Transcript IE_F10_lecture8

Lecture 8
Diode Applications and small signal
models
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Agenda
-Diode Regulator
-Zener Diode
-Design of regulator circuits
-Clamping circuits (DC-restoration)
-Diode Small Signal Models
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Application 2: Voltage Regulators:
Introduction to Reverse Breakdown
Increased reverse bias
eventually results in the diode
entering the breakdown
region, resulting in a sharp
increase in the diode current.
The voltage at which this
occurs is the breakdown
voltage, VZ.
2 V < VZ < 2000 V
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Reverse Breakdown Mechanisms
• Zener Breakdown
Zener breakdown occurs in heavily doped diodes. The heavy doping
results in a very narrow depletion region at the diode junction.
Reverse bias leads to carriers with sufficient energy to tunnel directly
between conduction and valence bands moving across the junction.
Once the tunneling threshold is reached, additional reverse bias leads
to a rapidly increasing reverse current.
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Zener Diode
I Z  iD
VZ  vD
Knee Current
Valid iff IZ > IZK
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Zener Diode Model
In breakdown, the diode is
modeled with a voltage source,
VZ, and a series resistance, RZ.
RZ models the slope of the i-v
characteristic.
Diodes designed to operate in
reverse breakdown are called
Zener diodes and use the
indicated symbol.
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Analysis of Zener Diodes
Choose 2 points (0V, -4 mA) and (-5 V, -3
mA) to draw the load line.It intersects with i-v
characteristic at Q-point (-2.9 mA, -5.2 V).
2.Using piecewise linear model:
I  I  0
Z
D
1.Using load-line analysis:
 20 V  5000I
D
D
20  5100I  5  0
Z
(20  5)V
I 
 2.94mA
Z 5100
Since IZ >0 (ID <0), solution is consistent
with Zener breakdown assumption.
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Regulator design
Rectifier
120 Vrms
vr 
Filter
R
C
vp
fRC
Less ripple  larger C
We can get less ripple without using a larger C with a Zener regulator
Unregulated
supply
RL
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Voltage Regulator using Zener Diode
constant voltage model
V V
(20  5)V
I  S Z
 3mA
S
R
5k
V
5V
I  Z
 1mA
L R
5k
L
I  I  I  2mA
Z S L
For proper regulation, Zener current must be
positive. If Zener current <0, Zener diode no
longer controls voltage across load resistor
and voltage regulator is said to have
Zener diode keeps voltage
“dropped out of regulation”.
across load resistor constant.
R R


V
R



For Zener breakdown
I  S V  1  1   0 L  VS  min
Z R
ZR R 
1

operation, IZ >0.
L

V

 Z

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Voltage Regulator using Zener Diode:
Example (Including Zener Resistance)
V  20V  VL  5V  VL  0
5000 100 5000
L
V  5.19V
L
Problem: Find output voltage and
Zener diode current for Zener diode
regulator.
Given data: VS=20 V, R=5 k, RZ=
0.1 k, VZ=5 V
Analysis: Output voltage is a
function of current through Zener
diode.
V  5V 5.19V  5V
I  L

1.9mA  0
Z 100
100
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Regulator Characteristics
Rth
R
+
Thevenin
rZ Equivalent
VOC
IL
+
VZo
Vth (VOC )  Vs
rZ
R
 Vzo
rZ  R
rZ  R
Rth  rZ // R
For best regulation
rZ <<<<  limitation on the value of Iz
VL  VOC  I L Rth
VL  Vs
rZ
R
 Vzo
 I L (rZ // R )
rZ  R
rZ  R
VL
Vs
Line Regulation
VL
I L
Load regulation
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Line and Load Regulation
Line regulation characterizes how sensitive output voltage is to input
voltage changes.
dV
Line Regulation  L mV/V
dV
r
S
Z
For fixed load current, Line regulation =
Rr
Z
Load regulation characterizes how sensitive output voltage is to changes
in load current withdrawn from regulator.
dV
Load Regulation  L Ohms
dI
L
For changes in load current, Load regulation =  (rZ R)
Load regulation is Thevenin equivalent resistance looking back into
regulator from load terminals.
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Selecting R to keep Zener Diode in the
Breakdown Region
R
+
IZ
IL
rZ
VZo
I Z  I Z min , when vs  vs min
I Z  I Z max , when vs  vs max
vs min  [VZo  I Z minrZ ]
R
I Z min  I L max
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Application 3: Peak Detector Circuit
(Clamping circuit)
As input voltage rises, diode is on and
capacitor (initially discharged) charges
up to input voltage minus the diode
voltage drop.
At peak of input, diode current tries to
reverse, diode cuts off, capacitor has no
discharge path and retains constant
voltage providing constant output voltage
Vdc = VP – Vd,on.
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Application 4: Clipping or Limiting Circuits
Clipping circuits have dc path between input and
output, whereas clamping circuits use capacitive
coupling between input and output.
The voltage transfer characteristic shows that
gain is unity for vI < VC, and gain is zero for vI >
VC.
A second clipping level can also be set or diodes
can be used to control circuit gain by switching
resistors in and out of circuits.
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Clamping or DC-Restoring Circuit
After the initial transient lasting
less than one cycle in both
circuits, output waveform is an
undistorted replica of input.
Both waveforms are clamped to
zero. Their dc levels are said to
be restored by the clamping
circuit.
Clamping level can also be
shifted away from zero by
adding a voltage source in series
with diode.
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Clamping or DC-Restoring Circuit
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Voltage Doubler
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Diode Small Signal Model
Diode Small Signal Model
Diode Small Signal Model (Example)
Example