Diod Thyristor

The structure of a diode thyristor of the n-p-n-p type is shown in Fig. 6.1a. As is seen, it has three p-n junctions, two of them (J_l and J₃) being forward-biased and the third (J₂), which is sandwiched in between, being reverse-biased. The outer p -region is called the anode, and the outer n -region is called the cathode.

Fig. 6.1. Diode thyristor: (a) circuit, (b) structure and (c) its model as a combination of two transistors

Minority carriers are injected across the for­ward-biased junctions J₁ and J₃ into the regions adjacent to the junction J₂ with the result that the resistance of J₂ is brought down.

The current-voltage characteristic of a thyristor, shown in Fig. 6.2, illustrates the events taking place in a thyristor when the applied voltage is raised. At first the current is small and rises slowly-this corresponds to portion O A of the characteristic. In this condition the thyristor may be deemed in the OFF state.

Fig. 6.2. Current-voltage characteristic of a diode thyristor

Around point A, that is, at a certain definite voltage (tens or hundreds of volts), called the forward breakover voltage U_fbo, the two factors produce an equal but opposite effect, and ba­lance each other. Now even a minute increase in the applied voltage gives the second factor an edge over the first, and the resistance of J₂ begins to decrease. Now the thyristor is driven to conduction (turned ON) in a cumulative fashion.

The current rises stepwise (portion AB of the characteristic) because an increase in voltage across J₁ and J₃ brings down the resistance of J₂ and the voltage across it. As a result, the voltages across J₁ and J₃ rise still more, and this leads in turn to a further increase in the current, a reduction in the resistance of J₂, and so on. This chain of events finally terminates in a condition which resembles the saturated state of a transistor-a heavy current at a low voltage (portion BC of the characteristic). The current in this state when the device is conducting (in the ON state) is mainly decided by the load re­sistance R_A connected in series with the device. Owing to the resultant heavy current, nearly all of the supply voltage is dropped across R_A.

Diode thyristors are specified in terms of the absolute maximum forward current I_{f max} (point C in Fig. 6.2) at which a small ON-state voltage, U_ON , still exists across the thyristor. If we reduce the current flowing through the device to below some value called the holding current, I_H (point B), the current will drop suddenly and the voltage will rise as suddenly, indicating that the device has jumped back to the OFF state corresponding to the portion OA of the cha­racteristic (called the forward blocking region ).

Дата добавления: 2015-11-14