EMC & Thermal Management
Figure 3: Temperature rise simulation with transformer-embedded heatsinking
of the current, so both are minimised by thin plates. The central, thicker plate in theory has no eddy currents if symmetrical with the windings, as the magnetic field from each core aperture cancels. Material for the plates can be copper for the highest performance or aluminium for cost-sensitive applications. The thermal conductivity of copper is better than aluminium by a factor of two but its electrical conductivity is higher by a similar ratio, so any residual eddy currents would produce lower losses in aluminium.
Practical results
Figure 4: Temperature rise simulation without transformer-embedded heatsinking
can be bonded or clamped together, but the pressure and small separations are not critical, except for the faces between the top and bottom U core sets. All other interfaces are not in the magnetic field path and small gaps are not material
although thermal coupling is better with closer contact.
Similar to steel laminations in 50/60Hz transformers, the thin plates in red do not form complete conducting loops and eddy currents would anyway be primarily
induced through the thin dimension of the plate. Eddy current is proportional to the area of the induced current loop and power lost is proportional to the square
Figure 5: Measured internal temperatures of a working transformer with heatsink plates
To confirm the performance of this approach to transformer heatsinking, a 50kW 24kHz converter was simulated with the plates embedded and then compared with a version without the plates. An actual transformer was then constructed and loaded and temperature measurements were taken. The converter is typical of an EV battery charger with a 700VDC input bus and an output of 417V at 122A. Figure 3 (left) is a simulated temperature map of the transformer with heatsink plates included (external view) while the image on the right shows a cross-section of the part with internal hot spots. The ambient temperature was set at 31°C and a maximum internal temperature of 56.2°C is indicated, a rise of around 25°C. The same transformer was simulated without the additional heatsinking and Figure 4 (left) and (right) are the two equivalent plots, showing an internal maximum temperature rise of 39°C, over 50 per cent higher than with the heatsink plates. Note that the temperature scaling is different between Figures 3 and 4. Practical measurements bear out the simulation (Figure 5), with embedded thermocouples recording a maximum internal hot-spot temperature of a little over 58°C, within 1.5°C of the simulation. The transformer heatsink approach patented by Murata as described promises to allow higher power from a given transformer assembly or lower temperature for the same power with a corresponding increase in reliability and lifetime. Safety margin to the material temperature limits is improved and agency certification is facilitated without necessarily resorting to specialised and expensive high-temperature insulation systems. A combination of simulation and practical measurements confirms the value of the approach.
https://www.murata.com/en-eu/products/ transformers/highpower-highfrequency- transformers
www.cieonline.co.uk Components in Electronics June 2022 23
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