Schematic Symbol for a Heat Sink

[sdscotto]

I've looked all over and can't find an official or standard schematic symbol for a heatsink.

Anyone have a favorite they can point me towards?

Thanks in advance,
scotto

[Uncle Vernon]

A heatsink generally wont be shown in a schematic, it plays no part electrically. If you wish to indicate additional information you could use something like the hot surface symbol.

[sdscotto]

A heatsink generally wont be shown in a schematic, it plays no part electrically.

I understand that generally it wouldn't be included, which is probably why a symbol was difficult to find.  My heatsink does have two through-hole pads that will be tied to ground, which is why I wanted to add it to the schematic.

If you wish to indicate additional information you could use something like the hot surface symbol.

An excellent idea, thank you!

[Alex]

The most common I have used and seen is a dashed line following the contour of a standard schematic symbol.

You can place an asterisk next to the symbol and as a footnote have something like: Power dissipation x, mounted on heatsink y, thermal resistance z, see mechanical BOM. You should also indicate if the heatsink is live or too hot to touch.

[Zero999]

I think it's a good idea to draw a heat sink and will do so in future.

[Neilm]

Firstly - why do you want to connect it to ground? Are you sure that this will not cause any EMC issues (if this is a commercial product).

Generally what I do with heatsinks is shown the mounting tab holes on the schematic and just draw a note that this is the heatsink.

Neil

[Zero999]

The heatsink should be connected to ground, if the circuit is connected to directly to the mains (i.e. not via an isolated SMPS or transformer) and it's possible the user could come into contact with it. I don't see how grounding the heat sink could cause EMC issues.

[Neilm]

You can get switching noise being picked up and transmitted by a heatsink. This can happen if the conenctions to ground are not good - or if your switching frequency is about the frequency that the HS resonates at. This will of course depend on the shape of the HS and how well bonded it is to the device.

Also - there is no legal reason for the heatsink to be connect to ground. When it is in a product, the product design should prevent the user from touching it - if it is not in a product then it can. Commerial SMPS quite often have parts that can be touched.

Yours

Neil

[Zero999]

Also - there is no legal reason for the heatsink to be connect to ground. When it is in a product, the product design should prevent the user from touching it - if it is not in a product then it can. Commerial SMPS quite often have parts that can be touched.

Where is there a requirement for the user to be prevented from touching the heat sink?

This is only necessary if the heat sink maybe shock or a burn hazard. There is no reason why there shouldn't be a vent in the case with holes large enough for a little finger, as long as there's no chance of the user coming into contact with hot or live parts. Earthing the heat sink will help to protect against shock and proper thermal design will protect against burns.

[Alex]

You can get switching noise being picked up and transmitted by a heatsink. This can happen if the conenctions to ground are not good - or if your switching frequency is about the frequency that the HS resonates at. This will of course depend on the shape of the HS and how well bonded it is to the device.

Time-varying signals or power rails capacitively couple from the device being cooled to the heatsink. A floating heatsink forms a dipole antenna in combination with the power planes of the PCB hence radiating emissions directly or after resonance depending on heatsink geometry and frequencies involved.

From an EMC viewpoint, connecting the heatsink to ground would indeed be best in some cases (read on). A more general best practice is to connect the heatsink to the rail or signal that is carrying the fluctuating signal in the first place. By doing this the parasitic current that would otherwise flow is eliminated (minimised). The connection method of the heatsink to this fluctuating voltage must be of a low impedance at the frequencies and harmonics involved (short, thick, isolated).

In the case where the heatsink cannot be connected to the fluctuating voltage source (e.g no thermally-conductive pads used), a suitably rated capacitor can be used to couple the heatsink and fluctuating voltage at the frequencies involved.

I hope this was useful and it clarified some thoughts and suspicions. I am no expert on this, so for more info you can have a look at "Minimising EMI from Heatsinks" by N.J.Ryan page 52-56 in the 2000 Annual Compliance Engineering Reference Guide. It is available online here: http://www.ce-mag.com/ARG/Ryan.html

[sdscotto]

I'll let everyone else debate the desirability or usefulness of having a schematic symbol for, and/or connecting to signals.

Thanks to Uncle Vernon's suggestion I am now using the following visuals for schematic and PCB layout.