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P-Channel MOSFETs, the Best Choice for High-Side Switching
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.
CIRCUIT APPLICATIONS
Switching Ground-Return Loads
The principal application of the p-channel, enhancement-
mode MOSPOWER FET is in switching power (or voltage) to
grounded (ground return) loads.
To 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. To 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.
N-Channel
P-Channel
V
DD
V
DD
V
GG
0
0
Load
Load
V
GG
– V
th
0
V
DD
– I
L
r
DS
(1a)
(1b)
FIGURE 1.
Comparing the Performance Between N-Channel and P-Channel Grounded-Load Switching
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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
– V
th
.
The equations describing performance of the n-channel
ground-switching MOSFET with a ground-reference gate drive
are based on the relationship between V
DD
and V
GG
:
If V
DD
≥
V
GG
, then V
O
= V
GG
– V
th
;
If V
DD
< V
GG
– V
th
. then V
O
= V
DD
– I
L
r
DS(on)
.
Sustaining a more acceptable gain with an output in direct
relation to V
DD
would require an isolated gate drive referenced
to the source (Figure 4). The dc bias option rectifies the pulse
of ac from the transformer and stores a “+” voltage on the
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
DD
(Figure 1b). To return gate control to a more acceptable
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.
A
R
B
C
Load
dc Bias
Options
FIGURE 4.
Floating Gate Drive
V
DD
15 V
V
DD
P-Channel
V
DD
– I
L
r
DS
N-Channel
N-Channel
TTL
N-Channel
Load
TTL
FIGURE 2.
Bootstrapping for N-Channel
Grounded-Load Switching
FIGURE 3.
Using An N-Channel Level-Shifter
Simplifies Driving From Logic
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Document Number: 70611
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Vishay Siliconix
Half-Bridge (Totem Pole)
A high-side p-channel MOSFET and a low-side n-channel
MOSFET tied with common drains (Figure 5) make a superb
high-current “CMOS equivalent” switch. One fault common to
such circuits has been the excessive crossover current during
switching that may occur if the gate drive allows both
MOSFETs to be on simultaneously.
+V
DD
+15 V
A resistively-coupled lower-power complementary pair
offers extremely low crossover current when the output stage
uses high-power MOSFETs. The Zener, Z1, and resistors, R1
and R3, act as a level shifter, properly driving the low-power
MOSFETs. The Zener may be selected according to the
equation
V
ZENER
= 2 V
DD
– V
th
where +V
DD
= –V
DD
P-Channel
+15 V
V
OUT
–15 V
N-Channel
Whatever crossover current that might occur in the low-power
drivers is dramatically reduced by the series resistor, R4.
Additionally, driving the high-power complementary pair using
this resistor divider scheme all but eliminates crossover
current in this critical output driver. This increases both the
driver’s efficiency and its reliability.
I
DD
–15 V
–V
DD
TABLE 1. N AND P CHANNEL DUAL
MOSFETS IN SO 8 PACKAGE
FOR OUTPUT STAGE
Part
Number
Si9939DY
FIGURE 5.
Low-Voltage Complementary
MOSPOWER Array
V
(BR)DSS
(V)
30/–30
20/–20
20/–20
r
DS
Max (W)
0.05/0.1
0.125/0.2
0.05/0.11
I
D
(A)
3.5/–3.5
3/–2.5
5/–3.4
Properly driving the MOSFET gates can minimize unwanted
crossover current at high supply voltages (both +V
DD
and –V
DD
)
(Figure 6).
Si9942DY
Si9928DY
+V
DD
R1
P-Channel
R2
N-Channel
TTL
Z1
R4
N-Channel
R3
–V
DD
FIGURE 6.
High-Voltage Complementary Pair Driven by Logic-Compatible MOSFET
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TABLE 2. POPULAR DISCRETE N AND P CHANNEL MOSFETS FOR INPUT DRIVER STAGES
AND OUTPUTS
P-Channel Devices
Part Number V
(BR)DSS
Min (V)
VP0300L
TP0610L
VP2020L
TP0101T*
TP0610T*
*Surface Mount Package
–30
–60
–200
–12
–60
N-Channel Devices
Package
TO-226AA
TO-226AA
TO-226AA
TO-236
TO-236
r
DS
Max (W)
2.5
10
20
0.65
10
Part Number V
(BR)DSS
Min (V)
VN0300L
2N7000
VN2010L
TN0200T*
2N7002*
30
60
200
20
60
r
DS
Max (W)
1.2
5
10
0.4
7.5
Package
TO-226AA
TO-226AA
TO-226AA
TO-236
TO-236
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Document Number: 70611
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