VISHAY AN804 Technical data
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P-Channel MOSFETs, the Best Choice for High-Side Switching
AN804
Historically, p-channel FETs were not considered as useful as
their n-channel counterparts. The higher resistivity of p-type
silicon, resulting from its lower carrier mobility, put it at a
disadvantage compared to n-type silicon.
Getting n-type performance out of p-type FETs has meant
larger area geometries with correspondingly higher
inter-electrode capacitances. Consequently, a truly
complementary pair—a p-channel and an n-channel device
that match in
all
parameters—is impossible.
Yet, despite its shortcomings, the p-channel MOSFET
performs a vital “high-side” switch task that the n-channel
simply cannot equal.
Used as a high-side switch, a p-channel MOSFET in a
totem-pole arrangement with an n-channel MOSFET will
simulate a high-current, high-power CMOS (complementary
MOS) arrangement. Although the p-channel MOSFET cannot
complement the n-channel in both on-resistance and
capacitance simultaneously, such combinations as the
low-threshold p-channel TP0610 and the n-channel 2N7000
together offer outstanding performance as a complementary
pair.
Switching Ground-Return Loads
The principal application of the p-channel, enhancementmode MOSPOWER FET is in switching power (or voltage) to
grounded (ground return) loads.
T o drive the FET properly , the gate voltage must be referenced
to its source. For enhancement-mode MOSFETs, this gate
potential is of the same polarity as the MOSFET’s drain
voltage. T o turn on, the n-channel MOSFET requires a positive
gate-source voltage, whereas the p-channel MOSFET
requires a negative gate-source potential.
During switching, a MOSFET’s source voltage must remain
fixed, as any variation will modulate the gate and thus
adversely affect performance. Figure 1 shows this
degradation by comparing n-channel and p-channel MOSFET
high-side switching.
V
GG
0
FIGURE 1. Comparing the Performance Between N-Channel and P-Channel Grounded-Load Switching
Document Number: 70611
10-Mar-97
P-ChannelN-Channel
V
DD
VGG – V
0
Load
(1a) (1b)
th
V
DD
VDD – ILr
DS
0
Load
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AN804
Vishay Siliconix
If an n-channel, enhancement-mode MOSFET were switching
a positive-polarity voltage to a grounded load, the output would
be limited to V
GG
– Vth.
The equations describing performance of the n-channel
ground-switching MOSFET with a ground-reference gate drive
are based on the relationship between V
≥ VGG, then VO = VGG – V
If V
DD
If V
< VGG – V
DD
th
. then VO = VDD – ILr
;
th
and VGG:
DD
DS(on)
.
Sustaining a more acceptable gain with an output in direct
relation to V
would require an isolated gate drive referenced
DD
to the source (Figure 4). The dc bias option rectifies the pulse
of ac from the transformer and stores a “+” voltage on the
A
gate-to-source capacitance of the MOSFET. The RC
determines the turn off time.
Bootstrapping the n-channel MOSFET (Figure 2) is
satisfactory for short turn-on times of a few milliseconds. In this
arrangement, both MOSFETs must have breakdown voltage
specifications that match or exceed the supply voltage.
Using a p-channel MOSFET may place some severe
restraints on the gate drive, since the gate must be close to
V
(Figure 1b). To return gate control to a more acceptable
DD
logic format, add an n-channel MOSFET (Figure 3).
Using an n-channel MOSFET in this way simplifies the gate
drive for a high-voltage, high-side, p-channel MOSFET.
Placing a Zener diode between the gate and supply ensures
that V
(BR)GSS
will not be exceeded. Again, both MOSFETs
must withstand the full rail voltage.
TTL
N-Channel
RC
B
FIGURE 4. Floating Gate Drive
V
DD
N-Channel
dc Bias
Options
TTL
Load
15 V
N-Channel
V
DD
P-Channel
Load
VDD – ILr
DS
FIGURE 2. Bootstrapping for N-Channel
Grounded-Load Switching
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FIGURE 3. Using An N-Channel Level-Shifter
Simplifies Driving From Logic
Document Number: 70611
10-Mar-97
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