Page 92 - Power Electronics Handbook

Isolating components 85

same source as the main power semiconductor. However, now the output
stage will see the full load voltage when the power triac is off, so it must be
rated for this value. Many optical couplers have a zero crossing detector
built into the package, enabling the power triac to be switched at the zero
crossing points of the a.c. mains, so minimising the generation of radio
frequency interference. The circuit of Figure 3.1qc) shows two
series-connected thyristors which are simultaneously fired. The blocking
voltage of the optical coupler output stages is reduced by the use of a
voltage-dividing resistor chain across the power thyriston, although now
these resistors provide a leakage path for the load current, which bypasses
the main power devices.

3.4.2 Isohting tram&-
Many different types of transformers are used in power circuits, such as
mains transformers used for d.c. power supplies, current transformers,
autotransformers and pulse transformers. Pulse transformers provide the
isolation between the low-power control circuit and the power semi-
conductor, and these are considered in this section, although the parameters
of all transformers are very similar.
The peak secondary voltage of a transformer will change between the
no-load and full-load currents, due to voltage drops in the secondary and
primary windings, even if the primary voltage is kept constant. This is
called the regulation of the transformer, and is defined as the change
measured as a percentage of the full-load voltage. The efficiency of the
transformer is an indication of how well it converts the input power into
output power, the difference between the two being dissipated as
transformer losses, generating heat.
Transformers need to withstand high voltages between the secondary
and primary. These voltages result in corona, dielectric failure, surface
creepage and flashover between points. Corona, being partial discharge
within the transformer, can destroy the insulation and also generates radio
frequency interference, which affects adjacent equipment and circuits. It
increases with the magnitude of the applied voltage or frequency.
Flashover is arcing between parts of the transformer and creepage is
flashover across the surface of the insulation, both of which can result in
high voltages in secondary circuits. The dielectric strength of the insulation
between the primary and secondary windings is usually measured as a
maximum withstanding voltage per unit thickness of insulation. Solids have
a higher dielectric strength than liquids and gases. In a transformer, the
presence of gas adjacent to a solid insulator presents a weakness in which
corona can be generated, and this limits the maximum voltage which can
be applied across the insulation system. It is therefore important to avoid
air gaps in series with the insulation. When several insulating materials are
used in series, the stress in each is inversely proportional to its dielectric
constant. Therefore the insulation with the lowest constant has the highest
stress, and this is usually air or gas. When a direct voltage is applied to the
material the voltage drop is mainly due to its resistivity, so the material
with the highest resistivity has the highest stress.
Transformers can be shielded electrostatically and electromagnetically.