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Buck converter for small wind turbine project

Harald Kapp

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First: I'm not an expert at switched mode supplies.

But: May I add some thoughts on your "test circuit"? To me it looks like a step-down (buck) regulator with some problems/deficiencies:

1) Yes, you will need some kind of voltage doubler or similar. The MOSFET needs a gate-source voltage > 0V, at least 5V if you use a logic gate MOSFET, 10 V - 20 V if you use a standard switching MOSFET (consult datasheet). Since the source of the MOSFET is at battery voltage (or higher during the conducting phase) it will need a gate voltage higher than the battery voltage by at least that amount.

2) Where does that control signal at point A come from? This signal should be generated from a stable regulating loop. If the MOSFET is turned on for too long a time, the inductor will saturate, being in effect a short circuit, and a large current will flow uncontrolled into the battery. Therefore you need some feedback from the output of the circuit to point A.
I suggest you use a specialized switch mode battery charger (Google or http://www.ti.com/lit/ds/symlink/uc3909.pdf as just one example)

3) For stable operation some storage capacitor at the input (after the bridge rectifier) of the circuit is required. This capacitor will smooth out fluctuations in the input voltage due to the rectification of the sinusoidal input voltage from the generator or short dips due to varying wind speeds.

4) I'd add some kind of undervoltage protection. If the voltage from the generator is less than the battery voltage, the step-down charger will not work because it is not able to force current into the battery. On the contrary, if the MOSFET is turned on with the battery voltage being higher than the generator voltage, current will flow from the battery into your circuit (how much current depends on the details of your circuit).

Regards,
Harald
 

Burnit0017

Jan 8, 2012
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Hi, PMA = permanent magnet alternator. Thank you for the help. This is a first attempt at this type of project. I watched a video that demonstrated the benefits of using a MPPT controller with a PMA. It appears to remove the negative affects of the PMA’s stator internal resistance and will result in better turbine performance.



All the examples of a MPPT controller that I could find use a micro controlled buck converter. I am starting with a very basic circuit. I am trying to determine if a micro controller is needed. Input A can be used to integrate a micro if needed. After reading your analysis it looks like it can be used with a voltage comparator to turn off the MOSFET when the input voltage drops below the battery voltage.

I am starting with a very basic circuit with a 5 amp limit. I need to determine the switching frequency that will prevent saturation of the air core inductor.

Again, thank you for the help, comments welcome.
 

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Harald Kapp

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Hi,
thanks for your explanation.
As far as I understand your schematic, the charge pump will be premanently active or inactive, depending on the state of the control signal A. For the circuit to work as a buck converter, however, you need to control the MOSFET by a kind of PWM signal. A simple voltage comparator will not suffice. The voltage on the battery is fixed by the battery. The voltage on the generator is fixed by the generator (and, of course) the current wind speed etc.).
When you turn on the MOSFET, a current will start to flow in the coil until it saturates. From that moment on the inductance will decrease rapidly, leaving effectively only the ohmic resistance of the coil's wire between generator and battery. And since the generator may put out much more voltage than the battery is rated for, this will result in a high current and subsequent destruction of the MOSFET and/or coil, battery etc.
You need to control the current through the coil and turn the MOSFET off before the coil saturates. There is a reason why switch mode power supply controllers are rather compelx devices. You might want to look up the basics of step down converters eg. here: http://www.national.com/en/power/buck_converters_step_down.html.

Regards,
Harald
 

Resqueline

Jul 31, 2009
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Like Harald says your diagram does not constitute a switch-mode circuit at all. It'll need considerable redesign.
Also, air-core inductors don't saturate, ever. They'll keep the same inductance until the insulation fries, but will radiate a lot of RF noise.
PMA's are essentially constant-current generators that are voltage limited only by their speed.
So to get twice the power out of one you just allow it to spin twice as fast and allow the voltage to double.
This behaviour is predictable and could perhaps be implemented in a regulator circuit being simpler than an MPPT.
 

Burnit0017

Jan 8, 2012
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Hi, thank you for added resource. This will be the first circuit to determine what else will needed for the buck converter. I should be able measure input current and compare it the output current. Because the controller circuit will be on a bread board it will be easy to make improvements after I have a working buck converter. At this time I am just trying to get the basic circuit working for some experience. I order a basic oscilloscope that I should have in about a week. This will give some time to fabricate the basic circuit and continue research. Thanks again. The video is just for fun.

