IC voltage regulators are versatile and
relatively inexpensive and are available with features such as a
programmable output, current-voltage boosting, internal short-circuit
current limiting, thermal shutdown, and floating operation for high
voltage applications-Voltage regulators comprise a class of widely
employed ICs. Regulator IC units contain the circuitry for reference
source, comparator amplifier, control device, and overload protection
all in a single IC. Although the internal construction of IC is somewhat
different from that explained in case of discrete voltage regulator
circuits, the external operation is almost the same.
A power supply can be built using a
transformer connected to the ac supply line to transform the ac voltage
to a desired level, then rectifying the ac voltage, filtering with a
capacitor and RC filter, if desired, and finally regulating the dc
voltage employing and IC regulator. The regulators can be selected for
operation with load currents ranging from hundreds of milli amperes to
tens of amperes, corresponding to power ratings from milliwatts to tens
of watts.
IC regulators are of the following types.
- Fixed output voltage regulators: positive and/or negative output voltage.
- Adjustable output voltage regulators: positive or negative output voltage.
- Switching regulators.
- Special regulators.
Except for the, switching regulators,
all other types of regulators are linear regulators. The impedance of
the active element of the linear regulator may be continuously varied to
provide a desired current to the load, on the other hand, in a
switching regulator a switch is turned on and off at a rate such that
the regulator provides the desired average current in periodic pulses to
the load. The switching regulators are more efficient than the linear
regulators. This is because there is negligible power dissipation in
switching elements in either the on or off state. Nevertheless, in
switching regulators the power dissipation is substantial during the
switching intervals (on to off or off to on). Also, most of the loads
(devices) cannot accept the average current in periodic pulses.
Therefore, most practical voltage regulators are of the linear type.
Voltage regulators, especially the
switching type, are employed as control circuits in pulse width
modulation, push-pull bridges, and series type switch mode supplies.
Almost all power supplies make use of some type of voltage regulator IC
because they are simple to use, reliable, cheaper in cost, and, above
all, available in a variety of voltage and current ratings.
For instance, the LM 309 is a fixed
positive regulator with an output of + 5 V, a maximum load current of 1
A, a load regulation of 15 mV, a source regulation of 4 mV, and a ripple
rejection of 75 db. For the adjustable regulators, LR and SR are given
in percentage rather than millivolts. The table also includes the
drop-out voltage, or the minimum permissible difference between the
input and output voltages. For example an LM 309 has a drop-out voltage
of 2 V. It implies that the input voltage must be at least 2 V greater
than output voltage i.e. input voltage must be at least 7 V, because its
output voltage is 5 V.New devices can supply load current from 100 mA
to more than 5 A. Available in plastic or metal packages, these
three-terminal voltage regulators have become extremely popular because
they are inexpensive and easy to use. Aside from a couple of bypass
capacitors, the new three-terminal IC voltage regulators do not need any
external component.
The integrated three-terminal voltage
regulators typically incorporate many of the functions discussed so far
in a single package, as illustrated in figure.
The error amplifier is used to maintain a
constant voltage through a negative feedback. The internal voltage
reference is tightly controlled during the fabrication of IC. So, the
nominal output voltage of most of the three-terminal voltage regulators
has tolerances that range from ± 6 % to better than ±2%. The series-pass
element is driven by the output of the error amplifier. IF acts as an
automatically controlled variable resistor. This resistance varies as
required for maintaining the output voltage constant. The series-pass
element is typically a BJT that is rated to pass the maximum load
current.
The basic connection of a three-terminal
voltage regulator IC to a load is shown in figure. The fixed voltage
regulator has an unregulated dc input voltage Vin, applied to one input terminal, a regulated output dc voltage, Vout from a second terminal, with the third terminal connected to ground.
No comments:
Post a Comment