1. Home
  2. Electronics Design
  3. Very small, very high ratio, boost converters

Very small, very high ratio, boost converters

Hi there - I need to build a very small, very high ratio boost converter. It needs to go on a PCB that can be no greater than 1cm in width and can be no greater than 1cm in height. Length is more flexible. It will have a 5VDC input and needs to have a 1KV, 1ma output. Regulation is not terribly important.

First I thought about just using a normal DC/DC boost converter and using a HV fet on the output. However, it seems all HV FETs are massive. The smallest relatively HV FET I could find was the STMicro STN1NK80Z, which is an 800V (I could probably get away with just an

800V output - but 1KV is my goal) 250mA FET in a SOT-223 (7x6.5mm) package. However, I noticed that RDSon is ~13 ohms. So since my boost ratio is about 200, that'd mean I'd have about 200ma on average (if I'm remembering my maths right) going through the FET, producing a 2.6V drop. So I don't think this FET is even going to be able to handle my 1ma load. So I'll probably have to go up to a larger FET. There are more FETs in the D-PAK (AKA TO-252) package available. The Infineon SPD06N80C3T looks much better 0.9 RDSon. A quick calculation shows my boost ratio ending up at about 208 once the RDSon is factored in (assuming the resistance of the inductor is much smaller than RDSon). So I'd have about 208ma going through the inductor. Now, for choosing the inductor, I may be completely off in the way I am calculating its value. This is the equation I came up with:

Einductor = .5LI^2 = output power * (1/ boost ratio) * (1/operating frequency)

If that is right, if I used a 100uH Sumida CDFH6D28, I'd have to operate it at 462KHz. I could put a couple of those guys in series, or find a higher inductance inductor, but already 462KHz seems nearly reasonable already. That circuit is seeming almost doable, though driving that big HV FET at that speed will take a decent amount of power.

However, I remember being told that boost converters with high boost ratios do not work very well for some reason that escapes me. So then I started thinking about transformers. However, I can't seem to find any that are this small. Heck I can't seem to find any reasonable sources for boost transformers (none of the big three - Digi-Key, Mouser, and Newark) seem to have any.

So then I started thinking about PZTs (piezo transformers). PZTs are typically very small with very high ratios - however again I can't seem to find anybody that sells any.

With all of this in mind - anybody have any recommendations as to a possible route for me to go? Are my calculations for the boost converter right? I am not typically a power guy - so I could definitely be messing some things up here.

Thanks!

-Michael

Reply to
Michael
Loading thread data ...

How about a boost to a lower voltage (100-200V?) followed by a Cockroft- Walton multiplier?

--
   Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1
Reply to
Wim Lewis

Stray capacitance is a killer in this kind of applicaiton. Can you use that to advantage by resonating the whole structure. Tap the drive way down on the inductor. Don't need hv fet any more.

Might also be interesting to look into the details of how they make stun-guns work. 9V in 250KV out.

--
Return address is VALID!
Bunch-O-Stuff Forsale Here:
http://mike.liveline.de/sale.html
Reply to
mike

Obviously, one step isn't going to cut it. The most that's practical off a single inductor is, oh, let's say 10:1. The duty cycle is already awful (ca. 90%). You can get peaks in the 20-50x range, at nil output current and insane loads to the driving transistor (since it has to handle all that voltage AND current).

The obvious solution is a transformer, which ends up having the same size as a single winding (an inductor), more or less. Put your ratio there and you get much happier duty cycle, current and voltage capacity.

You could probably tinker with a camera photoflash charger. Those are around 1cc. I don't know how much current they do, and 1.5 to 3V stepped up to 200-300V is quite a bit lower than 5V going to 1kV, so you'd probably need a new transformer at least. But sure, a self resonant sort of inverter, just what you need.

Tim

-- Deep Fryer: A very philosophical monk. Website @

formatting link

Reply to
Tim Williams

Maybe a CCFL backlight power supply? They're usually small and output

600V-ish.
Reply to
a7yvm109gf5d1

You don't say how important but I'm going to guess 10% or so.

[....]

When the duty cycle is large, the upper harmonics are large. You need an inductor that is not too lossy at all the frequencies that have much energy in them.

The pass transistor has to handle a large current when on and a high voltage when off. The same applies to the output diode.

You may need to abuse a transformer intended for audio if you want to get it from them. I'm thinking of something like a MET-23. I don't like this idea but I'll put it out there.

The turns ratio is sqrt(1600/3.2)= 22.4 which is a bit low for turning

5V into 1000, but lets push on with thinking about it.

The inductance on the 3.2V side will be more than 3.2 Ohms at 300Hz so:

3.2 / (2 * pi * 300) = 1.7mH Lets use 3mH below.

We can assume that the inductance is more than this. You didn't say if you were making many of these or just one but I will assume several so we do need to work with whatever the inductance of any one. At this point you'd need to but one and measure it to see if the rough guesses are right.

I'm going to suggest a half bridge series resonant design to make the secondary voltage hit about 250V or 330V so that only trippling or x4 is needed in the rectifier. Unfortunately, you will end up with quite a large AC current in the 3.2 Ohm winding when you do this. This really messes with the desire to keep things small because it pushes you up into bigger transistors.

You also need a 0.1uF capacitor that can take the high current.

All in all, I'd rather get a transformer intended for the purpose.

Here we hit the "use a PIC" suggestion. A micro can drive the switching devices and produce the needed timing. A voltage divider off the 1KV can do the feedback. The micro can also tune the switching rate onto resonance.

The transformer is rated at 65mW at 300Hz but if we run at, lets say

10KHz, we won't burn it out at a higher power level.
Reply to
MooseFET

...snip! there goes a lot of interesting design stuff!.....

Why build when you can buy? This sounds like an "off-the-shelf" device, for example:

formatting link
. If you acan build one cheaper and better, then you can put these companies out of business or work for 'em! My point is, custom electronics is great for those applications where nothing exists to satisfy cutomers, but if there are a host of products already out there, why go through the misery of design, testing, electrocution, and customer unhappiness? I used to work in a research outfit, and it used to drive us electronics guys nuts when labs would want to build stuff (wasting weeks and months of time), when they could buy a far superior thing from an electronics supplier. Why would you want to waste your precious time on something that highly skilled people have already perfected, and reduced the costs by economies of scale? If your "thing" had to withstand high vaccuum, or radiation, or other weird "one-of" situations, then go for your own design!

-Paul

Reply to
Paul

High ratio single-inductor boost converters are bad news. The fet is impossible to find, and the summed fet+inductor+diode capacitance eats up the flyback energy.

Use a low-ratio boost plus a C-W multiplier, or use a flyback transformer, or do both.

ftp://66.117.156.8/Z206.pdf

John

Reply to
John Larkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.