SMD ferrite beads to supress noise when driving mosfets

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incal
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Posted by incal: Sat. Feb 15, 2014 - 07:29 PM
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I found a very interesting element a ferrite bead in smd form.

It has 0.015ohm resistance until 1MHZ and after 1MHZ the resistance rise light parabolically to a value of 50Ohm at 100Mhz.

I read that , this can be used as a gate drive resistor series with a lower value resistor to drive mosfets and IGBTS and this method reduces parasitic noise.

What can be the downside. I attachted a datasheet of the ferrite bead. It looks like a purpose shaped bandwidth optimized inductor between 1MHZ and 100MHZ

Attachment(s): 

PDF icon Ferritebead.pdf

________________________________ We dream of a world where current does not need the voltage to flow.

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Learning and Information, General Electronics
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Kartman
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Posted by Kartman: Sun. Feb 16, 2014 - 02:47 AM
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This is used to hopefully eliminate oscillations when switching due to inductance in the drain circuit. When switching at high currents and frequencies, even a very small amount of inductance can have a significant effect.
Known as a 'gate stopper'.
You also see ferrite beads in the leg of the catch diode in flyback circuits to curb the snap on oscillations. These can be of the order of several megahertz.

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incal
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Posted by incal: Sun. Feb 16, 2014 - 09:07 AM
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The attached paper claims that it very usefull to add a ferrite bead to decrease the parasitic ossiltions even for high current high frequency gate drives (see figure 7 of the paper).
It says that the loop area between gate and emitter that acts as an antenna is the problem but not the proporly dimensined *compact* (not over the pcb splitted) inductance.

they exchanged 1ohm+ferrite bead with 3ohm gate resistor and the result is better. When properly selected the gate stop is not an issue. (figure 7: the mosfet has 25ns rise time).

Attachment(s): 

PDF icon Redution of Gate ossilations

________________________________ We dream of a world where current does not need the voltage to flow.

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Kartman
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Posted by Kartman: Sun. Feb 16, 2014 - 09:27 AM
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In light of your investigations, what is the downside? I'd say its necessary.

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