Texas Instruments SNVA006B pdf datasheet download

SNVA006B

Introduction to Power

Supplies
Introduction
Virtually every piece of electronic equipment, e.g., computers
and their peripherals, calculators, TV and hi-fi equipment,
and instruments, is powered from a DC power source,
be it a battery or a DC power supply. Most of this equipment
requires not only DC voltage but voltage that is also well
filtered and regulated. Since power supplies are so widely
used in electronic equipment, these devices now comprise a
worldwide segment of the electronics market in excess of $5
billion annually.
There are three types of electronic power conversion devices
in use today which are classified as follows according
to their input and output voltages: 1) DC/DC converter; 2) the
AC/DC power supply; 3) the DC/AC inverter. Each has its
own area of use but this paper will only deal with the first two,
which are the most commonly used.
A power supply converting AC line voltage to DC power must
perform the following functions at high efficiency and at low
cost:
1. Rectification: Convert the incoming AC line voltage to
DC voltage.
2. Voltage transformation: Supply the correct DC voltage
level(s).
3. Filtering: Smooth the ripple of the rectified voltage.
4. Regulation: Control the output voltage level to a constant
value irrespective of line, load and temperature
changes.
5. Isolation: Separate electrically the output from the input
voltage source.
6. Protection: Prevent damaging voltage surges from
reaching the output; provide back-up power or shut
down during a brown-out.
An ideal power supply would be characterized by supplying
a smooth and constant output voltage regardless of variations
in the voltage, load current or ambient temperature at
100% conversion efficiency. Figure 1 compares a real power
supply to this ideal one and further illustrates some power
supply terms.

Linear Power Supplies

Figure 2 illustrates two common linear power supply circuits
in current use. Both circuits employ full-wave rectification to
reduce ripple voltage to capacitor C1. The bridge rectifier
circuit has a simple transformer but current must flow
through two diodes. The center-tapped configuration is preferred
for low output voltages since there is just one diode
voltage drop. For 5V and 12V outputs, Schottky barrier
diodes are commonly used since they have lower voltage
drops than equivalently rated ultra-fast types, which further
increases power conversion efficiency. However, each diode
must withstand twice the reverse voltage that a diode sees in
a full-wave bridge for the same input voltage.

The linear voltage regulator behaves as a variable resistance
between the input and the output as it provides the
precise output voltage. One of the limitations to the efficiency
of this circuit is due to the fact that the linear device must
drop the difference in voltage between the input and output.
Consequently the power dissipated by the linear device is
(Vi–Vo) x Io. While these supplies have many desirable
characteristics, such as simplicity, low output ripple, excellent
line and load regulation, fast response time to load or
line changes and low EMI, they suffer from low efficiency and
occupy large volumes. Switching power supplies are becoming
popular because they offer better solutions to these
problems.

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