Archive for February 24th, 2010
Solar Power Pv
Wednesday, February 24th, 2010
Designing a buck converter for Solar PV to deliver load to 10 ohms resistor?
Let say, I have PV with open circuit voltage of 430 Volts and short circuit current of 18 amperes.
The maximum power point for given PV occurs at 350 Volts.
What should be the duty cycle, to deliver maximum power to this 10 ohms load? How I calculate this duty cycle? Will it be ratio between Output and PV input or Output/Max voltage of PV ?
I have some ideas, but some of this will depend on your particular application. Does the voltage or current delivered to the load need to be clean? Or is it just a heating element or something?
Look at your PV, and find out what the amperage is at the max power point. This will give the power available. Let’s say that it’s 350 volts, 16 amps. That’s 5.6 kW. Not a power to be trifled with, so I hope you know what you’re doing, and have adequate heat sinking and protections.
Ok, 5.6 kW, let’s say you lose some amount in your switch, so 5 kW into 10 ohms. That’s 500 volts. At this point, I have to ask whether you really want a buck converter. You would need a boost or buck-boost to get from 350 volts to 500 volts. And for efficiency’s sake, I would think that you’d go with a full-wave bridge for this kind of power.
One thing you will find with the solar array is that the max power point isn’t fixed, and varies with temperature and lighting conditions. Rather than fix the duty cycle, you will want to have an adaptive circuit to track maximum power delivery to the load. This would differ slightly from a traditional max power point tracker, which would track maximum draw from the array.
For a primer on switching power supplies, see if you can go to the National Semiconductor site and get hold of application note AN-556. But if this is more than just a theoretical school assignment, you really should hook up with a switching supply designer with experience. Not everything in practice matches theory. Like, you would think that if you drive the gate of a power FET harder, it would conduct better. Not always true in real life.
Now, if you don’t want to fuss with all this, and can afford to be approximate, if you could just get a 20 ohm load, you could just hook it unregulated across the array, and get close to optimal power transfer.
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