Power Characterization, Diodes and Controlled Rectifiers

7/25/2019 Power Characterization, Diodes and Controlled Rectifiers

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EE 123 Power ElectronicsDepartment of Electrical EngineeringUniversity of California Riverside

Instructor: Roman Chomko

Laboratory 1EE 123

Winter 2013

LABORATORY 1

M A N U A L

Power Characterization.

Diodes and Controlled Rectifiers

CONTENT

Objectives............................................................................................................................ 3

Hardware ............................................................................................................................. 3

Software .............................................................................................................................. 3

Parts .................................................................................................................................... 3

PART 1. Power Characterization.......................................................................................... 4

1.1 LTspice IV Software..................................................................................................... 4

1.2 Energy and Instantaneous Power.................................................................................. 41.3 Average and RMS Power.............................................................................................. 5

1.4 Average and RMS Power.............................................................................................. 7

1.5 Laboratory Procedures and Report ............................................................................... 91.5.1 Average and RMS Values of Waveforms...........................................................9

1.5.1 Instantaneous, Average, Apparent Power, and Power Factor...........................10

PART 2. Diodes and Controlled Rectifiers and Switches ................................................... 11

2.1 Switches as Controlled Power Regulators.................................................................. 112.2 Diode Rectifiers as Uncontrolled Switches ................................................................ 12

2.3 Voltage Controlled Switches ...................................................................................... 132.4 Thyristors Semiconductor Based Controlled Rectifiers .......................................... 14

2.4.1 Silicon-Controlled Rectifiers (SCR).................................................................14

2.4.2 Triacs.................................................................................................................152.5 Laboratory Procedures and Report ............................................................................. 16

2.5.1 i-v Characteristics of Diodes, SCRs and Triacs .............................................16

2.5.2 Diode, SCR and Triac Rectification with Resistive Loads...............................18

Presentation and Report .................................................................................................... 19

Prelab ................................................................................................................................ 19


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Lab 1 Power Characterization. Diodes and Controlled Rectifiers Manual

EE123 Power Electronics

University of California - Riverside

2


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Objectives

Theobjectives of Lab 1are:

1. Become familiar with LTspice IV software*environment;

2. Understanding of fundamental power characterization quantities such asinstantaneous, average, rms power, power generation and consumption,

power factor;

3. Understanding of the operation principles and i-v characterization offundamental power switching electronic components and their

characterization regular rectifiers (diodes) and controlled rectifiers

selected thyristors (SCRs and triacs);

4. Understanding of basic evaluation of power consumption by switchingpower components;

Hardware

none

Software

LTspice IV Spice Simulator

EE123 Power Electronics Spice Library

Parts

none

*LTspice IV is a registered trademark of Linear Technology


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PART 1. Power Characterization

1.1 LTspice IV Software

LTspice IV is a freeware computer software implementing a SPICE simulator

of electronic circuits, produced by semiconductor manufacturer Linear

Technology (LTC). LTspice IV provides a schematic capture and waveform

viewer with enhancements and models to speed the simulation of switching

regulators.

LTspice IV is node-unlimited and 3rd party models can be imported. Circuit

simulations based on transient, AC, noise and DC analysis can be plotted as well

as Fourier analysis. Heat dissipation of components can be calculated and

efficiency reports can also be generated.

Since SPICE simulators are all based on the same Berkeleys SPICE core, the

workflow within LTspice IV is very similar to that within other SPICE

simulators. More than that, electronic component model libraries developed forone simulator are easily transferrable to other simulators.

The primary references which describe the features of LTspice IV are the Help

file (LTspice IV Users Guide) and the LTspice Getting Started Guide which

along with the LTspice IV software can be downloaded from Linear Technology

site http://www.linear.com/designtools/software/. There are multiple LTspice IVtutorials available on-line, for example, LTspice Tutorial prepared by Terry

Sturtevant of Wilfrid Laurier University which can be found at

http://denethor.wlu.ca/ltspice/.

1.2 Energy and Instantaneous Power

Figure 1.1Power consumers and generators

Energy is a quantity which characterizes work, performed or to be potentiallyperformed. It is an indirectly observable quantity which in electronics isdetermined by the rate of change of power which is defined for shown voltage

polarities and current directions in (1).

Wiki: http://en.wikipedia.org/wiki/LTspice

Wiki: http://en.wikipedia.org/wiki/SPICE


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(1.1) ( ) ( ) ( ) dW

p t v t i tdt

= =

wherep(t) is the instantaneous power measured in Watts [W], v(t) is an

instantaneous voltage across an electric component, and i(t) is the instantaneouscurrent in the branch where the component is located.

