High-Current DC-DC switching regulator - Inductor Current limit

Thread Starter

pmd34

Joined Feb 22, 2014
527
I have been "playing around with" some DC boost switching regulator designs for use in a solar panel DC-DC converter typically 33V to 48V.
Typically I use a switching frequency of about 150Khz and an inductor of around 68uH - this seems an OK compromise for switching time losses, current ripple etc. (Modeling with TI Power Stage Designer)
Rather than 1 converter per panel (which I use now) I was looking at the feasibility for 1 central module. However I always seem to come up against inductor current limit / saturation current for off the shelf inductors. Typically this is in the area of about 10A.
So my question is, what is usually done for high power DC/DC switching converters? - Do you just use many in parallel, or is there some other solution?
(I realise I could increase the switching frequency which would then use a smaller inductance, which then usually has a higher operating current, but there is no significant BIG increase in current, and the switching MOSFET losses will start to increase also. So....?)
 

Ian0

Joined Aug 7, 2020
9,498
Having dismantled a Victron MPPT (100A output) I can tell you that they use an iron powder toroid of about 63mm diameter.
 

Thread Starter

pmd34

Joined Feb 22, 2014
527
Having dismantled a Victron MPPT (100A output) I can tell you that they use an iron powder toroid of about 63mm diameter.
You didn't happen to take any photos did you? I guess that must have pretty thick copper wire around it?
 

Ian0

Joined Aug 7, 2020
9,498
Sure, it would be very interesting to see, but I don't want to put you to too much trouble!
The first one is a 100A, with 63mm diameter toroids. The second is a 70A with smaller toroids, which we cut up to see if they were ferrite or iron powder.
EDF586C7-75A3-491A-AB5B-791C9927F2FD.jpegE5C123AE-0378-4DBA-B010-2DFC1922CEFE.jpeg
 

Thread Starter

pmd34

Joined Feb 22, 2014
527
The first one is a 100A, with 63mm diameter toroids. The second is a 70A with smaller toroids, which we cut up to see if they were ferrite or iron powder.
View attachment 307665View attachment 307666
Hmm thanks, that's very interesting to see! Not as big as I would have imagined for 100A, even using multiple strands of wire they must have significant resistive loses. I would also have thought they would use more strands of thinner wire to avoid additional losses due to the skin effect. (The maximum peak current is calculated from the core area, material saturation, and current I seem to remember?)
 

Ian0

Joined Aug 7, 2020
9,498
Only the AC component of the current causes skin affect losses. 90% of the current would be the DC component.

4 x 2mm wire = 12mm^2
Measures 52uH
Each will give 50A.
it is controlled by a STM32, presumably it’s an interleaved two-phase synchronous buck circuit.
It might be a Micrometals type 45 core, but black/black isn’t a colour code unique to Micrometals!
 

Thread Starter

pmd34

Joined Feb 22, 2014
527
Only the AC component of the current causes skin affect losses. 90% of the current would be the DC component.

Ahh of course.. I missed that one! Thanks!
What are your feelings on the core loss itself.. like if you were to use an air core inductor instead (but with much thicker wire)?
 

Ian0

Joined Aug 7, 2020
9,498
Ahh of course.. I missed that one! Thanks!
What are your feelings on the core loss itself.. like if you were to use an air core inductor instead (but with much thicker wire)?
Air-core inductor = radio transmitter.

If I’m correct about the core, it has a relative permeability of 100.
That would mean ten times as many turns.
That’s a lot of copper loss.
 

Thread Starter

pmd34

Joined Feb 22, 2014
527
Have you seen the "MICROLITE® Toroidal Cores" they seem to be a good option for cores with high saturation.
 

Ian0

Joined Aug 7, 2020
9,498
Have you seen the "MICROLITE® Toroidal Cores" they seem to be a good option for cores with high saturation.
Yes. Tricky to get in small quantities! (They were when I looked, that may have changed)
Iron powder can manage 1.5T.
For a buck regulator, it usually works out that copper loss dominates, and the hypothetical minimum loss point would be well above the saturation limits of the core.
 

Thread Starter

pmd34

Joined Feb 22, 2014
527
Yes. Tricky to get in small quantities! (They were when I looked, that may have changed)
Iron powder can manage 1.5T.
For a buck regulator, it usually works out that copper loss dominates, and the hypothetical minimum loss point would be well above the saturation limits of the core.
Mouser seems to have a stock of most of them, though they only go to 57mmØ
Thanks for all your help! Think I will have a play with a few designs and see how it goes!
 

MisterBill2

Joined Jan 23, 2018
17,790
Some solar power systems use an inverter per array so that each can operate at a max depending on individual illumination.
And even more important, redundancy improves reliability and system availability.
 

Ian0

Joined Aug 7, 2020
9,498
I agree, and it would also seem sensible for quite short strings of panels (4 or 5) rather than putting the whole lot in series to make 400V or so, so that one failed connection stops the entire system; but that seems to be what is popular.
 
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