Onsemi BU406G Silicon Power Transistor: Datasheet, Pinout, and Application Circuits
The Onsemi BU406G is a quintessential high-voltage NPN silicon power transistor designed for robust performance in medium-power switching and amplification applications. This component has established a long-standing reputation for its rugged construction and reliability in demanding environments, making it a common choice in power supply units, deflection circuits, and general-purpose power drivers.
This article provides a detailed overview of its datasheet specifications, pinout configuration, and typical application circuits.
Datasheet Overview and Key Specifications
The BU406G is characterized by its ability to handle high voltages and moderate currents. Its key absolute maximum ratings and electrical characteristics define its operational boundaries.
Collector-Emitter Voltage (VCEO): 400 V. This high voltage rating is crucial for its use in off-line power supplies and CRT deflection circuits.
Collector Current (IC): 7 A (continuous). It can handle significant current, enabling it to drive heavy loads.
Power Dissipation (Ptot): 60 W. This specifies the maximum power the device can dissipate without a heat sink at 25°C. Adequate heatsinking is mandatory for operation near this limit.
DC Current Gain (hFE): Typically 40 (at IC = 0.5 A, VCE = 4 V). This moderate gain means it often requires a driver transistor in high-current applications to ensure full saturation.
Transition Frequency (fT): 10 MHz. This indicates its useful frequency range for switching applications.
Important Note: The BU406G includes an integrated damper diode between its collector and emitter. This internal diode is a significant feature for inductive load switching, as it helps suppress voltage spikes generated when the current through a coil (like a relay or motor winding) is suddenly interrupted, thereby protecting the transistor.
Pinout Configuration
The BU406G is packaged in the robust TO-220 package, which is designed for efficient heat transfer to an external heatsink. The pinout, when viewing the component from the front (the side with the metal tab with a hole), is standard for many TO-220 transistors:
1. Pin 1 (Left): Base (B)
The control terminal. A small current flowing into the base controls a much larger current between the collector and emitter.
2. Pin 2 (Center): Collector (C)
The positive power terminal. It is typically connected to the load. Important: The metal tab of the TO-220 package is internally connected to the collector. This must be considered when mounting the transistor to a heatsink to prevent accidental short circuits.
3. Pin 3 (Right): Emitter (E)
The output terminal. Current flows out of this pin to ground.
Application Circuits
The BU406G's high voltage and current capabilities make it suitable for a variety of applications.
1. High-Voltage Switching Regulator
The BU406G is an excellent choice as the main switching element in flyback or buck converter power supplies. In such a circuit, the transistor is driven by a pulse-width modulation (PWM) signal from a controller IC (often through a smaller driver transistor due to its moderate hFE). It rapidly switches on and off to control the energy transfer from the input to the output, efficiently stepping down a high DC voltage (e.g., from a rectified mains input).
2. Inductive Load Driver
It is perfectly suited for driving inductive loads such as relays, solenoids, and small DC motors. The integrated damper diode is critical here. The circuit requires a base resistor to limit the control current. When the input signal is removed, the collapsing magnetic field in the coil generates a reverse voltage spike, which is safely clamped by the internal diode, protecting the transistor from damage.
3. Deflection Circuit (Historical Context)
Historically, a primary application for transistors like the BU406G was in the horizontal deflection stages of cathode ray tube (CRT) televisions and monitors. These circuits required a transistor capable of switching very high voltages at a high frequency (15.734 kHz for NTSC) to drive the deflection coils that moved the electron beam.
ICGOODFIND Summary
The Onsemi BU406G remains a highly reliable and versatile high-voltage NPN power transistor. Its 400V VCEO rating and 7A current handling capability, combined with its integrated damper diode, make it a robust solution for switching and medium-power linear applications. While its use in modern consumer electronics like CRT displays has diminished, it continues to be a valuable component in power supplies, motor controllers, and industrial equipment where durability and high-voltage performance are paramount. Proper design, including adequate base driving and heatsinking, is essential to unleash its full potential.
Keywords: Power Transistor, High Voltage Switching, Inductive Load Driver, TO-220 Package, Damper Diode.
