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

STMicroelectronics BUL128D-B

Name: BUL128D-B
Brand: STMicroelectronics
SKU: BUL128D-B
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BUL128D-B

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-220-3

Datasheet:: BUL128D-B.pdf

Description:: TRANS NPN 400V 4A TO-220

Quantity:: RFQ Add To RFQ list

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Product details

Overview

BUL128D-B from STMicroelectronics is a high-voltage, fast-switching NPN power transistor in TO-220 package, rated for V_CES = 700 V, I_C = 4 A, and P_tot = 70 W at T_C = 25 °C. It integrates an antiparallel collector-emitter diode and is optimized for electronic ballasts in fluorescent lighting and flyback/forward low-power SMPS.

For engineers reviewing the BUL128D-B datasheet, BUL128D-B pinout, BUL128D-B application, or BUL128D-B equivalent, key selection criteria include its 400 V V_CEO(sus) under inductive load, 2 µs storage time (inductive), 0.2 µs fall time, 1.78 °C/W R_thj-case, and cellular emitter structure enabling wide RBSOA with high switching speed.

Technical Context

This device employs ST's high-voltage Multi Epitaxial Planar process with planar edge termination and a cellular emitter layout to simultaneously achieve high voltage blocking (700 V V_CES), fast switching (t_s = 2 µs, t_f = 0.2 µs under inductive load), and robust RBSOA. Its integrated antiparallel diode eliminates need for external freewheeling diodes in single-transistor converters.

The transistor is characterized for operation up to T_J = 150 °C, with guaranteed V_CEO(sus) = 400 V at I_C = 100 mA and L = 25 mH, and exhibits tight parameter spread across lots-critical for consistent performance in mass-produced lighting and power supplies.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CES	700 V - Maximum collector-emitter voltage with base open; enables use in 400 V DC bus applications with safety margin.
I_C	4 A continuous - Sustained current handling at T_C = 25 °C; supports 15–25 W flyback converters.
t_s (inductive)	2 µs - Storage time under V_CC = 250 V, I_C = 2 A; determines minimum off-time in high-frequency SMPS.
R_thj-case	1.78 °C/W - Junction-to-case thermal resistance; allows 70 W dissipation with ≤125 °C case temperature rise.
V_CE(sat)	1.5 V @ I_C = 4 A, I_B = 1 A - Low saturation voltage reduces conduction loss in hard-switched topologies.
h_FE	12 (typ) @ I_C = 2 A - Sufficient DC gain for direct base drive without complex bias networks.

Pinout & Package

TO-220 package with vertical mounting orientation, insulated tab, and standard 3-pin configuration (Emitter, Base, Collector). Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector (viewed from front, leads down).

Pin/Terminal	Circuit Role	Design Meaning
Pin 1 (Emitter)	Current return path	Low-inductance connection point for emitter loop; tied to ground or source in common-emitter switch configurations.
Pin 2 (Base)	Control input	Receives drive current (I_B ≤ 2 A) to saturate transistor; requires active turn-off (V_BE(off) = −5 V) for fast switching.
Pin 3 (Collector)	High-side switching node	Connected to primary winding of flyback transformer or lamp inductor; handles 700 V transient spikes.

Key Features

Feature	Design Value
Integrated antiparallel C-E diode	Eliminates discrete freewheeling diode in single-transistor forward/flyback designs, reducing BOM count and layout area.
Cellular emitter + planar edge termination	Enables simultaneous high dV/dt immunity, wide RBSOA, and sub-µs switching times without secondary breakdown risk.
Low lot-to-lot parameter spread	Ensures consistent t_s, V_CE(sat), and h_FE across production batches-critical for automated lighting ballast calibration.
150 °C max junction temperature	Supports sealed or thermally constrained environments (e.g., compact electronic ballasts) without derating below full I_C.

Applications

Fluorescent Lamp Electronic Ballast

Flyback Converter (≤25 W)

Use Scenario: Driving resonant LC tank to ignite and regulate 36–40 W T8/T12 fluorescent lamps at 20–60 kHz.

IC Role / Device Role / Timing Role: Main switching transistor in half-bridge or single-ended topology; provides zero-voltage switching (ZVS) capability via integrated diode.

Use Value: Integrated diode enables self-oscillating operation and reduces component count by one diode per channel.

Use Scenario: Primary-side switch in isolated AC/DC adapters for consumer electronics (e.g., set-top boxes, routers).

IC Role / Device Role / Timing Role: High-voltage switch controlling energy transfer to secondary via transformer; operates at 50–100 kHz with fixed or variable frequency control.

Use Value: 700 V V_CES rating accommodates reflected transients from 230 VAC input without snubber redesign.

Forward Converter (Low-Power)

DC-DC Boost Pre-regulator

Use Scenario: Single-transistor forward converter supplying 12 V/2 A for industrial control modules.

IC Role / Device Role / Timing Role: Primary-side switch clamped by RCD network; conducts during main duty cycle, freewheels via integrated diode during reset.

Use Value: Integrated diode replaces external ultrafast recovery diode, lowering system cost and improving reliability over 100 khr lifetime.

