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Home > Products > Discrete Semiconductor Products > Transistors > Bipolar (BJT) > Single Bipolar Transistors

onsemi MJE521

Name: MJE521
Brand: onsemi
SKU: MJE521
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: MJE521

Manufacturer:: onsemi

Category:: Single Bipolar Transistors

Package:: TO-225AA, TO-126-3

Datasheet:: MJE521.pdf

Description:: TRANS NPN 40V 4A TO-126

Quantity:: RFQ Add To RFQ list

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Inventory:7,534

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Product details

Overview

MJE521 from STMicroelectronics is a silicon epitaxial-base NPN power transistor in SOT-32 (TO-126) package, rated for 4 A continuous collector current, 40 V V_CEO, and 40 W total dissipation at T_c ≤ 25 °C. It serves as a linear amplifier or switch in mid-power audio stages and general-purpose amplification circuits.

For engineers reviewing the MJE521 datasheet, MJE521 pinout, MJE521 application, or MJE521 equivalent, key selection criteria include its 40 V breakdown rating, 4 A DC current capability, pulsed 8 A peak current support, and thermal resistance of 3.12 °C/W junction-to-ambient - critical for heatsink-limited 5–20 W amplifier designs.

Technical Context

The MJE521 employs an epitaxial-base structure optimized for linear operation and switching in medium-power analog circuits. Its h_FE ≥ 40 at I_C = 1 A and V_CE = 1 V ensures stable biasing in Class AB audio output stages, while the 40 V V_CEO(sus) rating supports reliable operation under transient overvoltage conditions in unregulated supplies.

Designed for case-temperature–limited thermal management, it features a low R_thj-amb of 3.12 °C/W and maximum junction temperature of 150 °C. The SOT-32 package enables direct mounting to heatsinks via the collector tab, supporting conduction-cooled operation without isolation hardware.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	40 V - Maximum safe collector-emitter voltage with base open; defines rail voltage limit in amplifier output stages.
I_C	4 A - Continuous DC collector current; sets maximum quiescent and signal-current handling in Class AB drivers.
P_tot	40 W at T_c ≤ 25 °C - Total power dissipation limit when case is actively cooled; determines heatsink sizing baseline.
h_FE	≥40 at I_C = 1 A, V_CE = 1 V - Minimum DC current gain; ensures predictable base drive requirements in linear bias networks.
R_thj-amb	3.12 °C/W - Junction-to-ambient thermal resistance; used to calculate worst-case junction temperature under given ambient and heatsink conditions.
T_j(max)	150 °C - Absolute maximum operating junction temperature; constrains thermal design margin in sealed enclosures.

Pinout & Package

SOT-32 (TO-126) package with isolated collector tab for direct heatsink mounting; plastic body with three leads arranged in line. Collector connects electrically and thermally to the metal tab.

Pin/Terminal	Circuit Role	Design Meaning
1	Base	Control terminal for forward-biased injection; requires current-limited drive to maintain h_FE-based gain stability.
2	Collector	Main power terminal and thermal path; electrically connected to metal tab for heatsink interface and low-inductance high-current routing.
3	Emitter	Current return path; typically grounded or connected to emitter resistor for bias stabilization in common-emitter configurations.

Key Features

Feature	Design Value
Epitaxial-base construction	Enables high f_T and consistent h_FE across production lots, improving repeatability in audio driver biasing.
40 W power dissipation rating	Supports continuous operation in 10–20 W push-pull output stages without forced air cooling when mounted on ≥1.5 K/W heatsinks.
40 V V_CEO(sus) rating	Allows safe operation with ±18 V rails in transformer-coupled or bridged amplifier topologies with moderate supply ripple.
Low R_thj-amb (3.12 °C/W)	Reduces required heatsink surface area by ~35% compared to TO-220 devices with similar P_tot ratings.

Applications

Hi-Fi Audio Output Stage

DC Motor Driver (Small Actuators)

Use Scenario: Single-ended or push-pull output stage in 10–15 W stereo amplifiers using discrete transistor topology.

IC Role / Device Role / Timing Role: Linear power amplification device operating in Class AB mode with emitter-follower or common-emitter configuration.

Use Value: Delivers low-distortion output with h_FE ≥ 40 ensuring stable bias current under thermal drift, and 40 V rating accommodates typical ±15 V rails.

Use Scenario: H-bridge half-bridge switch controlling 12 V/2 A brushed DC motors in industrial positioning systems.

IC Role / Device Role / Timing Role: High-current saturated switch with fast turn-on/turn-off enabled by low base charge requirement.

Use Value: 8 A pulsed current capability handles motor stall transients; 4 A DC rating supports continuous 1.5 A load current with 50% duty cycle.

Regulated Power Supply Pass Element

General-Purpose Switching Amplifier

Use Scenario: Series pass transistor in adjustable 0–30 V/3 A linear bench power supply.

