EMC of SMPS and MOSFET switching speed

[eem2am]

smps mosfet

Hello,

In SMPS, if mosfet switches on/off very fast, then this is said to cause EMC problem.

Which is the best way to mitigate this? ..

is it to slow down the mosfet switching speed by inserting a higher value series gate resistor

or is it to use an RC snubber across the mosfets Souce_drain ?

Thankyou for listening.

 

[vaibhav.k]

emc mosfet

You cant slow down switching frequency by adding just resistor.Better way decrease the switching frequency.But due to that transformer size will increase.

 

[eem2am]

mosfet switching speed

hello vaibhav.k

thanks for replying, sorry if i misled about this.

I dont mean switching frequency i mean the actual time for the mosfet to come on....the rise time.

if rise time is very quick a bad EMC problem can arise. I was told to slow the rise time of mosfet with series gate resitor - but i wonder if an rc snubber is a better way to slow the rise time?

 

[FvM]

mosfet switching

To my opinion, the basic means to reduce EMC issues isn't reduction of switching speed but reduction of coupling. The loop formed by the switched current has to be minimized. RC snubbers are good for dampening of oscillations and overvoltages but less effective for switching speed moderation. If it's necessary to reduce rise and fall times, you better slow down the gate drive by series resistors. This always increases switching losses, unfortunately.

 

[eem2am]

smps with mosfet

hi,

i had this as an interview question at a company.

i said that mosfets should always be switched on with the highest possible rise time.

The interviewer seemed to revel in the fact that i had answered wrong, and told me that fast rise times give bad EMC problem,

but i was wondering if i could still have a fast rise time and somehow mitigate the emc problem somehow?

 

[FvM]

mosfet switching

For a given circuit layout, reduction of switching speed may be necessary means to achieve acceptable EMC properties. But I already clarified my view of EMC problems caused by fast switching.

 

[eem2am]

highest mosfet switching frequency

and so i can just have a high switching speed (tr and tf) and just put in a suitable common mode choke to filter out all the horrid common mode noise that my steep switching edges will create ?