Buck converter for small wind turbine project

[Burnit0017]

Hi, The circuit is working with a resistive load.

1. The mosfet source is connect to the high side of the load. The mosfet is a IRF510.

2. A IR2117 with bootstrap is being used and is referenced to Vs so the bootstrap capacitor voltage can be charged high enough to turn on the mosfet.

3. A 555 timer is used to control the IR2117 @ 10kHz with a Vcc of 12 volts.

4. A permanent magnet alternator is used to supply Vin and can range from 0 to over 200 volts depending on the permanent magnet alternators RPM.

5. A bench power supply is used to supply power to the IC’s. This will be changed later.

The problem is :
 When the resistive load is replace with a 12 volt battery the circuit does not work.
I do not understand why the circuit will work for a resistive load and not with a 12 battery as a load.  Any suggestion will be greatly appreciated.




http://www.ti.com/lit/ml/slup169/slup169.pdf

on page, 26 Figure 29 maybe the solution. What value should Dz  be?

[oztules]

"The problem is :
 When the resistive load is replace with a 12 volt battery the circuit does not work.
I do not understand why the circuit will work for a resistive load and not with a 12 battery as a load.  Any suggestion will be greatly appreciated."

What part does not work. Is the fet being pulsed but not turning on or is there no pulse, or have you not enough output voltage to drive the battery perhaps... ie if you have 50% pulse width and the input is not over 26v, youmay not have the output above 12v to charge the battery as a load?

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

[Burnit0017]

Hi, when was testing with the resistive load if there was any charge on the large input capacitor the MosFet would never turn on. The voltage of the input capacitor , Vin, would continue to increase as the PMA was rotated. The only way I could get the MosFet to turn on would be to completely discharge the input capacitor then the circuit would work, the circuit would turn and off as I manually operated the PMA.

When the battery is used for the load the MosFet never turns on and the large capacitor voltage, Vin, just increases. There is no output pulse from the IR2117.
 

[oztules]

I have never used that kind of circuit before.... that said, you sound like the bootstrap is not working. I suspect that it is because the charge pump cannot work on smooth DC. By discharging the cap, the pulsed DC input gets it going, and once running the pulses come from the loaded open and close interaction. (D S open closed) or, CBST discharging into the gate via VB

With the battery connected, at start there is no pulsing voltage to get the vs vg voltage needed. VB must come from Vin being 12v ( or whatever uvlockout is) above b+
 Dz could be 14v

What is the VB to VS differential when you cant get it to fire. Is it above lockout?

To test this, use an isolated supply to VB and VS on the high side driver, and see if that runs it. If so, your not getting pulses to the charge pump on  at start, only stable DC< 25v or so.

Can't have a charge pump without voltage fluctuation.

With the charge pump not working, the pm will only keep charging the filter cap up.... until  if vb is 12v or so above vs, it should get going.

I have never bothered with driver IC in high side only use, but you should be able to find where the voltage is not, and work from there.

If supplying isolated voltage from VB to VS, does not get the driver going... perhaps the driver chip is toast.

If you still can't get it going, try a toroid driver, just to get it a feel for a working circuit, and then work backwards to your exotic chip system.

Another alternative is to use  ( when all else fails) some of the pm ac to drive a small transformer to make a truly isolated supply, and drive the fet directly with this.... silly but very stable.


..............oztules
Edit: and lets assume you have Dstart around the right way

[Burnit0017]

Hi, I exchanged the IRF510 with a IRF540 and I am unable replicate the results shown in the last video.   It is a puzzling situation. It may require a different bootstrap capacitor or some other solution. I am unable to continue until a solution is found.  The circuit will function with a lower load but does not work with 50 watt load. The oscilloscope shows the IRF540 starts to turn on then very quickly turns off, then the Vin gains magnitude and the 555 timing signal shuts down.  Comments welcome.

[Burnit0017]

Hi, I was able to find all the needed parts to make all the required connections to interface the circuit to a 12 volt battery. I am using to bench power supplies, one to power IC’s and the other for Vin.

I had just a LED and 22kohm resistor from the mosfet Vs to ground and the mosfet was turning on and off.

With a capacitor at the output of the circuit no current would flow into the battery.  I removed the capacitor and I was able to measure 0.14 amps flowing into the battery.

I had attached a variable resistor to the 555 to be able to test different duty cycle, I adjusted the variable resistor to low and the 555 fried, so I have to purchase some replacements before I can continue testing. I will post more test results after I replace the 555. 

