SLVUBB4B November   2017  – February 2023

 

  1.   Abstract
  2.   Trademarks
  3. 1 Topologies Window
  4. 2 FET Losses Calculator
  5. 3 Load Step Calculator
  6. 4 Capacitor Current Sharing Calculator
  7. 5 AC/DC Bulk Capacitor Calculator
  8. 6 RCD-Snubber Calculator for Flyback Converters
  9. 7 RC-Snubber Calculator
  10. 8 Output Voltage Resistor Divider
  11. 9 Dynamic Analog Output Voltage Scaling
  12. 10Dynamic Digital Output Voltage Scaling
  13. 11Unit Converter
  14. 12Loop Calculator
    1. 12.1 Inputs
    2. 12.2 Transfer Functions
      1. 12.2.1  Output Impedance Transfer Function
      2. 12.2.2  Transfer Function VMC Buck Power Stage
      3. 12.2.3  Transfer Function CMC Buck Power Stage
      4. 12.2.4  Transfer Function CMC Boost Power Stage
      5. 12.2.5  Transfer Function CMC Inverting Buck-Boost Power Stage
      6. 12.2.6  Transfer Function CMC Forward Power Stage
      7. 12.2.7  Transfer Function CMC Flyback Power Stage
      8. 12.2.8  Transfer Function Closed Loop
        1. 12.2.8.1 Transfer Function Type II Compensation Network
        2. 12.2.8.2 Transfer Function Type II Transconductance Compensation Network
        3. 12.2.8.3 Transfer Function Type III Compensation Network
      9. 12.2.9  Transfer Function Isolated Type II Compensation Network With a Zener Clamp
      10. 12.2.10 Transfer Function Isolated Type II Compensation Network Without a Zener Clamp
  15. 13Filter Designer
    1. 13.1 Impedances
    2. 13.2 Transfer Functions
    3. 13.3 Filter Output Impedance
    4. 13.4 Damping Factor
  16. 14Additional Information
  17. 15Revision History

4 Capacitor Current Sharing Calculator

When connecting different kinds of capacitors in parallel at the input or output of a power supply, the RMS current going through each capacitor is different as it depends on the impedance of the capacitors across the entire frequency range. For exact results for the RMS current per capacitor, impedances and currents must be calculated for all harmonics of the switching frequency. The RMS current for each harmonic must be derived with a Fast Fourier Transformation (FFT) of the total current signal based on the ratio between total impedance and single-capacitor impedance at that harmonic frequency.

#SLVUBB48326 shows the Capacitor Current Sharing Calculator.

Note:

In Power Stage Designer, the impedances and the RMS currents are only calculated at the switching frequency. Thus, the resulting RMS currents are rough estimations.

GUID-98792DF0-9DFF-4BA5-8121-1B9339C31BB9-low.png Figure 4-1 Capacitor Current Sharing Calculator

The impedance for one capacitor at the switching frequency (n can be 1, 2, or 3 and refers to the capacitor index) can be calculated as indicated in #GUID-52184369-1A68-4F95-8C95-7C0EC84E18A8:

Equation 10. Z cap , n =   ESRC n + i   ×   2   ×   π   ×   f swi t ch   ×   ESLC n - 1 2   ×   π   ×   f switch   ×   C n

Typical ESL values for capacitors are from 1 nH to 7 nH. By assuming 6 nH/cm as parasitic inductance for a conductor, the inductance for a ceramic capacitor can be estimated by multiplying this value with the capacitor length. PCB traces and vias can increase this value slightly (see [1]).

The total impedance of three parallel capacitors at the switching frequency results as seen in #SLVUBB47688:

Equation 11. Z t o t a l = 1 1 Z c a p , 1 + 1 Z c a p , 2 + 1 Z c a p , 3

The RMS current of one capacitor, while neglecting all other harmonics besides the switching frequency, can be calculated as seen in #GUID-B13DFF4F-7176-4C44-82B3-972C5196E89E:

Equation 12. I r m s , c a p , n =   I r m s , t o t a l   ×   Z t o t a l Z c a p , n