pcb design - Why does this buck boost with dual MOSFETs fails output at certain input voltages at light load?

A buck boost converter for 11A 45V output and 33.0-55.0V input has been designed using the Webench Power Designer. The LM5176 based PCB design was adapted for dual MOSFETs (4 times 2 pieces) having the TI evaluation board SNVU547 in mind.

Wrong output at light loads

I measured at different input voltages in steps of 1.00V difference.

3.00A output = OK

No output errors were found at steps of 1.00V. Should I repeat with smaller steps?

1.0A output, output error at 46.00V input

It light loads of 1.00A the output is around 15 volts instead of 45 volts.

0.10A output, output error at 42.00, 43.00, 45.00 and 46.00V input

At even lighter loads of 0.100A the output voltage is around 32-40 VDC for 42, 43, 45, and 46 VDC input tests.

As soon as there is an issue, I do hear a "ticking" sound.

Single MOSFET = OK

I also have the same PCB populated with single (4 times 1) MOSFETs. That board has no issues. I even stepped that board in 0.1V steps over the full range and in 0.01V steps over the 46 to 42 VDC range without spotting any output issues at light 0.100A load.

What is the issue with the (design of the) dual MOSFET board?

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Schematic

Evaluation board

Prototype

The prototype is a 6-layer 4Oz printed circuit, where board top and bottom sides of the dual MOSFET version do look like this:

BOM

• Rvisns 1.98kΩ ERJ-6ENF1961V


• Cboot1,Cboot2 100nF ESR=1mΩ C0603C104M3VACTU



• Cbulk 68uF ESR=320mΩ EEV-FK1K680Q


• Ccomp 39nF C0603C393K5RACTU


• Ccomp2 750pF C0603C751J5GACTU


• Ccs 47pF ESR=147mΩ 06035A470JAT2A


• Cf 100nF 06035D104KAT2A


• Cin1, Cin2 15uF ESR=40mΩ 100V 100SXV15M


• Cin3…Cin14, Cout1…Cout15 4.7uF ESR=4.236mΩ 100V CGA6M3X7S2A475K200AB


• Cout16…Cout18 33uF ESR=25mΩ 63V 63SXV33M


• Cslope 1.2nF CL10C122JB8NNNC


• Css 22nF C0G GCM21B5C1H223JA16L



• Cvcc 1uF GRM188R61C105KA93D


• Dboot1 10A 0.91V MBRD10200CT-13


• Dboot2 20A 100V 0.455V STPS20M100SG-TR


• Df 100V 1.0A 1.05V STPS1H100A


• L1 10uH SER2915H-103KL


• M1,M3 80V BSC037N08NS5ATMA1


• M2 150V 50A BSC190N15NS3GATMA1


• M4 100V 5.3mΩ CSD19531Q5A


• Rcomp 4.64kΩ ERJP03F4641V


• Rcsg,Rcsp 100Ω



• Rf 10Ω


• Rfbb 20kΩ


• Rfbt 1.1MΩ


• Rpg 10kΩ


• Rt 86.6kΩ


• Ruvb 9.31kΩ


• Ruvt 249kΩ


• Rmode 93.1kΩ


• Rsense1 62mΩ // Rsense2 5.1mΩ -> Rsense 4.7112mΩ