Infineon BSS138WH6433XTMA1: Key Specifications and Application Circuit Design Guide
The Infineon BSS138WH6433XTMA1 is a widely adopted N-channel logic level enhancement mode MOSFET in a compact, surface-mount SC-70 (SOT-323) package. Its combination of low threshold voltage, high performance, and minimal footprint makes it an ideal choice for a vast array of low-power switching applications in modern electronics.
Key Specifications
Understanding the critical electrical parameters is fundamental to effective circuit design. Below are the standout specifications for the BSS138WH6433XTMA1:
Drain-Source Voltage (VDS): 50 V. This defines the maximum voltage that can be applied between the drain and source terminals, making it suitable for a range of low-voltage circuits, including those powered by 12V or 24V rails.
Continuous Drain Current (ID): 220 mA. This is the maximum continuous current the MOSFET can handle in its fully-on (saturated) state.
On-Resistance (RDS(on)): A remarkably low 3.5 Ω (max.) at VGS = 4.5 V and ID = 50 mA. A low RDS(on) is crucial as it minimizes power loss and voltage drop across the MOSFET when switched on, leading to higher efficiency.
Gate-Threshold Voltage (VGS(th)): Between 0.5 V and 1.5 V. This low threshold voltage is its defining feature, allowing it to be directly driven by 3.3V and 5V logic from microcontrollers (MCUs), FPGAs, and other digital ICs without needing a separate gate driver circuit.
Package: SC-70 (SOT-323). This ultra-small package is designed for high-density PCB designs, saving valuable board space.
Application Circuit Design Guide
The primary function of the BSS138 is to act as a switch, allowing a low-voltage logic signal to control a higher-voltage or higher-current load.
1. Basic Low-Side Switch Configuration
The most common application is the low-side switch. Here, the load (e.g., an LED, relay coil, or small motor) is connected between the positive supply rail (VDD) and the drain of the MOSFET. The source is connected directly to ground.
Operation: When the microcontroller's GPIO pin outputs a logic 'HIGH' (3.3V), the voltage at the gate (VGS) exceeds the threshold voltage, turning the MOSFET on. This creates a low-resistance path to ground, completing the circuit and allowing current to flow through the load. A logic 'LOW' (0V) from the MCU turns the MOSFET off, stopping the current flow.
Critical Design Component: The Gate Resistor (RG)
While sometimes omitted for very slow switching, a small series resistor (e.g., 10Ω to 100Ω) between the MCU pin and the gate is highly recommended. This resistor serves two key purposes:
It dampens high-frequency oscillations caused by the interaction of the MCU's output impedance and the MOSFET's inherent gate capacitance.
It limits the inrush current during fast switching transitions, protecting the MCU's output driver stage.
2. Level Shifting Circuit
Another powerful application of the BSS138 is in constructing a simple single-directional level shifter, for example, to convert a 5V signal to 3.3V or vice-versa.
Circuit Setup: The drain is connected to the lower voltage logic (e.g., 3.3V VCC) via a pull-up resistor. The source is connected to the input signal from the higher voltage domain (e.g., 5V MCU). The output is taken from the drain pin.
Operation: When the 5V input is LOW (0V), VGS is positive, turning the MOSFET on and pulling the output (drain) LOW to near 0V. When the 5V input is HIGH (5V), VGS becomes 0V (because source is at 5V and gate is at 5V), turning the MOSFET off. The pull-up resistor then pulls the output HIGH to 3.3V. Thus, a 0V-5V input is translated to a 0V-3.3V output.
General Design Considerations:
Flyback Diode: When driving inductive loads like relay coils or motors, a flyback diode (or freewheeling diode) must be placed in reverse bias across the load to protect the MOSFET from voltage spikes generated when the current is suddenly interrupted.
Power Dissipation: Ensure that the combined switching and conduction losses (ID² RDS(on)) do not exceed the package's power dissipation limits, especially in thermally challenging environments.
The Infineon BSS138WH6433XTMA1 stands out as an exceptionally versatile and efficient solution for logic-level switching. Its low threshold voltage and minimal on-resistance ensure seamless integration with modern microcontrollers, while its ultra-small SC-70 package addresses the needs of space-constrained designs. By adhering to fundamental design principles like using a gate resistor and flyback diode, engineers can reliably leverage this MOSFET to create robust and efficient electronic systems across consumer, industrial, and automotive domains.
Keywords: Logic-Level MOSFET, Low On-Resistance, Low-Side Switch, Level Shifter, SC-70 Package.
