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Advances and breakthroughs continue to improve

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Unformatted text preview: FET. Advances and breakthroughs continue to improve performances of both devices. Lower parasitic capacitances, lower RDS (on), RGint , Qg, Rthjc and faster switching times are being achieved in newer MOSFETs. Third generation IGBTs have lower forward voltage drop, lower gate Charge, lower trr of inverse parallel diode, shorter and lesser tail current, lower tf , smaller Eon a nd Eoff, improved current sharing amongst parallel devices and better Rthjc . 1.2 POWER LOSSES IN DRIVERS AND DRIVEN MOSFET/IGBT For determining the power loss in a Driver while driving a power MOSFET, the best way is to refer to the Gate Charge Qg vs. V GS curve for different values of V DS(off). PGATE=VCC*Qg*fsw Eq. 1.1 Wherein, Vcc is the Driver’s supply voltage, Qg is the total Gate Charge of the MOSFET being driven and fsw is the switching frequency. It is prudent then to choose a MOSFET with lower value of Qg and it is here that of low Gate Charge MOSFETs are preferred because they as well as the drivers incur lower losses. As far as switching losses in a MOSFET are concerned, there are some short time-intervals, during which finite VDS and finite ID coexist, albeit momentarily. When this happens during turn-on, the actual integration: ∫ VDS(t) ID(t) dt Eq. 1.2 Is defined as Turn-On switching energy loss. Likewise, during turn-off, when finite values of ID a nd VDS coexist, integration of: ∫ VDS(t) ID(t) dt Eq.1.3 IXAN0009 Is called Turn-off switching energy loss in a MOSFET. Amongst the responsible parameters determining these switching energy losses, Ciss, Coss and Crss affect the turn-on and turn-off delays as well as turn-on and turn-off times. For an IGBT, it would be similarly shown that: ∫ VCE(t)IC(t)dt Eq. 1.4 This equation represents switching energy loss. Needless to emphasize that the time interval for these integrals would be the appropriate time during which finite values of ID a nd VDS or VCE and IC coexist in a MOSFET or IGBT respectively. Thus average switching energy lost in the device can be computed as follows: MOSFET: Ps = 1/2*VDS *ID *fsw*(t on+t off ) Eq...
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