Unformatted text preview: he output N
Channel
and
PChannel
MOSFETs,
practically
eliminating cross conduction.
As
described
under
section
1.2,
a
MOSFET/IGBT driver incurs losses. Let us
derive formulae to compute this power loss in a
driver:
PD(on) = D x ROH x Vcc x Qg x fsw Eq. 5.2
ROH + RGext + RGint
PD(off) =(1D)xROLxVccxQgxfsw
Eq.5.3
ROL + RGext+ RGint
where:
ROH = Output resistance of driver @ output
High
ROL = Output resistance of driver @ output
Low
fsw = Switching frequency
RGext = resistance kept externally in series
with
Gate of MOSFET/IGBT
RGint = Internal mesh resistance of MOSFET/
IGBT
D = Duty Cycle (value between 0.0 to 1.0)
Qg= Gate Charge of MOSFET/IGBT
Total loss PD = PD(on) + PD(off)
Note also that in general, RGint is small and can
be neglected and that ROH = ROL for all IXD_
drivers. Consequently, if the external turnon
and turnoff gate resistors are identical, the total
driver power dissipation formula simplifies to:
PD=PD(on)+P D(off)= ROHxV cc xQgxfsw Eq.5.4
ROL + RGext
Let us review with some examples:
Assume that we are driving an IXFN200N07 for
a Telecom power supply application or for a
UPS/Inverter application at a switching
frequency of 20 kHz. Furthermore, RGext = 4.7
Ohms and gate supply voltage is 15V.
On page two of the IXD_409 Data sheet, we
read the value of ROH = 1.5 Ohms ( Maximum).
For Qg, refer to Data Sheet of the IXFN200N07
and go to Gate Charge vs. VGS curve and look
for value of Qg at Vcc = 15 V. You can read it as 640 nC. Substituting these values into Eq. 5.4 yields:
PD = 1.5 x 15 x 640 x 20,000 x 109
1.5 + 4.7
PD = 46.45 mW
Assuming an ambient of 50 o C in the vicinity of
IXD_409PI, the power dissipation capability of
IXD_409PI must be derated by 7.6mW/oC, which
works out to be 190 mW. The maximum allowable
power dissipation at this temperature becomes 975190=785 mW. However, as calculated above, we will
be dissipating only 46.45 mW so we are well within
the dissipation limit of 785 mW.
If one increases fsw to 500 kHz for a DCtoDC
Converter appl...
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 Winter '08
 Staff
 power supply, driver, gate drive, driver IC, MOSFET/IGBT

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