7. EMV Fachtagung. EMV-gerechtes Filterdesign. 23. April 2009, TU-Graz. Dr. Gunter Winkler (TU Graz) Dr. Bernd Deutschmann (Infineon Technologies AG)

Transcription

1 7. EMV Fachtagung 23. April 2009, TU-Graz EMV-gerechtes Filterdesign Dr. Gunter Winkler (TU Graz) Dr. Bernd Deutschmann (Infineon Technologies AG) Page 1

2 Agenda Filter design basics Filter Attenuation Placement of filter components Location of the filter Page 2

3 Part 1 Filter design basics Filter Attenuation Placement of filter components Location of the filter Page 3

4 The EMC problem Every EMC problem consists of three parts: System that generates interference System that is susceptible to the interference Coupling path System that generates the interference (Culprit) System that is susceptible to the interference (Victim) Power supply Data bus Page 4

5 The EMC problem Filters are often used to eliminate unwanted interference on cables and wires. Where should the filters be placed? At the culprit? Or at the victim?? How does a filter work?? How should the layout of the filter components be done?? What kind of filter should be used? Page 5

6 How does a filter work? Filters work by creating a discontinuity in the characteristic impedance seen by the noise signal that is travelling from the source to the load in order to reflect most of the noise energy back to where it came from or to absorb a part of it. discontinuity discontinuity Page 6

7 Creating a discontinuity Example 1 (inserting a serial impedance): Z Noise =100Ω Z Filter =1kΩ 1:10? Only about 10% of the noise signal reaches the load (=> an attenuation of around 20dB). Page 7

8 Creating a discontinuity Example 2 (inserting a parallel impedance): Z Noise =100Ω Z Filter =10Ω 10:1 Only about 10% of the noise signal reaches the load (=> an attenuation of around 20dB). Page 8

9 R-only filters: Serial impedance - single stage filter Equivalent circuit of a real resistor Frequency response of a real resistor R-only filters create a high series impedance but usually only achieve a few db attenuation. They are best used where the impedance of the noise source and the load is low. R filters lose their performance at high frequencies due to their parasitic shunt capacitance. Page 9

10 L-only filters: Serial impedance - single stage filter Equivalent circuit of a real inductor Frequency response of a real inductor L-only filters create a high series impedance but usually only achieve a few db attenuation. They are best used where the impedance of the noise source and the load is low. L filters lose their performance at high frequencies due to their parasitic shunt capacitance which creates a resonance. Page 10

11 C-only filters: Parallel impedance - single stage filter Equivalent circuit of a real capacitor Frequency response of a real capacitor C-only filters create a low parallel impedance and are best used where the impedance of the noise source and the load is high. C filters lose their performance at high frequencies due to their parasitic lead inductance which creates a resonance. Page 11

12 Selection guide for filters Suitable filters for different source/load impedances Load impedance Source impedance Page 12

13 Agenda Filter design basics Filter Attenuation Placement of filter components Location of the filter Page 13

14 Filter Attenuation The attenuation of a filter is used as a design criteria for choosing an adequate filter (datasheet). The output voltage is measured in a 50 Ohm system with and without the filter in the circuit. Page 14

15 Design example: Filter selection Filter Attenuation Requirement: Using the 150Ohm method (IEC ) the conducted EME of the DUT (74HCU04) should be below 42dBµV in the FM band. The conducted EME of an IC pin can be measured with an impedance network of 150Ω which represents the typical antenna impedance of lines, as specified in IEC Page 15

16 Filter Attenuation Example Measurement of a switching IC pin (74HCU04, 3V3, 1MHz) Att 0 db Ref 80.0 dbµv * RBW VBW SWT 10 khz 30 khz 100ms M1[1] dbµv MHz 1AP Clrw 70 dbµv 60 dbµv 50 dbµv 40 dbµv D dbµv M1 At 100MHz the measured Value of 59dBµV is 17dB above the limit of 42dBµV. 30 dbµv 20 dbµv 10 dbµv 0 dbµv -10 dbµv A filter should suppress the disturbance by 23dB (6dB below the limit). CF MHz Span 10.0 MHz Date: 11.APR :37:37 Page 16

17 Choosing the right filter Filter Attenuation Example Attenuation at 100 MHz: Trc1 S21 db Mag 10 db / Ref 0 db Cal 1 Trc1 S21 db Mag 10 db / Ref 0 db Cal 1 S21 Mkr MHz db S21 Mkr MHz db Mkr Mkr E6 1E7 1E8 1E9 1E6 1E7 1E8 1E9 Ch1 Start 150 khz Pwr -10 dbm Stop 2 GHz Date: 11.APR :42:55 Ch1 Start 150 khz Pwr -10 dbm Stop 2 GHz Date: 11.APR :47:22 T-Filter (Ferrite Beads, 100pF): 15.5 db not O.K. 6 Hole Ferrite Bead, 3 turn: 23.5 db O.K Page 17

18 Filter Attenuation Example Evaluation of the filter Suppression at 100 MHz: Att 0 db Ref 80.0 dbµv * RBW VBW SWT 10 khz 30 khz 100ms M1[1] dbµv MHz Att 0 db Ref 80.0 dbµv * RBW VBW SWT 10 khz 30 khz 100ms M1[1] dbµv MHz 1AP Clrw 70 dbµv 1AP Clrw 70 dbµv 60 dbµv 60 dbµv 50 dbµv 50 dbµv 40 dbµv D dbµv M1 40 dbµv D dbµv M1 30 dbµv 30 dbµv 20 dbµv 20 dbµv 10 dbµv 10 dbµv 0 dbµv 0 dbµv -10 dbµv -10 dbµv CF MHz Span 10.0 MHz Date: 11.APR :55:25 CF MHz Span 10.0 MHz Date: 11.APR :51:18 T-Filter: = 22dB O.K.? 6 Hole Ferrite: 59-42,4 = 16,6dB not O.K Page 18

