Tài liệu INTRODUCTION TO ELECTRONIC ENGINEERING- P4 docx

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Introduction to Electronic Engineering

61

Semiconductor Devices

When it is non-conducting, the thyristor operates on the lower line in the forward blocking state (off

state) with a small leakage current. The thyristor is in off state until no current flows in the gate. The

short firing pulse below the breakover voltage from the gate driver triggers the thyristor. This current

pulse may be of triangle, rectangle, saw-tooth, or trapezoidal shape.

When a thyristor is supplied by ac, the moment of a thyristor firing should be adjusted by shifting the

control pulse relative to the starting point of the positive alternation of anode voltage. This delay is

called a control angle or firing angle . In dc circuits, the use of thyristors is complicated due to their

turning on/off.

After the pulse of the gate driver is given, the thyristor breaks over and switches along the dashed line

to the conducting region. The dashed line in this graph indicates an unstable or temporary condition.

The device can have current and voltage values on this line only briefly as it switches between the two

stable operating regions. Once turned to the on state and the current higher than the holding current,

the thyristor remains in this state after the end of the gate pulse.

When the thyristor is conducting, it is operating on the upper line. The current (up to thousands of

amperes) flows from the anode to the cathode and a small voltage drop (1 to 2 V) exists between them.

If the current tries to decrease to less than the holding border, the device switches back to the non￾conducting region.

Turning off by gate pulse is impossible. Thyristor turns off when the anode current drops under the

value of the holding current.

Input characteristics. Fig. 1.50 illustrates the input characteristics of the thyristor. The curves show

the relation between the gate current and the gate voltage. This relation has a broad coherence area

with a width that depends on the temperature and design properties of the device.

UGC

IG

IG

Fig. 1.50

The gate current has an effect upon the form of the characteristic. The value of the breakover voltage

is the function of the gate current. The more is the gate current the lower is the voltage level required

to switch on the thyristor. Maximum breakover voltage of a thyristor reaches up to thousands of volts.

If the applied voltage exceeds the breakover level, SCR triggers without the gate pulse. This

prohibited mode should be avoided.

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Introduction to Electronic Engineering

62

Semiconductor Devices

Transients. Fig. 1.51 reflects the current and voltage transients of a thyristor when it turns on after the

gate pulse appears and turns off after the current direction changes. During the thyristor opening

process, the anode current will be distributed through the full crystal surface at the speed near 100

m/μs. The current distribution is not homogeneous. The local overloading is possible; therefore, the

growing rate of forward current IF should be limited by hundreds of amperes per microseconds. For

the best control of the thyristor firing process, the gate electrode has a specific spreading shape. The

turn-on process includes three time intervals − the turn-on delay t0, the current rise time t1, and the

current spreading time t2.

The turn-off process of the thyristor is similar to that of a diode. For that, the anode current must be

kept well below the hold current. The decreasing rate of the current depends on the circuit inductance.

The density of excess carriers will diminish by the recombination. Although the current direction

changes, the thyristor remains opened until the current attains its peak negative value IR(max). The

voltage of the device remains small and positive. During the next time intervals of the reverse

recovery time (t4, t5), the SCR will switch off and the reverse voltage UR is stabilized. At the end of the

turn-off process, the excess carriers remain in the medium layers and recombine until the forward

voltage appears.

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