161 sey Diode 919° Zener Diode sical ee eatY been discussed that when the reverse bias on a crystal diode is increased, a critical voltage, called breakdown voltage is reached where the reverse current increases sharply {0.2 high value. The breakdown region is the knee of the reverse characteristic as shown in Fig 9.39. The satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zener. Therefore, the breakdown voltage is sometimes called, Zener voliage and the sudden increase in current is known as Zener current. le eens KNEE: Fig. 9.39 The breakdown or Zener voltage depends upon the amount of doping. If the diode is ieavily doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage. On Te ot ind, a lightly doped diode has a higher breakdown voltage. When an ordinary crystal diode is properly doped so that it has a sharp breakdown voltage, it is called a zener diode. stal diode which has a sharp breakdown voltage is known as a Zener A properly doped c diode. Fig. 9.40 shows the symbol of a zener diode. It may be seen that it is just like an ordinary diode except that the bar is turned into Z-shape. The following points may be noted aboyt the zener diode (i) A zener diode is like an ordinary digde except that it is Properly doped so as to have a sharp breakdows voltage. (ii) A zener diode is always reverse connected i.e. it is always reverse biased. (iii) A zener diode has sharp breakdown voltage, called zener voltage V,, (iv) When forward biased, its characteristics are just those of ordinary diode. (v) The zener diode is not immediately burnt just because it has entered the *breakdown region, As long as the external circuit connected to the diode limits the diode current to less than ig. 9.40, burn out value, the diode will not burn out. 9.20 Equivalent Circuit of Zener Diode The analysis of circuits using zener diodes can be made quite easily by replacing the zener diode by its equivalent circuit The currents limited only by both external resistance and the power dissipation of =ener diode. FE esi 162 Principles of Electronics ST 70n" state, When reverse voltae across a zener diode is equalito”oramre TA breakdown voltage V,, the current increases very sharply. In this region, the curve 5 Tote vertical. It means that voltage across zener diode is constant at V; even though the CURE eS it changes. Therefore in the breakdown region, an tideal zener diode can be repressi ae battery of voltage V, as shown in Fig. 9.41 (i). Under such conditions, the Zener diode to be in the “on” state. . co + | * _ | , | ‘ & (i 23 Veena Equivalent circuit of zener for “on” state Fig. 9.41 ii) "OFF" state. When the reverse voltage across the zener diode is less than V,_ but greater than OV, the zener diode is in the "OFF" state. Under such conditions, the zener diode can be represented by an open-circuit as shown in Fig. 9.42 (ii). ses] fee J (i) Equivalent circuit of zener for “off” state Fig. 9.42 9.21 Zener Diode as Voltage Stabiliser A zener diode can be used as a voltage regulator to provide a constant voltage from a source whose voltage may vary over sufficient range. The circuit arrangement is shown in Fig. 9.43 (i). The zener diode of zener voltage Vz is reverse connected across the load R;, across which constant output is desired. The series resistance R absorbs the Output voltage fluctuations 0 as to maintain constant voltage across the load. It may be noted that the zener will maintain ‘a constant voltage Vz (= Eq) across the load so long as the input voltage does not fall below Vz. “) , Fig. 9.43, ey This assumption is fairly reasonable as the impedance of zener diode is quite small in the br region. Vivoldiallcamers shot of WIS F } =Semiconductor Diode 163 When the (equal t0 Vz) ever range. circuit is Properly designed, the to: n though ad voltage E, the input volt Femains essentially constant ‘age E, and load resistanc ¢ Ry, may vary over a wide (2 Suppose the input voltage increas zener diode is equivalent to remains constant at Vz (= £. es. Since the zener ji battery Vz as shown in F The excess vol s in the breakdown reg 8. 9.43 (ii). It is cle ion, the that output volt 's dropped across the series resistance R. This Will cause an increase in the value of total current J. The zener will conduct the incr ase of current in J while the lo: ad current remains constant, Hence. Output voltage E,, remains constant irrespective nf the changes in the input voltage £ (4) Now suppose that input volta ‘e constant but the load resistance R,, decreases. This will cause an increase in load curren The extra current can not come from the urce because drop in R (and hence source current will not change as the zener is within its regulatiy range The additional load current will come 1 nel Secteeel ened” curren joe Basequicuti vain utput voltage stays at constant value Voltage dr IR = E,-§ Current through & iy Applying ohm’s law, we ha 9.22 Solving Zener Diode Circuits The analys; zene H PI nalysis semiconductor diodes. The fi determine the state of zener diode whethe zener is in the "on" state ff" state. Next, th s re b ate mode Finally, the unknown quantities are determin n Ej and R,, fixed Sis the simplest case and is shown in Fig. 9.44 (i). He » the qpplied voltage (E;) as well as load R, 1s fixed. ‘The first step is to find tne state of zener diode. This can be determined by removing the zener from the circuit ant alculating the voltage V across the resulting open-circuit as shown in Fig, 9.4 RE RR, dual camera I'V2 V., the zener diode is in the in Fig. 9.45 (i). If V< Vgy Y83 Pro Sco) eh vivoZener Diode as Voltage Regulator 22S ‘Tedenr sci ues niosrn wands Waban end deere aa rer al deck wena tage ‘arsed be estos stl scree rota Rte yotige cas roa wl ote bons tp ae The ‘iditecreace go Ser ye maemo po ee caret iad es S03 Tt Season pot elteder enka ‘eee Bote mucin Yn ha’ Fram htc atm a ay tat re Seas regan RR ere rcs of so 3 wt te ele ees of tw cree fonny reg Syst wunmaremay cma nes noe tng ere ere aie cota ct ca be gt ec ie © wabine 4 wns Ye wart leet "wien Ty a bate ag at Peabo he etaon Ngee amet ecnn esr ae eo Fat mersate Characterstin Fie hom he curs ea tae aa doe. 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AVy erceeage ofineroplaion canbe calcued by= 77 an “100 he ouput vollage an Vy the laputvakage and AV 4 e change eetput ota for a pareular change mine wottage AV were’ ) Load Regulation In testy of regulation, np tage faed and te ead rsitance © vaeyng Cutout vol rear ware along asthe load vesicle it amtaeed above 8 tera ae Vine ¥ peor cincennns [8 | 00 im sabice Vg tte nll load resetoe voape [fe r=mow the load resistance and measisr the wotage across the Zener Diode) ant Vy, v2 the fal one reece tage: Design a Voltage Regulator When selecting tha zener ode be sre that ht maim power rang a nl wacom hems Aemarcamere ts Dew uct Power troe Zener voltage We seer oie stn eae You Inpu volage(n shoe) Vy Voluge across sens tesntance Y,Votage serous the ose resistance 1k Corte pasting ough me senes resixance |e Carrenpasung trough he Zar doce Current pasingsroug the ont ronstance Celctingvotape and curert ‘Tratotl cuanto a te touche tame a tht sgh fe oes eter 4 vs a ‘The caret tiough the load ester = sd he res ede curentie Wate voltage source in gieats than Vz he tage urea i loe than Ve Pt “TRF RI “