November 2020

12V to 6kV High Voltage transformer

The primay power source in my home made X-Ray machine is an high-voltage high-frequency transformer. This transformer is drived by a Mazilli ZVS circuit and it’s capable to output 6kV from 12V. It’s input circuit uses a center tapped coil drived in balanced way by two mosfets of the ZVS, with input voltage feeded into the center tap. This gives balanced sinusoidal waveform on it’s output. Sin wave helps the “slow” high voltage diodes of the HV multiplier chain to turn ON and OFF correctly minimizing RFI generation.

Transformer characteristcs:

  • Nominal allowable output 4.5-7.5kV @ 10mA from 10-15V@ 8A
  • 50kHz working frequency
  • Completly encased in epoxy
  • ZVS drive. Primary winding is a center tapped coil
  • ETD54 N87 ferrite core ungapped and #B66396W1022T1 Epcos coil former
  • Primary winding 4+4 turns of 2.5mm2 wire
  • Secondary HV winding 1400 turns 0.2mm dia CuEn

The realization of such kind of transformer presents many challenging problems to be solved. First one is insulation. I’m using standard ETD54 horizontal plastic coil former.

This requires an 8mm clearance/creepage free space between every single layer of the secondary high voltage winding and the side walls.

The insulation between ferrite core and first layer is insufficent because Epcos coil former is not rated for 6kV. I’ve fixed it wrapping a first insulation layer of 4 turns of 0.2mm thick PTFE tape. Each layer is made winding side by side 100 turns of the enameled wire. In total I’ve 14 layers with 0.4mm thick layers of PTFE between each coil winding layer.

The primary is periferic and very far from inner core. The need of a massive insulation between secondary and primary resulted in 3mm total thickness of PTFE tape. I’ve feared that positioning it near the core coud let me not enought space for the primary into the coil former. Winding the primary on external side of the coil former gives me some clearance even I exceed the coil former sides height. The HV output terminals are soldered to silicon insulated HV cable rated 10kV and the whole transfrmer is encased into epoxy. This prevents any kind of corona effect induced disasters like arcing, fire and smoke and gives to the whole trafo a mechanical rigidity and protection. I’ve made a custom cardboard mould for this.

The top of the transformer is separate from the bottom of the cardboard mould by a couple of insulating paper risers of 10mm diameter to let the epoxy to fill every side of the transformer with sufficent thickness