So that total energy generated or consumed with respect to some initial time t0is

given by the integral of power. The unit of energy is Joule [J].

(1.2)

0

( ) ( )

t

t

W t p d =

The sign of power determines whether a component generates or consumespower.

(1.3) if( ) 0, consumer

( ) 0, generator

p t

p t

>

>

1.3 Average and RMS Power

An average of a periodic waveform, and power in particular, is defined by thetime average of the waveform over one period T:

(1.4)0

0

1

( )

t T

t

P p t dt T

+

=

Note that a more general definition of the average of a periodic waveform states

that

(1.5)

0

1lim ( )

t

tt

P p dt

=

Both (1.4) and (1.5) provide the same result if averaging in (1.5) is done over

many period cycles when the steady-state is established.

Theroot-mean square (RMS)of a periodic waveformx(t) is defined by (1.6).

(1.6)0

0 0

2 21 1 ( ) lim ( )

t T t

rmst

t t

x x t dt x dT t

+

= =


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Figure 1.2The RMS values of commonly encountered waveforms.Taken from M. Rashid, Power Electronics: Circuits, Devices and Applications, 3

rded,

Prentice Hall, 2004.


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As far as theRMS value of powerwhich is also referred to as effective power, orapparent

power, it is defined as a product of the rms values of voltage and

current. This is to say, if

(1.7)0

0 0

2 21 1 ( ) lim ( )

t T t

rmst

t t

V v t dt v d T t

+

= = , and

(1.8)0

0 0

2 21 1 ( ) lim ( )

t T t

rmst

t t

I i t dt i dT t

+

= =

then

(1.9)rms rms rmsP S V I =

Note that the unit of the RMS of power Sis Volt-Amp [VA].

1.4 Average and RMS Power

The most suitable waveforms for practical analysis are sinusoidal waveforms. Forsinusoidal voltage and current:

(1.10) ( ) cos( )m Vv t V t = +

(1.11) ( ) cos( )m Ii t I t = +

where VmandImare amplitudes, is the angular frequency of oscillations and Vand I are phase delays, the instantaneous power is

(1.12)1 1

( ) cos( ) cos(2 )2 2

m m V V m m V V p t V I V I t = + +

which due to the amplitude limited oscillation of the second term in (12) leads to

theaverage(orreal, ortrue)power P measured in units of [W]:

(1.13)

0

1 1lim ( ) cos( )

2

t

ave m m V I t

t

P P p d V I

t

= = [W]

The apparent power Sof the sinusoidalvoltage and current waveforms gives

It is apparent because rms values of voltage and current are measured with multimeters, and

their product will give the result for the value of power. Note that apparent true.


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(1.14)1

2rms mV V= , and

1

2rms mI I=

(1.15)1

2rms rms m mS V I V I = = [VA]

Remember that

Another critical quantity is thepower factorwhich is defined as

(1.16) p.f. cos( )V I

P

S = =

Since cosine is an even function, its value will be independent of the sign of the

expression under cosine. However this is an important piece of information in theanalysis of power systems:

(1.17) ifp.f. 0, (current) leading (voltage)

p.f. < 0, (current) lagging (voltage)

>

True PowerP(= Pave) Apparent Power S(= Prms)


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1.5 Laboratory Procedures and Report

1.5.1 Average and RMS Values of Waveforms

a)

b)

Figure 1.3Schematic for measurement of average and rms values of miscellaneous

waveforms a), and a sample simulation output b).

1. Capture the schematic of Figure 1.3ausing LTspice and EE123 PE Librarysource components ee123_vsin, ee123_vsaw, ee123_vpwm, mathematical

components ee123_math_aveand ee123_math_rms. And the load resistor

from regular LTspice library.

2. Assuming amplitudes of all voltage generators to be Vp = 170V andfrequency fs= 60 Hz, measure the instantaneous, average and rms values

of a sinusoidal voltage across the load resistor RL over a long period of

time. The results will look similar to the ones shown in Figure 1.3a. Forsinusoidal waveform, compare the results with the theoretical formulas

(1.5) and (1.7). How many oscillation cycles it took for the average and

rms values to be well defined? If needed prolong the simulation time givenby the transient simulation parameters in .tran 0 xxx where xxx is the


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simulation time in seconds. At which point in time the amplitude ofoscillations become less then 5% of the peak value?

3. Change the load resistor voltage input label (Right-Click over the label) tosawtooth (VSAW) and PWM (VPWM) input sources, and repeat

calculations using formulas provided in Figure 1.2. For the PWM voltagegenerator repeat calculations with different duty cycles**: 10, 30, 50, 70.

