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| Electronic load control feedback loop - Use ACS712? |
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| SpottedDick:
--- Quote ---Paralleling MOSFETs in linear mode is known to not work well. --- End quote --- "Work well" is the issue here. First of all, these calculations where done well over a year ago. I just recently got started on this project. I didn't keep good notes. What follows is a mix of memory and some emails I found that hint at where the figures come from. I'm trying to remember exactly what way I calculated this, but emails back and forward between myself and an associate indicate we estimated around a ~20% load discrepancy between the MOSFETs when raw dog paralleled. We worked out that 3 MOSFETs of this type (can't find the part number now and they're soldered to a board) when properly controlled individually would give us 600W at 24V, and the limit on that was more the thermal constraints, which would only fully come out during thermal testing. We choose cheap MOSFETs as at the time component prices where high, and these older MOSFETs cost something like a fifth of the newer MOSFETs, that could handle higher current individually, but you still end up with the issue of getting that massive amount of heat out of a small case. 6 Cheaper older MOSFETs only handling about 70W load each, would: 1: Put us so far down on the SOA and °C/W curves that they should run relatively well. 2: Imbalances between MOSFETs don't matter too much as from point 1, we have tons of headroom. 3: 6 MOSFETs in this case style fit perfectly on easily obtainable Intel CPU coolers. 4: We now have tons of thermal area to get the heat out. I'm nearly sure I calculated with the right cooling (read: Huge radiator) and controlling them individually to balance them, we could do 1.5KW. I'm not saying this is the *right* way to do this. If I was building this for a contract, it would be individually controlled and not doing things like using CPU cooler mounts or using fucking Arduinos. However, as an open source "hey, you can build and maintain this yourself using cheap off the shelf parts!" project, I like the route I'm going with it. If it fails I'll learn something :) EDIT: Oh, and something I haven't mentioned before since I wanted to do more testing on it first, but since I've gone this far with the description I might as well. I have run this on long term 15A loads, and the difference between the spots marked orange and purple on this image was only 3°C. The MOSFETs are sandwiched between a PCB and a copper heat-sink plate with good thermal paste. Also, orange was on the outside of the case, Purple was on the top beside the actual heat sink :) |
| T3sl4co1l:
The correct way to do this, is to buy extra say 2-4x as many as needed, test Vgs(th) on every single one of them, and sort them. Select the set of 6 closest matching parts. Probably, gm needs to be matched as well; perhaps a 2-point test, doing Vgs(th) at say 1mA and 1A. Which will further narrow the pool. This is far more effort than just designing it right the first time, so it's never done this way -- except when absolutely required, which these days basically means repair of poorly-designed hi-fi audio amps. They still won't track well in operation, but well enough that capacity won't be degraded much, say -20% so you only need one extra to handle the total. A similar situation is had for paralleling diodes, or IGBTs (in switching duty) as their Vce(sat) curve is diode-like as well. At least unhooking the parallel source resistors, so they act independently to degenerate each individual transistor, and then taking the average over them by running a feedback resistor to each, helps reduce gm at high currents. Tim |
| Kleinstein:
The thermal image shows rather poor current sharing. Chances are the lower left MOSFET takes about 50% of the load, the upper middle one maybe 40% and the residual 10% is shared. It is hard to tell how much of the heating on the colder ones if just by thermal conduction from the neighbors. A circuit made to be duplicated by also less experianced persons should be reliable and not one of the maybe work if you are lucky. Trying to save a few cents for the OPs is nor really worth it - it is way to prone to failure. Even if it works in a few occasions, the failure rate will be very high. The current sharing may work at low voltage (e.g. 6 V) and it can work OK below 2 V, but the chances for failure go up with voltage. 70 W from a cheap MOSFET (like TO220) sounds overly optimistic and water cooling. |
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