Use Scenario: Boost switch in PFC pre-regulator stage for LED drivers or medical power supplies.

IC Role / Device Role / Timing Role: Fast-turnoff switch managing inductor current ramp; must sustain 400 V+ output while minimizing conduction and switching losses.

Use Value: 2 µs storage time and 0.2 µs fall time enable >80 kHz operation with <5% switching loss penalty vs. MOSFET alternatives.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
STBUL128D	Same die, TO-220FP (full-pack) variant with insulated tab; identical electrical specs but different mechanical mounting.	Required where electrical isolation between heatsink and collector is mandatory (e.g., floating ground systems).	Select when thermal interface requires insulating pad or ceramic washer; otherwise BUL128D-B is lower-cost standard version.
BU508A	Lower V_CES = 1000 V but higher V_CE(sat) = 2.5 V @ 4 A; slower t_s = 4 µs (inductive); no integrated diode.	Suitable for higher-voltage flybacks (>600 V DC bus) but demands external freewheeling diode and larger heatsink.	Choose only if >700 V blocking is essential and switching speed is secondary; avoid for ballast designs requiring integrated diode.

Compared with STBUL128D and BU508A, BUL128D-B offers optimal balance of integrated diode functionality, 700 V rating, and 2 µs storage time-making it uniquely suited for cost-sensitive, high-volume fluorescent ballasts and sub-25 W flyback converters where layout simplicity and thermal efficiency are prioritized.

Availability

BUL128D-B is available at Aetrix Electronics and suitable for electronic ballasts, low-power flyback converters, forward converters, and boost pre-regulators requiring stable component supply, consistent parametric performance across production lots, and TO-220 mechanical compatibility.

Supply support for BUL128D-B 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 power devices, microcontrollers, sensors, and automotive ICs.

BUL128D-B belongs to ST's high-voltage bipolar transistor product line, engineered specifically for lighting control and cost-optimized switch-mode power conversion in consumer and industrial equipment.

FAQ

What is the maximum safe operating frequency for BUL128D-B in a flyback converter?

BUL128D-B supports reliable operation up to 100 kHz in flyback topologies, based on its 2 µs storage time and 0.2 µs fall time under inductive load conditions (V_CC = 250 V, I_C = 2 A). At 100 kHz, minimum off-time exceeds 3× t_s, ensuring full charge removal from the base region and preventing second breakdown.

Does BUL128D-B require a base resistor for gate drive?

Yes-BUL128D-B is a bipolar transistor and requires controlled base current, not voltage drive. A series base resistor (e.g., 10–47 Ω) is necessary to limit I_B to ≤1 A during saturation and ensure clean turn-off with −5 V reverse bias. Unlike MOSFETs, it has no gate oxide and cannot be driven directly by logic-level signals without current amplification.

Can BUL128D-B replace a MOSFET in a 24 V input forward converter?

No-BUL128D-B is not suitable for low-voltage, high-frequency forward converters due to its relatively high V_CE(sat) (1.5 V) and base drive complexity. At 24 V input, conduction losses exceed those of logic-level MOSFETs, and switching efficiency drops above 200 kHz. It is optimized for ≥100 V DC bus applications like electronic ballasts and universal-input SMPS.

Is the integrated antiparallel diode rated for continuous conduction?

The integrated diode is rated for pulsed freewheeling only-not continuous conduction. Its forward voltage and thermal limits match the transistor's SOA: peak current ≤8 A (t_p < 5 ms), average current limited by package thermal resistance. For continuous diode conduction, an external ultrafast diode with matching V_F and t_rr must be added in parallel.

BUL128D-B Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

Package/Case:: TO-220-3

Packaging:: Tube

Product Status:: Obsolete

Transistor Type:: NPN

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

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

Vce Saturation (Max) @ Ib, Ic:: 500mV @ 1A, 4A

Current - Collector Cutoff (Max):: 250µA

DC Current Gain (hFE) (Min) @ Ic, Vce:: 12 @ 2A, 5V

Power - Max:: 70 W

Frequency - Transition:: -

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-220

BUL128D-B FAQ

1.How can I place an order for BUL128D-B through Aetrix?

Please submit a Request for Quotation (RFQ) for BUL128D-B 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 BUL128D-B reliable?

The price and inventory of BUL128D-B are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BUL128D-B is usually 5 days.

3.What payment methods are accepted for BUL128D-B?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BUL128D-B transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BUL128D-B?

BUL128D-B orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BUL128D-B 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 BUL128D-B?

For technical support, including BUL128D-B datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BUL128D-B requirements.

6.How does Aetrix verify that BUL128D-B is sourced from the original manufacturer or authorized distributors?

All BUL128D-B 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 BUL128D-B meets industry standards.

7.What is the process for return or replacement of BUL128D-B?

All BUL128D-B units undergo pre-shipment inspection (PSI). If there is an issue with BUL128D-B, 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 BUL128D-B part is unused and in its original packaging.

Return procedure for BUL128D-B:

1.Submit a request within 90 days.

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

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