Page 109 - Power Electronics Handbook

102 Electromagnetic compatibility

shown as a switch Sw. In this circuit L1 and are series line impedances,
which could be part of the supply transformer leakage reactance and the
self inductance of cables between this and the load. C1 and C, are stray
capacitances across the load and from the load to earth. C, and C4 are also
stray capacitances to earth and C, and C, appear between the lines. With
Sw open, the capacitors are charged to the peak instantaneous supply
voltage, and when the switch closes the capacitors discharge. This creates
an oscillatory system which produces a wide spectrum of unwanted
frequencies, the magnitude of the EMI produced being determined by the
peak energy stored in the capacitors at the instant the switch closes.
An especially strong source of EM1 is nuclear electromagnetic pulse
(NEMP), which results as a by-product of a nuclear explosion. The
frequency spectrum of NEMP covers a wide range from 10 IrHZ to 10 GHz
and is therefore difficult to protect against. The EMI resulting from NEMP
depends on the type of nuclear explosion: high altitude, air burst or surface
burst. In all cases the explosion releases high-energy gamma radiation
which collides with air molecules releasing free electrons, called Compton
electrons, resulting in a Compton current.
In a high-altitude explosion the Compton electrons spiral around the
earth's magnetic field and produce large current loops. The resulting fields
exceed 50 kV/m and rise times are less than 10 ns. Because of the height of
the explosion the EM1 field has a large coverage, for example an explosion
at a height of 500km in the centre of the USA will cover the whole of
North America. Therefore equipment which must continue to function in
the event of a nuclear explosion must be unaffected by NEW.
Air-burst explosions result in a small vertical dipole current and
relatively weak radiated fields. Surface burst, on the other hand, gives a
large vertical dipole current due to the asymmetry of the air-earth
interface, but has local coverage only.

4.4 Circuit design for EMC

Electromagnetic interference is generated in power circuits due to rapid
transitions and ringing. Oscillations can be damped by introducing
resistance if the source of resonance is isolated. Harmonics generated by
transformers can be minimised by using high-permeability material for the
core, although this would cause the device to operate at high flux densities
and result in large inrush current. Electrostatic shielding is often used in
transformers to minimise coupling between primary and secondary.
Interfering signals can often be bypassed to the case of circuits by
high-frequency capacitors, or metal screens used around circuitry to
protect them from these signals. Twisted signal leads, or leads which are
shielded, can be used to reduce coupling of interference signals.
The collapse of flux in inductive circuits often results in high-voltage
transients, causing interference in connecting circuitry. This is prevented
by providing a path for the inductive current to flow, such as through a
diode, zener diode or voltage-dependent resistor, as in Figure 4.3.
Emission from an electronic circuit and its susceptibility to these signals
is significantly affected by the layout of the circuit, usually on a printed