I'm designing a Lead Acid battery charger with 3 stages. For the 2nd stage (constant voltage stage) as it is written in literature: the voltage applied on the battery must be dependent on temperature. So I thought of making a buck converter. The mosfet is controlled by IR2111 driver via a PIC Microcontroller PWM (Look at the circuit). Input = 18 V.
When I used a resistive load (R = 3 Ohm or 10 Ohm) the circuit worked very well: The driver worked, the output voltage is flat (no ripples) and up to 15.5V (with a duty cycle of 95%), the current is up to 5 amps with the 3 Ohm load (without overheating) ...
Now when I use the battery as a load to charge it (I have a discharged battery of 12V and its voltage is 9V) it doesn't work at all :( The output voltage is imposed by the battery so I find Vout = 9V. !!! Even if I change the duty cycle nothing will happen, the driver can not drive the mosfet and (Vgs is not square and equals 9V: seems that is related to battery voltage)
It seems that I must add something or .. I don't know .. I you can help me I will be very thankful :)
This is the circuit that I used: