Download (mA) V Z Graph of zener current versus voltage Power supply

MALVINO
Electronic
PRINCIPLES
SIXTH EDITION
Special-Purpose
Diodes
Chapter 5
-VZ (Volts)
6
4
2
0
20
40
60
VZ
80
100
120
140
Graph of zener current versus voltage
-IZ (mA)
A zener diode voltage regulator
RS
Power
supply
VS
VZ
RL
This circuit will regulate when the Thevenin voltage
facing the zener diode is greater than the zener voltage.
RL
VTH =
V
RS + RL S
RS
Power
supply
VS
RL
VZ
Assuming the zener is conducting:
VS - VZ
IS =
RS
IL =
VZ
RL
IZ = I S - IL
A zener waveshaping circuit
R
Power
supply
RL
VZ + 0.7 V
-VZ - 0.7 V
Power
Zener diode
supply
RZ
second approximation
VZ
Applying the second approximation
RS
Power
supply
RZ
VS
RL
VZ
The deviation in load voltage from the ideal case:
DVL = IZRZ
The zener regulator also reduces ripple
RS
Power
supply
RZ
VR(in)
RL
VZ
Assuming that both RL and RS >> RZ:
VR(out) @
RZ
RS
VR(in)
Zener diode ratings
• Maximum power = PZ(max) = VZIZ(max)
• Available tolerances: 20, 10 and 5 percent
• Zener resistance, RZT, increases at the
knee of the characteristic curve
• A derating factor such as 3.2 mW per
degree for temperatures above 50 degrees
Celsius is typical.
1 kW
RS
V
-30
-20
-10
0
Q1
VS
-10
mA
Q2
-20
30 V
V
VS = 20
-30
What happens to VZ when VS varies from 20 to 30 volts?
Load lines provide a graphical solution.
RS
Power
supply
VS
Light-emitting diode
IS =
VS - VD
RS
The typical voltage drop for most LEDs is from 1.5 to 2.5 V.
RS
Power
supply
VS
Photodiode
Photodiodes are reverse biased and
conduct in the presence of light.
R1
Signal
source
V1
R2
V2
Power
supply
Optocoupler combines an LED and a photodiode.
High-frequency V
in
rectification
Vin
Vout
Charge storage can
cause poor performance
at high frequencies.
Vout
Tails
Hot-carrier
rectifier
Vin
Vin
Vout
Vout
Schottky diodes eliminate
tails at high frequencies.
Less reverse bias
(more capacitance)
More reverse bias
(less capacitance)
Varactor symbol
A varistor diode can be used to protect
line-operated equipment from voltage surges.
RS
Power
supply
VS
Current-regulator diode
RS can vary over a wide range and the current stays the same.
Other diode types
• Laser: emits coherent light
• Step-recovery: snaps off when reverse
biased
• Back: conducts better when reverse
biased
• Tunnel: has a negative resistance region
Other diode applications
•
•
•
•
Laser: CD players, communications
Step-recovery: Frequency multipliers
Back: Small-signal rectifiers
Tunnel: High-frequency oscillators