 

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Burnit0017

Jan 8, 2012
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Greetings, I tried a very basic experiment by hooking a 7200 MFD 50 VDC to the bridge rectifier and used the PMA to charge the capacitor. I was surprised how fast and how little effort was required to charge the capacitor to 50 VDC. I then discharged the capacitor using a piece of metal. This resulted in a very large spark and snap sound and the piece of metal had pitted scorch marks. I now have a new respect for the components I will be working with. Safety is a new priority for the project.
 

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Burnit0017

Jan 8, 2012
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Hi, The RC timing constant at the voltage multiplier output is not allowing a pulsed output. The output of the voltage multiplier has 1.5 volt ripple and is not discharging to zero, so the MOSFET will never turn off when point A is enabled. I am searching for alternatives. Ordering a gate driver is the obvious solution. Comments welcome.
 

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Burnit0017

Jan 8, 2012
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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.


high side, low side, I am starting to understand
 

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Burnit0017

Jan 8, 2012
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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.
 

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Burnit0017

Jan 8, 2012
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Hi, first test circuit update.

The circuit requirements are :
1. use a comparator to enable a oscillator when Vin is > the battery voltage.
2. the oscillator will enable the IR2117 to turn the MOSFET on and off.
3. when Vin is < the battery voltage the comparator will turn off the buck converter.
4. when the buck converter is off the input capacitor will charge until Vin is > the battery voltage and the cycle will start over.



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

I will post results when available. Comments welcome.
 

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Burnit0017

Jan 8, 2012
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Hi, the IR2117 have arrived and I completed all the high current connections of the buck converter. Using a Zener diode as Vcc to power just the nand gate oscillator I was able to illuminate a LED with a quarter turn of the PMA. Not great results but it is a start. I have to test the IR2117 with the oscilloscope to determine if HO is oscillating. I wired the test circuit and used a 12 volt halogen light as a test load and spun the PMA manually. The PMA was very easy to spin until the MOSFET turn on, then I was unable to spin the PMA. I believe the MOSFET is not turning off. My concerns are there is a lot energy stored in the input capacitor and I do not want to harm my oscilloscope. Are there any points I should avoid when testing the circuit????? The chassis ground and each channel ground are a the same common point, is there a safe method to use when testing the circuit with the scope? Comments welcome.
 

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Burnit0017

Jan 8, 2012
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Hi, The photo displays the Vs wave form on top and the IR2117 trigger input on the bottom. I am using switching diodes because I do not have any fast recovery diodes.

The only way I could get the MosFet to turn off is to connect Vs from the IR2117 to ground. Control signal is 10 kHz and I am using bench power supplies and a 12 volt light for a load.

I am making progress.
 

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BitHead

Mar 2, 2012
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I myself am pondering embarking upon a similar project using solar panels - going a bit further and using a buck/boost to get something out of the '12V and below' voltage on my input cap.
W.r.t your circuit... considering that you have the advantage of a non-ground-referenced source (isolated generator), would a less-complicated circuit be one that uses an N-channel low-gate-drive mosfet to discharge the input cap into the battery via the 'battery minus/diode' point? (no gate-boost circuit, gate drive directly with logic output, powered with input cap voltage)?
I.e. - the battery minus terminal (and diode) would be the 'high-side' load.
I'm not an expert - so I could be wrong - but I've gotten the impression that high-side circuits are used because 'everything' shares ground.
Sorry if I just turned your ship back to port. ;-)
Oh - one more thing - mosfets without a good bypass diode across them can latch up (if they aren't destroyed outright). I don't see one in your schematic - but maybe it's built in to the mosfet?
 

Burnit0017

Jan 8, 2012
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Hi, project update.

The test circuit is using to separate bench power supplies, one to power the 555 and the IR2117 and the other supplies power for Vin and acts as the PMA. The load is a 12 volt tail light and the oscillator is at 10 kHz.

I had a lot of problems because the bench power supplies that I am using are programmable. Once I configured them correctly most of the problems have been solved.

