Feature: Power
Figure 2: Internal construction of Murata’s new
transformer heatsink approach
and skin and proximity effects. The PDQP technique decreases losses, but a further technique (also patented by Murata) which embeds heatsink plates within the core and winding structure provides better control of internal transformer temperatures. The method suits high-power transformers where temperature rises might be high and the core is typically assembled from combinations of U, or U and I cores. Figure 1 shows the general approach, and, here, eight cores U7-8 make up the assembly with sandwiched metallic heatsink plates shown in blue and red. Figure 2 shows the internal construction,
at medium and low power as well as low frequencies. At higher power, however, even fractions of a percent of inefficiency can result in significant power loss, with correspondingly high average and hot- spot transformer temperatures. Tis can be problematic, especially if the advantage of small magnetics size has been realised by increasing the frequency, giving an overall small transformer surface area for heat dissipation. High temperatures can damage insulation and risk safety or, at best, force the use of unnecessarily high temperature ratings of wire insulation and bobbins to achieve agency safety certification. Ohmic resistance of copper windings also increases with temperature, leading to further losses and, in turn, even higher temperatures. One approach to minimise
temperature rise in transformers is to provide controlled paths for the heat to be led away. Ferrite cores used at high frequency have relatively poor thermal conductivity, typically 2-5W/mK compared with 400W/mK for copper, so temperature differentials across ferrite can be high, effectively thermally insulating the interior of a transformer. Thick windings of a few turns such as those found in planar construction can be used to lead heat away, but the approach is not effective for inner windings, which may often be high- voltage primaries with a relatively large number of turns of thinner wire.
Advanced approach Murata has recently made advances with winding arrangements for high- power, high-frequency transformers with a patented PDQP technology, which interleaves windings in a novel way, minimising leakage inductance
in this case using 12 U cores, with the top six removed for clarity. Te thicker central plate acts as a conduit for heat and can be mechanically attached or bonded to an external housing or ‘cold wall’ to provide heat sinking for the assembly’s interior. Te thinner plates in red can be bonded to the central plate or can protrude from the assembly for attachment to the external
Figure 3: Temperature rise simulation with transformer-embedded heatsinking
Figure 4: Temperature rise simulation without transformer-embedded heatsinking
www.electronicsworld.co.uk February 2023 21
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