Self In boost topology

[arvinfx]

I am building a boost circuit that convert 12v to 24v with 4A in output.

I am realy confused about inductor part! Is this topology like flyback? or forward?
In flyback transformer needs air gap but in forward don`t.
What should I do for self in my circuit? With air gap or without ?

 

[ronsimpson]

Boost = flyback
Needs air gap
You said "12v to 24v with 4A in output". watts in = watts out, If output is 4A then input is 8A.
If this is a non isolated supply; then 50%, The inductor will have 12V from input from 50%, and for 50% it will have 12V from input to output.

 

[crutschow]

A boost circuit can be flyback (if the inductor/transformer has two or more windings) or a boost switching-regulator type circuit which uses a single-winding inductor. In either case you need to be concerned about saturating the core and this may require an air gap.

 

[arvinfx]

Ok , Well do you have the formula for calculate air gap and also core selecting?

Actually this circuit needs more than 8A ( because of switching part drop voltage and some other parameters )

 

[crutschow]

No it's not a trivial calculation. But Google turned up may sources for this such as here.

 

[arvinfx]

I know about advanced formula . My circuit is this:
**broken link removed**

And also spice output:
**broken link removed**

What is the suitable formula for my circuit?

 

[ronsimpson]

It is hard to see what trace is what but, I think you peak current is about 10 amps.
The inductor needs to handle 10A with out saturation.
If you get a "10A" inductor it will probably run 100C and the inductance will be -30% at 10A. It is not a good idea to run parts at their limit. A 15 A inductor will be much cooler and the inductance will only drop a little.
If you build your inductor; then watch the flux level and keep it out of saturation. At 100khz you are running you may have to think about AC loss in the core.

 

[ronv]

I think I would buy one like this for $4 if you can.
https://www.electro-tech-online.com/custompdfs/2012/01/2300ll_series.pdf
Check the ripple current in C1 it may be to high.

 

[ronsimpson]

C1 will see 8 to 10A of ripple.
You need a input cap across the battery. It's ripple current is about 1A if you have a 70uH coil.

I would not use RonV's coil. It will be at 55C with 8A of current DC. Plus AC loss that will put the coil at 70 to 80C. Note the 68uH coil will be at 37.4uH +/- 20% at 8A. The peak current is going to be 10 or 11 amps and the inductance will be closer to 20uH. Running at 1/2 the inductance will cause the P-P ripple to increase 2:1. This will make the peak inductor current to go up one more amp.

 

[simonbramble]

Just to add something here. The terms 'Flyback' and 'forward' are normally reserved for transformer based designs. With a single inductor, this is just normally called a boost. A flyback stores the energy in the transformer, then dumps it into the secondary. An air gap enables the transformer to store more energy which is why a flyback needs an airgap. With a forward converter, put simply, you are imposing a square wave on the primary that is transformed to a square wave on the secondary. No (or very little) energy is actually stored in the transformer, so you dont need the airgap.

A boost converter is (technically) a flyback (it dumps its energy into the load when the FET switches off and the inductor voltage flies back), but dont use this term to refer to a boost converter as most people refer to flybacks as being transformer based

If you want to understand boost converters a bit more, please see my tutorial:
http://www.simonbramble.co.uk/dc_dc_converter_design/dc_dc_converter_design.htm
You can normally simulate most things like this in LTSpice. Here is my LTSpice tutorial:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

I hope this helps

Simon

 

[RCinFLA]

A flyback or boost inductor must store energy. A transformer needs to tightly couple magnetic field. The B-H curve on an inductor designed for storage is drawn out along the H mag-motive force axis. The area from the B-H curve and the vertical B axis is the amount of energy stored.

Most transformer 'E' cores are designed with very high permeability cores. These provide tight magnetic coupling for a transformer and their B-H curve is very steep with the ability to store very little energy. Torroid cores are designed with less permeability and are better choices for inductors. You can stretch out the B-H curve of a high permeability 'E' core by gapping the core. This does not change the saturation B field of the core but does allow more H force by reducing the slope of the B-H curve.

This link has good info on designing an inductor with a torroid to handle a given amount of current. If the core is not sufficient it will saturate and the inductance will severely drop.
https://www.cwsbytemark.com/index.php?main_page=index&cPath=206_222
https://www.cwsbytemark.com/index.php?main_page=page&id=117

 

[arvinfx]

Just to add something here. The terms 'Flyback' and 'forward' are normally reserved for transformer based designs. With a single inductor, this is just normally called a boost. A flyback stores the energy in the transformer, then dumps it into the secondary. An air gap enables the transformer to store more energy which is why a flyback needs an airgap. With a forward converter, put simply, you are imposing a square wave on the primary that is transformed to a square wave on the secondary. No (or very little) energy is actually stored in the transformer, so you dont need the airgap.

