Dear All,
I've designed a 12V, 1.5A LM5116-based DC-DC converter that needs to operate at Vin = 85V.
During testing, at 85Vin the LM5116 IC fails at currents >800mA. No other devices (MOSFETs etc) have failed yet, just the PMIC.
I measured the switch node at the MOSFETs (see measurement technique in photos), and the ringing does not appear to exceed 100V. But when measuring the switch node at the SW pin of the LM5116, at 600mA, the ringing is starting to exceed 100V. This is what I think is the current cause of failure.
If you look at the oscilloscope screenshot, the green line is the switch node at the MOSFETs, and the white line is the SW pin of the LM5116.
I've tried to design the layout as recommended by TI application notes and online resources, such as:
- Placing input capacitors as close as possible to the MOSFETs.
- Placing input capacitors as close as possible to the VIN pin on the LM5116
- Reducing the main power loop area as much as possible, HS MOSFET -> LS MOSFET -> CIN
- Large copper pours for lower resistance and better thermal performance.
One thing I believe could be improved in the next design revision is minimizing the loop area of the high-side gate drive loop, as the HO gate and SW traces are relatively long (~1.5 inches), and are are only 0.254mm / 0.01" thick.
So, here are my questions:
1) Do you think ringing on the SW pin is what is causing the failures? The peak voltage of the ringing increases as output current increases.
2) Do you think the poor layout of the high-side gate drive loop could be responsible?
3) What else could I try to achieve a higher output current without failure? Snubbers? Extra capacitance?
Attached is my filled out LM5116 quick-start design spreadsheet, and a PDF that contains the schematic, a 3D view of the layout and an image of each layer.
Thank you in advance for taking the time to look at my design.
Any help and advice would be greatly appreciated!
