Inductance and saturation Current Design

[dann11]

Hi! I am designing an inductor for a 1600W Boost converter. I want to use a material core that is only available to me right now, which is ETD59, as per computation I need an inductor with an Inductance value of .6mH and can work on a peak current of approx. 32A. I'm finding it hard to design this inductor due to the Inductance and saturation value problem. Whenever I tried to increase my inductance value to .6mH my saturation value decreases, and once I increase my saturation value the inductance value decreases. And suggestion guys on how will I compensate on this?

 

[E-design]

For this to work within safe limits will require a really large air-gap of > 6 mm, depending on material type.

With such large gap, your other problem may be the fill-factor. You won't be able to get the required number windings to fit on the bobbin.

 

[BradtheRad]

Perhaps it's worthwhile to consider making two or more interleaved converters?
Or full-H-bridge type converter?
You might get by with a reduced saturation rating.

 

[E-design]

It will help if we can get the full requirement specs.

 

[dann11]

For this to work within safe limits will require a really large air-gap of > 6 mm, depending on material type.

With such large gap, your other problem may be the fill-factor. You won't be able to get the required number windings to fit on the bobbin.

I did try to simulate, with an air gap of 8.5mm, my inductance value will be 273.64uH and a saturation current of 32.96A and will required a 50 turns. Inductance value is already acceptable since my computed min. boost inductor value is .266mH. But 8.5mm is quite a large gap and with the Area of the core it will not fit the 50 turns if I will be using an 13 strands of AWG#26. I am also using a 67KHz as my switching Frequency.

Do you have any other suggestion?

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Perhaps it's worthwhile to consider making two or more interleaved converters?
Or full-H-bridge type converter?
You might get by with a reduced saturation rating.

I see, but i have limited budget in which I can no longer add components or revised my circuit design.

 

[E-design]

What is your input and step-up output voltage?

 

[Warpspeed]

I have successively built a similar boost converter.

My design uses a U93/76/30 U core and an I93/28/30 I core with 3mm gap.
18 turns gave me 140uH with a greater than 50 amp (measured) saturation current.

LI squared at 30 amps peak works out to 126mJ
At 20 Khz that is 2.5Kw

 

[dann11]

What is your input and step-up output voltage?

Input varies from 85V-265V, output is 400V..

- - - Updated - - -

I have successively built a similar boost converter.

My design uses a U93/76/30 U core and an I93/28/30 I core with 3mm gap.
18 turns gave me 140uH with a greater than 50 amp (measured) saturation current.

LI squared at 30 amps peak works out to 126mJ
At 20 Khz that is 2.5Kw

Will lowering down my switching frequency can makes me come up with a higher inductance? but will these still lower down my saturation value?

What should I consider to work on this kind of design? higher inductance and a higher saturation current?

 

[Warpspeed]

My system has solar input, MPPT about 240 volts to 290 volts no load input dc, and output 230 volts regulated dc. Its a buck boost system so the input and output voltages can cross over.

You need to design the number of turns and core cross section to give you the required flux swing at the desired operating frequency.
Then size the gap to give the needed inductance.

Provided the peak flux swing is kept well below saturation, you should be o/k if the frequency is below about 30 kHZ.

If its faster than that, core heating may require a reduction in the flux swing.

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Oops made a mistake, should be a half LI squared.
30 amps and 140 uH is 63mJ not 126mJ

Currently running a current limit of 35 amps peak (current mode control) at 20 Khz which is about 86mJ and 1.7 Kw but it will only ever reach about 1.6Kw in normal use. Core saturation is 50+ amps, at 20 Khz the core runs stone cold.

 

[dann11]

You need to design the number of turns and core cross section to give you the required flux swing at the desired operating frequency.
Then size the gap to give the needed inductance.

Provided the peak flux swing is kept well below saturation, you should be o/k if the frequency is below about 30 kHZ.

If its faster than that, core heating may require a reduction in the flux swing.

I did made few adjustment in my design. I followed your suggestion to work with the number of turn, cross sectional area, frequency and magnetic flux.

I increased my the cross sectional area of my core material. I replaced my core to CS400090 to workout with the cross sectional increase, in parallel it would also increase my core permeabillity. I used two of CS400090 core and I also adjusted its number of turns and the switching frequency.

With my simulation, I am able to meet the min. value of my design inductor and its saturation current.

Thumbs up to that..

 

[Warpspeed]

If its run in discontinuous mode, the peak current will be very high, and the conduction loss in the mosfet needs to be watched.
A few years ago this was just not really possible with a single mosfet, but recent improvements in high voltage mosfet Rdson has made discontinuous mode much more practical.

Continuous current mode will give you more power and a lower conduction loss, but you slam right into nasty instability problems with the control loop.
Either DCM or CCM mode is certainly possible, but discontinuous mode and keeping less than 50% duty cycle avoids ALL the control loop problems.
A very fast integral loop will work with lightning speed and complete stability.

I developed my circuit with crappy cheap mosfets until I was sure it was working reliably and well, then switched to something a bit better once it was all sorted out.
I have ordered some IPW65R019C7 mosfets for it which should arrive any day now.
650 volts 75 amps and 17 milliohms in a TO-247 package is not too bad.