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 Post subject: Variable buck converter question
PostPosted: Sep Thu 19, 2019 6:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2397
Location: Spring Hill, FL 34608
I don't know if this belongs here, homebrew or in test equipment, but here goes.

I'm kicking around building another benchtop power supply, but this time a step up from using the LM317. I'm thinking something Arduino based, for a few reasons. So that leads me to a buck converter. I know you can buy the premade ones from Amazon really cheaply, but I'd like to build it myself (that being part of the fun here.) In the mean time, I've found formulas for picking the inductor and capacitor values, but they are based on a fixed output voltage and current. If I want to go variable, do I pick those based on the maximum output voltage and current I want to use or maybe something in between? That's the other half of building it myself instead of just buying the board, is the learning part, as in how to pick the values.

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 Post subject: Re: Variable buck converter question
PostPosted: Sep Thu 19, 2019 7:54 am 
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Joined: Feb Wed 07, 2018 6:52 pm
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Location: Stone Mountain, GA
Yes, max voltage and current.

I did a nicad battery charger with a PIC16 using the PWM out. Frequency was limited to 20kHz so the inductor was large. Worked fine and had boot and buck modes and also a discharge mode.

It was a bit slow to respond to changes. Fine for a charger

You might consider using the Arduino PWM to generate a DC voltage as a reference for a regular switching regulator.

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 Post subject: Re: Variable buck converter question
PostPosted: Sep Thu 19, 2019 7:25 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
I don't use switchmode converters for lab supplies.

1. They generate RFI unless you really work at getting a stable design and shielding.

2. Generally, they are used for fixed, known outputs. The choice of feedback circuit usually means you know something about the load, or you can have instability and oscillation.

Rich


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 Post subject: Re: Variable buck converter question
PostPosted: Sep Fri 20, 2019 4:31 am 
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Location: Perrysburg, OH, 43551 U.S.A.
You could also use a switching/linear regulator similar to the one shown in Figure 5 on page AN2-3 of this application note:
https://www.analog.com/media/en/technical-documentation/application-notes/an2f.pdf
I designed a variable output regulator using this concept that delivers zero to 22 volts from a 24 volt supply at 8 amperes for a client. I replaced the PNP Darlington with a MOSFET and added current limiting. The voltage control in that application was the 0-5v output of a DAC.
John

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 Post subject: Re: Variable buck converter question
PostPosted: Sep Fri 20, 2019 7:25 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
One of Radio Shack's small low cost technical books on building power supplies
and regulators, for hobby use has a good section on sizing SMPS inductors and
complete examples of stuff you can build.

It was written by David Lines.
Who wrote it so you could understand it.
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 Post subject: Re: Variable buck converter question
PostPosted: Sep Fri 20, 2019 9:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Spring Hill, FL 34608
I think I have that Radio Shack book buried around here somewhere. I'll look around for it.

With the suggestion of linear regulators, and doing some more research, I'm thinking the LT3080 might be a good way to go. Especially since they can be cascaded to allow higher current values.

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