The output from the 555 oscillator has a severe ringing that is transferred to Vs of the mosfet. But the boot strap appears to be working with the Vs from the IR2117 connected to Vs of the mosfet and the mosfet is turning on and off.

I am currently searching for away to supply the IC’s with Vcc. I am not sure if I should use a 12 volt linear regulator from the 12 volt battery bank or if there is a method to use the large input capacitor at Vin. The voltage at Vin can range form 0 to over 200 volts open circuit and I do not know how fast the buck converter will drain the cap at Vin when the mosfet is oscillating. Comments are welcome.
 

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twister

Feb 12, 2012
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Hi, PMA = permanent magnet alternator. Thank you for the help. This is a first attempt at this type of project. I watched a video that demonstrated the benefits of using a MPPT controller with a PMA. It appears to remove the negative affects of the PMA’s stator internal resistance and will result in better turbine performance.



All the examples of a MPPT controller that I could find use a micro controlled buck converter. I am starting with a very basic circuit. I am trying to determine if a micro controller is needed. Input A can be used to integrate a micro if needed. After reading your analysis it looks like it can be used with a voltage comparator to turn off the MOSFET when the input voltage drops below the battery voltage.

I am starting with a very basic circuit with a 5 amp limit. I need to determine the switching frequency that will prevent saturation of the air core inductor.

Again, thank you for the help, comments welcome.


You probably know that you don't need that to charge your batteries, right? My wind charger will produce over 90 volts, maybe more, but when it is hooked to the batteries, the batteries pull the voltage down. Be careful and keep one hand in your pocket when working on it.
Do you know that you won't need so much voltage if you put the mosfets on the low side?
 
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Burnit0017

Jan 8, 2012
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Hi, the PMA I fabricated has input and outputs on each stator. I connected the stators to form a 3 phase, 12 coils per phase. I am operating the PMA manually and I was able to reproduce the results in the video. I was using a Buck converter and it worked well for resistive loads but I am having trouble interfacing the circuit to a 12 volt battery. I am currently trying the SEPIC topology and I will post results when available. I have very little experience with power circuits and this is my first attempt.Comments are welcome


Video shows my results using a Buck converter and manual operation of PMA.
I was very skeptical but after reproducing the results, the circuit does converts a high voltage with low amps at the input to a lower voltage with higher current at the output, requiring less torque to operate the PMA.



 

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Burnit0017

Jan 8, 2012
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Hi, I had limited success using a IR2117 driving the mosfet in the buck converter with a resistive load. When I tried replacing the resistive load with a 12 volt battery the mosfet would start to turn on and immediately turn off. Using the app notes for the IR2117 I found the prescribed method for a battery charging circuit and I still was unable to get the circuit to function as a battery charger. There is something about charging the boot strap capacitor that I do not understand. But I am not giving up. The limited success I experienced with the resistive load dramatically showed me the benefits of adding such a circuit to the system.

I found a reference paper that shows a buck converter using a isolated power supply for the gate driver circuitry to drive a N channel mosfet for a battery charging circuit. Chapter 4. page 69

http://etd.ohiolink.edu/view.cgi?acc_num=akron1320692738

I ordered some small isolated power supply modules and I am continuing to work on the problem until a solution is found. I will post results when available.
 
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Burnit0017

Jan 8, 2012
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Buck converter for small wind turbine project: update

Greetings, using a isolated power supply and FOD3180 does work. I am using a blocking diode between the inductor output and the plus terminal of the 12 volt battery. The gate driver is triggering a N channel IRF540 in a Buck circuit configuration and is charging the battery, current about 1 amp. Then I replaced the battery with the 50 watt halogen light and the circuit was able to illuminate the light after I adjusted the duty cycle, current max about 4 amps. I was manually operating the PMA. So far this is a very happy day. The next step is to drive the PMA with the drill press to determine how the circuit behaves at a higher RPM.

Chapter 4. page 69

http://etd.ohiolink.edu/view.cgi?acc_num=akron1320692738

Isolated power supply with FOD3180 gate driver. N channel Buck converter using a IRF540 mosfet. Bottom wave form is gate of mosfet, top wave form is source of mosfet.
Input voltage max about 40 volts. Output current varies depending on duty cycle. Circuit still needs improvement but it is working. Input voltage is from a permanent magnet alternator and is being manually operated. Comments welcome.
 
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