EMC & Thermal Management
Overcoming thermal challenges in high power, high-frequency transformer design
As efficiency targets for power converters get tougher, the focus is turning to magnetic components to improve power density and reduce losses. Andrea Polti, global product manager – Magnetics, Murata, describes a patented approach to the thermal design of high-power transformers that reduce internal temperature rise
G
reat strides have been made in recent years in improving the efficiency of power converters using the latest semiconductor technology,
with wide band-gap devices in resonant converter topologies now allowing single- stage efficiency figures of better than 99 per cent. With a steady reduction in semiconductor static and dynamic losses, attention is increasingly focusing on the dissipation from passive components – particularly magnetics.
One of the desired benefits of higher converter efficiency is smaller, lighter and lower-cost heatsinking and overall packaging. However, to match this, magnetics in the form of energy storage inductors, filters and transformers also must reduce in size, and this is facilitated by increasing switching frequency. Filter and storage inductors passing net DC generally require less inductance as switching frequency rises, which can allow smaller cores and/or fewer winding turns for the same flux density. This produces little or no increase in total magnetic component losses if the AC component of the current is small. For transformers, core size can also reduce with increasing frequency for the same number of winding turns and flux density. However, because transformer current is all AC, eddy current and core hysteresis losses also rise substantially as frequency increases. Additionally, semiconductor dynamic losses increase with frequency so there is always a trade-off between system frequency, efficiency, temperature rise and size. Transformers can nevertheless be very efficient and losses are often disregarded at medium and low powers and at low frequencies. At higher powers, however, even fractions of a per cent of inefficiency can result in significant power loss, with correspondingly high average and hot-spot transformer temperatures. This can be problematic, especially if the advantage
22 June 2022
of small magnetics size has been realised by increasing frequency, giving a small overall transformer surface area available for heat dissipation to the environment. 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 yet further losses and in turn, higher temperatures still.
An 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 to 5W/m.K compared with 400W/m.K 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 are typical in a ‘planar’ construction can be used to lead heat away, but the approach is not effective for
Figure 2: Internal construction of the new transformer heatsink approach
inner windings which may often be high voltage primaries with relatively large numbers of turns of thinner wire.
A new approach reduces transformer internal temperatures Murata has recently made advances with winding arrangements for high-
Figure 1: Murata patented transformer heatsinking ar- rangement using embedded metal plates
Components in Electronics
power, high-frequency transformers with their patented PDQP technology, which interleaves windings in a novel way to minimise leakage inductance and skin and proximity effects. The PDQP technique provides a useful decrease in losses but the company has now patented a further technique to provide better control of internal transformer temperatures by embedding heatsink plates within the core and winding structure. 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. In this example, eight cores U7 – U8 make up the assembly with ‘sandwiched’ metallic heatsink plates shown in blue and red. Figure 2 shows the internal construction, in this case using 12 ‘U’ cores but with the top six removed for clarity. The central thicker 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 interior of the assembly. The thinner plates in red can be bonded to the central plate or can protrude from the assembly for attachment to the external thermal sink. The whole assembly
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58
