PCB inductor for DC DC converter

[daqq]

Hi guys,

I've been thinking: Switched induction DC DC these days work at really high frequencies, and can require very low inductances for their operation (1 uH, .68 uH...). This is effectively just a few turns of a copper wire even without a ferrite core.

My question is: Could a PCB pattern (spiral, toroid-ish, simple loop) be used as a coil for a converter? Without any ferrite core.

I know that in terms of EMC this is an awful and horrible idea, that it would scream noise far and wide. I also know that there are ferite cores meant for mounting into the PCB and making a pattern around it. I'm just wondering whether it can be done without the core and how effective it would be, what losses would there be and such.

Thanks,

David

[mikeselectricstuff]

High power DC-DC modules often use planar transformers - these are multiple PCB layers surrounded by a pair of ferrite cores. I'm not sure if there are any off-the-shelf ferrites available for this. Don't think you'd get much of use without ferrite.
One problem is it will use quite a lot of PCB area compared to a traditional inductor, and may need multiple layers to get enough turns.

[Paul Price]

As my Catholic priest once told me during my weekly Confession, "Just because you want to do it, don't mean you should do it!"

[nctnico]

If you push the frequency up then the inductor doesn't need to be big. About 10 years ago I've seen a very compact 150W converter. But then again its not expensive to have a transformer custom wound.

[SeanB]

I saw a SMPS made by Thompson, where they decided they did not need no ferrite core, just wound a giant big air cored inductor out of 12 SWG wire and used it. Ran at 1MHz as well, pretty impressive SMPS for a design made in the early 1960's, generated 5V at 40A on the one side and 5V at 40A at 400Hz on the identical other side. They also dispensed with capacitors, instead they used the decoupling caps on the boards to do the filtering, only having a 100uF cap on the output. Used 2 TO3 transistors and giant stud diodes on the power side, and even had overvoltage protection, courtesy of a pair of 5V6 50W zener diodes on the heatsink. you could keep your coffee hot on it even with a fan blowing on it.

[zappaMan]

One of my colleagues in my research lab works on planar magnetics.  They are especially useful when repeatable parasitic parameters are required for use in converters like LLC and other resonant topologies.  They also have much lower assembly costs.  http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=5617812&searchWithin%3Dp_Authors%3A.QT.Cove%2C+S.R..QT.%26searchWithin%3Dp_Author_Ids%3A37590063900

[ejeffrey]

It is good to have a core not just to increase the inductance, but it also lets you confine the flux within the core.  An air core inductor will cause a lot more interference in nearby circuits.  In transformers, the ferrite core also increases that coupling factor and reduces the leakage inductance.

[free_electron]

I know that in terms of EMC this is an awful and horrible idea, that it would scream noise far and wide. I also know that there are ferite cores meant for mounting into the PCB and making a pattern around it. I'm just wondering whether it can be done without the core and how effective it would be, what losses would there be and such.

Thanks,

David

Not necessarily. Its a matter of layou. You can very effectively trap the field.
Many manufacturers have off the shelf cores for that puropes. You need to mill slots in the pcb to drop the core through the board.
Companies like ericsson and victor who kake power modules running off 48 volts frequently emply 6 or 8 layer boards with integrated inductors.

Here's a little hi-tech bit : the new intel cpu has an on-board regulator... They integrate the inductors on the silicon and cover with a slab of ferrite.... Clocked at over 150MHz switching speed. It powers the core. So you can even do it on silicon.

[free_electron]

Catholic priest

Should shut up, as they frequently fall in the category 'do as i say, don't do as i do'

[M. András]

I know that in terms of EMC this is an awful and horrible idea, that it would scream noise far and wide. I also know that there are ferite cores meant for mounting into the PCB and making a pattern around it. I'm just wondering whether it can be done without the core and how effective it would be, what losses would there be and such.

Thanks,

David

Not necessarily. Its a matter of layou. You can very effectively trap the field.
Many manufacturers have off the shelf cores for that puropes. You need to mill slots in the pcb to drop the core through the board.
Companies like ericsson and victor who kake power modules running off 48 volts frequently emply 6 or 8 layer boards with integrated inductors.

Here's a little hi-tech bit : the new intel cpu has an on-board regulator... They integrate the inductors on the silicon and cover with a slab of ferrite.... Clocked at over 150MHz switching speed. It powers the core. So you can even do it on silicon.

wow i guess its a very effective onboard regulator. do you know somehow whats the effiency on that converter what they integrated?

[daqq]

Thanks for the replies guys.

I've been aware of planar transformers before (though I did not know their english names). They were the ones that inspired the silly question on core-less inductors.

Not necessarily. Its a matter of layou. You can very effectively trap the field.

On a board inductor without a core? The only way I can think of is a bizzare toroidal thingie made from vias and routing on both sides. Or is there a better configuration?

Thanks for the info on the Intel on chip regulator. That's VERY impressive. It would be great if they sold that die seperately

do you know somehow whats the effiency on that converter what they integrated?

http://hothardware.com/News/Haswell-Takes-A-Major-Step-Forward-Integrates-Voltage-Regulator/

[hlavac]

I seem to remember Autotrax DEX had a built in PCB spiral inductor generator...

[halfwalker]

I think I've seen an Altium Designer video showing a pcb-based planar transformer.  Hrm, might have been an example design ?  Been a while, I'll poke around tomorrow.

Hah, I knew it.