This article explains capacitor losses (ESR, Impedance IMP, Dissipation Factor DF/ tanδ, Quality FactorQ) as the other basic key parameter of capacitors apart of capacitance, insulation resistance and DCL leakage current.
There are two types of losses:
- Resistive real losses – these are real losses caused by resistance of leads, electrodes, connections etc. During current flow these losses are dissipated by Joule heat. Usually (unless it is intended by designed) the effort is to minimize these losses for maximum efficiency and high power load ratings.
- Reactance imagine losses – these are losses caused by capacitive reactance and inductive reactance “stored” in the component that can be reverted back
Cs,Rs or Cp,Rp What to use for impedance measure bridge set up ?
An impedance analyzer can measure the capacitance both in parallel or in series. The best fit circuit model will be depending on the value of capacitance of the capacitor.
When C is small and impedance is high, parallel impedance between C and Rp will become significantly higher than Rs. Thus the meter setting for measuring capacitance should be Cp. When C is large and impedance is small, parallel impedance for C and Rp is not as significant. Therefore, Cs should be used for the meter setting to measure capacitance.
A good rule of thumb to select the impedance setting is to use Cp for capacitor impedance values greater than 10kΩ and Cs for less than 10Ω.
ESR or DF ?
Another practical outcome is to think about what parameter is better for description of losses in capacitor – ESR or DF ? From the pure physics point of view, it does not matter, as together with a second parameter (capacitance or impedance), both parameter are describing the same stage of the capacitor. While ESR is an information about real losses only, DF combine information about real losses and reactive losses (but we need the second parameter anyway to describe the exact capacitor stage).
The same is true about ESR/DF frequency dependence. It does not matter if I use ESR or DF characterization for description at low or high frequencies from the basic physics. HOWEVER, the industry convention is to use DF for low frequency (120Hz or 1kHz) characterization, where dielectric losses are dominating and ESR for higher frequency (100kHz) behaviour, where resistive connection losses are the main part of the losses. You can find both DF and ESR values in manufacturer datasheets with reference to those frequencies.
Case study: you can hear people from industry saying: “that capacitor has a high DF” that means that the capacitor has a high losses in the lower frequency zone (120/1kHz) that could indicate some issue with dielectric material (impurities, delamination …). and of course, ESR at 120Hz/1kHz will be also high. The same is about ESR – when someone says: “ESR of the capacitor is a problem” he/she usually means reference to the standard measurement frequency 100kHz indicating problems with resistive losses (inter-connection issues, bad welds etc) … and of course DF at 100kHz would be also high.