Page 183 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 171

5.7 Component layout

The layout of the converter is very important as high voltage and current are
switched at high frequencies. This generates a great deal of EMI and voltage spikes,
and care should be taken to minimize the inductance so that the electric noise effect is
also kept to the minimum. Of course the worst-case scenario would be wrong
triggering of a device that may result in short circuit, which will probably cause
destruction of the system.

5.8 Protection ofsemiconductors ± snubber circuits

Proper use of the devices presented so far requires determination of semiconductor
losses since adequate cooling means have to be provided to keep the device tempera-
ture within rated values.
Generally, the semiconductor losses are grouped into three categories (Rockot, 1987):
1. conduction (on-state and dynamic saturation)
2. switching (turn-on and turn-off)
3. off-state.
The relative magnitudes of the conduction and switching losses are greatly dependent
on the type of the converter (i.e. resonant, quasi-resonant, PWM, etc.), the operating
frequency, the type of the load (i.e. linear or non-linear, resistive or inductive), and
certain characteristics of the switch itself (i.e. turn-on time, turn-off time, etc.). Off-
state losses are generally a very small portion of the total losses and are considered
negligible.
Snubber circuits are a typical way to minimize switching losses in converters
(McMurray, 1972; 1980; 1985). In general, snubber circuits are used for the reduction
of switching losses and associated stresses (i.e. protection against high dv/dt and di/
dt) of power semiconductor devices. The turn-on and turn-off circuits are placed in
series/parallel to the power switching devices, respectively. For instance, one major
purpose of using such circuits, especially for BJTs and GTOs is to keep the power
device within its safe operating area (SOA).
Two different types of snubber circuits can be considered as follows:
1. dissipative
2. non-dissipative (low-loss snubber).

The basic difference between them is as follows:
. In dissipative snubber circuits, the energy stored in reactive elements (limiting di/dt
inductor and limiting dv/dt capacitor) is dissipated in resistors and converted into
heat. This type is certainly not the best choice to achieve high switching frequencies
and/or high power levels.
. In non-dissipative (low-loss) snubber circuits, there are no substantial losses due to
resistors. In this case, losses are only caused by non-ideal device properties, such as
conduction and transient switching losses of the switching devices contained in the
snubber circuits.