BU508A NPN Transistor: The Ultimate High-Voltage Power Solution

The BU508A from Motorola stands as one of the most reliable high-voltage NPN bipolar junction transistors (BJTs) in the electronics industry. Designed for demanding applications, this power transistor combines exceptional voltage handling with robust current capacity, making it a preferred choice for engineers working on power electronics, industrial controls, and high-voltage switching systems.

Comprehensive Technical Specifications

Manufacturer and Product Details

• Manufacturer: Motorola Semiconductor (Now part of ON Semiconductor)
• Part Number: BU508A
• Transistor Type: NPN Bipolar Junction Transistor
• Package: TO-247-3 (Through-Hole Mounting)

Electrical Characteristics

• Collector-Emitter Voltage (V_CEO): 700V
• Collector Current (I_C): 8A Continuous
• Power Dissipation (P_tot): 125W
• DC Current Gain (h_FE): Typically 8-40 (depending on operating conditions)
• Transition Frequency (f_T): 7MHz
• Collector-Emitter Saturation Voltage: 1V typical @ 4.5A

Thermal Properties

• Maximum Junction Temperature: 150 C
• Thermal Resistance: Junction to Case 1.25 C/W

Why the BU508A Remains Popular Despite Obsolete Status

While officially marked as obsolete by manufacturers, the BU508A continues to be widely used due to:

• Proven Reliability: Decades of field performance in demanding applications
• High Voltage Capability: One of few transistors offering 700V breakdown voltage
• Robust Construction: TO-247 package provides excellent thermal performance
• Legacy Design Support: Many existing systems still require this specific component

Typical Applications

The BU508A excels in numerous power electronics applications:

• Switch Mode Power Supplies: Particularly in flyback converter designs
• CRT Display Circuits: Horizontal deflection and high-voltage generation
• Industrial Motor Controls: For switching inductive loads
• Electronic Ballasts: Fluorescent lighting applications
• High-Voltage Switching: Where 700V capability is required

Design Considerations

When implementing the BU508A in your designs, consider:

• Heat Sinking: Essential for maintaining safe operating temperatures at high power
• Drive Requirements: Base current of at least 1A recommended for saturation
• Snubber Circuits: Important when switching inductive loads
• Derating: Follow manufacturer guidelines for voltage and current derating

Modern Alternatives and Cross-References

While the BU508A remains available through secondary markets, designers may consider these modern alternatives:

• 2SC5386 (Toshiba)
• KSC5027 (Fairchild/ON Semi)
• MJW13007 (Motorola/ON Semi)

Purchasing Considerations

When sourcing BU508A transistors:

• Verify authenticity to avoid counterfeit parts
• Check for proper packaging and storage conditions
• Consider buying from authorized surplus dealers
• Evaluate minimum order quantities and pricing

Conclusion

The BU508A NPN power transistor represents an excellent solution for high-voltage, high-current applications requiring proven reliability. Its 700V collector-emitter voltage rating and 8A current capacity make it particularly valuable for power supply and industrial control designs. While newer components are available, the BU508A's performance characteristics and established track record continue to make it a preferred choice for many engineers.

For your next high-voltage design project, consider the BU508A's unique combination of capabilities. Our inventory includes genuine Motorola BU508A transistors with full traceability and quality assurance. Contact us today to discuss your requirements or place an order for this reliable power transistor solution.