POWER TRANSISTORS-MOSFET

POWER TRANSISTORS:

                              The power level diodes and transistors naturally differ from ordinary transistors and diodes only by the operating voltage and current values. The power transformers are designed in such a way to meet the voltage levels upto kVs and kAs and also it has less switching times in terms of nano seconds to few micro seconds.  The power level diodes and transistors have evolved over decades from their signal level counterparts to the extent that it can handle even more kilo volts and currents in even more faster switching time.

                            
CHARACTERISTICS CONSIDERED IN SELECTION OF POWER DIODE AND TRANSISTORS:

  • voltage rating
  • current rating
  • switching speeds
  • on state voltage
1.VOLTAGE RATING:
                           The maximum voltage that the device can operate, beyond which the device "breaks down" and the damage to the device occurs is called as voltage rating of the device.

2.CURRENT RATING:
                            The maximum current expressed as an instantaneous that a device can carry during its on state is called current rating.  Beyond this limit due to the excess heat produced the device will be destroyed.

3.SWITCHING SPEED:
                            The speed within which the device can make a transition from its on state to its off state is called switching speed.  In other words the time taken to make a device on and off is called switching speed.  Small switching times associated with the fast switching device result in low switching losses in other words fast switching devices can be operated at high switching frequencies with acceptable power losses.

4.ON STATE VOLTAGE:
                            The voltage drop across a device during its on state while conducting current is called on state voltage.  The smaller the on state voltage, the smaller will be the on state power loss.

                                  
                              Transistors are controllable switches, which are available in several forms for switching mode power 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 mode converters two types of transistors are primarily used: mosfet are typically used below a few hundred volts at switching frequencies in excess of 100 kHz, whereas IGBTs dominate very large voltage, current and power ranges extending to MW levels when the switching frequency are below few tens of KHz.  The following topics provide a clear overview of mosfet and igbt.

MOSFET:
   
                                The circuit symbol of the n channel mosfet are shown in the below diagram
          

                                          It consist of three terminals: drain(D),source(S),gate(G).  The forward current in a mosfet flows from the drain to the source terminal when the gate is biased.  Mosfets can block only the forward polarity voltage, that is a positive Vds.  They cannot block only the forward polarity voltage due to an intrinsic anti parallel diode, which can be used effectively in most switch mode converter designs.
                                     Mosfet i-v characteristics are shown in the below diagram

                                                                 For gate voltage below Vgs threshold , typically in a range of 2 to 4 v, a mosfet is completely off, as shown by the i-v characteristics and acts as an open switch. Beyond vgs threshold , the drain current  id through the mosfet depends on the applied gate voltage vgs, as shown by the transfer characteristics in the below diagram

                                                 To carry id would require a gate voltage of a value atleast equal to vgs(Io) as shown in the figure. Typically gate source voltage is maintained at 10v o keep the mosfet in its on stage and carrying id current. In its on state a mosfet offers a small resistor Rds and the drain current that flows through it depends on the external circuit in which it is connected.  The on state resistance is the inverse of the slope of i-v characteristics as shown in figure.

                                                  In applications at voltage below 200volts and switching frequencies in excess of 100 kHz , mosfet are clearly the device of choice because of their lower on state losses in low voltage ratings, their fast switching speeds, and a high impedance gate which requires a small voltage and charge to enable on/off transition. From the i-v characteristics

                                                 Rds α Vdss
                                            This equation explains why MOSFET in low voltage applications at less than 200 volt is an excellent choice.  The on state resistance goes up with the junction temperature within the device and proper heatsinking must be provided to keep the temperature below the design limit.


MOSFET REGIONS OF OPERATION:

                                   Most MOSFET devices used in power electronics applications are of the n-channel, enhancement type MOSFET. For the MOSFET to carry drain current, a channel between the drain and the source must be created. This occurs when the gate-to-source voltage exceeds the device threshold voltage VTh. For vGS > VTh, the device can be either in the triode region, which is also called ‘‘constant resistance’’ region, or in the saturation region, depending on the value of vDS. For given vGS, with small vDS (vDS < vGS -VTh), the device operates in the triode region (saturation region in the BJT), and for larger vDS …vDS > vGS -VTh), the device enters the saturation region (active region in the BJT). For vGS < VTh, the device turns off, with drain current almost equal to zero. Under both regions of operation, the gate current is almost zero. This is why the MOSFET is known as a voltage-driven device and, therefore, requires simple gate control circuit. The characteristic curves in V-I characteristics diagram show that there are three distinct regions of operation labeled as triode region, saturation region, and cut-off region. When used as a switching device, only triode and cut-off regions are used, whereas, when it is used as an amplifier, the MOSFET must operate in the saturation region, which corresponds to the active region in the BJT. The device operates in the cut-off region (off-state) when vGS < vTh, resulting in no induced channel. In order to operate the MOSFET in either the triode or saturation region, a channel must first be induced. This can be accomplished by applying gate-to-source voltage that exceeds vTh, that is,
                                                             
                                                             VGS > VTh
Once the channel is induced, the MOSFET can operate in either the triode region (when the channel is continuous with no pinch-off, resulting in drain current proportional to the channel resistance) or the saturation region (the channel pinches off, resulting in constant ID). The gate-to-drain bias voltage (vGD) determines whether the induced channel enters pinch-off or not. This is subject to the following restriction.For a triode mode of operation, we have

                                                                            vGD > VTh
                                                                             vGD < VTh
And for the saturation region of operation, pinch-off occurs when vGD ˆ VTh.
        In terms vDS, the preceding inequalities may be expressed as follows.
                         1. For triode region of operation
                                    vDS < vGS -VTh and vGS > VTh 
                          2. For saturation region of operation
                                     vDS > vGS -VTh and vGS > VTh …
                           3. For cut-off region of operation
                                      vGS < VTh …

SAFE OPERATION AREA:
                              
                                      The safe operation area (SOA) of a device provides the current and voltage limits the device must be able to handle to avoid destructive failure. Typical SOA for a MOSFET device is shown in Figure below. The maximum current limit while the device is on is determined by the maximum power dissipation,


                                  Pdiss(ON )=ˆIds…(ON)†Rds(…ON†)
                                                       safe operation area of MOSFET
                                                           
                                                          the on state resistance as a fraction of
                                                                            temperature
                                                   
                                                       
                                              As the drain-source voltage starts increasing, the device starts leaving the on-state and enters the saturation (linear) region. During the transition time the device exhibits large voltage and current simultaneously. At higher drain-source voltage values that approach the avalanche breakdown it is observed that power MOSFET suffers from a second breakdown phenomenon. The second breakdown occurs when the MOSFET is in the blocking state (off) and a further increase in vDS will cause a sudden drop in the blocking voltage. The source of this phenomenon in MOSFET is caused by the presence of a parasitic n-type bipolar transistor as shown in Figure of mosfet equivalent circuit.

                                                The inherent presence of the body diode in the MOSFET structure makes the device attractive for applications in which bidirectional current flow is needed in the power switches.
                                          MOSFET equivalent circuit with parasitic BJT

                                                  Today’s commercial MOSFET devices have excellent high operating temperatures. The effect of temperature is more prominent on the on-state resistance as shown in figure of on state resistance as a fraction of temperature.

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

                                                  As the device technology keeps improving, especially in terms of improved switch speeds and increased power handling capabilities, it is expected that the MOSFET will continue to replace BJTs in all types of power electronics systems.







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