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Power electronic control in electrical systems 161

valve especially for high and ultra-high power applications. If new devices
become available with higher voltage ratings, the number of the required
switches connected in series to produce the same valve will be reduced. This will
minimize the problems with the voltage sharing of the various switches in series,
will increase the reliability of the overall system and will minimize the problems
with their protection.
2. High current during conduction state. At the moment, when the current ratings of
a given converter must be met, a number of switches are connected in parallel. If
a device is available with high current ratings, the need for parallel connection as
well as the problem of current sharing can be eliminated.
3. Low off-state leakage current. In most cases such a requirement is not significant as
the already available switches exhibit almost negligible off-state leakage current.
4. Low on-state voltage drop across the switch. Even a relatively low voltage drop of a
few volts across the device at significant current flowing through the device can result
in high conduction losses. It is therefore important that such an on-state voltage drop
is as low as possible. This becomes more important when a number of switches are
connected in series to increase the power handling capability of the converter, as the
load current flows through a number of switches generating high conduction losses.
5. Low turn-on and turn-off losses. The ability to switch from on to off state and vice
versa with minimum overlap between the current and voltage waveforms means
that the switching turn-on and off losses are low. When such characteristics are
combined with the low conduction losses, cooling requirements and other aux-
iliary components may be reduced or even eliminated in certain applications
making the converter simpler, smaller, more efficient and simply less expensive.
6. Controlled switching characteristics during turn-on and turn-off. This means that
overcurrent control becomes simpler and easier, the stresses on the device and
other parts of the converter such as load, transformers, etc. can be reduced along
with EMI generation, the need for filters and snubber circuits.
7. Capability to handle its rated voltage and current at the same time without the need
for derating. This will mean snubberless design, i.e. the required extra snubber
components (resistor±inductor±capacitor±diode) to protect the switch and shape
its switching waveforms, can be eliminated. Therefore, if the design does not
require all these components, a simpler configuration, more efficient and more
reliable will result.
8. High dv/dt and di/dt ratings. This will eliminate or reduce the size of the snubber
circuits. Of course EMI generation will limit how fast the current and voltage
waveforms can change but it is desirable that the switch has large dv/dt and di/dt
ratings to eliminate the previously mentioned snubber circuitry.
9. Ability to operate in high temperatures. This will also eliminate the cooling
requirements and simplify the converter's structure.
10. Short-circuit fault behaviour. This will mean that the converter will still be able to
operate when a number of switches are connected in series allowing designs that
have redundancy factors especially in high and ultra-high power applications.
11. Light triggering and low power requirements to control the switch. This will allow
fibre optics to be used to control the switch. In most cases the power to drive the
switch is taken from the power circuit itself and the low power requirements will
minimize the losses of the system.