19 Filter Attenuation Example Choosing the right filter (embedding): Attenuation with simulated 150Ω impedance network at 100MHz: Trc1 S21 db Mag 10 db / Ref 0 db Cal 1 Trc1 S21 db Mag 10 db / Ref 0 db Cal 1 S21 Mkr MHz db S21 Mkr MHz db Mkr 1-30 Mkr E6 1E7 1E8 1E9 1E6 1E7 1E8 1E9 Ch1 Start 150 khz Pwr -10 dbm Stop 2 GHz Date: 11.APR :12:22 Ch1 Start 150 khz Pwr -10 dbm Stop 2 GHz Date: 11.APR :04:49 T-Filter: 27,9 11,7 = 16,2 db not O.K.? 6 Hole Ferrite : 30-11,7 = 18,3 db not O.K Page 19

20 Filter Attenuation Example Looking at the output of the HCU04 Top: Output HCU04, Bottom: Output 150Ω impedance network T-Filter: The input impedance of the filter modifies the rise time of the ICs output pulse 6 Hole Ferrite Bead: The rise time of the ICs output pulse is unchanged Page 20

21 Part 3 The filter basics Filter Attenuation Placement of the filter components Location of the filter Page 21

22 Placement of filter components DC/DC converter Design example: Placement of filter components Input PI-Filter Output PI-Filter 15uH 2.2uH 47uF 22uF 47uF 47uF TLE6365 Step Down Voltage Regulator Source: Philipp Schröter, Untersuchung und Optimierung der Verkopplung von Ein- und Ausgangsfilter für einen DC/DC Konverter Master thesis, TU-Ilmenau, 2007 Page 22

23 Placement of filter components DC/DC converter TLE6365 Step Down Voltage Regulator (12V -> 5V) same circuitry same voltage same placement area same components different placement of the components Board A Board B Page 23

24 Conducted Emission measurement Setup for the conducted emission measurement according to CISPR 25 EMI Receiver Power supply DUT LISN CISPR25 Page 24

25 Comparison of board A and board B CISPR 25 LISN measurement Amplitude [dbµv] Frequency [MHz] Board A Board B Class I Class II Class III Class IV Class V Page 25

26 Placement of filter components Where does the difference come from? Answer: Coupling between the filter components Coupling between the capacitors and the inductor L C C Coupling between the two capacitors Page 26

27 Placement of filter components Coupling between the two capacitors: L1 i1 i2 C1 C2 Page 27

28 Placement of filter components Optimal placement of the capacitor: L1 C2 i1 C1 Place C2 90 degrees to C1 => minimal coupling Page 28

29 Coupling between C1 and L1: Placement of filter components L1 C2 C1 Coupling from L1 to C1 due to wrong placement of C1 Page 29

30 Placement of filter components Optimal placement of capacitor and inductor: L1 C2 C1 Place C1 in parallel to the flux lines of L1 Page 30

31 Placement of filter components example PI-Filter (470nF + 10µH + 470nF) Trc1 Mem2[Trc1] 0 S S21 S21 db Mag db Mag 10 db / 10 db / Ref 0 db Ref 0 db Cal Smo Smo Mkr MHz db Mkr E6 1E7 1E8 1E9 Ch1 Start 150 khz Pwr 0 dbm Stop 2 GHz Date: 17.APR :29:13 Filter attenuation at 10MHz is 20dB higher only by turning the right capacitor by 90degrees. Page 31

32 Part 4 Filter design basics Filter Attenuation Placement of filter components Location of the filter Page 32

33 Location of the filter Wrong placement Capacitive coupling between the protected line C and the unprotected line B Page 33

34 Disturbance source on a PCB Location of the filter Should a filter be placed close to the disturbance source inside the instrument? where the signal line leaves the PCB? where the signal line leaves the device? Page 34

35 Location of the filter Disturbance source outside the device The current induced by the outer disturbance should be guided to the case as close as possible to the input. If a split ground plane is used to isolate a protected ground from filter ground, signal lines crossing the gap increase susceptibility against electromagnetic fields and raise the electromagnetic emission. Page 35

36 Conclusion Murphy's Filter Law 1: The filter that turns out to be necessary will be completely incompatible with the rest of the product! either it will be too expensive, too large to fit in the case will require expensive retooling or it will have too large leakage currents that present safety hazards. Murphy's Filter Law 2: The filter attenuation that is specified by the filter manufacturer turns out to be much less when the filter is used in the application! The filter attenuation is characterized using a 50Ohm system But does your application also provide a 50Ohm source and load impedance? It is impossible to rely on manufacturers specifications unless working in a controlled 50Ohm system. Page 36

37 Conclusion Murphy's Filter Law 3: The attenuation of the filter would have been perfect if it had been connected to the right reference point! either there is a plastic case => case is no good reference point or there is a two layer PCB without a solid ground/signal return plane choose the right reference point in relation to the kind of interference (common mode / differential mode) Murphy's Filter Law 4: Using the best components for a filter is nothing without knowing how the place them! the quality of a self-made filter strongly depends on the placement of each filter component and the layout of the interconnections always consider the capacitive and inductive coupling between each filter component Page 37