Power Electronics: Course Overview

• 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.

Diode rectifiers:

• 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.

Dc–dc converters:

• 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 \(t_1 = \delta \cdot T\).

Dc–ac converters:

• 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.

Ac–dc converters:

• 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.

Ac–ac converters

• 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.

• Power Diodes and Switched RLC Circuits

• Diode Rectifiers

• Power Transistors (MOSFETs, JFETs, IGBTs)

• DC–DC Converters (or Choppers)

• DC–AC Converters (or Inverters)

• Resonant Pulse Inverters

• Multilevel Inverters

• Thyristors

• Controlled Rectifiers

• AC Voltage Controllers