SLOA339 July   2024 TAS2764 , TAS2764 , TAS2780 , TAS2780 , TAS2781 , TAS2781

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Capacitor Only Connected at the Class-D Outputs
    2. 1.2 LC Filter Connected at the Class-D Outputs
    3. 1.3 Ferrite Bead Filters
  5. 2EMI Filter Considerations
    1. 2.1 Impedance Consideration of EMI Filter
    2. 2.2 Device Reliability Constraint for High Output Voltage
    3. 2.3 EMI Filter Current Reliability
  6. 3Post Filter Feedback
  7. 4Loop Stability With Post Filter Feedback
  8. 5User Guide: EMI Filter Modeling and Post Filter Feedback Validation Tool
  9. 6Summary
  10. 7References

5 User Guide: EMI Filter Modeling and Post Filter Feedback Validation Tool

Since it is relatively cumbersome to do multiple calculations as shown previously to estimate the EMI filter R,L,C parameters and cross the stability criteria for loop stability with post filter feedback for each dc bias current setting, TI has built a custom excel based EMI filter validation tool using simple excel spreadsheet to validate the same to make it easier for users to cross verify (Ferrite Bead Impedance Calculator PFFB Work).

The first step involves back calculating the EMI filter R,L,C model to estimate ω0 and Q. For this, the user must input the following from the data sheet of the bead, available online.

  1. Rdc (dc impedance of the bead)
  2. Peaking frequency (Fpeak)
  3. Peak impedance (Zpeak)
  4. Additional Frequency (Freq2) and impedance at that frequency (Z at freq2)
  5. Cpar

For example, consider the case of 2506036017Y2 as per the data sheet for DC bias current=0, the following parameters can be estimated: Rdc= 0.12, Fpeak= 180MHz, Zpeak=675. Additionally if we chose Freq2= 30MHz and Z at Freq2=300. These inputs are given as input to the excel workbook (Green color) and the excel sheet output provides the complete bead R,L,C model as well as the ω0 and Q. Let us consider Cpar=100pF.

Snapshot of EMI R, L, C Model and ω_O, and Q Calculator Figure 5-1 Snapshot of EMI R, L, C Model and ω_O, and Q Calculator

The outputs Rdc, Rpar, CEQ and LEQ calculated are modeled as per Figure 5-1. Further the calculated ω0 and Q can be given as inputs to the excel sheet to validate if the EMI filter is acceptable for use in post filter feedback configuration

As observed for these values the ω0 falls in the 10MHz-20MHz range and the (ω0 / Q) = 2.36e6 which is >7.7e5 as per Figure 5-1. Hence satisfying the PFFB stability criterion. The two columns Check for ω0 input range and ω0/Q Check are used to validate the same (as shown in Figure 5-2). If both criteria are met as observed in the excel sheet, a green-highlight cell indicates that the filter is valid for use.

Snapshot of EMI Filter Validation Excel for PFFB Stability Figure 5-2 Snapshot of EMI Filter Validation Excel for PFFB Stability

If there is no green-highlight then the filter is not valid for use and if the ω0 > 1.5MHz then Q factor has to be reduced to allowable limit as per Table 4-1. Post modifications in bead or filter cap parameters, the calculator can again be used confirm the stability criteria is satisfied.