onsemi BD438S

Overview

BD438S 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 motor drivers and relay control circuits.

For engineers reviewing the BD438S datasheet, pinout, applications, or equivalent options, key selection factors include its -45 V breakdown rating, -0.2 V to -0.6 V saturation voltage under 2 A load, -1.2 V base-emitter ON voltage, 3 MHz f_T, and TO-126 thermal performance with case temperature derating above 25°C.

Technical Context

The BD438S operates as a complementary PNP counterpart to the NPN BD437, enabling push-pull and H-bridge configurations. Its safe operating area (SOA) supports pulsed collector currents up to -7 A with 300 µs pulse width and 1.5% duty cycle, while junction temperature remains within 150°C limits.

Designed for linear and switching use, it features low V_CE(sat) (-0.2 V min at I_C = -2 A, I_B = -0.2 A), controlled h_FE spread (30–140), and guaranteed -100 µA ICEO at -45 V V_CE, ensuring stable biasing in high-voltage analog stages and robust turn-off in saturated switch applications.

Key Specifications

Pinout & Package

BD438S is housed in a TO-126 plastic power package with integral metal tab for heatsink mounting. The case is electrically connected to the collector terminal.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJD438T4G and BD438, the BD438S offers identical core performance in through-hole TO-126 form with higher thermal capacity (36 W vs. 20 W), making it preferred for high-reliability through-hole motor and relay drivers where heatsink attachment is critical.

Availability

BD438S is available at Aetrix Electronics and suitable for industrial motor drives, relay control circuits, and linear regulator designs requiring stable component supply and long-term manufacturability.

Supply support for BD438S 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 devices, analog ICs, and interface solutions before its acquisition by ON Semiconductor in 2016.

The BD438S belongs to Fairchild's BD43x series of medium-power PNP transistors, designed specifically for complementary switching and linear applications in industrial controls, power supplies, and motor interfaces.

FAQ

What is the maximum collector-emitter voltage rating for BD438S?

The BD438S has a guaranteed V_CEO rating of -45 V at T_C = 25°C. This value represents the maximum voltage the device can sustain between collector and emitter with base open, and is validated per Fairchild's absolute maximum ratings table. Operation beyond this voltage risks avalanche breakdown. Derating applies at elevated case temperatures per the datasheet's power derating curve.

Is BD438S pin-compatible with BD437?

No - BD438S is a PNP transistor while BD437 is its NPN complement; they share identical TO-126 pinout (Emitter-Collector-Base), but polarity and biasing requirements differ. BD438S pin 1 is Emitter, pin 2 is Collector, pin 3 is Base - same physical layout as BD437, but used in mirrored circuit roles such as high-side vs. low-side switching.

What is the typical V_CE(sat) of BD438S at 2 A collector current?

The BD438S 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 directly reduces conduction losses and improves efficiency in switching applications, especially when driving inductive loads like relays or small DC motors.

Does BD438S require a heatsink in normal operation?

Yes - BD438S dissipates up to 36 W at T_C = 25°C, but its thermal resistance (R_θJC) requires external heatsinking for sustained loads above ~5 W. Without a heatsink, power handling drops sharply above ambient temperature; the TO-126 metal tab must be mounted to a thermally conductive surface with thermal compound to maintain junction temperature below 150°C.

What is the DC current gain (h_FE) range for BD438S?

The BD438S has a specified h_FE range of 30 to 140, depending on test conditions: minimum 30 at V_CE = -1 V, I_C = -500 mA; maximum 140 at V_CE = -1 V, I_C = -2 A. This spread allows robust base drive design across production lots while maintaining predictable switching thresholds and linear gain behavior.