Typical Series Regulated Power Supply
Figure 3 shows the basic feedback circuit principle used in Agilent series regulated power supplies. The ac input, after passing through a power transformer, is rectified and filtered. By feedback action, the series regulator alters its voltage drop to keep the regulated dc output voltage constant despite variations in the ac line, the load, or the ambient temperature.
The comparison amplifier continuously monitors the difference between the voltage across the voltage control resistor, RP, and the output voltage. If these voltages are not equal, the comparison amplifier produces an amplified difference (error) signal. This signal is of the magnitude and polarity necessary to change the conduction of the series regulator which, in turn, changes the current through the load resistor until the output voltage equals the voltage (EP) across the voltage control.
Figure 3. Series Regulated Constant Voltage Power Supply
Since the net difference between the two voltage inputs to the comparison amplifier is kept at zero by feedback action, the voltage across resistor RR is also held equal to the reference voltage ER. Thus the programming current IP flowing through RR is constant and equal to ER/RR. The input impedance of the comparison amplifier is very high, so essentially all of the current IP flowing through RR also flows through RP. Because this programming current IP is constant, EP (and hence the output voltage) is variable and directly proportional to RP. Thus, the output voltage becomes zero if RP is reduced to zero ohms.
Of course, not all series regulated supplies are continuously adjustable down to zero volts. Those of the OEM modular type are used in system applications and, thus, their output voltage adjustment is restricted to a narrow range or slot. However, operation of these supplies is virtually identical to that just described.
Pros and Cons of Series Regulated Supplies
Series regulated supplies have many advantages and usually provide the simplest most effective means of satisfying high performance and low power requirements.
In terms of performance, series regulated supplies have very precise regulation properties and respond quickly
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