how to calculate inductor value of buck boost converter for range of dutycycle

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azarutz

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Hi i am designing an inverting buck-boost converter . I have to calculate inductor value for range of duty cycle (0.436 to 0.73) i searched in book it only explained calculating inductor value for single duty cycle . Is there any way to calculate inductor value for range of duty cycle .

MrAl

Well-Known Member
Hi,

One of the main points is the ripple current though the inductor, and also the peak current. So these points are the main points to be kept in mind throughout the process of selecting an inductor. The ripple current affect the filter capacitor because it has to be rated for at least that ripple current level, and the peak current affects the inductor mostly because it causes the inductance to drop and when the inductance drops it causes more ripple current which again affects the capacitor. The ripple current also affects the output ripple voltage so you've got to keep an eye on that when you have to spec the output ripple voltage which is also usually the case because the design usually has to be able to work with a certain load which is only allowed so much ripple riding on the DC output.

Because we usually have an input voltage range and fixed output what you would do for a complete analysis is start with say the lowest input and calculate the ripple current and peak current, then go to the highest input and check again, then maybe a middle point. This helps not only choose the inductor but also gives you an idea about how stable the circuit is going to be.

A short cut is to figure out the ripple and peak currents at a 50 percent duty cycle, because that is theoretically the worst case. With an input voltage that gives you the correct output voltage when there is 50 percent duty cycle you get an idea what the ripple and peak current is going to be so you can adjust the value of the inductor to suit.

It's always a good idea to do a simulation too, even if the simulation does not include feedback. Including the feedback helps investigate the stability.

So to recap, the main point is the ripple current and peak current. The ripple current works with the capacitor (and load) and that's where the ripple voltage comes from so the ripple current has to be kept under some given level to keep the capacitor from overheating and also working with the capacitor value to keep the ripple voltage low. The peak current causes a decrease in inductance which in turn causes more ripple current (lower inductance=higher ripple current) and that more ripple current causes more stress in the capacitor and also more ripple voltage. So there will be some minimum inductor value you want to use that gives you the required ripple voltage and ripple current, even if the DC load current goes up to max.
Of course the DC rating of the inductor has to be able to handle the average current too without overheating, and the drop in inductance can not be too large with the rated load current.

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azarutz

New Member
Hi thanks for reply . You explained very clearly . Can you suggest me a good simulator for buck boost design . I simulated in proteus it didnt give output but that circuit works in matlab.

MrAl

Well-Known Member
Hi,

Did you try LT Spice yet?

azarutz

New Member
Hi MrAl, this is my ltspice file

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MrAl

Well-Known Member
Hi,

Ok, i dont see the statement for your switch. For example:

.model mysw SW(Ron=0.1,Roff=1meg,Vt=1,Vh=0.2)

then rename the switch (SW) to mysw. Try that, then run it.
There's more help on this in HELP on the menu.

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azarutz

New Member
Hi thanks for reply . still i get same voltage no boosting operation i included as u said i didn't figure it out . i want to get 54 V output still i get 30 to 33 V for 45V input. I varied Duty Cycle result no changes . is any prob in schematic ?

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MrAl

Well-Known Member
Hi,

Oh ok, well well if you want buck boost you have to include a diode between the inductor (top) and cap (top) with the cathode to the inductor top and anode to cap top. Diode should be a Schottky type.
You also will have to reverse the polarity of the cap because the output will be negative.
Also, dont forget to set the ton time in your pulse source.

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azarutz

New Member
Hi after including diode and reversing capacitor polarity still no help .No variation of output after varying duty cycle .No boosting operation .

MrAl

Well-Known Member
Hi again,

You wont be able to see a boost unless there are certain requirements met. For one, the inductor series resistance can not be too high. That's because it has to be able to draw a certain level of peak current in the time it has to be charged (Ton of the switch) and if there isnt enough time (because the resistance limits the current vs time) then the inductor can never charge up to the required current level in order to produce a boost with a certain load value.
To see it work better either lower the inductor series resistance or increase the load resistance. You can then see a boost and then experiment to see how the series resistance affects the boost level and also how the load limits the boost too with that certain inductor series resistance.

Here's my file that does show a boost...you can try this and take it from there...

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azarutz

New Member
Hi thanks for circuit you helped me a lot. I have some doubts which i not aware of . Is it necessary that average voltage across inductor and average current through capacitor should be zero? and when i give 20V input i get more ripple voltage at output then 45V input why it happen ?

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MrAl

Well-Known Member
Hi again,

If the average voltage across the inductor was not near zero it would mean we would have a net DC voltage across the coil which would lead to an infinite current draw. There's always some series resistance to limit current, but it would probably still be too high.
In order to get an average current though the cap that was not near zero it would require a voltage that is continually rising, which we cant have because any physical system will have some limitation.

About the ripple, you can check the diode to make sure it has a voltage rating that is well above the operating voltage. You can also check to see that the cap is not reversed as i think i forgot to reverse it in my circuit i posted previously.
In a simulation i did i saw an increase in absolute ripple voltage when i went from 20v to 40v input, not a decrease, however percentage wise it was a decrease (ripple did not double it was less than that).

azarutz

New Member
Hi i see it in a pdf in internet . It says about volt-sec balance if average voltage across inductor is zero then it satifies volt-sec balance , and also for current in capacitor , i actually dont understand about it .
I reversed the capacitor polarity it looks fine and ripple also reduced after changing diode and i reduced load resistance to 10 ohm and also inductor to 2mH it also works fine for voltage variation of 20V to 70V to get constant 54V at output by varying duty cycle only .

MrAl

Well-Known Member
Hello again,

Oh that sounds good. So i guess you have it working now. Are you going to build this too?

azarutz

New Member
Hi,
No, it is same circuit only actually i am going to charge a battery after that converter so it load resistance will be very smal so i altered the inductor value and it works .
Thank you.

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