BSS138 MOSFET : Working & Its Applications

From many years we know various 3.3Volts devices such as ESP8266, ESP32, HC-05 Bluetooth module, Nokia 5110 LCD, BMP180 barometric pressure sensor, etc. To connect a 3.3V SPI or I2C 3.3 Volts to a 5 Volts device like Arduino which supports the bi-directional communication, the solution is to use either a logic level shifter or an external level shifter. The logic level shifter uses one BSS138 MOSFET per bus to support bidirectional level shifting and protect the low voltage side from the high voltage side. This article gives a brief description of the N-channel logic level MOSFET called BSS138.

The alternatives of BSS138 MOSFET are 2N7000 2N7002, NTR4003, FDC558, FDC666, and BS170. The equivalents of BSS138 N-channel MOSFET are IRF3205, IRF540N, IRF1010E, 2N7000, BS170N, FDN358P (P-channel MOSFET), BSS84 (P-channel MOSFET).

What is BSS138 MOSFET?

The BSS138 is a low resistance (3.5 ohms) and low input capacitance (40 pF) logic-level N-channel MOSFET in the SMD package. MOSFETs can also switch at a high speed of 20 ns. Due to their higher switching speed and low threshold voltage, these MOSFETs are mostly used in level shifting circuits.

It is a compact N-channel logic level MOSFET with a very low threshold voltage of 0.5 Volts, which makes it suitable for all low voltage and level shift applications. These MOSFETs also have low input capacitance and low on-resistance, making them efficient for switching circuits. Due to their compactness, these MOSFETs are commonly used in portable devices such as cell phones and other power management circuits.

The BSS138 is a bit pricey for its specifications, so for a cheaper alternative, check out the 2N7002. However, some compensation is required for the MOSFET’s on-state resistance and threshold voltage. These MOSFETs are available in an SMD package, allowing them to be used in smaller applications. The main disadvantage of MOSFET is the low drain current. It can provide 200mA continuous current and up to 1A peak current at maximum threshold voltage. If exceeded, the MOSFET would get damaged.

BSS138 MOSFET Pin Configuration/Pin Diagram:

The BSS138 is a 3-terminal or 3-pin N-channel logic-level MOSFET. The BSS138 pin configuration/pin diagram is shown in the figure below.

• Pin 1: Source: The current flows out through this terminal.
• Pin 2: Gate: The MOSFET biasing is controlled by this terminal.
• Pin 3: Drain: The current passes in through this terminal.

BSS138 Specifications:

The BSS138 specifications and characteristics are given below.

• It is an N-channel MOSFET logic level with low on-state resistance.
• The continuous drain current ID is 200mA.
• Drain-Source voltage VDS is 50V.
• The minimum gate threshold voltage VGS is 0.5V.
  The maximum gate threshold voltage VGS is 1.5V.
• On-State resistance is 3.5ohms.
• Turn On and Turn Off time is 20ns each.
• Available in SOT23 SMD package.

The absolute electrical and thermal characteristics @T=25°C:

• Drain-Source voltage VDSS: 50V.
• Gate-Source voltage VGSS: ±20V.
• Continuous drain current ID: 0.22A.
• Pulsed drain current: 0.88A.
• Maximum power dissipation: 300mW.
• Operating and storage junction temperature range: -55°C to +150°C.
• Maximum lead temperature for soldering purpose: 300°C.
• Thermal resistance: 350°C/W.
• Input capacitance @ VDS=25V, VGS=0V, f=1MHz: 27pF.
• Output capacitance @ VDS=25V, VGS=0V, f=1MHz:13pF.
• Reverse transfer capacitance @ VDS=25V, VGS=0V, f=1MHz:6pF.
• Gate resistance: 9 ohms.

Circuit Diagram/How to Use?

Now, let’s know the circuit diagram/how to use N-channel BSS138 logic level MOSFET as a bi-directional level shifter. Consider the below circuit diagram consisting of a BSS138 N-channel MOSFET with an internal drain-substrate diode (one must be present). Divide the circuit into two parts: the left, or low voltage side, and the right, or high voltage side.

Both sides have different supply voltages and logic levels. Pull the bus on the low voltage side to HIGH to 3.3 Volts and the device power is 3.3 Voltage supply on this side. The bus on the right side is pulled HIGH at 5V and the device power is 5V. The MOSFET’s gate terminal must be connected to a low voltage supply of 3.3 V. The source and drain terminals of the MOSFET are connected to the low voltage bus and the high voltage bus respectively. This simple circuit works as a bidirectional logic level shifter.

Now let’s see how the above circuit works as a bi-directional logic level converter. To get a clearer picture of the task, let’s divide its working into 3 states.

Standby State

Firstly, when there is no signal from either side, it will generate either a high output signal or take it as input. In practice, this means that there is nothing to lower the signal level, so we need to pull the output to logic high (for example 3.3V for Raspberry Pi, 5V for Arduino) at both ends.

The gate-to-source junction voltage VGS (pin 1 and pin 2) is 0V (both 3.3V), therefore the MOSFET is in the OFF state. Hence the output on the LV1 side is based on a resistor R1 (10k) which can pull to 3.3V, and the output on the HV1 side is based on R2 which can pull to 5V. So both ends are at a higher logic level.

3.3Volts Side is Pulled Low

If you want to get the output of low voltage from the low voltage side, then connect the output to 0V through an open drain. This pulls up the Q1 source to 0V and 3.3V at the gate terminal, which makes the MOSFET turn ON. Then the high output (at the gate terminal) from Q1 goes low. Thus, this results in a logic low-level output on the high voltage side.

5 Volts side is Pulled Low

Now let’s know how to pull the bus low to handle the transition from a high level on both sides to the 5V side. The higher side now has a lower output by pulling the end down through the open drain. The MOSFET’s drain-substrate diode substrate is pulled down (as seen in the MOSFET schematic) so that it can flow from the source to the drain (opposite to the conducting current, when it works as a switch).

Then, when the voltage of the source drops below the voltage of the gate, the MOSFET turns ON. When the MOSFET is in an ON state, both LV and HV sides are at 0V, providing a logic low level on each side.
When these 3 states are combined, the logic levels can be shifted in both directions, and hence it works as a bidirectional logic level shifter.

Applications of BSS138/Where to Use?

The following are the applications of BSS138.

• Used in low voltage and low current applications.
• Used in bidirectional logic level shifter circuits.
• Used in DC-DC converters.
• Used in applications where low on-state resistance is needed.
• Used in e-mobility applications
• Used in power management applications.

Please refer to this link to know more about BSS138 MOSFET Datasheet

Thus, this is all about an overview of BSS138 N-channel logic-level MOSFET. These are designed to minimize the on-state resistance and provide fast switching performance. These are more reliable and rugged types used in low current and low voltage applications such as servo motor control, power drive circuit, and other switching applications.