* bad PCB layout- What is the electronic component of the power supply make the emission
Maybe, maybe not. It depends on EMI of the whole power supply.- If i use a power supply with higher capacity, is the emission decreased?
Not necessarily. If you see FM interferences, it's more likely the computer main board, graphic card or display.i known the reason is the switching power supply inside the computer
It is hard to say, it is not as simple as the transformer emitting EMI. Any current that form a loop will emit EMI, just like loop antenna. High speed current spike make it that much worst. PCB layout play a vital part in EMI emitting. Ground plane usually help, but still have to be careful not to have cuts in the wrong place. This is a big subject called signal integrity and they actually have engineers specialized in chasing down EMI. It's all electromagnetics.
If you design your own SMPS, you can be in control and if you know what you are doing, it's really not bad at all. BUT if you buy OEM product, then you are really at the mercy of the vendor. I was the manager of EE for semi-conductor analytical instruments, design a few of those like time of flight mass spectrometer. They used to use rackful of analog power supplies to avoid noise. I went in the completely changed them to SMPS, the rack shrink down to a rack mount box. But careful work on the layout of pcb, we did not have a hint of problem in noise in the system. We ran into problem in CE test and mostly chased down to those OEM SMPS. We designed a lot of HV SMPS ourselves because of the requirements, we used them in the most sensitive places and never have problem.
So, I don't think anyone can really answer your question, it's all on the design. I can only say, the transformer is NOT the only problem....not even the major problem. I would be more worry about the MOSFET driving the transformer, the Source of the MOSFET where a lot of current dumping into the ground. It's all about the layout of the pcb. get a book on Signal Integrity. This is one of the book I used, easy to understand, like reading a story book. It wrote in very simple way, mostly avoid the deep EM theory.
https://www.amazon.com/High-Speed-Digital-Design-Handbook/dp/0133957241/ref=sr_1_2?s=books&ie=UTF8&qid=1529734824&sr=1-2&keywords=howard+johnson+in+books
Thanks for your response
So my question is: What is component of the power supply emitting EMI the most ?
If i use a power supply with higher capacity, is the emission decreased ?
now....that the biggest noise causator in an smps is a fast fet swith on....and fast fet switch off......slow up that fet and you decrease emi....but at the expense of more switching loss.
most of the SMPS I've seen never have a ground plane
Hopefully both assumptions are not true for the majority of SMPS...if the filter cap is far from the driver IC
yes i couldnt agree more, but if you have done all that as much as you can, and still have problems, then slowing up fet switch on is next up on the menu. Certainly slowing the fet up increases switching loss, so yes, its definetely a last resort thing....but often it does come to that i find...As you will know, sometimes the constraints mean you are limited in pcb layers and space etc, and you just cant get the good layout that you want.Making the switch slow and inefficient isn't the best way, in my opinion. As Alan pointed out, EMC issues are caused by a fast signal that goes "the wrong way", due to routing and cabling that isn't EMC-aware . So the EMC problem should better be solved by fixing that current path and introducing filters if needed, not by making an inefficient supply with slow switching times.
Not always......eg think of ceramic capacitors where the higher value ones have higher stray inductance.....If i use a power supply with higher capacity, is the emission decreased ?
Hi,
In detail it is more complicated.
Your gate drive path (red) shows only one situation: charging the C_GS of the Mosfet.
* But another path is gate-source. With it's miller effect.
Then there are the two discharge paths:
* via the GND pin of the driver and source of Mosfet
* via the GND pin of the driver and the source of the Mosfet.
These are four red paths.
And there are transormer primary paths
And transformer secondary paths
You should include in your analysis.
Hopefully both assumptions are not true for the majority of SMPS...
If they are true... they are a good source of EMI emmissions.
Klaus
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