Attiny25/45/85 FW Development Thread

[wight]

@Fritz t. Cat

Cooling for linear mode FET drivers: Is it practical to use metal core printed circuit boards for drivers, as everyone (but the very cheapest) does for the LED?

[…] high frequency spikes […]

[…] low viscosity freon […]

(a) I don’t think so. Getting the heat away from the FET and into the body of the light is the only practical thing to do. I doubt that non-DTP MCPCBs will perform better here and they are expensive and typically single sided (no spring pad!). DTP is much trickier for soldering and manufacturing AFAIK. (b) The spikes shouldn’t occur in linear mode when implemented decently. © Fluorinert?

@Mike C

That’s what my timing is for. As I time how long it takes before the light starts to cool down after the LED is off I can use this time to start lowering output in advance, and also take into consideration the lag time the sensor would expect to see the results of lowering. I’m thinking I’d use the timing when on the table, in still air and in room temperature would be a bench mark as the slowest reaction time to lowering for each host. Reaction time would be faster if holding or mounted on a bike while riding fast. I’m still just testing, but it looks like have come further with it.

So you do your lowering only once the temp reaches the ceiling? Is your lowering visible, like a mode change, or do you slowly fade?

After reading both your existing approach and TK’s I think it’s reasonable to say that a properly tuned PID should be much better than either. That said, PID tuning escapes many people. It’s escaped me so far, but I haven’t made a serious try at it. There should be plenty of demos showing PID responding to interesting situations.

@Sharpie

If you look at the datasheet you’ll see that they are supposed to be interchangeable with standard SO8 (leaded) footprint. But for best thermal path, the drain pads should be extended underneath so the slug on the bottom can be reflow soldered. I think Wight is already doing this on his latest layouts.

Is also quite a large component for leadless SMD, so soldering to the pad on the bottom might help releive any stress on the solder joints due to thermal expansion/contraction. And of course, you can’t just bend a pin up to e.g. add a gate resistor etc.

This is true, apparently the intention was to make high-power parts footprint-compatible with the pre-existing low-power MOSFETs already being packaged in SO8.

I’ve been providing the thermal pad since the first FET driver I showed off. (it also provided a DPAK footprint in the same place! [Certainly this would make locating the Power-SO8 FET during soldering difficult.])