Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 110)

Publication GMC-RM001A-EN-P — July 2001
A-8 Noise Control Supplement
Switch-Mode DC Power
Supplies This section describes the advantages and disadvantages of
switch-mode DC power supplies and how to reduce common-mode
noise.
Background Information
Switch-mode power supplies have become very popular due to their
small size and low weight compared to the traditional 50/60 Hz
transformer/rectifier/capacitor construction. They achieve this by
rectifying and smoothing at line voltage then driving the primary of a
transformer at high frequency by switching transistors. The low
voltage secondary is then rectified and smoothed again to produce the
required DC voltage. This high frequency operation allows the use of
a much smaller transformer and smoothing capacitor than those
required at 50/60 Hz.
The disadvantage is that the transistors produce transients similar to
those of a PWM drive (given the same line voltage) but since only DC
output is required, the rectification and smoothing process largely (but
not always completely) contains the noise currents within the power
supply frame.
Differential-mode noise is always well controlled, typically less than
50mV peak to peak. However, potentially troublesome common-mode
noise is common. In addition, plastic cased supplies have a significant
noise radiation field around them, which is easily picked up on
sensitive wiring.
Noise is also propagated back into the AC line.
IMPORTANT
Switch-mode power supplies do not always isolate
noise and may generate common-mode noise on
both AC and DC lines.