[SOLVED] Inductor design for boost converter

[elecTomas]

Hi all,

I am designing a 5kW boost converter. The inductor size should be 2.3mH/23A. Any recommendations about kind of inductor, core, etc.

Magnetics design is not my best friend...

 

[FvM]

More information required: Switching frequency, ripple current. Up to 10 or 20 kHz, laminated iron is suitable, for higher frequencies, ferrite cores are suggested.

 

[treez]

You can buy a core with an AL value.
This helps you to know how many turns you need to wind to get your required inductance.

AL = L(nanohenrys) / Turns^2

If you have money, there are plaenty of transformer winding companies that will accept it and make you what you want...agw.co.uk aero stanrew, and others

 

[elecTomas]

Switching frequency will be 20kHz, later on a redesign may be done to increase the switching freq, anyway the first aproach will be for 20kHz. Ripple current will be 20% of the current, so 5A.

Thanks!

 

[Orson Cart]

Large toroid (powered iron) is one choice, easy but large/heavy, E55 or E65 ferrite core (see ebay) with centre gap will do the job. Post full details so we can calc the turns, wire size, gap etc... e.g. Vin, Vout, 20kHz, 5kW

 

[elecTomas]

Large toroid (powered iron) is one choice, easy but large/heavy, E55 or E65 ferrite core (see ebay) with centre gap will do the job. Post full details so we can calc the turns, wire size, gap etc... e.g. Vin, Vout, 20kHz, 5kW

Thanks for your help, but it seems that I finally found a good solution for it. I could get a Kool Mu E core 60permeability from Magnetics, that allows high storage capability with reduced size compared to ferrite cores and also high saturation current. With 160 turns, single layer, winding factor of 65%, size 80x20x80cm, I get an inductor of 1mH and saturation current above 20A. So it seems that magnetics part is already solved!

Anyway, thanks a lot!

 

[Orson Cart]

What is the resistance of 160 turns?, (say R) what is the dissipation at 20A? 400xR, if R=0.1 ohm then dissipation = 40W, 80W for R=0.2 ohm...
too high for the choke size? possibly...?

 

[elecTomas]

What is the resistance of 160 turns?, (say R) what is the dissipation at 20A? 400xR, if R=0.1 ohm then dissipation = 40W, 80W for R=0.2 ohm...
too high for the choke size? possibly...?

The DC resistance is approximately 120m ohm, but the converter has a ~3kW power rating, so the losses due to the DCR of the inductor are lower than 1%. I ll implement and test it to check if theory matches with experimental tests, if it doesnt, I ll look for another solution.

Right now I m trying to figure out how to estimate the AC resistance of the inductor, any idea?

 

[Anna Conda]

What is the ratio (and values) of ac current (ripple) to DC current at full power? knowing the current and the freq and the proposed wire diameter, the AC res can then be calculated...
48W is likely way too high for a single toroid, unless you can tolerate 175C operation.... and there is plenty of fresh air for convection cooling...

 

[elecTomas]

What is the ratio (and values) of ac current (ripple) to DC current at full power? knowing the current and the freq and the proposed wire diameter, the AC res can then be calculated...
48W is likely way too high for a single toroid, unless you can tolerate 175C operation.... and there is plenty of fresh air for convection cooling...

The ripple current is 6.5A peak-to-peak allowing a 25% ripple ratio. The frequency is 25kHz.

Regarding the core type is a Kool Mu 60 E core. Which solution would you recommend me to decrease the losses?

Thanks!

 

[elecTomas]

According to the previously given numbers, you're already exceeding core saturation current, which would normally suggest a reduction of windings count and/or a larger core. Presumed you have already fully utilized the available copper room.

I also can't reproduce your core specification. You wrote of "size 80x20x80cm" which sounds a way too big. If it's actually 80x20x80 mm, e.g. 8020 E core, neither the saturaturation current nor the inductance numbers seem to fit.

Sorry for the cm/mm error, obviously it is 80x20x80 mm.

Regarding the system specifications, sorry the values I sent were from a previous calculation. Finally the specs are:

DC sat current: 20 A
Ripple current pk-pk: 5 A
Ripple ratio: 25%
Freq: 25kHz
Inductance: 1mH
Turns: 169
Winding factor: 67%
Wire area: 4.445mm^2

Sorry for the mistakes...

 

[FvM]

According to the previously given numbers, you're already exceeding core saturation current, which would normally suggest a reduction of windings count and/or a larger core. Presumed you have already fully utilized the available copper room.

I also can't reproduce your core specification. You wrote of "size 80x20x80cm" which sounds a way too big. If it's actually 80x20x80 mm, e.g. 8020 E core, neither the saturaturation current nor the inductance numbers seem to fit.

- - - Updated - - -

O.K., the winding inductance calculation refers to an Al of 35 nH/n². But where do you see it in the core data?

Or did you make an additional air gap?

