The cross sectional view of a diac showing all its layers and junction is shown above figure.Diac is a two electrode device,it can conduct in either direction.Terminals are denoted by T1 and T2.word diac stands for ‘diode for ac‘.The four layer are pn pn and pn pn’. Symbol Principle of Operation When T1 is positive with respect to T2,the layers p-n-p-n starts conducting.This happens when voltage of T1 is more than break over voltage VB01.Once the conduction starts,the current through the diac becomes very large and has to be limited by the external resistance in the circuit.When T2 is positive with respect to T1 the layers p-n-p-n’ conducts.This happens when the voltage of T2 exceeds break over voltage VBO2.In both the cases the current during blocking regions read more

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From the above characteristics,latching current for large power GTO is several amperes here 2A as compared to 100-500mA for conventional thyristors of same rating.If gate current is not able to turn on the GTO,it behaves like a high voltage,low gain transistor with considerable anode current.This leads to a noticable power loss under such conditions. read more

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Advantages of GTO over Thyristor Commutation circuit is not needed Fast switching speed More di/dt at turn on Higher efficiency because losses in commutation circuit is eliminated Circuits using GTO  are compact Lesser acoustical and electromagnetic noise due to elimination of choke of commutation Disadvantages of GTO Higher latching and holding current Higher on state voltage drop and power losses Higher gate current Higher gate circuit losses Lower reverse voltage blocking capacity Inspite of all above disadvantages,GTOs are being used in a variety of application such as variable frequency inverter circuits,electric traction and steel mills.Rating available are upto about 6kV and 6kA. read more

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The voltage and current wave forms of a GTO is show in figure below.The positive and negative pulses are shown. When a positive signal is applied,GTO starts conducting.Before initiation of conduction anode current(iA)is zero and anode- cathode voltage VAK is the peak reverse voltage.When conduction starts rises iA to full value and the VAK becomes very small(equal to on state voltage drop which is about 1V or so).when a negative gate signal is applied,the anode current becomes zero and the VAK rises to peak reverse voltage. The total turnoff time is composed of three distinct times,storage time(ts ),fall time(tf) and tail time(tt). Initiation of turn off process starts immediately on the application of negative gate signal.  The time elapsing between application of negative gate read more

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GTO is a special thyristor which can be turned on by a positive gate signal and can be turned off by a neagative signal.Evidently the use of GTO in power electronic circuit eliminates the need of forced commutation circuit because turnoff is achieved by applying a negative circuit. The two transistor analogy of a GTO Two transistor analogy of transistor is shown in figure below When a positive signal is applied,a GTO switches into conduction state like the ordinary thyristor.However in ordinary thyristor the current gains of NPN and PNP transistors are very high so that gate sensitivity for turn on is very high and on state voltage drop is low.However in GTO,the current gai of PNP transistor is low so that turn of fis possible if significant current is drawn from the gate.When a read more

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Bipolar junction transistor have low power losses but have long switching time.(especially at turnoff).MOSFETs have very fast switching characteristics(low turn on and turn off times)but have higher power losses.IGBT combines the advantages of MOSFET and BJT.Thus an IGBT has low switching times as well as low power losses.Its called as IGBT or GEMET or COMFET(conductivity modulated field effect transistor). Configuration The configuration of an IGBT is shown in the figure below In many respects it is similar to a vertical diffused MOSFET(VDMOS).Main difference is the presence of p+ as injecting layer.Next is n+ layer(buffer layer).There is a p-n junction (j1) between these layers and two more junctions(j2 and j3).Thus this IGBT configurations has  a parasitic SCR.Turn  on  of this read more

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The drain characteristics of enhancement type MOSFET is given below.This depicts the variation of drain current(ID)with drain to source voltage(VDS)for different values of gate to source voltage(VGS). The lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almost zero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device can operate in the ohmic,active or cutoff(break down)region.The rising part of curve (fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a resistor,when operated in this region.The drain current is almost constant when the device operates in the active region.when VDS exceeds the rated value,avalanche breakdown occurs and the device is in the breakdown region. Transconductance curve of enhancement MOSFET is read more

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This type of power MOSFET is widely used in digital computers. The construction of enhancement type power MOSFET is shown in figure above .This is an n-channel device.However as seen in the figure,the p-substrate extends right up to the silicon dioxide layer.Thus there is no n-channel between drain and source. The above figure shows the connections.When the gate voltage is zero,drain battery tries to push free electrons from the source to the drain.However the p-substrate has only a few thermally produced free electrons and some electrons due to surface leakage.Therefore the drain current is almost zero.When gate is positive(VGS>0)the gate attracts free electrons into the p-substrate region these free electrons recombine with holes near silicon dioxide layer.When VGS is large read more

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