DC-DC 3v to 400v step up - Page 1
Forum section: electronics / beginners

author: acorn222
author_rank: Newbie
timestamp: July 08, 2019, 07:32:44 pm
content: 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

author: Buriedcode
author_rank: Super Contributor
timestamp: July 08, 2019, 08:25:38 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2019, 08:38:07 pm
content: 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.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 08, 2019, 08:59:03 pm
content: Quote from: Ian.M on July 08, 2019, 08:38:07 pmIt's 2 W not 20 W

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 08, 2019, 09:02:48 pm
content: Quote from: acorn222 on July 08, 2019, 07:32:44 pmThis 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

author: fcb
author_rank: Super Contributor
timestamp: July 08, 2019, 09:12:11 pm
content: 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 13V to 12-24V. Many COTS solutions for this or easy to use IC's (simple switchers/TI/etc..)Stage 212/24V to 400V. Probably a little flyback design built using something like a Wurth WE-FLEX transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2019, 09:32:42 pm
content: Quote from: ejeffrey on July 08, 2019, 08:59:03 pmOoops! 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.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 08, 2019, 10:22:14 pm
content: Quote from: Ian.M on July 08, 2019, 09:32:42 pmIf I had a dollar for every time I messed up a calc by an order of magnitude I'd have 10 cents! <rimshot>

author: Psi
author_rank: Super Contributor
timestamp: July 08, 2019, 11:35:26 pm
content: This looks like it would work for youhttps://www.aliexpress.com/item/33007743089.htmlUS $4.97Size:2.5*4.5cminput 5v :output voltage 280-620v adjustableinput 3V: output voltage 200-450V adjustableDunno about the output current though, probably get 2W from it.

author: acorn222
author_rank: Newbie
timestamp: July 09, 2019, 10:16:07 am
content: Hi again, thanks for all of your responses, It has made it a lot clearer for me to see what I need to do

author: JJalling
author_rank: Unknown
timestamp: July 09, 2019, 05:48:09 pm
content: Quote from: blueskull on July 09, 2019, 02:11:14 amCan you suggest some papers or books to read, to be able to achieve this size? Maybe even component suggestions.BR Jonas

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2019, 01:54:35 am
content: 5 milliamps at 400 volts is 2 watts. At that power level, an inverter may be a better choice than a flyback converter.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2019, 03:39:41 am
content: Quote from: blueskull on July 10, 2019, 01:56:32 amWith 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.

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2019, 04:18:32 am
content: 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 orCoilcrafttransformer is much higher current but good.