Diode

1
Chap. 3 Diodes

• Simplest semiconductor device
• Nonlinear
• Used in power supplies
• Voltage limiting circuits

2
3.1 Ideal Diodes
Reverse bias (off)
Forward bias (on)
3
I-V characteristics of an ideal diode
4
Ideal diode operation
5
Ideal diode operation
diode on diode off
6
Ideal diode operation
Vin 24 sinwt
24
12
Vout
on off on
off
30?
Diode conducts when 24 sinwt 12
sinwt 12/24 wt 30?
7
Exercise 3.4(a)
I
V -
2.5KW
5V
Find I and V Assume diode is on. V 0, I
5V/ 2.5KW I 2mA, implies diode is on. Correct
assumption
I
2.5KW
5V
8
Exercise 3.4(b)
I
V -
2.5KW
5V
Find I and V Assume diode is off. VD - 5, ID
0 implies diode is off. Correct assumption V
5, ID 0
2.5KW
5V
9
Exercise 3.4(e)
Find I and V
3
(Start with largest voltage) Assume D1 on, then
D2 will be off, and D3 will be off V 3V, and I
3V/1KW 3mA. Check assumption, VD1 0,
on VD2 -1, off VD3 -2, off Correct
assumption (old-style OR gate)
2
1
V -
I
10
3.6 Zener diodes

• Designed to break down at a specific voltage
• Used in power supplies and voltage regulators
• When a large reverse voltage is reached, the
  diode conducts.
• Vz is called the breakdown, or Zener voltage.

11
Typical use of Zener diode

• The Zener diode will not usually conduct, it
  needs Vs gt 12.5V to break down
• Assume Vs fluctuates or is noisy
• If Vs exceeds 12.5V, the diode will conduct,
  protecting the load

12
Solving ideal diode problems(determining if the
diode is on or off)

• Assume diodes are on or off.
• Perform circuit analysis, find I V of each
  diode.
• Compare I V of each diode with assumption.
• Repeat until assumption is true.

13
Prob. 3.9(b)
Are the diodes on or off?
Assume both diodes are on. 10V (10K)I1 I1
10V/10K I1 1mA 0 (5K)I2 - 10V, I2
2mA Current in D2 I2 2mA, on Current in D1
I1 - I2 -1mA, off Does not match assumption
start over.
I1
I2
14
Prob. 3.9(b)
Are the diodes on or off?
I
Assume D1 off and D2 on. 10V (10K)I (5K)I
-10V 20V (15K)I I 20V/15K 1.33mA Current
in D2 I 1.33mA, on Voltage across D1 10V -
10K(1.33mA) -3.33V, off Matches assumption
done.
15
I-V characteristics of an ideal diode
16
Solving ideal diode problems(determining if the
diode is on or off)

• Assume diodes are on or off.
• Perform circuit analysis, find I V of each
  diode.
• Compare I V of each diode with assumption.
• Repeat until assumption is true.

17
Prob. 3.10(b)
Is the diode on or off?
Assume diode on. 15V (10K)I1 (10K)(I1-
I2) 15 (20K)I1 - (10K)I2
1 0 (10K)(I2- I1) (10K)(I2- I3) 0
-(10K)I1 (20K)I2 - (10K)I3 2 0 (10K)(
I3- I2) (10K)I3 10 -10 -(10K)I2 (20K)I3
3
I3
I1
I2
Put 3 into 2. -5 -(10K)I1 (15K)I2, Put 1
into this equation, solve for I2. I2 0.875mA,
Current through diode is negative! Diode cant be
on.
18
Prob. 3.10(b)
Assume diode off. 15V (10K)I1 (10K)I1 I1
0.75mA I2 0 0 (10K)I3 (10K)I3 10 I3
-0.5mA
V1
V2
I3
I1
I2
Find V1. V1 (10K)I1 7.5V Find V2. V2
-(10K)I3 5V Voltage across diode is V2 - V1
-2.5V, diode is off
19
3.2 Real diodes
Characteristics of a real diode
breakdown
Forward bias
Reverse bias
20
Reverse bias region

