lecture 11 (dc converters) - overhead

8/10/2019 Lecture 11 (DC Converters) - Overhead

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University of British Columbia Elec Machines & Electronics

EECE 365 Winter 2012 Lecture 11: Power Electronics

Nathan Ozog 2013 Page 1 of 15

Power Electronics

Types

DC ConverterTo raise ( ) or Lower ( ) voltage

AC to DC Rectifier

To converter AC voltage/current to DC voltage/currentMay control voltage level as well

DC to AC Inverters

To convert DC voltage/current to AC voltage/current

Basic Components

Power DiodeSimplest switch

Passive

Controlled by the voltage/current that is imposed on it

Conducts current in the forward directionBlocks current in the reverse direction

Like an electrical valve


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Thyristor

aka. Silicon Controlled Rectifier (SCR)Similar to a diode

Has some control

The can only turn ON the thyristorCannot turn it off


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Transistors

Active switchesCan be turned on and off via their base or gate

BJT MOSFET IGBT

Ideal Operation

BJT: Bipolar Junction TransistorSimplest transistor

Moderately robustModerately fastRequires base current

MOSFET: Metal Oxide Field Effect TransistorNot very robust cannot block large voltages or conduct large

currentsExtremely fast

No gate current required

Fragile

IGBT: Insulated Gate Bipolar TransistorRobust

Slow

No gate current requiredConstructed from a MOSFET and BJT

SCR: Silicon Controlled RectifierExtremely RobustVery Slowbase current required to turn on only

Constructed from two BJTs


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Pulse Width Modulated (PWM) DC to DC ConverterBasic Idea: turn on and off a DC voltage source

We control the voltage

Switching Quadrants

We describe converters by the number of Quadrants they operate in

=aV


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One Quadrant Converter

Positive .Forward .

Forward only

Allows us to only be able to control DC machineas a Motorin 1 direction


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One Quadrant Converter


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Two Quadrant Converter

Positive onlyForward or Reverse .

Allows us to only be able to control DC machineas a or .in 1 direction


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Motor Mode


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Generator Mode


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Four Quadrant Converter

Positive or negative .Forward or Reverse .

Forward and Reverse .

Allows us to only be able to control DC machineas a or .

in directions

21 ssa iii ==

43 dda iii ==43 ssa iii ==

21 dda iii ==


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Motor Mode


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Generator Mode


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( )tVVrmss sin2=

Active (Thyristor Controlled) Rectifiers

Converters AC to DCCan control output with of thyristor

Single Phase-Half Wave RectifierThyristor begins conducting when turned on at angle .Thyristor stops conducting when .

Output voltage is:

Where

rmst VV

20

0

( ) ( ))cos(12

1

0

+== rmsT

tt

VdttV

TV


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( )tVVrmss sin2=

Single Phase-Full Wave Rectifier

Thyristors and begin conducting when turned on at angle

Thyristor stops conducting when 0s

V

Thyristors and begin conducting at angle .

Thyristor stops conducting when 0sV

Output voltage is:

Where

rmst VV

220

0

( ) ( ))cos(121

0

+== rmsT

tt

VdttV

TV


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( )tVVrmsa sin2=

( )o= tVV rmsb sin2

( )o+= tVVrmsc sin2

Three Phase-Full Wave Rectifier

Output voltage is:

rmst VV

630 ( )dttV

TV

T

tt = 01

lecture 11 (dc converters) - overhead

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