Chapter 2
DIODE part 3
ZENER DIODE
MENJANA MINDA KREATIF DAN INOVATIF
objectives
¾Diode zener –Introduction
¾ Diode zener
ener – I-V
I V characteristic
characteristic, data sheet
¾Diode zener
zener-applications
applications
Simple Regulator ‐ Introduction
Simple Regulator •The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change.
•Used to provide a stable reference voltage for use in power supplies and other equipment
supplies and other equipment
This particular zener circuit will work to maintain 10 V across the load
This particular zener circuit will work to maintain 10 V across the load.
Zener Diode
•A zener diode is much like a normal diode, the exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown
operates in reverse breakdown.
•This typical characteristic curve yp
illustrates the operating range for a zener.
•Its forward characteristics are just like a normal diode
just like a normal diode
Zener Diode
‰The zener diode’s breakdown y
characteristics are determined by the doping process.
‰Low voltage zeners less than 5V ‰
l
l
h
operate in the zener breakdown range Those designed to operate
range. Those designed to operate more than 5 V operate mostly in avalanche breakdown range.
‰Zeners are available with voltage breakdowns of 1.8V‐2.0V
l
b kd
f 1 8V 2 0V
This curve illustrates the minimum and
maximum ranges of current operation that the zener can effectively maintain its voltage
the zener can effectively maintain its voltage.
Data sheet
Data sheet
As with most devices, zener diodes have given characteristics such as temperature coefficients and
characteristics such as temperature coefficients and power ratings that have to be considered. The data sheet provides this information
sheet provides this information.
Data sheet
Data sheet
Application ‐ regulator
Application In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation.
Voltage regulator
Voltage regulator
VZ
I =
RL
L
V PS − V Z
II =
Ri
IZ = II − IL
The characteristics of the Zener diode determines VL.
Zener diode diode ‐ analysis
• Three types of Zener diode circuit
analysis
– Fixed VS and RL
– Fixed VS and variable RL
– Variable VS and fixed RL
Zener diode
diode as a Voltage regulator
as a Voltage regulator
1. Assume zener diode in open circuit
2 Determine the voltage across the zener 2.
Determine the voltage across the zener
diode, VL
3 If V
3.
If VL>Vz, zener diode zener diode ‘ON’
ON hence V
hence VL=Vz
4. If VL<Vz, diode is not a voltage regulator and VL value remain
value remain
Voltage regulator
Voltage regulator
‐ Fixed V
Fixed VS and R
and RL
The applied dc voltage is fixed, as the load resistor.
The analysis :
1.
Determine the state of the Zener diode by removing it from the network
and calculating the voltage across the resulting open circuit.
Voltage regulator
Voltage regulator
2. Substitute the appropriate equivalent circuit and solve for the desired
unknowns.
- For the on state diode, the voltages across parallel elements must be the
same.
VL=VZ
The Zener diode current is determined by KCL:
IZ = IR – IL
The power dissipated by the Zener diode is determined by:
PZ = VZ IZ
- For the off state diode, the equivalent circuit is open-circuit.
Voltage regulator
Voltage regulator
- Fixed VS and Variable RL
Rs
+
Vs
-
Step 1- get the RLmin so that zener is on.
RLVs
VL =
RS + RL
RL min
RSVZ
=
VS − VZ
- if RL ≥ RLmin, zener
diode ‘on’,, so that VL=VZ
Step 2: Calculate the IZ using KCL: 2 condition
1. If RLmin , then ILmax and IZmin because of constant I1
2. If RLmax, then ILmin and IZmax
Voltage regulator
Voltage regulator
I Z min or max = I1constant − I L max or min
VS − VZ
=
I
Where 1
RS
and
I L max
; Izmax taken from data sheet
Izmin = 0, if not given
VZ
=
RL min
or
RL max
VZ
=
I L min
Voltage regulator
Voltage regulator
Load regulation
When RL = ∞ (open cct), load current is zero.All current pass through zener diode.
When RL is connected, current is divided between diode and RL.As RL increases, IL
decrease and IZ increases, and vice versa.
Iz can decrease until Izmin, any lower diode will not able to regulate.At Izmin IL is maximum. Therefore, RL must be minimum.
Total current through R is constant.
Voltage regulator
Voltage regulator
Example on load regulation
What is ILmax and Ilmin to maintain regulation? Wh i i i
What is minimum R
RL?
Given rated value :
1mA < IZ < 50mA
Solution
When IL=0, (R
( L = ∞) , I
) Z =Izmax=IT
IZ max = IT =
VIN − VZ 24 − 12
=
= 25.5mA
R
470
This value is less than 50mA, RL can be removed without disturbing regulation.
ILmax occurs when IZ is minimum
IL max = IT − Iz min = 25.5mA
A − 1mA
A = 24.5mA
A
Minimum value of RL is
RL min =
VZ
I L max
=
12
= 490Ω
24.5mA
Voltage regulator
Voltage regulator
- Variable VS and fixed RL
Rs
+
Vs
-
St 1Step
1 gett the
th VSmin so that
th t zener is
i on.
( R + RS )VZ
RLVs
VS min = L
VL =
RS + RL
RL
if VS ≥ VSmin, zener
diode
d
ode will
w ll ‘on’,
on , so that
VL=VZ
Step 2: Calculate the IZ using KCL: 2 condition
1. if VSmin , then I1min and IZmin because of constant IL
2. if VSmax, thenI1max and IZmax
Voltage regulator
Voltage regulator
I Z min or max = I1min or max − I Lconstant
where
VL
IL =
RL
and I1min
; Izmax= Pzmax/Vz
VS min − VZ
=
RS
or
VS max = I1max RS + VZ
Voltage regulator
Voltage regulator
Line regulation
What is the range of input voltage VIN if the zener diode is to maintain regulation with zener current
regulation with zener current between 0.25mA and 100mA?
Zener current range : Izmin = 0.25mA to Izmax = 100mA
At minimum zener current, I
At
minimum zener current Izmin,
VRmin =IzminR = (0.25m)(220) = 55 mV, But VR = Vin – Vz,;
So Vin(min) = VRmin+Vz = 55m+10 = 10.055V
At maximum zener current, Izmax ,
VRmax =IzmaxR = (100m)(220) = 22 V, But VR = Vin – Vz,;
S Vin(max) = V
So V
(
) VRmax+V
Vz = 22+10 = 32 V
22 10 32 V
Percent regulation
Percent regulation
Defination:
Line regulation
Load regulation
g
VNL – Output voltage at minimum load current (RL = ∞, I.e. IL = 0)
VFL – Output voltage at maximum load current
Zener limiting
Zener diodes can used for limiting just as normal diodes. Recall p
p
in previous chapter studies about limiters. The difference to consider for a zener limiter is its zener breakdown characteristics.