Laboratory 3 Diode Characteristics

Transcription

1 Laboratory 3 Diode Characteritic BACKGROUND A diode i a non-linear, two terminal emiconductor device. he two terminal are the anode and the cathode. he circuit ymbol of a diode i depicted in Fig he diode i aid to be forward-biaed if the voltage v acro it i poitive. he diode i reverebiaed if v i negative. An ideal diode behave a follow: It act a an open-circuit (i = 0) when it i revere-biaed. It act a a hort circuit (v = 0) if a poitive current i i paed through it. he diode i aid to be cut off when revere-biaed, and aid to be turned on when forward-biaed. Fig. 3-1: Circuit ymbol of a diode Diode rating he forward current rating and the peak revere voltage (PRV) rating are two important diode rating. he forward current rating i the maximum average current that can be paed continuouly through the diode. he PRV i the maximum revere voltage that can be applied to the voltage. he diode can uffer revere breakdown if thi rating i exceeded. he 1N4001 diode deigned for rectifier application ha an average forward current rating of 1A and a PRV of 50V. Diode characteritic he ideal diode behavior decribed above give one a baic undertanding of how a diode function. he i-v characteritic of a real diode are however not a imple a decribed above. In the forward-bia region the i-v relationhip i decribed a follow: v / i = I ( e 1) Eq. 3-1 In the above equation, i i the forward current, v i the forward voltage, I i the aturation current, and V = k/q i the thermal voltage (k = i Botmann' contant, i the abolute temperature in Kelvin, and q = i the electronic charge). he value of V at room temperature (20 o C) i 25.2mV. he contant n in the diode equation above ha a value between 1 and 2 depending on the material and phyical tructure of the diode. he aturation current I i mall - it i of the order of A for mall ignal diode. When appreciable current i flow in the forward direction (i>> I ), Eq. 3-1 i approximated by the following exponential relationhip:

2 i v / = I e Eq. 3-2 In logarithmic form the above equation become a linear relationhip between ln(i) and v. We have v ln( i ) = + ln( I ) Eq. 3-3 hu the emilog plot of the i-v characteritic of a diode (with the log cale ued for i) i linear. he value of the diode parameter n can be determined from the lope of thi line, a decribed in the ection `Semi-log graphical conideration'. In the revere-bia region (v<0), the diode current i ideally ero. In reality there i a mall negative diode current (of the order of nanoamp for mall ignal diode). he data heet of a 1N4001 diode how that it maximum revere current i 5μA at a temperature of 25 o C. If the revere voltage exceed the PRV rating of the diode, the diode breakdown and the revere current become large. he diode will be detroyed in the revere-breakdown region if the power diipation rating of the device i exceeded. Zener diode A ener diode (unlike a normal diode) i deigned for operation in the revere breakdown region. he circuit ymbol of a ener diode i depicted in Fig he relationhip between V and I (in the revere-breakdown region) i almot linear. he following equation i ued to decribe the relationhip: V = V + I r Eq. 3-4 o Here, r i the dynamic reitance of the ener at the operating point (V, I ). V o i the voltage at which the traight-line approximation of the i-v characteritic interect the horiontal axi. Zener diode pecification include a tet point (V, I ) in the breakdown region, and a dynamic reitance r at thi point. A 1N4739A ener diode ha a power rating of 1W, V = 9.1V, I = 28mA, and r = 5Ω. Fig. 3-2: Circuit ymbol of a ener diode Uing an ocillocope to obtain i-v characteritic of a two terminal device he circuit depicted in Fig. 3-3 can be ued to diplay the I-V characteritic of a twoterminal device on the ocillocope creen. he device under tet i connected in erie with a reitor R which erve a a current ening a well a a current limiting reitor. A ine wave of uitable amplitude and frequency of everal hundred Hert (from a ignal generator) i connected acro the erie combination. he ocillocope i et to X-Y mode. he voltage acro the current ening reitor i fed to the vertical cope input

