56
\$\begingroup\$

I am designing the power supply circuit for a system that needs several supplies, my questions are:

  • Is it possible to replace all electrolytic caps (Mostly 100uF) by ceramic ones? What are the limitations of ceramic?

  • Should I use a 2x voltage rating for ceramics as it's done for electrolytic?

  • What about ripple current rating? Is it an important factor when choosing ceramics as in electrolytic?

Added 1/9/2014: More about ceramic limitations

I found this excellent video submitted by Dave at EEVBlog showing the limitations of different types of ceramic caps and how they are affected by the applied voltage and the bias voltage too. Worth watching!

\$\endgroup\$
  • 5
    \$\begingroup\$ could you find 100uF ceramic caps? \$\endgroup\$ – Standard Sandun Oct 9 '12 at 13:19
  • \$\begingroup\$ heres one au.element14.com/tdk/c3225x5r0j107m/… \$\endgroup\$ – geometrikal Oct 9 '12 at 13:22
  • \$\begingroup\$ @sandundhammika on digikey there is up to 680 uF but it's overpriced. 100uF are available at around $2. Voltage rating affects the price too. \$\endgroup\$ – Abdella Oct 9 '12 at 13:22
  • 5
    \$\begingroup\$ @geometrikal: When you don't want to throw away your money? \$\endgroup\$ – endolith Oct 10 '12 at 0:05
  • 1
    \$\begingroup\$ @Abdella, re your edit, First, tantalum is a type of electrolytic. Second, if tantalums weren't used in commercial products, nobody would bother manufacturing them. \$\endgroup\$ – The Photon May 2 '13 at 15:42
73
\$\begingroup\$

100 µF is really pushing the limit for ceramic caps. If your voltages are low, as a few volts to 10 or maybe 20 volts, then paralleling multiple ceramics may be reasonable.

High capacitance ceramic caps have their own set of advantages and disadvantages. The advantages are much lower equivalent series resistance and therefore much higher ripple current capability, usefulness to higher frequencies, less heat sensitivity, much better lifetime, and in most cases better mechanical ruggedness. They have their own problems too. The capacitance can degrade significantly with voltage, and the denser (more energy storage per volume) ceramics exhibit piezo effects often called "microphonics". In just the wrong circumstance, this can lead to oscillation, but that is rare.

For switching power supply applications, ceramics are usually a better tradeoff than electrolytes unless you need too much capacitance. This is because they can take much more ripple current and heat better. The lifetime of electrolytes is severely degraded by heat, which is often a problem with power supplies.

You don't need to derate ceramics as much as electrolytes because the lifetime of ceramics is much larger, to begin with, and is much less a function of the applied voltage. The thing to watch out for with ceramics is that the dense ones are made from a material that is non-linear, which shows up as a reduced capacitance at the higher ends of the voltage range.

Added about microphonics:

Some dielectrics physically change size as a function of the applied electrical field. For many, the effect is so small that you don't notice and it can be ignored. However, some ceramics exhibit a strong enough effect that you can eventually hear the resulting vibrations. Usually, you can't hear a capacitor by itself, but since these are soldered fairly rigidly to a board, the small vibrations of the capacitor can cause the much larger board to also vibrate, especially at a resonant frequency of the board. The result can be quite audible.

Of course, the reverse works too since physical properties generally work both ways, and this one is no exception. Since applied voltage can change the dimensions of the capacitor, changing its dimensions by applying stress can change its open-circuit voltage. In effect, the capacitor acts as a microphone. It can pick up the mechanical vibrations the board is subjected to, and those can make their way into the electrical signals on the board. These types of capacitors are avoided in high sensitivity audio circuits for this reason.

For more information on the physics behind this, look up properties of barium titanate as an example. This is a common dielectric for some ceramic caps because it has desirable electrical properties, particularly fairly good energy density compared to the range of ceramics. It achieves this by the titanium atom switching between two energy state. However, the effective size of the atom differs between the two energy states, hence the size of the lattice changes, and we get physical deformation as a function of applied voltage.

Anecdote: I recently ran into this issue head-on. I designed a gizmo that connects to the DCC (Digital Command and Control) power used by model trains. DCC is a way to transmit power but also information to specific "rolling stock" on the tracks. It is a differential power signal of up to 22 V. Information is carried by flipping the polarity with specific timing. The flipping rate is roughly 5-10 kHz. To get power, devices full wave rectify this. My device wasn't trying to decode the DCC information, just get a little bit of power. I used a single diode to half wave rectify the DCC onto a 10 µF ceramic cap. The droop on this cap during the off half-cycle was only about 3 V, but that 3 Vpp was enough to make it sing. The circuit worked perfectly, but the whole board emitted a quite annoying whine. That was unacceptable in a product, so for the production version, this was changed to a 20 µF electrolytic cap. I originally went with ceramic because it was cheaper, smaller, and should have a longer life. Fortunately, this device is unlikely to be used at high temperatures, so the lifetime of the electrolytic cap should be a lot better than its worst case rating.

