SWITCHMODE™ Power Supply Reference Manual

SMPSRMThe Power MOSFETPower MOSFETs are the popular choices used aspower switches and synchronous rectifiers. They are, onthe surface, simpler to use than BJTs, but they have somehidden complexities.A simplified model for a MOSFET can be seen inFigure 17. The capacitances seen in the model arespecified within the MOSFET data sheets, but can benonlinear and vary with their applied voltages.C DGC GSC ossFigure 17. The MOSFET ModelFrom the gate terminal, there are two capacitances thedesigner encounters, the gate input capacitance (C iss ) andthe drain–gate reverse capacitance (C rss ). The gate inputcapacitance is a fixed value caused by the capacitanceformed between the gate metalization and the substrate.Its value usually falls in the range of 800–3200 pF,depending upon the physical construction of theMOSFET. The C rss is the capacitance between the drainand the gate, and has values in the range of 60–150 pF.Although the C rss is smaller, it has a much morepronounced effect upon the gate drive. It couples thedrain voltage to the gate, thus dumping its stored chargeinto the gate input capacitance. The typical gate drivewaveforms can be seen in Figure 18. Time period t1 isonly the C iss being charged or discharged by theimpedance of the external gate drive circuit. Period t2shows the effect of the changing drain voltage beingcoupled into the gate through C rss . One can readilyobserve the “flattening” of the gate drive voltage duringthis period, both during the turn–on and turn–off of theMOSFET. Time period t3 is the amount of overdrivevoltage provided by the drive circuit but not reallyneeded by the MOSFET.TURN–ONt3V DRTURN–OFFt1 t2t2 t1t3V GS0V thV plV DSI G0+0–Figure 18. Typical MOSFET Drive Waveforms (Top: V GS , Middle: V DG , Bottom: I G )http://onsemi.com17