Unit - 1

Power Switching Devices

Q1) Draw and explain working operation of n-channel E MOSFET (8M)

A1) MOSFET (Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor). MOSFET has three terminals source(S), drain (D), and gate (G). The gate (G) terminal of MOSFET is insulated from channel. The circuit symbol and construction is shown below.

Fig (a) symbol of n-channel EMOSFET (b) basic structure of n-channel EMOSFET

Working of n-channel enhancement type MOSFET

The n-channel Enhancement MOSFET is similar to that of p-channel Enhancement MOSFET but only operationally and constructionwise these two are different from each other. In N Channel Enhancement MOSFET a lightly doped p-type substrate forms the body and source and drain regions are heavily doped with n-type impurities. Here body and source commonly to the ground potential. Now, a positive voltage is applied to the gate terminal. Due to positive gate and corresponding capacitive effect, free electrons i.e. minority carriers of the p-type substrate get attracted towards the gate and form a layer of negative uncovered ions there just below the dielectric layer by recombining these free electrons with holes. If we continually increase the positive gate voltage, after the threshold voltage level, the recombination process gets saturated and then free electrons start to accumulate at the place to form a conductive channel of free electrons. The free electrons also come from the heavily doped source and drain n-type region.

Now if we apply a positive voltage at the drain, current start flowing through the channel. The resistance of the channel depends on the number of free electrons in the channel and the number of free electrons in the channel again depends on the gate potential of the device. As the concentration of free electrons forms the channel, and the current through the channel gets enhanced due to increase in gate voltage, it is named as N – Channel Enhancement MOSFET.

Q2) Explain two transistor terminology for thyristor (8M)

A2) The thyristor is designed by cascading two transistor as shown below. The collector current from the NPN transistor is fed directly to the base of PNP transistor, while the collector current of PNP transistor is fed to the base of NPN. These interconnected transistors rely on each other for conduction.

So for one of the transistors to conduct, a base current is required. When the thyristor’s anode terminal is negative with respect to cathode, the NP junction becomes forward biased and the PN junction becomes reverse biased.

Fig: (a) basic structure (b)two transistor model (c) two transistor circuit

Here, the flow of reverse current is blocked until a breakdown voltage is applied. After breakdown voltage, it starts to conduct without the application of gate signal. This is one of the negative characteristics of thyristor as it triggers into conduction by a reverse break over voltage.

When the anode terminal is made positive with respect to cathode, the outer junctions are forward biased and the centre NP junction is reverse biased and blocks the forward current. So to trigger it into conduction, a positive current is applied to the base of transistors.

The two transistors are connected in a regenerative loop and this force the transistor to conduct to saturation. Thus, it can be said that a thyristor block current in both the direction of an AC supply in it’s OFF state and can be turned ON by the application of positive current to the base of transistor.

Q3) Draw and Explain V-I characteristics of Thyristor (8M)

A3) The V_I characteristics of thyristor is shown in fig below. Thyristor can either be forward biased state or reverse biased state.

Forward Biased State

When anode is made positive, the PN junctions at the ends are forward biased and centre junction (NP) becomes reverse biased. It will stay in blocked (OFF) mode (also known as Forward Blocking Stage) till the time it is triggered by Gate current pulse or the applied voltage reaches the forward breakover voltage. The thyristor can trigger ON by following two methods.

1. Triggering by Gate Current Pulse – When it is triggered by the gate current pulse, it starts conducting and will act as a close switch. The thyristor remains in the ON-state, i.e. it remains in the latched state. Here the gate loses its control to turn off the device.

2. Triggering by Forward Breakover Voltage – When a forward voltage is applied, a leakage current starts to flow through the blocking (J2) in the middle junction of thyristor. When voltage exceeds the forward break over voltage or critical limit, then J2 breaks down and it reaches to the ON state.

When the Gate current (Ig) is increased, it reduces the blocking area and so the forward break over voltage is reduced. It will turn ON when a minimum current called latching current is maintained .When the gate current Ig=0 and anode current falls below a certain value called holding current during the ON state, it again reaches to its forward blocking state.

Fig: V-I characteristics of Thyristor

Reverse Biasing state

If the anode is negative with respect to cathode, i.e., with the application of reverse voltage, both PN junctions at the end i.e. J1 and J3 become reverse biased and the centre junction J2 becomes forward biased. Only a small leakage current flows through it. This is the reverse voltage blocking mode or OFF state of Thyristor.

