Figure 23. Protection Circuits, Switching Type Supply
Additional Protection - Although not shown on Figure 23, all Agilent switching supplies contain some form of overcurrent protection, usually a current foldback circuit. Also included are remote sensing protection resistors and input protection components for the comparison amplifier.
SPECIAL PURPOSE POWER SUPPLIES
High Voltage Power Supply
Normal series, switching, or SCR regulation techniques may not be suitable for all-semiconductor, short-circuit-proof power supplies with outputs greater than 300 volts. For example, in a series regulated supply, shorting the output terminals would place the rectifier voltage (more than 300 volts) across the series transistors. Utilizing a sufficient number of high voltage series transistors to achieve output of several thousand volts would be too costly and unreliable. Even the preregulator circuit of Figure 7 is not suitable for a higher voltage supply, because a shorted output causes the rectifier capacitor to discharge through the series regulator, and the energy stored in this capacitor is enough to destroy the power transistors in the regulator.
High voltage Agilent supplies utilize a circuit technique that extends the usefulness of series regulating transistors rated for 30 volts to short-circuit-proof power supplies rated for outputs of well over 3000 volts. As shown in Figure 24, the basic technique consists of placing a well-regulated low voltage power supply in series with a less-well-regulated high voltage supply. Notice, however, that the amplified error signal from the voltage comparison amplifier is dependent upon the total output voltage -- not just the output of the low voltage power supply alone. Thus, the well-regulated "piggyback" supply continuously compensates for any ripple, load regulation, or line regulation deficiencies of the main power source by adjusting the voltage across its series regulator to maintain the total output voltage at a constant level.
