Introduction
the power semiconductor devices are backbone of power electronics circuit and controllers. The major type of semiconductor device used as switch in power electronics circuit and controllers are as follows:
- Diodes
- Bipolar junction transistor
- metal oxide semiconductor field effect transistor
- Silicon controlled rectifiers
- Triacs
- Gate turnoff thyristor
- Static induction transistor
- Static induction thyristor
- Mos controlled thyristor
An ideal gas which satisfies the following condition:
- It turns on and turns off in 0 time.
- In the on position, the voltage drop across the switch is zero.
- In the off position, the current through the switch is zero.
- Dissipates zero power.
- When on, it is capable of carrying large current.
Power diode
A diode has two layer of p and and semiconductor materials. This layer forma junction called PN junction. This layer are connected with ohmic contact or terminals, called anode and cathode. The structure of diode and its symbol and shown in figure
when the anode voltage is more positive then the cathode the diode is said to be forward biased, and it conduct current with the small voltage drop.
when the cathode voltage is more positive then the anode, the diode is said to be reverse biased, and it block the current flow.
the arrow on the diode symbol show the direction of conventional current when the diode conducts.
Current voltage characteristics of a diode
in this figure shown the current voltage characteristics of diode. This characteristics can be divided into three regions.
- Forward bias region :- in the forward bias region the diode current ID is very small if the diode voltage VD is less than specific value vt-d which is about 1 volt for silicon diodes. The diode conduct fully if we d is higher than the value of v TD. this voltage is known as the threshold voltage for the cut in voltage of the turn on voltage.
- Reverse biased region :- when the diode is reverse biased, small amount of current called the rivers leakage current flow as the voltage from anode to cathode is increased. It indicates that a diode has very high resistance in the reverse direction. The leakage current is of the order of microamperes aur milliamperes the leakage current increases slowly with the reverse voltage until the avalanche or zener voltage or breakdown voltage is reached.
- Breakdown region :- in the breakdown region the reverse voltage is high usually greater than 1000 volt. if the magnitude of the reverse voltage exceed a specified voltage called the breakdown voltage v b r the reverse current increases rapidly with a small change in reverse voltage beyond vbr.in normal operation, the reverse bias voltage should not reach the breakdown rating. in order to limit the rivers current in the breakdown region a current limiting resistor must be used in series with the diode to prevent its destruction.
Thyristor device
the thyristor controlled rectifier is the generic term and, therefore, a large number of power semiconductor device belong to this family. Some members of this family are silicon controlled rectifier, triac, gate turnoff thyristor, mos controlled thyristor.Silicon controlled rectifier
the silicon controlled rectifier is the most widely used device of the thyristor family. It was invented in 1957. Since then it has been widely used in industry in such applications as static switches, choppers, inverters, cyclone converters, heating, lightning and motor control. Different types of SCR are available with very white voltage and current rating.
An SCR is a four layer pnpn semiconductor device with alternating layer of p and n materials. There are 3 PN junction. The structure of an SCR is shown in figure and its symbol is shown in figure stop it has three terminal: the anode a , the cathode k and the gate.terminal a and k are its power terminal and G control terminal.
Triac
A triac three terminal bidirectional SCR which can conduct current in both the forward and reverse direction full stop it can be considered as to Sc are connected back to back with the common getconnection the symbol of triac and its equivalent circuit are shown in figure..
The current voltage characteristics for triac is shown in figure when terminal mt2 is positive with respect to terminal mt1, the Triac can we turn on by supplying a positive gate signal between gate G and terminal mt1.if terminal mt2 is negative with respect to terminal mt1, it is turned on by applying negative gate signal between gete G n-terminal mt1. Once the triac turns on the gate signal can be removed and just like and share the triac remains on until that triac main current falls below the holding current.
Triacjournalists which more slowly than SCR and are available only at lower voltage and current rating. It slow speed limit the operating frequency to few hundred Hertz.as a result their use is largely restricted to low and medium power application in 50 or 60 hz circuit. Triacare used in application such as light dimmer, heater control, juicer mixer, blenders, vacuum cleaners, speed control of motors and solid state AC relays.
Insulated gate bipolar transistor
the insulated gate bipolar transistor is a hybrid power semiconductor which combine the attributes of the BJT and MOSFET. The insulated gate bipolar transistor combines the low on state voltage drop and high of state voltage characteristics of the BJT with the excellent switching characteristics, simple gate drive and high input impedance of the MOSFET.
