Page 38 - Power Quality in Electrical Systems

Power-Quality Standards 21

i s
High Output
V s C Bus frequency voltage(s)
DC/DC
Figure 2.8 A switching power supply that draws high frequency components
from the AC power line.

Through design combinations of switching methods and EMI filtering,
we can reduce but never completely eliminate the high frequencies
injected into the AC line. These harmonics injected into the AC line are
sometimes called “conducted emissions.” Another effect of high-
frequency harmonics injected onto the AC line is that the AC line will
now radiate electromagnetic interference.
Another implementation that generates high-frequency harmonics
on the line is the boost converter power factor correction circuit
(Figure 2.9). This circuit is used in many high-power converters in the
front end. This circuit draws high power factor current from the line,
but the high-frequency switching of the MOSFET generates har-
monics drawn from the line as well. The typical spectrum of the line
current waveform for a DC/DC converter is shown in Figure 2.10.
The Federal Communications Commission (FCC), in their Rules, sub-
part J, sets limits for the conducted emissions allowable on power lines
injected from line-connected equipment [2.10]. Class A covers indus-
trial equipment, and class B covers residential equipment. Shown in
Figure 2.11 is the FCC standard, which sets limits on the conducted
noise injected onto the AC line in the 450 kHz to 30 MHz range. The
Canadian agency CSA has similar limits as the FCC.
Other agencies regulating conducted EMI are the International
Electrotechnical Commission (IEC) and the International Special
Committee on Radio Interference (CISPR) [2.11]. CISPR has no regu-
latory authority but has been adopted by most European countries.

i s

V s
PFC
control

Figure 2.9 Boost converter power factor correction circuit.