Download SIMULATION WITH THE BUCK TOPOLOGY ECE562: Power Electronics I COLORADO STATE UNIVERSITY

SIMULATION WITH THE BUCK TOPOLOGY
ECE562: Power Electronics I
COLORADO STATE UNIVERSITY
Modified in Fall 2009
ECE 562 Buck Converter (NL5 Simulation) Laboratory 1
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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:
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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.
ECE 562 Buck Converter (NL5 Simulation) Laboratory 1
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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 100µH.
R is an ideal resistor from the library. Set to 5Ω.
D1 is an ideal diode from the library. Set to 700mV (Diode drop).
C is an ideal capacitor from the library. Set to 10µF.
O1 is 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.
V2 is 0.5 volt reference for the Schmitt trigger comparator O1. Set V2 to 500mV.
V3 is 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 R1 to 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 Right several times to move the selected components to the right
Press the Right Mouse Button to 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 Space to switch to drawing mode (cursor changes to diamond with square at center).
Press Arrow Right several times to draw short wire to the resistor.
Select the right edge of the resistor (R1).
Press Arrow Right several times to draw short wire to the diode.
Press Space to switch to de-select drawing mode (cursor changes to diamond).
ECE 562 Buck Converter (NL5 Simulation) Laboratory 1
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Using the components window, Set R1 to 1mΩ. 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 F6 or 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 ‘D1’
and 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 F6 to 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 results
mathematically.
Hint:
𝑉𝑉𝑜𝑜
𝑉𝑉𝑖𝑖𝑖𝑖
=
2
4𝐾𝐾
(1+�1+ 2 )
𝐷𝐷 𝐼𝐼
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 the
transient settings to start at 700uS and run to 850 uS)
ECE 562 Buck Converter (NL5 Simulation) Laboratory 1
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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 (remember
that 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-topeak 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 5Ω to 500Ω.
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 5Ω and the inductor to 200uH. Also change the
transient data settings to run from 800uS to 1000uS. Add a trace for the capacitor’s 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 5Ω resistor is series with the capacitor, what happens
to 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 configurations
of 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.
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