Op amp improves power supply's regulation
Jimmie D Felps, Hewlett-Packard, Colorado Springs, CO -- EDN, June 6, 1996
A typical power supply has a voltage-regulation specification of 2 to 3% for all conditions, including initial setting, line, load, and temperature. You can improve regulation if the power-supply has remote-sensing capability, however. The circuit in Figure 1a adds circuitry to a typical 5V power supply to regulate the voltage between +VL and -VL to 5.00V±0.01V (±0.2%) over a 0-to-70°C temperature range.
The circuit connects the -S (-sense) connection to the load just as you would in a remote-sense configuration. However, unlike the standard configuration, error-amplifier IC1 drives the +S connection. IC1 overrides the power supply's internal regulation loop to maintain better voltage regulation than the power supply is capable of on its own. The nominal voltage between +S and -S is 5V. However, because of the regulation conditions listed above, this voltage can vary ±0.10V, or ±2%. IC1 compares +VL to a 0.1% voltage reference (IC2) and increases or decreases the voltage at +S to satisfy the loop conditions while maintaining the voltage between +VL and -VL to very near 5.00V.
Because there are no resistive voltage dividers at the input of IC1, precision resistors are not necessary. D1 and D2 limit the amount of correction that IC1 can make to avoid possible damage to the power supply. The diodes also prevent the circuit from activating the overvoltage protection that may be internally connected to +S. To prevent IC1 from oscillating, R1 provides isolation from the capacitance that probably exists at +S. R1 also provides current limiting and attenuates the loop gain. IC1 functions as a voltage follower above 15.9 Hz (C1 and R3 form a zero), so the amplifier doesn't affect the power supply's internal loop stability.
An auxiliary power-supply output provides the 12/15V power for IC1 and IC2. If such a supply doesn't exist, you can step up the +V output using a voltage doubler or a simple dc/dc converter, such as the Maxim MAX662.
If the power supply has a margining feature, fewer components are necessary (Figure 1b). A typical implementation of the margining feature is an additional input to the power supply that increases the power supply's output voltage 5% when connected to -S and decreases its output voltage 5% when connected to +S. To obtain this improved regulation circuit with a margining power supply, you should connect +S to +VL and connect R1 to the margining pin instead of +S. D1 and D2 are not necessary. (DI #1873)
