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Paralleling capacitors to reduce high frequency output voltage ripple
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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 10X improvement (-20 dB.)
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High frequency ripple
Switch waveform (scope trigger)
Vout ripple w/ 20 MHz bandwidth (bw)
5 mV /div 10 mv p-p
HF spikes ignored !
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Use Zoom function to measure ring frequency
20 MHz bw
200 MHz bw
2 GHz bw
Timebase Zoomed traces
300MHz ring
10 mV/div
100 mV/div
100 mV/div
Need to add 470 pF 0603 bypass SRF ~ 300 MHz
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Continue the method
20 MHz bw
200 MHz bw
2 GHz bw
10 mV/div
100 mV/div
100 mV/div
115 MHz ring
Measured after adding a 470 pF 0603 but before adding 2200pF 0603
Timebase Zoomed traces
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Continue the method
20 MHz bw
200 MHz bw
2 GHz bw
10 mV/div
100 mV/div
100 mV/div
60 MHz ring
Measured after adding a 470pF 0603 and a 2200pF 0603 but before 4700pF 0805
Timebase Zoomed traces
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Results after 3rd added small capacitor
Measured after adding a 470pF 0603, 2200pF 0603, and 4700pF 0805
Ring ~377MHz
20 MHz bw
200 MHz bw
2 GHz bw
10 mV/div
100 mV/div
100 mV/div
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Final amplitude improvement results
20 MHz bw
200 MHz bw
2 GHz bw
10 mV/div
10 mV/div
10 mV/div
20 mV p-p @ 20MHz bw
80 mV p-p @ 200MHz bw
After 470pF 2200pF 4700pF
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Starting point for comparison - 3 caps removed
20 MHz bw
200 MHz bw
2 GHz bw
10 mV/div
200 mV/div
200 mV/div
696mVp-p @ 200MHz bw
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Final schematic and bill of materials: 15 minutes later
Cost Approx < $0.03 USD
3.3µH 6AL1
220µF
0.0075 ohm6.3V
COUT1
GNDGND
+3.3VDC OUT
800 kHz Buck Switcher
Spe
cial
ty P
olym
er
470 pFNP0 (C0G)
COUT2470 pFNP0 (C0G)
COUT34700 pFX7R
COUT4
GNDGND
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
1.2VDC OUT @ 5A
2200pF
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Bench requirements• 2GHz bw / 20Gsps 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 4x passive probe)
CH2 - 20MHz
CH3 - 200MHz
CH4 - 2GHz
SOURCE 3 CHANNEL INPUT
50 OHM COAX49.9
RSOURCE
1.2VDC SOURCE
Use C0G (NP0) 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
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