The thyristors can be subdivided into eleven types:

  1. Forced Commutated Thyristor (FCT)

  2. Line Commutated Thyristor (LCT)

  3. Gate-Turn-Off Thyristor (GTO)

  4. Reverse-Conducting Thyristor (RCT)

  5. Static Induction Thyristor (SITh)

  6. Gate-Assisted Turn-off Thyristor (GATT)

  7. Light-Activated Silicon Controlled Rectifier (LASCR)

  8. MOS Turn-Off Thyristor (MTO)

  9. Emitter Turn-Off Thyristor (ETO)

  10. Integrated Gate-Commutated Thyristor ((IGCT)

  11. MOS-Controlled Thyristor (MCT)

The thyristors consist of at least four semiconductor layers: two P-type semiconductor layers and two N-type semiconductor layers with P-N-P-N configuration.

After applying Anode-Cathode voltage when the circuit is closed, the first and third semiconductor-junctions are forward biased; but the second semiconductor-junction is reverse biased.

The third semiconductor-layer is called the Gate by which one external pulse is injected to break the obstacle and hence the thyristors are being triggered. The main motto is to take control over the conduction with respect to time hence the profile and the average output can be controlled strategically. The overall efficiency of the thyristors depend upon the Gate-triggering. The phase difference between the Anode-Cathode Voltage and the Conduction that is the Anode current is called the firing angle.

Thyristor characteristic depends upon both the Anode-Cathode Voltage as well as the Anode current. The level of Anode current above which the thyristor maintains its conduction is called the Latching Current. The level of Anode current below which thyristors do not conduct is called the Holding Current. So other than the Anode-Cathode Voltage greater than the Threshold Voltage of semiconductor devices, the thyristors need a certain amount of Anode current for the confirmed conduction.


Ideal Characteristics of a Switch

  1. In the ON-state when the switch is ON, it must have

    1. The ability to carry a high forward current, tending to infinity;

    2. A low ON-state forward voltage drop, tending to zero;

    3. A low ON-state resistance, tending to zero, hence, low power loss.

  2. In the OFF-state when the switch is OFF, it must have

    1. The ability to withstand a high forward or reverse voltage, tending to infinity;

    2. A low OFF-state leakage current, tending to zero;

    3. A high OFF-state resistance,tending to infinity, hence low power loss.

  3. During the turn-ON and turn-OFF process, it must be completely turned ON and OFF instantaneously so that the device can be operated at high frequencies. Thus it must have

    1. A low delay time, tending to zero;

    2. A low rise time, tending to zero;

    3. A low settling time, tending to zero;

    4. A low fall time, tending to zero.

  4. For turn-ON and turn-OFF, it must require

    1. A low gate-drive power, tending to zero;

    2. A low gate-drive voltage, tending to zero;

    3. A low gate-drive current, tending to zero.

  5. Both turn-ON and turn-OFF must be controllable. Thus it must turn ON with a gate signal and must turn OFF with another gate signal.

  6. For turning ON and OFF, it should require a pulse signal only, that is, a small pulse with a very small width, tending to zero.

  7. It must have a high dvdt, tending to infinity. The switch must be capable of handling rapid changes of the voltage across it.

  8. It must have a high didt, tending to infinity. The switch must be capable of handling a rapid rise of the current through it.

  9. It requires very low thermal impedance from the internal junction to the ambient, tending to zero so that it can transmit heat to the ambient easily.

  10. The ability to sustain any fault current for a long time is needed; that is, it must have a high value of heat loss component, tending to infinity.

  11. Negative temperature coefficient on the conducted current is required to result in an equal current sharing when the devices are operated in parallel.