3 and the voltage acro the device under tet i fed to the horiontal cope input. he ocillocope etting can be adjuted to obtain a diplay of the I-V characteritic of the device under tet on the creen. Note that a vertical cale etting of V volt/div correpond to a current cale of V/R amp/div. For example, if the vertical cale etting i 5V/div, and the value of R i 1kΩ, then the current cale i 5mA/div. Note that a ingle ended cope probe can be ued acro R ince the lower terminal of R i connected to ground (the - terminal of the ignal generator). However, neither terminal of the device under tet i at ground potential. herefore, a differential cope probe mut be ued acro the device under tet. Fig. 3-3: Circuit ued to obtain device I-V characteritic Let u invetigate component value election for the cae where the device under tet i a diode whoe forward characteritic i required over the current range 0-10mA. Let u et R=1k. here i a 10V drop acro the reitor when the maximum current of 10mA flow through it. here i alo roughly a 0.7V drop acro the diode. he ignal generator hould thu be et to an amplitude (ero to peak) of 10.7V. If thi voltage etting i not poible with the equipment at hand, chooe a lower value for R. PRELAB 1. A PSpice chematic diagram for obtaining the I-V characteritic of a diode i depicted in Fig Perform a PSpice imulation of thi circuit (weep the voltage ource over the voltage range 0 to 15V). Ue Probe to obtain a plot of diode current i veru diode voltage v. At roughly what forward voltage doe the diode begin conducting? 2. Ue Probe to obtain a plot of ln(i) veru v. Ue the lope of the graph (beyond the cut-in region) to determine the value of the diode parameter n. 3. Fig. 3-4 i eentially a half-wave rectifier circuit. Simulate the circuit uing a inuoidal voltage ource (VSIN) of amplitude 10V and frequency 60H. Ue Probe

4 to obtain a plot containing 3 ubplot, namely, the ource voltage waveform, the load voltage waveform (voltage acro R), and the diode voltage v. 4. From your plot in tep 3 above, determine the voltage acro the diode when it i conducting. What i the peak value of the load voltage waveform? Explain why the peak value i what it i. What i the maximum revere voltage appearing acro the diode? Fig. 3-4: PSpice chematic for obtaining diode I-V characteritic IN LAB 1. Your intructor will demontrate how to ue the curve tracer to obtain a diode i-v plot. Ue the curve tracer to obtain the i-v characteritic of the 1N4001 diode in your lab kit. Obtain a printout of the characteritic for your lab notebook. Ue curor to obtain data point on the curve (beyond cut-in). abulate thi data in your lab notebook. Obtain a emi-log plot of i veru v (fit a traight line to the data point). Determine the value of the diode parameter n from the lope of the line (ee the ection below on emi-log graphical conideration). Aume that the temperature in lab i 70 o F i.e., 21.1 o C. Alo ue your graph to determine the value of the diode aturation current I. 2. Contruct the diode rectifier circuit. Ue the Wavetek ignal generator to power the circuit with a 60H inuoidal ignal. Ue the ocillocope to meaure the amplitude of the inuoid and adjut it to 10V. Obtain a plot of the load voltage. Determine the average value of the load voltage. Alo obtain a plot of the diode voltage (think about how you could do thi). Label your plot appropriately and pate them in your lab notebook. 3. he intructor will demontrate how the i/v characteritic of a ener diode may be obtained on an ocillocope creen. Obtain a printout of the i/v characteritic (in the revere-breakdown region) of the 1N4739 ener diode in your lab kit. Check the ener tet current and voltage pecification againt your meaurement.

5 Semi-log Graphical Conideration A i clear from Eq. 3-3, there i a linear relationhip between ln(i) and v when the diode current i >> I. Meaured data point (V d, I d ) on the diode characteritic beyond cut-in can be plotted on emi-log paper uing the log cale for i. hi emi-log plot i expected to be linear, and a line of bet fit" for thee data point can be drawn. Due to meaurement error thi line will not pa through all the data point. Let (V d1, I d1 ) and (V d2, I d2 ) be two point on the line of bet fit. From Eq. 3-3 we have VD1 V ln( I = + ln( I ) and ln( I ) = + ln( I ) Eq. 3-5 Subtraction of the econd expreion above from the firt yield VD1 V = Eq. 3-6 ln( I ln( I ) he denominator of the above equation involve natural log while the log axi of a emilog plot i bae 10. he following i the relationhip between the natural log and log bae 10 of a number x: log10( x) ln( x ) = Eq. 3-7 log ( e) Subtituting thi into Eq. 3-6 yield the following equation: 10 V D1 V = log10( e) Eq. 3-8 log10( I log10( I ) he denominator of the above equation reduce to 1 if the two data point on the line of bet fit are elected to be a decade apart (i,e., I d1 = 10 I d2 ). he value of the diode parameter n can thu be eaily determined uing Eq he diode aturation current I can be determined uing the value of determined above and any point on the line of bet fit (ee Eq. 3-5).