Author Topic: Buck converter for small wind turbine project  (Read 69464 times)

0 Members and 1 Guest are viewing this topic.

Offline oztules

  • Forum Advisors
  • Hero Member
  • ****
  • Posts: 1177
  • Karma: +105/-8
  • Village idiot
Re: Buck converter for small wind turbine project
« Reply #15 on: January 22, 2012, 03:37:21 pm »
Oh dear, thats a fet cooking waveform you have there.

You will be well advised to use a dedicated PWM chip...... from any computer power supply, and start with something that will actually work.

Use the op amps in the chip for your multiplying (change the feedback gain) and switch off - function/current limiting, and the chip will do the house keeping better than discrete circuitry. You can also use the deadtime control for remote operation / micro control/ override/control.... or any combination of the above. They are very accommodating in this way.

Totem pole transistors will give you pretty good low impedance fet switching, but a dedicated driver will be even simpler, with even less problems to worry about.

I really don't think this circuit is worth the perseverance.... sorry :(


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

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #16 on: January 22, 2012, 05:13:49 pm »
Hi, there is nothing to be sorry for. The circuit worked for a small DC motor driver, but it is clearly not going to work as a MOSFET gate driver, so it time to move on and find a suitable MOSFET and gate driver IC.  Thank you for the help I have been receiving. I will post updates when available. At some point it will work. Thanks again.

Offline ghurd

  • Global Moderator
  • Sr. Member
  • ******
  • Posts: 442
  • Karma: +22/-0
    • GHurd Solar
Re: Buck converter for small wind turbine project
« Reply #17 on: January 22, 2012, 08:57:07 pm »
The slopes are killing the fets.
A ghurd controller can have an output like this (but he is not able to entertain more thoughts about MPPT at this time),
G-



Offline rossw

  • Senior Moderator
  • Hero Member
  • *******
  • Posts: 879
  • Karma: +35/-0
  • Grumpy-old-Unix-Admin
Re: Buck converter for small wind turbine project
« Reply #18 on: January 22, 2012, 09:15:04 pm »
The slopes are killing the fets.
A ghurd controller can have an output like this (but he is not able to entertain more thoughts about MPPT at this time),
G-



You say "5 amps per division" on the vertical axis.... you're obviously not referring to gate drive here then? (In which case it'd be 5 volts per division? And if it were 5V/div,  that'd be close to 13V gate drive even at the low end of the slope - which should have been quite enough to saturate drain/source??)

So are you saying that the slope on the leading edge of the "high" part of the cycle - current starts high, then decays a bit, then holds steady? What do you attribute that to?

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #19 on: January 22, 2012, 09:35:59 pm »
Hi, a TC4420CPA has been recommended for the gate drive. I am just trying to get a basic buck converter to work at this time. It has been a number of years since I have thought about any circuit operation and all problems will find a solution.

Because I am using cmos nand gates and a 12 volt deep cell for Vdd, the output of just the nand oscillator may be enough to drive the mosfet gate. The data sheet for the IRF510 says Vgs at 10 volts has a continuous current at I drain of about 5 amps but I am not sure if  I am reading the information correctly.  I should know more soon, thank you for all the help I have been receiving.   

Offline oztules

  • Forum Advisors
  • Hero Member
  • ****
  • Posts: 1177
  • Karma: +105/-8
  • Village idiot
Re: Buck converter for small wind turbine project
« Reply #20 on: January 23, 2012, 01:55:27 am »
It's not just the voltage, it's the switch current.

If you must use cmos 4000 series chips, at least use a hex schmitt trigger, all paralleled, and you may achieve the drive necessary.

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

Offline frackers

  • Full Member
  • ***
  • Posts: 239
  • Karma: +9/-0
  • If it moves - computerise it!
Re: Buck converter for small wind turbine project
« Reply #21 on: January 23, 2012, 03:53:29 am »
I'm using an MC34151 dual driver chip because they are cheap and a stock item at the local emporium!! Works well driving four off IRF1405 75amp MOSFETS in my dump load controller.

You can check out the circuit and layout at http://gilks.ath.cx/~g8ecj/New_Turbine/new_controller.html which ensures low stray currents, proper decoupling of the driver chip, opto isolator drive and gate protection using high speed schottky diodes and damping resistors.

Not sure how fast it will run but my 50MHz scope can only just see the edges when switching at just under 1KHz


Robin Down Under (or are you Up Over!)

Offline oztules

  • Forum Advisors
  • Hero Member
  • ****
  • Posts: 1177
  • Karma: +105/-8
  • Village idiot
Re: Buck converter for small wind turbine project
« Reply #22 on: January 23, 2012, 05:17:47 am »
Nice project Frackers.....

