RF noise from switching

From time to time we seem to be overwhelmed with designs presented to us that all suffer from a common element.

People love to create converters based on app notes - and for a while the 4 switch buck-boost was a classic example - where the designer thinks that high speed gate drive must be good gate drive because it increases efficiency and lowers required heat-sinking.

The hard unfortunate truth is - aggressive gate drive generates RF noise that tends to get into many things and upset control - it can also cause voltage over stress on diodes and fets in the power circuit. And you can forget about passing EMC.

For any circuit there is a sweet spot of gate drive ( turn on R and turn off R ) that gives the designer the best compromise between noise generation and mosfet dissipation.

This often sits at the higher end of dissipation that a budding engineer has not really allowed for.

The root cause of this trend seems to be that app notes very rarely mention the effect of overly aggressive gate drive on noise generation, which often impacts directly on nearby associated control circuits ( esp current sensing ) - and of course impacts the ability to pass EMC standards - and can be a fet killer too.

So - to summarise:

• think about keeping layout tight in any power stage, de-coupling !
• think about adding snubbers to switch nodes - easier to do now than later !
• think about worst case dissipation ( fets, diodes, snubber R's ) - design for this from the start !
• allow for modifying gate drive in your design and layout the pcb to make this easy !
• don't assume application notes will get you to a working design !
• If you have access to an experienced power electronics engineer - use the advice !

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