672 Chapter 19
Internal Output Impedance. The impedance pre-
sented to the equipment receiving the power supply
voltage. In the operation of many devices, it is neces-
sary that the internal power supply impedance be as
near to zero as possible. Since most load devices consist
of both passive and active elements, the current drawn
from the supply consists of an ac component superim-
posed on the dc output of the supply. The ac component
is generally of a nonsinusoidal nature. The output
impedance in ohms over a wide range of frequencies is
used to determine the regulation of output voltage of a
power supply with respect to load variations. Power-
supply output impedance (Zo) may be defined as
(19-2)where,
Eac is the sinusoidal voltage across the power supply
terminals,
Iac is the sinusoidal current flowing through a series
loop consisting of the power supply and load
equipment.
Static Line Regulation. The variation in output volt-
age as the input voltage is varied slowly from rated min-
imum to rated maximum with the load current held at
the nominal value.
Dynamic Load Regulation. The variation in output
when the load change is sudden. The power supply may
be unable to respond instantaneously, and an additional
momentary excursion in the output voltage may result,
subsiding afterward to the static load regulation level.
The positive and negative excursion limits are superim-
posed on the static line and load regulation region. The
positive and negative components are not necessarily
equal or symmetrical. The most stringent rating is for a
change from no load to full load or from full load to no
load.
Dynamic Line Regulation. The momentary additional
excursion of output voltage as a result of a rapid change
in input voltage.
Thermal Regulation. Variations in the output voltage
over the rated operating temperature range due to ambi-
ent temperature variations influencing various compo-
nents of the power supply. This is also known as
thermal drift.
19.2 Power Supplies19.2.1 Simple dc Power SuppliesThe simplest type of dc power supply is a rectifier in
series with the load. As more rectifiers are installed into
the circuit, along with filters, the power supply becomes
more sophisticated. The rectifier in series with the load
supply will always remain simple and have poor regula-
tion and transient response. Table 19-1 shows various
power supplies and their characteristics. To determine
the value of the parameter in the left column, multiply
the factor shown in any of the center columns by the
value in the right column.19.2.2 One-Half Wave SuppliesA one-half wave unit can be connected directly off the
ac mains, Fig. 19-2A, or off the mains through a trans-
former, Fig. 19-2B. Since a rectifier only passes a cur-
rent when the anode is more positive than the cathode,
the output waveform will be one-half of a sine wave,
Fig. 19-2C. The dc voltage output will be 0.45 of the ac
voltage input, and the rectifier current will be the full dc
current; the peak inverse voltage (piv) across the recti-
fier will be 1.414 Vac, and the ripple will be 121%. In
the transformerless power supply, the 115 Vac power
line is connected directly to the rectifier system. This
type of power supply is dangerous to both operating
personnel and to grounded equipment. Also, power sup-
plies of this type will cause hum problems that can only
be solved by the use of an isolating transformer between
the line and power supply.19.2.3 Full-Wave SuppliesThe full-wave supply is normally used in electronic cir-
cuits because it is simple and has a good ripple factor
and voltage output. A full-wave supply is always used
with a transformer. Full-wave supplies may be either a
single-phase center tap design or a full-wave bridge. In
either case, both the positive and negative cycles are
rectified and mixed to produce the dc output.
The center tap configuration, Fig. 19-2D, uses two
rectifiers and a center-tapped transformer. The Vdc is
approximately equal to Vac where Vac is from one side
of the transformer to the center tap. Because the output
is from each half wave, ripple is only 48% of the output
voltage and at twice the input frequency. Each rectifier
carries one-half of the load current. The piv/rectifier is
2.828 Vac.ZoEac
Iac--------=