LMZ1420xH High Frequency Radiated Emissions

Hi Emanuel,

We're looking into this right now and will have a reply for you soon.

Thanks,

Anston

Thanks, I can send the schematics if needed and the plots

Yes please send schematics, layout and plots.

Here is the circuit and PCB layout

A few explanations skipped:

C15 is a tantalum capacitor.

This is the second revision of the circuit, we use the LMZ series in many applications with similar setup and layout, never had such issues.

This circuit requires an extremely low standby current and the load can go down to 10mA.

As mentioned, we had conducted emissions issues in the low load and took certain measures to avoid this, among other things the addition of the common mode choke, low frequency ferrite beads and increased increased capacitors.

The old setup used the LC filter recommended but if compared to the one above used only C6, C7, C9,C10,C11 and I1.

The evident 70MHz peak close to the allowed limit showed also there, barely passing the test but the CE wad bad.

The CE is the reason we keep the frequency around 330KHz.

These were the results of the old version:

The 68MHz is there and so another peak at about 330MHz. There is a raise also at 150MHz, quite small but needs attention, see below.

The new version passes quite well the CE test, but still gives the peaks now at 150MHz and 300MHz.

The customer requires testing the operation of 2 units (each in its own housing, totally separated)

When testing 2 units everything is amplified and fails. Here we can see the 70MHz back and big, at 150MHz completely fails and at 300MHz avery big peak.

Quite strange behavior, we never expected noise in this range and in fact based on many projects with the LMZ series we newer observed such high frequency noise. In fact we were very happy with the success we had filtering the clock from the CE tests almost completely.

I will really appreciate your assistance.

Can you set the frequency of one of the two converters to a slightly different frequency?  If the two frequencies are set at least 100kHz apart, they will not add together in quasi-peak mode in the spectrum analyzer.

Hi Joe,

The converters are in individual devices, it is not that they are on the same board or even in the same box.
As this is a product manufactured in quantities, there is no way to control which pair will be used in the same setup.
The other thing is that the peak at 300MHz is there at the same amplitude with one or two devices, the one that accumulates when two are connected is the 150MHz.
Same with the 70MHz peak which I think is the source of everything.
What I don't understand is how such high frequencies noises result from a 300KHz clock.... black magic indeed.
I can say for sure that there is nothing else in the circuit that might be the potential source.

Emanuel

Understood. I think what your seeing is the amplification of very high-order harmonics of your switching frequency. This can be due to a resonant circuit formed by the parasitic components of the low-frequency filters. Spice modeling could be useful but time-consuming. I noticed the return path for the capacitors is on the top layer, this forms a loop that can make a good antenna. Is there a ground plane below the top layer?

Like this:

The PCB is a simple double sided type, the bottom is almost in full a ground plane, can see the part under the filter and LMZ in my layout drawing above, the blue drawing in drawing.
I use this setup in a lot of designs with the same input filter (simplified) and same layout, the difference in this circuit is the inductor, I used a smaller package type and more capacitors than usual. The radiated emissions are going out through the cables, I am almost sure about his, the plots shown are with the antenna in horizontal polarization, in vertical they are below the limits.
So you think this is a layout problem? Not some resonant frequency of one of the filters?