02-11-2012, 01:14 PM
Understanding Linear Power Supply Operation
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Overview of linear power supply operation
The basic design model for a power supply consists
of a control element in series with a rectifier and
load device. Figure 1 shows a simplified schematic
of a series-regulated supply with the phase-controlled
pre-regulator depicted as a power switch and
the series element depicted as a variable resistor.
The phase-controlled pre-regulator minimizes the
power dissipated at the series element by maintaining
a low and constant voltage drop across the series
element. Feedback control circuits continuously
monitor the output and adjust the series resistance
to maintain a constant output voltage. The variable
resistance series element of the supply shown
in Figure 1 is actually produced by one or more
power transistor operating in the linear (class A)
mode; supplies with this type of regulator are often
called linear power supplies. Linear power supplies
have many advantages. Because they provide sufficient
power with stable regulation and little noise,
they usually are the simplest, most effective
solution for providing bench power.
Output characteristics
An ideal constant-voltage power supply would have
zero output impedance at all frequencies. Thus,
as shown in Figure 3, the voltage would remain
perfectly constant in spite of any changes in output
current demanded by the load.
The ideal constant-current power supply exhibits
infinite output impedance at all frequencies. Thus,
as Figure 4 indicates, the ideal constant-current
power supply would accommodate a load resistance
change by altering its output voltage by just the
amount necessary to maintain its output current
at a constant value.
The output of this power supply can operate in
either constant-voltage (CV) mode or constantcurrent
(CC) mode. Under certain fault conditions,
the power supply cannot operate in either CV or
CC mode and becomes unregulated.
Unregulated state
If the power supply should go into a mode of operation
that is neither CV nor CC, the power supply
is unregulated. In this mode the output is not
predictable. The unregulated condition may be
the result of the AC line voltage below the specifications.
The unregulated condition may occur
momentarily. For example, when the output is
programmed for a large voltage step, the output
capacitor or a large capacitive load will charge up
at the current limit setting. During the ramp up of
the output voltage the power supply will be in
the unregulated mode. During the transition from
CV to CC, as when the output is shorted, the unregulated
state may occur briefly during the transition.
Extending the voltage and current range
A power supply may be able to provide voltages and
current greater than its rated maximum outputs if
the power-line voltage is at or above its nominal
value. Operation can be extended typically up to 3%
over the rated output without damage to the power
supply, but performance cannot be guaranteed to
meet specifications in this region. If the power-line
voltage is maintained in the upper end of the input
voltage range, the power supply will probably operate
within its specifications. A power supply is more
likely to stay within specifications if only one of
the voltage or current outputs is exceeded.