Download PN Junction Diodes and Applications

UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201 CIRCUIT THEORY 1
PN JUNCTION DIODES AND APPLICATIONS
WHAT IS A PN JUNCTION DIODE?
A PN Junction diode is a 2-terminal semiconductor electronic On/Off switch that allows current
flow in only one direction. The circuit symbol is shown below. The terminals are called the
Anode (A) and the Cathode (K). The state of the diode switch (open or closed) is determined by
the polarity of the voltage across it.
Circuit Symbol and Typical Terminal Identification
Common types of Diode Packages
Figure 1. Some common Diode Packages
CIRCUIT BEHAVIOR OF THE PN JUNCTION DIODE
The behavior of electronic devices, such as the pn junction diode, can be described by their VoltAmpere, or (V-I) Characteristic. This is a plot of the current flowing through the device as a
function of the voltage across the device. Since the resulting plot of the diode’s V-I Characteristic
is not a straight line, the diode is referred to as a “non-linear” device. A typical V-I Characteristic
for a pn junction diode is shown here.
Figure 2. A typical PN Junction diode V-I Characteristic
Through experimentation, the current through a diode as a function of the voltage across it can be
mathematically modeled by the following “diode equation”
i  I 0 (e
 v 


 VT 
-1)
where
i = the current through the diode in Amperes (A)
v = the voltage across the diode from Anode (+) to Cathode (-) in volts
VT = 25 mv (25 X 10-3 volts) at room temperature (25°C)
I0 = a constant with units of current
η = a constant ranging from 1 to 2 depending upon the diode material
This representation does not account for the reverse breakdown region!
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APPLICATION AS A RECTIFIER
Consider the circuit shown below.
XSC1
Ext Trig
+
_
B
A
+
_
+
_
D1
V1
1N4007GP
R1
1kΩ
5 Vpk
1kHz
0°
Figure 3. A PN Junction Diode as a Half-Wave Rectifier
When the applied sinusoidal voltage is on its positive half – cycle, conventional current will want
to flow in the clockwise direction (the direction of the arrow in the diode symbol) and the voltage
polarity across the diode will be + on the Anode and – on the Cathode. This condition is known
as “forward bias”. The diode will act as a short circuit and essentially all of the applied voltage will
appear across the resistor R1. The applied voltage and the voltage across the resistor R1 are
shown on the oscilloscope display below. Note that the resistor voltage is the “positive half –
cycle” of the applied voltage, hence the circuit is called a “half – wave rectifier”.
Figure 4. Waveforms for the Half – Wave Rectifier circuit.
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If the diode was installed in the opposite direction, we would observe the following.
XSC1
Ext Trig
+
_
B
A
+
_
+
_
D1
V1
1N4007GP
R1
1kΩ
5 Vpk
1kHz
0°
Figure 5. A “negative” Half – Wave Rectifier.
In each of the above circuits, the input signal, having an average value equal to zero was
converted into a “unidirectional” signal having a non – zero average value. It can be shown that
the average value of a “half – wave rectified” sinusoid having a maximum value of Vm has an
average value (also known as the DC value) equal to
Vaverage  VDC 
4
Vm

ANOTHER RECTIFIER APPLICATION
The circuit shown below is a “full – wave” bridge rectifier.
XSC1
Ext T rig
+
_
B
A
+
2
_
+
_
D3
V1
4
5 Vpk
1kHz
0°
1
3
MDA2502
R1
1kΩ
Figure 6. A Full – Wave Bridge Rectifier with the output waveform.
In this circuit, the output waveform consists of both half – cycles of the input voltage. The
average value of the voltage across R1 will be
Vaverage  VDC 
This is twice the value of the half – wave case.
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2Vm

AN IMPROVEMENT IN THE AVERAGE (DC) VALUE
The output voltages of the half -- wave and full – wave rectifiers have average values other than
zero, but their instantaneous values, as seen on the waveforms, are not constant. One method to
“smooth – out” the voltage (make it look more like DC) is to install a capacitor in parallel with the
load resistor R1. This “filter” capacitor will charge up to the maximum value of the output voltage
during the time that the diode conducts, and will discharge into the load when the diode is off. If
the time constant of the capacitor and load resistor is long when compared with the period of the
rectified sinusoid, the capacitor discharge time is longer and the output voltage is smoother. This
is shown in the circuits below.
XSC1
Ext T rig
+
_
B
A
+
_
+
_
D1
V1
5 Vpk
1kHz
0°
1N4007GP
+
-
3.536
U2
V AC 10MOhm
C1
2.2µF
R1
1kΩ
+
3.705
-
AC Input Voltage
U1
V DC 10MOhm
DC Output Voltage
Figure 7. Half – Wave Rectifier with a 2.2 μF Filter Capacitor.
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XSC1
Ext T rig
+
_
B
A
+
_
+
_
D1
V1
5 Vpk
1kHz
0°
1N4007GP
+
-
3.536
U2
V AC 10MOhm
C1
10µF
R1
1kΩ
+
4.165
-
AC Input Voltage
U1
V DC 10MOhm
DC Output Voltage
Figure 7. A Half – Wave Rectifier with a 10 μF Filter Capacitor.
The DC value of the output voltage increases and the waveform gets “smoother” when the value
of the filter capacitor is increased.
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THE EXPERIMENT AND REQUIRED RESULTS
1.) Construct the Half – Wave Rectifier circuit shown in Figure 3 on your breadboard. When
installing the diode, look for a “band” at one end of the package. That end is the Cathode (-).
2.) Apply a 5 V 1 kHz sine wave. Observe and record the input and output voltage waveforms.
Use your Digital Multimeter to measure the rms value of the input voltage and the DC level of the
output voltage. Compare the measured values with the expected (calculated) values.
3.) Run a simulation of the circuit in MultiSim and compare your results. Comment on any
differences and suggest a reason (or reasons) for any difference.
4.) Install a 2.2μF capacitor across the resistor (watch the polarity signs). Observe and record
the output voltage waveform. Measure the DC level of the output voltage using the Digital
Multimeter. Compare your results with a MultiSim simulation.
5.) Repeat Step 4 using a 10 μF capacitor.
6.) Construct the Full -- Wave Bridge Rectifier shown in Figure 6. Apply a 5 V 1 kHz sine wave
and observe and record the output voltage waveform. Measure the DC level of the output voltage
and compare with the expected (calculated) value.
7.) Compare your results with a MultiSim simulation. Can you account for any differences?
8.) Install a 10 μF capacitor across the resistor (again observing the polarity). Observe and
record the output voltage waveform. Measure the DC level of the output voltage using the Digital
Multimeter. Compare your results with the MultiSim simulation.
REQUIRED RESULTS FOR EACH GROUP TO BE HANDED IN
Step 2
Input and Output voltage waveforms
Measured and calculated values of rms input voltage and DC output voltage
Step 3
MultiSim simulation
Step 4
Output voltage waveform
Measured value of DC output voltage
MultiSim simulation
Step 5
Output voltage waveform
Measured value of DC output voltage
MultiSim simulation
Step 6
Output voltage waveform
Measured and calculated value of DC output voltage
Step 7
MultiSim simulation
Step 8
Output voltage waveform
Measured value of DC output voltage
MultiSim simulation
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