Frank, that is an interesting and very good analysis. Your obvious deep knowledge makes it "easy" for you to see the situation. On the other hand, not everyone has such a deep understanding. It might have been useful for the designer to model the system in a CAE tool that can handle multiphysics. The interaction of electrical and thermal effects is very important in electronics, and as you found out, often overlooked.
The design process for a switching power supply design does take temps and ripple current into account for the derating of the capacitor. The various switcher controller manufacturers have simulation and design automation software services available to make a design very effective even for a novice.
In my experience most all output filtering capacitor failures are one of two issues. Problem one and the most prevalent is poor capacitor performance due to electrolyte formulation, or counterfeit components. In one issue we had a supplier that attempted to replicate the Panasonic electrolyte with disastrous results. Not one of their capacitors met the lifetime rating listed in the datasheet. The parts were failing at 1/10 of the rating on the sheet. Needless to say this was a big issue costing the company a significant amount of money.
The second issue is that the environment experienced is not what it was designed for. We have had units placed in very high temp ambients, far greater than the posted ratings. A rule of thumb is that every 10C increase in temp results in a reduction in life by 1/2. Given that you bought the unit used, you cannot rule out if it was placed in a high temp environment.
Switching power supply design is a very mature specialty, which can be fraught with pitfalls just like any other activity. While bad designs are out there, sometimes the explanation is much more simple than the derating choices made by the engineer.
The use of a single diode past the switching stage points to a flyback design. If that is indeed the case, full wave rectification is not an applicable concept, and the assumpton that the charge/discharge current in the capacitor has a 50% duty cycle would not be correct except at a specific (and low) input voltage. Flyback designs work well and are quire common, but they do need a decent HF rated output capacitor.
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..
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.
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.
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.
After reading the article again, it must be a high-frequency transformer with DC rectification. I suppose they would do this to limit the bandwidth of noise. Is this topology common in consumer satelite gear? It is unfortunate that the designer went to so much trouble to limit the noise of the power supply but was sloppy designing the last stage. Good power supply design requires expertise in so many disciplines, it is hard to find someone that has mastered them all. It is not something you learn in school. It is only learned by doing.
In response to Howman, the power supply may have been a flyback. My designs are all radio frequency circuits and switching power supplies create too much noise for my applications and I stay away from them. I have a hard enough time with noise from linear regulators!
When I got into electronics, everything used vacuum tubes. Tubes were inefficient, and even worse, they had a limited life, as they slowly deteriorated. It was not uncommon to see tube-testers in drug stores, so that people could test their own tubes. Occasionally, the electrolytic capacitor would fail and the radio would have a 60 cycle hum. (We called them cycles back then!) As long as you replaced bad tubes, the radio or television would last forever.
With the advent of transistors, it seemed that the problems of tube technology would disappear. But the early transistors were germanium and they got leaky after months or years. Early radios actually had sockets for the transistors, party because of the failings of germanium transistors and partly as a carry-over from tube technology.
Today's semiconductors are very reliable. Just as long as the ratings are not exceeded, the failure rate is very low, surprising considering the complexity and large scale integration. The weakest part is the aluminum electrolytic capacitor. Unlike most parts, it has a memory of abuse by overheating, whether internal or external. Overheating is often caused by high current in the capacitor. Careful design can often reduce the current without sacrificing performance. Miniaturization of capacitors has exacerbated the problem.
Some of the receivers that I designed have been in the field for over thirty years, and to my knowledge, no electrolytic has ever failed. In my home I have had a number of consumer products fail and it has always been the aluminum electrolytic.
Consumer and commercial products should be designed to last at least as long as the technology is viable. The designer should assume that aluminum electrolytics are the weak part in the system and design the circuit to minimize the stress on these parts. If the aluminum electrolytics do not fail, the product could easily outlast the technology or even the owner.
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.
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.
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.
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.
Lots of products have been designed that last for 50 years, or at least 20 years. And none of them was anywhere near the price peak for the era. The difference is in the design philosophy, where the intention now is for consumer electronics items to be obsolete in six months, and then to fail at between seven and nine months. I cal this an attitude devoid of any integrity, and with an adequate amount of publicity some makers could be out of business, which might possibly serve as a lesson to others. But probably not, since policy decisions seem to be made by boards of directors and upper managers who also lack integrity.
