Whats the best way of achieving HV DC.

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This is for the Power electronics gurus...

What is the best way to step up 24v DC to around 300v DC??

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

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How much power do you need ?
How well must it be stabilized ?
How high has the efficiency to be ?
Must it be isolated ?
How many units will you produced ?
What safety has to be guarantied ?

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As Ossi suggests, one needs to 'qualify and quantify'. Once we know the constraints, then a much more concise answer can be given.

Anyway, say you need 300V for a xenon flash tube, the average disposable camera uses a 'ringing choke' converter to generate this. It is probably about as simple as you will get, but it is not really good for large wattages.

If you want low current, a 'boost' converter is another choice. At higher wattages, the inductors get physically large.

For higher wattages, a push-pull converter would be a reasonable choice. The cheapo DC->AC inverters that convert your car 12V to 240VAC are usually this kind.

Make Google your friend and you'll find plenty of references on the 'web for these things.

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ossi wrote:
How much power do you need ?
How well must it be stabilized ?
How high has the efficiency to be ?
Must it be isolated ?
How many units will you produced ?
What safety has to be guarantied ?

How much power do you need ? about 200W
How well must it be stabilized ? doesnt really matter
How high has the efficiency to be ? as high as possible
Must it be isolated ? not realy
How many units will you produced ? One
What safety has to be guarantied ? Safe

Im running a 300 volt DC motor from 12volt SLA (or LiPO) batteries (i could run them in 24v too). I have to use this motor because its a very high efficiency Maxon motor connected to drive gear i need.

I'm googling as we speak, but thought a little more input could steer me in the right direction more quickly. :)

Application: Solar powered bicycle :)

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

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Welcome then. Developing a high efficiency switchmode voltage
converter for that power value isn't really a beginner's task.
Either buy it, or schedule at least a man month (plus a pile of
wasted semiconductors and possibly even transformers) for the job.

You could try recycling the transformer of a (larger) computer
power supply.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Quote:

How high has the efficiency to be ? as high as possible

This will come out with a price as high as possible !
But I know what you mean: It should have a
good efficiency.

I would start with a push-pull, eventually
driven from an AVR together with a MOSFET-driver.

300V means, that there are probably no cheap Schottky diodes. So either choose silicon diodes or
(best efficiency) synchronous rectification.
But normally that makes not much sense at 300V.

Are you experienced in design of
transformers and inductors ?

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Also, and this is no joke, wear safty goggles when developing this, and an IR thermal probe will help keep down the burns when you touch a chip that's cooked up to 400 degrees. This recommendation comes from watching professional power supply designers screw up. 300V at 200 Watts sounds scary to me.

Smiley

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I would use a PFC controller aproach, as for example MC33262 from On Semiconductor. One power mosfet, one diode, no microcontroller. The modern tv and pc power supply have this bust converter. You just need to adjust a little bit the parameters and the protections.
George.

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I think a boost-converter from 24 to 300V has a
relatively extreme duty-cycle. That leads
to relatively high peak currents. So probably
efficiency will not be the best possible.

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For boost converters efficiency has a direct relation to the ratio of Vout:Vin. In this case because Vout is is so high compared with to Vin (1:12.5) I doubt you will get anything much better than 60%-70% if the rest of the parameters are optimal. Is there no other lower voltage motor you can use with the same gears?

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One can use a transformer with a boost converter, in which case you can get closer to 50% duty cycle, but you then get some hundred volts extra over your rectifiers. And the magnetic core needs to be awfully large, even if air gapped.

My first thought is, since regulation isn't that important, a simple forward converter will work. Look up MAX253, that's a (tiny) forward converter. Only you need some series inductance after the rectifiers, and you need fast overcurrent cutout and undervoltage lockout on your driver transistors, otherwise the thing will blow up when you attempt to turn it on. If the transformer is wound two-layer on a suitable toroid (we're talking something small, like a 50mm, since the core doesn't have to carry the output current. The size will be dictated by how fat wires you can poke through it), it'll fit in your hand, and 95-98% is feasible.

Expect to blow it several times during development. To me a man month sounds really short.

Pay attention to noise emission, with converters in this power class it becomes awful if you don't design it in from the start. The police officer knocking on your door kind of awful.

/Kasper

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Tony, if you also need a regulator to control output-power of the motor, you can also merge the step-up with the controller, and make it one module.

A challenging project !

Nard

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AS I mentioned earlier, get one of those cheapo dc-ac inverters. Internally you'll get around 300VDC and you'll avoid all the nasties of designing such a supply.

