TPS54560B - Problem Ringing&Jiggling at primary switching nodes, harmonics etc.

[Houseman]

Hi all.
In order to have from a 5-12V input voltage a dual supply +/- 30V output I have built around TI's TPS54560B an inverting buck/boost converter following suggested PMP22343 reference design here
https://www.ti.com/tool/PMP22343 and trying to obey all the specs rules shown in the attached PDF's, however I get a very noisy output but what is worst the waveform is oscillating in a sort of manner and the output is full of harmonics of the main 150kHz switching frequency.
Here attached the schematic:

I have tried to use same layout with 4 layer ground planes with sufficient vias

however during tests (used a LOAD to have 0,125mA output current as suggested by the TIDT158.PDF test reports provided here: https://www.ti.com/lit/pdf/tidt158)  I got following problems:
•   Comparing to the test output voltage ripple is higher due to spikes on the positive and negative waveform at the output caps
VDD:

VSS: even worser


•   The switching frequency is constantly oscillating between 149 and 151 kHz.

•   Primary switching node at D1 shows some sort of ringing (?) (input is +12V)


 
•   Same for Primary Switching Node D2


 
•   But what is worse is that the waveform observed ad D1 and D2 are constantly going back and forth in an oscillating manner. Here a magnification of the waveform:

Magnification 1: at 300mA current it get worse


 
Magnification 2:

 
•   Finally the spectrum analysis of the output voltage of both VDD and VSS rails shows a huge harmonics of the 150kHz switching frequency:
VDD:



VSS:



Could You please help me find out what is going on?
best regards and thanks.
Steve

[georges80]

How are you probing the signals? In particular, how and where and how (with a lead or ?) are you connecting the ground of the scope probe to the circuit?

What I'm leading to is whether what you see is 'real' (at least the magnitude) or a measurement artifact causing by your probing setup.

cheers,
george.

[mtwieg]

I think most of what you're observing can be explained fairly easily:

Those "spikes" on VDD are likely due to inductive pickup by your probe, and depend on where exactly you connect the probe (both GND and tip) to the circuit, and how long those connections are.

The shape of the ripple waveforms on VDD and VSS should be roughly the same (assuming their load current is the same and they have similar bypass capacitors). Not sure if their different appearance is a sign of trouble, or the result of how you're testing them.

The "ringing" on the switching nodes is likely due to operating in discontinuous current mode (DCM). Such behavior is expected in discontinuous current mode, and generally isn't considered a problem.

As for the scope capture with lots of jitter, it looks like the underlying waveform is sometimes in DCM and sometimes in CCM. So there may be some underlying instability. Hard to tell what the cause is with that waveform alone. But then your following waveform seems completely stable? Can you try measuring the COMP pin waveform (relative to the TPS54560 GND pin)?

As for the harmonics on the output, I'm not sure what you were expecting. Both VDD and VSS are expected to be rich in harmonics.

[Houseman]

Thank You both for the reply. All measures I was performing are all made with tip and barrel method like the picture:


What I am thinking is maybe the load was connected with 10cm twisted wire. could this be the cause?
What can I do to kill all the harmonics that affect the signal?
Best regards