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| CUSTOMER
SERVICE / Technical guide |
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1.
LINEAR POWER SUPPLIES
A power supply is a device that converts an
ac voltage in a dc voltage supplying a certain
power and normally providing a galvanic isolation
between input and output. The main component
parts of a linear power supply can be seen
in the figure 1.
1.1
Transformer
It provides the necessary isolation between
input and output. It also adapts the voltage
difference between input and output by means
of an appropriate transformation ratio.
1.2
Rectifier and filter
The rectifier converts ac voltage into dc
voltage. The basic operation of this component
can be understood by observing the voltage
and currents in figure 2. In these graphs,
it can be seen that the voltage at the VC1
filter capacitor is not purely direct current,
but rather that it contains a ripple that
cannot be discarded. This is due to the fact
that the capacitor is charged and discharged
at each ac voltage period. Also, you should
notice that the Is current waveform is a pulse
current. The pulse is produced by the charge
of the filter capacitor . When the secondary
voltage drops below the capacitor voltage,
current flow through the secondary is stopped
and then the load current is supplied by the
capacitor. The fact that the transformer current
is a pulse current causes its rms value to
be higher than a sinusoidal wave value and,
therefore, the apparent power at the power
supply input is considerably higher than the
rms power.
1.3
Linear regulation
The linear regulator can be a discrete one
or an integrated circuit, depending on the
requirements of each design, but broadly it
should meet the following requirements:
• To provide a stable output voltage
when input voltage, load or temperature changes
occur.
• To minimize the voltage ripple resulting
from rectification.
• To provide a current limitation that
protects the assembly from possible overloads
or short-circuit conditions at the power supply
output
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