Electronics > Projects, Designs, and Technical Stuff
Modifiying EMC filter in offline SMPS in order to pass conducted EMC scan
(1/3) > >>
ocset:
Hi,
We have done an offline, 240VAC PFC’d Flyback LED driver. We have a slight failure of conducted emissions at 30MHz, as attached. In order to combat this, we wish to use two common mode chokes instead of just one like we have at the moment. That is, we wish to have a 10mH common mode choke, and a 500uH common mode choke in cascade…we believe that the smaller one will help attenuate our 30MHz problem.
The problem is that we currently only have one PCB and the scan session is on Monday. The PCB is very densely populated. So anyway, we cant layout a new PCB in time. …What about if we de-solder the existing common mode choke, and then use twisted pair “flying wires to “jump” over to a bit of stripboard, on which we will mount the two replacement common mode chokes as discussed?
Will this modified setup create so many other EMC problems that it won’t be a valid and worthwhile test?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Another point is that we would like to experiment with different values of diff mode inductors. As such, how good (or bad) a practice would it be to have these diff mode inductors coming out on flying wires so that we can change value quickly and take a fresh conducted mode scan?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Also, suppose we depopulated the entire EMC filter of the offline SMPS….(the section upstream of the mains rectifier bridge), and then made one using the same components, but on a piece of stripboard, and then wired this stripboard into the PCB, (so it again had a filter) how much different would the scan of the top post be, when dont with the "replacement" stripboard filter?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I think you can see what I am getting at here…….the situation of needing to vary the EMC filter components and then re-scan.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Having to make a whole new PCB every time an EMC filter component is changed, and then re-EMC-scanning, is not terribly practical.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Also, I have often seen offline SMPS EMC filters, with 22pF (or so) ceramic capacitors across the diff mode inductors. The purpose of these is to have a component that you can vary in order to reduce particular peaks on the conducted EMC scan. What do you make of this practice?
Also, at one company, they had done a 150W Boost BCM PFC stage, and across the boost inductor, they had a ~100pF ceramic capacitor in series with a ferrite bead. When asked about it, they simply said it failed if that was not there, so they were leaving it in. This was at a very large successful multinational TV company.
SiliconWizard:

--- Quote from: treez on July 27, 2019, 04:22:08 pm ---We have a slight failure of conducted emissions at 30MHz, as attached.

--- End quote ---

Isn't there one at ~70kHz as well?
coppercone2:
well if you have a decent LISN and you have frequency information, perhaps you can just see what the magnitude change at your interest frequency is with your own equipment (as in direction of change) to get an idea of what is happening (despite not having accurate amplitude information). I think so long the LISN is built good, at 30MHz you won't get incorrect delta sign information.

Goes up goes down might be enough to choose components.
MagicSmoker:

--- Quote from: treez on July 27, 2019, 04:22:08 pm ---...We have a slight failure of conducted emissions at 30MHz, as attached. In order to combat this, we wish to use two common mode chokes instead of just one like we have at the moment. That is, we wish to have a 10mH common mode choke, and a 500uH common mode choke in cascade…we believe that the smaller one will help attenuate our 30MHz problem.
...

--- End quote ---

This is the bass ackwards approach: reduce the noise emission first, then tweak the filter components, if necessary. That said, it's tough to prevent stray capacitance from interfering with your inductors and stray inductance from interfering with your capacitors at 30MHz.

Snappy recovery and/or junction capacitance ringing in the secondary rectifier is the what I would look for first, followed by ringing between switch capacitance and transformer leakage inductance occurring at the instant of turn-off. Consequently, the RC damper across the secondary diode or the RCD clamp/snubber across the switch (or, better, transformer primary) should be checked more closely. It may very be that only some component values in these networks need to be adjusted, rather than resorting to a second stage of common mode filtering.



T3sl4co1l:
Put an RC on the switch node.

Tim
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod