______________________________________________________________ Solid State Power Supplies
4-91. Ideally, the output of most power supplies should be a constant voltage.
Unfortunately, this is difficult to achieve. The following are the two factors that can cause
the output voltage to change.
The first factor is that the AC line voltage is not constant. The so-called 115 volts
AC can vary from about 105 volts AC to 125 volts AC. This means that the peak
AC voltage to which the rectifier responds can vary from about 148 volts to
177 volts. The AC line voltage alone can be responsible for nearly a 20 percent
change in the DC output voltage.
The second factor that can change the DC output voltage is a change in the load
resistance. In complex electronic equipment, the load can change as circuits are
switched in and out. In a television receiver, the load on a particular power supply
may depend on the brightness of the screen, the control settings, or even the
channel selected. These variations in load resistance tend to change the applied DC
voltage because the power supply has a fixed internal resistance. If the load
resistance decreases, the internal resistance of the power supply drops more
voltage. This causes a decrease in the voltage across the load.
4-92. Many circuits are designed to operate with a particular supply voltage. When the
supply voltage changes, the operation of the circuit may be adversely affected.
Consequently, some types of equipment must have power supplies that produce the same
output voltage regardless of changes in the load resistance or changes in the AC line
voltage. This constant output voltage may be achieved by adding a circuit called the
VOLTAGE REGULATOR at the output of the filter. There are so many different types of
regulators in use today that it is impossible to discuss them all.
4-93. A commonly used FIGURE OF MERIT for a power supply is its PERCENT OF
REGULATION. The figure of merit gives us an indication of how much the output voltage
changes over a range of load resistance values. The percent of regulation aids in the
determination of the type of load regulation needed. Percent of regulation is determined by
the following equation:
- E full
Percent of Regulation =
This equation compares the change in output voltage at the two loading extremes to the
voltage produced at full loading. For example, assume that a power supply produces 12
volts when the load current is zero. If the output voltage drops to 10 volts when full load
current flows, the percent of regulation is computed by using the following formula:
Percent of Regulation =
= 12 10 x 100
10 x 100
23 June 2005