Page 50 - Power Electronics Handbook

Thyristors 43

1.9.2 Thyristor characteristics
Figure 1.25(a) shows the static thyristor characteristics. In the reverse
direction it behaves like a diode, blocking voltage until the reverse voltage
V, is reached, when avalanche breakdown occurs. In the forward direction
the thyristor also blocks voltage until it breaks down into conduction at VI.
The thyristor will go into conduction so long as the current through it is
greater than a value called the latching current. Thereafter its
characteristics are similar to those of a diode, the device remaining in
conduction provided the anode-to-cathode current does not fall below a
value called the holding current. This holding current is lower than the
latching current.
The larger the gate current, the smaller the voltage at which the thyristor
breaks down into forward conduction. When the thyristor is fired its gate
current is made very large, so the device switches rapidly into conduction
at a relatively low anode voltage. Once again the anode current must be
greater than the latching current for it to remain in conduction in the
absence of gate drive, and it must not fall below the holding current or the
device will turn off.
The voltage rating of thyristors can be specified by three terms, the
repetitive blocking voltage, the breakdown voltage or peak repetitive
voltage, and the non-repetitive peak voltage. These three may be
considered in the forward or reverse directions. The repetitive blocking
voltage is that which is normally applied to the device. The breakdown
voltage causes the device to break over, but causes no damage provided
the power through it is limited. The non-repetitive peak voltage can result
in damage to the thyristor if it is applied frequently.
The current rating of the thyristor is determined by the thermal
dissipation which this causes through it. RMS current determines the rating
of the device, but in most applications the average current delivered to the
load is more important. Therefore data sheets usually give average
currents, but since the form factor (RMVAverage) varies with the
conduction angle of the thyristor a series of curves exist, which show the
maximum average current for various conduction angles. This is illustrated
in Figure 1.25(b).
The shorter the time for which the current flows, the greater its possible
overload value, as shown in Figure 1.23(c), and this is called its surge
current rating. If rated load was flowing in the device, then it will already
be hot and can therefore carry a lower surge current.
The surge current capability of the thyristor is also specified as its Pt
rating, and this is primarily used to determine the value of protective fuses,
as explained in Chapter 5.
The gate current in a thyristor causes a gradual spread of the turned-on
area of the silicon chip. Therefore if the anode current is allowed to build
up too rapidly it will result in current crowding through a small area of the
device, causing localised heating and bum-out. The dildt rating of the
thyristor specifies the maximum value of the permitted rate of rise of
current, and construction techniques which can be used to increase these
are described in the next section.