Lecture 4 ver2 diode_app

Chapter 2:
Diode Applications (cont.)
Prepared by:
Ikhwan Hafiz Muhamad
ikhwanh@ump.edu.my
Block 1, E10-C13

Copyright Ikhwan H Muhamad, Universiti Malaysia Pahang © 2010 1

Recap Last Lecture

HALF-WAVE RECTIFIER CENTER-TAPPED FULL BRIDGE FULL-WAVE
WAVE RECTIFIER RECTIFIER

V p (sec)
V p ( out ) V p (sec) 0 . 7V V p ( out ) 0 . 7V V p ( out ) V p (sec) 1 . 4V
2
V p ( out ) 2V p ( out )
2V p ( out )
V AVG V AVG V AVG

PIV V p (sec) PIV 2V p ( out ) 0 . 7V PIV V p ( out ) 0 . 7V


Diode Applications

1. Half-wave & full-wave rectification
2. Power supply filter & regulators
3. Clippers & Clampers


Power Supply Filters and Regulators
Sub-Chapter Outcomes

• Explain and analyze the operation and
characteristics of power supply filters and
regulators
• Explain the purpose of a filter
• Describe the capacitor-input filter
• Define ripple voltage & calculate the ripple
factor
• Discuss surge current & voltage regulation


Power Supply Filters
AL
Difference
AC & DC?

• Filter is a capacitor in a power supply used to reduce
the variation of the output voltage rectifier


Capacitor-Input Filter



Ripple Voltage: the small variation (undesirable) in
the capacitor voltage due to the charging &
discharging of the filter capacitor


Full rectifier easier to filter because of the shorter
time between peaks (smaller ripple)


Ripple Factor, r, is an indication of filter effectiveness (the
lower the ripple factor the better filter)

1
V r ( pp ) V p ( rect )
V r ( pp ) f RLC
r
V DC 1
V DC 1 V p ( rect )
2 f RLC



Eg. Determine the ripple factor for the filtered bridge
rectifier with a load as indicated in figure below

Answer: 0.079 @ 7.9%

Surge Current
At the instant the switch is closed,
voltage is connected to the bridge
and the uncharged capacitor
appears as a short.
 Produce ISurge through D1 & D2
 ISurge could destroy the diodes

 RSurge used to prevent the diodes
from destroy.
 RSurge < RL;

V p (sec) 1 . 4V
R surge
I FSM


Voltage Regulators
Voltage Regulators
 Combination of capacitor-
input filter with integrated
circuit voltage regulator

 Operation: Input
78XX Output
from
i. the input to the regulator rectifier
is first filtered with a
capacitor to reduce the
ripple to<10%
ii. The regulator reduce the
ripple to a negligible
amount (acceptable
level)

 3 terminals: input, output &
reference

Voltage Regulators
Percent Regulation – present the performance of
a voltage regulator

• Line regulation specify how much change occurs in the
output voltage for a given change in input voltage

Line regulation V OUT
100 %
V IN

• Load regulation specify how much change occurs in the
output voltage over a certain range of load current values.
Load regulation
V NL V FL
100 %
V FL


Exercise

1) A certain 7805 regulator has a measured no-load
output voltage of 5.18V and a full load output of 5.15V.
What is the load regulation expressed as a percentage?

2) If the no-load output voltage of a regulator is 24.8V
and the full-load output is 23.9V, what is the load
regulation expressed as a percentage?

14

Review section

1) When a 60Hz sinusoidal voltage is applied to the
input of a half-wave rectifier, what is the output
frequency?
2) When a 60Hz sinusoidal voltage is applied to the
input of a full-wave rectifier, what is the output
frequency?
3) What cause the ripple voltage on the output of a
capacitor-input filter?
4) If the load resistance connected to a filtered power
supply is decreased, what happens to the ripple
voltage?

15

5. A certain rectifier filter produce a dc o/p voltage of
75V with peak-to-peak ripple voltage of 0.5V.
Calculate the ripple factor?

6. A certain full-wave rectifier has a peak o/p voltage of
30V. A 50uF capacitor-input filter is connected to the
rectifier. Calculate the peak-to-peak ripple and the
dc o/p voltage developed across a 600ohm
resistance.

7. What is the percentage of ripple for the rectifier in
question 6?

16

Diode Applications

1. Half-wave & full-wave rectification
2. Power supply filter & regulators
3. Clippers & Clampers


Clippers and Clampers
Sub-Chapter Outcomes

• Explain and analyze the operation of
diode limiting & clamping circuits
• Explain the operation of diode limiters
• Determine the output voltage of a biased
limiter
• Explain the operation of diodes clampers


Diode Limiters (Clippers)

Diode Limiters
• A diode network that have the ability to “clip”
(cut short/crop) off portions of signal voltages
above or below certain limits, i.e. the circuits
limit the range of the output signal.
• Such a circuit may be used to;
– protect the input of a CMOS logic gate against
static
– eliminate amplitude noise
– fabricate new waveforms from an existing
signal


Diode Limiters

• 2 general of clippers:
a) Series clippers
b) Parallel clippers
• Parallel Clippers
Diode in a branch parallel to the load.
• Series Clippers
Diode is in series with the load.



