Infineon BSS138E6327: Key Specifications and Application Circuit Design Considerations

The Infineon BSS138E6327 is a widely adopted N-channel enhancement mode MOSFET, housed in a compact SOT-23 (TO-236) package. Its popularity stems from an optimal blend of key electrical characteristics, making it a versatile solution for a multitude of low-power switching applications, particularly in portable electronics, IoT devices, and as a level shifter.

Key Specifications

Understanding the absolute maximum ratings and electrical characteristics is paramount for reliable circuit design.

Drain-Source Voltage (V_DS): 50 V. This defines the maximum voltage that can be applied between the drain and source when the device is off.

Continuous Drain Current (I_D): 220 mA at a case temperature of 25°C. This is the maximum continuous current the channel can handle.

On-Resistance (R_DS(on)): A critical parameter for efficiency. The BSS138 offers a typical R_DS(on) of 3.5 Ω at V_GS = 4.5 V and I_D = 50 mA. Lower R_DS(on) minimizes conduction losses and voltage drop across the switch.

Gate Threshold Voltage (V_GS(th)): Ranging from 0.5 V to 1.5 V. This low threshold voltage is a defining feature, enabling the MOSFET to be fully turned on by low-voltage logic signals (e.g., 3.3V or even 1.8V microcontrollers).

Total Gate Charge (Q_g): Typically 0.7 nC. A low gate charge allows for very fast switching speeds and reduces the drive current requirements from the microcontroller GPIO pin.

Application Circuit Design Considerations

While simple to use, several design considerations ensure robust performance.

1. Gate Driving: Although the BSS138 can be driven directly from a microcontroller GPIO pin due to its low V_GS(th) and Q_g, a series gate resistor (e.g., 10 - 100 Ω) is highly recommended. This resistor dampens ringing caused by parasitic inductance and limits inrush current into the gate during fast switching, protecting the microcontroller's output driver.

2. Level Shifting Circuit: One of the most common applications for the BSS138 is a simple, unidirectional level shifter (e.g., translating 1.8V/3.3V logic to 5V). The circuit relies on the MOSFET's threshold voltage and the pull-up resistor on the high-voltage side. Selecting an appropriate pull-up resistor value (typically 1kΩ to 10kΩ) is a trade-off; a lower value provides faster edge times but draws more current when the output is low.

3. Load Switching: When used as a low-side switch to control a load (e.g., an LED, relay coil, or motor), the low R_DS(on) ensures minimal voltage drop and power dissipation. Designers must ensure the load current remains well below the absolute maximum I_D rating, considering derating for elevated ambient temperatures.

4. Protection: The BSS138 lacks an intrinsic body diode. Designers must be cautious of voltage spikes from inductive loads (like relays or motors). Implementing a flyback diode or a snubber circuit is essential to protect the MOSFET from voltage transients exceeding its V_DS rating.

5. PCB Layout: For switching applications, a good PCB layout is crucial. Minimizing parasitic inductance in the drain and source loops by keeping traces short and direct reduces voltage overshoot and electromagnetic interference (EMI).

ICGOODFIND

The Infineon BSS138E6327 stands out as an exceptionally efficient and cost-effective solution for low-voltage, low-current switching and logic level translation. Its optimal combination of a low threshold voltage, low on-resistance, and a compact package solidifies its position as a fundamental component in modern electronic design, from consumer gadgets to complex embedded systems.

Keywords: Low Threshold Voltage MOSFET, Level Shifter Circuit, SOT-23 Package, On-Resistance (RDS(on)), Load Switching