need some help designing a Sinewave inverter

You're going about it wrong.

Find an LCD tv set with an external power supply, or modify the existing set. Then figure out the voltages it needs, and supply those directly.

What you propose is to convert a 12v DC source to high voltage AC, so the tv set can then convert that AC to a low voltage DC source. Chances are good that the tv set is using a switching supply.

Figure out how to run the set off 12vdc directly, and feed a battery into it. You may need some regulators to supply some other voltages, depending on what's in the tv set. Or, replace the existing switching supply with a supply that takes 12vdc and supplies the needed voltages.

Michael

But the >>Why ?

1. Topology - how about inverting the 12V into AC then into a transformer. Then you can simply run the transformer "backwards".

e.g., if I was going to do it:

12VDC->PWM->Step Up-> Filter->sinewave

It shouldn't be hard to find a 230V:12V transformer and hence you just run it in reverse(although you might get a higher voltage since it won't be exact).

This method uses cheaper mosfets because of the low voltage(as it is easier to drive as there is no HS gate... you'll be able to use both n-ch and p-ch). Of course th current is larger but shouldn't be a bit problem.

In the US I could just take a basic wall-wart and use that. (The difficult thing is you'll need a 10A wall-wart which I've not ever seen... but you could take a few and parralel them)

1. Snubber - For AC these are basic cap + res in series. It acts as a simple HP filter and shunts HF spikes across the switches. There are other methods. For DC one can use diodes as any reverse spike will flow through the diode instead of trying to flow through the switch. Most mosfets have built in diodes and depending on the currents involved you don't have to do anything but makes sure you get the direction right.

There are lot's of inverter circuits out there that you might want to take a look at. If I were really going to do this I would follow what you suggest and simply use a boost converter. Getting the 325V shouldn't be an issue. The duty cycle will be about 3% which is a bit low but doable.

To get the inductor/transformer for these circuits you have to worry mainly about three things. DC resistance, Inductance(many calculators on the net for it), core saturation. (this is assuming your buying off the shelf)

You can compute the total magnetic flux based on the core type and current.

Should help.

If you use wall-warts or they may saturate and not be as effective but by paralleling them you'll reduce the current and hence the total flux through each individual core.

Some things just aren't ever going to be simple.

John

Paralleling the primaries does not reduce flux density. Reducing excitation voltage, or increasing frequency, does.

All others things being constant, reducing the load current taken from a transformer secondary *increases* flux density in the core.

John

Haha, you really have no knowledge of electronics do you? When are you going to learn the difference between current and voltage?

Given 12VDC and an H-bridge, the best you'll get out will be about

8.5VAC rms.

But you can probably find catalog transformers that'll come close to that, and can be used.

Less expensive MOSFETs, but a much more expensive (and bigger) transformer. Most switching circuits that I see for 12V use all N-channel MOSFETs with high-side drivers, because good high power N-channel FETs are enough easier to find than good high power P-channel FETs it makes the driver circuit worthwhile.

It'd be cheaper & smaller to find one power transformer, though.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

A flyback transformer is probably your best bet; get most of your step-up from the transformer, and the last little bit from the flyback.

Alternately, use a transformer with a turns ratio greater than 320:12, and control the duty cycle down.

(Come to think of it, you could do the latter with a self-oscillating circuit and avoid the need for a controller -- just accept that your

320VDC rail will be unregulated and be happy).

For this you may find it easiest to wind your own. Cores are available for just this sort of thing.

Get isolated 320VDC, drive your AC with an H-bridge, and you shouldn't need a transformer on the HV side -- just output filtering.

Find similar circuits and copy them shamelessly. It helps if you understand them before you copy them.

If you can find copies of the ARRL Handbook there's a big line-to-12V switcher that appears in there -- many of the principals of design are the same, even if you're going in the opposite direction.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Did you mean PWM -> Filter -> Step Up ? this would allow me to use a 50Hz transformer (quite bulky for 50 Hz,

100 Watts. may need a trolley ;-) ). If you did mean PWM->Step Up-> Filter, then i am not sure if we get off-the shelf transformers that do 12:230 at the PWM freq. I am now doing a search on Coilcraft/Pulse/Magnetica.eu etc The Magnetica.eu site has a 1KW inverter designed by ST ! having a look at it now.

If it makes it a wee bit easier to get a right transformer, i am ok to add one more battery and make the input 24V.

I believe winding a transformer is some kind of black magic involving lots of trial-n-error not to mention getting the correct core. Are there any caveats here ? If it comes to that, i am ok with winding my own transformer as well; only that i may tie myself in coils & "learning" would be much more than i had bargained for :-)

This is what my original plan is. Get the 320VDC, then using PWM+H bridge on it get the 230V and then filter it. and the most important question i have is how to get the 320VDC out of

12VDC(or even 24VDC) let me have a look at the ST design i hit upon...

Thanks

On the plus side, transformers for switchers are fairly small, with not too many turns (at least compared to the monster you'd need at 60Hz).

So the experiments won't take as much time.

And finding the smallest damn core that'll just barely work to keep costs down when you make 100K units is a _lot_ harder than finding a core that's plenty big enough to work for one stinking project.

Come to think of it -- you may even be able to harvest the transformer out of a PC power supply of suitable wattage, and adapt it to your use.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

I took two semisters of Electrical Machinery in college, which included a lot of transformer theory and practice. I got A's [1]. I've designed transformers, too, from 60 Hz to over a GHz. How about you?

How's this for a pick-and-place machine?

ftp://jjlarkin.lmi.net/PP5.JPG

That transformer weighs more than one of my employees.

John

[1] My average on tests was about 60. But the class average was around 15, and the prof graded on the curve. I did learn a lot about transformers.

by

oing

I went to military school where they graded on a curve and always failed the 2 lowest people out of maybe 25 each week. They called it the "Tube list" because they were using vacuum tube computers to grade the tests. If you failed 3 weeks in a row you were "tubed out" and looking for a new job. One of my friends was on the brink of tubing out and asked for assistance, so I sat next to him and let him copy my answers to the test. He got a better grade than I did that week.

-Bill

here's a 60VA transformer at 1.5KG

100VA should be less than twice that, but you may need more than 100VA it depends on the power factor of the TV.

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