Zener Diode and diode are turned on during the negative cycle, and the charges is stored in the capacitor. During the positive cycle, the diode is turned off and the capacitor acted as DC voltage source, which shifted the output voltage up.
During positive cylce
The voltage stored in capacitor is
Vc= Vp+ VB -Vd
During negative cycle
Voutput = -Vp - Vc
