Hello there internet.

I have an issue where I am getting Mhz resonance the moment the power mosfets are activated.
This is causing interference to the point where my mouse touch pad on the laptop stops working, the digital system controlling the buck converter is also affected (stops working for a period of time then comes back to life) amoung other electronic equipment starts to malfunction which are also near the device. I monitored the 3.3v linear regulators output voltage on the digital system and i get these spikes on the output. The digital system is powered by the battery and has the same GND as the neg side of the battery.


I need to minimize this interference. The oscillation/resonance is only occurring the first moment the power mosfet is activated.

Voltage example on the inductor when mosfets are activated:
(cell block = 12v) I could of taken a photo of the oscilloscope, but im lazy

I am using a toroidal inductor:


Any toroidal inductor experts out there who can advise on reducing noise from a toroidal inductor?

Maybe cover the whole ferrite core with more enameled wire, making sure the entire core is not exposed? or maybe the opposite?

Maybe its the eddy currents causing the resonance?

Anyone else experience this phenomenon?

Inductor has about 600uh
test was performed @ 50khz 33% duty cycle.
RMS current measured = 0.25 amps
35v cap = 4000uf
100v cap = 8000uf

 

Your schematic shows no resistors on the gate connections. Those help to stop gate 'ringing', which could be the cause of your problem. Google "mosfet gate ringing" for more information.

 

Your schematic shows no resistors on the gate connections. Those help to stop gate 'ringing', which could be the cause of your problem. Google "mosfet gate ringing" for more information.

Oops, should of had that in the schematic. Yes I do have resistors on the gates. My Bad!

Thanks shortbus for the reply, appreciate it.

Today I found the cause.

Because the digital system is being powered by the very source that is being charged by the inductor, it was causing PWM communication issues to the mosfet driver. Basically, the surge from the inductor was going through the digital system causing a feedback loop, which was causing the oscillations which caused the interference.

Having the digital system powered separately and isolated from the battery has fixed this issue.

Note taken to others out there, dont design a charging system thats powered directly by the very thing that your trying to charge! Bad idea!