Thyristor: Introduction, Construction, Turn-ON Methods, and VI Characteristics

Introduction to Thyristor

• Thyristor denotes a family of semiconductor devices used for power control in dc and ac systems.
• Silicon controlled rectifier (SCR) is the most widely used device and oldest member of the thyristor family.
• Thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube.
• The name thyristor is derived as: "THYRatron + transISTOR = THYRISTOR".
• Note: The name thyristor came into existence after a formal decision at a conference held by IEC (International Electrotechnical Commission) in 1963. Prior to that, It was called silicon controlled rectifier (SCR).  

Constructional Details of Thyristor

• The threaded portion is for the purpose of tightening the thyristor to the frame (or heat sink) with the help of a nut as shown in Fig. 1a.
• Gate terminal is usually kept near the cathode terminal.
• Heat sink is used in large current application to provide the cooling for thyristor.
• Now SCRs of voltage rating of 10kV and an rms current rating of 3000A are available, which have power handling capacity of 30MW.
• Such a thyristor can be switched ON by a low supply of about 1A and 10W.
• It is so called as SCR, because silicon is used for its construction and its operation as a rectifier can be controlled.

Turn-on Methods of Thyristor

With anode positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques:

1) Forward Voltage Triggering

• In Fig. 2a, When forward voltage is applied between anode and cathode with gate circuit open, Junction J_2 is reverse biased.
• As a result, depletion layer is formed across junction J_2. The width of this layer decreases with an increase in anode-cathode voltage.
• If forward voltage across anode-cathode is gradually increased, a stage comes when the depletion layer across J_2 vanishes.
• At this moment, reverse biased junction J_2 is said to have avalanche breakdown and the voltage at which it occurs is called forward break-over voltage V_BO as shown in Fig. 2b.
• At this voltage, thyristor changes from OFF-state to ON-state characterized by low voltage drop across thyristor with large forward current. This forward current is limited by the load impedance.
• In practice, the transition from OFF-state to ON-state obtained by exceeding V_BO is never employed as it may destroy the device.
• |Forward break-over voltage V_BO| is nearly equal to |Reverse break-down voltage V_BR|
• In practice, V_BR is slightly more than V_BO and both are temperature dependent.
• Therefore, V_BO is taken as the final voltage rating of the device during the design of SCR applications.

2) Gate Triggering

• A thyristor with forward break-over voltage (Say 800V) higher than  normal working voltage (Say 400V) is chosen.
• This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode with gate open.
• However, when turn-ON of thyristor is required, a positive gate voltage between gate and cathode is applied.
• When positive gate current is applied between gate and cathode terminals, a significant number of electrons from n_2 layer cross junction J_3, and diffuse through p_2 layer.
• This leads to reduction of the width of depletion layer around junction J_2.

3) dv/dt Triggering

• Space charges exists in the depletion region near junction J_2 and therefore junction J_2 behave like a capacitance.
• If forward voltage is suddenly applied, a charging current i_c through junction capacitance C_j may turn-ON SCR.
• i_c = dQ/dt = d/dt(C_j*V_a) = C_j*d/dt(V_a) + V_a*d/dt(C_j)
• As C_j tends to constant
• i_c = C_j*d/dt(V_a)
• If d/dt(V_a) is high then i_c will be high.

4) Temperature Triggering

• It is also known as thermal triggering.
• During forward blocking, most of the applied voltage appears across reverse biased junction J_2.
• This voltage across J_2, associated with leakage current, would raise the temperature of this junction.
• With increase in temperature, width of depletion layer also decreases. This further leads to more leakage current and therefore, more junction temperature.
• With cumulative process, at some high temperature, depletion layer of this reverse biased junction vanishes and the device gets turn-ON.

5) Light Triggering

• For light triggered SCR, a recess is made in the inner p layer.
• When this recess is irradiate, free charge carriers (pairs of holes and electrons) are generated just like when gate signal is applied between gate and cathode.
• The pulse of light of appropriate wavelength is guided by optical fibers for irradiation.
• If intensity of this light thrown on the recess exceeds a certain value, forward biased SCR is turned-ON.
• Example: LASCR.

VI Characteristics of a Thyristor

Thyristor has three basic modes of operation as shown in Fig. 2b:

1) Reverse Blocking Mode

• Cathode is made positive with respect to anode with switch S open.
• Thyristor is reverse biased between 0 to V_BR.
• In this mode, thyristor may be treated as an open switch.

2) Forward Blocking Mode

• Anode is positive with respect to the cathode with gate circuit open.
• Thyristor is in forward blocking mode between 0 to V_BO.
• In this mode, thyristor may also be treated as an open switch.

3) Forward Conduction Mode

• Anode to cathode forward voltage is increased with gate circuit open.
• Thyristor is in forward conduction mode between I_L to Ia.