STMicroelectronics STN951

Overview

STN951 from STMicroelectronics is a PNP silicon planar power transistor in SOT-223 package, rated for -60 V V_CEO, -5 A I_C, 1.6 W P_tot, with V_CE(sat) as low as -50 mV at I_C = -100 mA/I_B = -5 mA, used in low-voltage switching circuits such as relay drivers and emergency lighting control stages.

For engineers reviewing the STN951 datasheet, STN951 pinout, STN951 application, or STN951 equivalent, key selection criteria include verified V_CE(sat) performance at high current, h_FE stability across I_C = -10 mA to -10 A, thermal resistance (R_thj-amb = 78 °C/W on 1 cm² PCB), and SOT-223 mounting compatibility with medium-power DC-DC and regulator designs.

Technical Context

The STN951 uses a "Base Island" layout in PNP planar technology to achieve high DC current gain (h_FE = 150 min at I_C = -10 mA; 65 min at I_C = -5 A) while maintaining ultra-low saturation voltage - down to -50 mV at light load and -350 mV at I_C = -5 A/I_B = -200 mA. Its f_T of 130 MHz supports fast switching in resistive-load applications.

Thermal design is constrained by R_thj-amb = 78 °C/W (on 1 cm² PCB), with T_j(max) = 150 °C and T_stg = -65 to 150 °C. Breakdown ratings are symmetrical: V_(BR)CBO = V_(BR)CEO = -60 V, and V_(BR)EBO = -6 V, enabling robust operation in low-voltage, high-current linear and switching modes.

Key Specifications

Pinout & Package

SOT-223 package: surface-mount, medium-power plastic case with exposed emitter pad (Pin 2) for thermal and electrical connection; standard footprint compatible with automated assembly and reflow profiles per JEDEC J-STD-020.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJD2955T4 and ZTX951, STN951 uniquely balances SOT-223 mountability, -5 A capability, and sub-350 mV V_CE(sat) up to full rating - making it optimal for space-constrained, medium-power PNP switching where thermal budget and efficiency are co-critical.

Availability

STN951 is available at Aetrix Electronics and suitable for emergency lighting systems, voltage regulators, and relay driver circuits requiring stable component supply, RoHS-compliant packaging, and proven thermal performance on standard FR4 PCBs.

Supply support for STN951 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, MCU, power, and sensor solutions for industrial, automotive, and consumer markets.

The STN951 belongs to ST's legacy power bipolar transistor product line, engineered specifically for high-gain, low-saturation switching in cost-sensitive, medium-power linear and PWM applications - emphasizing manufacturability, thermal reliability, and JEDEC-standard packaging.

FAQ

What is the maximum safe continuous collector current for STN951?

The absolute maximum continuous collector current (I_C) is -5 A at T_amb = 25 °C with proper PCB copper area (≥1 cm²). Derating is required above 25 °C ambient: current must be reduced by 63 mA/°C beyond 25 °C due to R_thj-amb = 78 °C/W and T_j(max) = 150 °C. At 70 °C ambient, max I_C drops to approximately -3.2 A.

Does STN951 require a heatsink in typical SOT-223 applications?

No external heatsink is required if mounted on ≥1 cm² of 2-oz copper on FR4 PCB, which achieves the specified R_thj-amb = 78 °C/W. With this layout, the device dissipates its full 1.6 W rating at 25 °C ambient. Smaller copper areas increase thermal resistance proportionally - e.g., 0.25 cm² raises R_thj-amb to ~150 °C/W, limiting usable power to ~0.8 W at 25 °C.

Can STN951 be used as a direct replacement for older NPN transistors in existing designs?

No - STN951 is a PNP device with inverted polarity: emitter connects to higher potential, collector to load/ground. Replacing an NPN (e.g., BC337) requires circuit redesign - including reversing biasing, swapping supply references, and validating h_FE and V_CE(sat) behavior under identical operating conditions. It is not a functional or pin-compatible substitute for NPN topologies.

What is the guaranteed minimum DC current gain (h_FE) at 5 A collector current?

The datasheet guarantees h_FE ≥ 65 at I_C = -5 A, V_CE = -1 V, and T_case = 25 °C. This value is measured under pulsed conditions to avoid self-heating. At elevated junction temperatures (>85 °C), h_FE decreases - typically to ~45 at T_j = 125 °C - so base drive must be sized conservatively for worst-case thermal conditions.