Power Ground is the net that typically represents the negative voltage of the power stage. Since high and "noisy" currents flow in this GND it is not recommended to use it as a reference for digital and analog circuits such as the ones present in the control part of the DC/DC converter. This is why the control part is connected to the "signal ground".
Depending on the converter Signal and Power grounds can be connected or not:
- Isolated converters may have signal and power ground isolated from each other.
- In non isolated converters Signal and Power grounds are typically connected but ONLY IN ONE POINT in the PCB. This is crucial to avoid ground current loops that would worsen the converter performance.
- In low power/low voltage converters Power Ground and signal ground can be the same, typically refereed to a common ground plane.
Ernest