High frequency loss is an important index of film capacitor, which directly affects the reliability of the whole machine. This paper introduces the conception of capacitor loss, the composition of loss and the influence of external factors on loss. In this paper, the experimental data are used to show that the high frequency loss of film capacitor increases with the increase of test frequency and the relationship between them is not linear. By analyzing the production process of film capacitor, the reason of high frequency loss and the measures to be taken are pointed out. And the main parameters of important process are determined by 0.618 method and orthogonal test method.
What makes a film capacitor high frequency?
The entire goal in choosing a capacitor is to make sure that it acts as close to a real capacitor as possible. Real capacitors have parasitic resistance (called effective series resistance, or ESR) and parasitic inductance (called effective series inductance, or ESL). Capacitors also have some leakage resistance across the two plates in the capacitor, but this is generally large enough that it can be ignored in high frequency applications, especially when working with large capacitors.
So what does this mean for your capacitors? Essentially, it means that every capacitor is really a series RLC circuit. This means it has some resonance frequency when driven with a periodic signal. At low frequency, the impedance provided by the capacitor is dominant, and your capacitor will exhibit close to ideal behavior. At sufficiently high frequency, the ESL value takes over, and the impedance starts to appear inductive. This produces an effect known as self-resonance at just the right frequency.
Equivalent high frequency capacitor model
This means that the important characteristic distinguishing different capacitors for different frequency ranges is the capacitor’s self-resonant frequency. At this particular frequency, the capacitor will exhibit its minimum impedance and a very strong current response.
For PCBs that will operate a high speeds and high frequencies, the selection of capacitors becomes very important. With high speed digital signalling, capacitors should be selected such that they have ideal capacitive impedance up to the signal’s knee frequency (0.35 divided by the 10%-90% rise time). In other words, the self-resonant frequency should be greater than the knee frequency. With high frequency analog signals, any capacitors should be chosen such that the relevant frequencies in the system are lower than the self-resonant frequency.
This is important from a design aspect as you need your capacitors to act like ideal circuit elements, otherwise you might miscalculate the capacitance you are providing in a particular circuit. It is also important from a power integrity and signal integrity aspect. Capacitors used for bypassing/decoupling are intended to suppress power fluctuations and ringing in a power bus or signal chain when transistors switch, but a mis-sized capacitor can produce ringing due to self-resonance rather than suppressing it.
What is film capacitor loss?
Any real capacitor, under the action of electric field is to consume energy. The capacitor converts a part of the stored or transmitted electrical energy into heat, and a part of it heats the capacitor and increases its temperature. The other part is consumed in the surrounding environment, usually we put the capacitor under the action of electric field, unit time due to heat and energy consumption called capacitor loss. The loss of capacitors is an important index to measure the quality of capacitors. The greater the loss, the more serious the heat of capacitors will be, indicating that the efficiency of energy transfer is worse. In the limit, there is a risk of capacitor destruction, and the higher the frequency of use, the greater this risk.
Composition of loss
The loss of capacitor is mainly composed of dielectric loss, leakage loss and metal loss. When the capacitor is not working at high temperature and very low frequency, the influence of leakage loss can be ignored. That is to say, under normal circumstances the loss of capacitor is mainly composed of dielectric loss and metal loss.
The energy loss of dielectric part of capacitor is mainly composed of conductance loss, polarization loss and ionization loss. The energy loss of the metal part of the capacitor is mainly composed of the loss of the lead line of the capacitor, the loss caused by the effective resistance of the capacitor plate and the loss caused by the contact resistance.
Loss frequency characteristics of film capacitors
According to the national standard, the test voltage of the instrument is set to 0.1V or 0.3V when testing the loss factor of the film capacitor. The test frequency is 1KHZ and the loss of MYlar capacitor is required to be less than 0.0080 (Max. 0.0100). The loss factor of polypropylene film capacitors is required to be less than 0.001. These capacitors are often used in DC, low voltage, low frequency of use. However, some electronic equipment use occasions such as color TV line reverse circuit, switching power supply circuit. Their use frequency is higher in dozens of KHZ or so, and some circuits have AC components, so the high frequency performance of capacitors is required to be good, that is, the loss of capacitors in high frequency conditions to be small. If the high frequency loss is large, the capacitor itself will consume energy during the use of the capacitor, causing the capacitor core to heat up, and eventually leading to the capacitor core film shrinkage, capacitor failure. The high frequency loss of film capacitor increases with the increase of test frequency, and the relationship between them is not linear. The high frequency loss of polypropylene film capacitor is less than that of polyester capacitor. Polypropylene film capacitors are suitable for high frequency circuits because of their low high frequency losses.
How to reduce the high frequency loss of film capacitor in production process
The loss of film capacitor in high frequency state is mainly metal loss, mainly including: capacitor lead loss, capacitor plate loss and contact loss. At present, the lead wire used for film capacitors is tinned copper clad steel wire or tinned copper wire, their own loss is very small, can be ignored. So the loss of film capacitor at high frequency is mainly plate loss and contact loss.
Some high frequency capacitors for your next design
In addition to the standard capacitor selection criteria, you should focus on locating the self-resonant frequency of a candidate capacitor, if it is listed in a datasheet. If you cannot find this value in your datasheet, then you should at least locate the ESR and ESL values. You can then quickly calculate the self-resonant frequency for a series RLC circuit using these values (ignore the leakage resistance for simplicity):
Self-resonance frequency of a capacitor.
Once you locate the various specifications, you can use the above equation to quickly check that a given capacitor will have a sufficiently high self-resonant frequency. You can read more about proper sizing for bypass/decoupling capacitors in this article.
Some other important aspects to consider are:
1. ESR frequency stability: As ESR is a parasitic effect, it can also be a function of frequency. This will affect the shape of the capacitor’s measured impedance curve to some degree. This will also affect sizing in some very high frequency applications.
2. Surface-mount vs through-hole: This becomes extremely important at microwave and higher frequencies. The pin on a through-hole capacitor can act like a strong resonator at extremely high frequencies (mmWave), meaning it will radiate strongly like an antenna. Keep this in mind.
3. Temperature and voltage stability: Capacitance (and all other ratings) can change with the input voltage level and temperature.
4. First series resonance (FSR) and first parallel resonance (FPR): These are the lowest rated frequency value at which S11 and S21 are rated for the capacitor in question.
