Solar Freaks

Hi,

I'm designing a MPPT for a school project and I have some difficulties dimensioning the inductor and capacitor for the boost converter. I'll explain the project in short.

We intend to charge a NiMH battery (required constant charging voltage = 21V, the battery voltage itself is 14,4V) using a small solar panel. We want to do this with a PIC-processor controlled MPPT. The voltage of our solar panel at maximum power point is aprox. 18V so we need a boost converter. The problem is that I don't know how to use the boost converter design equations. An equation for the inductor that I found says the following:

L=

(Vout - Vin + Vdiode)*(1-duty_cycle)
-------------------------------------------
min_load_current * frequency

If I'm not mistaking, the input voltage (coming from the solar panel) can be considered to be constant because the MPPT will keep the solar panel working at it's maximum power point, so Vin = 18V. I want Vout to be a constant 21V so Vout = 21V. But what should I use for the duty cycle and minimum load current? The duty cycle is constantly being varied by the MPPT so how can I use this formula?

Am I using the wrong formula? And if not, how can I correctly use it?

Thanks in advance

Yellow! I'm working on a similar circuit but at much lower voltages. We have about 1.5V input from PV and want to boost to 4.4-5V for cell phone charging. I'm thinking that you have to figure out what the duty cycle range will be, and calculate the range of inductances you will need. (The ideal inductor size will vary so you have to just pick the one that fits best). You probably need to choose the highest inductor value - oversizing the inductor should be fine. Someone should confirm that though. Good luck.

Hi Rabin,
I think the issue that you should be take into account is about input voltage.according to change the irradiation and temperature your input voltage will be change.so you don't have fix input voltage every time.
next,the main duty of MPPT is load matching .with use of boost converter you have this load matching :
Rin=(1-D)^2.Rload
So,you have to calculate the equivalent battery load also.to know your DC/DC converter work in which range of duty cycle.