DC Resonant Charging
This will explain how DC resonant charging circuit works. Let's start with a simplified circuit shown in Figure 1 and the resulting waveform in Figure 2 to understand its operation.
Figure 1. Simplified Resonant Circuit Model
Figure 2. Resonant Waveform (Simplified)
The initial capacitor tank voltage is zero and the switch (SW1) is open. The instant switch is turned on, the inductor (L1) starts to store the energy while the capacitor (C1) is being charged to 100% of source voltage. Once the capacitor reaches 100% of source voltage, the inductor releases the energy it stored and continue to charge the capacitor to 200% with the source in series with the inductor. Once it reached 200% of source voltage, the process would go reverse and return the energy back to inductor from the capacitor. Resulting repeated process of energy transfer between the capacitor and the inductor due to being 180º out of phase with each other, thus it would oscillate. Due to the loss in circuit (R1) converting electrical energy into heat, it acts as a damped oscillation circuit as you would expect in real world.
Since because it's impossible construct a spark gap would fire at right time when the peak capacitor voltage is reached, this would call for "De-Q-ing" circuit. See Figure 3 and 4 circuit with "De-Q" diode and the resulting waveform below.
Figure 3. resonant Charging Circuit with "De-Q" Diode
Figure 4. Resonant Charging Waveform with "De-Q" Diode
The "De-Q" diode block the reversed energy from returning to charging inductor, thus the capacitor is held at 200% of source voltage. Due to some loss in the circuit, it may actually be 190% of source voltage in real world, assuming 95% efficiency. The finial charged voltage may actually be lower or higher depending on factors such as how long RPG conducts/breaks, the remaining charge left in capacitor (either negative or positive polarity), the inductor being saturated, etc.
The instant capacitor is being discharged by Tesla coil primary coil, the resonant charging of capacitor starts right then. This process is repeated by RPG firing, see Figure 5 and 6 below.
Figure 5. Repeated Process of Resonant Charging
Figure 6. Close-up View of Waveform
Depending on how Tesla coil system performs, i.e. RPG conduction time, damped oscillation decay time, etc, it could affect the performance of DC resonant charging. If we could delay the DC resonant charging by placing high voltage SCR or Thrysistor, the DC resonant charging can be triggered at desired time. See Figure 7 below for simplified trigger circuit example and resulting waveform.
Figure 7. SCR-Trigged DC Resonant Charging Circuit
Figure 8. RPG and SCR Fire Timing Waveform
SCR trigger timing can be controlled by Rotary Spark Gap disc position sensor for synchronization. It's even possible to "skip by number" SCR firing rate with a counter circuit for lower pulse rate while the RPG is at ideal speed. Single pulsing mode is also possible.
Why use DC resonant charging system?
1. High performance power transfer from DC power supply to tank capacitor with lower loss.
2. Doubles voltage from DC power supply.
3. Improved power factor on 60Hz power system, especially for 3 phase-system.
4. RPG synchronization with line power is not required at all.
5. Variable output power without heavy variac, only by varying RPG speed.
6. Step-up transformer as low as 5kV can be used with a good result.