Aluminum electrolytic capacitors have a finite life. They consist of two strips of aluminum foil coated with a paste made of aluminum oxide, water, and other chemicals. If the water dries out, the capacitor's value drops and the internal resistance increases. At 25o C, the useful life is many years, but as the temperature increases, the life drops exponentially.
Talk about incorrect tech!
Aluminum electrolytic capacitors are NOT made of "two strips of aluminum foil coated with a paste of aluminum oxide, water, and other chemicals".
FAR FROM IT!
Aluminum electrolytic capacitors are made of carefully "doped" aluminum foil. The "doping" depends on the designed end use of the capacitors and amounts to a "trade secret" usually carefully guarded by the capacitor manufacturer. Funny thing here is that this was discovered (and confirmed in the lab) by a former General Electric scientist named "Alwin". I do not remember the man's first name. No matter. Alwin discovered how aluminum electrolytics actually operate.
For many years the major producers of these capacitors, Mallory, Sprauge, GE, Cornell-Dubilier, and others proclaimed that the anodes of these capacitors were "roughened" by etching the surface of the foil to increase the area. Nobody ever took the trouble to truly understand what was going on. Then, Doctor Alwin (yes, he was a certified PhD) did an experiment. He carefully dissected a small area of "formed" (etched) aluminum foil. Alwin gold-plated the foil to protect the "roughened" surface, dissolved the aluminum in acid, and examined what remained in a scanning electron microscope.
What he saw were infinitesimally tiny "tubes" of gold. These tubes extended from what had been the surface of the aluminum foil at right angles to the surface and branching off in myriads of directions, but ALWAYS at 90° angles. To Alwin it appeared to be a plumber's nightmare.
However, Alwin had discovered exactly how the area of the anode foil was multiplied. Apparently, the folks at GE didn't know (or care) and Alwin left GE. He came to the Capacitor Division of General Instrument Corporation in Tazewell, VA in about 1971 when I was Marketing Manager of the GI Division. I had already written several technical books for Howard W. Sams & Co. and I used this information in the 1978 revision "ABC'S of Capacitors". See pages 74 & 75.
At GI, we took the knowledge and performed dozens of experiments with aluminum foil by altering the alloy of the pure aluminum with iron, foil thickness, applied voltage, etching chemicals, cathodic chemicals, and the paper impregnated with the chemicals. As a result, we were able to "tailor" the etching and oxide formation PRECISELY to the intended use of the capacitors. This caught our primary competitors completely off-guard and we changed GI from an "also-ran" in the capacitor business to the BIG DOG ON THE BLOCK.
Where is GI today? Gone. The Capacitor Division was wiped out by the GI CEO who wanted to be KING OF ICs and be done with passive components such as capacitors. I split and became Publisher of Electronic Products magazine (then part of Cox Broadcasting).
Getting back to Sherlock Ohms...
No doubt there was a crappy capacitor in the circuit (or a good capacitor was misapplied). However, I can tell you I learned a whole bunch as a designer of electronic flash systems back in 1949-1952. I had a lab full of exploded chards of electrolytics before I learned how to manage them.
I do volunteer works on weekend at Electronics recycle (since 1995), I fixed many PC, Monitors, TV that have failed due to bad bulging caps. The age of these failed products are about 2~3 years. Even Apple/Lenavo/Samsung/LG/Dell/Acer/HP/Sony. products have failed due to bad caps on the MB and in the PS. The sad is that we see 2~3 years old Flat panel TVs/Monitors are dropped off due to failed caps.
I use PANASONIC FM/FC/FR series which are made for switching Power supply for the repair works, I have yet to get the units back from the people we have donated the repaired units too due to same failing caps.
It is just poor quality caps are being used. Good site to see about bad caps: badcaps.net
It's all about cost. Consumer products are designed and manufactured to price points. It is not about the character of individual design engineers or manufacturers. They just give consumers what most of them want: CHEAP.
Design tasks are often not completed (the way some of us think of a completed design) because of cost or time constraints. The very best components are not used because they are just too expensive for the application.
It would be nice if the editor would learn to type the degree (°) symbol. It is very easy, unless the editor is using some weird kind of computer: ALT-248. Hold down the ALT key, and type 248 on the numeric key pad. Make sure the Num Lock is on.
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