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176 E364xA User’s and Service Guide
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 6-6 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.
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