Page 108 - Power Electronics Handbook

Sources of EM1 101
noise resulting from electrical storms. Above loMHz they are primarily as
a d t of cosmic noise and solar radiation. Lightning, which is caused by a
sudden ditwharge of high potential between earth and a cloud, or a cloud to
cloud, can d t in interference to ground or airborne equipment, and
damage ifa direct hit occurs. Perk currents can exceed 5OkA with rate of
ri# in the region of 100 Wp, giving field strengths in nearby conductors
of greater than 200 kV/m. The voltage induced in antenna systems, having
a large physical length, is especially high and these are also prone to direct
StrikCS.
Man-made EMI can be intentional or unintentional. In both cases it is
the variation of the voltage and current which produces EMI, whose
magnitude depends on the value of the cumnt, the length of the
COWhlCtofS, the rate of change of voltage and current, and the physical
position of the conductom relative to each other and any earth plains.
Examples of intentional EM are radar, radio, television, and pagers.
Comprniea near airfields suffer moat from EMI resulting from radar,
d y in the frequency mges 6OOMHz, 1OOOMHz and 10OOOMHz. Field
strengths approach 2OOV/m, with buildings giving only a low level of
shielding.
The most common source of radio interference, and the one which is on
the increase, results from mob& radio. A variety of frequencies are used
but the power levels do not exceed about 50 W. However, the source, such
as police cars patrolling the street, can be very close to the equipment
being interfered with, so the field strmgth can exceed 2OV/m.
Unintentional man-made interference is caused by sources such as
switches, relays, motors, and fhmrescent lights. The inrush current of
tradormcrs during tum-on is another source of interference, as is the
rapid collapse of current in inductive ekments, resulting in transient
voltages. Integrated circuits also 8enerate EM due to thek high operating
speeds and the close proximity of circuit elements on a silicon die, giving
stray capacitive coupling elements.
The generation of EMI in power semiconductor circuits can be
illustrated by the circuit of Figure 4.2, in which the power semiconductor is

plla 4.2 Stray capacitance md inductance in a typical thyristor control system