Test: Power Electronics - Electrical Engineering (EE) MCQ

dv/dt protection:

• When the SCR is forward biased, junctions J₁ and J₃ are forward biased and junction J₂ is reverse biased. This reverse-biased junction J₂ exhibits the characteristics of a capacitor.
• If the rate of the forward voltage applied is very high across the SCR, charging current flows through the junction J₂ is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. 
• dv/dt rating of thyristor indicates the maximum rate of rise of anode voltage that will not trigger the device without any gate signal. We use a snubber circuit to control this limit.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of a gate signal can be prevented by using a snubber circuit.
• Snubber limits the dv/dt across the switching device during the turnoff of the device.

Concept:

• Need of series connection of SCR is required when we want to meet the increased voltage requirement by using various SCR’s.
• When the required voltage rating exceeds the SCR voltage rating, a number of SCR’s are required to be connected in series to share the forward and reverse voltage.
• When the load current exceeds the SCR current rating, SCR are connected in parallel to share the load current.
   

Application:
According to the question:

Given that
SCR 1 voltage = 350 V; current = 6 A
SCR 2 voltage = 300 V; current = 9 A
SCR 3 voltage = 250 V; current = 12 A
Let us take the total current to be ‘I’
Current through resistor R in shunt with SCR 1 is

I₁ = I – 6

Similarly, current through resistor R is shunt with SCR 2

I₂ = I – 9

And, current through resistor R in shunt with SCR 3

I₃ = I – 12

Now, the string voltage becomes

V_s = I1R + I2R + I3R

Vs = (I - 6) R + (I - 9) R + (I - 12) R

Vs = (I - 6) R + (I - 6) R – 3 R + (I - 6) R – 6 R …..(1)

Note that

We should consider the extreme case from the calculation of resistance R for voltage equalization in string of SCR.

In extreme case, the voltage drop across SCR 1 (or the one having highest voltage drop) will be maximum forward blocking voltage.

∴ V_­max = 350 = (I - 6) R     ----(2)

Put (I - 6) R = 350 in equation (1), we get

V_s = 350 + 350 – 3 R + 350 – 6 R

(350 + 300 + 250) = 1050 – 9 R

900 = 1050 – 9 R

9 R = 1050 – 900

R = 150/9Ω

R = 16.66 Ω

Therefore the value of equalising resistance to be used across each thyrister is 16.66 Ω.

Type A chopper (first quadrant chopper):

• When chopper is ON, V₀ = V_s and current flows in the direction of the load (as shown in fig).
• When chopper is off, V₀ = 0 but I₀ continues to flow in the same direction through freewheeling diode, thus V₀ and I₀ is always positive.

Type B chopper (Second quadrant chopper):

• In type B chopper, load must always contain DC sources.
• When chopper is ON, V₀ = 0 but load E drives the current through the inductor and the chopper and thus inductor stores energy during the time T_ON of the chopper.
• When the chopper is off, V₀ = E+L^di/dt thus V₀ will be more than V_s and thus diode D_B will be forward biased and begins conducting and power starts flowing to the source.

Note: No matter the chopper is ON or Off, current I0 will be flowing out of the load (opposite direction to the given circuit shown) and treated as negative.

Since V₀ is +ve and I₀ is -ve, Type B chopper is IInd quadrant chopper

Type C chopper (two quadrant type A chopper):

• Type – C chopper is obtained by connecting type A chopper and type B chopper in parallel.
• In this chopper, V₀ is always +ve as F.D. is connected across it.
• In this type of chopper average value of current may be +ve or -ve.
• For regenerative breaking and motoring, there type of chopper configuration is used.

Type D chopper (Two quadrant Type B chopper)

• When the two choppers are ON, V₀ = V_s
• Average output voltage V₀ will be +ve when the chopper turns ON time ‘T_ON’ will be more than their turn off time ‘T_off’, otherwise it will be negative.
• As the diodes and the choppers conduct current only in one direction, the direction of load current will always be positive.

Type E chopper (four quadrant chopper):

Only in type E chopper, both current and voltage remains negative i.e. when CH₃-CH₂ are ON and CH₂-D₄ conducts (In III^rd quadrant)

Concept:

Concept:

Ripple frequency at the output = m × supply frequency

f_o = m × f_s

Where m = types of the pulse converter

Calculation:

A three-phase full-wave AC to DC converter is a 6-pulse converter

Number of pulses (m) = 6

f_o = 6 × supply voltage frequency

∴ f₀ = 6 x 50

f₀ = 300 Hz

DC chopper

• It is a power electronics device that is used to convert pure DC into pulsating DC.
• The average value of output voltage (pulsating value):

where, D is the duty cycle

Calculation

Source voltage (V_s) = 220 V

V_ch = 2 V

When the chopper is in an ON state:

When the chopper is in an OFF state:

V_o = 0V

The waveform is given below:

The average value of output voltage is:

Concept

When S₁ is closed, the main diode is ON and D_m is OFF.


 

In the above circuit, the inductor current is the output current.

where, V_S = Source voltage

T_ON = ON period

L = Inductor

Calculation

Given, VS = 220 V

TON = 100 μs

L = 220 μH

I_o = 100 A

MOSFET:

• MOSFET can carry DC current in both directions.
• The MOSFET channel itself can conduct current in either direction.
• And in addition, it has this built-in body diode that can conduct a negative current.
• Due to the integral body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "ON".
• So the MOSFET by itself can be a current bidirectional two-quadrant switch.

