IRLZ44N Logic-Level N-Channel MOSFET Low-Side DC Switch

The circuit is organized as an N-channel low-side power switch. The load (M1, representing a brushed DC motor, solenoid, or high-power LED array) connects between the positive DC supply rail (+12 V nominal) and the drain terminal of the N-channel MOSFET Q1 (Infineon IRLZ44NPbF). The source terminal of Q1 connects directly to system ground (GND). Because the source voltage is permanently referenced to 0 V, the gate-to-source drive voltage (VGS) is identical to the gate terminal voltage (VG), allowing direct drive from single-ended microcontroller logic outputs without requiring bootstrap floating gate drivers or high-side charge pumps.

When the microcontroller GPIO is at logic LOW (0 V) or unpowered, pull-down resistor R2 (10 kΩ) ties the gate to ground. Because VGS = 0 V, which is well below the gate threshold voltage range (VGS(th) = 1.0\dots 2.0 V), the inversion channel is completely closed. Drain cutoff leakage current is negligible (IDSS ≤ 25\ µA at 25°C), holding the load completely de-energized. During microcontroller bootloader execution, firmware reset, or ISP programming, GPIO pins default to high-impedance input mode; R2 prevents electrostatic charges or capacitive Miller coupling (Cgd) from unintentionally charging the gate into partial conduction.

When the microcontroller drives the GPIO pin HIGH (+3.3 V or +5.0 V), current flows through series damping resistor R1 (100 Ω) to charge the input capacitance (Ciss = 1700 pF, total gate charge Qg = 48 nC max at VGS = 5 V). Series resistor R1 limits the peak inrush current extracted from the microcontroller I/O pin (Ipeak ≤ 33 mA at 3.3 V, 50 mA at 5.0 V), preventing latch-up and internal output driver damage. As the gate voltage crosses the threshold voltage and traverses the flat Miller plateau (VGP ≈ 2.5\dots 3.0 V), the drain-to-source voltage abruptly collapses to near ground, establishing full channel conduction.

With a 5.0 V gate drive, the IRLZ44NPbF achieves a guaranteed on-resistance of RDS(on) ≤ 25 mΩ at ID = 25 A (TJ = 25°C). Even with a 3.3 V gate drive, the channel achieves an on-resistance under 35\dots 40 mΩ for moderate currents (ID ≤ 10 A). For a continuous load current of ILOAD = 5.0 A, the static conduction drop across the transistor is only VDS = ID · RDS(on) = 5.0 A × 0.025\ Ω = 0.125 V, resulting in continuous power dissipation of Pcond = ID2 · RDS(on) = (5.0 A)2 × 0.025\ Ω = 0.625 W. This low dissipation allows continuous operation with minimal heatsinking.