Section B9: Zener Diodes

Transcription

1 Secton B9: Zener Dodes When we frst talked about practcal dodes, t was mentoned that a parameter assocated wth the dode n the reverse bas regon was the breakdown voltage, BR, also known as the peak-nverse voltage (P). Ths was a bad thng before the whole avalanche breakdown, large current, overheatng devce and total destructon thng... Well, guess what? Under specfc fabrcaton condtons, a dode may be created that wll not be destroyed f the breakdown voltage s exceeded, as long as the current does not exceed a defned maxmum (to prevent overheatng). These devces are known as zener dodes and they are desgned to have an avalanche characterstc that s very steep. The characterstc curve and dode symbols for the regular and zener dodes are gven to the rght (Fgure 3.38 of your text). n the forward bas regon, the zener behaves lke a regular dode wthn specfed current and/or power lmts. The magc of these devces comes n when we get nto the reverse bas regon. As prevously mentoned, the zener s desgned to have an almost vertcal avalanche characterstc at the breakdown voltage herenafter also called the zener voltage, and t s deal for use n voltage regulaton. The lmtng (maxmum) power for a zener dode s gven by P z = z zmax and s a functon of the desgn and constructon of the dode. The knee of the curve (the current for whch v D = Z ) s generally approxmated as 10% of zmax, or zmn =0.1 zmax. There are two dstnctly dfferent mechansms that may cause breakdown n a zener dode: 1. Above approxmately eght (8) volts, the predomnant mechansm s avalanche breakdown, also referred to as mpact onzaton or avalanche multplcaton. Ths process begns wth thermally generated mnorty carrers that acqure enough knetc energy to break covalent bonds and create an EHP through collsons wth crystal atoms. The free carrers created through ths collson contrbute to the reverse current and may also possess enough energy to partcpate n collsons, creatng further EHPs and the avalanche effect.

2 2. The hgh feld emsson or zener breakdown mechansm s the second method of dsruptng the covalent bonds of the crystal and ncreasng the reverse bas dode current. The reverse voltage where ths occurs s determned by the dode dopng and occurs when the depleton layer feld s large enough to break covalent bonds and cause the number of free carrers due to EHP generaton to multply. Ether of these effects, or a combnaton of the two, sgnfcantly ncreases the current n the reverse bas regon whle havng a neglgble effect n the voltage drop across the juncton. Although breakdown and dsrupton and words of that order have been lberally used n the prevous dscusson, please realze that the zener process n not nherently destructve unless the maxmum power dsspaton specfed for the devce s exceeded. Zener Regulator As mentoned earler, the characterstcs of the zener dode make t deal for applcaton as a voltage regulator. Placng the zener dode n parallel wth the load as shown n Fgure 3.39 (reproduced to the rght) ensures an essentally constant output voltage even though the load current and the source voltage may vary. The key to the desgn of ths voltage regulator s to choose the resstor R to keep the zener n the breakdown regon, whle ensurng that the dode current never exceeds zmax. Your text derves an expresson for ths crcut parameter by developng the nodal expresson for the zener current and defnng the two extremes for Z n terms of the nput/output condtons: 1. zmn occurs when the load current s maxmum and the source voltage s mnmum. 2. zmax occurs when the load current s mnmum and the source voltage s maxmum. Crcut analyss technques yeld an expresson for the resstor of nterest as v s Z n Equaton 3.56, where R = and R= Z+ L. Equatng the characterstcs for the extremes of Z ( zmn and zmax ) n the expresson for R yelds: R

