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Circuit Design for DC to DC Buck-Boost Converter Answered

Sir, I am Anand from india, I will made a project based on bicycle. I have take 5 to 80v From my Bicycle pedals. Generator power is in AC 3-Phase. I want to made next step. in this I want to this 3-Phase AC 5 to 80V are constant At 48V DC. I put the 4 batteries in series. 12V 20Ah battery I use. I want to charge this batteries please help me. Thank you.



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Jack A Lopez
Jack A Lopez

4 years ago

Hello Anand,

I do not have time to give you a design that is complete in every detail, or even one that is mostly complete.

What I will offer instead, is a broad overview, in the form of a hand-drawn block diagram of how I imagine the major blocks of this project. This should be attached as picture at the bottom of this post.

Looking at my block diagram, and moving from left to right:

The alternator feeds a 3-phase rectifier. Next, the DC current from the rectifier fills a capacitor, named Cin. Next, Cin provides the DC input to the buck-boost converter. Next, the buck-boost converter feeds Cout. Finally, Cout provides the DC current to charge the battery.

So, naturally you are wondering about the details for the block in the middle labeled "Buck-Boost Converter". I mean that's essentially the title of your question here. Like, "Duh. Right?" The part inside that block, that's the part you want to know about.

But I figured the place to start is with a overview of how that block, the buck-boost converter, fits into all the other stuff you mentioned.

Next I tried Google(r) Images, trying to see if someone else posted a nice, easy, buck-boost converter, one that looked like it ?might? work well with the input (approx 5 to 80 V) and output voltage(approx 48 V) you mentioned.

Also guessing the maximum power throughput will be less than 200 watts. A guess. Actually, this site guesses higher

But the trouble is, the cost of a power converter tends to scale with max power througput. You know, more power demands bigger transistors, bigger inductor, bigger capacitors, etc.

So, the design gets really expensive if you've got an Olympic athlete powering your bike.

Anyway, back to the subject of me looking at Google(r) Images for circuit diagrams, I didn't find much.

I think the most interesting (to me) thing I found was an image from a Youtube video, which is somebody I don't know, showing off a buck-boost converter, that was purchased in module form from a Chinese eBay seller.

It never ceases to amaze me, the circuits that are available these days, sold on eBay, in the form of an almost complete module.

It's like cake-mix for making circuits, and it's the sort of thing that can make your life easier if, if... you can find an existing module that is a good match to your application.

Regarding these buck-boost modules built around the LTC3780 IC, these are probably NOT A GOOD MATCH for your application since the ranges for input voltage, and output voltage, are like 0 to 30 VDC, and the input, output, voltages you want are mostly outside that range.

But I think the video will be interesting anyway, especially if you are new to the subject of switchmode power converters, or Chinese "cake-mix" circuit modules.

By the way, any switching converter you can think of, will typically be built around some particular IC (integrated circuit).

Actually, that might be a good question to ask this form, specifically:

Can anyone here recommend a good switching converter IC, for to convert (approx 5 to 80 VDC input) to (approx 48 VDC output) at a max power throughput of around 200 watts?

Then when you get some recommendations for ICs, you can look up the data sheets, e.g. using a place like
for those ICs, and in those data sheets you should find some circuit diagrams for applications for that IC.

I remember I built a switching converter with a SG3524, and got it to work kind of good. It's a old IC, but I think at the time it was recommended to me as sort of a "general purpose" IC for making switching converters. I don't know if it could be used for your thing, or not, but I thought I would mention it. I mean, look up the data sheet for it, and see if it looks meaningful to you.


4 years ago

Across the whole range of voltage you will generate you can't.

Think of it this way.

Power is measured in watts = Amps x Volts. So if you want the same wattage to charge with a variable voltage then your going to need to increase the current as the voltage drops. BUT if your generating power with your pedaling as your voltage output drops so will the available current. You want something for nothing and your not going to get that.