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Topic: What is a part called, that rectifies powerful PWM (10 A)? (Read 4311 times) previous topic - next topic

dwightthinker

Sorry about misleading you with the AC coil.
I should have known better, having designed switchers in the past.
Still, most of these down switchers shown could be used for
what you have in mind without using a digital resistor.
You can take the PWM output and clean it up through a RC filter
to control the switching regulator control pin.
It can be treated just like a linear regulator.
If you don't mind a big heat sink, a linear regulation setup
would also work fine.
Dwight

Unknownsymbols

#16
Jan 05, 2016, 01:40 am Last Edit: Jan 05, 2016, 01:51 am by Unknownsymbols
Well, you mean the regulator itself will be dissipating much heat, and will need the big heatsink - not the RC-circuit? And the power dissipated will be on orders of the same 100 W? If that, it's not doing to suit. Such a size (10 cm3) for just regulator is too big. And of course the system mustn't lose all power that's not powering it.
And you see, applying the digital resistor is not the hardest - all the harnessing is the hardest. So linear entry to that point i guess isn't simplifying it.

I'm now wondering what must the inductor be, if it's just a flyback. The parameters must be like: diode: >= 10 amperes, and inductor >= 10 amperes, right? But how much henries? Perhaps what formula is used.
I get the notion, eeeeh, like it was about RC here at 25 minutes; the more L, the more time it'll be settling the curve (at any change of PWM), but the less L, the less the compensation will be. And i mustn't go over some critical scale. But around what. (I can't pretty much try variants, because big inductors are hard to find.)

dwightthinker

#17
Jan 05, 2016, 02:08 am Last Edit: Jan 05, 2016, 02:41 am by dwightthinker
I didn't say the linear would be practical for your application.
For the switcher the size of the inductor depends on the frequency
of the switcher and the ripple you are willing to tolerate on the output.
Even though, you don't want a full turn-on/turn-off, I would suspect
for your application a 2% or 3% ripple is well within the range you
can tolerate.
You don't want to run the switcher at the PWM frequency of the
adruino. That would require a really large inductor. You just want
to make a relatively smooth DC to control the switching regulators
input control. Let the switcher go to a higher frequency.
Dwight


Unknownsymbols

#18
Jan 05, 2016, 12:54 pm Last Edit: Jan 05, 2016, 01:06 pm by Unknownsymbols
I think i don't intend to control a switcher regulator - but a transistor, and for that need the inductor. If i need to raise the frequency, maybe i could use the driver of the Mosfet. But sure it isn't critical. I could even do without the smoothener, so i guess even 10% ripple will be not so bad, ?

The size of the inductor should be ultimately fittable in the 5'25 computer bay together with the Arduino and all other small parts with it. Also i read there can be several (smaller) coils combined.

So i found this formula - is it it? ind. for bucker
L = (V0 × dt) / (dI × F) = (V0 × 10) / (I0 × F) for 1/10 ripple dI/dt = I0/10, if i need that oscillation on the current to be 1 10th of the main amplitude. It means L ≈ 12 / 62500 (the max. freq. of Ard.) ≈ 0,0002 H? And for 2% it would be 0,001 henry. These coils don't seem very big, isn't it n't?

MarkT

200uH is fairly big, 1mH is large.  Remember you need a high efficiency ferrite-cored gapped
inductor for switching usage, and if you look for 1mH 10A rated inductors you'll find they are
not small or cheap.  High current means thick wire means fewer turns means larger core, and
the energy-storage requirement indirectly means larger cores too (even though the energy is
mainly stored in the magnetic field outside the ferrite, its vital the ferrite doesn't saturate, and
ferrite doesn't need much magnetic field before it saturates.

