While not suitable for ultra-miniaturized modules operating in severe environmental conditions, air insulation offers a lightweight repairable structure that minimizes parasitic capacitance losses for most applications. We have developed HV structures that incorporate equipotential grading and electrostatic shielding of sensitive components so that we achieve excellent stability and accuracy. All of our HV assemblies are based on the well-known Cockcroft-Walton voltage multiplier concept, (or variations, thereof), to achieve high DC outputs while minimizing peak transformer secondary voltages.
The use of air allows for forced cooling of HV components when required. Forced air cooling allows us to incorporate an increased value of series protection resistance (where practical), which minimizes peak discharge currents when an arc or overload occurs. (NOTE: Some models or applications require external series protection resistance.) This not only protects the HV components and the customer’s load, but also reduces the discharge energy that occurs during an arc and minimizes the electromagnetic interference (EMI) pulse that can damage or disrupt sensitive controls and microcontrollers. All these techniques improve the reliability of the entire high voltage assembly, as well as the control and power elements of the complete power supply structure.
Above 150kV our designs utilize an open-air “stack” that eliminates the HV connector and cable that would be massive at these voltages. Toroidal terminals and equipotential surfaces are used to minimize the electrostatic fields. For units of 150kV and below, we mount the HV assembly in a proprietary HV insulated enclosure whose walls can withstand the full voltage. This enclosure is made of fire retardant materials and is designed to provide a uniform surface gradient to minimize corona. This, in turn, is mounted in a grounded chassis.
One of the problems with increasing the conversion frequency in HV supplies is the reflected parasitic capacitance. This is formed by the proximity of surfaces to ground. In a large HV structure, reflected parasitic capacitance can be sizable. If solid or liquid encapsulation is employed, this capacitance is much higher than in air since the dielectric constant of air is 1.0 while most encapsulants are on the order of 3-4.5. Capacitance is directly proportional to the insulation dielectric constant.
Our HV transformers typically have 6kV or less peak voltage on the secondaries and employ special universal winding techniques to produce a self-supporting large diameter winding which has the proper voltage gradients. In addition, we typically employ large window U-cores which give enough space for the proper gradients.