DC/DC Buck Converter Board

Here's the latest design for a solar panel DC/DC buck converter. Ultimately something like this will be used to employ a MPPT (Maximum Power Point Tracking) algorhythm. This design works well for current limiting power from the panel to hold the battery at a specific voltage. The current software runs a 3 stage charging pattern (bulk, saturation and float charge levels) that is configurable to support any battery.

Current issues are voltage spikes. These occur at the P-MOSFET gate as well as the battery voltage and regulated 5V power for the PIC. Also the capacitors C1 and C2 get very hot when current is continually running through the system (even 500mA). I'm thinking that for a brief moment the battery is shorted through Q1 and Q2. This occurs when the PWM changes Vgs to control the MOSFET, at the time of transition.
















DC/DC Buck Converter Schematic

This plot shows the gate voltage at the bottom and the battery voltage at the top. Note the different volts per division for each channel.

When the gate is switched low, which allows the P-MOSFET to conduct and the panel to charge the battery; the battery voltage is seen to drop dramatically about half a volt. You would expect it to rise when the MOSFET first starts to conduct.













This plot shows the 5V regulated board power. A slight ripple can be seen despite the 7805 regulator. I think in certain cases this can be extreme and reset the PIC.

The PCB built up. The two small blue capacitors seen in the front are the parts with the thermal issues. I think this is due to the current spikes when Q1 and Q2 allow the battery to short. Perhaps a larger resistor in R4 would limit the current to a max of 1.5 ~ 2 amps and that would alleviate some of the issues with the high side driver circuit.