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

STMicroelectronics BU931

Name: BU931
Brand: STMicroelectronics
SKU: BU931
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BU931

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-204AA, TO-3

Datasheet:: BU931.pdf

Description:: TRANS NPN DARL 400V 15A TO-3

Quantity:: RFQ Add To RFQ list

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Product details

Overview

BU931T from STMicroelectronics is an AEC-Q101-qualified NPN power Darlington transistor designed as a high-voltage ignition coil driver for automotive electronic ignition systems. It features 500 V V_CES, 10 A continuous collector current, 125 W total dissipation at T_C = 25 °C, and operates up to 175 °C junction temperature in TO-220 package.

For engineers reviewing the BU931T datasheet, BU931T pinout, BU931T application, or BU931T equivalent, key selection criteria include V_CEO(sus) = 400 V under inductive switching, V_CE(sat) ≤ 1.8 V at 10 A/250 mA drive, ruggedness against repetitive avalanche stress, and thermal resistance R_thJC = 1.2 °C/W for engine bay mounting.

Technical Context

The BU931T integrates a monolithic Darlington pair with integrated antiparallel freewheeling diode (V_F = 2.5 V @ 10 A), enabling direct inductive load switching without external flyback protection. Its multi-epitaxial planar bipolar process ensures stable h_FE ≥ 300 at I_C = 5 A and supports pulse operation up to 20 A peak.

Designed for ignition coil primary-side switching, it sustains 400 V during turn-off (V_CEO(sus)) with storage time t_s ≤ 15 μs and fall time t_f ≤ 0.5 μs under 7 mH inductive load, 12 V supply, and 47 Ω base-emitter resistor conditions.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CES	500 V - Withstands ignition coil back-EMF spikes without breakdown when base is open.
I_C	10 A continuous - Supports standard 12 V automotive ignition coils drawing up to 8–10 A primary current.
V_CE(sat)	1.8 V max @ 10 A/250 mA - Limits conduction loss to ≤18 W at full load, critical for thermal management.
R_thJC	1.2 °C/W - Enables direct heatsink mounting with minimal thermal interface resistance for engine compartment use.
t_s / t_f	≤15 μs / ≤0.5 μs - Ensures fast turn-off to prevent coil saturation and enable precise spark timing control.
h_FE	≥300 @ I_C = 5 A - Reduces required base drive current, easing MCU or driver IC output stage design.
T_j max	175 °C - Rated for under-hood environments where ambient exceeds 125 °C near exhaust manifolds.

Pinout & Package

BU931T uses TO-220 type A package with isolated tab (collector-connected). The case is electrically connected to the collector terminal, requiring insulating mounting hardware when mounted to grounded heatsinks.

Pin/Terminal	Circuit Role	Design Meaning
Collector (Tab)	Main power output node	Electrically tied to metal tab; must be insulated from chassis unless system design permits collector-ground connection.
Emitter	Power return path	Low-inductance emitter connection essential for stable switching; routed directly to battery ground or low-impedance ground plane.
Base	Control input	Receives current-driven signal (up to 1 A); requires series resistor to limit IB and ensure hFE-based saturation.

Key Features

Feature	Design Value
Integrated freewheeling diode	V_F = 2.5 V @ 10 A - Eliminates need for external flyback diode, reducing BOM count and PCB area in ignition modules.
AEC-Q101 qualification	Validated for automotive temperature cycling, humidity, and mechanical shock - meets OEM requirements for engine control unit deployment.
High V_CEO(sus)	400 V @ I_C = 100 mA - Sustains voltage during inductive turn-off without clamping circuitry, simplifying ECU power stage design.
Rugged bipolar process	Withstands repetitive unclamped inductive switching (UIS) events - critical for durability in misfire-prone or cold-start conditions.
TO-220 isolation	Collector-isolated tab with creepage/clearance per ECOPACK® - enables safe high-voltage isolation in compact engine bay layouts.

Applications

Gasoline Engine Ignition Control

Distributorless Ignition System (DIS)

Use Scenario: Direct drive of primary winding in single-cylinder or multi-coil-on-plug (COP) gasoline engines.

IC Role / Device Role / Timing Role: High-current, high-voltage switch controlling energy buildup and release in ignition coil primary.

Use Value: Delivers precise spark timing with ≤15 μs storage time and 400 V sustaining capability, supporting variable dwell control.

Use Scenario: Paired switching in dual-ignition DIS modules driving two cylinders simultaneously (waste-spark).

IC Role / Device Role / Timing Role: Primary-side power actuator synchronized to crankshaft position sensor signals.

Use Value: 10 A continuous rating and 20 A peak support simultaneous firing of paired cylinders without thermal derating.

