STMicroelectronics BUX87

Overview

BUX87 from STMicroelectronics is a high-voltage NPN power transistor designed for switching in flyback and forward single-transistor low-power converters. It features V_CEO = 450 V, V_CES = 1000 V, I_C = 0.5 A continuous, h_FE = 12–50 (at 40–50 mA), and R_thj-case = 3.1 °C/W - enabling robust operation in offline AC/DC adapter primary-side circuits.

For engineers reviewing the BUX87 datasheet, BUX87 pinout, BUX87 application, or BUX87 equivalent, key selection considerations include its 450 V V_CEO rating, 40 W T_c = 25 °C power dissipation, 20 MHz f_T, and SOT-32 (TO-126) package thermal performance in isolated DC-DC topologies.

Technical Context

The BUX87 employs high-voltage multi-epitaxial planar technology to achieve high switching speeds and reliable voltage withstand. Its internal structure supports pulsed V_CE(sus) = 450 V at I_C = 10 mA with t_p = 300 µs, and exhibits t_s = 4.5 µs storage time and t_f = 0.5 µs fall time under defined resistive-load switching conditions.

It operates with base-emitter saturation voltage V_BE(sat) = 1 V at I_C = 0.2 A / I_B = 20 mA, and collector-emitter saturation voltage V_CE(sat) = 0.8–1 V across 0.1–0.2 A, supporting efficient hard-switched converter designs up to 150 °C junction temperature.

Key Specifications

Pinout & Package

SOT-32 (TO-126) package: three-terminal through-hole plastic case with metal tab connected to collector for direct heatsinking. Mounting requires insulated washer or thermal pad when heatsink is grounded.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar high-voltage NPN power transistor applications.

Compared with BU508A and 2SC4027, the BUX87 uniquely balances 450 V operational rating, 20 MHz f_T, and controlled h_FE spread - making it optimal for universal-input, 60–100 kHz flyback converters needing predictable saturation and thermal behavior without overdesigning drive or heatsinking.

Availability

BUX87 is available at Aetrix Electronics and suitable for AC/DC wall adapters, industrial control power supplies, LED driver reference designs, and appliance standby power supplies requiring stable component supply and long-term manufacturability.

Supply support for BUX87 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, specializing in power discrete devices, microcontrollers, and analog ICs for industrial, automotive, and consumer markets.

The BUX87 belongs to ST's high-voltage bipolar transistor product line, engineered specifically for reliable, cost-effective switching in low-to-medium power offline SMPS topologies where ruggedness and thermal predictability are prioritized over ultra-high speed.

FAQ

What is the maximum recommended switching frequency for BUX87 in flyback applications?

The BUX87 achieves usable gain up to 20 MHz (f_T), but practical flyback operation is limited to 60–100 kHz due to storage time (t_s = 4.5 µs) and associated turn-off losses. Operating above 100 kHz increases switching loss disproportionately and risks thermal runaway without aggressive heatsinking.

Is BUX87 suitable for use in automotive applications?

No - the BUX87 is not qualified to AEC-Q101 and lacks automotive-grade screening, temperature range extension beyond 150 °C, or guaranteed parametric stability under automotive stress profiles. ST explicitly restricts non-automotive-grade parts like BUX87 from safety-critical or engine-bay applications.

How should the metal tab of the SOT-32 package be handled electrically?

The metal tab is internally connected to the collector terminal. When mounted to a heatsink, electrical isolation (e.g., mica washer + thermal paste or insulating thermal pad) is mandatory unless the heatsink is floating or intentionally biased to collector potential. Failure to isolate risks short-circuiting the collector to chassis ground.

Does BUX87 require a base resistor for safe operation in a flyback controller circuit?

Yes - a series base resistor is essential to limit peak base current (I_B ≤ 0.3 A, I_BM ≤ 0.6 A) and prevent second breakdown. For I_C = 0.2 A saturation, a 220 Ω resistor with 5 V drive yields ~23 mA I_B, satisfying h_FE ≥ 12 while staying within safe operating area limits.