I'm just posting these pictures for someone. This is my water heating system at 100W
(1/4 array power) on my test panels. This is PWM from a capacitor bank that is kept
at the 60V array power point. The top trace is the DRAIN voltage on the FET. Low, the
water heater element is turned on. High, displays the capacitor bank voltage.
The lower green is the AC component of the capacitor ripple voltage. Downward slope is
the discharge of the capacitor bank when the FET turns on the heating element. Upward
slope is when the FET turns off and the capacitor bank recharges. Hope a picture is
worth a thousand words.
Please continue your education by watching this video on adiabatic capacitor charging
which is similar. This professor has many excellent videos on electronics for the beginner.
https://www.youtube.com/watch?v=ZtgOboxMNoUThe second screen shot shows the voltage of the timing capacitor in green at 25W. This
capacitor is shorted out till the storage capacitor bank voltage reaches the desired
value. It takes the timing capacitor 1.6ms from being unshorted to reach the turn on
value of 5.3V. With the output now on, it keeps increasing till the storage capacitor
bank voltage drops sufficiently. At that time the timing capacitor is shorted out which
turns the outputs off. The timing capacitor remains shorted till once again the voltage
on the storage capacitor bank reaches a sufficient value. The process then begins over
again.
