Thyristor

Thyristor

Thyristor is a four-layer semiconductor device of p-n-p-n structure with three junctions namely J1,J2,J3 and J4 as shown in figure below

When the anode is made positive with respect to the cathode, the junction J1 and J3 are forward biased but junction J2 is reversed biased. Therefore, only a small leakage current will flow from anode to cathode. The thyristor is then said to be in forward blocking mode(or off state). When the cathode is made positive with respect to the anode, the junction J2 is forward biased and J1 and J3 are reversed biased. Therefore, only a small leakage current will flow from the cathode to anode. This is the reverse blocking state of the thyristor.

If the anode to cathode voltage Vak is increased to a sufficiently large value, the reverse biased junction J2 will break. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage Vbo. Since, the other junctions J1 and J3 are already forward biased, there will be free movement of carriers across all three junctions resulting in a large forward anode current. The device will then be in conducting state. The voltage drop will be due to the ohmic drop in the four layers and it is small and typically around 1 volt. In the conduction state, the anode current is limited by an external load impedance. The anode current must be more than a value known as latching current in order to maintain the required amount of carrier flow across the junctions, otherwise, the device will revert back to the blocking condition. The latching current(Il) is the minimum anode to cathode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned and gate signal has been removed.

Once a thyristor conducts, it behaves in a similar way to that of a diode and there is no control over the devices. The devices will continue to conduct because there is no depletion layer on the junction J2 due to the free movement of carriers. However, if the forward anode current is reduced below a level known as the holding current(Ih), a depletion layer will be developed across the junction J2 due to reduced number of carriers and the thyristor will be in blocking state. The holding current(Ih) is in the order of milliamp and is less than the latching current(Il). Holding current(Ih) is the minimum anode current to maintain the thyristor in the on-state.

If the cathode to anode voltage is increased to a sufficiently high value, the reversed biased junction J1 and J3 will break. This is known as a reverse breakdown of the thyristor and the corresponding voltage is known as reverse breakdown voltage(Vbr). A thyristor can be turned on by increasing the forward voltage beyond break over voltage, but such a turn on could be destructive. In practice, the thyristor is turned on by applying a gate signal to the gate terminal of it.

Turn ON Turn OFF Time | Switching Characteristics

A forward biased thyristor can be turned on by applying a positive voltage between gate and cathode terminal. As the gate signal, current value is increased, the forward blocking voltage gets decreased(This is depicted in the diagram below). But it takes some transition time to go from forward blocking mode to forward conduction mode. This transition time is called the turn on time of SCR and it can be subdivided into two small intervals as delay time (t_d) and rise time(t_r).

Turn on time is defined as the time taken between 10% of peak gate current and 90% of device steady current.

Here,

Ton = Td + Tr

Where Td = Delay time, Tr = Rise time

The delay time is defined as the time interval between 10% of peak gate current and 10% of device steady current.

Rise time is defined as the time interval between 10% to 90% of device steady current.

Points to care

Gate signal should be removed after the thyristor is turned on. A continuous gate signal will increase power loss in the device.

While a thyristor is reversed biased, the gate signal shouldn't be applied otherwise the device may get damaged due to increased reverse current.

The width of gate pulse must be longer than the time required for device current to rise above holding current. In normal practice, the pulse width is made greater than turn on time of thyristor.

Protection of Thyristor

1) di/dt Protection: If the rate of rising of anode current is very fast compared to the spreading velocity of the charges across the p-n junction, a localized hot spot will get developed and the device might get damaged due to overheating. Therefore, there must be a protection scheme to limit the value of di/dt to a safe value and it is limited by adding an inductor of suitable value in series to the thyristor.(Formulae: di/dt = Vs/Ls).

2) dv/dt Protection:If the value of dv/dt is very high during the turn on process, the insulating material of the device might get damaged. The value of dv/dt can be limited to a safe value by connecting a snubber circuit across the device as shown in the figure below.

The value of dv/dt can be calculated by:

dv/dt = 0.632 * Vak(steady))/ (Rs * Cs)

Types of thyristor

Thyristors are basically classified based upon the physical construction, turn on and turn off behavior.Few of them are :

1. Phase control thyristor
2. Fast Switching thyristor
3. Gate turn-off thyristor
4. Bi-directional thyristor

Phase control Thyristor : This type of thyristor generally operates at line frequency and is turned off by natural commutation(refer nxt topic for more explanation). The turn off time of it is around 50 to 100 micro-seconds. And it is must suitable for low-speed switching application.

Fast Switching Thyristor : This type of thyristor operates at high frequency and is turned of by force commutation(refer nxt topic). They have fast turned off time, generally in the range of 5 to 50 micro-seconds. And it is mostly applied in choppers, inverters,etc.

