Recently, there has been some discussion about this video by TH3D which claims that the MOSFETs used by BIGTREETECH on their boards are fakes. One of the factors considered was appearance: the MOSFETs used on the boards look different from the ones ordered via LCSC.
I myself had that similar experience with fake MOSFETs which looked different. You can read about it here. But more important than appearance is performance. I reached my conclusion that I had a fake MOSFET not just because of the appearance, but also because the MOSFET was not able to draw the amount of current that it was supposed to be capable of. It was not performing within specifications.
TH3D has also done some "extreme" testing of boards in the past, where they put Chinese boards against their EZBOARD and see how much current can be drawn by the MOSFET on the boards before smoke comes out. Well, surprise surprise, their EZBOARD outperformed the rest. But is that really a good comparison?
First, how much power a board can pull through the mounted MOSFET depends on many things. The thing is, boards are rated for a safe amount of current. For example, the MRR ESPx boards are designed to operate up to 15A, and that limit is not due to the MOSFET. It is due to the screw terminals being used as well as the heat being dissipated by the MOSFET at 15A. So an experiment that draws 30A for a board designed to operate at up to 15A is an experiment that is designed to fail the board. (I know the MRR ESPA can draw 24A without going up in smoke, but that is not what the board is rated for.)
Second, even if a MOSFET is rated for 100A, it does not mean it can safely draw 100A of current just like that. It must be mounted properly on a heatsink that can help dissipate that kind heat. For example, the WSK220N04 MOSFET used on the MRR ESPx as well as BIGTREETECH's SKR boards. It is rated for up to 220A. But with a RDSON of 3.2mohm and a junction to ambient thermal resistance of 62.5 degC/W, you are looking at a temperature rise of 9,680 degC when it is pulling 220A mounted on a 1 sq. inch copper area. There will be a fire if not instant disintegration.
The temperature rise of a MOSFET is governed mainly by its RDSON, thermal resistance and heatsink area. For the same MOSFET, being mounted on a larger copper area will allow heat to dissipate better, which means a lower temperature rise for the same amount of current being drawn. Adding a heatsink has the same effect. It is why those big external MOSFETs have huge heatsinks. It is also why MOSFETs mounted on 3D printer control boards usually have a higher temperature rise, because the boards they are mounted on usually do not have that 1 sq. inch or more of copper area for that MOSFET. The smaller the copper area, the higher the temperature rise.
The other factor is the MOSFET itself. Saying the SKR boards are not as good as the EZBOARD is... comparing an apple to an orange. They use different MOSFETs, so the temperature rise will obviously be different. Just for comparison, let's take the WSK220N04 and the IRLS3036. The former has a RDSON of 3.2mohm at VGS of 10V and junction to ambient thermal resistance of 62.5 degC/W. The latter has RSDON of 2.4mohm at VGS of 10V and junction to ambient thermal resistance of 40 degC/W.
When both are mounted to the same copper area of 1 sq. inch, drawing a current of 30A should result in an approximate temperature rise of 180 degC for the WSK220N04. But the IRLS3036 is expected to have an approximate temperature rise of 86.4 degC. But the price of the IRLS3036 is US$2.66 per unit, while the WSK220N04 costs only US$1.16 (all prices from LCSC for better comparison). Obviously, a more expensive MOSFET is going to outperform a cheaper one. I am not sure what MOSFET is used on the EZBOARD, but given that it is said to be overspec, it is likely to be an expensive MOSFET with a very low RDSON and thermal resistance. Which also means that the EZBOARD will see a much lower temperature rise than the SKR boards at the same current being drawn. But the EZBOARD also costs a few times more.
Okay, I think I have written enough about what I think about this issue. I will not question the intentions behind TH3D for doing those "extreme" tests and making their videos. I am no mind reader. I just hope to add my own view on MOSFET performance based on what I know.
I myself had that similar experience with fake MOSFETs which looked different. You can read about it here. But more important than appearance is performance. I reached my conclusion that I had a fake MOSFET not just because of the appearance, but also because the MOSFET was not able to draw the amount of current that it was supposed to be capable of. It was not performing within specifications.
TH3D has also done some "extreme" testing of boards in the past, where they put Chinese boards against their EZBOARD and see how much current can be drawn by the MOSFET on the boards before smoke comes out. Well, surprise surprise, their EZBOARD outperformed the rest. But is that really a good comparison?
First, how much power a board can pull through the mounted MOSFET depends on many things. The thing is, boards are rated for a safe amount of current. For example, the MRR ESPx boards are designed to operate up to 15A, and that limit is not due to the MOSFET. It is due to the screw terminals being used as well as the heat being dissipated by the MOSFET at 15A. So an experiment that draws 30A for a board designed to operate at up to 15A is an experiment that is designed to fail the board. (I know the MRR ESPA can draw 24A without going up in smoke, but that is not what the board is rated for.)
Second, even if a MOSFET is rated for 100A, it does not mean it can safely draw 100A of current just like that. It must be mounted properly on a heatsink that can help dissipate that kind heat. For example, the WSK220N04 MOSFET used on the MRR ESPx as well as BIGTREETECH's SKR boards. It is rated for up to 220A. But with a RDSON of 3.2mohm and a junction to ambient thermal resistance of 62.5 degC/W, you are looking at a temperature rise of 9,680 degC when it is pulling 220A mounted on a 1 sq. inch copper area. There will be a fire if not instant disintegration.
The temperature rise of a MOSFET is governed mainly by its RDSON, thermal resistance and heatsink area. For the same MOSFET, being mounted on a larger copper area will allow heat to dissipate better, which means a lower temperature rise for the same amount of current being drawn. Adding a heatsink has the same effect. It is why those big external MOSFETs have huge heatsinks. It is also why MOSFETs mounted on 3D printer control boards usually have a higher temperature rise, because the boards they are mounted on usually do not have that 1 sq. inch or more of copper area for that MOSFET. The smaller the copper area, the higher the temperature rise.
The other factor is the MOSFET itself. Saying the SKR boards are not as good as the EZBOARD is... comparing an apple to an orange. They use different MOSFETs, so the temperature rise will obviously be different. Just for comparison, let's take the WSK220N04 and the IRLS3036. The former has a RDSON of 3.2mohm at VGS of 10V and junction to ambient thermal resistance of 62.5 degC/W. The latter has RSDON of 2.4mohm at VGS of 10V and junction to ambient thermal resistance of 40 degC/W.
When both are mounted to the same copper area of 1 sq. inch, drawing a current of 30A should result in an approximate temperature rise of 180 degC for the WSK220N04. But the IRLS3036 is expected to have an approximate temperature rise of 86.4 degC. But the price of the IRLS3036 is US$2.66 per unit, while the WSK220N04 costs only US$1.16 (all prices from LCSC for better comparison). Obviously, a more expensive MOSFET is going to outperform a cheaper one. I am not sure what MOSFET is used on the EZBOARD, but given that it is said to be overspec, it is likely to be an expensive MOSFET with a very low RDSON and thermal resistance. Which also means that the EZBOARD will see a much lower temperature rise than the SKR boards at the same current being drawn. But the EZBOARD also costs a few times more.
Okay, I think I have written enough about what I think about this issue. I will not question the intentions behind TH3D for doing those "extreme" tests and making their videos. I am no mind reader. I just hope to add my own view on MOSFET performance based on what I know.
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