HVPS overall design
The design of the high voltage power supply is a little unconventional, but by convention, most are. In order to simplify the transformer design, I'm going with a hybrid structure, with an unregulated H-bridge inverter fed by a regulated buck converter, all powered by the system's 110V supply.
The buck converter reduces the 110V input to around 50V, to allow regulation of the outputs by adjusting this voltage. If it looks unusual, that's because this buck converter is upside down — that's just done to make it easier to control, with a low-side MOSFET and current sense resistor. The inverter already requires gate drive transformers regardless of where it sits on the rails so there are no real drawbacks to this approach.
The inverter uses a MOSFET H-bridge to create a 100Vp-p square wave from the output of the buck converter. A frequency divider creates its 25kHz drive waveform from the 50kHz buck converter drive. This keeps them in phase lock, preventing beat frequencies from developing between them.
The transformers and rectifiers multiply the square wave by a ratio, then rectify it. The transformers are kept as simple as possible so there is plenty of space for good isolation; both transformers must handle 2000V (for the filament transformer, in the form of isolation voltage). +8kV is developed by an external multiplier pumped by the 4kVp-p AC output from the transformer.
The control circuit, shown below (not present in the block diagram), generates all drive waveforms and provides the control loop to regulate the output voltages.
The control circuit is based around a UC3843 PWM controller. This old chip provides a synchronizable oscillator, a voltage reference, error amplifier, current-mode sense comparator, and an output capable of driving a MOSFET directly.
Regulation is taken from the +2kV output, through a voltage divider and into the voltage sense input of the UC3843. An external frequency compensation circuit and slope compensation circuit (latter not shown) keep the control loop stable.
The 50kHz PWM signal from the UC3843 is fed to a 4013 flip-flop configured as a frequency divider, which produces a 25kHz square wave at 50% duty cycle regardless of the pulse width driving the MOSFET. This signal is buffered by a gate driver IC, producing a strong 24Vp-p output that is then isolated and split by a pair of gate drive transformers and sent to the gates of the H-bridge.
Two transformers are used by this circuit:
High voltage transformer
(Detailed posts to come on the transformer construction!)
The high voltage transformer is a fully custom transformer, with a single primary and a single secondary, wound on a pair of Fair-Rite 9277012002 U-shaped cores. It uses a sectioned winding for the secondary, with the full 2000V output split into six 333V sub-windings separated by pieces of insulating material. This reduces the voltage stress seen by the insulation of the magnet wire to no more than 167V across one layer of enamel (remember that there are two layers of enamel between two adjacent wires).
The filament transformer does not produce high voltages, but it needs to be sufficiently spacious to isolate the primary from the secondary to a working voltage of 2000V. The current plan is for this transformer to be a modified Signal HCTI-1000-2.4 toroidal inductor, with a secondary added using high voltage wire. Whether the ferrite material is suitable for this use remains to be seen.