 PM   #6
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Default Re: 8.5w Psu

Quote:
Originally Posted by PeteS in CA View Post
They're basically an output capacitor for a discontinuous flyback power supply output, a relatively stressful application.
So how do you combat that other than replacing it when it dies? Use a higher quality cap in the first place? Use a larger capacitance cap?
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Old 05-03-2021, 06:03 PM   #7
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Default Re: 8.5w Psu

The VIP22a Vcc cap needs to supply high ripple current for the MOSFET's gate drive. 10uF 50V is more typical than 1uF and should have a ceramic cap in parallel to deal with 60kHz.
If you upsize to 10-47uF it makes the power-on delay much longer, as it is charged at 1mA up to start-up voltage say 14.5VDC
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Old 05-05-2021, 10:57 PM   #8
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Default Re: 8.5w Psu

I think it's just a shit design an you can't really upgrade it without redoing the entire thing.
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Old 05-06-2021, 11:16 AM   #9
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Default Re: 8.5w Psu

Quote:
Originally Posted by jayjr1105 View Post
So how do you combat that other than replacing it when it dies? Use a higher quality cap in the first place? Use a larger capacitance cap?
Use a high quality low impedance capacitor, e.g. Nichicon PW or PA series or Nippon Chemicon LXZ series. Or if cooling is good, maybe even Nichicon HE or Chemicon KY or KZE series.
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Old 05-06-2021, 02:24 PM   #10
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Default Re: 8.5w Psu

Nichicon PW ESR:
1uF 50V [email protected], ripple 40mA 5x11
1uF 100V [email protected], ripple 30mA 5x11

10uF 25/35V [email protected], ripple 180mA 5x11
10uF 50V [email protected], ripple 120mA 5x11

I would upsize the part, it's almost always 10-47uF. 1uF is just a small value part and not capable of enough ripple current, no matter what brand/series.
I didn't expect the higher voltage parts to be lesser spec'd (for the same physical size).
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Old 05-11-2021, 07:26 PM   #11
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Default Re: 8.5w Psu

Quote:
Originally Posted by jayjr1105 View Post
Those bootstrap caps always go bad in low wattage smps. Not sure if that 10uf cap will work in it's place but it's worth a shot.
Only the crap cap brands.
Good brands usually tend to last for many many years without issues.

Quote:
Originally Posted by redwire View Post
I didn't expect the higher voltage parts to be lesser spec'd (for the same physical size).
It always is...
That is, if you want to keep the same physical size and same capacitance, but want higher voltage rating in that physical size (and from the same cap series), then the ESR and ripple current specs will always suffer as a result.

Quote:
Originally Posted by brethin View Post
I think it's just a shit design an you can't really upgrade it without redoing the entire thing.
It's fine.
If you don't ever want to deal with electrolytic cap failures for the startup cap, then easiest thing to do is replace it with something reliable, like film type. Obviously, of course, this will come at the expense of higher cost and much bigger physical size. Actually, it will become challenging to find film caps above 2.2 to 3 uF that can still fit the circuit. But for lower capacities, it's an option.

Another option would be electrolytic + MLCC in parallel.

Last edited by momaka; 05-11-2021 at 07:35 PM..
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Old 05-11-2021, 08:41 PM   #12
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Default Re: 8.5w Psu

You might get away with just an MLCC (if you can find one in the correct value that will fit). The VIPer22A datasheet and one of ST's evaluation boards show ceramic caps.
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Old Yesterday, 10:21 AM   #13
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Default Re: 8.5w Psu

MLCCs with Z5U or Y5V ceramic are crap in any application where the in-circuit capacitance matters. Between the tempco and the voltage coefficient, the in-circuit capacitance could be just 20% of the rated capacitance.

Even with X7R it's best to have a rated voltage 5X-10X to circuit voltage.
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