Effects of MOSFET Gate Driving on Conducted Emissions in a Flyback LED Driver

One of the key components in switching power supplies is the main switch, of which MOSFET is commonly used. High dv/dt and di/dt are inherent in MOSFET switching operation, and so it is a major source of electromagnetic emissions that may fail the standard limits. In this paper, effects of MOSFET gate driving on conducted emissions are examined. Measurements are made on a prototype of flyback LED driver. It is found that increasing gate resistance could suppress the drain turn-off oscillation voltage. By experiment, an external gate resistance necessary to reduce emission below limit lines is obtained. Anti-parallel diode speeds turn-off up but increases EMI level. It was confirmed that the noise upper 20MHz was originating from the drain voltage spike. Instead of anti-parallel, the diode can be also connected in parallel to the gate resistance. In this way, it can facilitate faster MOSFET turn-on transients, and therefore power efficiency had shown an improvement but without increasing EMI level.