POWER ELECTRONICS

EE 312‐Engr.Talha Ahmed Khan

Introduction to Power Electronics

• Power Electronics = Power + Control + Electronics• Control deals with the steady state and dynamic

characteristics of closed loop systems.• Power deals with the static and rotating power

equipment for generation, transmission and distribution of electrical energy.

• Electronics deals with the solid state devices and circuits for signal processing to meet the desired control objectives.

• A system in a steady state has numerous properties that are unchanging in time.

• For example: The flow of fluid through a tube, or electricity through a network, could be in a steady state because there is a constant flow of fluid, or electricity. Conversely, a tank which is being drained or filled with fluid would be an example of a system in transient state, because the volume of fluid contained in it changes with time.

• Dynamic characteristics tell us about how well a sensor responds to changes in its input. For dynamic signals, the sensor or the measurement system must be able to respond fast enough to keep up with the input signals.

• Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material. Common solid-state devices include transistors, microprocessor chips, and DRAM.

• Power Electronics Def: The applications of solid state electronics for the control and conversion of electric power.

• – Primarily based on the switching of power semiconductor devices.

• Modern power Electronics uses – Power semiconductors that can be regarded as the muscle

• – Microelectronics that have the power and intelligence of brain

• Power Electronics have revolutionized the concept of power control for power conversion and for control of electrical motor drives.

• • Applications:• – Heat controls• – Light controls• – Motor controls• – Power supplies• – And many more … (Table 1.1)• • History Milestones:• – 1948 Invention of Transistors• – 1956 Invention of SCR (PNPN) or Thyristor• – 1958 Commercialization of SCR

• Power Electronics Revolution is giving us the ability to shape and control large amounts of power with ever increasing efficiency.

• 1.2 Power Semiconductor Devices• Till 1970s, Thyristor(SCR) was the major

device used for power control in industrial appliances.

• After 1970s, a lot of power semiconductor devices were developed.

• Major classification:• – Power Diodes• – Power Transistors• – Thyristors

Power Semiconductor Diodes

• Power diode plays important role in power electronics circuits for conversion of electric power.

• Diode acts as a switch to perform various functions• – Switches in rectifiers• – Free wheeling in switching regulators• – Charge reversal of capacitors• – Energy transfer between components• – Voltage isolation• – Energy feedback from load to power source

• A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode) is a diode used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.

Charge Reversal

• When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased.

• Power diodes have• – similar V I characteristics as ordinary signal ‐

diode• – More power, voltage and current handling

capabilities

• The Static induction thyristor (SIT, SITh) is a thyristor with a buried gate structure in which the gate electrodes are placed in n-base region. Since they are normally on-state, gate electrodes must be negatively biased to hold off-state.

• A gate turn-off thyristor (GTO) is a special type of thyristor, a high-power semiconductor device. GTOs, as opposed to normal thyristors, are fully controllable switches which can be turned on and off by their third lead, the GATE lead.

N = 2 for Si diode,n=2 for Ge.

Power Transistors

• – Power Electronics deals mostly with the transistor operation as a switch

• – Controlled on & Controlled off characteristics‐ ‐• – For Switching, Transistor is operated in

saturation region ↓ on state Voltage drop ‐• – Can be operated at much speeds as

compared to thyristors.But lower power ratings• – Used in dc dc and dc ac convertors‐ ‐

Quiz 1

1.Define the following.• Fast recovery diodes• Schottky Diode• Silicon Carbide Diodes2.Classify the power transistors.

THYRISTOR

• A Thyristor (silicon controlled rectifier or SCR) is a little like a transistor. When a small current flows into the GATE (G), this allows a larger current to flow from the ANODE (A) to the CATHODE (C). Even when the current into the gate stops the thyristor continues to allow current to flow from anode to cathode.

Function of the Gate Terminal

• The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

• When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor,avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).

• If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.

Thyristor Turn-on