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Power Electronics

This stack is concerned with power switching devices, converters.

QuestionAnswer
What is the goal of power electronics? Efficient conversion, conditioning or processing and control of electric power using solid-state semiconductor devices in order to supply high-quality power to the load causing minimum pollution of environment.
What is a converter? A converter is a power electronic circuit which converts one form of electric energy to another form of electric energy.
Which are the major components used in a converter? Power switching semiconductor devices and energy storing circuit elements like inductors and capacitors.
Which circuit element is not preferable in a converter? Why? Resistive elements should be avoided in converters because they cause power loss and reduce the efficiency of energy conversion.
List different types of converters. Rectifiers (controlled & uncontrolled), AC Voltage Controllers (ACVC),Inverters, DC-to-DC Converters (Choppers), Cycloconverters
What is the function of rectifiers? Conversion of A.C. power at fixed voltage and frequency to D.C.
What is the function of ACVC? Conversion of A.C. power at fixed R.M.S. voltage and frequency to A.C. power at variable R.M.S. voltage at the same frequency as the input.
What is the function of inverters? Conversion of D.C. power to A.C. power at variable R.M.S. voltage and frequency which may be fixed or variable depending upon the application.
What is the function of a cycloconverter? Conversion of A.C. power at fixed R.M.S. voltage and frequency to A.C. power at variable R.M.S. voltage and variable frequency.
What is the function of a D.C.-to-D.C. converter? Conversion of D.C. power at constant voltage to D.C. power at variable voltage.
What are the two main blocks in a power processor? Power circuit and control circuit.
What is the function of the control circuit in a power processor? To turn on/off the switching devices present in the power circuit on the basis of feedforward, feedback, and reference signals.
List some of the popular power semiconductor switches used in modern power converters. Power diodes, power MOSFETs, SCR, GTO, IGBT, TRIAC.
List important goals of designing a modern power processor. Higher energy efficiency, compactness, light weight, high reliability, reasonable price.
Why switching on a resistive load can damage the semiconductor switch. Switch could be damamged due to di/dt through the switch exceeding the rated value of the switch.
Why switching off an inductive load can damage the semiconductor switch? Switch could be damaged due to very high impulse voltage appearing across it when the switch is opened.
How is di/dt in a switch limited? By connecting an inductance in series with the switch.
Why do we need to reduce burning of fossil fuels? To preserve them for posterity and to minimize environmental pollution.
What is the justification for improving the energy efficiency of equipment at the consumer's end by using power electronics? Every 1 unit of energy saved at the consumer's end saves approximately 6 units of fuel at the generating site.
What is the bulk of the load in power systems? Induction motors driving fan or pump.
Why are D.C. motors being replaced by induction motors in industries? Because they are costly and require more maintenance. Induction motor speed can be now smoothly varied using power electronic drives.
Where are slip-ring type of induction motors used? In grinding mills.
Which type of motor is used in electric vehicles? BLDC motor.
What is a fully controlled switch? A semiconductor switch which can be turned on and off by applying a control signal is said to be fully controlled.
What is a semi-controlled switch? A semiconductor switch in which either on or off state can be achieved by applying a control signal is called a semi-controlled switch.
What is an uncontrolled switch? A semiconductor switch with no control input is said to be uncontrolled.
Give example of an uncontrolled switch. Power diode.
Give examples for a semi-controlled switch. SCR and TRIAC
Give examples for fully-controlled switches. IGBT, MOSFET and GTO.
How does a power diode differ from a small signal diode. Power diode has p-i-n structure where as a small signal diode has p-n structure.
What is i-layer in a power diode? Why is it used in power diodes? i-layer refers to intrinsic semiconductor layer. This layer gives higher voltage blocking capability to the power diode.
Which parameter of the power diode limits the frequency of switching of the diode? The reverse recovery time.
What is trr? Reverse Recovery Time.
Define trr of a power diode. It is the time duration for which the diode current will be negative when the diode is turned off.
What is the function of power switching semiconductor devices in power converters? Power switching semiconductor devices are used in converters to establish path for the flow of electric power.
List different power losses occurring in power semiconductor devices. On-state power loss, off-state power loss, gate power loss, and switching power loss.
Why in modern power electronics the trend is to operate the semiconductor switches at higher and higher frequencies? By operating switches at higher frequencies, the sizes of energy storing elements like L and C will be reduced. This makes the converter circuit compact and light weight.
What is the typical value of trr of a standard recovery power diode? About 25 micro-seconds.
What is the typical value of trr of a fast recovery power diode? Less than 25 micro-seconds.
Give examples for level-triggered switches. BJT, MOSFET, and IGBT.
Give examples for pulse-triggered switches. SCR, TRIAC, GTO.
Give examples for unilateral switches. SCR, BJT, MOSFET, IGBT, GTO, LASCR, diode.
Give example for bilateral switch. TRIAC.
What is a power diode module? This is a module having power diodes connected as a single-phase or three-phase bridge or as one arm of the bridge.
List different types of power diodes. Standard recovery diode, fast recovery diode, Schottky diodes, silicon carbide diodes.
Which type of power diode does not have a p-i-n structure? Schottky diode.
What are the main features of Schottky diodes? trr is around few nano-seconds and on-state voltage drop is very low of the order of 0.15-0.45 V and hence Pon is very low.
What are the main demerits of Schottky diodes? Available only in 50-200 V range. They are costlier as compared to ordinary power diodes.
What is the main application of Schottky diodes? In SMPS.
What is a silicon carbide diode? It is a Schottky diode made using silicon-carbide instead of silicon.
What are the merits of silicon-carbide diodes as compared to Schottky diodes? Extremely low power loss, low reverse leakage current, very low trr, can operate at junction temperature > 225 Deg.Centigrade, higher PIV rating.
What is the most important demerit of silicon-carbide diodes? Very expensive.
What is an SCR? SCR is a semi-controlled power switching semiconductor device.
Specify the SCR structure and its terminals. SCR has 4-layer p-n-p-n structure and three terminals --- anode, cathode, and gate.
Which are the power terminals in SCR? Anode and cathode are the power terminals.
Across which terminals is the control signal applied in an SCR? Between gate and cathode.
What is the normal method of turning on an SCR? By injecting a positive gate current into the gate.
What happens when positive gate current is injected when the SCR is reverse biased? When SCR is reverse biased, it remains in the off state even if gate current is injected.
Where are LASCRs used? In converters used in HVDC transmission systems.
What is LASCR? Light Activated SCR. This device has a window through which light radiation can be made to fall on the junction to turn on the device.
Created by: rsamurti