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ENHANCEMENT TYPE MOSFET
Here current control in an n-channel device is now affected by positive gate to source voltage rather than the range of negative voltages of JFET’s and depletion type
A slab of p-type material is formed and two n-regions are formed in the substrate. The source and drain terminals are connected through metallic contacts to n-doped regions, but the absence of a channel between the doped n-regions. The SiO2 layer is still present to isolate the gate metallic platform from the region between drain and source, but now it is separated by a section of p-type material.
Fig. 5: Structure of n-channel enhancement type MOSFET
If VGS 0V and a voltage is applied between the drain and source, the absence of a n-channel will result in a current of effectively zero amperes. With VDS set at some positive voltage and VGS set at 0V, there are two reverse biased p-n junction between the n-doped regions and p substrate to oppose any significant flow between drain and source.
If both VDS and VGS have been set at some positive voltage, then positive potential at the gate will pressure the holes in the p-substrate along the edge of SiO2 layer to leave
the area and enter deeper region of p-substrate. However the electrons in the p-substrate will be attracted to the positive gate and accumulate in the region near the surface of the SiO2 layer. The negative carriers will not be absorbed due to insulating SiO2 layer, forming an inversion layer which results in current flow from drain to source.
The level of VGS that results in significant increase in drain current is called threshold voltage VT . As VGS increases the density of free carriers will increase resulting in increased level of drain current. If VGS is constant VDS is increased; the drain current will eventually reach a saturation level as occurred in JFET.
Power MOSFET’s are generally of enhancement type only. This MOSFET is turned ‘ON’ when a voltage is applied between gate and source. The MOSFET can be turned ‘OFF’ by removing the gate to source voltage. Thus gate has control over the conduction of the MOSFET. The turn-on and turn-off times of MOSFET’s are very small. Hence they operate at very high frequencies; hence MOSFET’s are preferred in applications such as choppers and inverters. Since only voltage drive (gate-source) is required, the drive circuits of MOSFET are very simple. The paralleling of MOSFET’s is easier due to their positive temperature coefficient. But MOSFTS’s have high on-state resistance hence for higher currents; losses in the MOSFET’s are substantially increased. Hence MOSFET’s are used for low power applications.
determines the voltage blocking capability of the device. On the other side of n substrate, a metal layer is deposited to form the drain terminal. Now p regions are diffused in the epitaxially grown n layer. Further n regions are diffused in the p regions as shown.
SiO2 layer is added, which is then etched so as to fit metallic source and gate terminals.
A power MOSFET actually consists of a parallel connection of thousands of basic MOSFET cells on the same single chip of silicon.
When gate circuit voltage is zero and VDD is present, n p junctions are reverse biased and no current flows from drain to source. When gate terminal is made positive with respect to source, an electric field is established and electrons from n channel in the p regions. Therefore a current from drain to source is established.
Power MOSFET conduction is due to majority carriers therefore time delays caused by removal of recombination of minority carriers is removed.
Because of the drift region the ON state drop of MOSFET increases. The thickness of the drift region determines the breakdown voltage of MOSFET. As seen a parasitic BJT is formed, since emitter base is shorted to source it does not conduct.
The switching model of MOSFET’s is as shown in the figure 6(a). The various inter electrode capacitance of the MOSFET which cannot be ignored during high frequency switching are represented by Cgs , Cgd & Cds . The switching waveforms are as
shown in figure 7 . The turn on time td is the time that is required to charge the input capacitance to the threshold voltage level. The rise time tr is the gate charging time from this threshold level to the full gate voltage Vgsp . The turn off delay time tdoff is the time required for the input capacitance to discharge from overdriving the voltage V1 to the
pinch off region. The fall time is the time required for the input capacitance to discharge from pinch off region to the threshold voltage. Thus basically switching ON and OFF depend on the charging time of the input gate capacitance.
Fig.7: Switching waveforms and times of Power MOSFET
The turn-on time can be reduced by connecting a RC circuit as shown to charge the capacitance faster. When the gate voltage is turned on, the initial charging current of the capacitance is
Where RS is the internal resistance of gate drive force.
COMPARISON OF MOSFET WITH BJT
MOSIGT OR IGBT
IGBT BASIC STRUCTURE AND WORKING
It is constructed virtually in the same manner as a power MOSFET. However, the substrate is now a p layer called the collector.
When gate is positive with respect to positive with respect to emitter and with gate emitter voltage greater than VGSTH , an n channel is formed as in case of power MOSFET.
This n channel short circuits the n region with n emitter regions.
An electron movement in the n channel in turn causes substantial hole injection from p substrate layer into the epitaxially n layer. Eventually a forward current is established.
MOSFET is formed with input gate, emitter as source and n region as drain. Equivalent circuit is as shown below.
Also p serves as collector for pnp device and also as base for npn transistor. The two pnp and npn is formed as shown.
when gate is applied VGS VGSth MOSFET turns on. this five the base drive to T1
Therefore T1 starts conducting. The collector of T1 is base of T2 . Therefore
regenerative action takes place and large number of carriers are injected into the n drift region. This reduces the ON-state loss of IGBT just like BJT.
When gate drive is removed IGBT is turn-off. When gate is removed the induced channel will vanish and internal MOSFET will turn-off. Therefore T1 will turn-off it
T2 turns off.
Structure of IGBT is such that R1 is very small. If R1 small T1 will not conduct therefore IGBT’s are different from MOSFET’s since resistance of drift region reduces
when gate drive is applied due to p injecting region. Therefore ON state IGBT is very small.
Static V-I characteristics ( IC versus VCE )
Same as in BJT except control is by VGE . Therefore IGBT is a voltage controlleddevice.
Transfer Characteristics ( IC versus VGE )
Identical to that of MOSFET. When VGE VGET , IGBT is in off-state.
