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 timeintervals,
during which finite VDS and finite ID coexist, albeit
momentarily. When this happens during turnon, the
actual integration: ∫ VDS(t) ID(t) dt
Eq. 1.2
Is defined as TurnOn switching energy loss.
Likewise, during turnoff, when finite values of ID a nd
VDS coexist, integration of: ∫ VDS(t) ID(t) dt Eq.1.3 IXAN0009 Is called Turnoff switching energy loss in a
MOSFET. Amongst the responsible parameters
determining these switching energy losses, Ciss,
Coss and Crss affect the turnon and turnoff
delays as well as turnon and turnoff 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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 Winter '08
 Staff
 power supply, driver, gate drive, driver IC, MOSFET/IGBT

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