3 R s mn z s max z = = (Equaton 3.58) L max + z mn L mn + z max usng the approxmaton zmn =0.1 zmax, strrng lberally and rearrangng... z max ( ) + ( L mn z s mn L max s max z = (Equaton 3.61) s mn z s max ) The varaton n load current (or, equvalently, load resstance) and source voltage may be gven or measured. Be aware that these dervatons are based on the rule of thumb relatonshp between the maxmum and mnmum zener current. f another relatonshp s defned, you ll have to back up to Equaton 3.58 and rework Equaton Once zmax s known, the second expresson n Equaton 3.58 may be used to solve for R. Equvalently, Equaton 3.58 (or Equaton 3.61) may be solved for zmn by rearrangng our rule of thumb to be zmax =10 zmn : z mn ( 10 L mn z s mn L max s max z = (Equaton 3.61: Modfed) s mn ) + 9 z ( s max ) n ths case, the frst expresson n Equaton for 3.58 may be used to solve for R. One fnal note before we leave ths topc. Be aware that t s not wrtten n stone that every combnaton of source voltage and/or load current wll result n a vable crcut. As llustrated n Example 3.6(b), sometmes there s no resstor R that wll make the regulator operate correctly. As your author states, somethng n the desgn condtons wll have to change an ncrease n source voltage or a reducton n load current for the desred output voltage (Ths s obvously assumng that you are totally commtted to the zener voltage!). Full-Wave Zener Regulator The flterng dscusson n Secton B7 ntroduced the reducton of output rpple by usng a parallel RC combnaton. As shown n Fgure 3.41 of your text (reproduced below), ths process may be enhanced by ntroducng a zener regulator after the RC flter. Note that n the crcut above, the resstor n the RC combnaton s no longer the load, but s what s known as a

4 bleeder resstor and s denoted R F. The purpose of R F s to provde a dscharge path for the capactor f the load s the removed. t s desred that ths resstor absorb as lttle power as possble when the crcut s n operaton, so t usually has a very hgh resstance (hgher resstance, lower current for same voltage). Anyway, we can stll use Equaton 3.52 to solve for the capactor (now called C F to match the R F notaton), wth, of course, a couple of modfcatons: n Equaton 3.52, we were only concerned wth the maxmum swng of the source wth respect to the zero axs. However, snce the voltage across R wll not go to zero as long as the zener s operatng, the max term s replaced by smax - z. The equvalent resstance across the capactor C F s evaluated under forward bas condtons and s the parallel combnaton of R F and R. However, snce R F s generally very much larger than R, ths parallel combnaton s approxmately equal to R. The remander of the terms are as defned n Equaton 3.52: o s the peak-to-peak rpple allowable at the output, and o f p s the frequency of the rectfed waveform (twce the orgnal frequency for full-wave rectfcaton). Puttng ths all together, an expresson for the capactance n a full-wave zener rectfer s gven by C F f R s max Z = (Equaton 3.62) p Practcal Zener Dodes and Percent Regulaton We ve been toodlng along n ths dscusson actng lke the zener dode s gong to behave absolutely perfectly at all tmes the characterstc n the breakdown regon s perfectly vertcal (nfnte slope mples zero resstance),

5 t s gong to gve us an absolutely perfectly constant voltage as long as we don t exceed some smple power ratng, rght? Well, not exactly (bg surprse)... Actually, the zener s pretty well behaved and s extremely useful f used correctly. However... to account for the fact that the curve s not perfectly vertcal, we must nclude a nonzero zener resstance n seres wth our deal dode as shown n Fgure 3.42 and reproduced to the rght. The effect of ths resstor (R Z ) s to cause the output voltage to devate from the constant value of the deal zener voltage ( Z n the fgure) What we ve got now at the output s the practcal zener voltage, Z = Z + Z R Z. Your text llustrates a numerc example of ths effect n Equaton By usng the two extremes of zener current ( Zmax and Zmn ), t can be seen that the output of a practcal zener dode vares between outmax and outmn. Snce we have a nondeal output, the concept of percent regulaton s ntroduced. Percent regulaton s defned as the total varaton over the nomnal (desred) value. For voltage regulaton of a zener dode, ths translates to % Regulaton out max out mn = * 100 (Equaton 3.64: Modfed) outnomn al The lower the percent regulaton, the better the regulator an deal zener dode would have a percent regulaton of zero ( outmax = outmn ).