Fairchild SEMICONDUCTOR AN-7502 - Manual

©2002 Fairchild Semiconductor Corporation Application Note 7502 Rev. A1
AN-7502
Power MOSFET Switching Waveforms:
A New Insight
The examination of power MOSFET voltage and current
waveforms during switching transitions reveals that the
device characterization now practiced by industry is inade-
quate. In this Note, device waveforms are explained by con-
sidering the interaction of a vertical JFET driven in cascode
from a lateral MOSFET in combination with the interelec-
trode capacitances. Particular attention is given to the
drain-voltage waveform and its dual-slope nature. The
three terminal capacitances now published by the industry
are shown to be valid only for zero drain current. For cases
where the gate drive is a voltage step generator with inter-
nal fixed resistance, the drain voltage characteristics are
inferred from the gate current drive behavior and compared
to observed waveforms. The nature of the “asymmetric
switching times” is explained.
A waveform family is proposed as a more descriptive and
accurate method of characterization. This new format is a
plot of drain voltage and gate voltage versus normalized
time. A family of curves is presented for a constant load
resistance with V
DS
varied. Gate drive during switching
transitions is a constant current with voltage compliance
limits of 0 and 10 volts. Time is normalized by the value of
gate driving current. The normalization shows excellent
agreement with data over five orders of magnitude, and is
bounded on one extreme by gate propagation effects and
on the other by transition time self-heating (typically tens of
nanoseconds to hundreds of microseconds).
Device Models
The keystone of an understanding of power MOSFET
switching performance is the realization that the active
device is bimodal and must be described using a model that
accounts for the dual nature. Buried in today’s power MOS-
FET devices is the equivalent of a depletion layer JFET that
contributes significantly to switching speed. Figure 1 is a
cross-sectional view of a typical power MOSFET, with MOS-
FET/JFET symbols superimposed on the structure.
Figure 2 is obtained by taking the lateral MOS and vertical
JFET from this conception and adding all the possible node-
to-node capacitances. Computed values of the six capaci-
tances for a typical device structure suggest that device
behavior may be adequately modeled using only three
capacitors in the manner of Figure 3. This is the model to be
employed for analysis and study.
FIGURE 1. CROSS-SECTION VIEW OF MOSFET SHOWING
EQUIVALENT MOS TRANSISTOR AND JFET
FIGURE 2. MOS TRANSISTOR WITH CASCODE-CONNECTED
JFET AND ALL CAPACITORS
FIGURE 3. FIGURE 2 SIMPLIFIED
Gate Drive: Constant Voltage or
Constant Current
Before moving on to the study of the equivalent circuit states
of the model, a gate-drive forcing function which is easy to
represent, relates to reality, and best illustrates device
behavior must be chosen. The choice may be immediately
narrowed to two:
(1) An instantaneous step voltage with internal resistance R,
Figure 5.
(2) An instantaneous step current with infinite internal resis-
tance, Figure 6.
SOURCE METAL
POLY GATE GLASS GATE OXIDE
0
10 VOLTS
DEPLETION EDGE
40 VOLTS
n+ DRAIN
JFET
n-
MOS
n+ SOURCE
p BODY
p+
GATE
C1
C2
C6
C3
C5
C4
SOURCE
DRAIN
GATE
C
GS
C
DS
SOURCE
DRAIN
C
x
Application Note October 1999
/Title
AN75
2)
Sub-
ect
Power
OS-
ET
witch
ng
ave-
orms:
New
nsi
ht)
Autho
()
Key-
ords
Inter-
il
orpo-
ation,
emi-
on-
uctor)
Cre-
tor ()
DOCI
FO
df-
ark
Page-
ode
Use-
ut-
ines