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Semiconductor diode

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ripple voltage, Vr and. Vr Q/C (L22) Eliminating Q from (L21) ... Capacitor size is half that of the half-wave case for the same load current and ripple voltage ... – PowerPoint PPT presentation

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Title: Semiconductor diode


1
Semiconductor diode
2
First approximation of a semiconductor diode
(Ideal-Diode)
iD
iD
R10 ?
R10 ?
Vs 10V
Vs 10V
iD 1A
iD 0A
3
Second approximation of a semiconductor diode
Diode on
iD
vD
Diode off
Vf ? 0.7V
iD
iD
R10 ?
R10 ?
Vs 10V
Vs 10V
iD 0.93 A
iD 0A
4
Third approximation of a semiconductor diode
Diode on
iD
vD /iD Diode body (bulk) resistance Rf
vD
Diode off
Actual Characteristics
Vf ? 0.7V
iD
iD
R10 ?
R10 ?
Vs 10V
Vs 10V
iD 0.62 A (say Rf 5 ?)
iD 0A
5
How to Find Out the Exact Diode Operating
point?
6
Example of Diode Load Line
If R 200 ? , Vss 3V in the what is iD and vD?
Operating point
7
Answer to the Diode Load Line Problem
iD 9.5 mA, vD 1.1 V
8
Multiple Ideal-Diode Circuit Analysis
  • Assume either a on (short) or off (open)state
    for a diode
  • Determine the current direction in an on diode.
  • Determine the voltage polarity of an off diode.
  • If the assumption is correct, current flows from
    anode to
  • cathode in all on diodes cathode voltage
    should be positive
  • with respect to anode in all off diodes.
  • In a n diode circuit 2n such combinations are
    possible

9
Example on Diode Circuit
10
What are the currents in D1,D2,D3,D4?
Assume ideal diodes
11
Answer and Another Question
iD1 2 mA, iD2 0 mA, iD3 0 mA, iD4 5 mA
What should be the minimum value of the current
source in the previous figure 10.17(c) in order
to make D3 conduct?
12
Rectifier Circuits
  • Rectifier circuits convert AC voltage to DC
    voltage
  • The output DC voltage magnitude is controlled by
    controlling the input
  • AC voltage
  • Usually classified by two types Half-wave and
    Full-wave
  • Has many applications such as battery charger,
    power supply.

13
Half-Wave Rectifier Circuits
  • During the positive half-cycle of the source the
    diode conducts
  • During the negative half-cycle the source diode
    blocks
  • Current through the diode looks similar to the
    voltage across RL

14
A simple Battery charger-Example of a Rectifier
  • Can be used to charge a car battery from the
    alternator

15
Smoothening the Output Voltage of a Rectifier-Add
a Capacitor across Load
16
Design of Filter Capacitor
  • Assume capacitor takes negligible time to charge
  • Assuming constant load current , the charge lost
    by the capacitor is
  • Q
    ? ILT (L21)
  • The charge lost in the capacitor is reflected in
    the load voltage as
  • ripple voltage, Vr and
  • Vr ?
    Q/C (L22)
  • Eliminating Q from (L21) and (L22) one gets
  • C ILT
    / Vr (L23)
  • where T 1/f (f frequency of the input
    sinusoidal source)

17
Design of Filter Capacitor(Contd..)
  • Load Voltage VL Vm- Vr /2 where Vm peak of
  • the sinusoidal source

18
Full-wave rectifier
  • Normally used with center-tap transformer
  • Rectifies both the positive and negative half
    cycle
  • Capacitor size is half that of the half-wave case
    for the same load current and ripple voltage

Vin
Vin
Vout
19
Diode-bridge Full-wave rectifier
  • Can be used without a transformer
  • Rectifies both the positive and negative half
    cycle
  • Capacitor size is half that of the half-wave case
    for the same load current and ripple voltage
    (why?)

Vin
Vout
Vin
Vout
20
Comparison of Rectifier Circuits
With 12 transformer turns ratio With Large
C Peak Inverse Voltage (also with Large C)
 
21
SPICE simulation of Full-wave Bridge Rectifier
Circuit
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