Semiconductor Devices: Voltage Doubler and Multiplier

[@chrisleon27]

Interesting circuit

[@StealthParrot]

We use a 'somewhat' similar method of generating high voltage DC using a pulse forming network. 208VAC is stepped up to approx 11KV with a large plate transformer. This is rectified to DC and when a switch (thyratron) closes current flows through a diode stack and charge choke then into a 25KV capacitor. When the current flows into the cap a large electromagnetic field is generated around the choke. When the cap is fully charged, current flow stops, the emf collapses and generates and equivalent current flow into the capacitor. Back flow is prevented by the diodes so the capacitor effectively doubles its voltage to about 22KV. Another switch (thyratron) is used to discharge the cap into a klystron to create high power RF.

[@ElectronicswithProfessorFiore]

Out of curiosity, what is the application?

[@StealthParrot]

@@ElectronicswithProfessorFiore Oh sorry, this is for Linear Accelerators for Radiation Therapy. The extremely high voltage requirement is for generating the high power RF necessary to accelerate electrons under high vacuum to near the speed of light. When the electrons hit a fixed tungsten target, they emit photons at energies required for radiation treatment. But it's neat since you were talking about voltage doublers/multipliers and the technique is somewhat similar, although definitely not for bench top applications. 🙂

[@ats89117]

The old Cockcroft-Walton voltage multiplier is great for ionizing the air and zapping vermin, but the efficiency is terrible (would have been interesting to show this by looking at the battery power vs the load power as a function of time). That's mostly due to the diodes, but also partially because of the inefficiency of charging capacitors from voltage sources. The last point might be a good topic for a future lecture as many practicing engineers don't understand that transferring charge between two identical capacitors (without an intervening inductance), one fully charged and the other with no initial charge, results in loss of half the initial energy regardless of the resistance between them.

[@ElectronicswithProfessorFiore]

Certainly not a particularly efficient circuit from multiple angles (just look at the waveforms in the latter third of the video), but it is an interesting application for students to see. I may do a deeper dive in the future as you suggest, but at nearly 20 minutes, I think this serves as a decent intro.