11
\$\begingroup\$

After looking around, I have found quite a few things about how DC bias reduces effective capacitance for a ceramic, or electrolytic capacitor, however nothing seems to talk about the effect on film capacitors. I've found a couple of papers about film caps (such as: TDK Film capacitors - General technical information), and they mention frequency and temperature, but I can't find anything about DC bias reducing capacitance.

I have obviously checked the data sheet of the component I plan on using (TDK - MKT Film capacitors), but there isn't anything about DC bias in that (unless I've just failed at spotting it).

This suggests to me that they aren't affected by DC bias, but that seems a bit of a bold claim, and I can't see that stated anywhere.

Are film capacitor values affected by DC bias? If so, can anyone point me to some data about what the roll off is?

I am planning to use some film capacitors in an application with 48-60V DC across the capacitors and a ripple current around 30 amps.

\$\endgroup\$
4
  • \$\begingroup\$ If it isn't mentioned in the data sheet, you obviously don't need to bother about it. \$\endgroup\$
    – Leon Heller
    Commented Mar 16, 2017 at 9:32
  • \$\begingroup\$ See en.wikipedia.org/wiki/Dielectric_absorption . Note that some - especially polyester film and PEN - has larger dielectric absorption than others. That's not a complete list, I don't see polycarbonate on there. Note that the effect here is a "memory" for the DC voltage rather than a change in capacitance value. \$\endgroup\$
    – user16324
    Commented Mar 16, 2017 at 9:32
  • 5
    \$\begingroup\$ @LeonHeller Not sure if you're being sarcastic here, but surely there are thousands of datasheets where certain information is omitted, either due to oversight or because "everyone should know this". \$\endgroup\$
    – pipe
    Commented Mar 16, 2017 at 9:41
  • \$\begingroup\$ Capacitance dependency on voltage is a moot point in your application. You should be more concerned with healing characteristics: is the dielectric self-healing when disrupted, or does it short? \$\endgroup\$
    – glen_geek
    Commented Mar 16, 2017 at 17:34

2 Answers 2

Reset to default
10
\$\begingroup\$

Film capacitors have a very stable capacitance over DC bias. Look at this:

Film vs. Ceramic

But it comes to happen that film capacitors are not the only ones immune to DC bias:

Capacitor comparison

So, the actual question is why do ceramic capacitor have such a bad behaviour with DC bias? And the answer lies in the dielectric.

In order to achieve high capacitance values in small capacitor sizes, a high-K dielectric materials like Barium Titanate have to be used. Unfortunately Barium Titanate also has ferroelectric properties that reduce its capacitance value under DC bias:

When spontaneous polarization is reversed under no voltage stress (no DC bias), MLCCs achieve a high capacitance. However, if an external bias is applied to the spontaneous polarization process, the free reversal of spontaneous polarization is much more difficult. As a result, the capacitance gained is lower compared to the capacitance before the application of the bias. This is why capacitance decreases when DC bias is applied-hence the term DC bias characteristic.

Sources:

Quote: Understanding DC Bias Characteristics in High-Capacitance MLCCs.

Images: this comparison document from NIC Components.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ That is interesting. I have also got some electrolytic caps in the design (for somewhere else which has lower current ripple), but had a graph showing that the DC bias we were looking at dropped the capacitance by about 30% for that particular cap... May just be a cheap capacitor... And "good" is very much relative... \$\endgroup\$
    – Puffafish
    Commented Mar 16, 2017 at 14:30
  • \$\begingroup\$ Just a note, those ceramic capacitors are going to be class 2 MLCC's. For low values(below about 200pF), a C0G ceramic is going to equal or beat most other capacitors. For whatever marketing?) they totally ignored the class 1 ceramics. \$\endgroup\$
    – GB - AE7OO
    Commented Jan 5, 2020 at 20:57
3
\$\begingroup\$

Film caps dC/dV is very low, and only relevant in signal processing/filtering/audio when you want the lowest possible distortion.

For your high current application, you can consider the cap value constant, no problem.

Considering the high ripple current, make sure you use enough caps, and properly rated ones.

\$\endgroup\$
2
  • 3
    \$\begingroup\$ But is dC/dV really the right measurement here? As far as I understand (granted, that's not very far), the effect seen on the infamous MLCC capacitors can not really be measured as "dC/dV". \$\endgroup\$
    – pipe
    Commented Mar 16, 2017 at 9:40
  • \$\begingroup\$ Well, for high-K ceramics the effect is so extreme you might not even call it dC/dV but for films it'll be much less than the capacitor tolerance. \$\endgroup\$
    – bobflux
    Commented Mar 16, 2017 at 9:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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