I see from the comments there is some discussion about why switching power supplies sometimes whine. Some of that could be due to the ceramic caps, but magnetic components like inductors can also vibrate for two reasons. First, there is force on each bit of wire in the inductor proportional to the square of the current thru it. This force is sideways to the wire, making the coil vibrate if not held in place well. Second, there is a magnetic property similar to the electrostatic piezo effect, called magnetostriction. The inductor core material can change size slightly as a function of applied magnetic field. Ferrites don't exhibit this effect very strongly, but there is always a little bit, and there can be other material in the magnetic field. I once worked on a product that used the magnetostrictive effect as a magnetic pickup. And yes, it worked very well.

\$\endgroup\$
  • 1
    \$\begingroup\$ Are microphonics the high pitched whines I occasionally see people reporting as problems with some GPU cards? \$\endgroup\$ – Dan Neely Oct 9 '12 at 19:16
  • \$\begingroup\$ @DanNeely I think Olin is referring to the fact that the capacitance of ceramic capacitors can change as they physically vibrate. Sound can cause the capacitance to change. So if the capacitance happens to be affecting a generated sound, and the sound affects the capacitance, you get a feedback loop that will have unintended effects. \$\endgroup\$ – Stephen Collings Oct 9 '12 at 19:20
  • \$\begingroup\$ At least, I assume the effects would be unintended. If you're designing around the microphonic effect of a ceramic cap, you're probably doing something wrong. \$\endgroup\$ – Stephen Collings Oct 9 '12 at 19:20
  • \$\begingroup\$ @DanNeely - The high-pitched wine is caused by magnetostriction in the device's inductors. I don't think I've ever seen a capacitor make noise (well, except for when they fail catastrophically). \$\endgroup\$ – Connor Wolf Oct 9 '12 at 21:21
  • \$\begingroup\$ @Remiel - It's a common consideration in high-performance audio, and one of the many reasons they don't use ceramic caps in the audio path. \$\endgroup\$ – Connor Wolf Oct 9 '12 at 21:23
18
\$\begingroup\$

There are a couple of reasons not to switch a design from electrolytics to ceramics that haven't been mentioned yet:

  • Some linear regulator designs require the higher ESR of the electrolytic on their output capacitor to maintain stability.

  • Ceramics are less robust than electrolytics when subjected to board flexure. Especially in the large sizes, say 1206 and above, like you'll need for values above 10 - 20 uF with reasonable WV, ceramics are easily cracked if there is any flex in the board. The damaging flex could happen in the field, or it could happen with some methods of singulating the boards out of the panel they're manufactured in.

\$\endgroup\$
14
\$\begingroup\$

Pursuant to OP's derating questions, and further to Olin's fine answer:

IPC-9592A (which is a standard for high-reliability power conversion devices) cites the following derating guidelines:

Fixed ceramic MLCCs:

  • DC voltage <= 80% of manufacturer's rating
  • Temperature: Minimum 10°C below manufacturer's rating
  • Size: Sizes larger than 1210 not recommended due to cracking potential

Aluminum electrolytic capacitors:

  • DC voltage <= 80% of manufacturer's rating (<= 90% for 250V or higher devices)
  • Life / endurance: >= 10 years at 40°C, 80% load for Class II devices (datacenter stuff), or 5 years for Class I devices (consumer grade)

The life/endurance rating of an aluminum electrolytic capacitor is a function of all it's stresses - voltage, ripple current and ambient temperature. If the cap is in good airflow, it can take more ripple and maintain a long service life. A hot cap won't have a long life.

For ceramic capacitors, it's also about temperature. The ambient temperature and ripple current will result in a temperature rise. That's not to say that ceramics don't age - certain dielectric materials (Class 2 materials like X7R and Y5V) degrade in capacitance over time - Class 1 materials are largely immune to this.

Also, as Olin stated, certain dieletric materials suffer from significant capacitance roll-off as a function of DC bias voltage. Again, Class 2 materials suffer from this, Class 1 materials largely don't.

Essentially, if you use either type of capacitor, keep the maximum voltage under 80% of the stress.

The much lower ESR of ceramic capacitors (vs. electroytic caps) has a feedback loop stability implication. Assuming your converter will be a switcher and have an output L-C filter, a type-3 compensation network may be required to stabilize the converter.

The low ESR causes the open-loop gain to roll off at -40 dB/decade for a long interval (the ESR zero is pushed out as the ESR drops) necessitating +20dB / decade gain in the compensation network for the frequency cross-over to be at -20dB / decade (which is one of the three loop stability criteria that power designers look for, along with gain margin and phase margin).

\$\endgroup\$
2
\$\begingroup\$

I may be wrong, but switching to ceramic bulk caps will create an anti-resonance between the bulk caps and the smaller decoupling caps. Unless they are chosen carefully, the inductance of the bulk caps will resonate with the capacitance of the decoupling caps. This does not happen with Tantalum and electrolytic caps, because the ESR of those devices damps the resonance. Again, I could be wrong, as I have never tried this in practice.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.