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Urgency in the need for corona-free customs magnetics sol'ns

28 Apr 2016  | Steve Taranovich

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A lot of designers consider transformers as if they were "garden variety" resistors in so many designs. They would choose one that meets the circuit need for transforming electrical energy from one circuit to another without a direct electrical connection using mutual induction between two windings and having the necessary isolation voltage needed for the system design. However, there is more to a transformer than it actually is.

Corona inception voltage

Figure 1: Corona inception voltage is the lowest voltage at which continuous corona of specified pulse amplitude occurs as the applied voltage is gradually increased. Corona inception voltage decreases as the frequency of the applied voltage increases. Corona can occur in applications as low as 300V. Corona extinction or extinguish voltage is the highest voltage at which continuous corona of specified pulse amplitude no longer occurs as the applied voltage is gradually decreased from above the corona inception value. Thus, once corona starts, the voltage must be decreased to get it to stop.

I met Kevin Bradley, marketing director, from Bicron Electronics Co., a Connecticut-based manufacturer of high-reliability, custom magnetics, at APEC this year. They announced an improvement in their line of VoltBoss corona-free transformers. Why is that important?

First of all, most engineers think of high voltage transmission lines when they hear the term corona. Have you heard of Corona bloom, a corrosive effect of partial discharge, is a leading cause of failure in magnetics components? Bicron's latest generation of VoltBoss transformers have been developed to eliminate corona failure (dielectric breakdown) for voltages up to 4.5kV in an amazingly small one sq. in. footprint.

This phenomena, frequently misunderstood by many designers as only applying to large, high voltage magnetics devices, is equally important in smaller magnetics devices in operating environments as low as 300V employed in equipment that must operate reliably over their lifetime, often decades.

Corona is caused by ionisation of the media (usually air) surrounding an electrode/conductor. It is a function of the voltage and the surrounding relative humidity and air density. Corona is generated when a localised discharge resulting from the gaseous ionisation in an insulation system when the voltage exceeds a critical value. That critical value was previously assumed to be about 1kV but it can even be exhibited at levels down to 300V.

Corona is not just a function of an Electric Field. It depends upon the surface and radius of curvature of the electrode/conductor. It is also dependent upon the rate of decay of the electric field away from the conductor/electrode. So the conductor with the smallest electric field at its surface is not the obvious solution.

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