Widely used in medium power applications such as DC and AC motor drives,
UPS systems, Power supplies for solenoids, relays and contractors.
Though IGBT’s are more expensive than BJT’s, they have lower gate drive requirements, lower switching losses. The ratings up to 1200V, 500A.
SERIES AND PARALLEL OPERATION
Transistors may be operated in series to increase their voltage handling capability. It is very important that the series-connected transistors are turned on and off simultaneously. Other wise, the slowest device at turn-on and the fastest devices at turn-off will be subjected to the full voltage of the collector emitter circuit and the particular device may be destroyed due to high voltage. The devices should be matched for gain, transconductance, threshold voltage, on state voltage, turn-on time, and turn-off time. Even the gate or base drive characteristics should be identical.
Transistors are connected in parallel if one device cannot handle the load current demand. For equal current sharings, the transistors should be matched for gain, transconductance, saturation voltage, and turn-on time and turn-off time. But in practice, it is not always possible to meet these requirements. A reasonable amount of current sharing (45 to 55% with two transistors) can be obtained by connecting resistors in series with the emitter terminals as shown in the figure 10.
The resistor will help current sharing under steady state conditions. Current sharing under dynamic conditions can be accomplished by connecting coupled inductors. If the current through Q1 rises, the l di/dt across L1 increases, and a corresponding voltage of opposite polarity is induced across inductor L2 . The result is low impedance path, and the current is shifted to Q2 . The inductors would generate voltage spikes and they may be expensive and bulky, especially at high currents.
BJTs have a negative temperature coefficient. During current sharing, if one BJT carries more current, its on-state resistance decreases and its current increases further, whereas MOSFETS have positive temperature coefficient and parallel operation is relatively easy. The MOSFET that initially draws higher current heats up faster and its on-state resistance increases, resulting in current shifting to the other devices. IGBTs require special care to match the characteristics due to the variations of the temperature coefficients with the collector current.
di/dt AND dv/dt LIMITATIONS
Transistors require certain turn-on and turn-off times. Neglecting the delay time td and the storage time ts , the typical voltage and current waveforms of a BJT switch is shown below.
During turn-on, the collector rise and the di/dt is
During turn off, the collector emitter voltage must rise in relation to the fall of the collector current, and is
The conditions didt and dv/dt in equation (1) and (2) are set by the transistor switching characteristics and must be satisfied during turn on and turn off. Protection circuits are normally required to keep the operating di/dt and dv/dt within the allowable limits of transistor. A typical switch with di/dt and dv/dt protection is shown in figure (a), with operating wave forms in figure (b). The RC network across the transistor is known as the snubber circuit or snubber and limits the dv/dt . The inductor LS , which limits the didt , is sometimes called series snubber.
Let us assume that under steady state conditions the load current IL is free wheeling through diode Dm , which has negligible reverse reco`very time. When transistor Q1 is turned on, the collector current rises and current of diode Dm falls, because Dm will behave as short circuited. The equivalent circuit during turn on is shown in figure below
SCR-Principle of Operation
The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig.a. The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit. Essentially the device is a switch. Ideally it remains off (voltage blocking state), or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal.
The thyristor then switches on and current flows and continues to conduct without further gate signals. Ideally the thyristor has zero impedance in conduction state. For switching off or reverting to the blocking state, there must be no gate signal and the anode current must be reduced to zero. Current can flow only in one direction. In absence of external bias voltages, the majority carrier in each layer diffuses until there is a built-in voltage that retards further diffusion. Some majority carriers have enough energy to cross the barrier caused by the retarding electric field at each junction. These carriers then become minority carriers and can recombine with majority carriers. Minority carriers in each layer can be accelerated across each junction by the fixed field, but because of absence of external circuit in this case the sum of majority and minority carrier currents must be zero. A voltage bias, as shown in figure, and an external circuit to carry current allow internal currents which include the following terms:
The current Ix is due to
Turning-off Methods of an SCR
As already mentioned in previous blog post, once the SCR is fired, it remains on even when triggering pulse is removed. This ability of the SCR to remain on even when gate current is removed is referred to as latching. So SCR cannot be turned off by simply removing the gate pulse.
There are three methods of switching off the SCR, namely natural commutation, reverse bias turn-off, and gate turn-off.
(a) Natural Commutation : When the anode current is reduced below the level of the holding current, the SCR turns off. However, it must be noted that rated anode current is usually larger than 1,000 times the holding value. Since the anode voltage remains positive with respect to the cathode in a dc circuit, the anode current can only be reduced by opening the line switch S, increasing the load impedance RL or shunting part of the load current through a circuit parallel to the SCR, i.e. short-circuiting the device.
(b) Reverse-bias Turn-off :A reverse anode to cathode voltage (the cathode is positive with respect to the anode) will tend to interrupt the anode current. The voltage reverses every half cycle in an ac circuit, so that an SCR in the line would be reverse biased every negative cycle and would turn off. This is called phase commutation or ac line commutation. To create a reverse biased voltage across the SCR, which is in the line of a dc circuit, capacitors can be used. The method of discharging a capacitor in parallel with an SCR to turn-off the SCR is called forced commutation. In power electronic applications one advantage of using SCRs is that they are compact. The control equipment is also compact if integrated circuits are used. There has also been an attempt to miniaturize capacitors used for forced commutation and for filtering. The former use is important because the currents can be high and thermal dissipation takes high priority in design considerations. Small sizes of capacitors are at present being achieved by the use of metalized plastic film or a plastic film and aluminium foil.
(c) Gate Turn Off : In some specially designed SCRs the characteristics are such that a negative gate current increases the holding current so that it exceeds the load current and the device turns-off. The current ratings are presently below 10 A and this type will not be considered further.