Feature: Power
Figure 5: Measured internal temperatures of a working transformer with heatsink plates
thermal sink. Te whole assembly 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 be primarily induced through the plate’s thin structure. Eddy current is proportional to the area of the induced current loop, and power lost is proportional to the square 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 fields from each core aperture cancel each other out. Material for the plates can be copper for 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 similarly higher, so any residual eddy currents would produce lower losses in aluminium.
Practical results To confirm the performance of this approach to transformer heatsinking, a 50kW 24kHz converter was simulated with embedded plates and without them. An actual transformer was then constructed and loaded, and temperature measurements 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 tempe-
rature map of the transformer with included heatsink plates (external view), whereas 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.
22 February 2023
www.electronicsworld.co.uk Te same transformer was simulated
without additional heatsinking; Figure 4 shows the two equivalent plots, where an internal maximum temperature rise of 39°C can be clearly seen – 50% higher than that with heatsink plates. Note that the temperature scaling is different between Figures 3 and 4. Measured internal temperatures of a
working transformer with heatsink plates are shown in Figure 5, with embedded thermocouples recording a maximum internal hot-spot temperature of just over 58°C, within 1.5°C of the simulation. Tis Murata transformer heatsink approach
promises higher power from a given transformer assembly or lower temperature for the same power with a corresponding increase in reliability and lifetime. Safety margins of the material temperature limits are improved, and agency certification is facilitated without turning to specialised and expensive high-temperature insulation systems. A combination of simulation and practical measurements confirms the value of the approach.
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