Overview of power electronics

Question 1

What is the definition of power electronics?

Answer 1

The study of electronic circuits which are able to efficiently process and transfer electric power using semiconductor switching devices

Question 2

What is the function of power electronics?

Answer 2

To convert electrical power from the form supplied by a source to the form required by a load.

Question 3

What are the types of DC-DC converters

Answer 3

Buck converters

Boost converters

Question 4

What are the types of AC-DC converters

Answer 4

Uncontrolled rectifier

Line frequency-controlled rectifier

Question 5

What are the types of DC-AC converters?

Answer 5

Line frequency-controlled inverter

High frequency-controlled inverter

Question 6

What are the properties of a diode?

Answer 6

> Reverse current is blocked and there is very little leakage current
Quickly recovers from the reverse-biased condition (good for high frequency applications)
An uncontrollable semiconductor switch.
Uncontrolled because it is turned on and off by the voltage across it and current through it.
We cannot externally control it.
The power capability varies
Forward voltage drop of 0.8V – 1V

Question 7

What are the properties of a Thyristor?

Answer 7

> Also called a Silicon Controlled Rectifier (SCR)
> Semi-controllable diode
> Can be turned on independently
> Cannot be turned off independently
> Very high-power handling capability

Question 8

What are the properties of a transistor?

Answer 8

> Fully controllable switch
Can be controlled by applying a gate voltage
Medium to high power capabilities.
The power capabilities may improve over time as technology advances

Question 9

How does a thyristor function?

Answer 9

> If a signal is applied to the gate, then it will turn on.

> It will only turn off when the current flowing through it is negative

Question 10

What are the 3 types of transistors used in power electronics?

Answer 10

> Power BJT (Bipolar Junction Transistor)
Power MOSFET
IGBT (Insulated gate bipolar transistor)

Question 11

What are the properties of power BJT?

Answer 11

> Not commonly used because it does not switch as fast as the other technologies
High current rating (up to 400A)
High voltage (up to 1500V)

Question 12

What are the properties of power MOSFET?

Answer 12

> N-channel is mainly used (almost all the time)
Fast switching and is efficient at switching at a high frequency.
Low current rating (up to 50A)
Medium voltage (up to 600V)
On state voltage drop is high

Question 13

What are the properties of IGBT?

Answer 13

> Switches faster than BJT but has a lower on-state voltage drop than MOSFET devices
High voltage (up to 3300V)
High Current (up to 1200A)

Question 14

What are the attributes of an ideal switch?

Answer 14

> Zero transitional period between on and off switching states
Voltage drop across switch when it is on is 0
Current through the switch when it is off is 0
Infinite power control ratio
Max Voltage drop across switch when it is on is ∞
Max Current through the switch when it is on is ∞
Infinite power handling capability

Question 15

What are the attributed of a practical switch?

Answer 15

> Finite switching time
When turned on there is a finite voltage drop across it
When it is turned off, small leakage current may flow through it

Question 16

What is the linearised switching characteristics circuit?

Answer 16

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Question 17

What is assumed to be the transition time when compared to the time period?

Answer 17

t_on ≫ t_c(on) or t_c(off)

Question 18

How does back EMF occur?

Answer 18

When you have an inductive-resistive source like a motor:
> When the switch closes, current builds up
> When the switch opens, the current is forced to become 0A and causes the voltage across the inductor to shoot to infinity.
> This causes the switch to experience an over voltage and can fail.

Question 19

How can you negate back EMF?

Answer 19

A parallel diode