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Chapter 7 Tutorial
Output Characteristics
151
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 Agilent
E3633A and Agilent E3634A. 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.