460 mV forward drop at 20 A — the efficiency floor
The STMicroelectronics FERD40U45CG-TR is a dual common-cathode Field Effect Rectifier Diode (FERD) rated 45 V reverse and 20 A average rectified current per diode. The 460 mV maximum forward voltage at 20 A sets the conduction loss per leg — at full load each diode dissipates about 9.2 W, so the D2PAK tab must be soldered to a sufficient copper area to keep the junction below the 175°C absolute maximum. The FERD structure delivers a lower Vf than a conventional fast-recovery diode of similar voltage rating, but the reverse leakage at 45 V is 1.8 mA — higher than a Schottky of comparable current. This is the trade-off: lower forward loss at the expense of higher off-state leakage, which matters most in lightly loaded or high-temperature standby conditions.
Common-cathode pair in D2PAK — layout and thermal routing
The two diodes share a common cathode on the tab, so the D2PAK footprint connects the output rail directly to the large copper pour — the tab is the thermal path. The TO-263-3 (D²Pak) body has two leads plus the tab; the anode pins are on the outer leads. The 0.50 mm pitch of the lead frame is standard for D2PAK, and the package is compatible with standard reflow profiles for a surface-mount assembly. The 45 V reverse rating gives headroom for 24 V rails with transients, but not for 48 V telecom buses — that needs a higher Vr part.