Power Transistors-Mosfet

POWER TRANSISTORS:

                              The ability degree diodes together with transistors naturally differ from ordinary transistors together with diodes alone past times the operating voltage together with electrical flow values. The ability transformers are designed inwards such a agency to run across the voltage levels upto kVs together with kAs together with equally good it has less switching times inwards price of nano seconds to few micro seconds.  The ability degree diodes together with transistors direct hold evolved over decades from their indicate degree counterparts to the extent that it tin handgrip fifty-fifty to a greater extent than kilo volts together with currents inwards fifty-fifty to a greater extent than faster switching time.

                            

CHARACTERISTICS CONSIDERED IN SELECTION OF POWER DIODE AND TRANSISTORS:

• voltage rating
• current rating
• switching speeds
• on soil voltage

1.VOLTAGE RATING:

                           The maximum voltage that the device tin operate, beyond which the device "breaks down" and the harm to the device occurs is called equally voltage rating of the device.

2.CURRENT RATING:

                            The maximum electrical flow expressed equally an instantaneous that a device tin send during its on soil is called electrical flow rating.  Beyond this boundary due to the excess estrus produced the device volition live destroyed.

3.SWITCHING SPEED:

                            The speed inside which the device tin brand a transition from its on soil to its off soil is called switching speed.  In other words the fourth dimension taken to brand a device on together with off is called switching speed.  Small switching times associated amongst the fast switching device outcome inwards depression switching losses inwards other words fast switching devices tin live operated at high switching frequencies amongst acceptable ability losses.

4.ON STATE VOLTAGE:

                            The voltage driblet across a device during its on soil piece conducting electrical flow is called on soil voltage.  The smaller the on soil voltage, the smaller volition live the on soil ability loss.

                                  

                              Transistors are controllable switches, which are available inwards several forms for switching vogue ability electronics applications:

• MOSFET(METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTORS}
• IGBT(INSULATED GATE BIPOLAR TRANSISTORS)
• IGCT(INTEGRATED GATE CONTROLLED THYRISTORS)
• GTO(GATE TURN OFF THYRISTORS)
• BJT,SIT,MCT AND SO ON

                                In switching vogue converters 2 types of transistors are primarily used: mosfet are typically used below a few hundred volts at switching frequencies inwards excess of 100 kHz, whereas IGBTs dominate really large voltage, electrical flow together with ability ranges extending to MW levels when the switching frequency are below few tens of KHz.  The next topics supply a clear overview of mosfet together with igbt.

MOSFET:

   

                                The circuit symbol of the n channel mosfet are shown inwards the below diagram

          

                                          It consist of 3 terminals: drain(D),source(S),gate(G).  The forwards electrical flow inwards a mosfet flows from the drain to the rootage lastly when the gate is biased.  Mosfets tin block alone the forwards polarity voltage, that is a positive Vds.  They cannot block alone the forwards polarity voltage due to an intrinsic anti parallel diode, which tin live used effectively inwards most switch vogue converter designs.

                                     Mosfet i-v characteristics are shown inwards the below diagram

                                                                 For gate voltage below Vgs threshold , typically inwards a hit of 2 to four v, a mosfet is completely off, equally shown past times the i-v characteristics together with acts equally an opened upwards switch. Beyond vgs threshold , the drain electrical flow  id through the mosfet depends on the applied gate voltage vgs, equally shown past times the transfer characteristics inwards the below diagram

                                                 To carry id would require a gate voltage of a value atleast equal to vgs(Io) equally shown inwards the figure. Typically gate rootage voltage is maintained at 10v o maintain the mosfet inwards its on phase together with carrying id current. In its on soil a mosfet offers a pocket-size resistor Rds together with the drain electrical flow that flows through it depends on the external circuit inwards which it is connected.  The on soil resistance is the inverse of the gradient of i-v characteristics equally shown inwards figure.

                                                  In applications at voltage below 200volts together with switching frequencies inwards excess of 100 kHz , mosfet are clearly the device of selection because of their lower on soil losses inwards depression voltage ratings, their fast switching speeds, together with a high impedance gate which requires a pocket-size voltage together with accuse to enable on/off transition. From the i-v characteristics

                                                 Rds α Vdss

                                            This equation explains why MOSFET inwards depression voltage applications at less than 200 volt is an fantabulous choice.  The on soil resistance goes upwards amongst the junction temperature inside the device together with proper heatsinking must live provided to maintain the temperature below the blueprint limit.

