AN-7017 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – Reducing Power Losses in MOSFETs

www.fairchildsemi.com
AN-7017
Reducing Power Losses in MOSFETs by Controlling
Gate Parameters
Alan Elbanhawy, Fairchild Semiconductor
Introduction
The gate Equivalent Series Resistance (ESR) has been
recognized as a factor affecting the losses in MOSFETs for a
number of years. Controlling the losses is exceedingly
important as devices need to produce the highest power
conversion efficiency and lowest losses to operate efficiently
at the target application. From an applications standpoint, an
increase in losses can lead to overheating in the actual
system requiring additional components such as heat sinks
and fans. Additional components add to the overall cost and
size of the end application. This application note explore
ways to control these losses in MOSFETs.
What this application note will do:
• Provide guidance in selecting the right MOSFET for a
specific application
• Optimize overall design by correct selection
• Explore the three distinct effects of gate ESR
The Effect of Rg on Switching
Current Distribution
One of the first effects this application note will be explore is
current distribution across the die during turn on and off. The
gate ESR (Rg) together with the gate-source capacitance
(Cgs) are distributed parameters across the MOSFET die.
V1
V2
V3
Id
R
R
R
Vd ⁺
–
C
C
V4
R
C
V5
R
C
C
Figure 1. Simplified schematic for the
solution of the current distribution
7
6
5
4
3
2
1
0
2.5e-09
3.5e-09 4.0e-09 4.5e-09 5.0e-09 5.5e-09 6.0e-09
Figure 2. Current in each of the five segments
6
5
4
3
2
1
0
5e-09
4e-09
1
2
3
4
5
6
3e-09
7 ^2e-09 _Cgs
Rg
Figure 3. The first and last segment currents for
different combinations of Rg and Cgs
REV. A 9/30/05