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Author Topic: Step down converter  (Read 2657 times)
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alien
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« on: October 14, 2009, 09:30:37 09:30 »

I am having a transformer with primary-230Vac/secondry-100Vac,I want to get 22Vdc/1Amps regulated out of this.....so what will be the best cheap,robust/reliable circuit  solution for this.....linear regulation will dissapate too much heat....in my mind the idea of using MC34063A striked but its specification says its only for 40Vdc(maximum) operation....so any one here having experience with such stuff?
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jestanoff
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« Reply #1 on: October 14, 2009, 03:39:06 15:39 »

Try TOP242-250 series. In this case there is no need to 230/100V transformer. Standard DC-DC converters as MC34063A have input voltage to 55-60V max.
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alien
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« Reply #2 on: October 14, 2009, 08:36:57 20:36 »

ok,I know about top switch etc but I am looking after a cheap solution because this 100Vac is available there and will remain even if I do not use it(its also used for some other purposes)...so why not take the advantage of it Smiley
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leptro
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« Reply #3 on: October 15, 2009, 10:18:02 10:18 »

A nice solution is to use a diode bridge (to have positive voltage) ,a mosfet and a zener diode to reduce your 100V AC to 40V DC  and use a step down converter like lm2595 or else.

I have seen this schematic in a elektor in the past ( may be 07/08 2004 ) but i'am not at home to verify.

You need only 4 additional part for this.

please let me know if i have to serch the schematic for you.

Regards.

Finally here is a link :
http://www.elektor.com/magazines/2004/december/5-volts-from-the-mains.57307.lynkx


« Last Edit: October 15, 2009, 10:35:25 10:35 by leptro » Logged
jestanoff
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« Reply #4 on: October 15, 2009, 07:10:35 19:10 »

   
If you have a free PWM and ADC ports on the microcontroller, you can use it with power MOSFET transistor to make a buck converter.
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alien
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« Reply #5 on: October 16, 2009, 11:26:30 11:26 »

Guys i am still waiting for some more suggestions.....in mean time I am also trying the solution pointed by leptro.
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thecic
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« Reply #6 on: October 21, 2009, 04:26:02 16:26 »


Hi alien,

could you tell more about the transformer like VA and is it a toroid (ring) shaped transformer.

If you have a toroid transformer you could go for a quick and half dirty solution, not meaning low performance though, as you need only 22 Volts and 1 Ampere you could actually add your own winding to it.

Start by finding out how many volts per turn your transformer will give by make adding 10 turns and measure with a multimeter how many volts you get, if you get 2 Volts on 10 turns (eg. 0.2 V per turn) but need 22 Volts you now can calculate easily how many turns you need which in this case is  22/0.2 = 110 turns.

Remember when you measure with a multimeter you see only the RMS value of your VAC, the final rectified voltage when smoothed out with electrolytic capacitors is square root of 2 (= 1.414...),
eg. 22 x 1,4 = 31 (Unloaded! The final voltage will drop a bit all depending on your transformer.)
This is a bit high so we can back off to say 95 turns (95 * 0,2 = 19 ===> 19 * 1,4 = 26,6 Volts)
The comes the full bridge rectifier, as there will always be 2 diodes in series and each diode have a voltage drop of 0,7 Volt so you will lose 1,4 Volts all together, so let's calculate again
...26,6 - 1,4 = 25,2 volts

Now when we have found proper winding turns ratio we need an old and decent proven linear voltage regulator and I would suggest you to find a LM337 rated at 1,5 Amperes, this is such a common regulator you will find it almost from any electronic component shop.

Here is the PDF document of the LM337
http://focus.ti.com/lit/ds/symlink/lm337.pdf

Well, there's a starter!

Cheers
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alien
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« Reply #7 on: October 21, 2009, 05:17:57 17:17 »

Hi thecic,
I appriciate your efforts for posting such a worked out post...but thing is....main reason for using this 100+ volts winding is because voltages at secondary side may varies down to 90 volts so to make sure we always get 30+ volts from this winding this much higher rated secondry is used...I hope you its more clear now.
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thecic
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« Reply #8 on: October 21, 2009, 05:42:57 17:42 »

Hi alien,

ok I see... if the voltage may vary by 10% that would be for my previous example you need additional 2,5 Volts and 12-13 turns more on your winding, if you do it your self.

