EMC & Circuit Protection
Figure 3: More sketches of heatsinks
Figure 4: Yet more sketches of heatsinks
attractive, and help designers quickly get an overview of whether they might have problems at any frequency.
A “cutting plane” can be chosen, at any angle, to view the 3-D animation in 2-D, for more precise observations and measurements, and this is what has been used to create Figure 6 which has also chosen the video frame corresponding to the first resonant frequency that appeared – at 5.32GHz.
Figure 5: Even more sketches of heatsinks I recommend < 30/ƒMAX bond spacing
(ƒMAX in MHz gives the bond-spacing in metres, GHz gives it in mm). 3-D field solvers can quickly reveal problems with heatsink bonding, and also help to find solutions quickly. Figure 6 shows a simulation of the emissions from an example ‘floating’ heatsink, very kindly done for me by Paul Duxbury, using FLO/EMC (now
part of CST Studio Suite https://www.3ds. com/products/simulia/cst-studio-suite). One of the common output formats from 3-D field solvers is a three-dimensional view of the fields around an object, coloured to represent intensity, and fluctuating as the simulated frequency is ramped through the selected range. The resulting colour animations are very
In Figure 6, the heatsink is assumed to be floating above the device’s RF Reference Plane (i.e. a solid copper ‘ground’ plane used as the circuit’s common return power rail) with the thin wire centred under it being the simulated source of stray return currents. Because nothing else is nearby, the electric near-fields around the heatsink are symmetrical. In the far-field (see Part 2.4 of [1]) these complex near-fields will become a simple expanding spherical shell. Figure 7 shows the same heatsink
as Figure 6, at the same frequency, but this time with zero-Ω bonds to the RF Reference Plane at each of its four corners. The near-fields are now noticeably closer to the RF Reference Plane – indicating lower far-field emissions – but of course from this one video frame we don’t know whether the emissions have now worsened at a different frequency.
Field solvers can also plot the field strengths as the measured frequency is varied, at any specified measuring points, and Figure 8 shows a number of such graphs plotted on the same axis, for different combinations of heatsink bonding, at one point.
The vertical axis of Figure 8 shows the far-field emissions at a specified point, but note that it uses a linear scale whereas most emissions measurements use a dB (i.e. logarithmic) scale. (dB scaling will be a display option). It shows continues on page 16
Figure 6: The electric near-fields around a ‘floating’ heatsink at its resonant frequency
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Figure 7: Fig. 6 but with heatsink RF-bonded directly to Reference Plane at each corner Components in Electronics September 2024 15
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