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EMC & Circuit Protection


Solving some EMC issues in heatsinking


By Keith Armstrong, CEng FIET Senior MIEEE ACGI, Cherry Clough Consultants


Heat sink resonances Stray return currents will flow in a heatsink, at least due to capacitive coupling through its TID (thermal interface device, such as a silicone- impregnated fibreglass pad) from the device(s) it is cooling. These currents cause heatsinks to suffer dV/dt, making them radiate EMC emissions as ‘accidental antennas’. If a heatsink has a resonant frequency close to a fundamental or harmonic frequency used by whatever it is cooling, its EMC emissions can increase by 20dB or more at those frequences, because it is now a ‘tuned accidental antenna’ (see Part 4 of (1)


).


Resonance effects only become significant when the longest heatsink dimension (D) exceeds 30/fMAX (ƒMAX in MHz gives D in metres, ƒMAX in GHz gives D in millimetres). The lowest resonance frequency ƒRES = 150/D, where D is the longest ‘three-dimensional’ diagonal (D in metres gives ƒRES in MHz, D in millimetres gives it in GHz). For example, e.g. a 54mm cube heatsink could have its first (lowest) resonance at around 1.6 GHz and possibly causing problems for GPS reception.


Accurate analysis requires computer- aided simulation, taking into account: (a) the heatsink’s geometry / shape (b) types and locations of the heatsunk semiconductors


(c) the proximity of PCB planes, chassis, enclosures, etc.


(d) any electrical connections from heatsink to the semiconductor’s circuit


Square or cube shaped heatsinks tend to have the highest resonant frequencies, which is good if their lowest resonant frequency, ƒRES, is well above the highest frequency emissions we want to control,


14 September 2024 Figure 1: Heatsink shapes (viewed from above)


RF-bonding heatsinks to reduce emissions


The increased EMC emissions caused by stray return currents flowing in heatsinks can be reduced by careful design of the electrical bonding between the heatsink and the common return rail (which I call the RF Reference Plane) of the circuit powering the heatsunk devices. When heatsinking RF and digital ICs, and – increasingly – SIC and GaN power switching devices, a very low bonding inductance is important, and it can also help ensure the heatsink resonates at frequencies comfortably higher than ƒMAX.


Figure 2: Some sketches of heatsinks


fMAX. Figure 1 summarises these very basic guidelines.


But symmetrical heatsinks have a higher gain, or “Q”, at their ƒRES and can significantly increase emissions if their ƒRES is not well above our ƒMAX. In such cases, we make them rectangular, and avoid simple ratios of length:width:height


Components in Electronics


(such as 1.5, 2 etc.) to reduce their Q. The “Golden Mean” (approximately 1.618) is a good choice when there are no other constraints – and looks nice, too. Where heatsink resonances could lie within (or near to) the frequency range concerned, the best place for the IC or power transistor is generally in the centre


The radiated emissions from a heatsink increases with the bonding network inductance to the power of 3.5 (approx.), so for a transistor running at <1MHz, a single short bond to the closest point on its power rail is usually adequate. But, for higher frequencies, evenly distributed bonds are important, as they can have much lower emissions than if they are poorly distributed but achieve the same overall inductance.


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of the heatsink’s base – usually the best position for good thermal performance too. Edge locations cause greater resonant gain, and so cause higher levels of emissions.


Vertical fins reduce the resonant gain for the direction in which they run, so it is usually best for the fins to run along the longest heatsink dimension. Fins running perpendicular to a resonant dimension increase its resonant gain, which is not desirable, see Figures 2, 3, 4 and 5. Vertical pins increase the resonant gain of all resonant dimensions, which is a pity because “pin” heatsinks are often more efficient than finned types.


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