1. Selection of Power Filters for Switched Mode Power Supplies
Konstantin Kostov
Jukka-Pekka Sjöroos
J. Kyyrä and T. Suntio
Power Electronics Laboratory
Helsinki University of Technology

2. Selection of Power Filters for Switched Mode Power Supplies
Content
Insertion Loss
EMI measurements
EMI measured at the line & neutral
CM and DM noise measurements
Comparison between different IL data
Conclusions
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

3. Selection of Power Filters for Switched Mode Power Supplies
Insertion Loss (IL)
Calculations
IL measuring set up
IL data from power filter manufacturers:
Usually 50 Ω / 50 Ω source and load impedance
Also based on CISPR 17 the so-called ”approximate worst case” IL measurements:
0.1 Ω / 100 Ω
100 Ω / 0.1 Ω
The closed loop system may even become unstable when the design infinite-precission controller is implemented with a fixed-point digital processor due to the finite precission of the parameter representation resulting from the FWL
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

4. EMI Filter CY = 4.7 nF LCM = 4.2 mH CX1 = CX2 = 100 nF Lleak = 37 H
CM & DM equivalent circuits
CY = 4.7 nF
LCM = 4.2 mH
CX1 = CX2 = 100 nF
Lleak = 37 H
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

5. Measured and Calculated CM IL
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

6. Measured and Calculated DM IL
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

7. Conducted Emissions Measurements
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

8. Buck Converter Used in Measurements
fs = 250 kHz
Uin = 35 V
CE = 220 F
Uo = 12 V
L = 190 H
Io = 1 A
C = 100 F
Iin = 0.38 A
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

9. EMI from a Buck Converter (Line)
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

10. EMI from a Buck Converter (Neutral)
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

11. Selection of Power Filters for Switched Mode Power Supplies
Measuring CM EMI
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

12. Selection of Power Filters for Switched Mode Power Supplies
Measured CM EMI
CM and DM
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

13. Selection of Power Filters for Switched Mode Power Supplies
Measuring DM EMI
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

14. Selection of Power Filters for Switched Mode Power Supplies
Measured DM EMI
CM and DM
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

15. CM Noise Attenuation Measured, Calculated and Actual
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

16. DM Noise Attenuation Measured, Calculated and Actual
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

17. Selection of Power Filters for Switched Mode Power Supplies
Conclusions
The attenuation of a passive EMI filter operating with a SMPS can be about:
The CM IL data for 0.1  / 100  source / load impedance.
The DM IL data for 100  / 0.1  source / load impedance.
The actual IL can be a lot lower than the standard 50  / 50  IL data.
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

18. Measured CM and DM EMI (with Tektronix A6312 Current Probe)
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

19. Selection of Power Filters for Switched Mode Power Supplies
DM Noise Attenuation Y3 Z2 Y1
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

20. Selection of Power Filters for Switched Mode Power Supplies
CM Noise Attenuation Z2 Y1
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

21. Components’ Impedances
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018

22. EMI Filter Topology and Equivalent Circuits
Usually π-filters are used
DM equivalent circuit
CM equivalent circuit
Selection of Power Filters for Switched Mode Power Supplies
22 September, 2018