Diodes Solution

$35.00

Description

1 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.

2 Introduction

2.1 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.

ID

ID

Anode Cathode

VD

VD p n

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

1

VD

nVT

≈ IS exp

VD

nVT

(1)

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).

(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

2.2 DC Power Supply

Oneofthemost commonlyused applicationsofdiodesisDCpowersupply,which convertstheAClinevoltage intoaregulatedDCvoltage. Figure 3shows theblockdiagramofatypical DCpowersupply.TheACvoltageis firstpassedthroughatransformertostep itdowntoalower voltage, then rectified using diodes. Theresulting DCvoltage ispulsatingand hence isthen filtered toremoveorreducetheripples,producingaconstantDCvolt- age. Additionalcircuitry may beadded toprovidevoltage regulationsothat thedesiredvoltage ismaintained, independentoftheloadcurrentdrawn.

AC

line

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 AC Vs

Vs

line

Vo

C RL

line

Vs

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

Table1:Power SupplyDesign Specifications

Output Voltage

Vo

Maximum Output Current

Io ,max

Maximum Ripple

% of Vo

Transformer Type

Center-tap or Single

Typicalspecificationsforapower supplyaregiven inTable1.Sincethemaximum rippleisobserved atthemaxi- mum loadcurrent,theworst-caseloadresistancecanbecalculatedas

Vo

RL = I

o ,max

(2)

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

1

C =

2fi RL Kr

(3)

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).

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

Vˆs ≈ Vo + 1.4 (4)

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

Vˆs ≈ Vo + 0.7 (5) Note that in Fig. 4(b), the total voltage at the secondary winding is 2Vs .

3 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 .

Figure 5: Power supply circuit

4 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.

5 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.

6 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.

7 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.