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Driving (AC) mosfet switch directly from MPU using GDT
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beduino:

--- Quote from: T3sl4co1l on October 27, 2019, 01:56:41 am ---The wires contribute negligible dissipation area.  The transistor assembly is dominant, and can dissipate a couple watts in free air.

--- End quote ---
Mosfet assembly, since mounted ~0.5mm above PCB while drain tap sits on wire connector (3mm inside hole and more than 7mm outer ring) also helps dissipate heat with additional copper layer on opposite side.

However, it is clear when you soldering connectors with copper wire inside a lot of heat is transfered into wire since you have to be carefull to keep connector during soldering in a position to avoid solder flow into the wire.

In this case, where we have not to much heat to dissipate this heat transfer into wire can be not significant.

I think the only way to debunk theory that those wires with connectors on mosfet drains can not be used as heatsinks is to use thermal camera  :-//

Anyway, estimated amount of copper in wires connected to those two mosfet drains in my AC mosfet switch is around 10g copper in wire per each side which is significant thermal mas in comparision to TO220 package taps coper volume, so I'm confident without any doubts that it must be significant heat transfer into those wires  :o
Some IRF840 sellers says it weight 1.5g total vs ~10g copper in wires not included connectors.

It is a pity Simon didn't made experiment using thermal camera if he thinks I'm wrong.
Maybe it could be interesting topic for Dave fundamentals fridays everybody likes?  :bullshit:
 
T3sl4co1l:
Thermal cam just shows heat distribution.  Doesn't show heat flows.  Soldering wires requires a lot of thermal mass, but not nearly so much steady-state heat (not that you'd ever be cooking a soldering joint for minutes at a time, anyway!).

I could equally well turn around your argument by asking how far down the wire you have to hold it, when soldering -- does the heat conduct along the whole length, or just a dm or two? ;)

Which, again, because of thermal mass, is a similarly flawed argument; the heat will indeed conduct further down in the steady state.  But not by much.

In any case, if a thermal cam is used, we can at least estimate the heat dissipation by measuring the surface area of the item in question, assuming an emissivity (mostly irrelevant to heat dissipation at low temperatures, but obviously quite important to the camera) and convectivity (heat dissipation into the air, the dominant sink), and combining everything to get a total power figure.  Hopefully this meshes with the actual electrical power dissipation...

Tim
beduino:

--- Quote from: T3sl4co1l on October 29, 2019, 01:22:37 am ---I could equally well turn around your argument by asking how far down the wire you have to hold it, when soldering -- does the heat conduct along the whole length, or just a dm or two? ;)

--- End quote ---
Soldering 3mm connectors takes a few seconds while clamped over copper wire, so heat conduction along the whole length of a few meters link is not possible at such high speed, but ac mosfet switch in my case creates heat for hours continuously which means the closer the mosfet drains taps, wire temperature will be higher, than at some point depending on disipated heat power closer to room temperature and heat disipated into free air.

Additionally, as I've already read in other posts, those a few meter long wires on each side of ac mosfet switch connected directly to mosfet taps probbaly should help (thanks to its  inductance) limit (di/dt)  ::)

T3sl4co1l:
Heh... heh heh...

A few meters won't limit mains dI/dt in any meaningful way, but mains itself is usually modeled as ballpark 100uH, including the inductance of all the wiring up to the pole/pad transformer (and a bit behind that as well, but because medium voltage distribution is done at a pretty modest impedance, it doesn't account for much inductance; the local wiring dominates).

But that same limit in dI/dt is also what blows up your transistors.  It stores energy at turn-on, and has nowhere to go at turn-off.

In short, if you had enough switching speed or inductance to matter, you'd know it very quickly. ;)

Tim
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