SWITCHMODE SERIES NPN POWER TRANSISTORS# Technical Documentation: BUX85 High-Voltage NPN Power Transistor
Manufacturer : ON Semiconductor
Document Version : 1.0
Last Updated : October 2023
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The BUX85 is a high-voltage, high-current NPN bipolar junction transistor (BJT) designed for demanding power switching and amplification applications. Its robust construction and electrical characteristics make it suitable for:
* High-Voltage Switching : Primary use in offline switch-mode power supplies (SMPS), electronic ballasts, and DC-DC converters where collector-emitter voltages (`V_CEO`) up to 800V are required.
* Deflection Circuits : Historically and in specialized designs, used in the horizontal deflection output stages of CRT (Cathode Ray Tube) monitors and televisions, handling high-voltage flyback pulses.
* Inductive Load Driving : Effective for driving relays, solenoids, motors, and other inductive loads where voltage spikes from back-EMF are a concern, provided appropriate snubber or clamping circuits are used.
* Linear Power Amplification : Can be used in Class A or Class B audio amplifier output stages for high-voltage rail designs, though thermal management is critical due to power dissipation in linear mode.
* Ignition Systems : Applicable in capacitive discharge ignition (CDI) systems and other high-energy pulse generation circuits.
### 1.2 Industry Applications
* Industrial Power Supplies : Used in the primary-side switching elements of industrial-grade AC-DC power supplies and uninterruptible power supplies (UPS).
* Lighting : Employed in high-intensity discharge (HID) lamp ballasts and professional lighting control equipment.
* Legacy & Specialized Display : Maintenance and repair of legacy CRT-based test equipment, radar displays, or in the design of new specialized analog display systems.
* Automotive (Aftermarket/High-Power) : Certain high-power aftermarket automotive applications, such as high-output ignition systems or power converters.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Voltage Rating : `V_CEO` of 800V and `V_CBO` of 1000V allow operation directly from rectified mains voltages (120VAC, 230VAC) with sufficient safety margin.
* High Current Capability : Continuous collector current (`I_C`) rating of 15A supports substantial power levels.
* Robust Construction : The TO-220 package offers good thermal coupling to a heatsink, essential for handling its 125W power dissipation (`P_tot`) at `T_case` = 25°C.
* Saturation Characteristics : Features a relatively low collector-emitter saturation voltage (`V_CE(sat)`), typically around 1.5V at high current, which improves efficiency in switching applications by reducing conduction losses.
* Cost-Effectiveness : For its voltage and current class, it is often a cost-effective solution compared to equivalent high-voltage MOSFETs in certain frequency ranges.
Limitations:
* Switching Speed : As a bipolar device, it has slower switching characteristics (turn-on time `t_on`, turn-off time `t_f`) compared to modern power MOSFETs or IGBTs. This limits its practical frequency in switching applications to the tens of kHz range, making it unsuitable for high-frequency (>100 kHz) SMPS designs.
* Drive Requirements : Requires significant base drive current to achieve and maintain saturation, especially at high collector currents. This increases complexity and losses in the drive circuit.
* Secondary Breakdown : Susceptible to failure from secondary breakdown if operated outside the Safe Operating Area (SOA), particularly in linear (active) mode with high voltage and current simultaneously.
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