Grading sheet for Experiment 4: Buck-Boost Converter

Abstract-1 point

Part I Output Voltage vs. Duty Cycle-3 points Measurements 1 pt Theoretical Equation for Vo Yz pt Plot of Vo vs. %DC 1 pt Discussion/Comparison Yz pt

Part II Ripple vs. Switching Frequency-2 points 1 current ripple waveform Yz pt 1 voltage ripple waveform '!4 pt extra credit Current ripple equation/theory Yz pt IL,ripple plot vs.fs Yz pt Voltage ripple equation/theory '!4 pt extra credi t Vo,ripple plot vs.fs ~ pt extra credit .~

Discussion Y2 pt

Part III Discontinuous Current Mode-1.5 points RL.critical at DCM boundary '!4 pt RL,critical theoretical calculation '!4 pt ILwaveform at DCM boundary '!4 pt Mosfet wavefonn in or at DCM '!4 pt Diode wavefonn in or at DCM '!4 pt Discussion Y4 pt

Part IV Efficiency-2.5 points Efficiency Measurements 1 pt Efficiency Plot 1 pt Discussion of Buck and Boost Yz pt

7A

Matthew Fuhs Mike Kuiken Richard S Carrillo Samuel Annor EE4743 POWER ELECTRO ICS LAB

EXPERIMENT 4: BUCK-BOOST CONVERTER

Abstract: In this lab, we observed the characteristics of a buck-boost converter. First we

noted the output voltage with respect to the duty ratio nearly matched the theoretical values. Next we measured output voltage ripple along with input current ripple at different switching frequencies ranging from 50 to 100 kilohertz. The current ripple was also very close to theoretical values. Continuing, we observed the effect of a varying load on the converter and its discontinuous conduction mode and boundary conditions by measuring input current and MOSFET voltage at different load resistances. Finally, we determined the efficiency of the buck-boost converter at different frequencies between 50 and 100 kilohertz. The efficiency averaged 89 percent but declined to 86 percent at 100 kilohertz.

Body: 4.5.1 Varying Duty Ratio:

In this section we measured the voltage over a load resistor at each duty ratio interval of 10% up to 70 and a constant input voltage of 10.0 I volts in order to compare its theoretical load voltage to the actual measured voltage.

The theoretical values were calculated using the following fonnula.

Yout = D Yin I (l-D)~vhere Yin = 10.01Y. /

I Experimental Theoretical

Vload(V) Duty ratio (%) Vload(V)

Duty ratio (%)

0 0 0 0 1.642 10 1.112 10

202.855 20 2.503 4.50 30 4.29 30 6.30 40 6.67 40 9.48 50 10.01 50

14.26 60 15.02 60 22.07 70 23.36 70

Load Voltage Vs Duty Ratio with Yin = 10.01 V

35.----------------------------,

JO

25

~ 20

~ o > "0

~ 15 ..J

- Theoretical

Measured

10

0'-==-­ -1

o 0.1 0.2 0.3 04 0.5 06 0.7 0.8

Duty Ratlo(-;o}

As can be seen, the experimental and theoretical values do not exactly match but are very close. The largest contributor to the mismatch of these values is from the non-ideal properties of the main circuit elements which are the inductor, capacitor, MOSFET and diode. Additional minor contributors are test equipment accuracy and other circuit elements on the test board. From the graph it is also noted that Yout does indeed depend on duty cycle and Yout will increase above Yin at a 50 percent duty ratio.

4.5.2 Varying Switching Frequency: In this section, at a 50% duty ratio and R10ad = son, we obtained the waveforms of inductor current ripple and output voltage ripple while varying the switching frequency. The switching frequency was set at 50 kHz, 60 kHz, 80 kHz and lOO kHz. This test was done in order to show how the two waveforms relate as switching frequency increases. Since the ripple voltage was small compared to the average DC level, the oscilloscope was set to AC coupling to block the DC. On the next page is a table of the results. Note CS5 is a current sense test point on the test board and was used to measure the current through the inductor. The voltage at this test point is converted tc)' amperes by multiplying the measured voltage by the given conversion factor of 2A/Y. v

The theoretical values were calculated from the following two formulas. Note that Yin ttlAe remained lO.OlY during the entire test. "otl-"(1,,y\C\G'.~~6..

/ ~ '("'~:~;\~I~ ()V'

.ML = YinD/LFs where Fs is the switching frequenc~SinceD = 0.5, L=l0C@1 and r ~\V\<\ LJ ~\~l Yin = 10.0 1Y this equation reduces to ~IL = 50()50/ Fs. Additionally, to make units ",,0 ~~'i('l~ f()JJ" match the measured values in rnA, ~IL will be multiplied by 1000. \ 'f\t\(,\(G~

~Yout= ,

1 100r".'1/

OTDFTEJ{l\1I 'EI> tadn, ! LA. _J\wV\~ weft \V'et) \t\oro.eWJ'mIV'ltlll ,

.. o.OOs 5.00W' Auto I

~1

.",;;;b.=X--'--=_2=0"'-.;...;;0...;;.:u-=.s --") l/b.X = 50.000kHz J b.YC 1) = -463mV )

.A. Mode 'I.A. So,urce J' X Y J Yl JU Y2 J J " Normal -,,, 1 " v " 46B.BmV" 6.3mV" Yl Y2

Figure I: Example of CSS waveform

J

Switching Freq. (kHz)

Theoretical I L Ripple (mApk-pk)

Theoretical VL Ripple (mVpk-pk)

Measured I L

Ripple (mApk-pk) Measured Vout Ripple (mVpk-pk)

50 1000.001 1038 139 60 833.3341667 850 113 80 625.000625 606 89.1

100 500.0005 504 78.1

Inductor and Vout Ripple vs. Switching Frequency

140 . 1400

130 1300

120 , 1200

110 1100

:>i' c. :>i'

.>< 100 1000 ". c. ""c. .. Theoretical VL Ripple (mVpk-pk) > « .s 900.s Measured Vout Ripple (mVpk-pk)

90 ~

~ -Theoretical I L Ripple (mApk·pk)c. c.a: 800 ~ Measured I L Ripple (mApk-pk) ..

