onsemi BD435STU

Overview

BD435STU from ON Semiconductor (formerly Fairchild) is a medium-power NPN epitaxial silicon transistor in TO-126 package, rated for 32 V V_CEO, 4 A DC collector current, and 36 W power dissipation at T_C = 25°C. It serves as a switching or linear amplifier in power supply regulation, motor control, and relay drivers where robust thermal performance and reliable saturation behavior are required.

For engineers reviewing the BD435STU datasheet, pinout, applications, or equivalent options, key selection criteria include its 32 V breakdown rating, 0.2–0.5 V V_CE(sat) at I_C = 2 A / I_B = 0.2 A, h_FE range of 40–130, TO-126 thermal interface capability, and compatibility with complementary PNP BD436 devices.

Technical Context

The BD435STU operates as a single NPN bipolar junction transistor optimized for medium-power linear and switching applications. Its design emphasizes stable DC current gain (h_FE ≥ 40 at I_C = 10 mA, V_CE = 5 V), low saturation voltage (V_CE(sat) ≤ 0.5 V under 2 A/0.2 A drive), and rugged safe operating area up to 36 W at case temperature 25°C.

It features a maximum junction temperature of 150°C, emitter-base voltage limit of 5 V, and pulsed collector current capability of 7 A. The device is not intended for RF or high-frequency amplification, with f_T specified at 3 MHz under V_CE = 1 V, I_C = 250 mA - confirming its classification as a general-purpose power switch rather than a broadband amplifier.

Key Specifications

Pinout & Package

BD435STU is housed in a TO-126 plastic package with integral metal tab for heatsinking. The package provides mechanical stability and thermal path via the collector-connected tab, requiring electrical isolation when mounted to chassis or shared heatsinks.

Key Features

Applications

Equivalent & Alternatives

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

Compared with BD435STU, the BD435 (BULK) offers identical performance in a different packaging format, while MJD44H11G provides higher voltage headroom and thermal capability at the cost of board space and layout change - making BD435STU optimal for cost-sensitive, space-constrained 24 V industrial switching where TO-126 is standardized.

Availability

BD435STU is available at Aetrix Electronics and suitable for power supply regulation, motor control, and relay/solenoid driving applications requiring stable component supply, long-term manufacturability, and consistent parametric performance across production lots.

Supply support for BD435STU 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

ON Semiconductor (formerly Fairchild Semiconductor) is a global supplier of energy-efficient semiconductor components, specializing in power management, analog, sensors, and discrete devices for automotive, industrial, and consumer markets.

The BD435STU belongs to the BD433/435/437 family of medium-power NPN transistors designed for industrial linear and switching applications where ruggedness, thermal reliability, and complementarity with PNP counterparts (BD434/436/438) are essential.

FAQ

What is the maximum collector-emitter voltage rating for BD435STU?

The BD435STU has a maximum collector-emitter voltage (V_CEO) rating of 32 V at T_A = 25°C. This rating applies under DC conditions with base open and defines the absolute upper limit for voltage across collector and emitter in switching or linear operation. Exceeding this value risks avalanche breakdown and permanent device failure. Derating is required above 25°C ambient per the power derating curve in the datasheet.

Does BD435STU have a built-in flyback diode for inductive load driving?

No, BD435STU does not include an integrated flyback diode. When used to switch inductive loads such as relays or solenoids, an external fast-recovery or Schottky diode must be connected across the load (cathode to V_CC, anode to collector) to suppress inductive kickback. Failure to do so may exceed V_CEO or V_CBO ratings and cause destructive voltage spikes damaging BD435STU.

What is the typical DC current gain (h_FE) of BD435STU at 10 mA collector current?

The typical DC current gain (h_FE) of BD435STU is 85 at V_CE = 5 V and I_C = 10 mA, with a guaranteed minimum of 40 and maximum of 130 under those conditions. This wide range reflects process variation and necessitates design margin in base drive calculations - especially critical when ensuring saturation at higher currents like 2 A where h_FE drops to ≥30.

Can BD435STU be used in parallel for higher current handling?

BD435STU is not recommended for direct paralleling without individual emitter resistors due to positive thermal feedback and h_FE mismatch. Even with matched units, unequal current sharing can occur because V_BE decreases with rising temperature, causing one device to hog current. If higher current is required, use a single higher-rated device (e.g., MJD44H11G) or implement active current balancing circuitry.

What is the thermal resistance from junction to case (R_θJC) for BD435STU?

The junction-to-case thermal resistance (R_θJC) for BD435STU is not explicitly published in the Fairchild BD433/435/437 datasheet. However, based on TO-126 package characteristics and power dissipation data (36 W at T_C = 25°C), R_θJC is estimated at approximately 0.7 °C/W. For precise thermal design, users should refer to ON Semiconductor's updated documentation or perform empirical thermal testing with the actual heatsink interface.