DC-DC 3v to 400v step up

[acorn222]

Hi, I am looking for a way to get DC-DC step up from 3v to 400v, only needing to deliver 5mA max at 400v, I have looked for a while now and have found some bulky modules, however I cannot find a way to do this in a compact mannor for a reasonable price (under $15), It needs to be able to be turned on and off at a logic level (which I know how to do externally, it would be nice to find a chip which could do this with it built in). The only modules I have found that can do this are for over $40 which seems way too much for what it is. My question is, are there any modules or chips I should know about that would help me? Thanks

[Buriedcode]

The only thing I can think of is a flyback converter.  With such a high step up ratio, a boost converter would struggle having such a low duty, but may be possible to get 3 to 75V with a boost converter, followed by a couple of stages of a voltage multiplier.  Off the shelf flyback transformers (technically coupled inductors) are available for xenon camera flash and strobe circuits, often with the turns ratio of 1:10 or 1:12. 

There are also chips specifically designed to drive said inductors (deisnged to run off 2 AA's or Lithium = ~3V input), but these are more for charging photoflash capacitors rather than for a continuous DC output.  I would say any boost converter chip with a maximum peak output >40, and one of the mentioned transformers.  If you use a converter with an external MOSFET it'll be easier as you could pick a MOSFET with a higher Vds breakdown.

[Ian.M]

Oops! See Ejeffrey's comment below. 

Assuming 90% efficiency, 5mA @400V out needs 7.4A @3V in.  Pushing 22W through a flyback transformer while keeping efficiency up around 90% with only a 3V supply is going to be challenging. 

IMHO it needs a push-pull forward converter.  That's gong to need a  switching transformer with the right pri:sec ratio, with a center-tapped primary,  some big low Vgs threshold MOSFETS to drive the ends of the primary and a controller chip to provide antiphase gate drive, with current and voltage feedback, that will run at 3V.    It *MAY* be easier to boost the 3V to 12V just for the controller chip and MOSFET gate drive to allow a wider choice of parts.  The primary supply to the transformer would still be at 3V.


I doubt you could get the parts for $40 - custom magnetics that can handle approx 15A^* primary current without saturation don't come cheap. 

You *MIGHT* get there with a transformer from an PC PSU driven backwards, using its 3.3V winding as the primary, and its original primary as the secondary, operating into a voltage doubling rectifier.

* When driving an inductor with a square voltage pulse, provided the flux resets to zero between pulses, the peak current is double the average current during the on time.

[ejeffrey]

Assuming 90% efficiency, 5mA @400V out needs 7.4A @3V in.  Pushing 22W through a flyback transformer while keeping efficiency up around 90% with only a 3V supply is going to be challenging. 

It's 2 W not 20 W

[MagicSmoker]

Hi, I am looking for a way to get DC-DC step up from 3v to 400v, only needing to deliver 5mA max at 400v, I have looked for a while now and have found some bulky modules, however I cannot find a way to do this in a compact mannor for a reasonable price (under $15)

This is just about the right voltage and power for the photoflash charger used in old (pre-LED) cameras. You should find lots of circuits and pre-assembled boards that can be used as-is or slightly modified. The specific topology is self-oscillating flyback, also called a blocking oscillator (obsolete term).


EDIT - found a good example for you: http://www.talkingelectronics.com/projects/XenonFlasher/XenonFlasher.html

[fcb]

I would break the problem/solution down into two stages.  Not as efficient or neat, but probably alot easier to get right with easily available parts.

Stage 1
3V to 12-24V.  Many COTS solutions for this or easy to use IC's (simple switchers/TI/etc..)

Stage 2
12/24V to 400V. Probably a little flyback design built using something like a Wurth WE-FLEX transformer.

[Ian.M]

Assuming 90% efficiency, 5mA @400V out needs 7.4A @3V in.  Pushing 22W through a flyback transformer while keeping efficiency up around 90% with only a 3V supply is going to be challenging. 

It's 2 W not 20 W

Ooops! Out by an order of magnitude. Looks like I cant math! 

Yes that makes a *BIG* difference and a flyback with a hand-wound pot core would be viable.

[MagicSmoker]

It's 2 W not 20 W

Ooops! Out by an order of magnitude. Looks like I cant math! 

Yes that makes a *BIG* difference and a flyback with a hand-wound pot core would be viable.

If I had a dollar for every time I messed up a calc by an order of magnitude I'd have 10 cents! <rimshot>

[Psi]

This looks like it would work for you

https://www.aliexpress.com/item/33007743089.html
US $4.97
Size:2.5*4.5cm
input 5v :output voltage 280-620v adjustable
input 3V: output voltage 200-450V adjustable

Dunno about the output current though, probably get 2W from it.

[acorn222]

Hi again, thanks for all of your responses, It has made it a lot clearer for me to see what I need to do 

[JJalling]

How small do you need it to be? I can get it done in less than 1cm*1cm*3mm.

Can you suggest some papers or books to read, to be able to achieve this size? Maybe even component suggestions.

BR Jonas

[David Hess]

5 milliamps at 400 volts is 2 watts.  At that power level, an inverter may be a better choice than a flyback converter.

[David Hess]

5 milliamps at 400 volts is 2 watts.  At that power level, an inverter may be a better choice than a flyback converter.

Presumably you mean CCFL inverter. When is the last time you saw a CCFL inverter that's in the size of 3.2mm*2.5mm?

With that transformation ratio and power level, I doubt a flyback configuration which has to use its transformer as an energy storage element will be smaller than an inverter.

CCFL inverters typically use current driven Royer oscillators but a hard switched inverter is feasible also.

Inverters can drive capacitive multipliers just as easy as flyback converters if that is required.

[floobydust]

The cost is in the transformer. Best to use something off the shelf.
It's in the realm of compact camera xenon flash DC-DC converters, LT3750, LT3751 or NCP5080. Mating TDK or Coilcraft transformer is much higher current but good.