Page 118 - Power Quality in Electrical Systems

Switch Mode Power Supplies 101

+
i s C R L V O
–
V s
C BUS
D
(a)

Figure 7.2 An offline flyback converter that draws its power from the line through a full-
wave rectifier. (a) The switch is turned on and off at a frequency that is high compared
to line frequency. The switch is ON for a fraction of a switching cycle of D percent. (b) The
circuit, from On-Semiconductor Application Note AN-1327 [7.2], with the input rectifier
not shown.
[© 2002, On Semiconductor, reprinted with permission]

a DC output voltage, and typical applications include battery chargers,
high voltage power for displays, and other low-cost power supplies.
A full-wave rectifier at the front end of the flyback converter converts the
AC input waveform to a DC waveform, which is filtered by the bus capac-
itor, C BUS . The voltage across the bus capacitor is used to periodically ener-
gize the primary side of the isolation transformer. When the switch turns
off, energy is dumped to the output capacitor. This process repeats itself
over and over at the switching frequency, f . This switching action gen-
sw
erates frequency components at multiples of the switching frequency f to
sw
be drawn from the line through the full-wave rectifier. The bus capacitor
will have some series inductance, and at high frequencies some of the
switch current can bypass the bus capacitor and enter the utility line.
Another type of DC/DC converter is the boost converter (Figure 7.3). The
boost converter is being used more and more in consumer and industrial