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ECE 562 Buck Converter (NL5 Simulation) Laboratory 1 Page 1

SIMULATION WITH THE BUCK TOPOLOGYECE562: Power Electronics I

COLORADO STATE UNIVERSITY

Modified in Fall 2009

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PURPOSE: The purpose of this lab is to simulate the Buck converter using NL5

to

better familiarize the student with some of its operating characteristics. This lab will

explore some of the following aspects of the buck converter:

Discontinuous Conduction Mode Inductor sizing Differential voltage across the inductor Time it takes for the converter to reach steady state Output Ripple voltage and selection of the capacitor. Ripple current through the capacitor Equivalent Series Resistance (ESR) of the output capacitor. Effects of changing and removing load resistance Effects of the ON resistance of the switch Efficiency Effects of changing frequency

NOTE: The simulations that follow are intended to be completed with NL5. It is assumed

that the student has a fundamental understanding of the operation of NL5. NL5

provides

tutorials for users that are not experienced with its functions.

http://nl5.sidelinesoft.com/downloads/manual.pdf

PROCEDURE:Part 1: Open NL5; From pulldown menu select File->Open.

Under pulldown menu File->Open, Open Buck.NL5 under the sub- folders Examples->Demo.

http://nl5.sidelinesoft.com/downloads/manual.pdfhttp://nl5.sidelinesoft.com/downloads/manual.pdfhttp://nl5.sidelinesoft.com/downloads/manual.pdf

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Figure 1 - Buck .nl5 "Stock" Demo

From the pulldown menu, select File->Save As and save as buck_lab1_Q1

Modify the component values shown in Figure 1.Select pulldown menu, Windows->components.

Figure 2 - component window for setting component values.

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V1 is a DC voltage source (VDC) from the source library. It needs to be set for 24

volts.L is an ideal inductor from the library. Set to 100H.

R is an ideal resistor from the library. Set to 5.

D1 is an ideal diode from the library. Set to 700mV (Diode drop).C is an ideal capacitor from the library. Set to 10F.

O1is an ideal comparator used to turn the switch S1 on and off.

By varying the width of V3 below, its output will act as a Pulse Width Modulator.

S1 is a voltage controlled switch, a standard component in the library.V2is 0.5 volt reference for the Schmitt trigger comparator O1. Set V2 to 500mV.

V3is Pulsing source. Set to values listed below using the components editing window.

This sets it to a switching frequency of 25kHz with a 50% duty cycle.

Add R1to simulate RON of the switch.

Entering and editing can be done using keyboard keys, mouse, or both. Here are step-by- step

instructions how to add component R1.

Press and hold the right mouse button and drag a rectangle to encompass components

{D1,L,C and R} and then release the mouse.

Press Arrow Rightseveral times to move the selected components to the rightPress the Right Mouse Buttonto move the red cursor in the empty space between the switch

and diode node.

Select the resistor from the component bar (located under letter R tab)Select the right edge of the Switch.

Press Spaceto switch to drawing mode (cursor changes to diamond with square at center).

Press Arrow Rightseveral times to draw short wire to the resistor.Select the right edge of the resistor (R1).

Press Arrow Rightseveral times to draw short wire to the diode.

Press Spaceto switch to de-select drawing mode (cursor changes to diamond).

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Using the components window, Set R1to 1m.The schematic should look like below.

Figure 3 - Schematic of Buck converter with R1 added to simulate switch resistance

Ensure all the wires are connected to components.Now press F6or the Green Arrow in the green rectangular toolbar at the top middle to

start the Transient response.

Figure 4 - Transient analysis of schematic above

Right click on the upper right hand legend showing V(C) and V(D1) and select data from the

popup. Uncheck the box next to I(L). Now under Add new trace window, select V and D1and click the green cross next to the word add. Select Close from the bottom right of the

window.

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Figure 5 - Window for setting data display for Transient analysis

From the pulldown menu, select Transient->Settings and set the Screen parameter to .001 to

show 1mS of data.

Figure 6 - Setting the transient data to show 1mS of data for the full screen

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Press F6to run the transient analysis again and the results should be as below.

(Right click on Transient data window traces and select Zoom->Vertical->Fit to screen.)

Figure 7 - This shows the output voltage rising to 12V and settling down to 10V for the Buck converter. We

can also see the voltage across the diode during the switch off time is near 0.7 volts and during the switch on

time is near the input voltage. (NL5 measures the diode voltage with the plus voltmeter reference on the

anode and the minus voltmeter reference on the cathode (line of diode))

Right click on the Transient data legend and select data.

