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SMPS failure ( capacitor bulge & burst )

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srevish

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Dear all,

I using this particular SMPS for more then 3 years but having same failure mode frequently.

I have attached this schematic please make a note of it.

Kindly give your valuable feed back whether the problem is due to the components quality ( may try for better brand ) or the failure is due to the design.

I have mentioned the frequent failure components with the red circle in the schematic attached.


I would appreciate if you can give us any ideas for the design change.
 

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It all points to the mains being too high. The MOV = 320V, 230 V Ac ~ 345 peak, change MOV. Whats the power rating of this PSU ?, Its just that the voltage drop across the 20 ohms , should protect the reservoir capacitor against excessive ripple - the voltage ratings seem to be OK.
Frank
 

What is the temperature rating in the electrolytic capacitors?

Avoid 85C components, always use 105C or higher rated ones. Reputable brands too.
 

I would use 275V varistor. You can focus also on Low ESR capacitors. My first guess without looking on schematic was to increase voltage rating, but 450V is enough, there should something with voltage on input. What kind of housing you use for this power supply and what temperature is inside typically? How many turn-on/off cycles between failures power supply was facing(10,100,...) ?
 
Last edited:

I agree with Chuckey, when the varistor breaks down your AC mains is too high, and/or there are repeating large transients on the mains. You should have premature failures in other electronic equipment also in case of too high a mains voltage.

when replacing the capacitors, make sure you use low ESR capacitors. That are capacitors specially made for switching applications.

As Sieger also mentioned, high internal temperature may also lead to premature capacitor failure.
 

The ESR should be low because ripple current times
ESR is your internal heater. You might want to look
at the application and any macro duty factor ratings,
if you're spending a lot of time at high current then
the ripple heating might be your problem. The bulging
implicates high internal temperature, figuring its
source is on you.
 

chuckey said:
It all points to the mains being too high. The MOV = 320V, 230 V Ac ~ 345 peak, change MOV. Whats the power rating of this PSU ?, Its just that the voltage drop across the 20 ohms , should protect the reservoir capacitor against excessive ripple - the voltage ratings seem to be OK.
Frank
Click to expand...


For this similar circuit I have tried MOV with 420V, in this case MOV did not failed but I had the failure with the resistor 20E, capacitor 10uF, 22uf & TNY.
 

sieger said:
I would use 275V varistor. You can focus also on Low ESR capacitors. My first guess without looking on schematic was to increase voltage rating, but 450V is enough, there should something with voltage on input. What kind of housing you use for this power supply and what temperature is inside typically? How many turn-on/off cycles between failures power supply was facing(10,100,...) ?
Click to expand...

Dear Sir,

Is there is any particular reason for choosing 275V for MOV,

This PCB will be mounted on the plastic box.

The typical temperature will be 55 C
 

Re: SMPS failure ( capacitor bulge & burst )

dick_freebird said:
The ESR should be low because ripple current times
ESR is your internal heater. You might want to look
at the application and any macro duty factor ratings,
if you're spending a lot of time at high current then
the ripple heating might be your problem. The bulging
implicates high internal temperature, figuring its
source is on you.
Click to expand...


I have selected the Nichicon make with low impedance & temperature level of 105 C , this capacitor is specially for power switching application. But they have not mentioned about the ESR in the datasheet.

Shall I select this capacitor

- - - Updated - - -

I have selected the Nichicon make with low impedance & temperature level of 105 C , this capacitor is specially for power switching application. But they have not mentioned about the ESR in the datasheet.

Shall I select this capacitor
 

I was thinkig for 275V AC rating (for clarification). It gives fair margin vs nominal line voltage 220-240 and remove unwanted stuff.
 

Re: SMPS failure ( capacitor bulge & burst )

chuckey said:
You still have not let on the power rating of the 24V line. This will reflect on the ripple current in you capacitors.

Frank
Click to expand...

The power rating is 18W.

- - - Updated - - -

chuckey said:
You still have not let on the power rating of the 24V line. This will reflect on the ripple current in you capacitors.

Frank
Click to expand...

How to calculate the ripple current ?
 

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