  12. Low price is a very important consideration for reduced cost of the power electronics equipment.

Characteristics of Practical Devices

Pon = 1Ts0tONpdt
Psw = Fs ( 0trpdt + 0tspdt + 0tfpdt )
Pd = Pon + Psw + Pg

Switch Specifications

The characteristics of practical semiconductor devices differ from those of an ideal device. The device manufacturers supply data sheets describing the device parameters that are important to the devices. The most important among these are:

  • Voltage Ratings
    Forward and reverse repetitive peak voltages, and an ON-state forward voltage drop.

  • Current Ratings
    Average, root-mean-square (rms), repetitive peak voltages, non-repetitive peak, and OFF-state leakage currents.

  • Switching Speed or Frequency
    Transition from a fully non-conducting to a fully conducting state (turn-ON) and from a fully conducting state to a fully non-conducting state (turn-OFF) are very important parameters. The switching period Ts and frequency Fs are given by 
    Fs = 1Ts = 1td + tr + ton + ts + tf + toff

  • didt Rating
    The device needs a minimum amount of time before its whole conducting surface comes into play in carrying the full current. If the current rises rapidly, the current flow may be concentrated to a certain area and the device may be damaged. The didt of the current through the device is normally limited by connecting a small inductor in series with the device, known as a series snubber.

  • dvdt Rating
    A semiconductor device has an internal junction capacitance Cj. If the voltage across the switch changes rapidly during turn-ON, turn-OFF and also while connecting the main supply the initial current, the current Cjdvdt flowing through Cj may be too high, thereby causing damage to the device. The dvdt of the voltage across the device is limited by connecting an RC circuit across the device, known as a shunt snubber, or simply snubber.

  • Switching Losses
    During turn-ON the forward current rises before the forward voltage falls, and during turn-OFF the forward voltage rises before the current falls. Simultaneously the existence of high voltage and current in the device represents power losses. Because of their repetitiveness, they represent a significant part of the losses, and often exceed the ON-state conduction losses.

  • Gate Drive Requirements
    The gate-drive voltage and current are important parameters to turn-ON and turn-OFF a device. The gate-driver power and the energy requirement are very important parts of the losses and total equipment cost. With large and long current pulse requirements for turn-ON  and turn-OFF, the gate drive losses can be significant in relation to the total losses and the cost of the driver circuit can be higher than the device itself.

  • Safe Operating Area
    The amount of heat generated in the device is proportional to the power loss, that is, the voltage-current product. For this product to be constant power = voltage X current and equal to the maximum allowable value, the current must be inversely proportional to the voltage. This yields SOA (Safe Operating Area) limit on the allowable steady-state operating points in the voltage-current coordinates.

  • Heat Loss for Fusing
    This parameter is needed for fuse selection. The heat loss of the device must be less than that of the fuse so that the device is protected under fault current conditions.

  • Temperatures
    Maximum allowable junction, case and storage temperatures, usually not lesser than 155 degree Celsius and not  greater than 195 degree Celsius for junction and case, and between - 45 degree Celsius to 170 degree Celsius for storage.

  • Thermal Resistance
    Junction-to-case thermal resistance, Qjc; case-to-sink thermal resistance, Qcs; and sink-ambient thermal resistance, Qsa. Power dissipation must be rapidly removed from the internal wafer through the package and ultimately to the cooling medium. The size of semiconductor power switches is small, not exceeding 148 mm, and the thermal capacity of a bare device is too low to safely remove the heat generated by internal losses. Power devices are generally mounted on heat sinks. Thus, removing the heat cost of equipment.

Device Choices

Although there are many power semiconductor devices, none of them have the ideal characteristics. Continuous improvements are made to the existing devices and new devices are under development. For high power applications from the AC 50 to 60 Hz main supply, the phase control and bidirectional thyristors are the most economical choices. COOLMOSs and IGBTs are the potential replacements for MOSFETs and BJTs, respectively, in low and medium power applications. GTOs and IGCTs are most suited for high-power applications requiring forced commutation. With the continuous advancement in technology, IGBTs are increasingly employed in high-power applications and MCTs may find potential applications that require bidirectional blocking voltages.