SWITCHMODEâ¢ Power Supply Reference Manual

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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
SMPSRMThe time needed to switch the MOSFET between onand off states is dependent upon the impedance of thegate drive circuit. It is very important that the drive circuitbe bypassed with a capacitor that will keep the drivevoltage constant over the drive period. A 0.1 F capacitoris more than sufficient.Driving MOSFETs in SwitchingPower Supply ApplicationsThere are three things that are very important in thehigh frequency driving of MOSFETs: there must be atotem–pole driver; the drive voltage source must be wellbypassed; and the drive devices must be able to sourcehigh levels of current in very short periods of time (lowcompliance). The optimal drive circuit is shown inFigure 19.V GLOADV GLOADR onR offa. Passive Turn–ONb. Passive Turn–OFFV GLOADV GLOADc. Bipolar Totem–poled. MOS Totem–poleFigure 19. Bipolar and FET–Based Drive Circuits (a. Bipolar Drivers, b. MOSFET Drivers)http://onsemi.com18
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Page 3 and 4: SMPSRMON Semiconductor and are regi
Page 6 and 7: SMPSRMIntroductionThe never-ending
Page 8 and 9: SMPSRMThe Flyback-Mode ConverterThe
Page 10 and 11: SMPSRMFactors 3, 4 and 5 have a dir
Page 12 and 13: SMPSRM+N1 N2C inV inControlTSWDL OC
Page 14 and 15: SMPSRMInterleaved Multiphase Conver
Page 16 and 17: SMPSRMV CCOSCChargeClock RampV erro
Page 22 and 23: SMPSRMThe Magnetic ComponentsThe ma
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Page 28 and 29: SMPSRMSnubbers and ClampsSnubbers a
Page 30 and 31: SMPSRMThe Active ClampAn active cla
Page 32 and 33: SMPSRMIn this design, both the powe
Page 34 and 35: SMPSRMA Power Factor Correction (PF
Page 36 and 37: SWITCHMODE Power Supply ExamplesSMP
Page 38 and 39: V_SyncH_SyncMonitorMCUSYNC PROCESSO
Page 40 and 41: SMPSRMACLineRectifier+Start-upSwitc
Page 42 and 43: http://onsemi.com41Part No. VRRM(V)
Page 44 and 45: SMPSRMApplication: Compact Power Fa
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Page 48 and 49: SMPSRMFeatures:70 W output power fr
Page 50 and 51: SMPSRMFeatures:Off power consumptio
Page 52 and 53: SMPSRMApplication: 110 W Output Fly
Page 54 and 55: SMPSRMApplication: Efficient Safety
Page 56 and 57: SMPSRMApplication: AC-DC Battery Ch
Page 58 and 59: SMPSRMLiterature Available from ON
Page 60 and 61: SMPSRMWeb Locations for Switching-M
Page 62 and 63: Analog ICs for SWITCHMODE Power Sup
Page 64 and 65: SMPSRMON SEMICONDUCTOR STANDARD DOC

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SWITCHMODEâ¢ Power Supply Reference Manual