Heavy-Duty Diesel Preheat Control

Two-Stroke Marine Engine Ignition

Use Scenario: Driving glow plug preheat circuits in commercial diesel engines during cold cranking.

IC Role / Device Role / Timing Role: High-reliability power switch managing 12 V/8–10 A resistive loads for 10–30 s durations.

Use Value: 175 °C T_j rating and 125 W dissipation allow sustained operation without forced cooling in confined engine compartments.

Use Scenario: Compact ignition module for outboard marine engines exposed to saltwater, vibration, and wide ambient swings.

IC Role / Device Role / Timing Role: Ruggedized primary switch replacing electromechanical points in legacy two-stroke platforms.

Use Value: AEC-Q101 qualification and TO-220 mechanical robustness ensure reliability in harsh marine environments with minimal maintenance.

Equivalent & Alternatives

The following parts are listed as comparable options for similar high-voltage Darlington ignition driver applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
STBU931P	TO-247 package, R_thJC = 0.8 °C/W, same electrical specs	Better thermal performance for high-duty-cycle or multi-cylinder applications	Select when heatsink space allows larger TO-247 and junction temperature margin is critical.
ON Semiconductor MJD127G	Lower V_CES = 100 V, I_C = 8 A, no integrated diode	Requires external flyback diode; unsuitable for 400+ V ignition systems	Only viable for low-voltage auxiliary loads (e.g., fuel pump drivers), not primary ignition.

Compared with BU931P, the BU931T trades 0.4 °C/W higher thermal resistance for lower profile and cost-effective TO-220 mounting; versus MJD127G, it delivers 4× higher voltage blocking and integrated protection essential for modern ignition coil interfaces.

Availability

BU931T is available at Aetrix Electronics and suitable for gasoline engine ignition control, distributorless ignition systems (DIS), and heavy-duty diesel preheat control requiring stable component supply across automotive production lifecycles.

Supply support for BU931T 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 automotive, industrial, and power discrete solutions.

The BU931T belongs to ST's high-voltage power transistor product line, engineered specifically for robust, high-energy switching in automotive engine management systems under extreme thermal and electrical stress.

FAQ

Is BU931T pin-compatible with BU931P?

No. BU931T uses TO-220 package with 3 leads in inline configuration, while BU931P uses TO-247 with 3 leads in staggered layout and different mounting hole spacing. PCB footprint and heatsink interface are not interchangeable without redesign.

Does BU931T require a base current limiting resistor?

Yes. Base current must be limited to ≤1 A DC and ≤5 A peak. For 10 A collector operation, typical design uses 70–250 mA base drive; a series resistor (e.g., 47–100 Ω) is mandatory to prevent thermal runaway and ensure h_FE-based saturation.

Can BU931T replace BU931 in existing designs?

Yes, BU931T supersedes BU931. Per ST's revision history, BU931 was consolidated into BU931T in Rev 5 (Oct 2017); both share identical electrical specs and TO-220 packaging-BU931T is the current production part number.

What is the maximum recommended gate/base drive frequency for BU931T?

BU931T is not a MOSFET and has no gate-it is a current-driven bipolar Darlington. Switching frequency is limited by storage time (t_s ≤ 15 μs) and thermal constraints. Practical maximum for ignition use is ~100 Hz (6000 RPM, 4-stroke); higher frequencies cause excessive heating due to slow turn-off.

BU931 Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

Package/Case:: TO-204AA, TO-3

Packaging:: Bulk

Product Status:: Obsolete

Transistor Type:: NPN - Darlington

Current - Collector (Ic) (Max):: 15 A

Voltage - Collector Emitter Breakdown (Max):: 400 V

Vce Saturation (Max) @ Ib, Ic:: 1.8V @ 250mA, 10A

Current - Collector Cutoff (Max):: 100µA

DC Current Gain (hFE) (Min) @ Ic, Vce:: 300 @ 5A, 10V

Power - Max:: 175 W

Frequency - Transition:: -

Operating Temperature:: 200°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Chassis Mount

Supplier Device Package:: TO-3

BU931 FAQ

1.How can I place an order for BU931 through Aetrix?

Please submit a Request for Quotation (RFQ) for BU931 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 BU931 reliable?

The price and inventory of BU931 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BU931 is usually 5 days.

3.What payment methods are accepted for BU931?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BU931 transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BU931?

BU931 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BU931 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 BU931?

For technical support, including BU931 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BU931 requirements.

6.How does Aetrix verify that BU931 is sourced from the original manufacturer or authorized distributors?

All BU931 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 BU931 meets industry standards.

7.What is the process for return or replacement of BU931?

All BU931 units undergo pre-shipment inspection (PSI). If there is an issue with BU931, 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 BU931 part is unused and in its original packaging.

Return procedure for BU931:

1.Submit a request within 90 days.

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

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