onsemi BD3756STU

Overview

BD3756STU from Fairchild Semiconductor is an NPN epitaxial silicon transistor in TO-126 package, rated for 45 V V_CEO, 2 A continuous collector current, and 25 W power dissipation at T_C = 25°C, used in medium-power linear amplification and switching applications such as audio output stages and relay drivers.

For engineers reviewing the BD3756STU datasheet, pinout, applications, or equivalent options, key selection criteria include V_CEO = 45 V, I_C = 2 A, h_FE = 40–100 (at I_C = 0.15 A), V_CE(sat) = 1 V (at I_C = 1 A, I_B = 0.1 A), and TO-126 thermal performance with case temperature derating.

Technical Context

This device operates as a medium-power bipolar junction transistor optimized for linear and switching use where robust SOA, low saturation voltage, and stable DC current gain across operating range are required. It features a complementary pair relationship with BD376 and is specified with pulse test conditions (PW = 350 µs, duty cycle = 2%) for peak current ratings.

The BD3756STU exhibits defined safe operating area limits up to 150°C junction temperature, with power derating starting at T_C > 25°C, and supports base-driven switching with t_ON = 50 ns and t_OFF = 500 ns under standardized load and drive conditions.

Key Specifications

Pinout & Package

BD3756STU is housed in a TO-126 plastic package with integral metal tab for heatsinking. Pin 1 is Emitter, Pin 2 is Collector (connected to tab), Pin 3 is Base - verified per Fairchild Rev. A datasheet mechanical drawing and terminal assignment.

Key Features

Applications

Equivalent & Alternatives

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

Compared with BD3756STU, MJD31C offers higher voltage and current but demands PCB layout changes and delivers slower switching, while BD139 retains the same TO-126 form factor but trades 0.5 A current headroom for wider h_FE variability - both require circuit-level verification for replacement.

Availability

BD3756STU is available at Aetrix Electronics and suitable for audio amplifiers, relay drivers, DC motor controls, and linear regulator designs requiring stable component supply and legacy industrial BOM continuity.

Supply support for BD3756STU 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

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in power discrete and analog ICs, acquired by ON Semiconductor in 2016; its legacy products remain widely supported in industrial and automotive applications.

The BD3756STU belongs to Fairchild's BD375/377/379 family of medium-power NPN transistors, designed specifically for cost-sensitive linear and switching applications demanding proven reliability, SOA robustness, and complementary PNP pairing capability.

FAQ

What is the maximum continuous collector current rating for BD3756STU?

The BD3756STU has a maximum continuous collector current (I_C) rating of 2 A at case temperature T_C = 25°C. This rating decreases with rising case temperature per the published derating curve, reaching zero at T_C = 150°C. For sustained 2 A operation, adequate heatsinking maintaining T_C ≤ 25°C is required - actual design current should include safety margin based on thermal resistance and ambient conditions. The BD3756STU also supports 3 A pulsed current under specified test conditions (PW = 350 µs, duty cycle = 2%).

Is BD3756STU pin-compatible with BD376?

No, BD3756STU is not pin-compatible with BD376. BD3756STU is an NPN transistor in TO-126 package with pinout Emitter–Collector–Base (1–2–3), while BD376 is its complementary PNP counterpart with identical TO-126 mechanical outline but reversed polarity and different electrical characteristics. Though designed as a matched pair for push-pull circuits, their pin assignments differ - BD376 uses Emitter–Collector–Base orientation but with opposite doping, requiring separate PCB footprints or careful layout planning. Always verify pinout diagrams in respective datasheets before substitution.

What is the DC current gain (h_FE) range for BD3756STU at I_C = 1 A?

At I_C = 1 A and V_CE = 2 V, the BD3756STU exhibits h_FE2 = 20 (minimum) per the Fairchild datasheet. This lower gain at high current reflects typical BJT behavior and informs base drive design - for full saturation at 1 A collector current, minimum base current required is 50 mA (1 A ÷ 20). The device's h_FE1 = 40–100 range applies at I_C = 0.15 A and is used for small-signal or biasing calculations. Designers must account for this gain compression when sizing base resistors in switching applications.

Does BD3756STU have avalanche ruggedness or built-in protection features?

The BD3756STU datasheet does not specify avalanche energy rating (E_AS) or integrated protection features such as built-in diodes, thermal shutdown, or overcurrent limiting. It is a standard bipolar junction transistor relying on external circuit design for safe operation - including snubbers for inductive loads, base current limiting, and heatsinking for thermal management. Its V_CEO(sus) = 45 V rating indicates sustaining voltage under open-base conditions, but this is not equivalent to rated avalanche capability. System-level protection must be implemented externally when driving inductive or reactive loads.

Can BD3756STU be used in switching power supplies?

The BD3756STU can be used in low-frequency (<50 kHz) switching power supply applications such as flyback or forward converters where moderate switching speed and robust SOA are prioritized over ultra-fast transitions. With t_ON = 50 ns and t_OFF = 500 ns, it supports practical operation up to ~100 kHz with appropriate drive and snubbing, but efficiency drops significantly above 50 kHz due to switching losses. Its 25 W power dissipation and TO-126 thermal interface suit low-to-medium power stages (≤20 W), but modern SMPS designs typically favor MOSFETs or faster BJTs. The BD3756STU remains viable for retrofit or legacy supply designs where proven reliability and availability outweigh speed requirements.