Diode - UWO二极管-西安大略大学共40页文档

0 = (10K)( I3- I2) + (10K)I3 + 10
-10 = -(10K)I2 + (20K)I3
3
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 can’t be on.
22
Constant-voltage drop model
I-V characteristics and equivalent circuit
+
-
0.7V
0.7V
23
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.
Find I and V (Start with largest voltage) Assume D1 on, then D2 will be off, and D3 will be off
+
V = 3V, and I = 3V/1KW = 3mA.
V
Check assumption,
VD1 = 0, on
24
Constant-voltage drop with resistor model
I-V characteristics and equivalent circuit
0.7V
+ 0.7V 50W
-
25
Prob. 3.9(b) (using constant voltage-drop model)
16
Prob. 3.10(b)
V1
V2
I3 I1
I2
Assume diode off.
15V = (10K)I1 + (10K)I1 I1 = 0.75mA
I2 = 0
0 = (10K)I3 + (10K)I3 + 10 I3 = -0.5mA
Find V1. V1 = (10K)I1 = 7.5V Find V2. V2 = -(10K)I3 = 5V Voltage across diode is V2 - V1 = -2.5V, diode is off
1
I-V characteristics of an ideal diode
2
Ideal diode operation
on off on off
3
Ideal diode operation
diode on diode off
4
Ideal diode operation
Vin = 24 sinwt 24
12 Vout
on
off
30
on
off
Diode conducts when 24 sinwt = 12 sinwt = 12/24 wt = 30
5
Exercise 3.4(a)
I
5V
2.5KW
+
V
Find I and V
-
Assume diode is on.
I
5V
2.5KW
V = 0, I = 5V/ 2.5KW I = 2mA, implies diode is on. Correct assumption
15
I1 I2
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
I3
0 = (10K)(I2- I1) + (10K)(I2- I3)
0 = -(10K)I1 + (20K)I2 - (10K)I3 2
6
Exercise 3.4(b)
I
5V
2.5KW
5V
2.5KW
+
V
Find I and V
-
Assume diode is off.
VD = - 5, ID = 0
implies diode is off.
Correct assumption
V = 5, ID = 0
7
+3 +2 +1
I
Exercise 3.4(e)
Matches assumption; done.
13
I-V characteristics of an ideal diode
14
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
3.2 Real diodes
Characteristics of a real diode
breakdown
Reverse bias
Forward bias
18
Reverse bias region
•A small current flows when the diode is reversed bias, IS •IS is called the saturation or leakage current IS •IS 1nA
12
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
•-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)
11
I1 I2
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.
Chap. 3 Diodes
•Simplest semiconductor device •Nonlinear •Used in power supplies •Voltage limiting circuits
0
3.1 Ideal Diodes
Forward bias (on)
Reverse bias (off)
26
Prob. 3.9(b) (using constant voltage-drop model)
Are the diodes on or off?
I
Assume D1 off and D2 on.
Hale Waihona Puke 10V = (10K)I + 0.7 + (5K)I -10V 19.3V = (15K)I I = 19.3V/15K = 1.29mA
21
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.
•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
20
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
-
VD2 = -1, off
VD3 = -2, off
Correct assumption (old-style OR gate)
8
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.
10
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.
19
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.
I1 I2
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.
9
Typical use of Zener diode
•The Zener diode will not usually conduct, it needs Vs > 12.5V to break down •Assume Vs fluctuates or is noisy •If Vs exceeds 12.5V, the diode will conduct, protecting the load