Page 166 - Power Electronic Control in Electrical Systems

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154 Power semiconductor devices and converter hardware issues

transistors such as the bipolar junction transistor (BJT) and the metal oxide
semiconductor field effect transistor (MOSFET). New hybrid devices such as
the insulated gate bipolar transistor (IGBT), the gate turn-off thyristor (GTO),
the mos-controlled thyristor (MCT), and many others have recently been intro-
duced.
In the next sections the various power semiconductors are presented.

5.2.1 Diode

The circuit symbol of the diode is shown in Figure 5.1(a). The diode as mentioned
earlier belongs to the family of uncontrolled devices that allow the current to flow in
one direction only, that is from the anode (A) to the cathode (K).
The diode's typical steady-state i±v characteristics along with the ideal ones are
depicted in Figure 5.1(b) and (c) respectively. The operation of the device can be
described in a simplified way as follows. When the voltage across the diode from
the anode to the cathode v AK (v AK ˆ v D ) with the polarity shown in Figure 5.1(a)
is positive (forward biased), the diode starts to conduct current whose value is
controlled by the circuit itself. Furthermore, a small voltage drop appears across
the diode. When the voltage across the diode v AK becomes negative, the device stops
conducting with a small current (leakage current) flowing from the cathode to the
anode. It should be noted that the ideal i±v characteristics should not be used as part
of a design procedure but only to explain the operation of a given circuit.
The typical two-layer structure of the diode is shown in Figure 5.1(d). It is a single
junction device with two layers in a silicon wafer, a p-layer lacking electrons and an
n-layer doped with a surplus of electrons.
If a diode is conducting and a reverse bias voltage is applied across, it turns off as
soon as the forward current becomes zero. However, the process of a diode turning
off is not a straightforward one. The extended time required for a diode to turn off
and the phenomena happening during such time are known as diode reverse recovery.
Practically, the time required for a diode to turn off ranges from between a few
nanoseconds to a few microseconds. This depends on the technology used to man-
ufacture the device and on the power ratings of the device.
A basic explanation of the reverse recovery phenomena can be given with the
assistance of Figure 5.2. The reverse recovery current reaches its maximum value
(I RM )after atimeinterval t r from the zero crossing point. By then, sufficient carriers
have been swept out and recombined and therefore current cannot continue to increase.
It starts then to fall during the interval t f . The sum of the two intervals t r and t f is also
known as reverse recovery time t rr . It is also known as the storage time because it is the
required time to sweep out the excess charge from the silicon Q rr due to reverse current.
This phenomenon can create large voltage overshoots and losses in inductive circuits.
The time t rr characterises the diodes as fast recovery, ultra fast recovery and line
frequency diodes. Proper design of the power circuit can influence the behaviour of
the device during reverse recovery and limit the negative effect of the described problem.
The diode has no low power control terminal like other semiconductors and
therefore is less susceptible to electronic noise problems. However, it still must be
protected against overcurrent, overvoltage and transients. For overvoltages, snubber