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Quote:
AS I mentioned earlier, get one of those cheapo dc-ac inverters. Internally you'll get around 300VDC and you'll avoid all the nasties of designing such a supply.

Seconded.

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Hi guys,

I'm building this with a team of student engineers. We have all had experience in DC-DC topologies but only low voltage, like 30 volts and below. I have the tools, the resources, and the man hours to design this.

We have looked into a lower voltage motor of the same type, but is going to cost us 1100USD. And efficiency isn't as good. More I²R losses in the winding and commutator i think.

Ill be happy if we can achieve 90% efficiency with this.

Currently we are looking at the Push-Pull type (non-synchronous version, to keep it simlpe). I've also thought of using an AVR to generate the necessary duty cycles, this will be beneficial later if i wanted to add speed control.

The only thing that worries me is the winding on the low-volatge side, it seems if i have a duty too high, i will create a short after the inductor chargers up.

And, what happens when i disconnect the load? where does the inductor energy go if i maintain a duty on the LV winding?? ...do i have to decrease/increase the duty cycle when the load is decreased/increased??

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

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Buy one of these and scrape the labels off. Put your team's name on it. Profit!

Seriously, it pays to look at other designs for inspiration. It doesn't make sense to step the voltage up that high for that application.

I like cats, too. Let's exchange recipes.

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here's another
http://www.made-in-china.com/sho...

and another

http://www.discountpv.com/electr...

I could go all day with this it appears.

I like cats, too. Let's exchange recipes.

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Quote:
The only thing that worries me is the winding on the low-volatge side, it seems if i have a duty too high, i will create a short after the inductor chargers up.

This is true. An important part of the design is never to let the inductor saturate. The inductor size and the operating frequency (maximum ON time) are the design concerns here.

Quote:
And, what happens when i disconnect the load? where does the inductor energy go if i maintain a duty on the LV winding?? ...do i have to decrease/increase the duty cycle when the load is decreased/increased??

The inductor energy goes into the output, driving up the voltage as high as necessary to get rid of it! There's no escape, and if you don't have an output capacitor to absorb the energy the voltage will rise high enough to break down the weakest component. This is where you need feedback loop to regulate the output by turning off the drive when the output is high enough. Again, inductor size and frequency matter here. The smaller the inductor and the shorter the period, the less energy you have to dump on the last cycle, and the smaller the output transient.

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I might be wrong but I think a push-pull has
no problems with no-load since the energy
can flow back into the primary supply, just like
an ordinary voltage-fed transformer is no-load stable.

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A push pull does not store energy. With no load, it won't draw any current except for the magnetizing current.

Transformer design is a black art. And tedious and time consuming too. So many variables that all influence each other. A multitude of unit systems with lots of 'magic' conversion factors. Calculations only give ballpark figures from which you can start, but experimentation is required to optimize it all. Oh well, that's what makes it so interesting :)

>> This << site also has a few calculators, including a core selector that selects an appropriate core given a set of parameters like input voltage, output voltage, current, operating frequency etc.

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I you REALLY want to design such a power supply, grab one of the cheapy inverters I've spoken of, pull it aprt and reverse engineer it. There's no micro involded and the parts of reasonably standard. This will give you a working baseline design you can then modify. The low cost of the inverter will be saved many times over in reducing the risk and thus the time it takes to achieve a working design.

Basically, they usually consist of a TL494 for the control ic and a bunch of mosfets and a transformer. There's some comparators to do low voltage shutdown etc. A few hours to sketch out the circuit and a few more to analyse and understand it will gain you more knowlege in trying to design from first principles. You'll also get a lesson on how to lay out the circuit board as this is a critical part of successful operation. Once you understand how it all works, then you can do your tests and start to modify the design to suit your requirements.

Whilst I don't fully agree with Jayjay, designing inductors/transformers successfully requires knowlege and experience. With power circuits, experience usually means you've blown some expensive semiconductors up in the process and lots of smoke. So wear your safety glasses and be real careful - if necessary get a perspex box to put over the unit when testing. You'll want to see where the smoke comes from.

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Quote:

Either buy it, or schedule at least a man month (plus a pile of
wasted semiconductors and possibly even transformers) for the job.

Quote:

...it pays to look at other designs for inspiration.

there were discussions maybe a year or so ago on one of these forums about an OpenServo or OpenDrive or something like that. there was lots of good info there on making the raw HV DC supply.

But I can't remember the name (to use in Google) and I don't have the link and ...

Perhaps someone remembers?

That failing, perhaps look at the Atmel motor driving kit schematics, or the ones at ST7MC.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.