In figures on the circuit symbol of an n-channel insulated gate bipolar transistor and its equivalent connection of mosfet and BJT. Insulated gate bipolar transistor has three terminal, the gate, the collector and the emitter E
Gate turnoff thyristor
Gate turnoff thyristor is a member of the thyristor family shown in figure the structure and the circuit symbol of gate turnoff thyristor. It has three terminal the anode cathode and the gate. like and share, the gate turnoff thyristor can be turned on by short duration positive get current pulse. once in the on state the return of thyristor main stay on without any further get current however unlike the SCR, the gate turnoff can we turn off by applying a large negative get cathode voltage there for causing a sufficiently large negative get current to flow. This negative get current it is typically as large as up to one third the and not current being turn off. Both on state and off state operation of the gate turnoff thyristor can we, therefore, controlled by the gate current.
since the gate turnoff thyristor can be turned off by negative gate pulse, no separate forced commutation circuit is required. This result in the improved efficiency of the converter. The cost, size and weight of the converter also reduced.
Power transistor
transistor with high voltage and current rating are known as power transistor. A transistor is a three layer PNP or NPN semiconductor device with two junction. Transistor have two basic type of applications; amplification and switching. in power electronics, where the main objective is the efficiency control of power transistor are invariably operated as switch.
two types of power transistor are extensively used in power electronics circuits;
- Bipolar junction transistor
- Metal oxide semiconductor field effect transistor
Power bipolar junction transistor
in order to handle large power of electronic circuit, NPN power transistor are normally used full stop the structure and symbol of an NPN transistor as shown in figure this type of transistor is called bipolar junction transistor as amplifier transistor. A transistor has three terminal the base of the collector and the emitter.
when a transistor is used as a switch to control power from the source to the load, terminal c and e are connected in series with the main power circuits. terminal b and c are connected to a driving circuit that control the on and off action. a small current through the base emitter junction turn on the collector to emitter paath.
This paath mein carry many times more current than the base emitter junction.
Metal oxide semiconductor field effect transistor
power metal oxide semiconductor field effect transistor is a very fast switching transistor it is a unipolar, majority carrier, non junction, voltage controlled device.
metal oxide semiconductor field effect transistor are available in both the n channel and p channel types. the n channel device are available with higher voltage and current ratings in figure the symbol of an channel MOSFET. It has three terminal the gate, the source and the drain. The gate is isolated by a silicon oxide sio2 layer and, therefore, the gate circuit input impedance is extremely hai. the source as is always at the potential nearest the gate the drain is connected to the load.
when a small positive voltage vgs is applied to the gate the transistor switch on if there is no voltage at the gate, the device turn off. Thus, the gate voltage control the on and off condition.
Static induction transistor(SIH)
Static induction transistor is a high power high frequency device full stop it is essentially a solid state version of vacuum triode. In figure show the symbol of static induction transistor.
static induction transistor is normally on device, and negative gate voltage hold it off.it is faster than MOSFET, but voltage drop is quite higher. the static induction transistor is used for high frequency and higher power application it is a majority carrier device and can be easily parallel like MOSFET. The the turn on and turn off times are very small, of the order full stop the current and the voltage rating of static induction transistor can be up to 300 ampere and 1200 voltage static induction transistor are most suitable for high power, high frequency applications.
Static induction thyristor (SITH)
the static induction thyristor is also called field control thyristor it consists p-i-n diode. The static induction thyristor symbol is shown in figure
Mos control thyristor (MOS)
metal oxide semiconductor field effect transistor control thyristor is a four layer pnpn device full stop in this figure ab show the symbol and equivalent circuit of for the the MCT. The two slightly different symbol for the mct not whether the device is a p-MCT OR n-MCT.the difference between the two arises from the different location of the control terminals.
In an mos control thyristor comma and SCR and two MOSFET are combined into single device. The two MOSFET have the same get terminal, which is the gate of the the mos control thyristor.they also have the same source terminal, which is the anode of the mos control method. The n channel MOSFET q offwhich is connected between the anode and one of its internal layers turn the device off full stop the p channel MOSFET q on, connected between the gate and the anode, turn the device on.
To turn on the MCT,it is forward biased, that is making the anode positive with respect to the cathode and applying and negative voltage between gate and the anode. When on, the voltage drop across the MCT is very small of the order of 1 volt and the anode current is limited only by the load resistance. Once the MCT turn on, it will remain on even if the gate voltage is removed. If the MCT is on, the application of a positive voltage to the gate turn the device off.
In this figure show the current voltage characteristics of the MCT full stop it is seen that the answer it has many of the properties of GTO. Namely a low voltage drop in the own state at relative lehigh current and latching characteristics full stop the main advantage of the MCT over GTO are as follows
- Much simpler drive requirements
- Fastest switching speed
the MCT are presently available in voltage rating upto 1500 volt with current rating of 50 ampere to a few hundred ampere.
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