How to make 76V from 38V

[MrEmbedded]

Hi, this is probably a simple question but i need a supply voltage of 70+ V. The "start voltage" is a battery with 9V. I am currently using MC34063 from ST electronics (dc-dc step-up converter). This circuit delivers 38V output.

How can i step up the voltage further? Can I invert the 38+ (to 38+/-) or is there any other solution for this problem?

Grateful for all answers, mr_embedded.

 

[BrianG]

You need to specify how much power and / or current you require at 76V. Clearly you can't get +/- 38V by using two of these chips from the same 9V source as there is no isolation between input and output. If your current requirement is low and you don't need good regulation at 76V you could use a capacitive 'charge pump' technique to add on to your 38V supply and get approximately twice the 38V.

If you use a different type of switch mode supply (e.g. a forward converter) this has a transformer to isolate input from output, so you can specify the input to output turns ratio of the transformer to get the higher 76V output that you need.

 

[VEBill]

"...+70v ...a battery with 9V..."

If that's a typical 9-volt alkaline battery (~0.5 A-hr), then you'll need to be very careful with efficiency. It'd be easy to come up with a DC-DC step-up 'solution' that would drain the battery in minutes.

 

[OperaHouse]

Is his a single application? I and most everyone else have dozens of switching wall wart power supplies around. Take the small transformer out of one and use it in reverse. A 34063 should easily drive it. Use about a 220pf timing cap.

 

[ecreek]

You might try stacking 38v on another 38v if the ground on the upper can be floated. Let us know if you solved the problem.

 

[MrEmbedded]

I am still struggeling with the same problem. Main problem is to get 80 V out of 5 V. Can someone help me find a schedule with a typical application?

Grateful for your help,
MrEmbedded

 

[VEBill]

You still need to answer the question "how much power and/or current" ?

 

[MrEmbedded]

At 80 V maximum current will be 25 mA (continuous).

 

[VEBill]

About two watts. With inefficiencies, probably about 3 watts input. That's enough to rule-out some very low current approaches.

Search Google Images for the following search terms:
1) DC-DC converter circuits
2) Voltage multiplier

Google Images is nice because you can scan the circuits right on the results page.

Option 1 should find you a suitable chip with reference design.
Option 2 would be combined with an oscillator of your own design.

Option 1 would be better.

 

[OperaHouse]

Seems like you are not wanting help, but someone to do the design for you. What have you tried so far? A transformer like I suggested would be the most efficient way. Since you are embedded why don't you just drive a FET off your micro at about 100K. A simple comparator or zener and opto isolator can be used to control the voltage.

 

[waross]

Cheap and dirty;
Two batteries and two convertors in series. Advantage, double the run time before changing batteries.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MrEmbedded]

I've tried so far with a MC34063 but it is limited to 40 V maximum output, are you guys suggesting I would use two of those in series?

Thanks so far!
mr_embedded

 

[IRstuff]

http://para.maxim-ic.com/en/search.mvp?fam=stepup&tree=powersupplies

seems like MAX668 and a bunch of others would be good candidates. Unclear why this is dragging on so long.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[OperaHouse]

"MC34063 but it is limited to 40 V" and the thought of putting two in series clearly indicates you have a very limited understanding of electronics. First 9V then 5V and 4.5V. What hobby application needs 80V @ 25ma? Was 25ma picked out of a hat?

 

[Compositepro]

How about using nine 9 volt batteries. They are really easy to put in series. You don't even need any special connectors.

http://www.youtube.com/watch?v=I2fD-hYVLxE

 

[jimkirk]

A basic boost circuit using power components (fast diode, switch & cap) that can handle your output voltage is the slightly tricky part. The control portion and input can be 5 volts easily.

Here's a Nixie tube power supply that turns +9 to +12 volts into +170 at about the power level you're looking for. The control circuit is a common 555 timer. Just about any SMPS chip that uses an external diode and switch could do this.

Modifying it for +5 volts in and 76 volt out shouldn't be too difficult.

http://www.ledsales.com.au/kits/nixie_supply.pdf

 

[OperaHouse]

That is a nice example of a typical circuit. The 555 can be replaced easily with a number of other switching regulators. Details and numbers matter to engineers. The OP hasn't told us if the the supply needs to be floating or what polarity it is to common, how precise it has to be and the quantity involved. Every 1ma is another 100mw of converter power. Quantity can really dictate design. I worked at a place where component insertion costs were 47 cents each. I agonized over adding a 1/3 cent resistor. Automatic insertion makes lots of cheap components a better solution. Real savings can occur when you the idle time most micros have to drive the power supply and battery charger.

 

[MrEmbedded]

Thank you guys for all your answers.