onsemi BCW31

Overview

BCW31 from Fairchild Semiconductor is an NPN general-purpose bipolar junction transistor (BJT) designed for amplification and switching in low-to-medium current applications. It features a 32 V V_CEO, 500 mA I_C, DC current gain (h_FE) of 110–220 at 2 mA, V_CE(sat) ≤ 0.25 V at 10 mA/0.5 mA drive, and operates across –55°C to +150°C. It is commonly used in signal amplification stages of consumer audio preamps and LED driver bias circuits.

For engineers reviewing the BCW31 datasheet, pinout, applications, or equivalent options, key selection considerations include its SOT-23 package footprint, low saturation voltage for efficient switching, f_T = 200 MHz for RF-capable small-signal use, thermal resistance of 357°C/W, and compatibility with standard 2.9 mm × 1.3 mm PCB land patterns.

Technical Context

The BCW31 is a silicon planar epitaxial NPN transistor fabricated using Fairchild's Process 10. Its design targets general-purpose linear amplification and digital switching up to 300 mA DC collector current, with verified operation under pulsed conditions per factory consultation guidance.

It exhibits low noise figure (10 dB at 1 kHz, 0.2 mA), moderate output capacitance (4.0 pF at 10 V), and stable h_FE across temperature - enabling predictable gain in bias networks and active filters without external compensation.

Key Specifications

Pinout & Package

SOT-23 plastic surface-mount package (3-pin, 2.90 mm × 1.30 mm × 1.14 mm max height), marked "D1", with gull-wing leads and standard JEDEC MO-178 footprint.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN general-purpose amplifier applications.

Compared with BCW31, BC846B offers higher voltage tolerance and gain but may require revalidation of bias networks due to h_FE spread; MMBT3904 trades 150 mW power capability for broader availability and tighter cost control in high-volume logic interface roles.

Availability

BCW31 is available at Aetrix Electronics and suitable for audio signal amplification, LED driver bias control, industrial sensor interface, and logic-level translation requiring stable component supply and long-term manufacturability.

Supply support for BCW31 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 management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016.

The BCW31 belongs to Fairchild's legacy general-purpose BJT product line, engineered for reliability and consistency in cost-sensitive linear and switching applications across consumer, industrial, and computing markets.

FAQ

What is the maximum continuous collector current rating for BCW31?

The BCW31 has a maximum continuous collector current (I_C) rating of 500 mA at T_A = 25°C. Derating applies above 25°C ambient per its 2.8 mW/°C thermal derating coefficient. At 100°C ambient, usable I_C drops to approximately 300 mA to maintain junction temperature ≤150°C. This rating is validated for DC and low-duty-cycle pulsed operation; consult Fairchild for high-frequency switching profiles.

Is BCW31 suitable for RF amplifier applications?

Yes, BCW31 is suitable for RF amplifier applications up to ~200 MHz due to its specified f_T of 200 MHz at I_C = 2 mA and V_CE = 5 V. Its 4.0 pF C_obo and low noise figure (10 dB at 1 kHz) support VHF-band small-signal amplification, though layout parasitics and bias stability must be carefully managed. It is not recommended for high-power RF output stages.

What is the pin configuration of BCW31 in the SOT-23 package?

The BCW31 uses standard SOT-23 pinout: Pin 1 is Base, Pin 2 is Emitter, and Pin 3 is Collector - confirmed by Fairchild's datasheet diagram and marking "D1" orientation. This matches JEDEC TO-236AB and is identical to BC846B and MMBT3904, enabling direct footprint reuse in many designs.

Does BCW31 have a specified thermal resistance value?

Yes, BCW31 has a specified R_θJA of 357°C/W when mounted on a standard FR-4 PCB (40 mm × 40 mm × 1.5 mm). This value assumes no copper pour or thermal vias. For improved thermal performance, designers should add thermal pads and vias under the collector pad, which can reduce effective R_θJA by 30–50% depending on layout.

Can BCW31 replace BC846B in existing designs?

BCW31 is not a direct replacement for BC846B due to lower V_CEO (32 V vs 65 V) and narrower h_FE range (110–220 vs 200–450). While pinout and package match, substituting BCW31 may cause premature breakdown in 5 V+ rail applications or gain-dependent instability in high-precision bias networks. Validation of voltage margins and AC performance is required before substitution.