6 Tutorial176 E364xA User’s and Service GuideUnregulated stateIf 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. Theunregulated condition may be the result of the ac line voltage below thespecifications. The unregulated condition may occur momentarily. For examplewhen the output is programmed for a large voltage step; the output capacitor or alarge capacitive load will charge up at the current limit setting. During the rampup of the output voltage the power supply will be in the unregulated mode. Duringthe transition from CV to CC as when the output is shorted, the unregulated statemay occur briefly during the transition.Unwanted signalsAn ideal power supply has a perfect dc output with no signals across the terminalsor from the terminals to earth ground. The actual power supply has finite noiseacross the output terminals, and a finite current will flow through any impedanceconnected from either terminal to earth ground. The first is called normal modevoltage noise and the second common mode current noise. Figure 6-6 shows thesimplified diagram of common mode and normal mode sources of noise.Normal mode voltage noise is in the form of ripple related to the line frequencyplus 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 powerdevices and other noise sources will keep these values low.Common mode noise can be a problem for very sensitive circuitry that isreferenced to earth ground. When a circuit is referenced to earth ground, a lowlevel 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 currentflow multiplied by the impedance. To minimize this effect, the output terminal canbe grounded at the output terminal. Alternately, any impedances to earth groundshould have a complementary impedance to earth ground to cancel anygenerated voltages. If the circuit is not referenced to earth ground, common modepower line noise is typically not a problem.
