Industry and consumers are increasingly aware of the benefits of efficacious use of metered power, as costs and cost trends increase. Making the right choices for power converter design can produce rapid investment payback in applications such as for LED illumination and automotive electric vehicles, and can be optimized for low total cost of ownership (TCO).
The right power converter design choices may also be enabling for aerospace and other mission critical applications where otherwise larger than necessary burdens of size and weight would impose high penalties upon design of those propulsion systems, and reduce efficaciousness of mission-tasked power utility.
One right choice is to cost-effectively design for high conversion efficiency, producing subsequent reduction in package cooling requirements, and leading to smaller package volumes, reduced package weight, and reduced manufactured cost of package. Another right choice is to cost-effectively design for reduced maximum working voltage magnitudes in secondary circuits, producing subsequent reduction in insulation and isolation requirements especially for secondary circuits that may now qualify as SELV, leading again to reduced package volume, reduced package weight, and reduced manufactured cost of package.
Certain current fed topologies provide the advantage of reducing secondary maximum working voltage magnitudes so these levels roughly do not exceed the load voltage plus the forward voltage drop(s) of the rectifier arrangement. The applicable topologies must be full bridge rectified through their secondary circuit, so that conduction occurs over the full cycle. They must have current-compliant impedance presented by their secondaries to the secondary terminals of their transformers, so that the transformer output will be a voltage-compliant current source. When these criteria are met, the transformer’s secondary voltage will be clamped over the full cycle by the full bridge rectifier to not exceed the load voltage by more than necessary to supply the additional forward voltage drop of the rectifier elements. This accomplishment allows a large range of power converters to have their secondary circuits now classified as SELV, with the subsequent benefit of reducing the secondary packaging insulation requirements for these so-classified SELV secondary circuits to ‘functional’ insulation only. Benefits may result due to reductions to package volume, weight, and cost of manufacture.
A second advantage can also be obtained with the described topology, where higher efficiency than otherwise may result for secondary circuit classified as SELV. To realize this advantage, the load voltage must be optimized for higher magnitude, so that lower proportional power loss will occur due to the forward voltage drops of the rectifier elements. This leads to higher efficiency of secondary circuits with subsequent reduction in cooling requirements. In the case of variable output voltage power converters, the advantage would be seen to increase at higher output voltage levels that continue to remain compliant with SELV voltage limits. Resulting benefits are lower package weight, volume, and cost of manufacture.
The described topology may be implemented using various resonant topologies, including the ‘LLC’. Resonant topologies often impose higher voltage levels than otherwise in primary circuits, but aid in reducing power dissipation due in primary circuits to zero voltage switching (ZVS) of power switching devices, and due in secondary circuits to softer commutation of full bridge rectifier elements. Since primary circuits must burden the task anyway of insulation and isolation of hazardous voltage levels for safety purposes, the trade-off is not particularly limiting in its effect to the overall net benefits described.
For more information, contact Ray Maroon at Volt Amp Consulting, Inc., P.O. Box 190, Solana Beach, CA 92024, or