June 2006
Intermediate to advanced
528 pages
17h 12m
English
Content preview from Switching Power Supplies A - Z
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Chapter 5
10 20 30 40 50 60 70
1000
800
600
400
200
0
Estimated
Switching
Loss
(Watts)
Crss
(pF)
Qg
(nC)
pF
500
3.5
3
2.5
2
1.5
1
2.6
35nC
Si4442DY (Crss varied)
Vt=1.05V
fsw=500kHz
Vin=15V
Io=22A
Vdrive=4.5V
Rdrive
ON
=4 ohms
Rdrive
OFF
=2 ohms
Si4442DY
Watts
Figure 5-19: Varying the Crss of the Si4442DY
In other words, Qg will certainly affect driver dissipation, but it is not necessarily a good
indicator of the switching losses — it is more helpful to try and minimize Qgd (or Crss) when
selecting mosfets, rather than just looking for a “low-Qg” mosfet.
Note: In the worked example, we had estimated the losses to be 1.5 W. There we had a pull-up of 2 ohms,
and a pull-down of 1 ohm.
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