Figure 15. Ideal Constant Current Power Supply Output Characteristic
Any one of the four basic constant voltage regulators can also furnish a constant current output provided that its output voltage can be varied down to zero, or at least over the output voltage range required by the load. Besides the regulator, the reference and control circuits required for constant current operation are nearly identical to those used for constant voltage operation. As a result of these many common elements, most constant current configurations are combined with a constant voltage circuit in one Constant Voltage/Constant Current (CV/CC) power supply.
The following paragraphs describe the current feedback loop generally employed in Agilent Technologies CV/CC supplies. This particular approach to constant current, while sufficiently effective for most applications, does have limitations that are caused by its simplified nature. For example, although output capacitor Co minimizes output ripple and improves feedback stability, it also increases the programming response time and decreases the output impedance of the supply; a decrease in output impedance inherently results in degradation of regulation. If precise regulation, rapid programming, and high output impedance are required, improvements on the basic feedback loop are necessary as described under Precision Constant Current Sources later in this section.
Figure 16 illustrates the elements constituting a basic constant current power supply using a linear type regulator. As mentioned previously, many of these elements are identical to those of a constant voltage supply with the only basic difference being in their output sensing techniques. While the constant voltage supply monitors the output voltage across its output terminals, the constant current supply monitors the output current by sensing the voltage drop across a current monitoring resistor (RM) connected in series with the load .
The current feedback loop acts continuously to keep the two inputs to the comparison amplifier equal. These inputs are the voltage drop across the front panel current control and the IR drop developed by load current IL flowing through current monitoring resistor RM. If the two voltages are momentarily unequal, the comparison amplifier output changes the conduction of the series regulator, which in turn, corrects the load current (voltage drop across RM) until the error voltage at the comparison amplifier input is reduced to zero. Momentary unbalances at the comparison amplifier are caused either by adjustment of current control Ra, or by instantaneous output current changes due to external disturbances. Whatever the cause, the regulator action of the feedback loop will increase or decrease the load current until the change is corrected.
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