STMicroelectronics ST631K

Overview

ST631K from STMicroelectronics is a low-voltage PNP power transistor manufactured using epitaxial base technology, housed in an SOT-32 (TO-216) plastic package. It delivers V_CEO = –120 V, I_C = –1 A, and V_CE(sat) = –0.5 V at I_C = –500 mA / I_B = –50 mA, enabling use in CRT scanning velocity modulation and medium-power linear/switching stages.

For engineers reviewing the ST631K datasheet, ST631K pinout, ST631K application, or ST631K equivalent, key selection criteria include saturation voltage under defined drive conditions, thermal resistance (R_thJ-case = 10 °C/W), complementary pairing with ST600K, and SOT-32 mechanical compatibility for through-hole PCB layouts.

Technical Context

The ST631K operates as a medium-power PNP bipolar junction transistor optimized for linear amplification and hard-switching in analog and pulse-driven circuits. Its epitaxial base construction ensures stable h_FE (120–280 at I_C = –100 mA) and low V_CE(sat), critical for minimizing conduction loss in high-current, low-voltage rails.

Designed for operation up to T_J = 150 °C, it supports pulsed loads up to I_CM = –2 A (t_P < 5 ms) and features C_CBO = 40 pF at V_CB = –10 V, f = 1 MHz - suitable for moderate-frequency switching where capacitive loading must be controlled.

Key Specifications

Pinout & Package

Package: SOT-32 (TO-216), three-terminal through-hole plastic package with heat-dissipating tab (pin 2). Dimensions per JEDEC TO-216 outline: D = 15.4–16.0 mm length, B = 10.5–10.8 mm width, A = 7.4–7.8 mm height.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJD127 and BD140, the ST631K offers superior voltage margin (–120 V) in the compact SOT-32 form factor, making it optimal for space-constrained CRT and industrial 100 V applications where thermal design relies on PCB copper rather than discrete heatsinks.

Availability

ST631K is available at Aetrix Electronics and suitable for CRT display manufacturing, industrial relay drivers, linear audio amplifiers, and DC motor control systems requiring stable component supply and long-lifecycle support.

Supply support for ST631K 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, Switzerland, specializing in power management, analog, MEMS, and microcontrollers for industrial, automotive, and consumer markets.

The ST631K belongs to ST's legacy bipolar power transistor family, designed specifically for analog signal conditioning and medium-power switching in cost-sensitive, reliability-critical industrial and display equipment.

FAQ

Is the ST631K still in active production?

No - the ST631K is marked "Obsolete Product(s)" in its official datasheet (Rev 1, August 2005). However, Aetrix Electronics maintains legacy inventory with full traceability and offers engineering support for redesign evaluation and cross-reference alternatives.

What is the pin 2 connection in the SOT-32 package?

Pin 2 is the collector terminal and is internally connected to the metal tab of the SOT-32 (TO-216) package. This tab must be electrically isolated from other circuit nodes unless intentionally used as the collector node and thermally coupled to a heatsink or large copper area.

Can the ST631K replace the ST600K in a circuit?

No - the ST631K is a PNP device, while the ST600K is its complementary NPN counterpart. They are not interchangeable. Substituting one for the other would reverse polarity and cause immediate circuit failure or damage due to incorrect biasing and current direction.

Does the ST631K require a base resistor, and how is its value calculated?

Yes - a series base resistor is mandatory to limit I_B within –0.5 A absolute maximum. For reliable saturation at I_C = –500 mA, select I_B ≥ –50 mA (h_FE min ≈ 10), then calculate R_B = (V_drive − V_BE(sat)) / I_B, using V_BE(sat) = –1.2 V from datasheet.