onsemi KSD1943TU

Overview

KSD1943TU from ON Semiconductor (formerly Fairchild) is an NPN epitaxial silicon power transistor in TO-220 package, rated for 60 V V_CEO, 3 A I_C, and 40 W P_C at T_a=25°C, commonly used in linear power supplies and audio amplifier output stages.

For engineers reviewing the KSD1943TU datasheet, pinout, applications, or equivalent options, key selection criteria include DC current gain (h_FE = 400–2000), low V_CE(sat) (1.0 V @ I_C=2 A, I_B=0.05 A), thermal derating behavior, and TO-220 mechanical compatibility with heatsink mounting.

Technical Context

The KSD1943TU operates as a high-current, medium-voltage NPN switching and linear amplification device. Its design supports continuous collector current up to 3 A with junction temperature limited to 150°C, and exhibits stable h_FE across I_C = 0.1–10 A at V_CE = 4 V.

It features low saturation voltages-V_BE(sat) = 1.5 V and V_CE(sat) = 1.0 V under specified drive conditions-and maintains safe operating area (SOA) compliance up to DC operation at reduced voltage/current combinations per Figure 4 of the datasheet.

Key Specifications

Pinout & Package

Package: TO-220 (3-lead, straight lead form, insulated tab). Mounting surface is electrically connected to collector; requires insulating washer when mounted to grounded heatsink.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MJD1943G and 2SC5200, the KSD1943TU offers optimal balance of voltage rating, continuous current capability, and through-hole TO-220 mechanical robustness for industrial linear regulation and medium-power switching-without requiring PCB redesign or compromising thermal interface integrity.

Availability

KSD1943TU is available at Aetrix Electronics and suitable for linear power supplies, audio amplifier output stages, DC motor drivers, and relay/solenoid control circuits requiring stable component supply and long-term industrial availability.

Supply support for KSD1943TU 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

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

The KSD1943TU belongs to Fairchild's legacy high-power bipolar transistor family, designed specifically for cost-sensitive, thermally demanding linear and switching applications where ruggedness and predictable gain behavior are critical.

FAQ

What is the maximum continuous collector current rating for the KSD1943TU?

The KSD1943TU is rated for 3 A continuous collector current (I_C) at ambient temperature T_a = 25°C. This rating decreases with rising case temperature per the power derating curve (Figure 5), reaching zero at T_C ≈ 175°C. For reliable operation, heatsinking must maintain T_C within limits defined by the 40 W P_C rating and thermal resistance R_θJC.

Is the KSD1943TU pin-compatible with other TO-220 NPN transistors like the 2N3055?

No-the KSD1943TU uses Base–Collector–Emitter pinout (1–2–3), whereas the 2N3055 uses Emitter–Base–Collector (1–2–3). Swapping them without verifying pin assignment risks circuit damage. Always confirm pin order using the official KSD1943TU datasheet diagram before PCB layout or replacement.

Does the KSD1943TU have an insulated tab, and what does that mean for heatsink mounting?

Yes-the KSD1943TU TO-220 package has an electrically insulated metal tab connected to the collector. When mounting to a grounded heatsink, an insulating washer and shoulder washer are required to prevent shorting. Failure to insulate may cause collector-to-chassis shorts and catastrophic failure of the KSD1943TU and associated circuitry.

What is the typical DC current gain (h_FE) of the KSD1943TU at 2 A collector current?

While h_FE is specified as 400–2000 at I_C = 0.5 A, typical curves (Figure 2) show h_FE ≈ 120–180 at I_C = 2 A and V_CE = 4 V. Designers should verify actual gain under operating conditions, as h_FE declines with increasing I_C; the KSD1943TU remains usable for 2 A loads with appropriate base drive scaling.

Can the KSD1943TU be used in switching power supplies operating above 20 kHz?

The KSD1943TU is not optimized for high-frequency switching. Its transition frequency f_T is not specified in the datasheet, and its stored charge and fall time characteristics limit practical use to ≤20 kHz. For SMPS applications, MOSFETs or RF-optimized BJTs are preferred; the KSD1943TU is best applied in linear regulation or low-frequency switching where its high current and rugged SOA provide advantage.