 

[elecTomas]

According to the previously given numbers, you're already exceeding core saturation current, which would normally suggest a reduction of windings count and/or a larger core. Presumed you have already fully utilized the available copper room.

I also can't reproduce your core specification. You wrote of "size 80x20x80cm" which sounds a way too big. If it's actually 80x20x80 mm, e.g. 8020 E core, neither the saturaturation current nor the inductance numbers seem to fit.

- - - Updated - - -

O.K., the winding inductance calculation refers to an Al of 35 nH/n². But where do you see it in the core data?

Or did you make an additional air gap?

Yes, as you can see the core datasheet lacks of some information.
**broken link removed** you can fins all the info required for the design. This catalog includes the Permeability vs DC bias curves, from where I estimated the inductance

 

[FvM]

If I understand right, you are operating the core at about 20% of initial permeability, which is as I think a way from optional operation point. According to the core selector chart, K8044E026 should be used above 17 A to achieve 1 mH. I see that a design according to the core selector parameters results in worst case 50% of initial permeability.

Particularly it's impossible to estimate the core losses with DC und superimposed AC magnetization from the given data.

 

[elecTomas]

If I understand right, you are operating the core at about 20% of initial permeability, which is as I think a way from optional operation point. According to the core selector chart, K8044E026 should be used above 17 A to achieve 1 mH. I see that a design according to the core selector parameters results in worst case 50% of initial permeability.

Particularly it's impossible to estimate the core losses with DC und superimposed AC magnetization from the given data.

Yes, you are right and I am aware that the core will be operating with a 20% of initial permeability. But right now, from the cores I already have in the lab this is the one that better suits my specs. In case it becomes really necessary, I will go for a better core afterwards. Thanks for the advice about the K8044E026.

Regarding the DC resistance, I estimated it using the mean lenght of turn for the core, the number of turns and resistace per unit of length. Then with the max. current, I guessed the power loss due to the DC res.

So, for the AC resistance, no estimation possible with the data available? It is not extremely important to have the result, but it would be nice to have an idea before implementing it. But, as I said, this is the core I have that better suits my converter, so I will use it anyway, at least for the 1st prototype.

Thanks for your help!

 

[Orson Cart]

What are the input/ output voltages please?

Kool Mu -26 is about 0.1 watt per cc at 25kHz, 100mT peak AC excitation, 450mW per cc at 0.2T peak excitation, which is getting up there, essentially no core losses for DC or 50/60Hz operation. So ideally keep the L as high as you can to keep the ripple current (and hence ripple B) down to 0.02 to 0.05T peak AC excitation according to the loss graphs..


It is a slightly iterative method to design with these cores, you need to pick your lowest acceptable L for a given peak I (including ripple current) and work from the tables to find a (large in your case) core, the A.turns per mag path length, and work back to find the turns, work out the resistance of the turns that will fit and see if the R is too high.

If too high go to a larger core... usually operate there cores at 50% saturation at the worst case Ipk.

Then you can calc the ac core losses...

You may need to parallel inductors if there isn't a large enough core to keep the wire losses down to reasonable...

Starting with the largest Kool-mu toroid, 165mm OD (2.2kg) , the -26 has a nominal Al = 78nH/t^2, giving 113T for 1mH
the AT are therefore 2260 @ 20A, giving an effective AL at 20A, of 70nH/t^2 i.e. 893uH at 20A thru.
The ID = 101mm, for a single full winding you can use up to 2.8mm dia wire,
putting into my winding program, if we use 2.62 dia wire (better fit), the MLT = 134mm, giving 19 watts at 20 amp at 100 degC for the 113 turns. Lil bit hard to wind, but can be done...Still pretty high but maybe acceptable if there is air flow.
OK, so core losses at 25kHz, you say ripple is 2.5A pk on 20A at 25kHz, 1mH, from the graphs, the losses will be (B @ 2260 AT/m (41.2cm) = 0.18T so the peak flux ripple is 0.0225T, gives Pcore = 6mW / cc x 407cc = 2.5W approx. If go to more layers on the toroid can use bigger wire to get the Pcu down. (Or more strands of a lighter wire - easier to wind..)

Two cores glued in parallel gives Al = 156 nH/t^2, and using 100 turns gives 1.52mH (@20A), and a corresponding drop in ripple flux and cores losses, and allows wire to go to 3mm dia, (Pcu = 19.8W @ 20A).
Core losses will be a little less - say 2W approx. Just more surface area to get rid of the Cu losses...

 

[FvM]

Kool Mu -26 is about 1 watt per cc at 100kHz, 100mT peak AC excitation, which is getting up there, no core losses for DC or 50/60Hz operation.

I had expected a certain dependency on DC bias, but I see that the Magnetics documents suggest to calculate core losses from ΔB only. Thanks for clarifying.

 

[elecTomas]

Thanks all you for the help! It has been really helpful. The inductor is already built and soon will be tested along with the converter!

 

[Orson Cart]

Well, tell us the details of the as built choke, and the temp rise...!