Chapter 3
Special purpose Diodes
Zener Diodes
Electronics I PHYS 230
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Obj ti
Objectives
¾ Describe the characteristics of a zener diode and
analyze its operation
¾Find the equivalent circuit of the zener diode by using:
a)) The ideal diode model
b) The practical diode model
Electronics I PHYS 230
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3.1 Zener Diodes
¾A zener diode is a Si pn Junction
¾Zener Diode is designed for operation in the reverse region
¾The breakdown voltage is controlled carefully by controlling
the doping level
The symbol for
a zener diode
Electronics I PHYS 230
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Zener Diodes
Forward
characteristics
are just like a
normal diode.
¾The normal operation
regions for a rectifier
diode are shown as
shaded area.
¾The normal operation region for a Zener diode
is shown as shaded area.
¾Zener diode is designed to operate in reverse
breakdown
¾The zener diodes breakdown characteristics are
determined by the doping process.
Electronics I PHYS 230
¾Available with VBR = 1.8 V – 200 V
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Breakdown Characteristics
VZT
A Zener diode
operating
p
g in the
breakdown act as a
voltage regulator
because it maintain
nearly a constant
voltage
¾The reverse current IR is called IZ
¾The curve illustrates the minimum (IZK) and maximum (IZM)
ranges
g of current operation
p
that the zener can effectivelyy maintain
it’s voltage.
¾Below the zener knee, the zener breakdown voltage (VZ) remain
almost constant as the current increase.
Electronics I PHYS 230
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Zener Equivalent Circuit
a) Ideal Model:
VZ
IR
Ideal characteristics curve
¾Zener diode is represented by a dc voltage source
¾Zener diode does not produce an emf voltage
¾The diode does not have any internal resistance
Electronics I PHYS 230
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Zener Equivalent Circuit
b) Practical Model
∆VZ
ZZ =
∆I Z
¾Zener diode is represented by a dc voltage source and a
resistance which is called zener impedance (ZZ)
¾A change in the current (∆IZ) produces a small change in the
voltage (∆VZ)
¾By Ohm’s law, the ratio of ∆VZ to ∆IZ is the resistance ZZ
Example 3.1: What is the zener impedance (ZZ)?
Electronics I PHYS 230
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Zener Voltage Calculation (Practical Model):
if I Z < I ZT then,
VZ = VZT − ∆VZ
VZ = VZT + Z ZT × ( I Z − I ZT )
⇒ VZ < VZT
if I Z > I ZT then,
VZ = VZT + ∆VZ
VZ = VZT + Z ZT × ( I Z − I ZT )
⇒ VZ > VZT
Example 3.2: A zener diode has a ZZT of 3.5 Ω. The data sheet
gives VZT = 6.8 V at IZT = 37 mA and IZK = 1 mA. What is the voltage
across the zener terminals when the current is 50 mA? When the
current is 25 mA?
± ∆Vz
Temperature Coefficient (TC)
TC specifies the percent change in VZ for each degree
centigrade change in temperature.
∆VZ = VZ × TC × ∆T
Where:
TC is the temperature coefficient
VZ is the zener voltage at RT (25°C)
∆T is the change in temperature
+ve TC means that VZ increases with increasing temperature or
d
decreases
with
ith decreasing
d
i ttemperature
t
‐ve TC means that VZ increases with decreasing temperature or
decreases with increasing temperature
Example 3.3: TC= 0.05% / °C, T changed from 25 to 60°C, VZ =
8.5V at 25°C. What is VZ at 60°C ?
Electronics I PHYS 230
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Zener Power Dissipation
Zener diodes are specified to operate at a maximum power called
the maximum dc p
power dissipation
p
PD(max).
(
) The p
power dissipation
p
is given by:
PD = VZ I Z
Therefore,
I ZM
PD (max)
≅
VZ
Zener Diode Data Sheet
Review
¾Zener diode normally operates in the reverse region
¾Zener diode is used as a voltage regulator because VZ remains
almost constant as the current increase
¾The zener impedance
p
causes the voltage
g to varyy slightly
g y
∆V Z
ZZ =
∆I Z
¾TC specifies the percent change in VZ for each degree
centigrade change in temperature.
∆VZ = VZ × TC × ∆T
¾The power dissipation in a zener diode is given by
Electronics I PHYS 230
PD = VZ I Z
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