For the PWM case, what is the relationship between the duty cycle,

waveform amplitude and the rms value of the amplitude?

1.5.1 Instantaneous, Average, Apparent Power, and Power Factor

4. Capture the schematic of Figure 1.4 and with a wattmeteree123_meas_wattmeter_p_s_pfcomponent.

Figure 1.4Instantaneous, Average, Apparent Power and power factor measurements

5. Measure the load resistor power consumption and compare the results withthe ones obtained from theoretical calculations in (1.13), (1.15) and (1.16).

**Duty cycleis defined for a periodic pulsed signalas a percentage fraction of time when the

signal is ON with respect to the period of the signal: duty cycle = tON/ T x100%


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PART 2. Diodes and Controlled Rectifiers and Switches

2.1 Switches as Controlled Power Regulators

Consider a basic switch circuit of Figure 2.1. By periodically (or not) turning it onand off the average power supplied to the load resistor will change even thoug thesupply power source VS provides a constant voltage output. It is not surprising

since the voltage across the load resistor is a PWM (pulse-width modulated)

signal, and the average power is determined by methods already used in Part 1of

this laboratory.

a) b)

i

v

i

v

Figure 2.1Switch as a controlled regulator of average power supplied to a load a)basic circuit, and b)i-vcharacteristic

Even though the circuit is seemingly straightforward, it does a very clearunderstanding of the power dissipation in switching electric and electronic

components. Lets see this by example.

Example

Assume that the Switch has ON resistance Ron= 1 m, and isnominallyrated at W. It is the means to deliver power to a 1W resistor Rload= 100 from a powersupply VS = 10V which apparently consumes power PRload = VRload IRload =

V2

Rload/Rload1W. Note that even though the switch transfers and can handle 1W

of power to the load resistor, by itself it consumes only a negligible portion of it,that is, Psw= VswIsw10

-7W, well within its nominal rating.

However, assume now that the load resistance R load= so that Psw0.022/10-3

= 400 mW. This is to say, instead of just transferring power to the load resistor,now the switch acts as a power consumer, consuming 400 mW > 250 mW(nominal). It cannot dissipate this much power, and it will get burned a short

while after being turned on.


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2.2 Diode Rectifiers as Uncontrolled Switches

It is not difficult to observe from the i-v characteristic of a switch in Figure 2.1that its behavior is similar to that of an ideal diode with one exception the

switching action of a switch is independent of the nature of the power source, DC

or AC, while the diode switching action can only be achieved with AC power

sources. This is to say, a regular switch can provide a controlled switching action(rms voltage output depends on the duty cycle of the switching action) while a

diode can provide only an uncontrolled switching action.

Figure 2.2i-vcharacteristic of a diode a), and its models b)ideal, c)0.7 voltage drop,

and d)piece-wise linear (PWL) model.

Note in Figure 2.3that a diode switch just cuts-off the negative part of the voltagewaveform, and also that the average and rms output voltages cannot be modified.

(2.1)0

1sin ( )

2

mo avg m

VV V V t d t

= = =

(2.2) [ ]2

0

1sin ( )

2 2

mrms m

VV V t d t

= =

(2.3) 2 /2 /4rms m rms rms rms mI V R P V I V R= = =

i

v

i

v

i

v

iiddeeaallddiiooddee

00..77VVddrrooppddiiooddee

PPWWLL((ppiieecceewwiisseelliinneeaarr))ddiiooddee

a)

b) c) d)


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Figure 2.3The output of the diode based voltage rectifier

2.3 Voltage Controlled Switches

Consider a class of the relay typeelectromagnetic switches whose state of being

closed or open is controlled by the applied voltage. Compared to the regular

(mechanical) switch it has an extra third terminal.

The action of such switches on input waveforms is shown in Figure 2.2.

Figure 2.4Voltage controlled switch action


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2.4 Thyristors Semiconductor Based Controlled Rectifiers

The lack of diode applicability to controlled switching is compensated by the type

of rectifiers commonly known as thyristors. Thyristors refer to the class of

semiconductor devices which look like a sandwich of p-n layers (pnpn, pnpnpn,

etc.). Here we will consider only two types Silicon-Controlled Rectifiers (SCR)

and Triacs.

2.4.1 Silicon-Controlled Rectifiers (SCR)

Unlike the diode, the SCR will not begin to conduct as soon as the source

becomes positive. Conduction is delayed until a gate current is applied, which is

the basis for using the SCR as a means of control. Once the SCR is conducting,

the gate current can be removed and the SCR remains on until the current goes to

zero. The i-vcharacteristic of SCR is shown in Figure 2.5.