• The body diode of practical MOSFETs may not be a very good diode.
• So if we have a slow body diode we might not want to let it conduct and have to switch it off because of its slow switching time.
• Thus we have to put an external diode in series with a MOSFET that only lets the current flow in the positive direction.
• And then put in externally an antiparallel diode to get a current bidirectional switch.

Advantages of SCR:

• It can handle large voltages, currents, and power.
• The voltage drop across conducting SCR is small. This will reduce the power dissipation in the SCR.
• Easy to turn on.
• The operation does not produce harmonics.
• Triggering circuits are simple.
• It has no moving parts.
• It gives noiseless operation at high efficiency.
• We can control the power delivered to the load.

 
Drawbacks of SCR:

• It can conduct only in one direction. So it can control power only during the one-half cycle of ac.
• It can turn on accidentally due to the high dv/dt of the source voltage.
• It is not easy to turn off the conducting SCR. We have to use special circuits called commutation circuits to turn off a conducting SCR.
• SCR cannot be used at high frequencies or perform high-speed operations. The maximum frequency of its operation is 400 Hz.
• Gate current cannot be negative.

 
Applications of SCR: Controlled rectifiers, DC to DC converters or choppers, DC to AC converters or inverters, As a static switch, Battery chargers, Speed control of DC and AC motors, Lamp dimmers, fan speed regulators, AC voltage stabilizers.

Power electronic circuits can be classified as follows.

1. Diode rectifiers:

• A diode rectifier circuit converts ac input voltage into a fixed dc voltage.
• The input voltage may be single phase or three phase.
• They find use in electric traction, battery charging, electroplating, electrochemical processing, power supplies, welding and UPS systems.

2. AC to DC converters (Phase controlled rectifiers):

• These convert ac voltage to variable dc output voltage.
• They may be fed from single phase or three phase.
• These are used in dc drives, metallurgical and chemical industries, excitation systems for synchronous machines.

3. DC to DC converters (DC Choppers):

• A dc chopper converts dc input voltage to a controllable dc output voltage.
• For lower power circuits, thyristors are replaced by power transistors.
• Choppers find wide applications in dc drives, subway cars, trolley trucks, battery driven vehicles, etc.

4. DC to AC converters (Inverters):

• An inverter converts fixed dc voltage to a variable ac voltage. The output may be a variable voltage and variable frequency.
• In inverter circuits, we would like the inverter output to be sinusoidal with magnitude and frequency controllable. In order to produce a sinusoidal output voltage waveform at a desired frequency, a sinusoidal control signal at the desired frequency is compared with a triangular waveform.
• These find wide use in induction motor and synchronous motor drives, induction heating, UPS, HVDC transmission etc.

5. AC to AC converters: These convert fixed ac input voltage into variable ac output voltage. These are two types as given below.

i. AC voltage controllers:

• These converter circuits convert fixed ac voltage directly to a variable ac voltage at the same frequency.
• These are widely used for lighting control, speed control of fans, pumps, etc.

ii. Cycloconverters:

• These circuits convert input power at one frequency to output power at a different frequency through a one stage conversion.
• These are primarily used for slow speed large ac drives like rotary kiln etc.

6. Static switches:

• The power semiconductor devices can operate as static switches or contactors.
• Depending upon the input supply, the static switches are called ac static switches or dc static switches.

Construction:

• The SCR is a four-layer and three-terminal device.
• The four layers made of P and N layers are arranged alternately such that they form three junctions J₁, J_2, and J₃.
• These junctions are either alloyed or diffused based on the type of construction.

Doping level:

• The level of doping varies between the different layers of the thyristor.
• Out of these four layers, the first layer (P1 or P+) and Last layer (N2 or N+) are heavily doped layers.
• The second layer (N1 or N-) is a lightly doped layer and the third layer (P2 or P+) is a moderately doped layer.
• The junction J1 is formed by the P+ layer and N- layer.
• Junction J2 is formed by the N- layer and P+ layer
• Junction J3 is formed by P+ layer and N+ layer.
• Thinner layers would mean that the device would break down at lower voltages.

Formula:

Where, Vo is the output voltage

V_in is the input voltage

T_ON is the pulse width

Application:

Given,

V_in = 200 volts

T_ON = 200 µs

V₀ = 600 V
From equation (1),

or, 3T - 600 = T

Hence, T = 300 µs

If the Pulse width is half then, the new value of pulse width (TON') will be,

Hence,

Hence, the new value of output voltage (V0') will be,

In majority carrier devices conduction is only because of majority carriers whereas in minority carrier devices conduction is due to both majority and minority carriers.

1. MOSFET is a majority carrier device.

2. Diode is both majority and minority carrier device.

3. Thyristor is minority carrier device

4. IGBT is minority carrier device

1ϕ full converter with resistive load

Case 1: During +ve half cycle (α to π) 

T₁ and T₂ are forward-biased.

Vo = Vs

Case 2: During -ve half cycle (π+α to 2π) 

T₃ and T₄ are forward-biased.

Vo = -V_s

The waveform is given below:

The average output voltage is given by:

Here, Vs is the RMS value.

The average output voltage in terms of the maximum value is:

Insulated Gate Bipolar Transistor (IGBT)

• IGBT is a three-terminal device i.e. gate, collector, and emitter
• IGBT is a voltage-controlled device i.e. +ve gate to emitter voltage (V_GE) is required to turn ON the IGBT.
• IGBT has been developed by combining the best qualities of both BJT and Power MOSFET. Hence, an IGBT exhibits a high input impedance as a PMOSFET and has low ON-state power losses like a BJT.