When the reverse voltage is increased further, then at a certain voltage, avalanche breakdown of J1 and J2 occurs and it starts conducting in the reverse direction. The maximum reverse voltage at which a thyristor starts conducting is known as Reverse Breakdown Voltage.

Q4) What is IGBT? Explain its construction and working. (8M)

A4) IGBT stands for Insulated Gate Bipolar Transistor. IGBT is a fusion between BJT and MOSFET, having the input characteristics of a BJT and output characteristics of a MOSFET. It is a three-terminal semiconductor switching device that can be used for fast switching with high efficiency in many types of electronic devices. These devices are mostly used in amplifiers for switching/processing complex wave patters with pulse width modulation (PWM).

Operation of IGBT as a Circuit

Since IGBT is a combination of BJT and MOSFET lets look into their operations as a circuit diagram here. The below diagram shows the internal circuit of IGBT which includes two BJT and one MOSFET and a JFET. The Gate, Collector, and Emitter pins of the IGBT are marked below.

Basic structure

The collector of the PNP transistor is connected to the NPN transistor through a JFET, the JFET connects the collector of the PNP transistor and the base of the PNP transistor. These transistors are arranged in a way to form a parasitic thyristor set up to create a negative feedback loop. The Resistor RB is placed to short the base and emitter terminals of the NPN transistor to ensure that the thyristor doesn’t latch-up which leads to the latch-up of the IGBT. The JFET used here will signify the structure of current between any two IGBT cells and allows the MOSFET and supports most of the voltage.

Q5) Explain triggering/firing circuits of thyristor (8M)

A5) Triggering means turning ON of a device from its off state. Turning ON of a thyristor refers to thyristor triggering. Thyristor is turned on by increasing the anode current flowing through it. The anode current can be increase by following ways.

Forward breakover Voltage Triggering: - In this, the applied forward voltage is gradually increased beyond a point known as forward break over voltage VBO and gate is kept open.

Gate Triggering: - This method of thyristor triggering is widely used method because of ease of control over the thyristor gate triggering of thyristor allows to turn of the thyristor whenever we wish. Here a gate signal is applied to the thyristor. The thyristor will turn on when gate signal is applied to it and it will be in forward biased. Once the thyristor starts conducting, the gate loses its control over the device and the thyristor continues to conduct. This is because of regenerative action that takes place within the thyristor when gate signal is applied.

When the thyristor is forward biased, and a gate signal is injected by applying positive gate voltage is applied between gate and cathode terminals, then the thyristor is turn on.

Dv/dt Triggering:- In this type of triggering, if the rate of rise of anode to cathode voltage is high, the charging current through the capacitive junction is high enough to turn on the thyristor. A high value of charging current may destroy the thyristor hence the device must be protected against high dv/dt.

Light Thyristor Triggering: - In this type of triggering, the rays of light are allowed to strike the junctions of the thyristor. This results in an increase in the number of electron-hole pair and thyristor may be turned on. The light-activated SCRs (LASER) are triggered by using this method.

Thermal Thyristor Triggering: - If the temperature of the thyristor is high, it results in increase in the electron-hole pairs. Which in turn increase the leakage current α1 and α2 to raise. The regenerative action tends to increase (α1 + α2) to units and the thyristor may be turned on. This type turn on is not preferred as it may result in thermal turn away and hence it is avoided.

Q6) what are commutation techniques used for thyristor (8M)

A6)  Commutation means the process of turning off a thyristor. In all the commutation techniques, the anode current is maintained below the holding current for a sufficiently long time, so that all the excess carriers in the four layers are swept out or recombined.

There are mainly two types of SCR commutation techniques: Natural Commutation and Forced Commutation.

Natural Commutation of SCR:

Natural Commutation of SCR is the process of turning off an SCR without using additional commutation circuitry.  This commutation technique only occurs in AC circuit.  For better understanding, let us consider an SCR circuit energized from AC source.

When SCR is conducting, the current will pass through zero after every positive half cycle. After that, the AC source then applies a reverse voltage across the terminals of SCR till the beginning of second cycle. If the time of application of reverse voltage applied by the AC source is more than the SCR turn-off time, then SCR will get turned off.

