Using aircored inductors in boost converter / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Using aircored inductors in boost converter

Move Thread

LAN_403

TheMerovingian

Thu Feb 14 2008, 11:25AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I'm currently working for the ICCOM-CNR institute as a local sponsorship working on fuel cells. In particular I use some electrochemical methods to test catalists for DAFC. Having also a small background in electronics i design some power converters to power small devices from fuel cells (eg cell phones) with the purpose of attract fundings and investors showing some applications at congresses and fairs. Fuell cell stack voltages are very low, so i switched on controller ICs (Maxim) that require small voltages to work with bootstrapped output. Since the voltage is very low (0.9 - 1.5V) and the swithching frequency is insanely high (1MHz) i need a very small inductor with a high current rating (>2A). The inductor value ranges from 0.5uH to 1.5uH . I failed to design properly the inductor using a ferrite core, and the commercial inductors have a small current rating and are difficult to find here. I was suggested to try air core. My worries are that an air cored inductor at 1MHz will radiate noise all other the place and so have poor efficiency. So should i try air core or buy an appropriate inductor from eg CoilCraft ?

Mates

Thu Feb 14 2008, 11:57AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

TheMerovingian wrote ...

I'm currently working for the ICCOM-CNR institute as a local sponsorship working on fuel cells. In particular I use some electrochemical methods to test catalists for DAFC. Having also a small background in electronics i design some power converters to power small devices from fuel cells (eg cell phones) with the purpose of attract fundings and investors showing some applications at congresses and fairs. Fuell cell stack voltages are very low, so i switched on controller ICs (Maxim) that require small voltages to work with bootstrapped output. Since the voltage is very low (0.9 - 1.5V) and the swithching frequency is insanely high (1MHz) i need a very small inductor with a high current rating (>2A). The inductor value ranges from 0.5uH to 1.5uH . I failed to design properly the inductor using a ferrite core, and the commercial inductors have a small current rating and are difficult to find here. I was suggested to try air core. My worries are that an air cored inductor at 1MHz will radiate noise all other the place and so have poor efficiency. So should i try air core or buy an appropriate inductor from eg CoilCraft ?

What about to load 10 electrolytic caps to 1V in parallel and then reconnect them electronically in series and load a one cap for 10V? That could be much easier to constructâ€¦

GeordieBoy

Thu Feb 14 2008, 12:18PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Although the inductance value is low, the air core coil may still be quite large and need many turns.

You could try air-core, ferrite stick, iron-powder and maybe even a small gapped ferrite core design. You didn't say what the design constraints were, eg. Cost, efficiency, weight, temperature rise, EMI? A design using a ferrite stick or a tiny gapped Mn/Zn ferrite core set should give lower overall losses if done properly. You could also try winding the choke on a type-2 iron powder core from Micrometals. Something like 19 turns on a T30-2 should be close to 1.5uH and not too lossy at 1MHz.

All designs but the iron powder one will have a significant leakage field.

-Richie,

Sulaiman

Thu Feb 14 2008, 06:54PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

If I were you I'd design my inverter to operate somewhere between 280 kHz and 530 kHz,
above the LW and below the MW radio bands - less chance of consumer complaints.
If so then I'd go for a ferrite LBC (large-bobbin-core) inductor if you're desining a trough-hole pcb.
They're cheap, about the size of a 0.5W resistor, can handle fairly high currents and have a reasonable 'Q'.
e.g.

If you're designing smd then there is a huge range to choose from.

Assuming your design(s) will go into production, start with components suitable for mass-production.

Marko

Thu Feb 14 2008, 07:08PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hm, isn't it important which mode converter is designed to work in,

as far as I know powdered iron is not good for large AC components but is superior to ferrite in max flux density (which you may not need much at all) so it would be a choice for a continuous mode converter. And yes, toroidal cores with distributed airgap will leak less flux.

From other side discontinuous mode converter would need ferrite core because of large AC component, may have more loss due to skin effect, but efficiency of circuit is helped because of zero current turn on, and etc.

As usual it's lots of trade offs, I hope you guys fill me up if I'm wrong on something.

But I don't see a special reason why would you want to use air core if you don't need to, as it will always have lower efficiency per cost that cored inductors.

I failed to design properly the inductor using a ferrite core, and the commercial inductors have a small current rating and are difficult to find here.

Can you describe how did you exactly 'fail' it, how was the core dimensioned and gapped, more data?

Just few turns would in most cases be enough for what you described.

TheMerovingian

Thu Feb 14 2008, 10:45PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Marko wrote ...

Hm, isn't it important which mode converter is designed to work in,

as far as I know powdered iron is not good for large AC components but is superior to ferrite in max flux density (which you may not need much at all) so it would be a choice for a continuous mode converter. And yes, toroidal cores with distributed airgap will leak less flux.

From other side discontinuous mode converter would need ferrite core because of large AC component, may have more loss due to skin effect, but efficiency of circuit is helped because of zero current turn on, and etc.

As usual it's lots of trade offs, I hope you guys fill me up if I'm wrong on something.

But I don't see a special reason why would you want to use air core if you don't need to, as it will always have lower efficiency per cost that cored inductors.

I failed to design properly the inductor using a ferrite core, and the commercial inductors have a small current rating and are difficult to find here.

Can you describe how did you exactly 'fail' it, how was the core dimensioned and gapped, more data?

Just few turns would in most cases be enough for what you described.

I cannot calculate a proper value, since i don't know ferrite permeability.
Design a 1.5uH air cored inductor isn't tricky, but i don't know if i has some drawbacks

Dr. Slack

Sat Feb 16 2008, 06:58PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You don't need to know the ferrite permeability to design a flyback inductor to specific inductance.

Think about it.

If you are designing a flyback inductor, then it needs an air-gap, otherwise you are wasting almost all of the performance of the ferrite.

If it has an air-gap, then the air will dominate the ferrite (if you've designed it right), and to a first approximation you can consider the ferrite as infinitely permaeable.

As a small correction, reckon the total ferrite path as 10% of the reluctance of the airgap. It's probably in the right ball-park, and won't make a huge difference if you're wrong by a factor of several either way.

An air-gapped ferrite will give you best volume efficiency, an air-core would need to be big to compete on Q, and a non-air-gapped ferrite would use much ferrite and be very inaccurate.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob