The high-voltage field is measured by the term called the "E" field
gradient. The higher the number, the greater the risk. There is an adjunct
coefficient called the "utilization factor" that acts as a multiplier to the
gradient number and this utilization factor is dependent upon sharp edges and
Corona is generated by high concentrations of the voltage field, usually a
result of sharp points, small geometries and their associated spacings.
Utilization factors, based on the minimum voltage stress condition, are
obtained with a uniform voltage distribution across the insulating material.
Corona inception and associated problems can be avoided by the
(1) Specified geometries should be consistent with the voltages contained
within the power supply. A high-voltage "E" field gradient analysis should be
performed to ensure that the appropriate utilization factors were used with
the specific geometries.
(2) Component case and even conductor shapes can create concentrated
voltage fields and/or fracture planes. Component edges, corners and fasteners
should have a radius or a fillet.
(3) Positioning of all components, connectors and cabling should be
guaranteed by design and verified throughout the assembly process.
(4) Conductors exiting from high-voltage planes should not create
concentrated voltage fields at the exit point.
(5) Where connections are made using solder techniques, a minimum solder
ball diameter should be specified and controlled.
(6) Avoid the use of multiple insulating materials.
(7) Minimize interfaces.
(8) Establish and enforce cleanliness procedures to prevent
(9) Use vacuum impregnation and pressure curing techniques to minimize
voids in encapsulation materials.
(10) Prepare all surfaces for bonding using wet, dry, plasma, or etching
techniques and cleaning.
(11) Derate insulating materials based on maximum (not average) electrical
(12) Preassign boundaries of high-field intensity and define voltage
(13) The voltage gradients within a resin system should be less than 50
VDC/mil. Across an interface between insulating materials, the gradient should
be less than 25 VDC/mil. It is recommended that AC voltage gradients should be
less than one-half of the DC gradient.
(14) The use of laminated insulating barriers and printed wiring boards
should be avoided.
(15) Where printed wiring boards are unavoidably used in high-voltage
fields, they should be shielded or have barriers added. Corona testing is
mandatory to demonstrate the level of design margin. The voltage gradient
between conductors should be per MIL-STD-275, paragraph 5.1.4, which
recommends approximately 8 V/mil average.