Page 116 - Power Quality in Electrical Systems

Chapter
7

Switch Mode Power Supplies

In this chapter, we shall examine high-frequency switching
1
power supplies. Such systems are used extensively in pulse-
width modulated (PWM) inverters and in DC/DC converters.
Typically, switching frequencies are in the few-kiloHertz range
to upwards of a megahertz or higher. Due to the high
switching speed of these power supplies, and also due to the
fast rising and falling edges of voltages and currents, these
converters create significant high-frequency harmonics.

Background

Switch mode power supplies are used extensively in consumer and
industrial equipment such as personal computers and battery chargers.
These systems require fast switching waveforms with high di/dt and
2
dv/dt to ensure good power delivery efficiency. These fast switching
frequencies have a detrimental effect on electromagnetic compatibility
(EMC) because of conducted electromagnetic interference (EMI) to the
3
power line. For instance, high-frequency switching components can be
drawn from the AC power line if sufficient EMI filtering is not present.

1
In this chapter, we shall use the terms “switch-mode power supply,” “high frequency
switching power supply,” and “DC/DC converter” interchangeably. In each case, there is
switching going on at frequencies much higher than the line frequency.
2
In recent years, a trend has developed to require higher efficiency in offline power sup-
plies for many consumer products. Agencies such as the California Energy Commission
and the EPA (with its Energy Star program) are active in this arena.
3
The mechanisms by which high-frequency currents from the switching action couple
to the power line are beyond the scope of this work.
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