Most electronic and computer systems require DC voltage for their operation. A typical power supply for such systems converts AC line current to a regulated DC voltage. A "switch mode" type power supply first converts AC to an unregulated DC which is then controllably pulsed to a regulated DC output. For example, conversion of AC to an unregulated DC may be achieved by rectifying the AC and storing the non-regulated DC voltage in bulk capacitors. Current from the bulk capacitors is then periodically switched or pulsed across a primary winding of a transformer. The pulsating DC waveform is received by a secondary winding of the transformer, rectified, regulated, and stored in capacitors which provide the primary DC output voltage of the power supply.
"Regulation" of the DC output voltage refers to limiting changes or swings in the output voltage level. This is particularly important in many applications where small deviations from the design voltage level can affect the operation of the equipment being supported by such power supply. A voltage regulator which has been used successfully with switch mode power supplies has an error amplifier and magnetic amplifier (mag amp). The error amplifier monitors or tracks the output current and provides an error signal when the output deviates from the design voltage level. A mag amp uses one or more saturable reactors either alone or in combination with other components to control power gain. A saturable reactor is a coil with a magnetic core whose reactance may be controlled by applying a magnetic biasing current to control the blocking capability of the core. Typically, the magnetic biasing current is provided to the mag amp through a transistor which is controlled by the error signal. As the blocking capability of the core changes, the pulse width of current pulses passing therethrough will be changed.
For example, an increase in the output voltage increases the error signal from the error amplifier. The error signal holds the transistor on, increasing the magnetic biasing current to the saturable reactor. This increases the blocking capability of the saturable reactor which decreases the pulse width of the voltage pulse passing therethrough. Decreases in the pulse width lower the output voltage.
The reliability of switch mode power supplies is high. However, in the event of loss of AC power it is desirable to have a back-up battery which can be used to supply the required DC voltage. At a minimum this provides adequate time for the supported system to shut down. If the back-up battery has sufficient reserve, it may even provide enough power for continued operation of the system until AC power is restored. Such back-up batteries generally provide an unregulated output. For systems which provide a regulated output, a linear voltage regulator is employed. The use of two voltage regulators with separate error amplifiers, a mag amp post regulator for AC and a linear series pass regulator for DC operation, results in slight differences in the output voltage between the two modes of operation. Furthermore, linear voltage regulators used with the battery have some associated inefficiencies which result in excessive power dissipation or losses. Such losses can reduce the amount of time the back-up battery can maintain or hold up the output voltage.