• A small current flows when the diode is
• reversed bias, IS
• IS is called the saturation or leakage current
• IS ? 1nA
• -VZ is the reverse voltage at which the diode
• breaks down.
• VZ is the Zener voltage in a Zener diode
• (controlled breakdown).
• Otherwise, VZ is the peak inverse voltage (PIV)

IS
21
Forward bias region

• For Silicon diodes, very little current
• flows until V ? 0.5V
• At V ? 0.7V, the diode characteristics are
• nearly vertical
• In the vicinity of V ? 0.7V, a wide range of
• current may flow.
• The forward voltage drop of a diode is often
• assumed to be V 0.7V

• Diodes made of different materials have different
  voltage drops V ? 0.2V - 2.4V
• Almost all diodes are made of Silicon, LEDs are
  not and have V ? 1.4V - 2.4V

22
3.4 Analysis of diode circuits(Simplified diode
models) p. 159-162

• Ideal diode
• Constant-voltage drop model
• Constant-voltage drop model with resistor
• All use assumptions because actual diode
  characteristics
• are too difficult to use in circuit analysis

23
Constant-voltage drop model
I-V characteristics

• A straight line is used to represent the
  fast-rising characteristics.
• Resistance of diode when slope is vertical is
  zero.

24
Constant-voltage drop model
I-V characteristics and equivalent circuit

0.7V
-
0.7V
25
Constant-voltage drop with resistor model
I-V characteristics

• A straight line with a slope is used to represent
  the fast-rising characteristics.
• Resistance of diode is 1/slope.

26
Constant-voltage drop with resistor model
I-V characteristics and equivalent circuit

0.7V
? 50W
-
0.7V
27
Prob. 3.9(b) (using constant voltage-drop model)
Are the diodes on or off?
Assume both diodes are on. 10V (10K)I1
0.7 I1 9.3V/10K I1 0.93mA 0 -0.7 0.7
(5K)I2 - 10V, I2 2mA Current in D2 I2
2mA, on Current in D1 I1 - I2 -1.07mA,
off Does not match assumption start over.
I1
I2
28
Prob. 3.9(b) (using constant voltage-drop model)
Are the diodes on or off?
I
Assume D1 off and D2 on. 10V (10K)I 0.7
(5K)I -10V 19.3V (15K)I I 19.3V/15K
1.29mA Current in D2 I 1.29mA, on Voltage
across D1 10V - 10K(1.29mA) -2.9V,
off Matches assumption done.
29
Prob. 3.10(b) (using constant voltage-drop model)
Is the diode on or off?
Assume diode on. 15V (10K)I1 (10K)(I1-
I2) 15 (20K)I1 - (10K)I2
1 0 (10K)(I2- I1) - 0.7 (10K)(I2-
I3) 0.7 -(10K)I1 (20K)I2 - (10K)I3 2 0
(10K)( I3- I2) (10K)I3 10 -10 -(10K)I2
(20K)I3 3
I3
I1
I2
Put 3 into 2. -4.3 -(10K)I1 (15K)I2, Put 1
into this equation, solve for I2. I2 0.91mA,
Current through diode is negative! Diode cant be
on.
30
Prob. 3.10(b) (using constant voltage-drop model)
Assume diode off. 15V (10K)I1 (10K)I1 I1
0.75mA I2 0 0 (10K)I3 (10K)I3 10 I3
-0.5mA
V1
V2
I3
I1
I2
Find V1. V1 (10K)I1 7.5V Find V2. V2
-(10K)I3 5V Voltage across diode is V2 - V1
-2.5V, diode is off
31
3.7 Rectifier circuits
Block diagram of a dc power supply
32
Half-wave rectifier

• Simple
• Wastes half the input

33
Full-wave rectifier
VS gt 0
VS lt 0

• Current goes through load in same direction for
  VS.
• VO is positive for VS.
• Requires center-tap transformer

34
Full-wave rectifier

• Entire input waveform is used

35
Bridge rectifier
VS gt 0 D1, D2 on D3, D4 off VS lt 0 D3, D4 on
D1, D2 off

• A type of full-wave rectifier
• Center-tap not needed
• Most popular rectifier

36
Bridge rectifier

• VO is 2VD less than VS

37
Filter

• Capacitor acts as a filter.
• Vi charges capacitor as Vi increases.
• As Vi decreases, capacitor supplies current to
  load.

