NXP Semiconductors BUT12AX,127

Overview

BUT12AX from NXP Semiconductors (formerly Philips) is a high-voltage, high-speed NPN silicon diffused power transistor in an isolated TO-220F (SOT186A) package, rated for VCESM ≤1000 V, IC ≤8 A, Ptot ≤23 W, and tf ≤0.8 µs. It serves as a switching element in high-voltage DC-DC converters, motor control systems, and inverters where fast turn-off and high breakdown voltage are critical.

For engineers reviewing the BUT12AX datasheet, BUT12AX pinout, BUT12AX application, or BUT12AX equivalent, this device is selected for rugged high-voltage switching in industrial power supplies, flyback converters, and solid-state relay drivers requiring isolated heatsink mounting and sub-microsecond fall time.

Technical Context

The BUT12AX employs a diffusion-based NPN structure optimized for high-voltage blocking and fast switching under inductive loads. Its isolated mounting base enables direct heatsink attachment without electrical isolation components, while its 1000 V peak collector-emitter voltage supports operation across wide input ranges in offline converters.

Dynamic performance is characterized by carrier storage time (ts) ≤4 µs and fall time (tf) ≤0.8 µs under resistive load conditions, with verified safe operating area (SOA) up to 20 A peak collector current at VCE = 100 V. Thermal resistance from junction to heatsink is 3.9 K/W with heatsink compound, enabling stable operation at Tj ≤150 °C.

Key Specifications

Pinout & Package

Package: TO-220F (SOT186A), plastic single-ended package with isolated mounting base, 1 mounting hole, and 3 leads. Mounting base is electrically isolated from all terminals and rated for 2500 V RMS isolation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar high-voltage NPN switching applications.

Compared with BUT11AX and BU508A, the BUT12AX uniquely balances 1000 V blocking, 8 A current, and isolated TO-220F packaging-enabling compact, thermally efficient designs in 400–800 V industrial power stages without added isolation hardware.

Availability

BUT12AX is available at Aetrix Electronics and suitable for industrial flyback converters, AC motor drive half-bridges, and high-voltage solid-state relays requiring stable component supply and long-term lifecycle support.

Supply support for BUT12AX 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

NXP Semiconductors is a global semiconductor leader focused on secure connectivity solutions for automotive, industrial, and IoT applications, with roots in Philips' pioneering analog and power transistor development.

The BUT12AX belongs to NXP's legacy high-voltage bipolar power transistor family, designed specifically for ruggedized industrial switching applications demanding high breakdown voltage, fast dynamics, and integrated thermal isolation.

FAQ

What is the maximum junction temperature rating for BUT12AX?

The BUT12AX has a maximum junction temperature (Tj) rating of +150 °C, as specified in Table 3 of its product data sheet. This rating must be respected in thermal design-actual operating Tj depends on ambient conditions, heatsink performance, and power dissipation. Derating curves in Figure 1 show allowable power reduction above 25 °C heatsink temperature. The BUT12AX must not exceed +150 °C Tj during continuous operation to ensure reliability and avoid permanent degradation.

Does BUT12AX have a built-in base-emitter resistor?

No, the BUT12AX does not include any internal base-emitter resistor. It is a standard three-terminal NPN bipolar junction transistor requiring external base drive circuitry-including current-limiting resistors and, typically, a base-emitter clamp diode for inductive switching. The datasheet shows no internal resistor in the simplified outline or electrical characteristics tables, and all hFE and VBEsat measurements assume externally applied base current. Designers must provide discrete base bias components for BUT12AX.

Can BUT12AX replace BU2508DX in a 400 V DC-DC converter?

The BUT12AX is not a direct replacement for BU2508DX due to key differences: BU2508DX is a Darlington transistor with higher hFE (>1000) and lower drive requirement, whereas BUT12AX is a standard NPN with hFE = 10–35 and needs ≥1 A base current for full saturation. Additionally, BU2508DX has VCEO = 1500 V but lower speed (tf ≈ 2 µs). In a 400 V DC-DC converter, BUT12AX may work if base drive is redesigned-but switching loss and gate driver compatibility must be revalidated. The BUT12AX cannot be substituted without circuit modification.

What is the isolation voltage rating between the mounting base and terminals of BUT12AX?

The BUT12AX provides 2500 V RMS isolation voltage (Visol(RMS)M) between all three terminals (base, collector, emitter) and the external heatsink/mounting base, as confirmed in Table 6 of the datasheet. This rating applies under 50–60 Hz sinusoidal waveform, RH ≤65%, and clean/dust-free conditions. It enables safe mounting to grounded metal chassis in industrial equipment without additional insulating hardware-critical for meeting safety standards like IEC 61800-5-1.

Is BUT12AX still in active production by NXP?

The BUT12AX was documented as "Production" status in its Rev. 01 datasheet dated 16 June 2004, and remains listed in NXP's legacy product portfolio. While not featured in current marketing materials, it is supported through authorized distributors with verified stock and extended lifecycle management. Aetrix Electronics maintains traceable inventory and offers obsolescence mitigation support for BUT12AX, including cross-reference guidance and long-term supply planning for industrial customers relying on this proven high-voltage transistor.