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polarity protection

038

G. Kleine

In many cases, the battery or batteries in electronic equipment may

be inserted with incorrect polarity. It is, therefore, advisable to use

polarity protection such as shown in the diagrams. It should be

noted that although a Schottky diode may be used, this causes a

voltage drop of a few hundred millivolts, which in the case of a

3 V or 1.5 V battery supply is too much. The protection in the dia-

grams does not cause any reduction in the supply voltage.

The use of a MOSFET, p-channel or n-channel, as the case may

be, ensures that when the polarity is correct, the battery voltage is

applied to the load without any loss. For good efficiency, it is best

to use an n-channel MOSFET, although this has the disadvantage

of having to be inserted in the negative supply line. In cases where

this is impossible or impractical, a p-channel device must be used.

In the choice of MOSFET, it must be borne in mind that the

drain–source breakdown voltage, V(BR)DSS, must be larger than

64

Elektor Electronics

7-8/99

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P-MOSFET

ensure that, provided the polar-

ity is correct, the transistor can

transfer the battery voltage to

the load. Suitable types are cer-

tain HEXFETs from Interna-

tional Rectifier. In the case of

n-channel types, the IRF7401 in

an SO-8 case, the IRF7601 in a

Micro-8 case, and the IRLML

in a Micro-3 case are suitable.

Types suitable for p-channel

operation are the IRF7404 in

an SO-8 case, the IRF7604 in a

Micro-8 case, and the IRML6302 in a Micro-3 case.

Data sheets are available at: http://www.irf.com

D

S

R L

R L

G

U BAT

U BAT

G

D

S

N-MOSFET

994034 - 11

994034 - 12

the battery voltage to ensure that the transistor survives an incor-

rectly connected battery. At the same time, the gate threshold volt-

age, VGS(th) must be small compared with the battery voltage to

[994034]

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