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Buck converter in CCM....changed to faster diode and now EMC is worse.

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treez

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Hello,
We are using a 1W , LNK302 based High voltage buck converter in our offline 230VAC, 150W linear LED driver which uses linear mode led drivers (they successively switch in/out banks of leds to keep efficiency high). The 1W Buck supplies the bias power for the LED current controllers.
So, to summarise, the only SMPS on the 150W PCB is the 1W Buck converter.

From scoping, we know that the Buck converter is operating in Burst mode, and when it is switching, it is in continuous mode.
We recently changed the Buck diode from a US1J (trr=75ns) , to a STTH1R06A (trr = 25ns).

However, since this change, the conducted EMC scan has gotten worse as the attached shows.

Do you think this worsening could have been caused by the faster Buck diode?

The only other changes made were that we are now using a 220nF ceramic capacitor downstream of the mains rectifier bridge, whereas before we were using a 220nF X2 capacitor upstream of the mains rectifier bridge. Also, the other change is that we now have no “quiet node” copper under the switching node of the Buck converter, whereas before we did….so I suppose in the previous PCB there might have been better shielding of the switching node from the earthed heatsink on which the whole PCB sits.


STTH1R06A datasheet:
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00005135.pdf

US1J diode datasheet:
https://www.vishay.com/docs/88768/us1.pdf

LNK302 datasheet:
https://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf
 

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  • STTH1R06A.jpg
    STTH1R06A.jpg
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  • US1J.jpg
    US1J.jpg
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One result, N unknowns wants you to take a step back
and change one thing at a time taking data along the way.

The more abrupt ("faster") any edge is, the worse the
EM emissions. But assigning blame without data would
be premature.
 
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Thanks, i think your suspicion on the faster switching of the 25ns diode is good, ....and i know its a convoluted situation, but, to be honest, i would be amazed if changing from an X2 capacitor to a ceramic had made EMC worse.
Also, the bit of shielding copper that we had between the switching node and the earthed heatsink, also, i would be surprised if that made the initial PCB version so much better. After all, on the updated PCB, the switching node copper is only on the top layer of a 1.6mm thick PCB, so i cant see how the switching node could significantly capacitively couple to the earthed heatsink through the 1.6mm.

Im thinking now about putting a ferrite bead in series with the slower buck diode to give even better EMC performance...ot at least a series resistor, which will give some RC damping with the diodes stray capacitance.....might even use a series resistor and a 100p capacitor across the buck diode, for even better EMC performance....what d'you reckon.

Our product doesnt have a normal AC line filter, because our arena of sales is so cost sensitive...we just have a SMD CLC filter upstream of the buck to filter it that bit.

- - - Updated - - -

The attached shows the plan for slowing up the dv/dt of the LNK302 Buck....what do you think?
Also attached LTspice sim to "demo" it.
 

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  • LNK302 buck  with added RC.jpg
    LNK302 buck with added RC.jpg
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  • LNK302 buck with added RC.txt
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Last edited by a moderator:

Ferrite might at least give you a sensitivity / response
clue.

A question is, whether the emissions are current mode
(magnetic) or voltage mode sourced. For example a
fast diode with low series resistance will keep the
voltage overshoot down, but maximize current, and
the low series R increases the Q of "tanks" that involve
the diode. You may be seeing "tone bursts" that come
from ringing, as your EMI bad boy. Looking at the
board with a "sniffer" on the spectrum analyzer might
help localize the source, and using a "sniffer" that
does not change node C, with a 'scope, looking for
the same frequency (maybe not continuous, but look
for damped ringing cycles that just match) could put
you close to the source (process of elimination).

Here's a full bandwidth 'scope grab from one of my
CMOS POL DC-DC designs, a hard switching buck
with an EVK that used the lowest-ESR caps we could
find (and plenty of 'em). Note the harsh HF on the
magenta trace which is VIN. There's probably an ideal
Q for the input filter and importantly the VIN-GND
decoupling (a prime "tank" recirculating GND perturbations
back to VIN).

LateSync1b_twitch.png

With caps, and I suppose diodes, "more ideal" may
not be better for EMI. Efficiency, maybe.
 

A question is, whether the emissions are current mode
(magnetic) or voltage mode sourced.
Agreed. Also you haven't revealed what specific test those plots are for (common mode, differential mode, one mains leg, etc).
 
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Thanks, the plots are common mode and diff mode together...one mains leg only. (but live and neutral were the same in each case)
 

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