Introduction to Power Electronics

• Power electronics involves using solid-state electronics for electric power control and conversion.
• This field includes research, design, control, computation, and integration of nonlinear and time-varying electronic systems.
• Power electronics began with the mercury arc rectifier.
• Peter Cooper Hewitt invented it in 1902.
• It was initially used to convert alternating current (AC) to direct current (DC).

Main Issues in Power Electronics (P.E.)

• Meeting load requirements and achieving better control.
• Improving efficiency through reliable device operation and energy saving.
• Balancing volume, weight, and cost.
• Reducing interferences.

Advantages of Power Electronic Devices

• Easy and flexible operation due to digital control.
• Faster dynamic response compared to electromechanical converters.
• Lower acoustic noise relative to electromagnetic controllers, relays, and contactors.
• High efficiency because of low losses.
• Long operational life and minimal maintenance attributed to absence of mechanical processes.
• Compact control equipment.

Disadvantages of Power Electronic Devices

• Generates undesirable harmonics.
• Injects harmonics into power supply lines, affecting other loads and equipment.
• Introduces unwanted interference with communication circuits by electromagnetic radiations.

Diodes

• Diodes are made from silicon p-n junctions with an anode and a cathode.
• P-N junctions are formed using alloying, diffusion, and epitaxial growth.
• Modern diffusion and epitaxial processes allows for specific device characteristics.
• High mechanical and thermal reliability is an advantage of diodes.
• High peak inverse voltage handling.
• Low reverse current.
• Low forward voltage drop.
• High efficiency.
• Compactness.
• When the anode is positive relative to the cathode, the diode is forward biased.
• A diode conducts fully when the diode voltage exceeds the cut-in voltage, which is 0.7 V for silicon (Si).
• A conducting diode has a small voltage drop across it.
• When the cathode is positive relative to the anode, the diode is reverse biased.
• With reverse bias, a small leakage current flows.
• Leakage current increases with a rise in the magnitude of reverse voltage, continuing until avalanche voltage is reached.
• Avalanche voltage is also known as breakdown voltage.

Transistors

• Transistors are semiconductor devices that amplify and switch electronic signals and electrical power.
• Transistors consist of semiconductor material with at least three terminals for connection to an external circuit.
• A voltage or current applied to a pair of transistor terminals can change the current through another pair.
• Transistors can amplify a signal because the controlled (output) power is higher than the controlling (input) power.
• Transistors have 3 layers and 3 terminals: base, emitter, and collector.
• There are two junctions: collector-base (CB) and emitter-base (EB).
• Transistors come in two types: NPN and PNP.
• Common configurations include common base, common collector, and common emitter.
• The common emitter configuration is generally used in switching applications.

Switching Characteristics

• An application of transistors is in switching circuits.
• As a switch, a transistor operates in either a cut-off or saturation state.
• A transistor in the cut-off state is non-conducting.
• A transistor in saturation is in the conduction state.
• The non-conduction state occurs in the cut-off region, while conduction occurs in the saturation region.
• When base voltage (VB) rises from 0 to VB, base current rises to IB, but collector current does not increase immediately.
• Collector current increases when the base-emitter junction is forward biased and VBE > 0.6V.
• Collector current (IC) gradually increases to a saturation level IC(Sat).
• Delay time (td) is the time for the collector current to rise to 10% of its final value.
• Rise time (tr) is the time for the collector current to rise from 10% to 90% of its final value.
• The sum of delay time and rise time gives the turn-on time, ton = td + tr.
• When input voltage reverses from VB1 to –VB2, the base current changes abruptly.
• Storage time (ts) is the short interval during which the collector current remains constant.
• Reverse base current helps discharge minority charge carriers in the base region and removes excess stored charge from the base region.
• Once excess stored charge is removed, the base current will start to fall towards zero.
• Fall-time (tf) is the time the collector current takes to fall from 90% to 10% of IC(Sat).
• Turn-off time (toff) equals the sum of storage time and fall time, toff = ts + tf.

Thyristors

• Thyristors form by a solid-state semiconductor device composed of four alternating layers of N and P-type material.
• Thyristors act as bistable switches, conducting when the gate terminal receives a current trigger.
• Conduction continues as long as the voltage across the device is not reversed (forward-biased).
• Three-lead thyristors control current of its two leads by smaller current of its control lead.
• Two-lead thyristors switch on once potential difference between its leads is large enough (breakdown voltage).
• Reverse blocking mode, involves reverse bias.
• Forward blocking mode, involves forward bias.
• Forward Conducting mode, involves forward bias that is forced.