Forced Commutation of SCR:

Unlike natural commutation, an external circuitry is required to forcibly bring the SCR anode current below holding current and keeping SCR reversed biased for a period more than the SCR / thyristor turn off time. This technique is applied for DC circuit. The commutation circuitry for forced commutation comprises of Inductor and capacitor. Forced commutation is applied to Choppers and Inverters. There are several forced commutation techniques.

• Class-A Commutation (also known as Load Commutation)
• Class-B Commutation (also known as Resonant Pulse Commutation)
• Class-C Commutation (often called Complimentary Commutation)
• Class-D Commutation or Impulse Commutation
• Class-E Commutation or External Pulse Commutation

Q7) List an application of following devices. i) Power Diode ii) MOSFET iii) IGBT    iv) Thyristor (8M)

A7) i) Application of Power Diode

• Power Diode are used in Rectifiers
• Power Diode are used in Clipper Circuits
• Power Diode are used in Clamping Circuits
• Power Diode are used in Reverse Current Protection Circuits
• Power Diode are used in In Logic Gates
• Power Diode are used in Voltage Multipliers

Ii) MOSFET

MOSEFET technology is applicable to many types of circuit. Applications include:

• Linear power supplies
• Switching power supplies
• DC-DC converters
• Low voltage motor control

Iii) IGBT

IGBTs are used in various applications such as

• AC and DC motor drives
• Unregulated Power Supply (UPS)
• Switch Mode Power Supplies (SMPS)
• Traction motor control and induction heating
• Inverters

Iv) Thyristor

Thyristors may be used in many applications such as

• Power-switching circuits
• Relay-replacement circuits
• Inverter circuits
• Oscillator circuits
• Level-detector circuits
• Chopper circuits
• Light-dimming circuits
• Low-cost timer circuits
• Logic circuits
• Speed-control circuits
• Phase-control circuits, etc.

Q8) What is need of gate driver?

A8) The structure of an IGBT/power MOSFET is such that the gate forms a nonlinear capacitor. Charging the gate capacitor turns the power device on and allows current flow between its drain and source terminals, while discharging it turns the device off and a large voltage may then be blocked across the drain and source terminals. The minimum voltage when the gate capacitor is charged and the device can just about conduct is the threshold voltage (VTH). For operating an IGBT/power MOSFET as a switch, a voltage sufficiently larger than VTH should be applied between the gate and source/emitter terminals.

Power MOSFETs and IGBTs are simply voltage driven switches, because their insulated gate behaves like a capacitor.

Driving a Gate

As shown below, driving a gate consists of applying different voltages: 15V to turn on the device through S1, and 0V to turn off the device through S2.

The voltage at which the gate voltage remains during switching is known as the Miller voltage, Vgm. In most applications, this voltage is around 4 to 6V, depending on the level of current being switched. This feature can be used to control the switching waveforms from the gate drive.

Q9) Draw and explain V-I characteristics of Diode.

A9)

When anode is positive with respect to cathode then diode is in forward bias mode. Initially diode current is zero with increase in applied voltage (Vs) from zero. When Vs is equal to threshold voltage Or cut in voltage, the forward anode current is very small. Beyond cut in voltage the diode current increases rapidly and diode is said to be in conduct.

When cathode is positive with respect to anode, diode is  reverse biased. In this mode small reverse current called leakage current(mA) flows. As reverse voltage increases upto the breakdown voltage or Avalnche ,the leakage current also slowly  increases. If leakage current or reverse current is not limited by series resistance then it may destroy the diode.to avoid the breakdown of diode, the diode should operate below specific peak respective reverse voltage (VRM) as shown in waveform above.

Q10) Explain V-I characteristics of n channel EMOSFET

A10)

a)     Transfer Characteristics

This characteristics shows variation between drain current ID with respect to voltage across gate to source terminal VGS . From the fig it is clear that below threshold voltage VGST the MOSFET is OFF.

b)    Output Characteristics

This Characteristics shows variation of drain current ID as a function of applied drain to source voltage VDS keeping VGS constant for low values of  VDS,  the graph of ID-VDS is almost linear, indicates constant value of on resistance RDS=VDS/ID. For given value of VGS, if VDS increases the drain current is nearly constant showing flat response.

Fig: output characteristics