Why do both diodes turn off when passing t2 in the above multiplier? Shouldn't the diode connected downwards be on?
Rectification is always a voltage flowing to charge the rising capacitor. After passing the vertex, no current flows.
And when the voltage is -, it is charged to C1 through D1, and when it is +, it flows to C2 through D2.
Look at the picture and analyze it.