onsemi BD438STU

Overview

BD438STU from Fairchild Semiconductor is a PNP epitaxial silicon power transistor in TO-126 package, rated for -45 V V_CEO, -4 A continuous collector current, 36 W power dissipation at T_C=25°C, and DC current gain (h_FE) of 30–140 at I_C = -500 mA to -2 A. It serves as a medium-power linear amplifier or switching device in industrial power supplies and motor control circuits.

For engineers reviewing the BD438STU datasheet, pinout, applications, or equivalent options, key selection factors include its -45 V V_CEO rating, -0.2 V to -0.6 V V_CE(sat) at I_C = -2 A / I_B = -0.2 A, TO-126 thermal performance, and complementary pairing with BD437 in push-pull output stages.

Technical Context

The BD438STU operates as a high-voltage PNP bipolar junction transistor optimized for linear regulation and hard-switching applications. Its V_CEO = -45 V, V_CBO = -45 V, and V_EBO = -5 V define safe operating limits under reverse-biased conditions, while f_T = 3 MHz supports moderate-speed switching up to ~100 kHz in Class AB amplifiers or PWM-driven loads.

Thermal design relies on its TO-126 package with case-to-ambient thermal resistance (R_θJA) implied by 36 W P_C at T_C = 25°C and derating curve showing zero dissipation at ~200°C case temperature. Safe Operating Area (SOA) curves confirm pulsed capability up to -7 A with 10 ms pulse width and 1.5% duty cycle.

Key Specifications

Pinout & Package

BD438STU uses the TO-126 package: molded plastic case with metal tab for heatsinking, 3-pin through-hole configuration, and standardized pin assignment per JEDEC TO-126 outline.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PNP power transistor applications.

Compared with MJD438G and BD438, the BD438STU offers proven through-hole reliability and thermal interface flexibility in TO-126, while maintaining identical core electrical performance-making it optimal for industrial repair, prototyping, and mixed-technology boards.

Availability

BD438STU is available at Aetrix Electronics and suitable for industrial power supplies, motor control circuits, and relay driver modules requiring stable component supply and long-term obsolescence management.

Supply support for BD438STU 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 BD438STU belongs to Fairchild's legacy medium-power bipolar transistor family designed for robust linear regulation and switching in industrial and automotive-adjacent equipment.

FAQ

What is the maximum continuous collector current rating for BD438STU?

The BD438STU has a maximum continuous collector current (I_C) rating of -4 A at case temperature T_C = 25°C. This rating decreases with rising case temperature per the published derating curve, reaching zero dissipation at approximately 200°C. For sustained operation above 10 W, a heatsink is required to maintain T_C within safe limits. The BD438STU must be operated within its Safe Operating Area (SOA) boundaries to avoid secondary breakdown.

Is BD438STU pin-compatible with BD437?

No, BD438STU is not pin-compatible with BD437. BD438STU is a PNP transistor in TO-126 package with pinout Emitter–Collector–Base (1–2–3), while BD437 is an NPN transistor with identical TO-126 mechanical outline but reversed polarity and complementary electrical behavior. They are electrically complementary-not pin-compatible-and are intended for paired use in push-pull or H-bridge topologies, not direct substitution.

What is the typical V_CE(sat) of BD438STU under switching conditions?

The BD438STU exhibits a typical V_CE(sat) of -0.2 V at I_C = -2 A and I_B = -0.2 A, with a maximum of -0.6 V under the same conditions. This low saturation voltage minimizes conduction losses in switching applications such as relay drivers or PWM-controlled loads. Designers should verify base drive strength to ensure full saturation, as insufficient I_B will increase V_CE(sat) and thermal stress on the BD438STU.

Does BD438STU require a heatsink in linear regulator applications?

Yes, BD438STU requires a heatsink in linear regulator applications where power dissipation exceeds ~5 W. With a maximum P_C of 36 W at T_C = 25°C and thermal derating of ~0.36 W/°C above 25°C, even modest dropout voltages (e.g., 5 V at 1 A = 5 W) generate significant heat. Without forced air or a properly sized heatsink, junction temperature can exceed 150°C. Thermal design must account for R_θJC, R_θCS, and R_θSA to ensure safe operation of the BD438STU.

What is the DC current gain (h_FE) range of BD438STU across operating conditions?

The BD438STU has a DC current gain (h_FE) ranging from 30 to 140, depending on collector current and V_CE. At I_C = -500 mA and V_CE = -1 V, h_FE is typically 30–85; at I_C = -2 A and V_CE = -1 V, it drops to 30–40. This wide variation means base drive must be designed for worst-case h_FE to guarantee saturation. The BD438STU's h_FE is not guaranteed at very low or very high currents outside datasheet test conditions.