Paralleling capacitors to reduce high frequency output voltage

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Paralleling capacitors to reduce high frequency output voltage ripple 1

Paralleling capacitors to reduce high frequency output voltage ripple 1

A technique for reducing High Frequency output noise • If the output capacitor(s) is

A technique for reducing High Frequency output noise • If the output capacitor(s) is not ceramic; then adding a small ceramic(s) in parallel with the output will reduce high frequency ripple. • Choose a ceramic capacitor that has an impedance null (self resonance) that is the same as the frequency to be attenuated. • One, two or three small ceramics can give 10 X improvement (-20 d. B. ) 2

High frequency ripple Switch waveform (scope trigger) Vout ripple w/ 20 MHz bandwidth (bw)

High frequency ripple Switch waveform (scope trigger) Vout ripple w/ 20 MHz bandwidth (bw) 5 m. V /div 10 mv p-p HF spikes ignored ! 3

Use Zoom function to measure ring frequency Timebase Zoomed traces 20 MHz bw 10

Use Zoom function to measure ring frequency Timebase Zoomed traces 20 MHz bw 10 m. V/div 300 MHz ring 200 MHz bw 100 m. V/div 2 GHz bw 100 m. V/div 4 Need to add 470 p. F 0603 bypass SRF ~ 300 MHz

Continue the method Timebase Zoomed traces 20 MHz bw 10 m. V/div 200 MHz

Continue the method Timebase Zoomed traces 20 MHz bw 10 m. V/div 200 MHz bw 100 m. V/div 2 GHz bw 100 m. V/div 115 MHz ring 5 Measured after adding a 470 p. F 0603 but before adding 2200 p. F 0603

Continue the method Timebase Zoomed traces 20 MHz bw 10 m. V/div 200 MHz

Continue the method Timebase Zoomed traces 20 MHz bw 10 m. V/div 200 MHz bw 100 m. V/div 2 GHz bw 100 m. V/div 60 MHz ring 6 Measured after adding a 470 p. F 0603 and a 2200 p. F 0603 but before 4700 p. F 0805

Results after 3 rd added small capacitor 20 MHz bw 10 m. V/div 200

Results after 3 rd added small capacitor 20 MHz bw 10 m. V/div 200 MHz bw 100 m. V/div 2 GHz bw 100 m. V/div Ring ~377 MHz 7 Measured after adding a 470 p. F 0603, 2200 p. F 0603, and 4700 p. F 0805

Final amplitude improvement results 20 MHz bw 10 m. V/div 200 MHz bw 10

Final amplitude improvement results 20 MHz bw 10 m. V/div 200 MHz bw 10 m. V/div 2 GHz bw 10 m. V/div 20 m. V p-p @ 20 MHz bw 80 m. V p-p @ 200 MHz bw 8 After 470 p. F 2200 p. F 4700 p. F

Starting point for comparison - 3 caps removed 20 MHz bw 200 MHz bw

Starting point for comparison - 3 caps removed 20 MHz bw 200 MHz bw 2 GHz bw 10 m. V/div 200 m. V/div 696 m. Vp-p @ 200 MHz bw 200 m. V/div 9

Final schematic and bill of materials: 15 minutes later Cost Approx < $0. 03

Final schematic and bill of materials: 15 minutes later Cost Approx < $0. 03 USD 1. 2 VDC OUT @ 5 A 2200 p. F Remember to reserve locations on the schematic and PCB for these parts. You will not know the capacitor values until after you test the running power supply for ringing noise. Plan ahead 10

Bench requirements • 2 GHz bw / 20 Gsps Digital oscilloscope with zoom feature

Bench requirements • 2 GHz bw / 20 Gsps Digital oscilloscope with zoom feature and adjustable channel bandwidth. • Selection of small capacitors pre-characterized by Self Resonant Frequency. • High quality interconnections with controlled impedances. Example of 3 channel input adapter built for this tutorial (net 4 x passive probe) 11

Use C 0 G (NP 0) dielectric for high frequency shunt filter capacitors –

Use C 0 G (NP 0) dielectric for high frequency shunt filter capacitors – lower ESR – more stable over temp Start with manufacturer data sheets, then measure SRF on bench to confirm 12