Additional Insights Into Power Electronics

There are several striking features of power electronics, the foremost among them being the extensive use of inductors and capacitors. In many applications of power electronics, an inductor may carry a high current at a high frequency. The implications of operating an inductor in this manner are quite a few, such as necessitating the use of litz wire in place of single stranded or multi stranded copper wire at frequencies above 50kHz, using a proper core to limit the losses in the core and shielding the inductor properly so that the fringing that occurs at the air gaps in the magnetic path does not lead to electromagnetic interference. Usually the capacitors used in a power electronic application are also stressed. It is typical for a capacitor to be operated at a high frequency with current surges passing through it periodically. This means that the current rating of the capacitor at the operating frequency should be checked before it is used. In addition, it may be preferable if the capacitor has self healing properties. Hence an inductor or a capacitor has to be selected or designed with care, taking into account the operating conditions before its use in a power electronic circuit.

In many power electronic circuits diodes play a crucial role. A normal power diode is usually designed to be operated at 400Hz or less. Many of the inverter and switch mode power supply circuits operate at a much higher frequency and these circuits need diodes that turn ON and OFF fast. In addition, it is also desired that the turning off process of a diode should not create undesirable electrical transients in the circuit. Since there are several types of diodes available, selection of a proper diode is very important for reliable operation of a circuit.

Analysis of power electronic circuits tends to be quite complicated because these circuits rarely operate in steady state. Traditionally, steady state response refers to the state of a circuit characterised by either a DC response or a sinusoidal response. Most of the power electronic circuits have a periodic response but this response is not usually sinusoidal. Typically, the repetitive or the periodic response contains both a steady state part due to the forcing function and a transient part due to the poles of the network. Since the responses are non sinusoidal, harmonic analysis is often necessary. In order to obtain the time response, it may be necessary to resort to the use of a computer program.

Power electronics is a subject of interdisciplinary nature. To design and build control circuitry of a power electronic application, one needs knowledge in the following areas:

  • Design of analogue and digital electronic circuits, to build the control circuitry
  • Microcontrollers and digital signal processors for use in sophisticated applications
  • Many power electronic circuits have an electrical machine as their load. In an AC variable speed drive, it may be a reluctance motor, an induction motor or a synchronous motor. In a DC variable speed drive, it is usually a DC shunt motor.
  • In a circuit such as an inverter, a transformer may be connected at its output and the transformer may have to operate with a non sinusoidal waveform at its input.
  • A pulse transformer with a ferrite core is used commonly to transfer the gate signal to the power semiconductor device. A ferrite cored transformer with a relatively higher power output is also used in an application such as a high frequency inverter.
  • Many power electronic systems are operated with negative feedback. A linear controller such as a PI controller is used in relatively simple applications, whereas a controller based on digital or state variable feedback techniques is used in more sophisticated applications.