STMicroelectronics BUL704

Overview

BUL704 from STMicroelectronics is a high-voltage fast-switching NPN power transistor in TO-220 package, rated for V_CEO = 400 V, I_C = 4 A, P_tot = 70 W at T_c = 25°C, with t_s/t_f as low as 0.6/0.1 µs (inductive load), designed for electronic ballasts and PFC half-bridge circuits in fluorescent lighting systems.

For engineers reviewing the BUL704 datasheet, BUL704 pinout, BUL704 application, or BUL704 equivalent, key selection factors include its 400 V sustaining voltage under inductive switching, 1.78 °C/W junction-to-case thermal resistance, cellular emitter planar structure for RBSOA robustness, and compliance with EU Directive 2002/93/EC for environmental safety.

Technical Context

The BUL704 employs Multi-Epitaxial Planar technology with cellular emitter and planar edge termination to simultaneously achieve high voltage blocking (V_CEO = 400 V) and fast switching (t_s = 0.6 µs, t_f = 0.1 µs under inductive load). Its RBSOA is widened by structural optimization rather than trade-offs in saturation voltage.

It operates with V_CE(sat) = 0.5 V at I_C = 1 A / I_B = 0.2 A and h_FE = 10–28 across I_C = 10 mA to 2 A, supporting stable current gain in medium-power switching where thermal stability and dynamic consistency are critical.

Key Specifications

Pinout & Package

Package: TO-220 (3-pin, straight lead, insulated tab). Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector (tab-connected). Mounting tab is electrically connected to collector; isolation washer required when mounting to grounded heatsink.

Key Features

Applications

Equivalent & Alternatives

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

Compared with STBUL704 (identical performance, different packaging) and BU508AF (higher voltage but slower switching), the BUL704 offers the best balance of 400 V ruggedness, sub-microsecond switching, and thermal efficiency for 25–100 kHz fluorescent and PFC applications.

Availability

BUL704 is available at Aetrix Electronics and suitable for electronic ballasts, PFC converters, and medium-power SMPS requiring stable component supply, long-lifecycle availability, and traceable ECOPACK® compliance.

Supply support for BUL704 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, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

The BUL704 belongs to ST's high-voltage bipolar power transistor product line, engineered specifically for energy-efficient lighting and power conversion systems demanding reliable fast switching at 400 V class.

FAQ

What is the maximum safe operating frequency for BUL704 in a half-bridge PFC circuit?

The BUL704 supports reliable operation up to 100 kHz in hard-switched half-bridge PFC topologies, validated by its 0.1 µs fall time and 0.6 µs storage time under inductive load conditions. At 65–85 kHz, it achieves <5% switching loss contribution with proper gate drive and heatsinking. Operation beyond 100 kHz increases risk of thermal runaway due to reduced SOA margin at high dV/dt.

Is a heatsink mandatory for BUL704 at 4 A continuous current?

Yes - at I_C = 4 A and P_tot ≈ 50 W (accounting for V_CE(sat) and switching losses), the junction temperature exceeds 150°C without heatsinking. A heatsink with thermal resistance ≤1.2 °C/W (including interface resistance) is required to maintain T_J ≤ 135°C at 50°C ambient, per derating curve Figure 2 in the datasheet.

Can BUL704 replace BU2508DF in a 36 W fluorescent ballast?

No - BU2508DF has V_CEO = 1500 V and is optimized for series-resonant lamp ignition with high-voltage spikes. BUL704's 400 V rating suits steady-state PFC and inverter stages but lacks the overvoltage margin needed for cold-start ignition transients. Substitution risks premature avalanche failure during lamp strike.

Does BUL704 require negative base drive for fast turn-off?

Yes - the datasheet specifies V_BE(off) = –5 V in inductive test conditions (Figure 13) to achieve t_s = 0.6 µs. A –5 V reverse bias during turn-off accelerates minority carrier extraction from the base region. Without it, storage time increases significantly, raising switching loss and thermal stress in high-frequency operation.