chap6_voltageregulators_part1
DESCRIPTION
important in power suppliesTRANSCRIPT
Chapter 6Voltage Regulators
- Part 1-
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).
Line Regulation
Line regulation can be defined as the percentage change in the output voltage for a given change in the input voltage.
Line Regulation
Line regulation in %/V can be calculated using the following formula:
Δ means “a change in”.
%100xV
VregulationLine
IN
OUT
IN
OUTOUT
V
xVVregulationLine
%100/
(6-1)
(6-2)
Figure 6–3 Load regulation. A change in load current has practically no effect on the output voltage of a regulator (within certain limits).
Load Regulation
Load regulation can be defined as the percentage change in the output voltage from no-load (NL) to full-load (FL).
where
VNL = the no-load output voltage VFL = the full-load output voltage
Load Regulation
%100xV
VVregulationLoad
FL
FLNL
(6-3)
Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification.
RTH = ROUT
RLVTH = VNL
VOUT
Power Supply
In this case, VOUT can be found by applying the voltage divider rule:
LOUT NL
OUT L
RV V
R R
In terms of resistances, load regulation can be expressed as:
OUT
FL
Load regulation 100%R
R
Load Regulation
TYPES OF REGULATOR
The fundamental classes of voltage regulators are linear regulators and switching regulators.
Two basic types of linear 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.
Basic Linear Series Regulators
Series Regulator Circuit
Figure 6.5 Block diagram of the basic connection of a series regulator circuit.
• The control element is a pass transistor in series with the load between the input and output.
• The error detector compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.
Basic Op-amp Series Regulator
Figure 6.6 Basic op-amp series regulator.
• The resistor R2 and R3 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.
• 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.
REFOUT VR
RV
3
21 (6-5)
The closed-loop voltage gain is:
3
21R
RACL
Determine the output voltage for the regulator in Figure below.
EXAMPLE
Answer: VOUT = 10.2 V
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.
VIN VOUT
R1
Q1
R3
R2
Q2
Current limiter
R4
+
–
The current limit is:
L(max)4
0.7 VI
R
For example, a 1.4 Wresistor, limits current to about 0.5 A.
Series Regulator with constant-current limiting
5R5 OUT
5 6
RV V
R R
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.
+VIN VOUT
R1
Q1
+
–Q2
R4
D1
R3
R2
R6
R5
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:
Regulator with Fold-back current limiting