A boost converter is (technically) a flyback (it dumps its energy into the load when the FET switches off and the inductor voltage flies back), but dont use this term to refer to a boost converter as most people refer to flybacks as being transformer based

If you want to understand boost converters a bit more, please see my tutorial:
http://www.simonbramble.co.uk/dc_dc_converter_design/dc_dc_converter_design.htm
You can normally simulate most things like this in LTSpice. Here is my LTSpice tutorial:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

I hope this helps

Simon

Thank you simon and other guys
I will read your article on your site(I think it is useful).
I am using Pspice, what is diffrent between LT and Pspice?

 

[bountyhunter]

I am building a boost circuit that convert 12v to 24v with 4A in output.

We built a lot of boost converters that run off 12V. For 100W power range, a push-pull design is much better. Magnetics will be smaller, smaller caps, lower peak currents. It's a forward converter using two FETs with the transformer primary center tap tied to 12V.

 

[ronv]

Simon,
Great write-up. Real information is a joy.

 

[arvinfx]

We built a lot of boost converters that run off 12V. For 100W power range, a push-pull design is much better. Magnetics will be smaller, smaller caps, lower peak currents. It's a forward converter using two FETs with the transformer primary center tap tied to 12V.

I have already built a push-pull that convert 12 to +-35v and works great. It is a challenge for me

 

[MrAl]

I am building a boost circuit that convert 12v to 24v with 4A in output.

I am realy confused about inductor part! Is this topology like flyback? or forward?
In flyback transformer needs air gap but in forward don`t.
What should I do for self in my circuit? With air gap or without ?

Hi,


Yes, i think you are a bit confused about what to call the different topologies as well as the real function of an air gap.

The topology you seem to be after is usually called simply a DC to DC boost converter circuit. It uses an inductor, not a transformer.

An air gap in an inductor would be placed there only if the inductor without an air gap was going to saturate due to some DC current level. If you already have an inductor that can handle the DC current then you dont have to add an air gap, as with many over the counter inductor purchases.

If you are building your own inductor then you might want to include an air gap if you calculate that the flux density will go too high without one. This includes the DC current and the AC current, or simply the peak current. The nice thing about an air gap is that it reduces the flux density increase with DC current (DC current will always be present in a DC to DC converter like this) so as to avoid saturation, but the drawback is reduced permeability so more turns have to be added to make up for the loss of inductance the air gap causes.
The air gap has a profound effect on the overall operation of the construction because the core material is usually made of material that has a permeability much greater than that of air. Even with a core permeability only 100 times that of air, a 1mm air gap has the same effect on the permeability as an additional magnetic path length (of the core material) of 100mm. Quite an effect indeed. So to calculate the inductance you would use the regular inductor formula but include the effect of the air gap as a change in magnetic path length. Thus for the core above with a path length of 100mm that causes an inductance of 100uH (for example), with a 1mm air gap the inductance would be reduced to about 50uH because the 1mm acts like 100mm of core material. For a 10mm core, even 0.1mm could affect it the same way, but reducing the inductance by 1/2. For higher core permeability the effect will be even greater.

I've built up many constructions in the past, including one with four air gaps of very short length and some that weighed in at almost 100 pounds each, and found the results to be interesting, but what most people do these days is simply look for an inductor online that will work well in their application. You do a few calculations based on your input/output voltage and current requirements and operating frequency and then you go out and find an inductor that can handle the max current expected without changing inductance too much (the inductors inductance decreases with increased current level), and you're done.

 

[arvinfx]

Just to add something here. The terms 'Flyback' and 'forward' are normally reserved for transformer based designs. With a single inductor, this is just normally called a boost. A flyback stores the energy in the transformer, then dumps it into the secondary. An air gap enables the transformer to store more energy which is why a flyback needs an airgap. With a forward converter, put simply, you are imposing a square wave on the primary that is transformed to a square wave on the secondary. No (or very little) energy is actually stored in the transformer, so you dont need the airgap.

A boost converter is (technically) a flyback (it dumps its energy into the load when the FET switches off and the inductor voltage flies back), but dont use this term to refer to a boost converter as most people refer to flybacks as being transformer based

If you want to understand boost converters a bit more, please see my tutorial:
http://www.simonbramble.co.uk/dc_dc_converter_design/dc_dc_converter_design.htm
You can normally simulate most things like this in LTSpice. Here is my LTSpice tutorial:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

I hope this helps

Simon

It is a great note! amazing!
could you tell me more about these yellow underline values? what is these and where came from?
**broken link removed**
Best Regards

 

[ronsimpson]

The output current is 2.67A.
The current in the inductor ramps up and down where the average is 2.67A. The ripple is 40% (that is P-P) so it is +20% and -20%.

With an average of 2.67A and a peak of +20% then the peak is (2.67 x 1.2)
The minimum current is (2.67 -20%) or 2.67 x 0.8
So the current ramps from 2.14 to 3.2A. The peak to peak is 1.06A

 

[simonbramble]

Ron

You took the words right out of my .... erm... fingers...

Thanks! Hope 2012 is a good one

Simon

 

[simonbramble]

if the inductor is too low in value the change in current with time will be too 'steep'. If it is too high, the change in current will not be steep enough and the output regulation will probably suffer if you dump a sudden load on it. 40% is a sweet stop to trade off low inductor with good transient response