Ok, I did some reading and I don't understand the last number on each row, is it wattage?
Why wouldn't I want power dissipation?Alan B wrote: ↑Aug 02, 2018 7:34 pmThose are just some summary numbers from the datasheets, look at the full datasheets if you want more data. Those are just some examples of FETs that have been discussed or are being used. Power Dissipation is not what you want to be doing with your FETs, anyway. That's mostly an indication of the thermal path capacity.
So what is the benefit of the better mosfets if it only reduces heat by a little?fechter wrote: ↑Aug 03, 2018 8:34 amOf course things are always a little more complex in real life and the numbers will only get you so far. Another thing that has to be considered is the gate charge. If the gate charge is too large, it will slow down the switching speed but it also depends on the gate driver used. If the proposed replacement part has a lower gate charge or at least is pretty close, it will be fine.
Switching losses and resistance losses both contribute to heating. Using a lower Rds reduces the resistance losses. Slower switching will likely increase switching losses. If you found a part with 1/2 the Rds, the heating would be less but not 1/2 due to switching losses.
The other critical factor is heat dissipation. If the controller has adequate cooling, you can run it way harder. Having a higher than needed voltage rating will make it more immune to voltage spikes. Water cooling might be overkill, but you can also look at improving passive air cooling (better air flow, better heat sink fins, conduction to frame, etc.)
The AOT290L has a proven track record, but would only reduce the heating by a little.
but 12 to 18 fets is 50% gainAlan B wrote: ↑Aug 04, 2018 1:54 pmFET IR heating would be reduced by (3.5-2.7)/3.5 = 23%, and the dissipation rating of the AOT290L is higher so it can handle more heat, or the same heat with more thermal margin. The PhaseRunner uses AOT290L's, which is one reason the little 6 FET controller can handle as much power as it does while being very small in size.
FET IR heating may be the major source of heat in the controller. The other is switching losses which are harder to estimate, but should be less in a well designed controller. If IR losses weren't important in the controller, they would not bother to parallel FETs. Paralleling FETs reduces IR losses but not switching losses (providing the FETs are well driven).
Changing from 4110's to 290L's would be like adding 23% more parallel FETs to the controller. Since parallel FETs don't share current perfectly, this is about as much improvement as going from 12 to 18 FETs, but without the increase in size.
oh ok got it, thanks alotAlan B wrote: ↑Aug 04, 2018 7:06 pmNo, it is not a full 50% increase, because the FETs don't share current equally, so the designer cannot increase current that much. He is only allowed to take about half of the new FETs because of unequal current division. Now some hotrodders ignore this, and they often blow up controllers. Designing for more current and power is not quite that simple.