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Planar transformer for 250W converter topologies

19 Dec 2014  | Gerard Healy

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Analogous with current flow in a conductor, flux will follow the path of least magnetic resistance. The simulation reveals areas of high localized flux densities. In the context of transformer design, this results in a reduced effective core cross sectional area and greater core losses than previously assumed, so additional turns are required to achieve higher levels of transformer efficiency. Using these advanced simulation techniques and design capability, the core design has been optimised for a more uniform flux distribution to increase the effective core cross-sectional area. The simulation results are presented in figure 4.

 Simulation of the flux density

Figure 4: Simulation of the flux density in the optimised ER+ core.

For a comparable core size, the ER+ design increases the effective core cross-sectional area by 40%. This directly reduces the AC flux density/core loss for the same number of turns (increasing the transformer efficiency) or allows for a lower number of winding turns/copper loss (increasing the throughput power).

New flat coil winding technique
In addition to the negative impact on AC resistance, multi-sequential winding layers increase the leakage inductance of a transformer. At higher powers, this limits the converter efficiency so winding segments of fewer turns, connected in series/parallel to interleave the primary and secondary windings, are necessary to reach very low leakage inductance targets. This adds complexity and cost to the transformer design.

To address this, a patent-pending winding technique has been developed to match the complete interleave capability of multi-layer PCB technology while continuing to offer the cost and performance benefits of the existing flat coil planar technology. The flat coil windings are corkscrewed into each other, as demonstrated by the following illustrations in figure 5.

 Deep interleaved flat coil windings

Figure 5: Deep interleaved flat coil windings.

The outcome is a reduction in leakage inductance with a simplified and lower cost winding structure. Eliminating the interconnects for series winding configuration also reduces the pin count, a fact that was anticipated in the design of ER+ core discussed in the last section. The table illustrates the benefits of the deep interleave winding over the traditional construction.

 Comparison of planar solutions

Table: Comparison of planar solutions.

The reduction in the number of winding turns when using the optimised core and the complete interleaving of this new winding technique significantly increases the throughput power for a given transformer size. Selective coupling between the windings, where a very low level of leakage inductance is required, can be effectively implemented with this technique, resulting in higher efficiencies and greater power densities. For a given ER platform size, the power capability of a similar-sized ER+ platform is touted to increas by approximately 50%.

Two important advancements have been presented which yield a quantum leap in throughput power capability while continuing the trend in planar magnetics' price reduction. The ER+ platform is touted to provide 40% greater core cross-sectional area compared to the existing platform of comparable size, while the new deep interleave winding technique reduces the cost by 35% by simplifying the winding structure for a comparable coupling performance.

Since the number of required winding turns is inversely proportional to the core cross-sectional area, the number of turns can be reduced for a given performance level. This reduces both winding resistance and the leakage inductance so the power capability increases. This next generation planar design is scalable and extends the power capability of the planar platforms to 1 kW.

About the author
Gerard Healy is the Field Application Engineer for the Power Division in Europe. He has a BEng Electrical and Electronic Engineering from University College Cork and a MSC from the National Microelectronics Research Centre, Ireland.

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