I am trying to create a PWM signal to charge a 12V AGM lead acid battery from a 100W solar panel. Of course the charge controller has multiple functions depending on the charge of the battery but I am kind of stuck at initially creating the PWM signal itself. Is there a typical method used? I've only ever used a 555 timer to create a pwm signal, but I haven't noticed any 555 timers so far in pwm charge controller circuit designs. I see that sometimes transistors are used, and sometimes a configuration of capacitors and resistors and diodes. Also, does frequency matter?