- Mon Apr 20, 2009 8:34 pm
#71051
I am working on a wireless weather station that uses uses a solar panel and rechargeable battery for power. Typically a diode is used between the solar panel and battery to prevent back current from flowing through the solar panel at night. My plan was to replace the diode with a high-side load switch (P-channel MOSFET) allowing the following:
1- Read solar panel voltage (voltage divider to ADC on uC while solar panel is disconnected)
2- Eliminate the forward voltage drop of a diode (disconnect panel when voltage is below charging range)
3- Use PWM to reduce panel output if it exceeds charging range (since I am not using a regulator between the solar panel and battery).
The high side switch I intended to use was the Fairchild FDC6331 (http://www.fairchildsemi.com/ds/FD/FDC6331L.pdf). It is essentially a P-channel MOSFET with an N-channel driver. It felt like I was being clever by taking advantage of the uC to control the MOSFET and get multiple benefits. That is when it struck me that MOSFETs have body diodes that can conduct current back through them. It would seem like that would still allow the battery to discharge through the body diode to the solar panel at night. Thus, I started looking for similar applications to see if anyone had tried this before. I found the following design note from Linear Technology (http://www.linear.com/pc/downloadDocume ... 142,D14157) which seemed to suggest exactly my proposed solution.
Question-- What prevents the MOSFET body diode from discharging the battery through the solar panel at night?
1- Read solar panel voltage (voltage divider to ADC on uC while solar panel is disconnected)
2- Eliminate the forward voltage drop of a diode (disconnect panel when voltage is below charging range)
3- Use PWM to reduce panel output if it exceeds charging range (since I am not using a regulator between the solar panel and battery).
The high side switch I intended to use was the Fairchild FDC6331 (http://www.fairchildsemi.com/ds/FD/FDC6331L.pdf). It is essentially a P-channel MOSFET with an N-channel driver. It felt like I was being clever by taking advantage of the uC to control the MOSFET and get multiple benefits. That is when it struck me that MOSFETs have body diodes that can conduct current back through them. It would seem like that would still allow the battery to discharge through the body diode to the solar panel at night. Thus, I started looking for similar applications to see if anyone had tried this before. I found the following design note from Linear Technology (http://www.linear.com/pc/downloadDocume ... 142,D14157) which seemed to suggest exactly my proposed solution.
Question-- What prevents the MOSFET body diode from discharging the battery through the solar panel at night?