Demystifying Power Electronics: A Course Overview for Engineers

Introduction

Power Electronics: Power + Electronics + Control
Power: the static and rotating power equipment for the G, T, & D of electric energy
Electronics: the solid-state devices and circuits for signal processing to meet the desired control objectives
Control: the steady-state and dynamic characteristics of closed-loop systems
Power electronics may be defined as the application of solid-state electronics for the control and conversion of electric power
Based primarily on switching of the power semiconductor devices.
Power semiconductors improves power-handling capabilities and switching speed of the power devices tremendously.
Microprocessors has a great impact on control strategy for the power semiconductor devices

Power electronics \(\Rightarrow\) power semiconductors (muscles) + microelectronics (brain)
current direction from anode (A) to cathode (K)
Gate (G) to turn on or off the signal
without gate signal remains off-state (open circuit) and can withstand a voltage across the terminals A and K.

1900 - Introduction of the mercury arc rectifier
Until 1950s - Several devices for power control, such as metal tank rectifier, grid-controlled vacuum-tube rectifier, ignitron, phanotron, and thyratron
1948 - First revolution: Invention of the silicon transistor
- Bell Telephone Laboratories by Bardeen, Brattain, and Schokley
Modern microelectronics evolved over the years from silicon semiconductors
1956 - Major breakthrough by Bell Laboratories: invention of the PNPN triggering transistor, called thyristor or silicon-controlled rectifier (SCR).
1958 - Commercial thyristor by the General Electric Company
Due to the fusion of power electronics, the muscle, with microelectronics, the brain, many potential applications of power electronics are now emerging, and this trend will continue.

Control of Electric Power:
- For effective control of electric power or power conditioning, it is essential to convert electric power from one form to another.
Switching Characteristics of Power Devices:
- The switching characteristics of power devices play a crucial role in facilitating these power conversions.
Static Power Converters:
- Static power converters are responsible for performing the functions of power conversions.
Converter as a Switching Matrix:
- Conceptually, a converter can be envisioned as a switching matrix.
- In this matrix, one or more switches are turned on and connected to the supply source.
- The purpose is to achieve the desired output voltage or current through controlled switching actions.

Power electronics circuits can be categorized into six types.
1. Diode rectifiers
2. Dc–dc converters (dc choppers)
3. Dc–ac converters (inverters)
4. Ac–dc converters (controlled rectifiers)
5. Ac–ac converters (ac voltage controllers or Cycloconverters)
6. Static switches
The switching devices in the following converters are employed to demonstrate fundamental principles.
The switching action within a converter may involve multiple devices.
The selection of a specific device is contingent upon the voltage, current, and speed requirements of the converter.

Converts ac voltage into a fixed dc voltage
A diode conducts when its anode voltage \(>\) cathode voltage, and it offers a very small voltage drop, ideally zero, but typically 0.7 V.
Behaves as an open circuit when its cathode voltage \(>\) anode voltage, offering a very high resistance, ideally infinite, but typically 10 \(k\Omega\).
Output voltage is a pulsating dc, but it is distorted and contains harmonics.
Input voltage could be either single phase or three phase.

Also known as chopper or switching regulator
When transistor \(Q_1\) is turned on by applying a gate voltage \(V_{GE}\), the dc supply is connected to the load and the instantaneous output voltage is \(v_0=+V_s\).
\(Q_1\) is turned off by removing \(V_{GE}\), the dc supply is disconnected from the load and \(v_0=0\).
Average output voltage can be varied by controlling the duty cycle (\(\delta\)) and controlled by varying the conduction time \(t\) of \(Q_1\).
\(T\) is the chopping period, then \(t_1 = \delta \cdot T\).

Also known as an inverter
When MOSFETs \(M_1\) and \(M_2\) are turned on by applying gate voltages, the dc supply voltage \(V_s\) appears across the load and the instantaneous output voltage is \(v_o = +V_s\).

When \(M_3\) and \(M_4\) are turned on by applying gate voltages, the \(V_s\) appears across the load in the opposite direction, \(v_o = -V_s\).
If \(M_1\) and \(M_2\) conduct for one half of a period and \(M_3\) and \(M_4\) conduct for the other half, the output voltage is of the alternating form.
The rms value of the output voltage becomes \(V_{o(\text{rms})} = V_s\)
However, the output voltage contains harmonics which could be filtered out before supplying to the load.

Also known as controlled rectifiers
When thyristor \(T_1\) is turned on at a delay angle of \(\omega t=\alpha\), the supply voltage appears across load and \(T_{1}\) is turned off automatically when its current falls to zero at \(\omega t=\pi.\)

When \(T_{2}\) is turned on at a delay angle of \(\omega t=\pi+\alpha\), the negative part of the supply voltage appears the across the load in the positive direction and \(T_{2}\) is turned off automatically when its current falls to zero at \(\omega t=2\pi.\)
The average output voltage can be found from \(V_{o(\mathrm{AVG})}=(1+\cos\alpha)V_{m}/\pi\).
The average value of the output voltage \(v_{0}\) can be controlled by varying the conduction time of thyristors or firing delay angle, \(\alpha\).
The input could be a single- or three-phase source.

Also known as ac voltage controllers
To obtain a variable ac output voltage \(v_o\) from a fixed ac source
A TRIAC allows a current flow in both directions
It activates when a gate voltage is applied at \(\omega t = \alpha\) for positive current flow, and at \(\omega t = \pi + \alpha\) for negative current flow.
\(v_o\) is controlled by varying the conduction time or firing delay angle \(\alpha\) of the TRIAC.

Power devices can function as static switches or contactors, operating with either AC or DC supplies.
These switches are termed AC static switches or DC switches.

The design can be divided into four parts:
1. Design of power circuits
2. Protection of power devices
3. Determination of control strategy
4. Design of logic and gating circuits

Multiple conversion stages are frequently linked together to generate the intended output.
Mains 1 delivers the standard AC supply to the load via the static bypass.
The AC to DC converter charges the backup battery from mains 2.
The DC to AC converter provides emergency power to the load through an isolating transformer.
Typically, mains 1 and mains 2 are connected to the same AC supply.