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IntroductionPLT207 Power Electronics

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Text Book

“Power Electronics”, by Daniel Hart; McGraw-Hill

Int’l Edition

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Power Electronics

Electrical Power Processing is

multidisciplinary and deals with the

conversion and processing of

electrical power using power

electronics.

Power electronics circuits convert

electric power from one form to

another using electronic devices.

Conversion is done using electronic

switches, passive energy storage

components (capacitors &

inductors), and control systems.

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Power Electronics

Applications of power electronics range from high-power conversion equipment to everyday appliances, i.e. Heating and lighting control Induction heating Fluorescent lamp ballast Motor driver Battery chargers Electric vehicles, regenerative breaking Switching power supplies Uninterruptible power supplies (UPS) Electric power transmission Automotive electronics (Ignition, alternators) Energy storage (Flywheel, SMES, super capacitor) Power conditioning for alternative power sources: Solar cells, Fuel

cells, Wind turbines) etc..

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Converter Classification

The objective of a power electronics circuit is to match the voltage and current requirements of the load to those of the source.

Power electronics circuits convert one type or level of a voltage or current waveform to another and are hence called converters.

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Converter Classification

ac input/dc output – rectifier

-Half-wave rectifier, full-wave rectifier, etc.

dc input/ac output – inverter

-Half-bridge inverter, full-bridge inverter, multilevel inverter, etc.

dc input/dc output - dc-dc converter

-Buck, boost, buck-boost, etc.

ac input/ac output - ac-ac converter

-Matrix converter, cycloconverter, etc.

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Multistep Conversion

Power conversion can be a multistep process involving more than one type of converter.

For example, an ac-dc-ac conversion can be used to modify an ac source by first converting it to direct current and then converting the dc signal to an ac signal that has an amplitude and frequency different from those of the original ac source.

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Power Electronics Concepts

Example: to supply 3 V to a load resistance from a 9V battery.

One simple solution is to use a voltage divider.

Drawbacks:

Poor efficiency - power absorbed by the 2RL resistor is twice as much as delivered to the load.

Poor regulation - if the value of the load resistance changes, the output voltage will change.

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Power Electronics Concepts

Alternative solution:

Adding a switch which is opened and closed periodically.

If the switch is closed for one-third of the period, the average output voltage is one-third of the source voltage.

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Power Electronics Concepts

Advantages of alternative solution:

Ideal switch consume no power and hence all power supplied by battery is delivered to load making it 100 percent efficient.

when switch is closed, voltage is zero.

when switch is open, current is zero.

Output voltage can be regulated by controlling duty cycle.

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Power Electronics Concepts

Drawback of alternative solution:

The output is not a pure dc voltage. The voltage waveform containing a dc term (the average value) plus sinusoidal terms at frequencies that are multiples of the pulse frequency (harmonics).

To create a 3-V dc voltage, vx is applied to a low-pass filter.

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Power Electronics Concepts

An ideal low-pass filter allows the dc component of voltage to pass through to the output while removing the ac terms, thus creating the desired dc output.

If the filter is lossless, the converter will be 100% efficient.

In practice, the filter will have some losses and will absorb some power.

Additionally, the electronic device used for the switch will not be perfect and will have losses.

However, the efficiency of the converter can still be quite high (more than 90%).

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Power Electronics Concepts

A feedback control system would detect if the output voltage were not 3 V and adjust the closing and opening of the switch accordingly.

Controller

Inductor and capacitor

Electronic switch

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Electronic Switches

Two states:

on - ideally being a short circuit with voltage is zero

off - ideally being an open circuit with current is zero.

Real switches absorb some power.

Diode

Thyristor

Transistor

on

off

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Diode

Uncontrollable

on and off conditions are determined by voltages and currents in the circuit

forward-biased (on) when the current id is positive and reverse biased (off) when vd is negative.

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Diode

An important dynamic characteristic of a nonideal diode is reverse recovery current.

When a diode turns off, the current in it decreases and momentarily becomes negative before becoming zero.

This phenomenon may become important in high-frequency applications.

Silicon carbide (SiC) and schottky diodes

have very little reverse recovery,

resulting in more efficient circuits,

especially in high-frequency

high-power applications.

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Thyristor

Family of three-terminal devices - Controllable diodes:

silicon-controlled rectifier (SCR)

Triac

gate turn-off thyristor (GTO)

MOS-controlled thyristor (MCT)

Thyristors are capable of large currents and large blocking voltages for use in high-power applications, but switching frequencies cannot be as high as when using other devices such as MOSFETs.

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Silicon-Controlled Rectifier (SCR)

For the SCR to begin to conduct, it must have a gate current applied while it has a positive anode-to-cathode voltage.

After conduction is established, the gate signal is no longer required to maintain anode current.

The SCR will continue to conduct as long as

the anode current remains

positive and above

minimum value called the

holding level.

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Gate Turn-off Thyristor (GTO)

Like the SCR, is turned on by a short-duration gate current if the anode-to-cathode voltage is positive.

GTO can be turned off with a negative gate current.

Suitable for some applications where control of both turn-on and turnoff of a switch is required.

The negative gate turnoff current can be of brief

duration (a few microseconds),

but its magnitude must be

very large compared to the

turn-on current.

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Triac

The triac is functionally equivalent to two antiparallel SCRs (in parallel but in opposite directions).

is a thyristor that is capable of conducting current in either direction.

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MOS-Controlled Thyristor (MCT)

Functionally equivalent to the GTO but without the high turn-off gate current requirement.

The MCT has an SCR and two MOSFETs integrated into one device. One MOSFET turns the SCR on, and one MOSFET turns the SCR off.

The MCT is turned on and off by establishing the proper voltage from gate to

cathode, as opposed to

establishing a gate current

in the GTO.

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Transistor

Transistors are operated as switches in power electronics circuits.

Drive circuits are designed to have the transistor either in the fully on or fully off state.

Types of transistors:

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

BJT (Bipolar Junction Transistor)

IGBT (Insulated Gate Bipolar Transistor)

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MOSFET

Voltage-controlled device.

A sufficiently large gate-to-source voltage will turn the device on.

Have on-state resistances as low as a few milliohms.

Fast switching transition and used in converters operating into the megahertz range.

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BJT

Current-controlled device.

On state is achieved by providing sufficient base current to drive the BJT into saturation.

Zero base current results in an off transistor.

To carry high collector current, high base current is needed to drive BJT into saturation.

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Insulated Gate Bipolar Transistor (IGBT)

Integrated connection of a MOSFET and a BJT.

Voltage-controller device.

Drive circuit for the IGBT is like that of the MOSFET, while the on-state characteristics are like those of the BJT.

IGBTs have replaced BJTs in many applications.

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Emerging Transistors

Silicon-Carbide (SiC) power MOSFET

Enhancement-mode Gallium Nitride (GaN) power FETs

Improve power efficiency and increase power density.

Characteristics:

Fast switching – minimized switching loss

Low reverse recovery – minimized switching loss

Low Rds(on) – minimized conduction loss

Low Qg – minimized gate power consumption

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Switch Selection27

Depends on :

voltage level

current level

switching characteristics (operating point, turn-on & turnoff characteristics)

Computer Simulation

OrCAD Pspice (used in the text book)

Demo Ver. : http://www.cadence.com/products/orcad/pages/downloads.aspx#demo

PSIM

Matlab Simulink

PLECS

SABER

Proteus

etc..

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