6V Solid State Switch Solar Charge Control

This is a Solid State Switch upgrade to the 6A, 6V Relay Based 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 essentially identical. One drawback is the loss of the secondary output feature. For more information go the above link as this write-up contains mostly supplemental information.


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.


6V SSS Solar Charge Control Schematic

Bill of materials

6A, 6V SSS Based Charge Control BOM

Eliminate the relay life issue

While relay life is OK, it is definitely limited. I estimate the relay life to be approximately 1year, but I have no direct experience with such. This makes the solid state switch desirable.

MOSFET thermal considerations

This is an issue that I did not cover in the 6A, 12V SSS Based Solar Charge Control. With an Rdson of 0.07Ω, the FQP27P06 device dissipates appreciable power. Also, note that the Rdson is specified at a gate voltage of -10V, while in this circuit it runs at approximately -8V. At the lower gate voltage the Rdson may be higher. Fortunately, the typical Rdson specification is close to 0.055Ω, so we will use the maximum Rdson spec of 0.07Ω for our calculations.

Thermal impedances

Θj-c (junction to case) 1.25°C /W
Θc-h (case to heatsink) 0.5°C /W
Θh-a (heatsink to air) 11°C /W (natural convection)
Θj-a (total resistance) 12.75°C /W
MOSFET power dissipation @6A = I²R = 6² * 0.07 = 2.5W
MOSFET power dissipation @10A = I²R = 10² * 0.07 = 7W

Junction temperature calculation

Max Tj (junction temp) = Pmax (power) * Θj-a + Ta max (max ambient temp)
= 7 * 12.75 + 40°C = 129°C (this is well below the rated Tj max of 175°C)

Effect of parallel MOSFET devices

While the above exercise indicates an acceptable, conservative temperature rise using my favorite, inexpensive TO-220 heatsink, parallel devices have merit. Simply paralleling two FQP27P06 MOSFETs, reduces the Rdson by a factor of two to 0.035Ω. This also reduces the total conduction loss to half, but there is an additional benefit here in that the power is split between the two devices. So the worst case power dissipation of 7W @ 10A is now reduced to 1.75W per device.

According to the FQP27P06 datasheet, the thermal resistance (Θj-a) without a heatsink is 62.5°C /W. So at 1.75W, the maximum junction temperature calculates out to 149°C.

Simply put, NO HEATSINKs ARE REQUIRED in this case using parallel power devices. Also, depending upon how much you pay for the MOSFET and /or the heatsink, the cost is essentially ‘a wash.’ Now 149°C is definitely ‘toasty,’ and can scorch your finger, but laying the MOSFET power device down on a large PCB copper pad will conduct heat away and reduce the thermal resistance –while it consumes additional circuit board area, it makes it run cooler without the addition of a physical heatsink.

LED function and Schottky isolation diode

LED (D4) indicates that the battery is charging. Note that if D5 is changed to a Schottky type rectifier, a shunt resistor (2.2K or so) may be required across the LED to prevent diode leakage from turning it on. Schottky rectifiers, while offering substantially reduced forward voltage drop suffer from lower peak inverse voltage (PIV) rating as well as higher leakage current than the usual silicon rectifier.


To set the Max Voltage Adjust potentiometer (R12), start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 7.2V). At this point, turn R12 CCW until the LED extinguishes. When the LED comes on again at the next clock cycle, recheck the voltage at which it extinguishes.


For the future

  • SSS solar charge control with synchronous isolation rectifier
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge control schematics

Undocumented words and idioms (for our ESL friends)

(a) wash –idiomatic phrase –essentially equal or no difference –literally, it all comes out clean when it is washed (like laundry).

toasty –idom, slang –adjective –hot –literally the temperature of a slice of bread when it pops out of the electric toaster.

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