Applications of power electronics deals with the flow of electronic power. In order to achieve better efficiency the semiconductor devices used in power electronic system are operated as switches. One of the semiconductor device used in a power electronic system is thyristor. Few of the other devices used as switches are diodes, bipolar junction transistors(BJTs) , metal oxide semiconductor field effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), gate turn off thyristor(GTOs).
The term thyristor is a general name for a family of semiconductor device. Thyristor families consist of large number of switching devices.
A thyristor is a solid state power semiconductor device. It has four alternating layer and three junctions J1, J2, J3 of N and P type semiconductor material. A thyristor has three terminals. Namely anode, cathode and gate. Thyristor acts as a bistable switch, conducts when its anode is made positive with respect to cathode and gate signal (between gate terminal and cathode terminal) is applied.
Triggering means turning ON of a device from its off state. Turning ON of a thyristor refers to thyristor triggering. Thyristor is turned on by increasing the anode current flowing through it. The increase in anode current can be achieved by many ways.
1. Voltage Thyristor Triggering: – Here the applied forward voltage is gradually increased beyond a pt.known as forward break over voltage VBO and gate is kept open. This method is not preferred because during turn on of thyristor, it is associated with large voltage and large current which results in huge power loss and device may be damaged.
2. Thermal Thyristor Triggering: – If the temperature of the thyristor is high, it results in increase in the electron-hole pairs. Which in turn increase the leakage current α1 & α2 to raise. The regenerative action tends to increase (α1 + α2) to units and the thyristor may be turned on. This type turn on is not preferred as it may result in thermal turn away and hence it is avoided.
3. Light Thyristor Triggering: – These rays of light are allowed to strike the junctions of the thyristor. This results in increase in number of electron-hole pair and thyristor may be turned on. The light activated SCRs (LASER) are triggered by using this method.
4. dv/dt triggering: – If the rate of rise of anode to cathode voltage is high , the charging current through the capacitive junction is high enough to turn on the thyristor. A high value of charging current may destroy the thyristor hence the device must be protected against high dv/dt.
5. Gate triggering: – This method of thyristor triggering is widely employed because of ease C8 control over the thyristor gate triggering of thyristor allows us to turn of the thyristor whenever we wish. Here we apply a gate signal to the thyristor. Forward biased thyristor will turn on when gate signal is applied to it. Once the thyristor starts conducting, the gate loses its control over the device and the thyristor continues to conduct. This is because of regenerative action that takes place within the thyristor when gate signal is applied.
When the thyristor is forward biased, and a gate signal is injected by applying positive gate voltage is applied between gate and cathode terminals, then the thyristor is turned on.
Fig. shows the waveform of anode current after the application of gate signal. ton is the turn on delay time. The turn on delay time is the time interval between the application of gate signal and conduction of thyristor. The turn on delay time ton is defined as the time interval between 10% of steady state gate current 0.1Ig and 90% of steady state thyristor on state current 0.9IT.ton is the sum of delay time td and rise time tr. The delay time td is defined as the time interval between 10% of steady state gate current (0.1 Ig) and 10% of on state thyristor current (0.1IT). The rise time tr is defined as the time taken by the thyristor anode current from 10% of thyristor on state current (0.1IT) to 90% of on state thyristor current (0.9IT).
While designing gate thyristor triggering circuit following points should be kept in mind.
1. When thyristor is turned on the gate signal should be removed immediately. A continuous application of gate signal even after the triggering on and thyristor would increase the power loss in the gate junction.
2. No gate signal should be applied when thyristor is reversed biased; otherwise thyristor
3. The pulse width of the gate signal should le longer than the time required for the anode current to rise to the holding current value IH.
Thyristor can not be turned off by applied negative gate signal. To stop the conduction of the thyristor we have to bring the anode current flowing through the thyristor to a level below holding current level. Holding current may be defined as the minimum anode current required to maintain the thyristor in the on state without gate signal below which the thyristor stops conduction.
If we want to turn on the thyristor, the current flowing through the thyristor must be greater than latching current of the thyristor. Latching current is the minimum anode current required to maintain the thyristor in the on state with at gate signal. Here we should note that even the thyristor anode current falls below latching current (once it is turned on and gate signal is removed) thyristor does not stop conduction. But if it falls below holding current (Latching current is more than holding current) then thyristor turn off.
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