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| Lowering SMPS dissipation |
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| xavier60:
If Q20 is moved to the large heatsink, the capacitive coupling between the tab and the heatsink can be a great cause of EMI. Those thick aluminum oxide washers would help a lot. I would use a SIHP15N60E with another 10Ω at the Gate pin. https://uk.farnell.com/aavid-thermalloy/4171g/insulator-alum-oxide-ceramic-to/dp/2787815 When Q20 fails, how much damage is caused? With power supplies where the shunt resistors blow open, it makes a real mess. |
| spec:
--- Quote from: xavier60 on December 02, 2018, 03:55:22 am ---If Q20 is moved to the large heatsink, the capacitive coupling between the tab and the heatsink can be a great cause of EMI. Those thick aluminum oxide washers would help a lot. I would use a SIHP15N60E with another 10Ω at the Gate pin. https://uk.farnell.com/aavid-thermalloy/4171g/insulator-alum-oxide-ceramic-to/dp/2787815 When Q20 fails, how much damage is caused? With power supplies where the shunt resistors blow open, it makes a real mess. --- End quote --- I did not say that Q20 should be moved to a larger heatsink if that is what you are implying. Q20 can be kept pretty much where it is but the heat sinking can still be radically improved, I would suggest. But even so, I doubt that the situation is that critical. On the subject of inductance etc, it would be useful to establish what trace lengths, and thus inductance etc exist on the board as it is now. |
| xavier60:
The op mentioned "there is a nice big heatsink already on the board" earlier. It has some risk. |
| spec:
--- Quote from: xavier60 on December 02, 2018, 07:29:51 am ---The op mentioned "there is a nice big heatsink already on the board" earlier. It has some risk. --- End quote --- I see! One approach to make use of the large heat sink, without significantly moving Q20, is to fabricate an aluminum bridge between Q20's existing heatsink, which to put it bluntly is a joke, and the large heatsink. But rather than that, I would think that it would be best to get rid of Q20's existing heat sink and either buy or fabricate a completely new, but far lower thermal resistance heatsink. A bigger standard heat sink may fit- I haven't checked- or a standard heatsink modified may do. The revised heatsink may or may not bridge to the bigger heatsink. It would almost certainly need to be supported by the large heatsink. But in any case, I see this as a simple, low-cost approach to improve Q20 cooling, with minimal technical risk. Of course, a better NMOSFET can be explored as a parallel activity, but, as you illustrate, it is a complex task. I have done a bit of work on driving MOSFETs and EMC certification, but not to the extent that you obviously have, and I find your detailed explanations, not just on this thread, quite interesting and helpful. It is a difficult area which crops up often and does not seem to be well understood. The opposing requirements of MOSFET fast turn on/off and EMC further complicate the issue. Have you ever thought about doing a simple tutorial for EEV? :) |
| xavier60:
I think that the SiHP15N60E should be used also. If more EMI is noticed, the Gate resistor value can be increased. |
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