Agilent Technologies E3633A, E3634A manual Unregulated State, Unwanted Signals

Chapter 7 Tutorial

Output Characteristics

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.

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