| FORUM

FEDEVEL
Platform forum

USE DISCOUNT CODE
EXPERT30
TO SAVE $30 USD

USB C - VBUS filtering

Alaeddine , 01-17-2025, 03:59 PM
hi, in my project the board get its power from the USB. I wanted to know if i have to use a pi filter and if yes how to choose the values for the caps and the ferrite bead. In most Phil's videos he is using a 120R FB and in some reference designs 600R is being used
QDrives , 01-17-2025, 09:11 PM
What is the power consumption of your device?
Current carrying capability decreases when impedance increases for the same package dimensions.
As for the combination (capacitance and inductance) you need to be aware of resonances between them. This resonance may could your noise to be amplified instead of being dampend.
Alaeddine , 01-17-2025, 09:18 PM
5V with 500mA. Should i use only a capacitor to be on the safe side ? if so how to dimension the cap as well
QDrives , 01-17-2025, 09:28 PM
"*Should i use only a capacitor to be on the safe side ?*" -- No. Although you might want to opt for a 'real' inductor.
You make an AC simulation with the inductance and capacitance. Add a snubber to dampen the resonance. C of snubber ~10x C of system/net.
Alaeddine , 01-18-2025, 01:48 PM
i made a simulation with the murata tool and this is the Differential Mode Insertion Loss for two different Pi Filters. To be frank, I have not understand this bode. and for the target noise frequency i selected 20 MHz to 300 Mhz. I'm not sure if it's correct. This whole world is new to me.
For the blue line i'm using 100nF, 120R FB, 10uF capacitor
For the grey, i'm using 4.7uF, 120R FB, 4.7uF capacitor
QDrives , 01-18-2025, 05:06 PM
What DC resistance?
Which ferrite bead did you select?
Alaeddine , 01-18-2025, 05:49 PM
30 mOhms. this is the bead BLM18KG121JH1D
i played around with different combinations. Which sim is the most effective here ?
QDrives , 01-18-2025, 07:34 PM
The simulation that starts at 10kHz or lower!
https://www.youtube.com/watch?v=_9zfzDkNZR8
Estimated inductance of bead = ~5uH. With 5uF (50% derating due to DC and AC bias), you get a resonance at ~32kHz of +30dB!
Use our interactive Discord forum to reply or ask new questions.
Discord invite
Discord forum link (after invitation)

Didn't find what you were looking for?