6A, 12V SSS Based Solar Charge Control
This is a Solid State Switch upgrade to the 6A, 12V solar charge regulator. Solid state switching provides longer life, smaller size and higher efficiency than a relay. In addition, the clock frequency may be doubled in order to better track battery load /charging requirements. The parts cost is only slightly higher. One drawback is the loss of the secondary output feature.
The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases.
Schematic of the Solid State Switch Solar Charge Controller
Bill of materials
6a, 12v sss based charge control bom.xls
Eliminate the relay life issue
While relay life is probably OK, it is definitely limited. Check out the following Omron relay endurance specification. Mechanically, it is good for 100million operations. With a 10A, 250VAC load, it is rated for only 36,000 operations. Note that a form C contact is used, so the form A (normally open contact) specification does not apply. I believe (guess) that it is good for at least 400,000 operations in the 6A solar charge regulator application –this equates to approximately 1year’s operation. So you can see the advantage of going solid state.
Omron G5LE specification
Driving the MOSFET gate
Obtaining high speed MOSFET operation can be a challenge, but at such a low frequency (0.067HZ), driving the gate is a piece of cake. MOSFET turn-off is generally the issue, so I obtained an oscillograph. Turn-off occurs within about 80uS, and the active portion of the gate voltage is indicated by the step on the transition. R14 is what discharges the MOSFET gate capacitance, including the Miller capacitance in which the change in drain voltage is coupled to the gate thus slowing the transition. The gate turns on via R9 and the gate voltage is limited via zener D3 to 10V –otherwise the full solar panel voltage could appear between the gate and source and potentially damage the MOSFET.
Oscillograph
Source: http://www.electroschematics.com/10218/sss-solar-charge-control/
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