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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, enhancementmode 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 VDD
VDD VGG 0 0 Load Load VGG - Vth 0 VDD - ILrDS
(1a)
(1b)
FIGURE 1. Comparing the Performance Between N-Channel and P-Channel Grounded-Load Switching
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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 VGG - Vth. The equations describing performance of the n-channel ground-switching MOSFET with a ground-reference gate drive are based on the relationship between VDD and VGG: If VDD VGG, then VO = VGG - Vth ; If VDD < VGG - Vth . then VO = VDD - ILrDS(on). Sustaining a more acceptable gain with an output in direct relation to VDD 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 VDD (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
VDD 15 V
VDD
P-Channel VDD - ILrDS 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 10-Mar-97
AN804
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.
+VDD +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 VZENER = 2 VDD - Vth where +VDD = -VDD
P-Channel +15 V VOUT -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.
IDD -15 V -VDD
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
rDS Max (W)
0.05/0.1 0.125/0.2 0.05/0.11
ID (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 +VDD and -VDD) (Figure 6).
Si9942DY Si9928DY
+VDD R1
P-Channel
R2 N-Channel TTL
Z1
R4
N-Channel
R3 -VDD
FIGURE 6. High-Voltage Complementary Pair Driven by Logic-Compatible MOSFET
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AN804
Vishay Siliconix
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
rDS 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
rDS 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 10-Mar-97

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