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Unformatted text preview: ication, keeping other parameters the
same as above, now the dissipation would be 1.16
W, which exceeds the specification for IXD_409PI.
So in this case, it is recommended to use either the
IXD_409YI (TO 263 package) or IXD_409CI (TO220) package. Both these packages can dissipate
about 17 W with proper heat sinking arrangement.
The TO263 is a surface mount package and can be
soldered onto a large pad on a copper surface of a
PCB for achieving good heat transfer. For TO 220
package, a heat sink can be used.
Let us take another example of a boost converter,
using IXFK55N50 at VDS = 250 VDC and at I =
D 27.5 Amps. Assuming sw = 500 kHz, Vcc = 12 V.
f
From the curve of Gate Charge for IXFK55N50 in
the Data Sheet one can determine that Qg = 370 nC.
Let us set R
Gext = 1.0 Ohm. We use IXD_414YI or
IXD_414CI here, which can dissipate 12W. Here
typical value of ROH = ROL = 0.6 Ohm. Substituting
the above values in our equation, we compute the
power dissipation to be:
PD = 0.6 x 12 x 370 x 500 kHz x 109
0.6 + 1.0 + 0.0
PD = 0.83 W.
With adequate air circulation, one may just be able
to use the PDIP Package. However, it is
recommended to use TO 263 or TO 220 package
for reliable performance.
For the third example, considering driving a large
size MOSFET module VMO 58002F at fsw = 250
kHz. Let Vcc = 10 V, ROH = ROL = 0.6 Ohm, RGext=
0.0 Ohm. We read that Qg =2750 nC at Vcc = 10 V
off the VMO 58002F data sheet. Now:
PD = 0.6 x 10 x 2750 x 250kHz x 10 9
0.6+ 0.0 + 0.0
PD = 6.86 W IXAN0009 IXD_414YI (TO263) or IXD_414CI (TO220)
can easily drive this load provided adequate
heat sinking and proper airflow is maintained.
Comments above for mounting TO263 and/or
TO220 packages apply here as well. For
derating use 0.1 W/o C. So for an ambient
temperature of 50 o C, it works out to be 2.5 W.
As the limit of IXD_414YI or IXD_414CI is 12 W,
subtracting 2.5 W from this yields 9.5 W. So
6.86 W is still within limit. Thermal Impedance
(Junction to Case) is 0.55o C/W for TO 263 and
TO220, hence a rise in case temperature
should be within limit. If we increase Vcc to 15V,
conduction losses in MOSFET could reduce due
to lower RDS(on), but obtaining the same rise...
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 Winter '08
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