Solar MPPT - Not Working?

[maddin]

Hi!

On my search for a Non-µC-MPP-Tracker I found the solar_mppt.pdf at your board (I also attached it). Next Friday my team and I will participate in a solar-modelcar race and we are still working on the electronics. So need some help quickly. 
The circuit in the .pdf named above has been the cheapest I had found, so I ordered the parts and tried to make it work. But it seems like I have failed. In the past two or three weeks I acquired some knowledge about all this MPP and Buck-Converter stuff, but I don't have any experience with it. So it is kind of hard getting through it.
This morning I finished soldering and checked the circuitry. But it didn't work as I expected. Using an ordinary power supply at the input and a changeable load at the output I could not see any current flowing through my digital meters. I really do not have a clue, if I did a mistake or even if the circuit is really what I have searched for.
So I need to know, how this circuitry is supposed to work. That's what I read out of it:

- it keeps the solar panel working at an preset voltage (via P1; I could not make any adjustments with it in my test)
- it also works like a buck-converter (for instance: it keeps the solar panel working at 12V and the output voltage is adjusted to continuous 8.1V)

The solar-car is powered by a 20w solar panel and it needs to recharge a 7.2V NiHM battery during the race-breaks. I know that the panel is underpowered to fully recharge the battery, but it is not supposed to.   

My questions:

- My assumptions are right? Or do I need an extra buck-converter?
- The datasheet for the 78S40 shows another value (µF-Range) for C2 (controls switching freq.). What is the switching frequency supposed to be with the 1nF capacitor?
- Or: What is the minimum value for the inductor?
- How do I set input and output voltage to the value I need? (adapt R1/P1? What is P1 for?)
- Is there a chance to make it work the way I want it to? 

I hope somebody can help me with these questions. There is not much time left, but a long way to go... it's kind of frustrating. 

Greeting from Germany and sorry for my English

[WindJuggler]

Hi,

If I analyze the circuit I can conclude that it won't work properly. A buck converter needs to have the switch in the line.
I cannot figure out what type of topology it should be but the D1will stop the current and Q1 can do whataver it wants but it is not going to influence the output voltage except for no negative lead.

Try putting Q1 between C6 and D1 inline with L1. Take a look at the datasheet. It will tell you what to connect where.

I think that the design is not very good. You should probably try another schematic. The schematics provided by linear technologies are quite good. Just copy one of them. If you want you can also emulate the Linear schematics with LTSpice. These are really the fastest to get a working MPPT.

[Opera House]

That circuit will work fine, but it probably won't do what you want.  It regulates the input voltage which for a 12V panel should be about 19V.  There is no protection for the battery from overcharging.   You should be able to adjust the input voltage provided there is enough load on the output.  If not then you have zapped something.  Try debugging with a much higher inductor.  Slowing everything down often solves many problems. 

[maddin]

That circuit will work fine, but it probably won't do what you want.  It regulates the input voltage which for a 12V panel should be about 19V.

Let me get it right: that circuit will only keep the solar panel working at its MPP-Voltage?! So I definitely need an extra circuit to convert the input/output voltage of the MPP-Circuit to the needed 8,1V.
My panel has got its MPP-Voltage around 12,5V at 1,6A. What do I have to change in the circuit to fit it to my Solar Panel? Only the the voltage-divider at the Feedback-Input?

There is no protection for the battery from overcharging.

There won't be enough time during the recharge-pause to reach this point.

You should be able to adjust the input voltage provided there is enough load on the output.  If not then you have zapped something.

No load = no adjusting? Can there be too much/too less load? What's a good value for the inductor? I chose 150µH.

[dnix71]

Chargers for laptops, for example, put out double the voltage the laptop uses., typically 19v. Half runs the laptop and half charges the battery.

All you need is a zener in the mix. Take as much power as you can from the panel to charge the batteries, but have the zener cut it off to a safe level.

The batteries are protected by the zener. The motor, on the other hand can use the full voltage out from the MPPT with a blocking diode to protect the battery. The motor under load will drop the panel output voltage to a safe level anyway, since the panel is way undersized.

[tecker]

OK I'll take a crack at thid circuit
 FIrst the supply voltage is part of the sensing circuit I don't Know why
Indecently if there's not a  really small battery C4 is wasted here .
 I can't see why there is ever any need to put resistor networks across the PV
 adjustments are not necessary you really need to sense battery voltage . there's no boost and no buck just a regulation when the voltage gets too high and the panel is cut off .I 'll have to look at the chip closer to see if there's any way to get the Flip flop to work with battery voltage  and how to use the coil
   

[Opera House]

My panel has got its MPP-Voltage around 12,5V at 1,6A

Unless you have something extremely unusual, you are wrong.  In the unlikely event you are correct, operating at this power point will not gain any extra current over connecting it directly to the battery.  The losses will be too great with such little differential.  If there is no concern over excess current or overcharging it is fine to operate circuit as it is.

[maddin]

My panel has got the same MPP!
I guess you also tried that circuit? How did you make it work? Did you change anything? What is it supposed to do? What voltage/current should be measured at the output?

Because there was no time left and to be on the safe side, I finally put a simple voltage regulator between the panel and the battery/car-boardcircuit. That isn't really necessary to charge the battery, but I'm afraid to destroy the speed controller with overvoltage. So there is a lot of work on electronics left for next years race.

Thanks for your effort up to now!