You have to remember if the voltage can go down much on your grid ( I don't know where in the world you live but in many modern countries it is VERY strict how much the grid voltage may vary) it can also go up too much, and that might destroy your LM337 because of excessive voltage, think carefully now!
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alien
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« Reply #9 on: October 21, 2009, 06:08:45 18:08 »

Hi thecic,
yes friend I leave in a part of a world where mains volatge vary from 70Vac-300Vac in single phase 220v system...worst can be seen in peak summer seasons where mains volatges never go above 90-120.. :(and thats why here flourished a bussines for voltage stablization units which uses auto transformer with relays and gives 220vac o/p with +/- 45 regulation....ok thats another story....it was due to all these facts that in my stating post under this topic I asked for some regulator with such a vast input volatges range...
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jestanoff
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« Reply #10 on: October 21, 2009, 09:23:08 21:23 »

Dear alien,
Since the topic is still seeking a solution, please check out the attached project. I think it meet the requirements, only the input voltage is limited to 100V. To avoid damage to the DC-DC converter use single diode as a rectifier. Nashional Semiconductors offer 5 free samples of LM5116MH, therefore you have at least 5 free of charge attempts.
Regards

DC-DC converter 100V/40V
http://www.megaupload.com/?d=FLTE7IYL

DC-DC converter 100V/22V
http://www.megaupload.com/?d=PGAAHWEI
« Last Edit: October 22, 2009, 10:10:32 10:10 by jestanoff » Logged
thecic
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« Reply #11 on: October 23, 2009, 11:41:34 11:41 »

Hi Alien,

ok now I understand your situation better, thanks for clarifying your situation!
As jestanoff suggested use the TOPswitch circuit from the company Power Integration.
http://www.powerint.com/en/products/product-archive/topswitch
On that page you will find all the data sheets and application notes.
and they can work in the range 85-265 VAC, but as your power grid voltage can vary so much we would need some spcial solutions here as the SMPS industry don't really design much AC/DC applications above 265 VAC so we have to do some home brew, for instance you could use a MOSFET and a Zener at 260 Volt (after a full bridge rectifier) to limit the voltage fed to your SMPS regulator.
I have used the TOPswitch once and they are nice and cheap solutions that work very well, in your case you needed 30 Volts @ 1 Ampere so you would need the TOP233YN at least but then you must assure it gets sufficiently cooled, but if you love in a warm region with very high temperature you should evaluate your choice here, remember the temperature!
IMHO I would use these circuits and skip the transformer, you need to build the SMPS regulator anyway, why not sell the transformer and fund your SMPS project.
Regards
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alien
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« Reply #12 on: October 23, 2009, 02:17:03 14:17 »

Dear Thecic,
thanks again for you efforts...problem here is`nt of any funds etc for one/two off production but we wll be employing this scheme thousands of end products...for now I am experimenting with the elektor elektronics`s above mentioned article(as pre regulator) + 34063a based buck regulator(as post/sub regulator)and that seems to work for us ...but I will fully comment on that in coming days..about what performance we gets from it...regarding the top switch stuff...we already use those smart devices in our other product`s power supplies but they are littel bit expensive(for current discussed aplication).In last let me tell you about our climate to...as here mains volatge varies from vaery minimum to extreme same is the case with temperature here....in summers we have 42+or even 45 degree celcius....and in winter it goes down to -1 and lower... Grin
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thecic
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« Reply #13 on: October 28, 2009, 07:53:24 19:53 »

Hi Alien,

how does it go?
ah, I see you are gearing up for a real production, why didn't you tell that!  Grin
ok I can now understand better your reasons to save couple of Cents here and there.
Yes Elektor have some times some nice articles, but check always out the big manufacturers page, especially Texas Instrument, they bought Unitrode for some years ago, a specialised company in SMPS, the have LOTS of resources on any type of converters like buck, boost, buck/boost, flyback and so on.
About the climate, you are truly living in a hot place, subtract roughly 20 degrees and you know how our summers and winters are.
Well, let us know how it goes and if you need further help, remember to careful so you don't build a wide band radio transmitter, it's not fun if people can't receive radio or TV channels and other problems too as you know switching high voltages and currents creates RF energy so careful design of both SMPS and the PCB is important.
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