# Buck converter - 72V to 24V

Discussion in 'Homework Help' started by gilley, Dec 7, 2014.

1. ### gilley Thread Starter New Member

Nov 23, 2014
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I have a task to design buck converter that will convert 72V to 24V, no eletric current was specified.

Can someone give me the link where I can find some good info? Maybe even some help ?

Feb 17, 2009
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3. ### gilley Thread Starter New Member

Nov 23, 2014
23
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Thank you Jony,

I have one question for you; who/what does determinate switching frequency?

4. ### Papabravo Expert

Feb 24, 2006
10,420
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You do. Like everything else in engineering there are tradeoffs. This is one of them.

Nov 23, 2014
23
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6. ### Papabravo Expert

Feb 24, 2006
10,420
1,875
Have you read the datasheet carefully?
The "fixed" frequency mode mode has a nominal value of 127 kHz "under heavy load", with a minimum of 106 kHz and a max of 150 kHz. It also skips pulses at light loads.
In addition it can be synchronized to any frequency from 119 kHz to 200 kHz. So no you don't have to use 127 kHz if you don't want to.

7. ### gilley Thread Starter New Member

Nov 23, 2014
23
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update:

I have tried to design it by using instructions from this site:
http://powerelectronics.com/site-fi...archive/powerelectronics.com/mag/606PET25.pdf

And here are my parameters:

V input - 72V max

V output - 12 V switching frequency - 100 kHz ( LTC3703)

I max -1,5 A

However, my teacher has given me a 15μH inductor and said that I should use that one, and that is where my problems start.

When I go to design it using formula for inductor, LIR is extremely big ( somewhere around 4-5, instead between 0.2-0.5 like proposed on web site )

This is the inductor we are using

http://hr.rsdelivers.com/product/wu...-15-μh-20-16-a-idc/3674660.aspx?query=3674660

So can someone help me to get good LIR, or should I say that we can't use this inductor?

8. ### ISB123 Well-Known Member

May 21, 2014
1,240
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Why would he do that?You calculate inductor value depending on input voltage,output voltage,output ripple and output current.

9. ### gilley Thread Starter New Member

Nov 23, 2014
23
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Ummm sorry, why would he do what ? Give me 15μH inductor ? Because he has one in school and I guess he want's to use it .

10. ### ISB123 Well-Known Member

May 21, 2014
1,240
534
You get inductor value of ~10uH so your teacher is right 15uH would work fine.

11. ### gilley Thread Starter New Member

Nov 23, 2014
23
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Thanks a lot, may I ask you where did you find formulas for it ?

12. ### ISB123 Well-Known Member

May 21, 2014
1,240
534

$L=\frac{Vout}{ILmax}x(1-\frac{Vout}{Vin})$

Last edited: Dec 14, 2014
13. ### gilley Thread Starter New Member

Nov 23, 2014
23
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May I pm you ?

May 21, 2014
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Ok

15. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,019
1,123
Well using L=15μH for F=100kHz is not wise choice
Type in this site
http://schmidt-walter.eit.h-da.de/smps_e/abw_smps_e.html (please play for a while with this simulator)
Vin = 72 , Vout=12 , Iout = 1.5A, f= 100 and L = 15E-6 and you will see why.
Or simply do a quick calculation
ΔIcoil = (72V - 12V)/15μH * Ton = 60V/15μH * 1.7μs = 6.8A
So for sure your circuit will work in DCM.
If you want to use 15μH increase your switching frequency at least to 250kHz. Then we have
ΔIcoil = (72V - 12V)/15μH * Ton = 60V/15μH * 0.7μs = 2.8A
So for ILoad = 1.5A we have 1.5 - 2.8/2 = 0.1A freedom before circuit enter DCM. If the load decrease below 1.4A the circuit will enter DCM.
And now the final question, your coil saturation current is ??

16. ### gilley Thread Starter New Member

Nov 23, 2014
23
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Hi Jony, sorry , my english is terrible, I'm not sure I know what coil saturation is. But if it is what I think it is , it is 1.6 A, max I that coil can whitstand.

And yes, I have noticed before that ΔIcoil is to high ( 6.8A) , that was "confusing" me whole time. That's why I'm not sure that 15uH coil is the best choice for this.

17. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,019
1,123
Notice that LTC3703 data sheet recommend
L > 68μH for F = 250kHz for ΔIL = 0.4*Iload
L > 168μH for F = 100kHz

18. ### gilley Thread Starter New Member

Nov 23, 2014
23
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I'm aware of that but I'm not in charge of project, neither did I choose elements for it.

I will try to calculate it with f=500kHz ( ΔIcoil shoudl be around 1.328 A which is accpetable ) and over, and present him the results and try to convice him to change inductor, but in the end it is his decision

edit: I have come to conclusion that my mentor/teacher just said components without checking them. Because this obviously can't work with this swithcing frequency and inductor that has max saturation of 1.6A. I will consult with him in next few days and tell him about error, and will update you about info.

Thanks everyone for help.

Last edited: Dec 14, 2014
19. ### gilley Thread Starter New Member

Nov 23, 2014
23
0
Ok so another update, I have seen my mentor and he has assured me that 15uH inductor will suit out needs.

So he said to me that I should calculate maximum electrical current that our circuit will need ( the same circuit that we are designing buck converter for).

Basicly our circuit is a H-bridge with some optocuplers added. Can anyone provide me with basic principle ?

20. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,019
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But what will be a load for this H bridge? A motor? Do you know the type ?