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Hoiw to wind output inductor for a 'Simple Switcher'?

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picstudent

New Member
I am about to make a Lm2576 based buck switching power supply.

I need to wind a 330uH inductor capable of carrying almost 2Amps @ 12V.

I have a inductance meter but on which core and how?

I think it is to be on a toroid (From pictures by search). What is the diameter and how to wind, Any special sequence or orientation? There must be something to be taken care of.

Please suggest.

Thanks and regards
 

picstudent

New Member
That page gives info about winding Air core coils.
winding a 330uH Air core coil will be bulky(?)

Thanks
 

stevez

Active Member
I can tell you that radio amateurs use toroidal inductors for many projects though much of that is for RF transformers rather than SMPS inductors. Still, you'll find some information in amateur radio publications - particularly the Handbook for Radio Amateurs. Amidon Associates used to publish (for sale) a handbook that had a lot of good information but I'll stop short of saying that it is a tutorial on inductor design and construction.

You might inspect a Jameco catalog to see what they have - I know they list some inductors for SMPS. Have a look at the approximate size of the part then match it to a core size from Amidon or other suppliers. You might see that the cost of the core is the same as a complete inductor. You might also have a look at discarded power supplies as a source.

The challenge in designing this inductor is selecting the core material for the frequency and use. Wire size is somewhat related to the current - and you can only stuff so much wire on a core of a given size. The books do give some help with this including turns ratios and dimensions.

Good luck.
 

Electronworks

New Member
Personally I would either buy one or get one as a sample. Coilcraft and Coiltronics all give samples away.

For board layout, to get best performance, keep the inductor close to the switch (inside the chip) and try to build the circuit as tight as possible. Keep decoupling capacitors close to the chip too. With a high current switching power supply, all kinds of spikes can appear if you are not too careful.

Apart from that, you should be OK
 

indulis

New Member
Can you give more details about the buck... i.e. What's the input voltage? How much output ripple can be tolerated? Does it have to be a simple switcher? The inductor would be much smaller at a higher switching freqency.
 

picstudent

New Member
Inductors with 2Amps current capacity are not available here, So i have to wind.


My input voltage to this system is 40V to 60Volts.
LM2576 is a simple switcher, its frequency is not changable.(Correct me if am wrong!)

I am using this +12Volts to manage some displays (7Segment) and then derive a +5Volt from it using 78L05. The system has a 8 bit ADC to read voltage (PIC16F72)

How much ripple such a system can afford?

Thanks
 
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indulis

New Member
Yes it is fixed frequency, but if you were to use a LM5005, for example, the inductor could drop to 68µH (per Nationals design tool).

That's also a "crappy" duty cycle (low)... at 60Vin it's ~20%. With the LM2576 running at 52KHz the inductor has to supply 2 A for ~ 15.38µS off time, so that's ~.55A of ripple current or a little over 25% of your output current. To get that down, you need more inductance or a higher switching frequency.
 

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bountyhunter

Well-Known Member
I am about to make a Lm2576 based buck switching power supply.

I need to wind a 330uH inductor capable of carrying almost 2Amps @ 12V.

I have a inductance meter but on which core and how?

I think it is to be on a toroid (From pictures by search). What is the diameter and how to wind, Any special sequence or orientation? There must be something to be taken care of.

Please suggest.

Thanks and regards
I recall most of the inductors we used to ship with those are powdered iron cores because the 2576 is the lower operating frequency parts in the SS product line.

You can also use a gapped ferrite core. BTW, there's no law that says it has to be a toroid core, but those put out the least EMI.
 
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bountyhunter

Well-Known Member
I am about to make a Lm2576 based buck switching power supply.

I need to wind a 330uH inductor capable of carrying almost 2Amps @ 12V.

I have a inductance meter but on which core and how?

I think it is to be on a toroid (From pictures by search). What is the diameter and how to wind, Any special sequence or orientation? There must be something to be taken care of.

Please suggest.

Thanks and regards
330uH/2A is a big honking part. Not sure why you need so much inductance.
 

bountyhunter

Well-Known Member
Can you give more details about the buck... i.e. What's the input voltage? How much output ripple can be tolerated? Does it have to be a simple switcher? The inductor would be much smaller at a higher switching freqency.
national Semi has other simple switcher products at higher operating frequencies. The 2576 is the original product line so it's the lowest frequency.
 

picstudent

New Member
330uH/2A is a big honking part. Not sure why you need so much inductance.
btw National instruments provides a software(DOS) to calculate inductors and capacitors in their switching regulators. When I input 12V @ 2Amps the inductance computed is 330uH.

Thanks for all the info. Will start from these info. Not sure weather I will get samples from them here in INDIA.

Thanks.

btw any information on how to wind toroid for 330uH inductance?
 

bountyhunter

Well-Known Member
You need info on the core before you can wind it.
Correct. You have to make sure your peak current stays in the allowable operating range for flux density for the specific core. Max usable flux density varies for core materail (and type of core). A gapped core where there is an air gap in the flux path allows higher operating current but gives less inductance.
 

bountyhunter

Well-Known Member
btw National instruments provides a software(DOS) to calculate inductors and capacitors in their switching regulators. When I input 12V @ 2Amps the inductance computed is 330uH.
Yeah..... I worked on helping the team develop that software for a while. Suffice it to say, computer programs have to make assumptions about ripple current in selecting an inductor.

You can choose to allow more ripple current in the inductor if you want. If it was my design, I would just trade up to one of the higher frequency simple switchers (I recall they went up to about 150kHz at last count?). All the output components (inductor and output capacitor) shrink if you do.
 

picstudent

New Member
Correct. You have to make sure your peak current stays in the allowable operating range for flux density for the specific core. Max usable flux density varies for core materail (and type of core). A gapped core where there is an air gap in the flux path allows higher operating current but gives less inductance.
Here I have a problem with core.
I collected the data sheet from my ferrite source.

The part names start like T16.0 or T27.5 etc. They have inner dia outer dia height etc. So far Ok.

As it starts with T, it is not ferrite but powdered iron core. (right?)

The they have Le, Ae, and Ve parameters specified in cm2. What is that?

Also they have AL value two versions .

I will attach the page. Please help me to understand it. Al value seems to be too different from many tutorial examples.


Thanks .
 

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picstudent

New Member
Hello
I have located a calculator which accepts my datasheet values for AL and gives output which Tally with practical coil measurements in my inductance meter.
My T-21.0 core (Outer dia 21mm, inner dia 13mm height 12.5mm) Al value is 3530.

On winding 9 turns it gives 340uH in meter. I need 330uH Only.

Now another issue is saturation flux density. As per datasheet it is 480.
How to calculate the flux density when 2Amps flows through winding?

Thanks for the info
 
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bountyhunter

Well-Known Member
Here I have a problem with core.
I collected the data sheet from my ferrite source.

The part names start like T16.0 or T27.5 etc. They have inner dia outer dia height etc. So far Ok.

As it starts with T, it is not ferrite but powdered iron core. (right?)

The they have Le, Ae, and Ve parameters specified in cm2. What is that?

Also they have AL value two versions .

I will attach the page. Please help me to understand it. Al value seems to be too different from many tutorial examples.


Thanks .
It's been about 25 years since I designed my own inductors. I think Ae is cross sectional area of the core. I don't remember all the other various parameters. This is not trivial stuff, you have to have the equations and theory to understand how to make an inductor for a specific operating frequency and keep the flux density reasonable. I seriously recommend you go to Digi-Key or similar part supplier and buy an inductor. That way you will have a data sheet on it and know what it does.
 
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