:J 80 ...J

0 >

70 700

60 600

50 ~500

40 400

40 50 60 70 80 90 100 110

Switching Frequency (kHz)

From this data, it is observed both ripple amplitudes decrease as switching frequency increases. This is expected since there is less time for the inductor and capacitor time constants to ram...£.to their steady-state values. It is also noted that there is a larger relative difference between the ripple amplitudes at high frequencies as compared to lower frequencies. This is likely due to the variance in the inductor and capacitor time constants. If both time constants had the same frequency dependence the relative difference would remain unchanged. Furthermore, the theoretical and measured inductor current ripple closely coincides. Variations are considered to be due to the non-ideal properties of the inductor such as internal resistances. Though altt'mpled, lhe lhcon'lical \'0111 ripple calculations were nol slice 'ssful and therefore can IHH he

WV\~t Mo.tAe t1t1eM lAV\SlAC(fS~\?. Yo'!'- S~ 0 lr\\ j llJ\ d\Ac\ e-- WVV-\r-~ 0LA, Cq~G ~~ W\ t~ Sor LG\V\ lAelp ~OtA Lulfh d",

com part:'tl. 4.5.3 Varying Load:

With a duty ratio of 50% and switching frequency of 100kHz we increased the load resistance until the convet1er entered into discontinuous conduction mode (DCM). We then made observations of the inductor cUiTent, MOSFET, and diode voltage waveforms b he DCM border and well into DCM. The load resister for border conditions was 3236bms.

110~1 &: 0.005 2.~ Auto ~ 2 5.47Y (\/'l'j 'll.~CI

W\t\~hDowd ~OlA. U-peL\

t-o~U-

Duty( 1 ): No edges l Freq( 1 ): No edges FreQ( 2 ): No si nal

• Source J Clear J Frequency) Period Peak Peak 1 .....1 . ----,-M""c"""s-J.',__----f.__-.J L - .,..

Figure 2: Inductor current well into DCM

15.00'11 .. 0.005 2.0~ Auto ~ 1 10.3V V

Duty( 1 ): 2.6;;

Figure 3: Voltage across MOSFET well into DCM

110.0U .. 3.36~ 2.00'Jj Stop ~ 1 32.5V j

r"'"\.,. frr-!-~----\ r ,I

DutY(l): 70.17­~ I FreQ( 1): 543kHz

'.' Freq ( 2 ): No signal"~

Print to disk file: PR NT 1 1 )

Figure 4: Voltage across diode well into DCM

4.5.4 Determining Efficiency With a duty ratio of 50% we adjusted the load resistance so that the load current was equal to 0.25A. The switching frequency was then adjusted to 50 kHz, 60 kHz, 80 kHz, and 100 kHz. At each frequency the Vin, lin, Vout, and lout were measured so the efficiency could be calculated. Three multi-meters were used to make the measurements.v The input and output currents required dedicated series meters. The third meter was used to measure the voltages. Below are the measured values. Gt>o&.-d.~pnO'V'

../Set Measured Calculated

Frequency (kHz)

Vin (V)

lin (A)

Vout (V)

lout (A)

Pin =

Vinlin (W)

Pout =

Voutlout (W)

Efficiency = Pout/Pinx100 (%)

50 10.01 0.26 9.24 0.25 2.6026 2.31 88.76

60 10.01 0.26 9.26 0.25 2.6026 2.315 88.95

80 10.01 0.26 9.28 0.25 2.6026 2.32 89.14

100 10.01 I 0.27 9.3 0.25 2.7027 2.325 86.03

Buck-Boost EHiciency vs. Switching Frequency

89.50

89.00

8850

88.00 .

~ >­u

87.50c:

"-­ -

-----+=Etficiency = PouVP,nxl00 (%) u i: UJ

87.00

8650

86.00 .

8550 40 50 60 70 80 90 100 110

Switchin9 Frequency (kHz)

As shown, the Buck-Boost converter efficiency declines after 80 kHz. This may be attributed to switching losses. Both the Buck and Boost converters had slightly higher efficiencies which can be expected due to the higher component count of the Buck-Boost.

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Conclusion During this experiment, we successfully observed the characteristics of a buck­

boost converter. We verified the output voltage is dependent on duty cycle and that Yout will increase beyond Yin at duty cycles greater than 50 percent. Next, we measured the inductor current and output voltage ripple at different switching frequencies ranging from 50 to 100 kilohertz. It was noted that both ripples decreased in amplitude as the switching frequency increased. By varying the load, the DCM boundary was determined to be at 323 ohms. Waveforms of the input inductor current, along with voltages across the MOSFET and diode were taken well inside DCM. Finally, the efficiency was determined to peak at 89.14percent at 80 kHz and decline to 86.03 percent at 100 kHz.