I'm dealing with a PWM generator which drives a load at 12V with pretty high currents (5-10A)
The PWM frequency is in the band of 100-200 Hz. Which is the best choice and right configuration of ferrite beads to achieve the best filtering of EMI?
Electronic – EMI suppression low frequency
complianceemcferrite-beadpwm
Related Solutions
Best way to minimize EMI is to have the MOSFET close to the load for sure but also have the MOSFET driver close to the MOSFET. I have a concern that along 30cm the pulse distortion and introduced impedances might cause issues such as spurious ringing and / or inefficient FET turn-on and turn-off. If you can't get the MCU generating the PWM close to the FET then maybe use a driver circuit at the FET that can "square" the signal back up again and offer a low impedance drive to the FET.
You will also reduce EMI by having a good reservoir capacitor on the DC line feeding the MOSFET - it needs to be up close to the MOSFET so no pulses of large current are carried down the cable feed from the 12V power supply. A good reservoir capacitor would be a decent sized electrolytic (100uF plus) with 1uF ceramic and 1n ceramic in parallel. This maybe over the top but I wouldn't take chances on this.
You might also consider what effects the power PWM has on the load and if necessary apply inductor and capacitor filtering on the FET's output. I'm presuming that you have a fly-back diode on the output of the FET and if you haven't you'll likely need one.
Here's a circuit I did a few years ago that demonstrates a driver chip local to the MOSFET and an over-current protection in case the load shorted out. The MOSFET used is not as suitable as the BUK954 you mentioned on another question. I'd use that type for your job.
The load is assumed to be purely resistive. In fact the original circuit had two MOSFETs driving a transformer with a centre-tap to the positive supply rail and it didn't need fly-back diodes (it used snubbers) but your's might need a protection diode across the load.
Circled in red is a local +5V power rail used for the over-current monitor - this can easily be derived locally with a small linear regulator such as 78L05.
Also note the big array of capacitors to the left of the circuit that absorbs most of the transient energy locally produced by switching the load. The original design was switching at 600kHz so if you wanted to run faster than 20kHz this shouldn't be a problem but i wouldn't go above 100kHz for the sake of efficiency.
Related Topic
- Electronic – EMI Filtering on LVDS lines
- Electronic – LED + PWM EMC concerns
- Electrical – Filter high-frequency PWM but keep low-frequency modulation
- Electronic – Automotive battery input EMI filtering
- Electronic – Filtering power to stepper motor drivers for EMI
- Electronic – Question on EMI filtering
- Electronic – Ferrite bead attenuation frequency
Best Answer
Ferrite beads will prove fairly useless at such a low frequency. They tend to start becoming useful in the area above 10 MHz. I'd consider LC filtering but the precise way depends on your particular circuit. Here's some general info on ferrite beads: