ECEN Lab 6: Diodes Solution

Objectives

The purpose of this laboratory exercise is to investigate the basic properties and characteristics of semiconductor diodes. I-V characteristics, switching behavior, and rectification properties are examined, leading to the construction of a DC power supply.

Introduction

Diode I-V Characteristics

The diode allows current to flow in one direction only, similar to a one-way water valve. A semiconductor diode consists of a junction formed by contact between p-type and n-type semiconductor material. The terminal connected to the p-type material is called the anode, and the terminal connected to the n-type is called the cathode, as in Fig. 1. On most diodes, the cathode is marked by a band on the body of the device.



Anode

ID

ID

Cathode



VD

p    n

VD



Figure 1: Semiconductor Diode

When the anode is at a higher voltage than the cathode, the diode is forward biased, and current will flow through the diode from the anode to the cathode. When the anode is at a lower voltage than the cathode, the diode is reverse biased, and very little current will flow. The current flowing through the diode can be expressed as
ID=IS
exp
nVT
1
IS exp
nVT
(1)



VD




VD



where ID and VD are the current through the diode and voltage drop across the diode, respectively, as shown in Fig. 1, VT = kT =q is the thermal voltage, which is around 25 mV at room temperature, IS is the saturation diffusion current, which is a constant dependent on the diode’s geometry and material, and n is a device constant between 1 and 2. To examine the I-V characteristics of a diode, a test circuit as shown in Fig. 2(a) can be used. Measuring the voltage drop VD across the diode and the current through the resistor R for different values of Vi results in the exponential I-V characteristics shown in Fig. 2(b).








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✝✂
✡☞

✁
✂
✂✞✟✂✟✠(a)
✂✄☎✆✂
(b)


Figure 2: (a) Diode Test Circuit for I-V Characteristic (b) Resulting ID    VD plot


c    Department of Electrical and Computer Engineering, Texas A&M University

1
DC Power Supply

One of the most commonly used applications of diodes is DC power supply, which converts the AC line voltage into a regulated DC voltage. Figure 3 shows the block diagram of a typical DC power supply. The AC voltage is first passed through a transformer to step it down to a lower voltage, then rectified using diodes. The resulting DC voltage is pulsating and hence is then filtered to remove or reduce the ripples, producing a constant DC volt-age. Additional circuitry may be added to provide voltage regulation so that the desired voltage is maintained, independent of the load current drawn.



AC

line


Diode

Rectifier



Filter


Voltage

Regulator


VO



Load





Figure 3: Block diagram of a typical DC power supply


Figures 4(a) and (b) show two power supply circuits using full-wave rectification. If the transformer available is single-ended, the bridge rectifier composed of four diodes can be used to obtain full-wave rectified signal as in Fig. 4(a). If a center-tap transformer is available, full-wave rectification can be realized using two diodes as in Fig. 4(b). In both circuits, RL represents the load resistance, which is not a part of the power supply circuit. The filter is implemented using a single capacitor.




AC

LINE




VS






AC
Vs



VO




line
−Vs
C
R
L










 Vo

C    RL


N:1

N:1

(a)


(b)

Figure 4: (a) Bridge rectifier with a single-ended transformer (b) Full-wave rectifier with a center-tap transformer



Table 1: Power Supply Design Specifications

Output Voltage
Vo
Maximum Output Current
Io,max
Maximum Ripple
% of Vo
Transformer Type
Center-tap or Single


Typical specifications for a power supply are given in Table 1. Since the maximum ripple is observed at the maxi-mum load current, the worst-case load resistance can be calculated as

RL =

Vo
(2)

Io,max




Based on the maximum ripple and load specifications, value of the capacitor can be calculated as

C =
1
(3)

2fi RLKr



where fi is the frequency of the AC line (60 Hz in the US), Kr is the ratio of the maximum ripple to the peak output voltage (for example, if the maximum ripple specification is 10%, then Kr = 0.1).


2
For the bridge rectifier in Fig. 4(a), 0-to-peak voltage of Vs should be designed as

^
(4)
Vs    Vo + 1.4

whereas the peak Vs voltage in Fig. 4(b) should be designed as

^
(5)
Vs    Vo + 0.7

Note that in Fig. 4(b), the total voltage at the secondary winding is 2Vs .

Calculations

Design the power supply in Fig. 5 to have 3V output voltage (Vo ) with a maximum load current of 3mA and 10% maximum ripple, where Vs is a 250-Hz sine wave. Determine the peak amplitude of Vs and the value of C .


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Figure✁✂✄☎☎✆✝5:✞Power supply circuit✌
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Simulations

For all simulations, provide screenshots showing the schematics and the plots with the simulated values prop-erly labeled.

    1. Draw the schematics for the diode characterization circuit in Fig. 2(a) and perform a DC sweep of Vi from -1V to 1V. Export the simulation data to Excel, and plot ID as a function of VD .

    2. Draw the schematics for the power supply circuit in Fig. 5 with the calculated values, and obtain the time-domain waveform for the output voltage using transient simulation. Measure the peak output voltage, maximum ripple, and the peak current on the diodes and the load resistor.

Measurements

For all measurements, provide screenshots showing the plots with the measured values properly labeled.

    1. Build the diode characterization circuit in Fig. 2(a) and apply a ramp signal from -1V to 1V at 1Hz as the input (Vi ). Export the voltage measurements from the scope to Excel, and plot ID as a function of VD .

    2. Build the power supply circuit in Fig. 5 with the simulated component values, and obtain the time-domain waveform for the output voltage using the scope. Measure the peak output voltage and the maximum ripple.

Report

    1. Include calculations, schematics, simulation plots, and measurement plots.

    2. Prepare a table showing calculated, simulated and measured results.

    3. Compare the results and comment on the differences.



3
Demonstration

    1. Build the circuits in Figs. 2(a) and 5 on your breadboard and bring it to your lab session.

    2. Your name and UIN must be written on the side of your breadboard.

    3. Submit your report to your TA at the beginning of your lab session.

    4. For the diode characterization circuit in Fig. 2(a):

Apply a ramp signal from -2V to 2V at 1Hz for Vi , and export Vd+ and Vd    measurements to Excel.

Plot ID vs. VD in Excel.

    5. For the DC power supply in Fig. 5:

Show the time-domain output (Vo ) using the scope and measure the output peak voltage. Measure the maximum voltage ripple on the output waveform.



















































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