STMicroelectronics BU505

Overview

BU505 from STMicroelectronics is a high-voltage NPN multiepitaxial fast-switching transistor rated for V_CEO = 700 V, V_CES = 1500 V, I_C = 2.5 A, and P_TOT = 75 W at T_C = 25°C, with t_s = 2 μs and t_f = 350 ns under inductive load conditions; it is engineered for electronic ballasts in fluorescent lighting and switch-mode power supplies.

For engineers reviewing the BU505 datasheet, BU505 pinout, BU505 application, or BU505 equivalent, key selection criteria include its 700 V sustaining voltage under zero-base-drive conditions, 1500 V collector-emitter blocking capability, 2.5 A continuous collector current rating, rugged second-breakdown margin (I_s/b = 2 A at V_CE = 120 V), and TO-220 package thermal resistance of 1.67 °C/W.

Technical Context

The BU505 employs a multiepitaxial silicon structure to achieve high voltage standoff while maintaining fast switching performance. Its design supports inductive switching with controlled storage time (t_s = 2 μs) and fall time (t_f = 350 ns) under defined test conditions (V_clamp = 250 V, L = 200 μH, I_B1 = 0.7 A).

It operates with base-emitter saturation voltage V_BE(sat) = 1.3 V at I_C = 2 A / I_B = 0.9 A and exhibits collector-emitter saturation voltage V_CE(sat) = 1 V under identical bias, enabling efficient hard-switching in resonant and quasi-resonant topologies.

Key Specifications

Pinout & Package

Package: TO-220 (3-lead, straight lead, insulated tab). Case material meets ECOPACK® environmental compliance with lead-free second-level interconnect.

Key Features

Applications

Equivalent & Alternatives

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

Compared with BU2520DX and ST13007, the BU505 uniquely balances 700 V sustain voltage, 2.5 A current, and 2 μs storage time-making it optimal for cost-sensitive 50–60 kHz fluorescent ballasts where surge margin and thermal simplicity are prioritized over ultrafast switching.

Availability

BU505 is available at Aetrix Electronics and suitable for electronic ballasts, offline switch-mode power supplies, and AC line surge suppression circuits requiring stable component supply and legacy-design continuity.

Supply support for BU505 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, designing and manufacturing analog, digital, and mixed-signal ICs, discrete devices, and microcontrollers for industrial, automotive, and consumer markets.

The BU505 belongs to ST's high-voltage bipolar transistor product line, developed specifically for lighting control and offline power conversion where ruggedness, voltage margin, and predictable switching behavior are essential.

FAQ

Is BU505 still in active production?

No - BU505 is marked "Obsolete Product(s)" in ST's official documentation (Rev. 4, Aug 2005). It is no longer manufactured, but Aetrix Electronics maintains legacy inventory with full traceability and supports long-life industrial designs requiring exact form-fit-function replacement.

What is the recommended gate/base drive for BU505?

BU505 requires a peak base current of at least 0.9 A to ensure full saturation at I_C = 2 A. A low-impedance driver (e.g., discrete transistor pair or dedicated bipolar driver IC) with ≥1.3 V V_BE(sat) margin is recommended; base resistors must be sized to deliver this current from the available drive voltage.

Can BU505 replace BU2508DF in a fluorescent ballast?

No - BU2508DF has V_CEO = 800 V and different SOA characteristics. While both are TO-220 NPN transistors, BU505's lower V_CEO (700 V) and distinct storage/fall time profile may cause premature failure under identical operating conditions; redesign validation is required.

Does BU505 require a heatsink in 40 W ballast operation?

Yes - at 40 W output with typical 60% efficiency, power dissipation exceeds 25 W. With R_thJ-case = 1.67 °C/W and T_J(max) = 150°C, even at 25°C ambient, a heatsink with ≤10 °C/W total thermal resistance (including interface) is necessary to maintain safe junction temperature.