Environmental Engineering Reference
In-Depth Information
I
Anode
Gate
Cathode
Figure 4.20
Thyristor
Collector
Drain
Collector
Base
Gate
Gate
Emitter
Source
Emitter
BJT
MOSFET
IGBT
Figure 4.21
Transistor types
Thyristors
In contrast to the diode, the
thyristor
(Figure 4.20) has a control capability through a third
electrode known as the
gate
. Even if the thyristor is positively biased no current will fl ow
through the main anode-cathode circuit until a small
pulse
has been applied to the gate.
Thereafter, the thyrstor acts like the diode so that the current will continue to fl ow through
the anode-cathode circuit until that current reduces to zero. The thyristor itself cannot reduce
or switch off the current. Unlike the diode, however, after the current has reached zero, the
thyristor regains its capability to block current in the forward direction.
Thyristors were the fi rst commercially available controllable power semiconductor devices
and were used in converters of all sizes. They are still the cheapest controllable devices and
are used in very high power converters (hundreds of MVA) and in smaller cost-sensitive
applications.
Gate turn-off thyristors (GTOs) and integrated-gate commutated thyristors (IGCTs) are
devices based on the thyristor principle but capable of turning themselves both on and off.
These derivatives are expensive and are being superseded by advances in transistor
technology.
Transistors
Bipolar junction transistors (BJTs), metal oxide semiconductor fi eld effect transistors
(MOSFETs), insulated gate bipolar transistors (IGBTs) (Figure 4.21) are all signifi cantly
more expensive than simple thyristors, but have the big advantage that they can be turned
off by a control signal. Practical and economic considerations currently favour the use of
MOSFETs for small converters up to about 30 kVA, which includes most PV inverters. IGBTs
are used in converters up to about 10 MVA, which includes wind turbine applications.
