onsemi BUX85G

Overview

BUX85G from onsemi is an NPN silicon power transistor designed for high-voltage, high-speed switching in DC-DC converters, motor control systems, and switching regulators. It supports 450 V_CEO(sus), 2 A continuous collector current, 50 W power dissipation at T_C = 25 °C, and operates across −65 °C to +150 °C junction temperature range.

For engineers reviewing the BUX85G datasheet, pinout, applications, or equivalent options, key selection criteria include its 450 V sustaining voltage, TO-220 package thermal resistance (R_θJC = 2.5 °C/W), switching times (t_on = 0.3–0.5 µs, t_s = 2–3.5 µs), and h_FE = 30–50 at I_C = 0.1 A.

Technical Context

The BUX85G is a ruggedized, high-voltage NPN bipolar junction transistor optimized for hard-switching topologies requiring fast turn-on/turn-off with controlled storage time. Its 1000 V V_CES rating enables use in snubberless or high-transient industrial inverters.

It features low saturation voltage (V_CE(sat) ≤ 1.0 V at I_C = 1 A, I_B = 200 mA) and moderate f_T = 4 MHz, balancing switching speed and drive simplicity in medium-frequency (<100 kHz) power stages.

Key Specifications

Pinout & Package

Package: TO-220, Case 221A, Style 1 - 4-pin variant with pins 1 (Base), 2 (Collector), 3 (Emitter), 4 (Collector). Mounting tab is Collector-connected and electrically isolated via insulating washer when required.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJ15003G and BUW12A, the BUX85G uniquely balances 450 V sustain voltage, 2 A capability, TO-220 mountability, and sub-4 µs storage time-making it optimal for space-constrained, medium-power, medium-frequency industrial switching where voltage margin and thermal density are critical.

Availability

BUX85G is available at Aetrix Electronics and suitable for motor control systems, switching regulators, and DC-DC converters requiring stable component supply and long-term industrial lifecycle support.

Supply support for BUX85G 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

onsemi (formerly ON Semiconductor) is a global semiconductor supplier delivering energy-efficient, intelligent power and sensing solutions for automotive, industrial, cloud, medical, and IoT applications.

The BUX85G belongs to onsemi's high-voltage bipolar power transistor product line, engineered specifically for rugged, cost-effective switching in industrial power conversion and motor drive circuits where reliability under transient stress is essential.

FAQ

What is the maximum safe operating voltage for BUX85G in inductive switching applications?

The BUX85G is rated for V_CEO(sus) = 450 V under defined inductive test conditions (I_C = 100 mA, L = 25 mH). This is the maximum voltage it can sustain during turn-off in switching applications like converters and motor drives. Exceeding this value risks avalanche failure unless external clamping is used. The higher V_CES = 1000 V applies only under open-base, non-switching conditions and does not guarantee safe inductive switching. Always design with margin below 450 V for reliability.

Can BUX85G be used as a direct replacement for BU208A in a 250 V flyback converter?

No, BUX85G is not a direct replacement for BU208A. While both are TO-220 NPN transistors, BU208A has V_CEO = 1500 V and is optimized for very high-voltage, low-frequency applications. BUX85G's 450 V_CEO(sus) is insufficient for BU208A's typical use cases. Substituting BUX85G in a design originally using BU208A would likely result in catastrophic failure under normal operating voltage. Always verify voltage, current, and SOA requirements before substitution.

What is the recommended base drive current for BUX85G at 1 A collector load?

For reliable saturation at I_C = 1 A, the BUX85G datasheet specifies V_CE(sat) ≤ 1.0 V with I_B = 200 mA. Given its h_FE range of 30–50, a minimum base current of 20–33 mA is theoretically sufficient-but practical design requires ≥200 mA peak to ensure deep saturation and minimize switching losses. Use a current-source driver capable of delivering ≥200 mA with fast rise/fall times to meet the BUX85G's 0.3 µs t_on requirement.

Does BUX85G require a heatsink in continuous 2 A operation?

Yes, BUX85G requires a heatsink for continuous 2 A operation. At full I_C = 2 A and typical V_CE(sat) ≈ 0.9 V, conduction loss alone is ~1.8 W. However, total power dissipation-including switching losses-can exceed 10 W in real-world 20–50 kHz operation. With R_θJC = 2.5 °C/W and max T_J = 150 °C, even 10 W demands a heatsink keeping T_C ≤ 125 °C. Without forced air, a minimum 25 cm² aluminum heatsink is recommended for reliable long-term operation.

Is BUX85G suitable for Class D audio amplifier output stages?

No, BUX85G is not suitable for Class D audio amplifier output stages. Its f_T = 4 MHz and switching times (t_s = 2–3.5 µs) limit usable bandwidth to ~100 kHz, which is marginal for high-fidelity audio PWM (typically >300 kHz). Additionally, its SOA and thermal characteristics are optimized for power conversion-not linear amplification-and lack the matched pair stability required for push-pull audio output. Use dedicated audio power transistors (e.g., MJE15030/MJE15031) instead of BUX85G.