I've never ended up using  opto isolators to actively drive the driver circuits before... i found switching time was laggy... but i didn't try 1khz either I guess....

If you get the wave you say, then maybe i didn't persevere enough with them.... although I try to keep above 20khz, and they didn't seem keen to keep up with a clean wave.


.............oztules

Flinders Island...... Australia

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #23 on: January 25, 2012, 06:42:36 am »
Hi, is it possible to have a transistor configuration to oscillate Vgs?

When point “A” is enabled the voltage at point X will be 2 times the battery voltage.

MOSFET source is always at the plus value of the battery voltage and is consider ground with respect to Vgs.

The MOSFET requires  10 volts at the gate to turn on.

Use a resistive voltage divider to create a high and low side of the voltage at point X.

Configure a transistor circuit on the high side of the voltage divider to provide 10 volts to gate.

I do not have a lot of experience with transistor circuits, comments are welcome.

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #24 on: January 29, 2012, 02:26:00 am »
Hi, update of basic buck converter circuit.

When voltage at point C is greater than the battery voltage
     point A is enabled
     point X is 24 volts
The top nand gate oscillator and inverter toggle the transistor switches
turning the MOSFET on and off.

I am ordering gate driver IC’s.

I will post test results when available. Comments welcome.

     

Offline oztules

  • Forum Advisors
  • Hero Member
  • ****
  • Posts: 1177
  • Karma: +105/-8
  • Village idiot
Re: Buck converter for small wind turbine project
« Reply #25 on: January 29, 2012, 03:26:08 am »
You must reference the Vgs voltage to the source....  not ground.


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

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #26 on: January 29, 2012, 04:16:53 am »
Hi, I am having a lot of confusion trying to determine what voltage is required to turn on the MOSFET.

I am using a IRF510. The data sheet info says Vgs is +- 20 volts and the gate(thershold) is 2 to 4 volts.

If the source is connected to +12 volts, what voltage value is needed at the gate to turn on the MOSFET?

Offline oztules

  • Forum Advisors
  • Hero Member
  • ****
  • Posts: 1177
  • Karma: +105/-8
  • Village idiot
Re: Buck converter for small wind turbine project
« Reply #27 on: January 29, 2012, 05:20:04 am »
Try to drive it with a square wave. 0v for off and +13v saturated on.. this voltage is for the gate REFERENCED to the source.... nothing else matters to it, only the voltage between the source and the gate.

If you look at how a field effect device works you will see why this is. 
Long story short. We have a NPN structure. We'll call the first N doped silicon the source, the P doped one next to it (is isolated for this story), and then the last N doped silicon is our drain.
If we put our source to ground, and our drain to a positive... the electrons can go from the left N into the doped P but get stopped at the next P-N barrier (reversed biased diode really).

So nothing happens.

Now we place an insulated layer on top of the NPN silicon matrix... then a conductor that sits on that.... thats the gate. It is not connected to anything else at this stage.... so still nothing happens.

Now if we place a potential between the insulated gate +, and the source, we create an electric field. This effects the P layer (pushes the positive carriers away, creating a N inversion layer), and basically turns it into a N substrate.... now we have a NNN from source to drain, acting as a single block of N type material... the more positive .....the more conductive the new "N" layer so the resistance is all gone... it's turned on.

If we now take away the gate charge and drive it back to the same potential as the source... it turns back into a reversed biased diode... and the channel no longer conducts.

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.

Note the gate is insulated from the other substrate, which gives us a very high impedance input.

Beyond +-20v Vgs, will puncture the isolation to the gate, 3 volts is enough electric field to start turning the N channel on, and conduction begins. We need to turn it up to 12-15v to for  enough electric field to make darn sure it is fully on.

You do not want a partially turned on source drain path. You want it off or on..... nothing in between.

At least thats how I remember it. There are probably a few more things to worry about (body diode, capacitance etc), But thats the gusts of it I think.

Someone else can correct any blues.



................oztules

Edit spelling
Flinders Island...... Australia

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #28 on: January 29, 2012, 12:32:38 pm »
I re labeled the voltage divider.

Offline Burnit0017

  • Full Member
  • ***
  • Posts: 191
  • Karma: +0/-0
  • No Personal Text Set by User
Re: Buck converter for small wind turbine project
« Reply #29 on: January 29, 2012, 10:07:39 pm »

Hi, waveform verifying function of nand oscillator and npn transistors switching circuit.
circuit update: all the parts of the control circuit are working, switching frequency @ 4 k.
Next I will try to piece them together  and try to trigger the MOSFET.