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Boost Converter

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LAN_403

wpk5008

Mon Jan 03 2011, 05:05AM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

I was thinking about designing my own boost converter as the charging circuit for my capacitor bank. Unfortunately I don't know exactly what my source voltage will be or what my output voltage will be (plus I will probably be designing a lot of these over my life), so I was thinking about just solving everything in variables. That way I could make an excel spread sheet to see how each variable affects my output and maybe even make it reversible so I could get the component values I would need for specified inputs and outputs. I think that I should do at least the first four or five cycles to get a clear idea of what is happening, but unfortunately it got pretty complex pretty fast. I don't have software to make clean circuit diagrams right now, so I will spare you all a poorly drawn version in paint. My circuit is simply a source, inductor, transistor, diode, and capacitor; the most basic boost converter that you could imagine.

For the first time period I am assuming no initial conditions. Therefore, I solved for I(t=1) in the first cycle as such...
V/s - LsI = 0
V/s = LsI
I = V/(Ls^2)
I(t=1) = (tV/L)u(t)

For the second time period things get a little more complex. The capacitor comes into the equations because the diode is now forward biased. While the capacitor still does not have an initial condition the inductor now does. Therefore, I solved for I(t=2) as follows...
V/s - LsI + LI(t=1) - I/(Cs) = 0
VC + LCsI(t=1) = LCs^2I + I
I = (VC + LCsI(t=1))/(LCs^2 + 1)
Partial fraction expansion gave me
I = (VC - (LC)^(1/2)jI(t=1))/(-2j((LC)^(1/2)+j)) + (VC + (LC)^(1/2)jI(t=1))/(2j((LC)^(1/2)+j))

I really wish I could just scan my work and post a photo, but my scanner is at school too... sorry

As you all can see this started getting frustrating pretty fast... I know that this current in the s-domain will transfer to a cosine function with a phase shift, but my algebra is starting to get a little hazy on it. Then to figure out the voltage of the capacitor at that point I would have to integrate with respect to the second time period...

After this I would have to repeat the first step only this time there would be an initial condition on the inductor. Followed by repeating step two with an initial condition on the inductor and capacitor. Followed by another repeat of the first step and hopefully at that point the system would have stabilized... but I don't know that I can count on that? In which case I would have to reiterate until I have a stable current or voltage...

There has to be an easier way to work this out by hand? I doubt doing it in second order differential equations would be better than laplace transforms though... but this is the best way I could think of doing this with what I know at this time...

I'm honestly about at the point where it isn't worth running this in variables and just start throwing numbers in there and see how changes in them affect my answer, but I just feel like it would be so much nicer to be able to make an excel spread sheet that can do all that work for me. If any of you have any suggestions let me know, I've got a couple more days to work this out before I have access to PSPICE to test it anyways.

Speaking of PSPICE though, I was wondering if any of you know of a circuit emulating software that I could download. I'm a little leery of just downloading the first program that you find on google when you search "free PSPICE", as you can probably understand. So, do any of you know of a trustworthy website that has such freeware?

GhostNull

Mon Jan 03 2011, 05:39AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

What you do is make a circuit where, once the capacitor reaches it's desired voltage it stops charging.

Read through this:

It has a looong discussion on boost converters and it should answer all your questions

Dr. ISOTOP

Mon Jan 03 2011, 05:46AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

PSPICE has a free student version here

wpk5008

Mon Jan 03 2011, 07:09AM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

@GhostNull: I'm going to just ignore your first sentence. I did glance through your link though, and while it does hold a lot of valuable information, it does not appear to discuss the math behind a boost converter in detail, which is what the main topic of my post was. There was a link within that topic though to a site that I ran across a couple weeks ago and had forgotten about that essentially does what my excel spread sheet plan would to. However, I want to have a thorough understanding of what I build so I still intend on doing the math on my own, but that site should provide a good way to check my work.

@bwang: I ran into that that free version in a search about two weeks ago, have you downloaded it? Like I said I'm hesitant to download something from a website I don't know. Sorry to ask you if you were a guinea pig...

Sulaiman

Mon Jan 03 2011, 11:58AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Generally speaking,
- choose a boost-converter ic (you will probably change later so don't get too technical)
- read the manufacturers datasheet for that device
- read the manufacturers application notes for that device
- read the manufacturers application notes for similar devices
Now you're an expert on that ic is your reqirement practical/workable
if not choose another manufacturer and repeat.
Many manufacturers give simulators or models etc.

The ic manufacturers need designers to succed,
their 'Typical Application' can be copied exactly.
If it doesn't work correctly you made a mistake!
(I can't remember a major exception since 1984 !)
(but I do have poor memory)

Anyway, that's what I'd do.

P.S. I repair a lot of industrial electronics and generally designs in production follow quite closely to manufacturers 'Typical Application', usually with a few personalised 'tweaks'.

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