0195136047.pdf

354 SEMICONDUCTOR DEVICES

−VZ

VZ

Dr Df

RZ

RZ

i

v

1

0

i Break points

vv

i

(a) (c)

Ideal

diode

Ideal

diode

(b)

+

Figure 7.2.7Zener diode.(a)Device symbol.(b)Linearizedi–vcurve.(c)Circuit model.

EXAMPLE 7.2.6

Consider a simple zener voltage regulator with the circuit diagram shown in Figure E7.2.6(a).

iZ

VS vout

RS

iout

(a)

+

−

VS vout

VZ

RZ

RS

iout

(b)

+

− +

−

Figure E7.2.6

(a) For a small reverse resistanceRZ<< RSandVS−RSiout>VZ, show thatvout∼=VZ.

(b) For values ofVS=25 V,RS= 100 ,VZ=20 V, andRZ= 4 , find:

(i) voutforiout=0 andiout=50 mA.

(ii) The corresponding values of the reverse currentiZthrough the zener diode.

Solution

(a) WhenVS−RSiout>VZ, the zener diode will be in reverse breakdown. The forward

diodeDfin our model of Figure 7.2.7(c) will be off while the reverse diodeDris on. The

equivalent circuit is then given by Figure E7.2.6(b).

Straightforward circuit analysis yields