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Low Pass Filter to Prevent Ringing in PWM Signal

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mengghee

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pwm filter

hello,

I am currently designing a boost converter and i have also designed the pwm signal. but while i fed the pwm signal to the buffer, the output of the buffer has some ringing occur. so i need to design a filter for the ringing effect. the ringing that occurs in the circuit is around 15Mhz. so i suppose my cut off frequency for the filter is around 200khz as my pwm signal is around 50khz to 100khz. but i don't know how to calculate the resistor and capacitor value. and also .... i am wondering if this might effect my mosfet as my mosfet requires certain current to drive it. thanks ....

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mengghee
 

electronrancher

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filter pwm

No you don't want to filter here - slowing down the gate will drop your efficiency. You should investigate why you have ringing.

For an external switch you could have ringing if your buffer->gate connection is quite long and inductive. The best thing to do is to fix this path so it is short and low impedance.

If you can't fix this path a quick and dirty patch is to put a small (2-10 Ohm) resistor in series with the gate to make your lowpass - the gate itself is the Capacitor. However, like I mentioned before you would be degrading the efficiency somewhat by doing this.
 

    mengghee

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mengghee

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pwm filter lowpass design

thanks electronrancher, I have noticed one thing in the circuit. the ringing that occurs from the buffer to gate, occurs before the Rg which is used to drive the gate of the mosfet. if i probe after Rg. there is no ringing occurs. I am just wondering if this is considered okay ? ... thank you

regards,
mengghee
 

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