Weird boost converter waveform.

edmond_chan85 said:

To Ubergeeks63 : I am using PIC16 because I had other thing to add on in the future work. Such as MPPT for solar system. therefore, I need PIC16 with I/O ports to measure input current, input voltage, output voltage and output current to perform MPPT.
I am wondering what is the rating for Rg. 5-10ohms with 0.25W??or higher??

Click to expand...

i see... well i am going to tell you what i keep telling my boss to tell our customers: stop asking about your latest issue and tell us what you are trying to accomplish!

what you have then is a 100W 12V solar panel that you want to charge a 24V lead acid pack with ...

a typical solar panel puts out about 8A@12V and about 7A@17V, so a 250uH 10A inductor @100KHz is all you need and is within the capability of the PIC... it is still a crappy overpriced part for the job but it comes in a standard DIP package that you can handle.

you do NOT need a voltage measurement or multiply operation to do the MPPT since the battery is a relatively constant voltage, all you need is the charge current.

you DO need the battery voltage for the charge algorithm though.

dan

To mneary and Ubergeek63:

Let me give my actual design specification.
My solar panel Maximum power produced is about 75W. Open circuit voltage 21.4V Short Circuit current 4.75A. I mpp is 4.45A and V mpp is 17V.

I actually want to build a boost converter to step up unregulated PV voltage to a 24V constant voltage. I am NOT using this voltage to charge any batteries. It doesn't store any energy. I just want a constant 24V. Because in later work or maybe, I will connect this voltage with an inverter to produce 230V ac. This is what I need to do..

So my boost converter should be something like this:
Vin : 6V -23V
Vout : 24V
Power rating: 100W
Switching Frequency : 20kHz

Doubt is here..output power 100W but my input is just 75W maximum. Therefore, what I targeted is maximum power transfer from PV to output. The main thing and the most important is to get 24V constant.
Is it possible?

Thank you

Call me uninspired but I can't think of a 24V load that can work like that. My mind can only perceive batteries and electric grids as MPPT loads.

Doubt is here..output power 100W but my input is just 75W maximum.

Click to expand...

You're kidding, right?

The main thing and the most important is to get 24V constant.
Is it possible?

Click to expand...

24V constant is possible, and I thought you are already achieving that. When your load is the same and the input power changes, your output voltage is the only thing left and it must change.

And you said your efficiency is poor at high ratios which I hope is not a mystery any more.

What's the voltage under load?

21.4×4.75 is just above 100W.

edmond_chan85 said:

So my boost converter should be something like this:
Vin : 6V -23V
Vout : 24V
Power rating: 100W
Switching Frequency : 20kHz

Doubt is here..output power 100W but my input is just 75W maximum. Therefore, what I targeted is maximum power transfer from PV to output. The main thing and the most important is to get 24V constant.
Is it possible?

Thank you

Click to expand...

what you are describing is not an MPPT application. if your load does not need or store the all the available power, there is no reason to extract it.

I did look at the chip and you can get 100KHz PWM out of it for your MPPT... the thing you need to remember is that MPPT is a slow response system, but a 24V regulator is a fast response system and a single PIC16 will NOT likely be able to keep up with both let alone both and what ever load app you are contemplating.

Dan

The PIC16 should have plenty of horsepower but the objectives are at cross purposes.

You can operate at MPPT with varying input power when you have a load such as battery or grid connection that will accept all the power you give it. You will never achieve 24V constant unless your load is, for example, a battery with a 24V charging voltage.

You can generate 24V constant voltage if you have a power source which gives enough energy that the load wants. You will leave behind the energy that your 24V load does not want, and therefore will never be at MPPT.

You can easily do each of these, but not both.

Thank you all for the information and ideas.

To Hero999: That is the specification or test of my solar panel...
"Voltage under load", sorry I am not getting it..(sorry my english is not so good)..Actually I am designing that my solar panel will connect to a boost converter to get a constant output voltage of 24V. So voltage under load = 24V?


Ok. So let me make my 1st conclusion from what I got.

If I stick to my ideas for getting constant 24V with MPPT and not connecting is to batteries or grid, it is NOT working..
If doing MPPT, it should connected to batteries or grid and it should NOT be constant 24V.

Is that so?

edmond_chan85 said:

If I stick to my ideas for getting constant 24V with MPPT and not connecting is to batteries or grid, it is NOT working..
If doing MPPT, it should connected to batteries or grid and it should NOT be constant 24V.

Is that so?

Click to expand...

MPPT will get as much power as you can out of the solar panel ... if you are not charging batteries what are you doing with the power that exceeds your load requirements?

if you are charging batteries the charge level determines the output voltage. the output current times the output voltage divided by the MPPT efficiency would be equal to the power delivered by the solar panel. Therefore you can run a solar charging algorithm with minimal CPU power since you do not need to do ANY complex math.

Dan