Re: monitoring heatsink temperatures

legg wrote:

Grounding of heatsinks, directly or at RF frequencies, is something
that will depend on how your thermal, mechanical, EMC and safety
schemes are intended to inter-relate. Heatsinks for HF noise sources
should have a well-defined RF noise return, to reduce the area of
vertically polarized planes and horizontally polarized loops. This can
be established without direct connections, using safety-rated
components, if necessary.

Hi,

The heatsink is electrically insulated from the HF fets, will it still
be a HF noise source because of the AC currents through the fet cases?

I don't understand what you mean regarding the RF noise return, the
heatsinks are extruded aluminum, similar but larger to those in an ATX
PC computer power supply. Can you elaborate on the method of reducing
the area of the current loops in the heatsinks? I guess this is
primarily to meet RF emission specs?



The larger or more physically unstable the heatsink, the harder it
will be to avoid a physical interface to safety-grounded mechanical
supports.

Although you are not responding to other posters' suggestions re diode
junction sensors, these and other indicators ( passive PTC/NTC or
thermo-mechanical) are available in TO220 or single-screw packages at
a price. An empty TO220 body has a price. FPGA port occupancy and
interfacing have a price.

If your assembly uses a mixed technology of SMD and PTH components,
surface mount semiconductors or passive sensors can also be thermally
coupled to heatsink mounting hardware directly, or through an FR4
layer in multilayer PWBs.

Properly selected non-linear passive sensors can be interfaced to the
digital inputs of an FPGA or a dedicated local control circuit more
simply than a 'linear' sensor, with no apparent reduction in
performance. Simple controllers that operate independently of
fail-sensing circuitry can actually offer better options for system
survival and recoverability under single-fault abnormal conditions,
regardless of the absence of accurate spot temperature reports to 'n'
decimal places. Don't lose sight of your initial intended aims.


cheers,
Jamie


RL
.

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