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Buck Converter Design


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I need to design a buck converter with the following requirements, Input voltage 24V - 100V (2 - 10 Amps) and the output voltage shall be 12V (20 Amps). I started with the following circuit available in the net.

image.png.95c14b0b719f12ee2518d9cf90721190.png

The Diode, Inductor, Capacitor i have not yet selected, First question is how do i switch the MOSFET, do i need to interface a micro controller to calculate the duty cycle based on the output current, input voltage? I am new to hardware design. Please help.

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There is a lot to designing a dc to dc buck converter. For example see:

https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/2031.html

 

You can buy one for far less than the cost of the parts to build one. See:

https://www.amazon.com/Aceirmc-Converter-Adjustable-Regulator-Protection/dp/B0823MM1DV/ref=sr_1_6?crid=1AB7YZ4O9O8B0

You may find complete circuits on the internet. The inductor is the tricky part . What to use as a core and how to wind it for 20 amperes.

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Thank you for the reply, by help from other forums online i selected LTC3703 regulator, and from the application note in the data sheet https://pdf1.alldatasheet.com/datasheet-pdf/view/97352/LINER/LTC3703.html

image.png.664c827a9decd815ac8b429e46af4aa1.png

Can i use this circuit as is to meet the requirements i mentioned? I want current upto 10 Amps, can i change the circuit accordingly? Please advise.

 

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As this chip is made by Analog  and Analog owns LTspice simulator the circuit appears in the simulator and can be explored there. The circuit there is slightly different. Note it uses the FDS6680A mosfets. If you want 10 amperes make sure the mosfets can handle the current.

642473649_Buckconv3703.png.8899aaaaeb0a0a2821b4a08dd32e1ab1.png

Go here and put in your requirements then you will see some 10 ampere chips:

https://www.analog.com/en/parametricsearch/11491#/p5573=min|24&p5574=100|max&p5347=min|12&p5357=12|max&p5349=10|240&qsfv=vinmin|24_vinmax|100_vout|12_iout|10&p5362=Buck

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On 3/28/2022 at 7:26 AM, HarryA said:

The circuit there is slightly different. Note it uses the FDS6680A mosfets.

642473649_Buckconv3703.png.8899aaaaeb0a0a2821b4a08dd32e1ab1.png

On 3/28/2022 at 7:26 AM, HarryA said:

 

I have started going through the data sheet and trying to download ltspice simulation models for buck converter, but the above circuit i could not locate, have you created on your own? just for information.

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In LTspice on selecting components open the directory [Power Products] in the listing find LTC3705 when you click on it you will see "Open this macromodel's example circuit" that will display the circuit. 

You can select RUN and the output display will come up. You can poke around the circuit with the "red probe" and see the  various wave forms are there. You may know this already but other reads may not.

Perhaps if you click on some other types there you will find more converters of interest; as it displays a brief description of each.

The Analog site listed above is is busted. I got in once but never again. But if you search on LTC3703 off that page you will get some links that look like they may be helpful.

This link seems to work. Gets you to where you can enter you requirements.

https://www.analog.com/en/product-category/step-down-buck-regulators.html

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I saw the data sheet of the LTC3703 and there was a design example on page 27, i followed the same procedure.
LTC3703 Data Sheet
Specifications:
VIN = 24V to 100V, VOUT = 12V, IOUT(Max) = 10A, f = 250kHz

1. RSET
As per the formula in the data sheet
RSET(kOhm) = 7100/(250 - 25) = 31.5KOhm -> eq1

2. Inductor value for about 40% ripple current at maximum VIN
L = 12/250K*0.4*10(1 - 12/100) = 10.56uH. -> eq2

3. Minimum On-time
ton(min) = 12/(100*250kHz) = 480nS -> eq3
which is above LTC3703's 200ns Minimum on-time.

4. Top and Bottom MOSFET switch

Since the drain of each MOSFET will see the full supply voltage 100V (max) plus any ringing, choose a 125V MOSFET to provide a margin of safety.
 

1648979631066.png


PMAIN = 1.32W -> eq4
 

1648979669188.png


= 96.4DegC.

1648979697485.png


=1.84 -> eq5
TJ = 70 + 1.84*20 = 106.8

5. Current Limit Resistor

RDS(ON) = (25mOhm)/2 [1 + 0.009(105 - 25)] = 21.5mOhm
IMAX pin voltage = 10*0.0215 = 0.215V
RMAX = 0.215/12uA = 18k -> eq6

6. CIN is chosen for an RMS current rating of 5A (IMAX/2) at 85DegC.
7. For the output capacitor 2 low ESR OS-CON capacitors (18mF) are used.

The LTSPICE model is in progress, i have completed to the below level, the work is still in progress, i also need to choose the MOSFET. But based on the above steps, could you please provide comments.

design.pdf

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  • 1 month later...
On 3/26/2022 at 8:08 PM, HarryA said:

There is a lot to designing a dc to dc buck converter. For example see:

https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/2031.html

 

You can buy one for far less than the cost of the parts to build one. See:

https://www.amazon.com/Aceirmc-Converter-Adjustable-Regulator-Protection/dp/B0823MM1DV/ref=sr_1_6?crid=1AB7YZ4O9O8B0

You may find complete circuits on the internet. The inductor is the tricky part . What to use as a core and how to wind it for 20 amperes.
Hello, thank you so much for those links. I am working on my college project and for that I need some help regarding DC to DC bulk converter. I am so glad I have found your post. I also found the MasterBundles website online where I have found a lot of templates that I can use for my website. Now, I don't have to waste my time making templates.

Hello, thank you.

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