AN804 Datasheet, PDF (1/4 Pages) Vishay Siliconix – P-Channel MOSFETs, the Best Choice for High-Side Switching

AN804
Vishay Siliconix
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.
V_GG
0
N-Channel
Load
V_DD
V_GG – V_th
0
P-Channel
V_DD
Load
V_DD – I_Lr_DS
0
(1a)
(1b)
FIGURE 1. Comparing the Performance Between N-Channel and P-Channel Grounded-Load Switching
Document Number: 70611
10-Mar-97
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