38
Filter
Diode off
Diode on

• When the diode is off, the capacitor discharges.
• Vo Vpexp(-t/RC)
• Assuming t ? T, and T1/f
• VP - Vr Vpexp(-1/fRC) half-wave rectifier (t ?
  T)
• VP - Vr Vpexp(-1/2fRC) full-wave rectifier (t ?
  T/2)

39
Exercise 3.30

• Bridge rectifier, given
• VS 12V(rms)
• f 60 Hz
• VD 0.8V
• R 100W

(a) Find C for a ripple voltage of 1V p-p VP -
Vr Vpexp(-1/2fRC) VP - Vr 12?2 - 2(0.8) -
1 14.37 VP 12?2 - 2(0.8) 15.37 Solve for
C. C -T/2R ln (VP - Vr)/VP 1280mF
40
Exercise 3.30

• Bridge rectifier, given
• VS 12V(rms)
• f 60 Hz
• VD 0.8V
• R 100W

(b) What is the DC voltage at the output?
Vo VS - 2VD - 1/2Vr Vo 12?2 - 2(0.8) - 0.5
14.87V (average voltage)
(c) What is the load current?
Iavg Vo/R 14.87/100 0.15A
41
Exercise 3.30
(d) What is the diodes conduction angle? (What
fraction of the cycle is the diode on?)
Diode off
The other 2 Diodes on
2 Diodes on
VP - Vr VP cos(wt) wt cos-1(1-Vr/VP) wt
0.361 radians 20.7 degree Each diode is on for
20.7 degree and off for 360 - 20.7 339.3degree
42
Exercise 3.30
Vo
(e) What is the average diode current?
wt 0.361 radians using w 2pf t8.3320.7/180
0.96mS length of time diodes are on.
VS
ID
Iavg(diodes) 0.15360/20.72.6 A Iavg(each
diode) 1.3 A Imax(each diode) 2.6 A
43
Prob. 3.98
Given a half-wave rectifier with VS 20Vp-p
(triangular) VD 0.7V, R 100W C 100mF, f
1kHz
(a) Find the average DC output voltage
VP - Vr Vpexp(-1/fRC) VP - Vr 8.4V Average
Vo ? (9.3 8.4)/2 8.85
VP VS - VD 9.3V
8.4V
20V
44
Prob. 3.98
(b) How long is the diode on?
VP
Dt
Slope DV/Dt slope 20V/0.5mS DV VP -Vr
9.3 - 8.4 Solve for Dt Dt 0.022mS
20V
Diode on
45
Prob. 3.98
(c) What is the average diode current?
Charge supplied to capacitor Charge released by
capacitor (Idiode)Dt Iavg in load(2p - Dt
) 2p 1.0mS, Iavg in load Voavg/R 8.85/100
0.089A (Idiode)0.022mS
0.089A(1.0mS - 0.022mS )
Idiode ? 4A(average current)
46
Fig. 3.46 A variety of basic limiting
circuits.
47
Fig. 12.31 (a) A three-segment sine-wave shaper.
(b) the input triangular waveform and the
output approximately-sinusoidal waveform.
48
Fig. 12.19 (a) The bistable circuit of Fig.
12.17 with the negative input terminal of the op
amp disconnected from ground and connected to an
input signal vI. (b) The transfer characteristic
of the circuit in (a) for increasing vI. (c) The
transfer characteristic for increasing vI. (d)
the complete transfer characteristics.
49
Fig. 12.24 (a) Connecting a bistable
multivibrator with inverting transfer
characteristic in a feedback loop with an RC
circuit results in a square-wave generator. (b)
The circuit obtained when the bistable
multivibrator is implemented with the circuit of
Fig. 12.19(a). (c) Waveforms at the various
nodes of the circuit in (b). This circuit is
called an astable multivibrator.
50
Fig. 12.25 General scheme for generating
triangular and square waveforms.
51
Fig. 12.31 (a) A three-segment sine-wave shaper.
(b) the input triangular waveform and the
output approximately-sinusoidal waveform.