Figure 2.5i-vcharateristics of SCR

Figure 2-6shows the voltage waveforms for a controlled half-wave rectifier with a

resistive load. A gate signal is applied to the SCR at t= , where is the delay

angle. The average (dc) voltage across the load resistor is

(2.4)0

1sin ( ) (1 cos )

2 2m

o avg m

VV V V t d t

= = = +

(2.5) [ ]2

0

1 sin 2sin ( ) 1

2 2 2

mrms m

VV V t d t

= = +

(2.6)2/rms rms rms rmsP V I V R= =

i

v

iiddeeaall

a) b)

i

v

nnoonn--iiddeeaall


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Figure 2.6SCR based controlled rectification a), and its waveforms b).

2.4.2 Triacs

A very important class of controlled rectifiers are triodes which act like SCRs

except that their current can be turned on in both directions. The i-vcharacteristics

of triacs are shown in Figure 2-7.

Figure 2.7i-vcharacteristics of SCR

(2.7) 1 sin 222

mrms

VV

= +

(2.8)2/rms rms rms rmsP V I V R= =

i

v

iiddeeaallTTrriiaacc

a) b)

i

v

nnoonn--iiddeeaallTTrriiaacc


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Figure 2.8SCR based controlled rectification a), and its waveforms b).

Note the similarity in behavior between triacs and SCRs. Note that the i-v

characteristics cannot show a time-dependent nature of switching. So the analysis

of waveforms is essential in all practical applications.

2.5 Laboratory Procedures and Report

2.5.1 i-v Characteristics of Diodes, SCRs and Triacs

Figure 2.9Circuit for evaluating the i-vcharacteristic of diodes

1. Capture the schematic of Figure 2.9 using the EE123 PE Library diode

component ee123_diode. To obtain the i-v characteristic it is required tovary the DC voltage VDD, in our case from 0V to 20V in steps of 0.1V;

2. After the simulation is completed hover with the mouse cursor over overthe resistor to show its current. Then in the waveform viewer hover with

the mouse over the time axis (or just below it) until it changes to a ruler,


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then Left-Click to set the axis in Quantity Plotted from time to

v(vd) or whatever voltage label you assigned to the node (see Figure 2.9

for an example).

Figure 2.10Circuit for evaluating the i-vcharacteristics of SCRs and Triacs

3. Capture the schematic of Figure 2.10using the EE123 PE Libraryrectifiercomponents ee123_scrand ee123_triac.

4. The i-vcharacteristics of SCRs and triacs cannot be determined the waythey were determined for diodes. So what we will do, is we will make a

number of measurements for different input voltages using an arbitrary

AC voltage source which will cause different currents at different times.

Then if we plot current vs voltage, all possible permutations will be shown

in the waveform viewer. Note that it is critical to set the tdelay in VPWM

switching sources to a value other then zero. After running the simulation,

repeat the steps of 2. to obtain the i-v characteristics of SCRs and triacs.Note however that the x-axis voltage should show the voltage across the

rectifiers. In the case shown, v(vsin,vd1)which tells LTspice to show the

voltage difference between nodes VSINand VD1.

5. How thus obtained i-v characteristics compare to the ones shown inFigures 2.5and 2.7?


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182.5.2 Diode, SCR and Triac Rectification with Resistive Loads

Figure 2.11Circuit for plotting the waveforms of rectifiers resulting from the sinusoidalvoltage source.

6. Capture the schematic of Figure 2.11 using the EE123 PE Library diode

component ee123_diodeand thyristors ee123_scrand ee123_triac.

7. Plot the resulting waveforms and compare them to the sinusoidal voltagewaveform.

8. Explain the shape of waveforms in each case. How would you predict thisbehavior from the i-v characteristics of rectifiers. Shown in Figures 2.3,

Figures 2.6and 2.8, and also obtain in Procedures 2.5.1?


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Presentation and Report

Lab report must be presented according to the general EE123 labguidelines.

Prelab

1. Study the theoretical materials provided in this manual, and familiarizeyourself with lab procedures.

2. Download and install LTspice IV from Linear Technologys websitehttp://www.linear.com/designtools/software/

3. Familiarize yourself with the LTspice Users Guide;

4. Study the LTspice tutorials uploaded on iLearn and run a few test cases tounderstand how to conduct simulations.

Power Characterization, Diodes and Controlled Rectifiers

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