Static V-I Characteristics of a Thyristor
An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 4.2 (a). The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source Es which provides positive gate current from gate to cathode.
Fig. 4.2 (b) shows static V-I characteristics of a thyristor. Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. Typical SCR V-I characteristic shown in Fig. 4.2 (b) reveals that a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below :
Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, Fig. 4.2 (a), thyristor is reverse biased as shown in Fig. 4.3 (a). Junctions J1 J3 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small leakage current of the order of a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state, of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers a high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.
Note that V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is zero, Fig. 4.2 (b). In case load resistance is present, a large anode current associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at VBR.
Forward Blocking Mode : When anode is positive with respect to the cathode, with gate circuit open, thyristor is said to be forward biased as shown in Fig. 4.3 (b). It is seen from this figure that junctions J1, J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward leakage current, flows as shown in Figs. 4.2 (b) and 4.3 (b). In case the forward voltage is increased, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward breakover voltage VB0. When forward voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as an open switch even in the forward blocking mode.
Forward Conduction Mode : In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen from Fig. 4.2 (b) that this voltage drop increases slightly with an increase in anode current. In conduction mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop vT across the device is due to ohmic drop in the four layers.