# DC - DC (boost) Converter Design

Hello,

I'm trying to design a boost converter, Vin =9V, Vout = 30V. I've read several publications about it and I've noticed that some of them ask one to specify the peak inductor current, diode voltage drop etc..how do I know the peak inductor current if I don't yet know the value of the inductor?
Also, about the switching frequency, how do i know what switching frequency to use? Do I just pick something that seems reasonable?

I apologize if this is a stupid question but I want to get as much information as possible before building the circuit.
Thanks.

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NascentOxygen
Staff Emeritus
It is easy to make this, and you don't even need an inductor, providing you don't need it to supply any current to speak of.

But to be practical, how much current do you need it to supply at 30v? Will it be a steady load? How precise do you need the output voltage to be?

thanks for your help. I was reading about boost and I was confused about how what the value of the load should be? Also, how do I decide the switching frequencies?.
I know what I want my vin and vout to be but I'm confused about computing the rest of the values that I mentioned above.

What are you supplying the 30 volts to?

a dc motor. Is that what I will need to take into consideration as in the switching frequency and load resistor?

NascentOxygen
Staff Emeritus
A dc motor from a model train? Or what? Does it say on the motor how many amps or watts it needs? If not, are there other similar motors you can find that information from?

In any case, it's not a small project you are undertaking. A couple of extra batteries isn't feasible?

Do you have a circuit in mind? If so, I think you had better post it. (I probably can't help further, but if others are able to they will need to see a schematic as well as know the current that will be drawn at 30v.)

sophiecentaur
Gold Member
It is easy to make this, and you don't even need an inductor, providing you don't need it to supply any current to speak of.
What set up did you have in mind?

And, yes, I would agree with your above post, that several low voltage batteries, charged in parallel and then connected in series, would be a good solution. In fact, if your setup needs to be portable, the efficiency of a series parallel system would be better than using DC-DC inversion and the same total volume / weight of battery would be needed however the energy is to be stored.

NascentOxygen
Staff Emeritus
What set up did you have in mind?
A Cockcroft-Walton multiplier can boost a voltage for low-current biasing, etc. Totally unsuitable for what OP has in mind.

sophiecentaur
Gold Member
I did think about a 'Cocky Wally" actually.

actually, I just looked at the dc motor and it doesn't look quite feasible for me. I was just hoping to build a simple boost converter with inductor, capacitor, load...my main issue was figuring out the switching frequency. I'm not sure how to determine that. do I just guess? as far as application is concerned, I'm not sure about that yet.
I apologize if the lack of a specific application will make you unable to help me. It's just that this is all new to me and I'm not sure what to drive with 24 volts, as well as how to calculate the switching frequency. I've been reading lots of papers and it looks like many of them just pick one but how they do that..i don't know.
I'm really sorry for my lack of clarity but please help me in the best way you can. I'm just building one just to build one..I'm not sure about an application; however, I am open to suggestions

sophiecentaur
Gold Member
The choice of frequency may be made on the basis of the material available for the inductor core material - operating frequency, loss, ampere turns and relative permittivity and also best working frequency for the switching device- which will relat to the actual power. Then there's the interference problem if the design needs FCC type approval.

Why not just choose one that looks like what you need and try it? You'll learn a lot and be able to make a better informed choice for your next one.

NascentOxygen
Staff Emeritus
I was just hoping to build a simple boost converter with inductor, capacitor, load...my main issue was figuring out the switching frequency.
The frequency is probably the least of your concerns; it can be tweaked for best performance in the testing stage after the circuit has been put together. I expect it will end up being in the tens of KHz. The biggest hurdle will be getting your hands on a suitable inductor. Perhaps salvage one from the power supply in a discarded desktop computer? Though before you grab hold of anything in the PSU, make sure to short the terminals of every high voltage capacitor, using an insulated-handle screwdriver. Some are reputed to be able to hold a dangerous voltage for weeks. The inductor will come ready-wound with one or more windings of heavy wire, so preserve this to save yourself that work.

As for what to drive with 24 volts? For the testing stage, just a 220 ohm resistor and a voltmeter, I'd say. If you do manage a 24 volt supply, you could power a pair of car lights (maybe two low power interior light bulbs) in series.

With a bit of dexterity, you might be able to unsolder the big power transistors from the PSU and reuse them. And maybe the capacitors, too!

If you are improvising as your go along, expect to destroy some transistors in the process, especially if you don't have a CRO to monitor circuit operation closely.

You will have to post the schematic if you want further advice.

NascentOxygen
Staff Emeritus