windgen.org vawt discussion

GoVertical wrote:circuit will have a five amp maximum limit mainly because the only diodes I could find have a 3 amp rating. I am running two in parallel. 

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 I may be wrong but with diodes won't the power  just blow through the one that is weakest and cascade. not sure just asking 

 http://www.aliexpress.com/product-fm/488055373-150W-500W-wind-controller-12v-24v-switched-auto-wholesalers.html 

Hi, commercially availably MPPT.

sjh7132 wrote:

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I started my design with something like the 2106, but I had trouble getting it to bootstrap into operation if the load voltage was 24v or higher.  At least for the chip I was trying, the load voltage had to be below the supply voltage of the chip.    Do you know if that's a limit of the 2106?

 

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 Yes, I had the same problem on buck converter startup but found the solution in International Rectifier's Application Note AN-978 starting on page 22.

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

I have a buck startup task that handles getting the bootstrap kick started when the buck circuit goes from disabled to enabled.  The transition from disabled to enabled occurs when buck input voltage becomes greater than the buck output voltage ie the turbine output voltage is now high enough to start bucking.

The basic startup sequence is:  If Buck input voltage is greater than Buck output then start PWM duty cycle at 99% then after time delay X change PWM duty cycle to proper duty cycle.

When the gate driver receives the signal to activate the high side output, nothing will happen since the bootstrap capacitor has not been charged high enough ie 10V above Vs (Vs pin of driver chip connected to source side of high side mosfet and input of buck output inductor).

When the gate driver receives the signal to activate the low side, the low side N-Mosfet will turn on.  Vs (the connection to the buck's output inductor) will then be pulled to ground.  This does create a potentially dangerous condition since current is now being pulled from the output back through the inductor and through the low side FET to ground.  But it takes time for the current to build up throuh the inductor so as long as the low side FET is only on for a very short period of time then not a problem.  This is why PWM duty cycle is set to 99% on startup.  The high side is on for 99% of the period but the low side is only on for 1%.  This is long enough for the bootstrap capacitor to charge and be ready for the next cycle.

When the low side FET switches off there is the problem of back emf at Vs.  I followed the application note and put a zener across the bootstrap capacitor and intalled R1 as shown in Figure 25 .  The back emf will forward bias the zener and charge the cap.  This now guaranties that the bootstrap cap is charged to a voltage that will eventually be higher than Vs.

On the next cycle the high side FET will be turned on since the bootstrap capacitor has been charged and Vs drops back down to buck output voltage (battery voltage).

So after a few cycles the buck is working.  When the input voltage is just above the output voltage and PWM duty cycle is > 85%, the buck is usually operating in discontinuous mode so I shut off the low side mosfet to improve efficiency.

Then your gate drive voltage will have to be about 68v, but only at the right times or you burn out the gate.  I'd suggest looking at some of those mosfet driver chips that JeffD suggested.   If your load is only 12v I think they would work for you.