[Burnit0017]

Hi, I have verified that using a isolated power supply for the gate drive circuit will turn the N channel mosfet on and off in a Buck circuit configuration with a 12 volt battery as a load.  I configured the gate drive circuit with a 555 time and used a Schmitz triggered nand gate as a inverter. The 555 timer was configured with a variable 1 meg ohm pot so the duty cycle could be changed. The shorter I set the duty cycle the easier the PMA was able to spin and produced a very high voltage at Vin. Positive current was flowing into the battery. Under manual operation the best I could achieve was 0.7 amps flowing into the battery. The circuit will definitely benefit from using a microcontroller to adjust the duty cycle.  At this time I starting to work on  paralleling the mosfets and adding a microcontroller to the system. Comments welcome   

This is the part I find confusing:

When comparing the waveforms of the two gate driver circuit’s the circuit without the IR2117 triggers the mosfet all the time.
Video without IR2117 in gate driver circuit:
mosfet Vs is top waveform:

The circuit using the IR2117 only triggers the mosfet when the PMA provides a input voltage.
Video using IR2117, bottom waveform is mosfet Vs:

Waveforms in both videos are measured at the same point.

[Burnit0017]

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.

[boB]

I think you were smart to start using the  FOD3180  driver !

it's a great part.

boB

[Burnit0017]

Hi, Vee is connected to Vs of the mosfet and rides on Vs of the N channel mosfet. So it is functioning like the bootstrap capacitor of the IR2117 but the FOD3180 will work with both a resistive load and will also work as a battery charger. I am not sure if that makes any sense.  Basically I could only get the IR2117 to work with a N channel mosfet  with a resistive load. The FOD3180 and isolated power supply will work with a N channel mosfet with a resistive load and as a battery charger. I hope that helps anyone that is going to try a similar project. I know, as clear as mud.

 I am now working on adding a snubber to dampen the ringing. Any suggestion will be greatly appreciated, comments welcome.

[Burnit0017]

I think you were smart to start using the  FOD3180  driver !

it's a great part.

boB

http://www.midnitesolar.com/pdfs/classicDevUpdate3.pdf

Not a fan of art deco but I wish I could afford one of your controllers. Very nice!

[boB]

Yep.  That's it.

   BTW, it uses that same   FOD3180  driver.

boB

[MadScientist267]

I am now working on adding a snubber to dampen the ringing. Any suggestion will be greatly appreciated, comments welcome.

In a nutshell: Really fast Schottky diodes and several low ESR caps on both the output and input. At the output, place them as close to the coil and Schottky cathode as you can get them. At the input, as close to the tranny's drain as you can get them. Ground heavy and nearby, with the common point ideally at the cathode of the Schottky.

The more solid your input/output rails are, the better, as less power will be lost by radiation. The input is sometimes neglected somewhat, but is also important, as you need a solid source for nice sharp, hard pulses at the input, not just a place to catch them at the output.

I notice in the schematic you posted, there aren't even any caps in the output. You'll just be wasting your time sending those spikes up the battery leads - the inductance of the cables will absorb a fair amount of power, effectively making little more than a crude antenna. In all honesty, this is where most of the ringing you're witnessing is currently coming from.

Other tidbits that come to mind:

It's going to be difficult to get rid of the ringing with any real effect with the breadboard in the mix, thanks to parasitic everything that's inherent to using such things. 

If you still need the versatility of the breadboard for further refinements, you can set up the "grunt" components on a more permanent fixture such as perfboard so that you can make the power path traces heavy and close together, along with your strategically chosen and placed filtering caps. This should help significantly.

The other thing that will greatly affect performance is the length of the run from the MOSFET driver to the gate. The longer this is, the more parasitics you will have, the slower the effective slew rate will become, and the more power you'll waste in the tranny as heat. Drive it short and hard - ultimately, everything from the gate driver on needs to be as close together as possible to get maximum efficiency.

Hope this helps some.

Steve

[Burnit0017]

Hi, I completed fabrication of analog MPPT test circuit. Video shows verification of selectable duty cycle with fine adjustment of each for each selected duty cycle, 10% to 50%. Next step is to test with PMA and battery test load. I am hoping I did not damage the mosfets when I solder them in the circuit. 

Thank you for all the help I have been receiving, circuit should help me better understand requirements for MPPT algorithm.

  I connected four IRF510 in parallel, this is a first attempt and is just for the learning experience of paralleling mosfets. My PMA has inputs and outputs on each stator so I can change the internal resistance of the PMA. The variable duty cycle of the circuit will aid in testing of the different internal resistances of the PMA. I will post results when available.     
 

[Burnit0017]

Hi, just a quick video showing test setup. With the PMA at 200 RPM the input voltage was 170 volts, 1.5 amps input current, 12.34 output voltage, and 0.3 output current. Mosfets measured 82 degrees Fahrenheit. The circuit needs major improvements.  The input capacitor is only rated for 100 volts and I have to check my inductor calculation. So there are major loses and the buck circuit is not working as it should but the test fixture is working. I am using a 12 volt deep cell for a test load. It does show the control circuit is working and the mosfets are turning on and off. It is a start. Comments welcome.