i need boost converter because i want to charge a 12V/7AH battery even when the solar panel voltage is as low as 8V...i step it up to 13.8 using boost converter..i'l post my ckt diagram in sometime...i dont have a soft copy of it
Solar cell is an illumination based current source that is clamped by its inherent diode. A panel for charging a 12v lead acid battery is typically 36 series connected cells.
While it is possible to use a panel with less then 24 series cells with a boost circuit to charge a 12v battery it is very unusual set up.
Maximum power point for a solar cell is where it is loaded to point where just a small amount of illumination generated current goes down inherent diode. For silicon cell this is about 0.52 vdc at 25 deg C. This has a -2 mV/deg C temp coefficient.
Any switcher driven from a variable current source must monitor the input voltage to regulate the switcher's loading so not to collapse the input voltage. As illumination level changes on PV panel the switcher must adjust its loading.
The maximum power point voltage will not change too much but the available current will drop as illumination drops. Since there is some series resistance within the PV panel the maximum power point voltage will drop slightly as PV illumination level generates more current. The largest maximum power point voltage variation is due to temperature change on PV panel. This is about -0.33% per degree C.
Capacitor at input of switcher across PV array provides the averaging on current load on PV panel. It should be able to supply at least ten cycles of inductor charging without too much loaded voltage variation. The actual capacitor value will depend on the bandwidth of the feedback loop network from input voltage sense. (which you have none in your circuit).
The feedback control system on a switcher fed from a PV panel is very different then a standard SMPS.
For MPPT tracking you have to continously adjust switcher input voltage target value to account for MPP voltage variation due to temp and voltage drop due to PV panel Rs and wiring from panel.
Ideally you optimize the V*I of the input. A simple way is to just maximized the battery charge current on output of switcher. Again this has to be continously searched as PV illumination changes.
Battery charge limits is a secondary control loop that backs off on the switcher when desired full charge voltage is reached on battery.
Again, with a proper PV panel, the switcher is usually a buck circuit. The best switcher IC to select is one that has voltage feedback control and current limit control. Use the voltage feedback for the switcher input voltage monitor. This will need invertering as you want the switcher to back off as input voltage goes lower then reference MPP voltage which is opposite the normal output voltage monitoring.
Use the current limit control for battery charge limit voltage to cause switcher to back off as output voltage to battery gets higher then target reference.
The searching algorythm is a whole new topic. This is why a processor is normally used to control the switcher with three feedback control loops. (MPP input voltage, battery voltage limit, and MPP optimization which tweeks MPP input reference voltage target).