Download (a)What is the load regulation?

Chapter 6
Voltage Regulators
- Part 1-
POWER SUPPLIES (VOLTAGE REGULATORS)
Fig. 6.1 Block diagram showing parts of a power supply.
Power supply: a group of circuits that convert the standard ac
voltage (120 V, 60 Hz) provided by the wall outlet to constant dc
voltage.
Transformer : a device that step up or step down the ac voltage
provided by the wall outlet to a desired amplitude through the action
of a magnetic field.
Transformers convert AC electricity from one voltage
to another with little loss of power.
Transformers work only with AC and this is one of the
reasons why mains electricity is AC.
Step-up transformers increase voltage.
Step-down transformers reduce voltage.
Rectifier: a diode circuits that converts the ac input voltage
to a pulsating dc voltage.
The pulsating dc voltage is only suitable to be used as a
battery charger, but not good enough to be used as a dc
power supply in a radio, stereo system, computer and so on.
There are two basic types of rectifier circuits:
1. Half-wave rectifier
2. Full-wave rectifier
i) Center-tapped full-wave rectifier
ii) Bridge rectifier
In summary, a full-wave rectified signal has less ripple than a
half-wave rectified signal and is thus better to apply to a filter.
Filter: a circuit used to reduce the fluctuation in the rectified
output voltage or ripple. This provides a steadier dc voltage.
Regulator: a circuit used to produces a constant dc output
voltage by reducing the ripple to negligible amount.
VOLTAGE REGULATION
Two basic categories of voltage regulation are:
 line regulation;
 load regulation.
The purpose of line regulation is to maintain a nearly constant
output voltage when the input voltage varies.
The purpose of load regulation is to maintain a nearly constant
output voltage when the load varies.
Figure 6–2 Line regulation. A change in input (line) voltage does not
significantly affect the output voltage of a regulator (within certain
limits).
Figure 6–3 Load regulation. A change in load current has practically
no effect on the output voltage of a regulator (within certain limits).
Line Regulation
Line regulation can be defined as the percentage change in
the output voltage for a given change in the input voltage.
 VOUT
Line regulation  
 VIN

 x100%

(6-1)
Δ means “a change in”.
Line regulation in %/V can be calculated using the following
formula:

VOUT / VOUT x100%
Line regulation 
VIN
(6-2)
Load Regulation
Load regulation can be defined as the percentage change in
the output voltage from no-load (NL) to full-load (FL).
 VNL  VFL 
 x100%
Load regulation  
 VFL 
where
VNL = the no-load output voltage
VFL = the full-load output voltage
(6-3)
Load Regulation
Sometimes the equivalent Thevenin resistance of a supply
is specified in place of a load regulation specification.
In this case, VOUT can be found by applying the voltage
divider rule:
VOUT


RL
 VNL 

R

R
L 
 OUT
Power Supply
RTH = ROUT
VOUT
In terms of resistances, load
regulation can be expressed as:
 ROUT 
Load regulation  
100%
 RFL 
VTH = VNL
RL
Exercise ;
1) A power supply has an output resistance of 25 mW and a full load
current of 0.50 A to a 10.0 W load.
(a)What is the load regulation?
(b)What is the no load output voltage?
2) The input of certain regulator increase by 3.5V. As a result, the output
voltage increase by 0.042V. The nominal output 20V. Determine the line
regulation in both % and in %/V.
3) If a 5.0V power supply has an output resistance of 80mΩ and specific
maximum output current of 1.0A, what is the load regulation in both %
and in %/mA.
TYPES OF REGULATOR
Two basic types of voltage regulator are the series regulator and
the shunt regulator.
The series regulator is connected in series with the load and the
shunt regulator is connected in parallel with the load.
Figure 6.4
Series and shunt regulators.
Series Regulator Circuit
Figure 6.5 Block diagram of the basic connection of a
series regulator circuit.
The series element controls the amount of the input voltage
that gets to the output.
The output voltage is sampled by a circuit that provides a
feedback voltage to be compared to a reference voltage.
Transistor Series Regulator
Figure 6.6 Pass-transistor regulator.
The transistor series regulator is also called the pass-transistor
regulator because the load current passes through the series
transistor.
Since Q1 is an npn transistor, Vo is found as
VBE  VZ  Vo
(6-4)
Equation ( ) explains the response of the pass-transistor to a change in
load resistance as follows:
- If load resistance increases, load voltage also increases.
- Since the Zener voltage is constant, the increase in Vo
causes VBE to decrease.
- The decrease in VBE reduces conduction through the passtransistor, so load current decreases.
- This offsets the increase in load resistance, and a relatively
constant load voltage is maintained.
Basic op-amp Series Regulator
Control
Element
VREF
Error Detector
Sample
Circuit
Fig. 6.7 Op-amp series regulator circuit.
The resistor R1 and R2 sense a change in the output voltage and provide
a feedback voltage. The error detector compares the feedback voltage
with a Zener diode reference voltage.
Control
Element
VREF
Error Detector
Sample
Circuit
The resulting difference voltage causes the transistor Q1 controls
the conduction to compensate the variation of the output voltage.
The output voltage will be maintained at a constant value of:
 R1 
Vo  1  VREF
 R2 
(6-5)
Exercise ;
The output voltage for the series regulator circuit is:
 R 
VOUT  1  2 VREF
 R3 
(a) What is the output voltage for the series regulator?
(b) If the load current is 200 mA, what is the power
dissipated by Q1?
 R 
(a) VOUT  1  2  VREF
 R3 
 100 k 
 1+
 3.9 V
 47 k 
VIN
18 V
R1
Q1
+
VREF
–
3.9 V
(b) P = VI
= (18 V – 12.2 V)(0.2 A)
4.7 k
VOUT
100 k
R2
47 k
R3
D1
Series Regulator with constant-current limiting
Current limiting prevents excessive load current. Q2
will conduct when the current through R4 develops 0.7
V across Q2’s VBE. This reduces base current to Q1,
limiting the load current.
The current limit is:
I L(max)
0.7 V

R4
For example, a 1.4 
resistor, limits current
to about 0.5 A.
Q1
R4
VOUT
VIN
R1
Q2
+
R2
Current limiter
–
R3
Regulator with Fold-back current limiting
Fold-back current limiting drops the load current well
below the peak during overload conditions. Q2
conducts when VR5 +VBE = VR4 and begins current
limiting. VR5 is found by applying the voltage-divider
rule:
 R5 
VR5  
VOUT
R

R
6 
 5
An overload causes VR5
to drop because VOUT
drops. This means that
less current is needed
to maintain conduction
in Q2 and the load
current drops.
Q1
R4
VOUT
+VIN
R5
R1
+
–
R2
R6
Q2
D1
R3
End
- Part 1-