Gate Turn-off Thyristor(G.T.O) :

A Gate Turn-off Thyristor or GTO is a three terminal, bipolar (current controlled minority carrier) semiconductor switching device. Similar to the conventional thyristor, the terminals are an anode, cathode, and gate as shown in the figure below. As the name indicates, it has gate turn-off capability. These are capable not only to turn ON the main current with a gate drive circuit but also to turn it OFF. A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off. Observe in below figure that the gate has double arrows on it which distinguish the GTO from normal thyristor. This indicates the bidirectional current flow through the gate terminal. The main applications are in variable speed motor drives, high power inverters, and traction.

Bi-directional Thyristor(TRIAC) :

it is a bidirectional device that can pass the current in both forward and reverse biased conditions and hence it is an AC control device. The triac is equivalent to two back to back thyristors connected with one gate terminal as shown in the figure. TRI means that the device consisting of three terminals and AC means that it controls the AC power (i.e conduction takes place on both positive and negative half cycles and TRI + AC = TRIAC). Due to the bidirectional control of AC, triacs are used as AC power controllers, fan controllers, heater controllers, triggering devices for SCRs, three position static switch, light dimmers, etc.

Thyristor Firing Circuit

The power circuit of the thyristor is easily high with compared to gate signal generating circuit. Therefore, there should be isolation between main power circuit and gate signal generating circuit. An isolation transformer can be used for this purpose as shown below.

When a pulse of adequate voltage is applied to the base of transistor Q1, the transistor turns on and a step voltage of ‘Vc’ will appear across the primary winding of isolation transformer. Hence, a positive pulse will induce across the secondary winding of isolation transformer which is applied to the gate of the thyristor to be fired.

When the gate pulse to transistor Q1 is removed, the transistor turns off and a voltage of opposite polarity is induced across the primary winding During this period, the magnetic energy stored in the primary coil decays through the conduction from diode Dm. During this transient decay period, a corresponding negative voltage is induced in a secondary winding of pulse transformer.

In the case of an inductive load in the thyristor power circuit, the beginning of thyristor conduction instant isn’t well defined. In such case, it is necessary to have gate signals in the form of a train of pulses instead of single pulse. Such pulses can be generated by the following circuit.

The positive pulse generated by logic gate signal generator turns on the transistor and during this period, a voltage is also induced in auxiliary winding N3 whose polarity is opposite to the polarity of voltage in N1 and N2. Because of this reverse voltage, the transistor turns off. At the same time, capacitor C1 charges through R1 and turns on Q1 again.

Commutation

Commutation is the technique by the use of which the thyristor operating mode is changed from forward conducting mode to forward blocking mode. There are usually two types of commutation.

1. Natural Commutation
2. Forced Commutation

Natural Commutation:
Generally, if we consider AC supply, the current will flow through the zero crossing line while going from positive peak to negative peak. Thus, a reverse voltage will appear across the device simultaneously, which will turn off the thyristor immediately. This process is called as natural commutation as thyristor is turned off naturally without using any external components or circuit or supply for commutation purpose. This method of commutation is also called as source commutation, or line commutation, or class F commutation.

Forced Commutation

The thyristor can be turned off by reverse biasing the SCR or by using active or passive components. Thyristor current can be reduced to a value below the value of holding current. Since the thyristor is turned off forcibly it is termed as a forced commutation process.One of the types of Forced Commutation is:

Load Commutation: This is also known as self-commutation, or resonant commutation. .In this commutation,the source of commutation voltage is in the load. This load must be an under damped R-L-C supplied with a DC supply so that natural zero is obtained. The commutating components L and C are connected either parallel or series with the load resistance R as shown above with waveforms of SCR current, voltage and capacitor voltage.

The value of load resistance and commutating components are so selected that they form an under damped resonant circuit to produce natural zero. When the thyristor or SCR is triggered, the forward currents starts flowing through it and during this the capacitor is charged up to the value of E. Once the capacitor is fully charged (more than the supply source voltage) the SCR becomes reverse biased and hence the commutation of the device. The capacitor discharges through the load resistance to make ready the circuit for the next cycle of operation. The time for switching OFF the SCR depends on the resonant frequency which further depends on the L and C component

References

Electrical4u. (n.d.). switching-of-on-off-characteristics-of-scr-turn-on-turn-off-time. Retrieved from Electrical4u: www.Electrical4u.com

Electronic hub. (n.d.). Gate-turn-off-thyristor/. Retrieved from Electronicshub: http://www.electronicshub.org

Electronicshub. (n.d.). TRIAC. Retrieved from Electronicshub: http://www.electronicshub.org

Elprocus. (n.d.). Classification-of-thyristor-commutation-methods. Retrieved from Elprocus: https://www.elprocus.com/classification-of-thyristor-commutation-methods/

H.Rashid, M. (2013). THYRISTOR CHARACTERISTICS. India: PEARSON PUBLICATION.