The need for a gapped core is to store energy (a transformer doesn't store energy, so its
usually much smaller for a given inductance).  Highly magnetic materials are less able to store
energy than air/vacuum (it turns out), hence the gap.  The core serves to guide the magnetic
field so that it nearly all passes through all of the coil and delivers the field efficiently to the gap.
This allows much more compact (and thus low-resistance) winding.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dwightthinker

Where do you get 62500. The PWM is only 490, 980 if using an Uno.
Dwight

MarkT

62.5kHz is 8 bit fast PWM at full-whack, no prescaler division, from 16MHz.  You can go faster, but
you get fewer than 256 levels then.  100 levels only at 160kHz, etc.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dwightthinker

62.5kHz is 8 bit fast PWM at full-whack, no prescaler division, from 16MHz.  You can go faster, but
you get fewer than 256 levels then.  100 levels only at 160kHz, etc.
I see.
Many regulator chips don't go much beyond 100K and that is still a big coil.
Maybe he is looking at adding some large capacitors to reduce the ripple.
Doesn't the prescaler also effect baud rates?
Dwight

Unknownsymbols

#23
Jan 06, 2016, 10:28 am Last Edit: Jan 06, 2016, 11:06 am by Unknownsymbols
Yes. 1 mH is quite big. I thought i found a good example on digikey.com, but i mislooked it. Schurter DFK-40-0005 is 40 ∙ 40 ∙ 30 mm. Hammond 157D is also like a general adapter - that type of big. 300 uH is 30 ∙ 30 cylindric. By the way i couldn't find a way to select if they are air-gapped.... (and also they cost > 20 $, > than a lot of power suppliers themselves)
So i'm looking into raising the frequency. I supposed, but i don't know for real, can a MOSFET-driver increase the frequency?
(i know i could modulate it manually by waiting microseconds in Arduino program, or even completely inline, up to 16 Mhz / step but i'll need that Arduino to do a lot of other things, so that's not good.)

MarkT

Fixed-value inductors are usually gapped - otherwise the value is very temperature dependent and unstable.
Transformers and chokes are usually ungapped, the inductance value just has to be "large enough"
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dwightthinker

Yes. 1 mH is quite big. I thought i found a good example on digikey.com, but i mislooked it. Schurter DFK-40-0005 is 40 ∙ 40 ∙ 30 mm. Hammond 157D is also like a general adapter - that type of big. 300 uH is 30 ∙ 30 cylindric. By the way i couldn't find a way to select if they are air-gapped.... (and also they cost > 20 $, > than a lot of power suppliers themselves)
So i'm looking into raising the frequency. I supposed, but i don't know for real, can a MOSFET-driver increase the frequency?
(i know i could modulate it manually by waiting microseconds in Arduino program, or even completely inline, up to 16 Mhz / step but i'll need that Arduino to do a lot of other things, so that's not good.)
Use a regulator IC. It has a frequency controlled by a resistor and capacitor.
Just use the normal PWM for the control voltage to the switching regulator.
Why make it complicated trying to push code to do simple hardwares work?
Dwight

Unknownsymbols

Can you say what model of IC are you meaning?
If you mean any regulator, then i don't know where to attach PWM to there.

dwightthinker

Many regulators have an external reference.
That can come from a simple RC filter of the PWM from
the processor.
You wouldn't do this with a fixed regulator ( at least not
easily ).
Dwight

Unknownsymbols

#28
Jan 23, 2016, 12:59 pm Last Edit: Jan 23, 2016, 11:35 pm by Unknownsymbols
Well, i guess that's similar to a programmable DC-DC converter. It can be called adjustable, and the Vref pin will be used to set the output voltage. I didn't wrap my mind around it first, though i had seen a "reference pin" on some of them. So i think they differ in just that programmable have sort of higher level interface, like I2C, instead of Vref.
To use a RC circuit to smooth the control signal, i already know how. (The formula F=1/(2piRC)).

Maybe i'll eventually try it, but if i understand correctly, an adjustable converter will still require a lot of parts around it, which aren't easily in the datasheet. Maybe also an operational amplifier to bring the Arduino's 5 V to the needed 12 V.

....
Also difficultating the matter is that i'd need a buck converter as close as possible in output voltage to the input voltage. They seem to return at best 0,9. (Mosfet in this aspect is 0,97. And i can't yet find that high.) Or buck+boost, but they are only resistor-selected.

dwightthinker

Many regulators have a + and - input lead that in the diagrams
look like an op amp.
The - lead is usually a divider from the output and the
+ lead is the VRef.
It doesn't need to be 12 volts for a 12 volt out.
The divider can be 5 volts for 12 volts out.
The arduino can easily supply 0-5 volts.
An old simple linear regulator might be the LM723 ( as I recall ).
There are a number of newer switching regulators that would
work fine as well.
Dwight

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