550 V, 16 A N-channel MOSFET in TO-220FP full pack
The maximum gate threshold voltage is 5 V at 250 µA drain current, so a 10 V gate signal comfortably overdrives the channel into full enhancement. Gate charge is 31 nC at 10 V — a moderate figure that keeps the gate-drive energy requirement manageable for a standard bootstrap or isolated driver topology.
STMicroelectronics lists the STF18N55M5 as obsolete. The TO-220FP package and 550 V / 16 A rating are common enough that a pin-compatible replacement from the same or another manufacturer may exist, but no official cross-reference is provided here.
Input capacitance and switching speed
Input capacitance (Ciss) is 1260 pF maximum at 100 V drain-source bias. This is a moderate value that keeps the gate-drive switching losses in check for hard-switched topologies up to around 100 kHz. The 550 V rating gives plenty of headroom for universal-input power factor correction stages and for DC bus switching in intermediate-bus converters.