Try moving the capacitor in FRONT of the regulator, and use a smaller capacitor after it. Of course, can't move the 3300uF 6.3v in front, as you'll kill it with the 15v coming from panels... but consider something like 470uF 25v polymer or low esr capacitor in front and maybe 100uF 10..25v or something like that on the output, you won't need more.
With a big capacitor at the output of the regulator, you're forcing the regulator to pull a burst of current in order to fill the capacitor with energy and that's in addition to the initial demand of the motors of the fans.
you have 5v at 0.12v so two fans will be 5v at 0.24a ... that's 5 x 0.24 = 1.2 watts ... assuming the panels give you 15v then the buck regulator will probably pull around 1.5 watts from the panels, or 1.5w / 15v = 0.1 A (buck regulator will only be around 90% efficient, so account for that)
If your buck regulator has an enable pin, you could see if maybe you could use a voltage divider (2 resistors) to send the enable signal only when the input capacitor's voltage goes above some threshold.
For example, if the regulator turns on only when you have 1.2v or more on the enable pin and turns off at less than 0.3v or 0.6v , then you could choose two resistors to get a 10:1 ratio. This way, when the input capacitor is charged enough to have more than 12v, the regulator turns on and produces 5v.
You could also have a time delay circuit added to each fan... for example a combination of resistor and capacitor plus a mosfet
Pick resistor and capacitor values in such a way that voltage ramps up from 0 to something like minimum 2v in a second or two. Connect this to the gate of a mosfet which you basically use as a switch. When the voltage on gate is high enough, the mosfet turns on and therefore powers the fan.
For the second fan, pick the resistor and capacitor to ramp up voltage much slower, let's say giving around 2-5 seconds more. This way first fan turns on and starts spinning for a couple of seconds and the initial current burst is gone, and now second fan can start and pull some current.
Here's tutorial and explanations on how to use RC and how to calculate time :
https://www.electronics-tutorials.ws/rc/rc_1.htmlHere's part of the table on that page:
Time
Constant RC Value Voltage Current
0.5 time constant 0.5T = 0.5RC 39.3% 60.7%
0.7 time constant 0.7T = 0.7RC 50.3% 49.7%
1.0 time constant 1T = 1RC 63.2% 36.8%
2.0 time constants 2T = 2RC 86.5% 13.5%
Let's say you have a 470kOhm resistor and a 10uF capacitor and the regulator outputs 5v.
The time constant is T = R x C = 470k x 10 uF = 4.7s ( because you convert uF in Farads)
At 0.5 time constant (~2.4 seconds), the voltage on the output of the 10uF capacitor will be around 39.3 / 100 x 5v = 1.96v
At 0.7 time constant (~3.3 seconds), the voltage on the output of the 10uF capacitor will be around 50.3 / 100 x 5v = 2.5v
For the second fan, you can pick a higher resistor value and a bigger capacitor, to get something like 6-10 seconds for the time constant. Or, you connect the first fan directly to the output of the regulator and have it start right away, and use this RC + mosfet scheme to delay the second fan startup a few seconds.
Now, you need to pick a mosfet that has minimum turn on threshold voltage somewhere above 1.8v but which can also handle up to 5v on the gate (which shouldn't be a problem)
You have to look at the Vgs(th) aka Voltage gate source threshold in the datasheets ... that tells you the minimum voltage that must be between the gate and source pins of the mosfet for the mosfet to turn on.
For example BS270 has a typical 2.1v gate threshold voltage (max 2.5v) but also a minimum of 1v , so you may have to get a bunch of mosfets and test them and hand pick the ones which only turn on at higher voltages:
https://www.onsemi.com/pub/Collateral/BS270-D.PDF (digikey link:
https://www.digikey.com/product-detail/en/on-semiconductor/BS270/BS270FS-ND/974203)
A safer choice would be for example FQN1N50C which has a minimum threshold voltage of 2v :
https://www.onsemi.com/pub/Collateral/FQN1N50C-D.pdf (digikey link:
https://www.digikey.com/product-detail/en/on-semiconductor/FQN1N50CTA/FQN1N50CTACT-ND/1923111 )
However, this one also has a higher Rds(on) resistance of around 6 ohm ... it's like having a resistor in series with the fan... and this can cause fan to slow down a bit. BS270 would be a bit better because it has only 2 ohm internal resistance so it will heat less.
now you can simply have +5v ---[fan]---- [drain pin ]--- [mosfet]----[source pin] ---- ground ... and the gate of the mosfet connected to that RC circuit 5v --- [ resistor in series] ---- [capacitor between r and ground] ---- [gate ]
Here's a mosfet tutorial / explanation :