Author Topic: Buck converter for small wind turbine project  (Read 69172 times)

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Offline rossw

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Re: Buck converter for small wind turbine project
« Reply #30 on: January 29, 2012, 10:24:13 pm »
I re labeled the voltage divider.

I might be missing something here - but if that's an N-channel FET, and you have the drain to the supply, how do you ever plan to turn it on?

Somehow, you need to get the gate about 10V higher than the source - and without some extra supply, I just don't see how you can do it. When the FET is off, the source will be (close to) ground, and sure - you can easily make Vgs more than enough to turn it on... but as it starts to turn on, resistance Drain-Source lowers, and V(source) aproaches V(drain), so whatever you have on the gate is going to be not enough to keep the FET saturated, it'll turn itself off (or more correctly, stop turning itself on). It's a lot like an emitter resistor in a conventional BJT.

Or, am I missing something in your circuit??

Edit: Actually, it's even worse than I thought. In my mind, I saw the battery as a "load", not a 12V supply.
So the Source is never going to go "lower" than +12V, and your CMOS chip is driving the FET GATE - yet the CMOS is powered from the battery, so you can NEVER get the gate higher than the source, much less 10V higher.


Offline oztules

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Re: Buck converter for small wind turbine project
« Reply #31 on: January 29, 2012, 11:43:47 pm »
Exactly.... and the only way he can drive the gate 12v or more positive than the source will be with an isolated gate driver of some type. Simplest is the small torroid transformer coupling. Any other method that looks at ground and not just the gate source relationship, will get feedback from the changing value of the battery and the freewheel diode.

I was beginning to think it was just me.... but I'm not getting through... even explaining how the field effect was used for conduction... so all I can do is leave it to you and quote this from my last post

"Do you now see why you MUST reference the gate to the source. You cant generate the electric field above the matrix and turn the P into N unless it is with respect to the source.... nor can you turn it off again unless you bring it back to source potential."

It's that simple.


Pulls pin from grenade... holds between teeth... and slowly counts down :P


.................oztules
Flinders Island...... Australia

Offline GW@PE

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Re: Buck converter for small wind turbine project
« Reply #32 on: January 30, 2012, 05:40:19 am »
Hi Burnit,

Mosfets,mosfets,mosfets. 

It worries me when readers tackle probably the most difficult task, at the deep end [PWM].  Mosfets can lead users down a very steep and costly learning curve.  I also wince when I see large air cored inductors used in power ccts.  I also worry when I see Meg ohm resistors in mosfet gate drive ccts.

Mosfets can be used with high efficiency at switching frequencies above 500kHz, as I have indeed done in other applications.

Some good suggestions have been made by oztules and RossW, particularly re high side drivers and analogue PWM controllers.  National instruments produced many books and technical notes, as have Maxim, etc etc. 

BTW, the mosfet will turn ON, as a result of the charge pump feeding volts to the gate through the 100ohm resistor.  I however see no way of turning the mosfet OFF.  the propsed transistor with 1M resistors will not ever turn the mosfet OFF.

I am new to this forum, however I have a lot of experience elsewhere, and I am feeling my way a bit again after forced excile.

Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #33 on: January 30, 2012, 06:45:05 am »
Hi, this is a first attempt at this type of circuit. Funding to order gate driver IC’s will be available within the week.

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2041244_-1

Thank you to everyone for your time and efforts.

Offline Jimf

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Re: Buck converter for small wind turbine project
« Reply #34 on: January 30, 2012, 09:03:48 am »
Burnit,

Quote
Hi, this is a first attempt at this type of circuit. Funding to order gate driver IC’s will be available within the week.

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2041244_-1

I too have been looking for a way to high side drive a FET and thought this IC might provide a solution for the isolated drive voltage and fast switching required, but after looking at the spec. sheet it appears to only provide the switching.  You will still have to have an isolated voltage source to supply the driver IC.  Does anyone know of a IC that will do both?  Good luck on your project!

Jim

Offline GW@PE

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Re: Buck converter for small wind turbine project
« Reply #35 on: January 30, 2012, 04:04:23 pm »
Hi Burnit,

I have just looked a bit more at the cct you posted, and if the back emf developed by the coil becomes sufficient, then this will push the mosfet source voltage higher.  This will then reduce the effective gate to source voltage to a point where the mosfet will no longer be held fully on.

To have low tri-state and conduction losses, mosfets need to be switched fast and with authority.  This opens another can of worms with RFI & EMI radiation from wiring etc. 

The challenge will test your patience.

Gordon.




Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #36 on: January 30, 2012, 05:00:35 pm »
Hi, I redrew the circuit hoping it will be easier for me to explain.

Starting at the bottom: the PMA charges to input capacitor. I have tested the PMA just charging a capacitor. The capacitor charges up very fast and reaches 50 volts in a very short period of time. Because I am just charging a battery I do not believe a capacitor is necessary at the output before the battery, but it can be added if required. 

