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