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AN804

P-Channel MOSFETs, the Best Choice for High-Side Switching

Vishay Semiconductors 

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

+15 V

–15 V

P-Channel

VOUT

N-Channel

IDD

–15 V

–VDD

FIGURE 5. Low-Voltage Complementary

MOSPOWER Array

Properly driving the MOSFET gates can minimize unwanted

crossover current at high supply voltages (both +VDD and –VDD)

(Figure 6).

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 – ȧVthȧ

where +VDD = –VDD

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.

TABLE 1. NĆ AND PĆCHANNEL DUAL

MOSFETS IN SOĆ8 PACKAGE

FOR OUTPUT STAGE

Part

Number

Si9939DY

Si9942DY

Si9928DY

V(BR)DSS (V) rDS Max (W)

30/–30

20/–20

0.05/0.1

0.125/0.2

20/–20

0.05/0.11

ID (A)

3.5/–3.5

3/–2.5

5/–3.4

+VDD

R1

P-Channel

R2

Z1

R4

N-Channel

TTL

R3

N-Channel

–VDD

FIGURE 6. High-Voltage Complementary Pair Driven by Logic-Compatible MOSFET

Document Number: 70611

10-Mar-97

www.vishay.com S FaxBack 408-970-5600

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