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

STMicroelectronics BUL128

Name: BUL128
Brand: STMicroelectronics
SKU: BUL128
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BUL128

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-220-3

Datasheet:: BUL128.pdf

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

Quantity:: RFQ Add To RFQ list

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Inventory:4,195

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Product details

Overview

BUL128 from STMicroelectronics is a high-voltage fast-switching NPN power transistor designed for electronic ballasts in fluorescent lighting and low-cost switch-mode power supplies. It features 700 V V_CES, 4 A continuous collector current, 70 W total dissipation at T_c = 25 °C, and sub-microsecond switching times (t_s = 0.6 µs, t_f = 0.1 µs under inductive load), enabling reliable operation in high-frequency AC-DC conversion stages.

For engineers reviewing the BUL128 datasheet, BUL128 pinout, BUL128 application, or BUL128 equivalent, key selection criteria include its 400 V V_CEO(sus) sustaining voltage, group-specified h_FE (10–40), TO-220 package thermal resistance (R_thj-case = 1.78 °C/W), and validated performance in inductive-switching ballast circuits with V_clamp = 200 V.

Technical Context

The BUL128 employs ST's high-voltage Multi Epitaxial Planar technology with cellular emitter structure and planar edge termination to simultaneously achieve high switching speed and wide RBSOA. Its design targets medium-voltage (≤400 V) hard-switched applications where dynamic parameter consistency across lots is critical.

It supports pulsed operation (300 µs, 1.5% duty cycle) and is pre-selected into Group A (h_FE = 10–25) or Group B (h_FE = 14–40); both groups meet identical V_CE(sat), switching time, and SOA specifications under specified test conditions.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CES	700 V - Maximum collector-emitter blocking voltage with base open, enabling use in 400 V AC mains-derived topologies.
V_CEO(sus)	400 V - Sustaining voltage under inductive switching (IC = 100 mA, L = 25 mH), defining safe turn-off capability in ballast resonant tanks.
I_C	4 A - Continuous collector current rating at T_c = 25 °C, supporting 60–100 W fluorescent lamp drivers.
t_s/t_f (inductive)	0.6 / 0.1 µs - Verified storage and fall times with IB1 = 0.4 A, V_BE(off) = −5 V, R_BB = 0 Ω, enabling >100 kHz switching.
R_thj-case	1.78 °C/W - Junction-to-case thermal resistance, allowing 70 W dissipation with ≤125 °C case temperature rise above ambient.
h_FE (Group A/B)	10–40 - DC current gain binned at IC = 10 mA/VCE = 5 V and IC = 2 A/VCE = 5 V, ensuring predictable base drive requirements.

Pinout & Package

Package: TO-220 (3-pin, straight lead, insulated tab). Mounting requires electrically isolated heatsink due to collector-connected tab.

Pin/Terminal	Circuit Role	Design Meaning
1 (Emitter)	Current return path	Low-inductance emitter connection for stable switching node referencing; soldered directly to PCB ground plane in ballast designs.
2 (Collector)	Main power output terminal	Internally connected to metal tab; must be electrically isolated from heatsink to prevent short-circuit in common-emitter configurations.
3 (Base)	Control input	Receives current-driven turn-on signal; requires external base resistor to limit IB1/IB2 per switching waveform requirements.

Key Features

Feature	Design Value
Cellular Emitter Structure	Reduces carrier recombination delay and improves uniformity of current distribution during turn-off, directly enabling <1 µs storage time.
Planar Edge Termination	Suppresses edge breakdown at high V_CE, maintaining rated 700 V V_CES while preserving wide RBSOA for inductive load robustness.
Low Lot-to-Lot Parameter Spread	Dynamic parameters (t_s, t_f, V_CE(sat)) tightly controlled across production lots, reducing need for post-manufacturing binning in lighting OEM lines.
Pre-Selected h_FE Groups	Two guaranteed h_FE ranges (Group A: 10–25; Group B: 14–40) allow precise base drive design without margining for gain variation.

Applications

Fluorescent Lamp Electronic Ballast

Low-Cost SMPS (≤100 W)

Use Scenario: High-frequency (25–65 kHz) half-bridge inverter driving resonant LC tank to ignite and regulate 36–40 W T8/T12 lamps.

IC Role / Device Role / Timing Role: Main switching transistor in asymmetrical half-bridge; handles repetitive 400 V V_CEO stress and 2 A peak collector current per half-cycle.

Use Value: Sub-microsecond t_s/t_f minimizes switching losses at 50+ kHz, while 400 V V_CEO(sus) ensures reliable turn-off under lamp ignition overvoltage transients.

Use Scenario: Primary-side switch in flyback or forward converter for consumer adapters (e.g., 12 V/5 A output from 230 V AC).

IC Role / Device Role / Timing Role: Hard-switched NPN power switch operating at 40–100 kHz; clamped by external RCD snubber to 200 V.

Use Value: 700 V V_CES headroom accommodates reflected voltage spikes; TO-220 package enables cost-effective heatsinking for 70 W dissipation.

