Transcript Voltage Regulator.pdf

VOLTAGE REGULATOR
BY
N.S.SRIVATCHAN
ASSISTANT PROFESSOR
EEE DEPT
Voltage Regulator
• Zener diode is a voltage regulator device
because it is able to fix the output voltage
at a constant value (DC voltage).
• RS is to limit the zener current, IZ so that it
is less than the maximum current, IZM (to
avoid the zener diode from broken).
Zener as Regulator
RS
RS
+
+
VS
+
VZ
VS
-
-
-
A simple regulator circuit
+
VZ
-
A regulator circuit with load resistance
RL
Voltage Regulator
• How to determine whether the zener acts as a
regulator or not??
– Use Thevenin Theorem
– See example
• If VTH<VZ, regulation does not occur.
Voltage Regulator….
• Referring to zener I-V charateristic curve, if the
voltage across the zener diode zener is between 0VZ, the zener diode is operating in the reverse
bias region, thus it DOES NOT functioned as a
regulator.
• VTH must at least the same value as VZ (VTH  VZ)
so that the diode CAN function as a voltage
regulator because it is operating in reverse
breakdown region.
Zener I-V Charateristic
Reverse
breakdown
region
Reverse
biased
region
Zener Diode Regulator
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.
Thevenin Equivalent Circuit
Examples of zener as voltage regulator
Cases in Zener
Regulator Circuits
Zener Diode
• Three types of Zener analysis
– Fixed VS and RL
– Fixed VS and variable RL
– Variable VS and fixed RL
Zener Diode
- Fixed VS 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.
Zener Diode
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.
Zener Diode
- Fixed VS and Variable RL
Rs
+
Vs
-
Step 1- get the RLmin so that zener is on.
VL 
RLVs
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
Zener Diode
I Z min or max  I1constant  I L max or min
Where
V  VZ
I1  S
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
Zener Diode
- Variable VS and fixed
RL Rs
+
Vs
-
Step 1- get the VSmin so that zener is on.
VL 
RLVs
RS  RL
VS min 
( RL  RS )VZ
RL
if VS ≥ VSmin, zener diode will ‘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
Zener Diode
I Z min or max  I1min or max  I Lconstant
where
IL 
VL
RL
; Izmax= Pzmax/Vz
VS min  VZ or
and I

1min
RS
VS max  I1max RS  VZ
IC Regulator
• The 78xx (also sometimes known as LM78xx) series of devices is a family
of self-contained fixed linear voltage regulator IC. The 78xx family is a very
popular choice for many electronic circuits which require a regulated
power supply, due to:
i. ease of use and
ii. Low cost.
78xx and 79xx IC
When specifying individual ICs within this family, the xx is
replaced with a two-digit number, which indicates the
output voltage the particular device is designed to provide
(for example, the 7805 has a 5 volt output, while the 7812
produces 12 volts). The 78xx line are positive voltage
regulators, meaning that they are designed to produce a
voltage that is positive relative to a common ground.
There is a related line of 79xx devices which are
complementary negative voltage regulators. 78xx and
79xx ICs can be used in combination to provide both
positive and negative supply voltages in the same circuit, if
necessary.