The DC voltage produced by the half wave rectifier is less than the peak value of the Sine Wave. The size and ratings of the capacitors and transformers become very large if pure high voltage DC  is to be produced. Voltage Doublers  are used  when higher DC voltages are needed. A typical voltage doubler circuit due to Greinarcher is shown below

The capacitor C₁ is charged through rectifier D₁ to a voltage +V_hmax with polarity as shown in the figure during the negative half cycle. During the positive half cycle as the voltage of the transformer rises to V_hmax, the potential of the other terminal of C₁ rises to 2V_hmax. The capacitor C₂ is in turn charged through the Rectifier D₂ to 2V_hmax.  Depending on the time constant C₂R_L where R_L is the Load Resistance, the output voltage will be less than 2V_hmax.  The rectifiers are rated to a Peak Inverse Voltage of 2V_hmax and it is preferable that both C₁ & C₂ have the same rating to affect equal amount of charge transfer. The ripple content is straight away proportional to the load current.

Ripple is Calculated using the formula I/2*f[(1/C₂)+(2/C₁)].

 

References

1. High Voltage Engineering and Testing(2^nd Edition),Edited by:Ryan,Hugh M.2001 Institution of Engineering and Technology
2. High Voltage Engineering By M.S. Naidu,V.Kamaraju, McGraw-Hill Professional Publishing,2001