MOSFET REGIONS OF OPERATION:

                                   Most MOSFET devices used inwards ability electronics applications are of the n-channel, enhancement type MOSFET. For the MOSFET to send drain current, a channel betwixt the drain together with the rootage must live created. This occurs when the gate-to-source voltage exceeds the device threshold voltage VTh. For vGS > VTh, the device tin live either inwards the triode region, which is equally good called ‘‘constant resistance’’ region, or inwards the saturation region, depending on the value of vDS. For given vGS, amongst pocket-size vDS (vDS < vGS -VTh), the device operates inwards the triode part (saturation part inwards the BJT), together with for larger vDS …vDS > vGS -VTh), the device enters the saturation part (active part inwards the BJT). For vGS < VTh, the device turns off, amongst drain electrical flow nigh equal to zero. Under both regions of operation, the gate electrical flow is nigh zero. This is why the MOSFET is known equally a voltage-driven device and, therefore, requires unproblematic gate command circuit. The feature curves inwards V-I characteristics diagram exhibit that at that topographic point are 3 distinct regions of functioning labeled equally triode region, saturation region, together with cut-off region. When used equally a switching device, alone triode together with cut-off regions are used, whereas, when it is used equally an amplifier, the MOSFET must piece of occupation inwards the saturation region, which corresponds to the active part inwards the BJT. The device operates inwards the cut-off part (off-state) when vGS < vTh, resulting inwards no induced channel. In social club to piece of occupation the MOSFET inwards either the triode or saturation region, a channel must first live induced. This tin live accomplished past times applying gate-to-source voltage that exceeds vTh, that is,
                                                             

                                                             VGS > VTh

Once the channel is induced, the MOSFET tin piece of occupation inwards either the triode part (when the channel is continuous amongst no pinch-off, resulting inwards drain electrical flow proportional to the channel resistance) or the saturation part (the channel pinches off, resulting inwards constant ID). The gate-to-drain bias voltage (vGD) determines whether the induced channel enters pinch-off or not. This is bailiwick to the next restriction.For a triode vogue of operation, nosotros have

                                                                            vGD > VTh

                                                                             vGD < VTh

And for the saturation part of operation, pinch-off occurs when vGD ˆ VTh.

        In price vDS, the preceding inequalities may live expressed equally follows.

                         1. For triode part of operation

                                    vDS < vGS -VTh together with vGS > VTh 

                          2. For saturation part of operation

                                     vDS > vGS -VTh together with vGS > VTh …

                           3. For cut-off part of operation

                                      vGS < VTh …

SAFE OPERATION AREA:
                              

                                      The rubber functioning surface area (SOA) of a device provides the electrical flow together with voltage limits the device must live able to handgrip to avoid destructive failure. Typical SOA for a MOSFET device is shown inwards Figure below. The maximum electrical flow boundary piece the device is on is determined past times the maximum ability dissipation,


                                  Pdiss(ON )=ˆIds…(ON)†Rds(…ON†)

                                                       safe functioning surface area of MOSFET
                                                           

                                                          the on soil resistance equally a fraction of
                                                                            temperature
                                                   
                                                       
                                              As the drain-source voltage starts increasing, the device starts leaving the on-state together with enters the saturation (linear) region. During the transition fourth dimension the device exhibits large voltage together with electrical flow simultaneously. At higher drain-source voltage values that approach the avalanche breakdown it is observed that ability MOSFET suffers from a bit breakdown phenomenon. The bit breakdown occurs when the MOSFET is inwards the blocking soil (off) together with a farther increment inwards vDS volition drive a abrupt driblet inwards the blocking voltage. The rootage of this phenomenon inwards MOSFET is caused past times the presence of a parasitic n-type bipolar transistor equally shown inwards Figure of mosfet equivalent circuit.

                                                The inherent presence of the trunk diode inwards the MOSFET construction makes the device attractive for applications inwards which bidirectional electrical flow flow is needed inwards the ability switches.

                                          MOSFET equivalent circuit amongst parasitic BJT

                                                  Today’s commercial MOSFET devices direct hold fantabulous high operating temperatures. The outcome of temperature is to a greater extent than prominent on the on-state resistance equally shown inwards figure of on soil resistance equally a fraction of temperature.

                                                 As the on-state resistance increases, the conduction losses equally good increase. This large vDS…OW† limits the role of the MOSFET inwards high-voltage applications. The role of silicon carbide instead of silicon has reduced Vds.

                                                  As the device technology scientific discipline keeps improving, peculiarly inwards price of improved switch speeds together with increased ability treatment capabilities, it is expected that the MOSFET volition choke on to supersede BJTs inwards all types of ability electronics systems.

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