An ideal power supply maintains a constant voltage at its output terminals, no matter what current is drawn from it. The output voltage of a practical power supply changes with load current, generally dropping as load current increases. Power-supply specifications include a full-load current (IFL) rating, which is the maximum current that can be drawn from the supply. The terminal voltage when full-load current is drawn is called the full-load voltage (VFL). The no-load voltage (VNI.) is the terminal voltage when zero current is drawn from the supply, that is, the open-circuit terminal voltage. Figure 17-24 illustrates these terms.
One measure of power-supply performance, in terms of how well the power supply is able to maintain a constant voltage between no-load and full-load conditions, is called its percent voltage regulation:
More precisely, equation 17-31 deflnes’the percent output, or load, voltage regulation, since it is based on changes that occur due to changes in load conditions, all other factors (including input voltage) remaining constant. It is clear that the numerator of equation 17:-31 is the total change in output voltage between no-load and ful!-Ioad, and that the ideal supply therefore has zero percent voltage regulation. Figure 17-25 shows the Thevenin equivalent circuit of a power supply. The Thevenin voltage is the no-load voltage VNL, and the Thevenin equivalent resistance
is caJled the output resistance, Ro, of the supply. Many power-supply manufacturers specify output resistance rather than percent voltage regulation. Wc will show that one can be obtained from the other, if the full-load-voltage and full-load-current ratings are known. Let the full-load resistance be designated It is clear that the ideal supply has zero output resistance, corresponding to zero percent voltage regulation. Like the output resistance we have studied in earlier chapters, R; can also be determined as the slope of a plot of load voltage versus load current:
A power supply having output resistance 1.5 0 supplies a full-load current of 500 mA to a 50-0 load.
1. What is the percent voltage regulation of ‘the supply?
2. What is the no-load output voltage of the supply?
Assuming that the transformer and forward-biased-diode esistances negligible, find the percent voltage regulation of the power supply. The full-load current is 2 A at a full-load voltage of 15 V.
This example illustrates that the series resistance used in an RC 1T filter to reduce ripple can seriously degrade voltage regulation. Note that the no-load voltage in this example is, from equation 17-31, VNL = VR(VFL) + VFL = (1.066)15 + 15 :::;31 V, so there is a 16-V change in output yoltage between no-load and fullload This is an example of an unregulated supply, since there is no circuitry designed to correct, or compensate Jor, the effect of load variations. It would be unacceptable for most applications in whieh the load could vary over such a wide range.
Percent line regulation is another measure of the ability of a power supply to maintain a constant output voltage. In this case, it is a measure of how sensitive the outpu, is to changes in input, or line, voltage rather than to changes in load. The specification is usually expressed as the percent change in out put voltage that occurs per volt char gc in input voltage, with the load RL assunud constant. For
example, a line regulsion of 1%/V means that the output voltage will change 1% for each 1-V change ill input voltage. If the input voltage to a 2(1· V supply having that speciflc=tion WCl e to change by 5 V, then the output could be expected to change oy (5 V)(1 %:V) = 5%, or 0.05(20) :::;1 V.