The inverting comparator at the top is needed to enable the system and will disable the system when Vin falls below Vref. Fan out of the comparator should be able to drive two gates.

When point A is high the nand gate oscillator and the voltage multiplier will generate 24 volts across the voltage divider and the second nand gate oscillator will toggle the transistors and create signal Vgs to turn the MOSFET on and off. ( This is the part of the circuit that needs improvement)

The buck converter will continue to function until Vin is below Vref.

I have tested all the parts of the circuit separately and they do function. I am currently constructing the two nand oscillators on the proto board to try to determine if they will generate Vgs. Once I have the Vgs signal then I can add the MOSFET and test with the PMA.

A micro controller can be added after I have a basic circuit working, it will make the algorithm easier to develop once I am able to observe the affect the buck converter has on the PMA.

  Thank you for pointing out possible problem areas , it is good to have options. I am just working on one problem at a time. Thanks again. 
   



Offline rossw

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Re: Buck converter for small wind turbine project
« Reply #37 on: January 30, 2012, 05:47:41 pm »
I still don't think you're "getting" a critical thing.

The voltage on the gate is actually irrelevant.
It's the voltage on the gate *WITH REFERENCE TO THE VOLTAGE AT THE SOURCE* that will turn the FET on.

So, if you have 50 volts at the rectifier output (point (C) on your diagram), the drain will be at 50V.
The source will be at (lets say) 12V, when the FET is off.
So your charge pump makes 24V. OK. Fine, you hit the gate with that 24V, and at that instant Vs=12, Vg=24, Vgs=12 and the fet starts to turn on.

AS it turns on, the 50V on the drain starts appearing on the source. So it is now ramping from 12V up "towards" 50V. In super-slow-motion... source starts getting power through, it's now at 13V. Still 24V on the gate, Vgs now 11V. FET still switching on, source now 14V, gate still 24V, Vgs now 10V, FET still switching on, source now 15V, gate still 24V, Vgs now 9V, FET may still switching on but not so hard... source now to 16V, Vgs only 8V.... at 20V on the source, Vgs is now only 4V - you see what's happening??

This is ALL because you're driving the gate with a ground-referenced signal, not a source-referenced signal.

The design is fundamentally flawed, and NO amount of fluffing about with it will make it work "properly"

Similarly, turning it "off", as has already been stated - a 1 megohm resistor will bleed charge off quite slowly. It certainly won't be DRIVING the FET off. There's a reason this type of circuit isn't used in production. It just doesn't work.


Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #38 on: January 30, 2012, 06:01:20 pm »
Hi, thank you, I now understanding what you are saying. Does a gate driver IC solve the problem?

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2041244_-1

Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #39 on: January 30, 2012, 07:57:17 pm »
Hi, on page 22 and 23 there seems to be a solution. Vcc would be replaced by the PMA?
Comments welcome.

http://www.irf.com/technical-info/appnotes/an-978.pdf

Offline rossw

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Re: Buck converter for small wind turbine project
« Reply #40 on: January 30, 2012, 08:22:26 pm »
Hi, on page 22 and 23 there seems to be a solution. Vcc would be replaced by the PMA?

Figure 24 looks pretty close to what you want.

Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #41 on: January 31, 2012, 07:24:48 am »
Hi, this is a first attempt at this type of circuit.

I did find a paper that does cover charger design.

 http://www.ti.com/lit/ml/slyp089/slyp089.pdf

At the beginning it does explain why a p channel mosfet is preferred for this type of circuit. 

Thank you for the help it is really appreciated.   

Offline Burnit0017

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Re: Buck converter for small wind turbine project
« Reply #42 on: February 01, 2012, 06:17:56 pm »
Hi, the circuit is from the .PDF file. I am not sure if it will address the problem that rossw has pointed out.  Searching for circuit to create Vdr, maybe a charge pump can be used.
Comments welcome.


http://www.usna.edu/EE/ee320/Supplements/dcdc5_driver.pdf

I have ordered IR2117

Offline rossw

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Re: Buck converter for small wind turbine project
« Reply #43 on: February 01, 2012, 07:02:11 pm »
I am not sure if it will address the problem that rossw has pointed out.

It's still ground-referenced, and inherently will suffer a plethora of problems related to the difference between system ground and FET Source.

As you say, more-expensive and slower P-channel FET would overcome those problems but leave you a different bunch :)

Proper high-side driver is the real answer. Or, isolated supply and perhaps opto-isolated control of your oscillator.

Offline Janne

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Re: Buck converter for small wind turbine project
« Reply #44 on: February 01, 2012, 07:19:46 pm »
Or, isolated supply and perhaps opto-isolated control of your oscillator.

That is propably the easier way compared to the signal transformer. Isolating DC / DC converters of suitable power can be had for under 10$, for example digi-key has loads of those parts.
I actually ordered a few of those, when desperation dug into my own mppt project with signal transformer problems. But eventually I was able to get it to work before the converters arrived.
Beyond the wolf border