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barry
User Rank
Iron
Re: single output diode = flyback
barry   6/20/2013 9:51:15 AM
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hello all

  from my days of servicing tv's/monitors i recall the dreaded bulging/defective cap used in scan derived flyback transformer circuits.  the biggest cause was the manufacture using 85* caps instead of 105* caps that were made for high freq power supplies with a nasty pulse.   85* caps are only good in 60 hz circuits..

                                    barry

                                    

Paul Murk
User Rank
Bronze
Re: Capacitors
Paul Murk   6/19/2013 10:12:11 PM
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Good obsevation.

But remember the electrolytic is to store DC power. If you had premature failure, then this device has a power supply that has high level harmonics not intended for this capacitor to handle. I would recommend replacing the cap. naturally because it failed but I would also add a high frequency cap & resistor to ground to protect the storage cap. If you have an oscilliscope to observe the ringing from the flyback supply you could calculate the high frequency filter you need to prevent this to happen again. Sometimes Engineers are in a rush to sign off designs. This would be an example. But the issue is to improve the design and not have it occur again. Replacing the electrolytic is the fix but not the cure? Time will tell.

Howman
User Rank
Iron
Re: Capacitors
Howman   6/19/2013 6:42:57 PM
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Frank,

From what you described, I am pretty certain that the power supply is a flyback: two-windings on a ferrite core with a single rectifier between the secondary and a large bulk cap.  That is a classic flyback which is the most common topology for an offline isolated power suppy with an output power of 80W or less.

Why do you think it is not flyback?  Just curious.

William K.
User Rank
Platinum
Re: The complex issue
William K.   6/19/2013 5:37:16 PM
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RBedell, are you any relation to a Dwight Bedell who lived in Royal Oak back on the 1950s? He was an amatueur radio operator.

Noswad
User Rank
Gold
Old man river
Noswad   6/19/2013 3:46:41 PM
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Nothing will last forever. I am sure a designer can design a product to last for 50 years but the end cost for the consumer would be very high. We are used to paying cheap prices for stuff these days, even though we know it will only last a couple of years.

It has to do more with economics instead of engineering.

SSLGuru
User Rank
Iron
Power supplies
SSLGuru   6/19/2013 3:44:14 PM
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Great piece - thanks. The multi-discpline detail is what makes electronic product development so much fun... and so painfull.

 It struck me that were this a cable television or other set top box in the USA today you probably would not have been able to fix things quite so rapidly. (Guessing this applies in other continents too now). The Energy Star standard would make that front end voltage down-conversion somewhat less accessible visually. A bad design could still show bulging caps of course.

Also, the pesky unwanted noise emission issues arising from the new tighter energy standard today would likley mean that a senior engineer would have had to think perhaps just a tad more about the front end supply design these days. More thought... more components and of course all the more opportunity to accidetally screw things up.

 Chances are you could still see a big fat non-SMT cap in that same position though.

 

Thanks again,

Jon Connell, Brooklyn, NY

William K.
User Rank
Platinum
The Power supply as an afterthought?
William K.   6/15/2013 3:02:53 PM
It is often amazing that some piece of equipment, while having some specialized part of the circuit be well thought out and a good design, will then have a power supply that looks like it was dsigned in a real hurry by an inexperienced individual, or possibly designed by an accountant. And so under rated parts are tightly packed in an area with inadequate ventillation. And so the power supply fails while the balance of the system is OK. 

But good detective work on the fix.

RBedell
User Rank
Gold
The complex issue
RBedell   6/14/2013 1:21:04 PM
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Mr. Karkota states what I have thought to be obvious.  But as Naperlou, observes, that may not be true.  To the considerations of pathologies of failure, let me add a few of my theories.  The transformer output from a switching supply is more 'square' than the output of a linear transformer (translation: it has faster rise times).  These faster rise times translates to higher surging charge currents.  The inverse it also true; the faster fall times translates to higher surging discharge currents.  The ESR plus higher surge currents equals heat; as Mr. Karkota states.  But, the inductive component of the electrolytic capacitor plays a larger part as the frequency goes up.  While there are electrolytic caps specified with a low-ESR, few if any are spec'ed with a low-ESL (effective series inductance).  This ESL would cause uneven charge distribution down the length of foil in relation to the charge-discharge(C-D) frequency.  All other things being equal, the amount of capacitance 'seen' by the circuit will diminish with increase in frequency. 

To solve this problem; If 3000uF was needed for filtering on a switching supply; should one use a single 3000uF cap or three 1000uF caps?  The 1000uF's are the better option because the C-D currents would be distributed across the three caps.  A lower current is lower heat produced.

Additionally, derating the capacitance in relation to heat is obvious; but what about derating the cap voltage and  'uprating' the ESR?

Then there is how components are packed into smaller spaces creating higher temp operating environments.

Frank Karkota
User Rank
Iron
Capacitors
Frank Karkota   6/14/2013 11:04:44 AM
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A few comments to responses.

Yes, there are capacitors that are designed for switching power supplies that have long life, but unfortunately Chinese makers of consumer products usually put in the cheapest part they can find.  Panasonic has a full line of electrolytics for every application, but they are expensive.  Most failure of consumer products are because of faulty aluminum electrolytic capacitors.  It is the duty of the designer, unfortunately, to assume that cheap or couterfeit parts will be used.  Even if the the designer approves the prototypes, the capacitors can be changed or substituted for counterfeits in production.

This power supply was not a flyback.  It was used for isolation from the power line and had a two-winding transformer.  I understand the compromises of using a full wave rectifier in that it takes a center-tapped winding or four diodes.  You are correct that a flyback cannot use a full wave bridge and they usually spec low ESR capacitors for best efficientcy.  

I do not remember the value, but I think that it was 1000 uf.  I replaced it with a Panasonic capacitor or the same value.  The replacement capacitor was intended for power supply use.

Frank in North Bergen
User Rank
Iron
what were the values of capacitor
Frank in North Bergen   6/14/2013 8:47:45 AM
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Very good theory explaination of the bulging capacitor.

What was the original value capacitance of capacitor found in the original design? What value was it correctly replaced to?

 

Thanks

 

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