# Buck Converter Questions (DC/DC Converter)

Discussion in 'Homework Help' started by jegues, Sep 26, 2012.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Hello all,

I recently did a lab on buck converters and I have a couple a questions.

See the figures attached for lab and my work.

For Experiment 1:

The measured quantities for part b) were,

$I_{ave} = 0.2A, \quad I_{min} = 0.09A, \quad I_{max} = 0.28A, \quad Ripple_{pp} = 0.19A$

These values differ slightly from my theoretical calculated values, did I make any mistakes or are there any discrepancies?

Also, the Imin I calculated was negative, is this a problem? Also, I'm not sure if I am correctly the average current correctly, can someone verify?

The new switching frequency we measured for part c) was,

$f_{swnew} = 300Hz$

The theoretical frequency I calculated was 240Hz. Is this a large discrepency? Did I make any mistakes in the calculation?

For Experiment 2:
For part a),The measured quantities for 500 rpm are,

$V_{t} = 32.8V, \quad I_{a} = 0.82A, \quad \text{Duty cycle }= 32.8\%$

The measured quantities for 1000 rpm are,

$V_{t} = 59.81V, \quad I_{a} = 0.89A, \quad \text{Duty cycle}= 59.81\%$

The measured quantities for part c) are,

$I_{ripple,p-p} = 1.27A, \quad I_{ripple,avg} = 0.79A, \quad f = 292.51Hz, \quad \text{Duty cycle }=32.09%$

When we preformed this we did not observe any oscillations on the shaft speed but I'm not quite sure why. Is it simply because the switching frequency was high enough such that the motor wouldn't have time to react to any of the sudden changes in voltage?

Can someone clarify why that would be the case?

Thanks again!

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• ###### Lab2AT1.JPG
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Last edited: Sep 26, 2012
2. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Bump, still looking for help on this!

3. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Here are better resolution pictures of my work attached.

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4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
It would help to see a schematic of the buck converter. But this question:

In any buck I ever designed (quite a few) the min inductor current can not be negative, it goes to zero in discontinuous mode of operation. Not sure if that answers your question.

see attached refs

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