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TrenchFET Gen IV MOSFETs PowerPAIR Slide 4

The Qg charging curve further demonstrates the improvement from Gen III and Gen IV. Both SiRA04DP and SiR818DP have similar Rds(on), but Qg and Qgd have very different values. In the graph shown on this slide, the blue curve represents the gate charge of SiRA04DP, which is a Gen IV device. The plateau in the middle of curve is Qgd. Less Qgd allows the device to switch on and off faster, therefore switching loss potentially is less. Less Qgd also implies lower Cgd. When the low-side driver pulls down the gate voltage, the high dV/dt on the switch node tends to couple the charge through Cgd and drives up the gate voltage, to cause shoot-through. Lower Qgd and having a Qgd/Qgs ratio below one prevents the chance of shoot-through. A sample formula is: Cgd/Cgs < Vth/(Vds-Vth); Qgd/Qgs = Cgd*(Vds-Vth)/(Cgs*Vth) < 1; therefore, Qgd/Qgs < 1.

PTM Published on: 2014-01-30