IRF4905PBF

The IRF4905PBF from Infineon Technologies (formerly International Rectifier) is a -55V P-channel power MOSFET with 20mΩ on-resistance and -74A drain current. As a P-channel device, it excels at high-side switching: the source connects to the positive supply rail, and pulling the gate LOW (below the source by at least 10V) turns the MOSFET ON. This eliminates the need for a bootstrap charge pump that N-channel high-side drivers require, simplifying the gate drive circuit significantly. The 20mΩ RDS(on) is impressive for a P-channel MOSFET (P-channel typically has 2-3× the RDS(on) of an equivalent N-channel due to lower hole mobility), but it’s still 2.5× the 8mΩ of the N-channel IRF3205. At 20A load, the conduction loss is P = I²×RDS(on) = 400×0.02 = 8W — manageable with a moderate heatsink. The IRF4905 is the P-channel complement commonly paired with the IRF3205 (N-channel) in half-bridge and H-bridge motor drive circuits. The PBF suffix denotes the lead-free TO-220 package.

The IRF4905PBF is a P-channel enhancement-mode MOSFET. Current flows from source to drain (conventional current) when the gate is pulled LOW relative to the source by at least |VGS(th)| (2-4V). For minimum RDS(on) of 20mΩ, VGS must be at least -10V (i.e., gate is 10V below the source). For a high-side switch with source at +12V: the gate must be pulled to +2V or lower (12V − 10V = 2V) for full enhancement. A simple NPN transistor or N-channel MOSFET can pull the gate to GND (VGS = -12V, well beyond the -10V requirement). To turn OFF, the gate must be pulled up to near the source voltage (VGS ≈ 0V). A pull-up resistor (10kΩ) from gate to source ensures the MOSFET turns off when the NPN driver is off. Important: VGS must never exceed ±20V. In a 24V system, pulling the gate to GND gives VGS = -24V, exceeding the absolute maximum. A Zener clamp (15V Zener between gate and source) is required. The body diode points from source to drain (anode at source), so it conducts when drain voltage exceeds source voltage. In an H-bridge, the body diode serves as a freewheeling path. The 89ns reverse recovery time of the body diode is moderate — adequate for frequencies below 50kHz.

IRF4905PBF pin assignment (TO-220-3, front view, pins down): Pin 1 (left) = Gate, Pin 2 (center) = Drain/Tab, Pin 3 (right) = Source. This is a P-channel MOSFET — Gate must be pulled below Source to turn ON. Common application: Source to VCC, Drain to load, Gate to GND for ON and Gate to VCC for OFF. The tab is connected to Drain. Complementary N-channel: IRF3205 (55V/110A). For surface-mount, use IRF4905SPBF (D2PAK).

• High-Side Switch (12V): Source→+12V; Gate→NPN collector (emitter GND); 10kΩ pull-up gate-to-source; Load between drain and GND; MCU→NPN base→gate LOW→MOSFET ON
• Battery Disconnect: Source→Battery+; Drain→System VCC; Gate pulled LOW by control circuit = battery connected; Gate released = battery disconnected
• Reverse Polarity Protection: Source→Battery+; Drain→System; Gate→GND; if battery reversed, body diode blocks and MOSFET stays off
• H-Bridge Motor Drive: IRF4905 (P-ch) on high sides; IRF3205 (N-ch) on low sides; gate drivers for each; bidirectional motor control
• 24V System (with clamp): 15V Zener gate-to-source; Source→+24V; Gate driver pulls to +9V (VGS=-15V, within limits)

IRF4905PBF is a -55V/-74A P-channel MOSFET from Infineon in TO-220. The IRF4905SPBF provides the same in D2PAK surface-mount. The FQP47P06 offers lower 26mΩ RDS(on) at -60V/-47A from ONsemi. The IRFP460 is an N-channel alternative for complementary push-pull designs. The SUM110P06-25 from Vishay provides -60V/-110A with 18mΩ for extreme current applications. The IRF9540NPBF offers -100V/-19A for higher voltage at lower current. For logic-level drive, IRL4905PBF is not available — use FQP47P06 which reaches full enhancement at -5V VGS.