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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 .
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 Agilent E3631A. 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.