Load Regulation

______________________________________________________________ Solid State Power Supplies

VOLTAGE REGULATION

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.

LOAD REGULATION

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

E no

load

Percent of Regulation =

x 100

E

full load

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:

- E

E

no load

full load

Percent of Regulation =

x 100

E

full load

-

= 12 10 x 100

10

2

=

10 x 100

= 20%

23 June 2005

TC 9-62

4-29