AC Motor Speed Controller (Fan)

DC-DC Boost Converter (Industrial)

Use Scenario: Phase-angle controlled dimming circuit for universal AC fans using triac-compatible gate drive.

IC Role / Device Role / Timing Role: Fast-turnoff switch in zero-crossing detection and pulse generation stage; triggered synchronously with AC line.

Use Value: 9 V V_EBO rating allows direct coupling to microcontroller GPIO; low t_f (<0.2 µs) ensures clean edge timing for accurate phase control.

Use Scenario: Boost switch in 24 V input → 48 V output industrial power module handling 3 A output current.

IC Role / Device Role / Timing Role: Medium-voltage (400 V) boost transistor operating in continuous conduction mode at 60 kHz.

Use Value: Wide RBSOA and 1.78 °C/W R_thj-case support sustained 4 A I_C with minimal derating, reducing need for oversized heatsinks.

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
BUL118	Lower V_CES (600 V), same TO-220 package, 3 A I_C, slower t_s/t_f (1.0/0.3 µs)	Restricted to ≤250 V AC-input SMPS or lower-power ballasts (≤30 W)	Select when 600 V rating suffices and cost sensitivity outweighs need for 700 V margin or fastest switching.
BU2520A	Higher V_CES (1000 V), TO-220F package (non-insulated tab), 5 A I_C, similar t_s/t_f	Requires non-isolated mounting; suited for 400–600 V DC bus applications like UPS inverters	Choose for higher voltage systems where tab isolation is not required and 1000 V blocking is mandatory.

Compared with BUL118, BUL128 provides 100 V higher blocking and faster switching for 400 V AC-derived topologies; versus BU2520A, it offers insulated-tab mounting flexibility at the expense of 300 V voltage headroom-critical for cost-sensitive lighting and adapter designs.

Availability

BUL128 is available at Aetrix Electronics and suitable for fluorescent lamp ballasts, low-cost AC-DC adapters, and industrial DC-DC boost converters requiring stable component supply, consistent h_FE grouping, and proven TO-220 thermal performance.

Supply support for BUL128 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 discrete devices, microcontrollers, and analog ICs for industrial, automotive, and consumer markets.

The BUL series targets cost-sensitive, high-volume lighting and power conversion applications, emphasizing manufacturing consistency, switching speed, and ruggedness in TO-220 packaging for mainstream AC-DC infrastructure.

FAQ

Is BUL128 pin-compatible with other TO-220 NPN transistors like BD139 or TIP31?

No. While all use TO-220 packages, BUL128 has emitter-base-collector pinout (1-E, 2-C, 3-B), whereas BD139 and TIP31 follow collector-base-emitter (1-C, 2-B, 3-E). Incorrect mounting risks immediate failure due to collector-tab shorting. Always verify pin assignment using ST's official mechanical drawing (P011CI).

What does "Group A" and "Group B" mean for h_FE in BUL128 datasheet?

ST pre-sorts BUL128 into two DC current gain bins: Group A (h_FE = 10–25 at IC = 10 mA) and Group B (h_FE = 14–40 at same condition). Both groups meet identical V_CE(sat), switching time, and SOA specs. Designers select base resistors based on the shipped group's min/max h_FE to ensure reliable saturation.

Can BUL128 replace BU2520A in a 400 V DC bus application?

BUL128 is not recommended as a direct replacement for BU2520A in 400 V DC bus applications. Although both handle 400 V, BU2520A's 1000 V V_CES provides essential safety margin against transient overvoltages exceeding 700 V. BUL128's 700 V rating leaves insufficient headroom, increasing risk of avalanche failure under line surges or inductive kick.

Does BUL128 require a heatsink in 4 A continuous operation?

Yes. At 4 A I_C and typical V_CE(sat) ≈ 1.5 V, power dissipation exceeds 6 W. With R_thj-case = 1.78 °C/W and max T_j = 150 °C, even a modest 50 °C case temperature requires ≥56 °C/W heatsink thermal resistance. A standard TO-220 clip-on heatsink (e.g., 25–30 °C/W) is mandatory for sustained 4 A operation.

BUL128 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):: 100µA

DC Current Gain (hFE) (Min) @ Ic, Vce:: 14 @ 2A, 5V

Power - Max:: 70 W

Frequency - Transition:: -

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-220

BUL128 FAQ

1.How can I place an order for BUL128 through Aetrix?

Please submit a Request for Quotation (RFQ) for BUL128 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 BUL128 reliable?

The price and inventory of BUL128 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BUL128 is usually 5 days.

3.What payment methods are accepted for BUL128?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BUL128 transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BUL128?

BUL128 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BUL128 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 BUL128?

For technical support, including BUL128 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BUL128 requirements.

6.How does Aetrix verify that BUL128 is sourced from the original manufacturer or authorized distributors?

All BUL128 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 BUL128 meets industry standards.

7.What is the process for return or replacement of BUL128?

All BUL128 units undergo pre-shipment inspection (PSI). If there is an issue with BUL128, 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 BUL128 part is unused and in its original packaging.

Return procedure for BUL128:

1.Submit a request within 90 days.

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

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