Uncheck the voltage traces and add a trace to plot the current of L.

Press F6 to run the transient data analysis.Right click on Transient data window traces and select Zoom->Vertical->Fit to screen.

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Figure 8 - Add a trace to show the current of the inductor.

Figure 9 - From the picture above, we can see that the converter is operating in the continuous conduction

mode with an average operating current of about 2.3A and a peak in-rush current of about 5A

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Now right click on the graph and select table.

Again, right click on the graph and select cursors, check the Show box and then select close.

Figure 10- Same graph as above but with cursor and table mode enabled

Question 1:What is the peak operating current, average current, and what is the operating mode of

the converter? Verify mathematically the mode and the peak current.

Hint:Kcrit= (1-D) ; Ch 5 Buck converter

K = 2L/(RT)

(Vin-V)/L = Ipk/DTs

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Question 2:

What is the average output voltage of the converter at steady state? Verify your resultsmathematically.

Hint: = 2(1+1+ 42)

Figure 11 - The picture shows that the converter reaches steady state after about 0.5 mS with a average

output voltage of 11.5 volts

Question 3:

What is the peak-to-peak ripple on the output voltage after it reaches steady state? (Hint: set thetransient settings to start at 700uS and run to 850 uS)

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Figure 12 - It is apparent that the ripple is around 1.2 Vpp

Now change L from 100uH to 30uH and rerun the simulation. Remember, you can vary the time

display using the Transient->Settings pulldown menu.

Question 4:

What is the peak inrush current now? What is the operating mode of the converter (rememberthat you can observe this by zooming in?)

Figure 13 - From the picture we can see that the peak inrush current is almost 12 amps and the converter is

operating in the discontinuous conduction mode.

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Question 5:

What can be said about the differential voltage measurement across L?

Figure 14 - Differential voltage across L

Question 6:

How long does it take for the output voltage to reach its average voltage? What is the peak-to-

peak ripple on the output voltage?

Figure 15 - From the picture above we can see that the average output voltage is reached in approximately

110uS with an average value of about 13 VDC. The output also has about 4V peak to peak ripple.

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Now change the load resistance from 5to 500.Change the run time of the simulation to 200uS.

Question 7:

What happens to the output voltage and what observations can be made?

Figure 16 - From the picture above, we can see that the output voltage rises to the value of the input voltage,

24V. This shows that the output voltage of the buck converter is not only a function of the duty cycle,

inductor value and the capacitor value but also the load resistance.

Question 8:What operating mode is the converter in?

Figure 17 - From the picture above, we can see that the converter is operating in the discontinuous

conduction mode with a peak current ~10Amps.

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Now change the load resistance back from 5and the inductor to 200uH. Also change thetransient data settings to run from 800uS to 1000uS. Add a trace for the capacitors voltage.

Question 9:

How does the inductor value affect the output ripple?

Figure 18 - From the picture above we can see that the peak to peak ripple is now less then 1V and from the

inductor current we can see that the converter is once again operating in the continuous conduction mode.

We can also compare the peak-to-peak inductor current with the peak-to-peak inductor current when the

inductor was 100uH and see that the inductor current ripple has also decreased.

Question 10:

With everything else left as is, what would the minimum output capacitance need to be in order

to limit the output voltage ripple to 2 volts peak to peak?(Hint:Set the transient settings to trace the data from 800uS to 1mS)

Question 11:

What can be said about the current through the capacitor?

Question 12:

If the ESR of the capacitor is modeled by a 5resistor is series with the capacitor, what happensto the output voltage ripple and the capacitor current?

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Question 13-16:

Leave the resistor that was just added. What happens if the duty cycle of the converter is

decreased from 20usec ON time to 5usec ON time in V3 set up?

Is the converter still operating in the continuous conduction mode?What is the average voltage ripple increase?

Question 17-20:

What observations can be made from increasing the on resistance of the switch?

What can be said about the efficiency of the converter? Comment on the different configurationsof the circuits used throughout this lab.

What happens if the load resistance is removed?

(Hint:Set a very high value for the load resistance, i.e 10000Meg)

What can be observed by increasing the switching frequency to 100kHz?

Hint:With everything else left as is, change V3 Width and Period to 5u and 10u respectively.

Adjust the transient settings to adjust the trace time accordingly.