Parallel Clipper


Parallel Clipper

• Whenever the input is below 0.7V, the diode is
reversed biased and appears as an open.

• The output voltage, VOUT looks like the negative part
of the input, but with a magnitude determined by the
voltage divider formed by R1 and RL, as follows:

• If R1 is small compared to RL, then VOUT=VIN


Analysis Steps for Parallel Clipper

1. Define the input and output port
2. Determine the current flow of the circuit
3. Determine the applied voltage (transition
voltage) that will result in a change of state
for the diode from the “off” to “on” state
4. Draw the output waveform directly below
the applied voltage using the same scales


Example

What should you expect to see displayed on an
oscilloscope connected across RL in the limiter shown
above? Sketch the waveform for 2 cycles.

Noted: +Vp= +10V; R1 = 100 ohm, RL=1kohm, diode model = 1N4001

Parallel Clipper with Biased Limiters

By adding a DC source to the circuit, the voltage
required to forward bias the diode can be
changed.

AL
Matlab


Diode Limiting Circuits - Example

What is the output voltage of positive limiter shown? Sketch the
waveform

R1

10 V 1.0 k
RL 3.0 V
Vin 0 VBIAS = 0
100 k
2.3 V +
–

The diode is forward-biased when the output tries to go above +3.0 V.
This causes the output to be limited to voltages less than +3.0 V.

Eg: Determine the output voltage waveform for the circuit
below.

Answer: Clipped at ±5.7V


Voltage-Divider Bias


Voltage-Divider Bias
Eg: Describe the output voltage waveform for the diode
limiter in Figure below.

Answer: Clipped at +8.95V


Series Clipper


Example :
•Determine the Vo and sketch the o/p waveform
for the below network. Assume diode is ideal.

Vi RL
16
+ +
t Vi Vo
0 T/2 T

-16 V=4 V
-
-

Solution:
• + ve region Vi Diode state Vo
1 ON V
2 ON V
+ 3 ON V
+ 4 ON V
Vi Vo 5 OFF Vi
- 6 OFF Vi
V=4 V - 7 OFF Vi
16 OFF Vi

+ Vo

+ 16

Vi
Vo 4
- 0 T/2
t
V=4 V -

Vi Diode state Vo
-1 ON V
Solution (continued):
-2 ON V
• - ve region (always ON state) -3 ON V
-4 ON V
-5 ON V
+ + Vo
16 -6 ON V
Vo
Vi -7 ON V
- 4
t
T/2 T
V=4 V - 0
-16 ON V

Vo
16
The resulting o/p waveform
4
t
0 T/2 T

Example :
Repeat the previous example using a silicon diode with VD=0.7 V
Solution: RL

VD=0.7 V
+
+ Vo
Vi
- V=4 V
-

Vi VD V 0
Vi V VD
4 0 .7
3 . 3V

Solution (continued):
For i/p voltages greater than 3.3 V the diode open cct and Vo=Vi.

For i/p voltages less than 3.3 V the diode short cct and the
network result as;
RL

VD=0.7 V
+
+ Vo
Vo 4 0 .7
Vi
- V=4 V
- 3 . 3V

Vo
16

The resulting o/p waveform 3 .3
t
0 T /2 T


Summary of Series Clippers : Refer

Electronic Devices and Circuit
Theory: International Edition, 8th
Edition (Robert L. Boylestad) page 87.

40

Diode Clampers

• Known as DC-Restorer

• Widely use in analogue television
receivers

• A clamper is a network constructed of a
diode, resistor, and a capacitor that
shifts a waveform to a different dc level
without changing the appearance of
the applied signal

Diode Clampers
Positive Clamper


Diode Clampers
Negative Clamper

• Note:
• RC time constant should be at least ten times the period
of the input frequency.

Diode Clampers
Negative Clamper
Eg: Determine the output voltage waveform.
Assume that RC is large enough to prevent
significant capacitor discharge

Answer: Vdc = -23.3V


Diode Clampers

Summary of clamping circuits : Refer

Electronic Devices and Circuit Theory:
International Edition, 8th Edition
(Robert L. Boylestad) page 91.

46

Review section

1) What component in a clamping
circuit effectively acts as a battery?

2) What is the difference between a
+ve limiter and a –ve limiter?

Lecture 4 ver2 diode_app

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