Hello,
We designed an output 12V/2A (24-watt) POE by using LM5072-80. We encounter a strange problem after change
the MOSFET (Q1 in our design). We have to change MOSFET, Q1 (APT55T10G), to other MOSFET (SPN72T10T252TG)
due to the end of live of the original MOSFET.
By installing a new MOSFET (SPN72T10T252TG) on Q1, the output ripple will be significant (above 500mV) when the POE device
is loaded with some specific range of output current (such as 0.35A ~ 0.38A; 0.5A ~ 0.56A; 0.65A ~0.7A; 0.8A~0.85A).
We use Electronic loader to do the test. The output ripple-voltage looks fine (less than 90-mV) for most loaded output current.
It is, however, for some specific segments of loaded output current, the output ripple is significant. For example, ripple is around 45mV
for loaded 0.4A, yet the ripple voltage will be 500mV when POE output 0.35A. We compare the data sheet of both MOSFET, it seems
that the new MOSFET (SPN72T10T252TG) has better parameters (eg., lower RDS on, Gate capacitor).
Do you have any clue and idea on how to debug it? Is it related to stability issue? The data sheet of the MOSFET and schematic of POE
are attached.
