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Chapter 7 Tutorial
Output Characteristics
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7
Unregulated State
If the power supply should go into a mode of operation that is neither CV or 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.
Unwanted Signals
An ideal power supply has a perfect dc output with no signals across the terminals or
from the terminals to earth ground. The actual power supply has finite noise across
the output terminals, and a finite current will flow through any impedance connected
from either terminal to earth ground. The first is called
normal mode voltage noise
and the second common mode current noise . Figure 7-6 shows the simplified diagram
of common mode and normal mode sources of noise.
Normal mode voltage noise is in the form of ripple related to the line frequency plus
some random noise. Both of these are of very low value in the power supply. Careful
lead layout and keeping the power supply circuitry away from power devices and
other noise sources will keep these values low.
Common mode noise can be a problem for very sensitive circuitry that is referenced
to earth ground. When a circuit is referenced to earth ground, a low level line—related
ac current will flow from the output terminals to earth ground. Any impedance to
earth ground will create a voltage drop equal to the current flow multiplied by the
impedance. To minimize this effect, the output terminal can be grounded at the output
terminal. Alternately, any impedances to earth ground should have a complementary
impedance to earth ground to cancel any generated voltages. If the circuit is not
referenced to earth ground, common mode power line noise is typically not a problem.
The output will also change due to changes in the load. As the load increases the
output current will cause a small drop in the output voltage of the power supply due
to the output impedance R. Any resistance in the connecting wire will add to this
resistance and increase the voltage drop. Using the largest possible hook up wire will
minimize the voltage drop. Using the remote sense leads at the load will compensate
for lead resistance in the load leads.