SMPS secondary caps and MLCC - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: March 06, 2017, 12:16:50 am
content: Inhave seen far too many videos fixing SMPS with popped secondary electrolytic caps, and I have personally fixed a few of those too.What is preventing designers from putting a bank of MLCC as the secondary output filter caps then? They ain't gonna pop, and have much lower ESR.For one of my own designs I ran out of the specified 220uF tantalum cap for an output of a 5V SMPS and I installed an 47uF MLCC instead just to get the circuit up. Then my scope told me that the 47uF MLCC performed better than the 220uF tantalum.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 06, 2017, 04:09:59 am
content: I think there's issues with price and size in a lot of spaces. Even though the MLCCs are usually smaller than electros, they also don't stack, so they can take up more board space for the same amount of capacitance. It's also more expensive to get the high value MLCCs rated to larger voltages, so a 3.3v or 5v supply may be fine, but on a 12V or 24V supply, getting a few hundred mic of MLCCs could actually be considerably more expensive than an electrolytic. There could also be some limited high-vibration environments where power supply noise is a big concern where the potential for microphonic effects of the MLCCs would rule them out.For low voltage applications or in space constricted environments, a high value MLCC or two is a great option. I didn't really design boards at all before larger MLCCs became available, and given the longevity, polarization, and cost of tantalums.... I have yet to build one into a design. Either the bulk inexpensive capacitance of an electro, the stability of a polyester, or the versatility and size of MLCCs have covered every design I've laid out (not tons, but still).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2017, 05:25:11 am
content: 1. They're expensive.2. They're big. (I'm not kidding: type 2 dielectrics are about 1/3 the energy density of electrolytic! Type 1 dielectrics are competitive, but good luck getting them any cheaper...)3. The stability is awful (for type 2's). Expect -50% capacitance, or worse, at rated voltage, plus a modest tempco (depends on grade), and aging.4. And while you're wrestling with control loop stability over that wide range of capacitance, the ESR is very low, which is actually an issue in many power supply designs. (The canonical and provably best SMPS control system doesn't much care about ESR, but in more foolish, and sadly all too common, designs, ESR is a critical requirement for operation! How horrible, right?)5. Some of these problems can be dealt with, but they require special preparation. These things exist:https://en.tdk.eu/inf/20/10/ds/B58031_LP.pdfThey're an electret, i.e., the ceramic is under electric bias when heated, and that field is "frozen in" as it cools below Curie temperature. They cannot be reflow soldered, not in the usual way anyway, else the bias is lost. Shifting the maximum-capacitance point towards operating voltage provides all the advantages you want for a filter/bypass cap (minimum AC impedance at some operating DC bias), and energy storage is improved (you're storing more charge, and at higher voltages; E = integral Q dV).They're very expensive in any case -- special purpose, and AFAIK, no one has bothered to make one for low voltages.Tantalum has some problems as well: expensive, prone to ignition (an especially inconvenient feature, in a power supply that might have to withstand short circuit current surges...), and bulky (not quite as much density as electrolytic, though close). You can get rid of the ignition hazard, and reduce ESR a bit more, by choosing wet slug tantalums, but they're even more expensive (mil spec -- lowest leakage for their size, though!).It'sreally hardto beat electrolytics (or aluminum polymers) on all metrics. It's easy to beat them in special case scenarios (say, in small stuff where a tiny ceramic chip is hardly a cost adder; or a high voltage, super-compact inverter, where cost is no issue), but across the board, they're really quite amazing. Except for that pesky lifetime thing...Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2017, 07:30:50 am
content: What others said, plus:Quote from: technix on March 06, 2017, 12:16:50 amThis is simply not true.Granted, with everything properly designed, MLCC is reliable, but so is electrolytic (although not eternal, but still much better what everybody seems to think, recalling some specific manufacturing and/or design issues in certain products).Still, in real world, we still fix both failed elcaps, and failed MLCCs! Elcap failures are more frequent, but OTOH, they are storing orders of magnitude more energy, so I'm not sure about failure rate per energy storage!If the big elcaps were to be universally replaced with big MLCCs overnight, usually in large cases such as 1210, shorting failures would be much more prevalent than they are today. No one really knows, so this is just speculation.Designing for reliability is not completely trivial either with elcap or MLCC, elcap requires careful thermal analysis, MLCC requires mechanical and manufacturing analysis.Re control loops, IMO controller designers should get their shit together and stop designing stuff which requires significant ESR to work. That's just dumb, proper current mode control just doesn't care, and introducing power stage efficiency losses to compensate for the lack of brain is unforgivable. Fortunately, today, many if not most SMPS (as well as linear) designs are stable with zero or near-zero output cap ESR.ESR can still be an important stability and EMI factor when there are stray inductances involved.