Designing a buck-boost converter 1

[walker1]

I have +40-50V and I want an adjustable output of -7 to -22V, 0.2 to 1.2A

A buck-boost converter/inverter looks like being able to do just that.


My imidiate problem, though, is how to figure out the size of the coil.

I have so far seen 2 formulas, and tried to devellop one of my own.
Needless to say I get 3 different results
I am aiming at something like 35 kHz switching freq.

Also, an extreme low ripple on the output is not necessary, but some filtering is needed,
as the output peak voltage must not exceed -26V
This last may have something to do with the need for continuous current or not.
I know the scemes exist, but have so far found no details on the web or in various
app.notes.

 

[RedDerek]

For the fastest breadboarding effort, have you gone to the National Semiconductor WebBench site?

http://www.national.com/appinfo/power/webench/

This allows you to purchase a demo board that has parts for the design you need. Derek Koonce
DDK Interactive Consulting Services

http://www.dkoonce.com

 

[walker1]

I just tried.
The site cannot handle negative outputs/inverters it seems.

 

[JeanPierre]

It seems, there is electronics and electronics with coils! I am also troubled with calculating them, but found recently this wonderfull site:

http://henry.fbe.fh-darmstadt.de/smps_e/smps_e.asp#Ivw

!
You choose the topology of your converter, type in the given and desired values and in return you receive all the data, including the coil, number of turns and wire diameter! On top of it, you get a choice of existing (Siemens) coils that fit your application best! Much more one can not ask for... Ah, yes, all the theory as well as the formulas are also provided.

good luck

 

[elektron]

Hi Walker1

First, why are you restricted to the 35 KHz switching frequency? The higher the frequency, the lower the cost and the smaller the inductor. Also, it does not sound like you need a boost/buck converter. Boost is used when your input voltage is lower than you output voltage, a buck converter is used for an output voltage HIGHER than the input voltage. A boost/buck is generally used when the input voltages can vary above and below the output voltage. Flybacks are used generally in situations where there is a requirement for much higher voltages and lower currents, although there are many other uses for them.

I suggest trying the National Semiconductor website at

http://www.national.com

where they have a full switching power supply design center. All you have to do is enter your current, voltage, thermal requirements and some other generic information about your needs. Their system will return a complete, fully documented design, complete with a layout, bill of materials, test chart, schematic, recommended regulator chip, recommended inductor and even a source (Pioneer) where you can get a complete set of all parts with a PC board suitable for testing the design. The bill of materials they supply will give you multiple sources for most of the parts. I have one of their surface-mount boards and it works great, exactly as I needed. The use of their design center is free when you register, and they will send you 5 (maybe more) free samples of the chips. The complete set of parts including overnight Fedex delivery only costs around $30 (+ or -).

I hope this is helpful to you. If you need any further assistance, let me know.

 

[elektron]

From elektron,

Sorry, I did not notice the negative voltage requirement at first. You are correct in the buck/boost. The National site is a little strange at times, but once you get used to it, it seems to work fairly well.

The other site mentioned in Germany looks quite good. Try it, too.

 

[koolkid]

Use the flyback topology, or better still, use the forward topology so you don't have to deal with right-half-plane zero and associated stability.

The transformer of those topologies takes care of negative output voltage.

Word of caution: switch-mode power supplies are deceptively simple on paper. However, most electrical/electronics engineers who don't study switching converter theories will have a very hard time making a prototype work.

You have been warned.

Better buy a used SMPS or hire a consultant. A cheap one is available at

http://www.1stquadrant.com/services.htm