We are designing a 13V-30Vin / 12Vout buck converter using LM3485Q1MM/NOPB IC. In the first revision of our design, the converter was working with no issues except for the PMOS heating too much for our requirements (~60C temp rise).
A second design was made with the only change being the amount of copper/vias for better heat dissipation (i.e. very minor layout modification and same schematics) around the PMOS (See PCB revisions at the end of message). However, the Buck Converter is not working as well as the first revision. When using the same components, a high frequency signal is present in the off state of the buck converter. While the buck is still able to maintain output voltage, it is less stable (especially without a load) and the controller and the PMOS are heating a lot (~120C temp rise) because of the high frequency switching (see VGS signal and VGS high frequency signal in attachment).
VGS -> On and Off state:
VGS -> Off state noise:
We tried a lot of fixes:
- Disable overcurrent
- Increase/Decrease gate resistor
- Increase/Decrease inductor value
- Increase/Decrease output capacitance
The only way, we have been able to remove the high frequency switching in the off state/have stability with no load is to increase significantly the value of C16 from 100pF to a value higher than 2.2nF. However, it also increase the working frequency of the converter which increases the switching losses.
To be noted, the Stackup of the first revision is :
- (Top/Bottom) 2Oz / (Inner) 1Oz
- PCB printed with Elecrow PCB
The Stackup for the second revision is :
- (Top/Bottom) 2Oz / (Inner) 0.5Oz
- PCB printed with JLCPCB