IC Role / Device Role / Timing Role: Voltage-controlled current source regulating output voltage via feedback loop error amplifier.

Use Value: 40 W dissipation allows 10 V dropout at 3 A without exceeding T_j = 150 °C when heatsinked to 2.5 K/W.

Use Scenario: Switch-mode audio amplifier output stage in Class D gate driver interface or PWM modulator output buffer.

IC Role / Device Role / Timing Role: Fast-switching power transistor driving inductive loads with controlled rise/fall times.

Use Value: Epitaxial base reduces storage time, enabling clean switching up to 20 kHz without external Baker clamp.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
2N3055	TO-3 metal can package; higher P_tot (115 W), but R_thj-amb ≈ 2.5 °C/W only with large heatsink; h_FE min = 20 at I_C = 4 A.	Requires larger mechanical footprint and isolation hardware; better suited for >30 W linear supplies.	Select when higher power headroom and legacy board compatibility outweigh SOT-32 space savings.
TIP31C	TO-220 package; same V_CEO = 100 V, but lower P_tot = 40 W at T_c = 25 °C and R_thj-amb = 3.75 °C/W; h_FE min = 10 at I_C = 3 A.	Higher voltage margin but reduced thermal efficiency and gain consistency; common in cost-sensitive industrial controls.	Select when 100 V rating is mandatory and thermal margin permits larger heatsink volume.

Compared with 2N3055 and TIP31C, the MJE521 offers superior thermal resistance per unit footprint and tighter h_FE specification - making it preferred for compact, thermally constrained 10–20 W audio and switching amplifier designs where gain predictability matters.

Availability

MJE521 is available at Aetrix Electronics and suitable for Hi-Fi audio output stages, regulated linear power supplies, and DC motor driver circuits requiring stable component supply and long-term obsolescence mitigation planning.

Supply support for MJE521 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, power, microcontroller, and sensor solutions for industrial, automotive, and consumer markets.

The MJE521 belongs to ST's legacy bipolar power transistor product line, developed specifically for discrete audio amplifier and linear regulator applications demanding robust thermal performance and repeatable DC gain.

FAQ

Is the MJE521 still in active production?

No - STMicroelectronics has marked the MJE521 as obsolete per its official documentation (document revision September 2003). However, Aetrix Electronics maintains verified legacy stock with full traceability and provides extended lifecycle support including cross-reference engineering and obsolescence mitigation planning.

Can the MJE521 be used in surface-mount assemblies?

No - the MJE521 uses the through-hole SOT-32 (TO-126) package with a metal collector tab designed for mechanical screw-mounting to heatsinks. It lacks solder pads or land patterns compatible with reflow or wave soldering processes.

What is the safe operating area (SOA) limitation at 100 °C case temperature?

At T_c = 100 °C, the MJE521's maximum continuous collector current drops to approximately 2.5 A due to thermal derating. SOA curves in the original ST datasheet show second-breakdown limits of 20 V × 2 A for DC operation, requiring careful layout to avoid localized hot spots.

Does the MJE521 require a base-emitter resistor in switching applications?

Yes - a base-emitter resistor (typically 1–10 kΩ) is recommended to prevent leakage-induced turn-on during high-temperature operation or when driven by open-collector logic. The 4 V V_EBO rating confirms susceptibility to spurious conduction without this safeguard.

MJE521 Specifications

Product attributes: Attribute value

Manufacturer:: onsemi

Series:: -

Package/Case:: TO-225AA, TO-126-3

Packaging:: Bulk

Product Status:: Obsolete

Transistor Type:: NPN

Current - Collector (Ic) (Max):: 4 A

Voltage - Collector Emitter Breakdown (Max):: 40 V

Vce Saturation (Max) @ Ib, Ic:: -

Current - Collector Cutoff (Max):: 100µA (ICBO)

DC Current Gain (hFE) (Min) @ Ic, Vce:: 40 @ 1A, 1V

Power - Max:: 40 W

Frequency - Transition:: -

Operating Temperature:: -65°C ~ 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-126

MJE521 FAQ

1.How can I place an order for MJE521 through Aetrix?

Please submit a Request for Quotation (RFQ) for MJE521 on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for MJE521 reliable?

The price and inventory of MJE521 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for MJE521 is usually 5 days.

3.What payment methods are accepted for MJE521?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for MJE521 transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for MJE521?

MJE521 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your MJE521 order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for MJE521?

For technical support, including MJE521 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your MJE521 requirements.

6.How does Aetrix verify that MJE521 is sourced from the original manufacturer or authorized distributors?

All MJE521 products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that MJE521 meets industry standards.

7.What is the process for return or replacement of MJE521?

All MJE521 units undergo pre-shipment inspection (PSI). If there is an issue with MJE521, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The MJE521 part is unused and in its original packaging.

Return procedure for MJE521:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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