
AN-937 (v.Int)
3 and 5. Figure 3 shows the waveforms of the drain current, drain-to-source voltage and gate voltage during the turn-on interval.
For the sake of simplicity, the equivalent impedance of the drive circuit has been assumed as purely resistive.
At time, t
0
, the drive pulse starts to rise. At t
0
it reaches the threshold voltage of the HEXFET
s and the drain current starts to
increase. At this point, two things happen which make the gate-source voltage waveform deviate from its original “path”. First,
inductance in series with the source which is common to the gate circuit (“common source inductance”) develops an induced
voltage as a result of the increasing source current. This voltage counteracts the applied gate drive voltage, and slows down the
rate of rise of voltage appearing directly across the gate and source terminals; this in turn slows down the rate of rise of the
source current. This is a negative feedback effect: increasing current in the source produces a counteractive voltage at the gate,
which tends to resist the change of current.
The second factor that influences the gate-source voltage is the so called “Miller” effect. During the period t
1
to t
2
some voltage
is dropped across “unclamped” stray circuit inductance in series with the drain, and the drain-source voltage starts to fall. The
To Order
LOAD
"OPEN CIRCUIT"
DRIVE
PULSE
DRIVE CIRCUIT
RESISTANCE
G
INDUCTANCE
STRAY
SOURCE
INDUCTANCE
DRAIN-SOURCE
VOLTAGE
DRAIN-SOURCE
V
TH
"OPEN CIRCUIT" DRIVE PULSE
GATE-SOURCE
VOLTAGE
I
t
0
t
1
t
3
t
2
t
4
Figure 3.
Waveforms at Turn-On
DRIVE
+
-
I
D
+
-
I
S
THIS INDUCED VOLTAGE
SUBSTRACTS FROM THE
DRIVE VOLTAGE
RESULTING IN
RESULTING IN
THIS VOLTAGE RISING
MORE SLOWLY
RESULTING IN
SLOW RISE OF I
S
VOLTAGE DROP ACROSS
THIS L MEANS THAT THE
DRAIN VOLTAGE FALL
RESULTING IN
DISCHARGE OF
THIS CAPACITOR
RESULTING IN
MORE CURRENT
THROUGH THIS
RESISTANCE
Figure 4.
Diagrammatic Representation of Effects
When Switching-ON
DRAIN-SOURCE
VOLTAGE
G-S VOLTAGE
CURRENT
GATE VOLTAGE
GIVING I
V
TH
"OPEN CIRCUIT"
DRIVE PULSE
t
4
t
3
t
2
t
1
t
0
I
Figure 5.
Waveforms at Turn-OFF
Index