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Oven Performance Shows Flip Side of Thermal Management

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Re: Counterintuitive design requirements
Bill_Jaffa   10/2/2014 12:31:22 PM
These are great examples--thanks!

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Counterintuitive design requirements
GSKrasle   10/2/2014 12:29:18 PM
Some somewhat counterintuitive issues: 1) Burn-in fixture for power modules: both the modules and their load resistors mounted to a big heat-sink, with a PID controller and fan to regulate it at the target temperature (50C, I believe). Unfortunately, natural convection was too much, so I had to INSULATE the heat-sink! 2) Heat sink for a power-device that infrequently would briefly burn a lot of power. It was more important to suck the heat out of the case (low θc-HS) than to get it out to ambient. An unfinned “block” was a better choice than a finned one in the tight quarters; thermal mass (~capacitance) more important than thermal conductivity to ambient (~resistance). 3) Circuit in an oven for thermal stability: it has to be heated to considerably higher than the anticipated maximum ambient temperature, despite hit to reliability/longevity, or what’s the point? 4) Li-Ion batteries have a continuous degradation process that is not a function of cycles, but of time and temperature and state-of-charge, with a minimum at ~50% SOC, where the sensitivity to temperature is also minimal; in production, products should be stored and shipped at 50% not 100%. 5) A mundane example: a clean cast-iron pan is not as useful as a dirty (“seasoned”) one. Keep that scouring pad out of my kitchen! 6) Yes, switching regulators are more efficient than linear ones MOST OF THE TIME, but not always. If the quiescent power (for running internal clocks and such) is comparable to the needed delivery of power, a linear regulator is often a better choice. This comes-up with products that have a very low-power (but >0) “standby” mode, where it can be optimal to have, say, a tiny LDO in parallel with the big main regulator. 7) In a load-circuit, it is sometimes useful to dissipate power in the Vfs of a chain of big diodes instead of resistors. Heat-sinkable big diodes are scarce and expensive, but FETs are not. But I don’t want to give-away ALL my tricks!

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boblespam   10/2/2014 8:14:28 AM
Last time I was working with myself on an automotive design. On this design we had 12V High Side outputs that had to be diagnosed for Open Circuit, even while the output is OFF. For example: detect if some kind of safety actuator is still connected to the output, whithout activating it.

To do that, I had to generate a leakage current and detect if it's flowing or not through the output to see if the circuit is open or not.

But the customer also wanted that in sleep mode, the ECU draws only a few µAmps ! So on the same ECU I found Myself fighting against leakage currents on one side and deliberately generating some leakage on the other side, just like a pyromaniac fireman !

Now we have Smart High Side drivers that take care of diagnosing the Open Circuit failure while OFF (like Freescale MC35XS3400), so I don't have to design the leakage generator myself... and me (not the maniac one, the other) feels much better :-)

Are ESD Simulation Guns protected against ESD discharges ? Those who design these things must be totally schizophrenic... not like me of course !


Aargh, no more pill in the little white box... where is my ESD gun, me needs to zap some colleague !

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