Electronic – replace all electrolytic capacitors with ceramic

capacitordecoupling-capacitorpowerpower supply

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!

Best Answer

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.