Electronics > Power/Renewable Energy/EV's

Reducing radiated emissions in non synch buck

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Faringdon:
Hi,
Buck 12v to 5v at 1A...is it a really significant reduction in radiated emissions that you get by increasing the series fet gate resistor?

Why doesnt the LTC1624 datasheet show such a series resistor being used?
https://www.analog.com/media/en/technical-documentation/data-sheets/1624f.pdf

JoeyG:
-Get the evaluation board or the evaluation board data that may show the emissions.
-Compare your PCB layout to the evaluation board and account for any  differences.

Gyro:
Of course, everything gets quieter if you slow it down Treez.

Adding gate resistance slows down the switching speed of the mosfet. This of course makes it quieter... it also increases the thermal dissipation of the mosfet and reduces the overall efficiency of the circuit.

As JoeyG says, get the eval board. If your design (without the gate resistor) is significantly noisier then your layout is wrong / poor. LT obviously didn't find the need to slug the mosfet speed in their eval.

T3sl4co1l:
I have a sneaking suspicion LT leaves out the gate resistor for simplicity, assuming the user knows they mean to put one in, of appropriate value.  But then, I've tested several eval boards that shat out copious spikes in the ~100MHz range (i.e., some ~ns pulses every switching edge)...

Tim

Mark:
I'm using the LT3752 + LT8311 (active clamp forward).  The datasheet shows no resistors for the catch and forward mosfet gates, whereas the demo board DC1929AFA shows an optional resistor for the catch gate only. 
After observing a lot of ringing on the swtich node, I worked on a snubber based on TI's app note "Controlling switch-node ringing in synchronous buck converters" (SLYT465). 
In a meeting with LT's engineer, they said the RC snubber was the correct approach and they did not recommend using the catch gate resistor as it would hurt efficiency too much. 

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