Onsemi BD681G NPN Darlington Transistor: Datasheet, Pinout, and Application Circuits
The Onsemi BD681G is a robust NPN Darlington transistor designed primarily for medium-power linear and switching applications. Housed in a versatile SOT-32 plastic package, it is well-suited for use as an output driver in audio amplifiers, motor control circuits, and for driving heavy inductive loads like relays and solenoids. Its Darlington pair configuration provides it with an exceptionally high DC current gain, allowing it to be driven directly by low-current sources like microcontrollers or logic ICs.
Datasheet Overview and Key Specifications
The BD681G is characterized by its ability to handle substantial collector current and voltage. Key absolute maximum ratings from its datasheet define the boundaries of its operation:
Collector-Emitter Voltage (VCEO): 100 V – Suitable for a wide range of DC power supplies.
Collector Current (IC): 4 A (continuous) – Capable of driving significant loads.
Total Power Dissipation (PTOT): 40 W – Requires an appropriate heatsink for operation at high currents.
DC Current Gain (hFE): 750 (min) at IC = 2 A, VCE = 2 V – The hallmark of the Darlington pair, offering very high gain.
These ratings make the BD681G a reliable choice for applications requiring high-current switching without the need for a pre-driver stage.
BD681G Pinout Configuration
The transistor is supplied in a SOT-32 package. When viewing the component from the front (the side with the marking), the pins are arranged from left to right as follows:
1. Emitter (E)
2. Collector (C)
3. Base (B)
It is crucial to confirm this pinout with a multimeter or the manufacturer's datasheet before soldering, as incorrect connections can instantly destroy the device.
Application Circuits
1. Simple Relay or Solenoid Driver
One of the most common uses for the BD681G is to interface a low-power microcontroller (MCU) with a high-power device like a relay, solenoid, or DC motor. A simple circuit involves connecting the load (e.g., a relay coil) between the positive supply (e.g., +12V) and the transistor's Collector pin. The Emitter is connected to ground. A current-limiting base resistor (e.g., 1kΩ) is connected between the MCU's output pin and the Base of the BD681G. A flyback diode (e.g., 1N4007) must be placed in reverse bias across the relay coil to protect the transistor from voltage spikes generated when the inductive load is switched off.
2. Linear Power Amplifier (Class A)
While less efficient for power amplification than Class AB or D designs, the BD681G's high gain makes it a candidate for a simple Class A audio amplifier output stage. In such a configuration, the transistor is biased into its linear region. A small audio signal applied to the base is amplified to drive a small speaker. This circuit requires careful biasing and typically uses additional components for stability and to prevent thermal runaway.
3. LED Array Driver
For applications requiring high-brightness LED arrays that draw several amperes of current, the BD681G can act as a simple switch or a linear current regulator. When used as a switch, a PWM signal from an MCU can be applied to the base to efficiently control the brightness of the LEDs.
Important Considerations for Use
Heatsinking: Operating near its maximum current or power ratings mandates the use of a suitable heatsink. The metal tab is electrically connected to the Collector pin, so an insulating kit (mica washer, silicone pad) must be used if the heatsink is grounded or shared with other components.
Saturation Voltage: As a Darlington transistor, the BD681G has a higher collector-emitter saturation voltage (VCE(sat)) than a standard bipolar transistor (typically around 1.5V at high current). This results in higher power dissipation when fully on, which must be accounted for in the thermal design.
Switch-Off Speed: The Darlington configuration is inherently slower to turn off than a single transistor due to charge storage. This is generally not an issue for low-frequency switching (like turning a relay on/off) but may be a limitation in high-frequency PWM applications.
ICGOOODFIND: The Onsemi BD681G is a highly capable and rugged medium-power Darlington transistor that excels in simplifying circuit design by providing immense current gain from a small input signal. Its primary strengths lie in driving inductive loads and serving as a output stage driver in various industrial and consumer applications, though its use requires careful attention to thermal management.
Keywords: Darlington Transistor, High Current Gain, Medium Power Switching, Relay Driver, SOT-32 Package.
