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

STMicroelectronics BUL128-K

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

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Part No.:: BUL128-K

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-220-3

Datasheet:: BUL128-K.pdf

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

Quantity:: RFQ Add To RFQ list

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Inventory:7,805

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Product details

Overview

BUL128-K from STMicroelectronics is a high-voltage fast-switching NPN power transistor in TO-220 package, rated for 700 V V_CES, 4 A I_C, and 70 W P_tot at T_c = 25 °C, optimized for electronic ballasts in fluorescent lighting and low-cost SMPS.

For engineers reviewing the BUL128-K datasheet, BUL128-K pinout, BUL128-K application, or BUL128-K equivalent, key selection criteria include V_CEO(sus) = 400 V under inductive load, t_s/t_f ≤ 1 µs/0.4 µs (resistive), h_FE group A/B pre-selection, R_thj-case = 1.78 °C/W, and cellular emitter planar edge termination for stable SOA.

Technical Context

This device employs Multi Epitaxial Planar technology with a cellular emitter and planar edge termination to simultaneously achieve high switching speed and wide reverse-biased safe operating area (RBSOA). Its design targets repetitive inductive switching at V_clamp = 200 V with controlled storage time (t_s ≤ 1 µs) and fall time (t_f ≤ 0.2 µs).

The BUL128-K is pre-sorted into h_FE groups (A: 10–25, B: 14–40 at I_C = 10 mA), enabling consistent gain matching across production lots. It supports pulsed operation (300 µs, 1.5% duty cycle) and maintains low dynamic parameter spread-critical for reliable lamp ignition and sustained conduction in ballast circuits.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CES	700 V - Maximum collector-emitter voltage with base open; enables direct line-voltage switching in 230 VAC ballasts.
I_C	4 A continuous - Sustains lamp current in 40–65 W fluorescent fixtures without forced cooling.
P_tot	70 W at T_c = 25 °C - Requires heatsink mounting; derates linearly above 25 °C case temperature.
t_s/t_f (inductive)	0.6 / 0.1 µs - Enables high-frequency (>30 kHz) resonant ignition and zero-voltage switching in half-bridge topologies.
h_FE (Group A)	10–25 at I_C = 10 mA - Ensures predictable base drive requirements for discrete driver stages.
R_thj-case	1.78 °C/W - Dictates minimum heatsink thermal resistance needed to hold T_j ≤ 150 °C at full load.
V_CE(sat)	1.5 V at I_C = 4 A, I_B = 1 A - Limits conduction loss to <6 W, supporting >85% efficiency in compact ballasts.

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 sink terminal	Low-impedance return path; connected to ground or low-side switch node in half-bridge.
2 (Collector)	High-voltage output terminal	Internally bonded to metal tab; must be electrically isolated from heatsink and chassis.
3 (Base)	Control input	Receives current-driven signal (I_B up to 2 A peak); requires active turn-off for fast switching.

Key Features

Feature	Design Value
Cellular Emitter Structure	Reduces carrier recombination delay, enabling t_f ≤ 0.2 µs under inductive load without sacrificing ruggedness.
Planar Edge Termination	Extends RBSOA by suppressing premature edge breakdown at 700 V, improving reliability in overvoltage transients.
Low Lot-to-Lot Parameter Spread	Ensures consistent t_s, V_CE(sat), and h_FE across manufacturing batches-critical for automated ballast calibration.
Pre-Selected h_FE Groups	Group A (10–25) and Group B (14–40) allow matched base drive design across production runs without requalification.

Applications

Fluorescent Lamp Electronic Ballast

Low-Cost AC-DC SMPS

Use Scenario: High-frequency (25–65 kHz) half-bridge inverter driving parallel-tuned LC network for instant lamp ignition and stable arc maintenance.

IC Role / Device Role / Timing Role: Main switching transistor handling 400 V DC bus and 4 A peak lamp current; operates in hard-switched or quasi-resonant mode.

Use Value: Cellular emitter + planar termination ensures t_s ≤ 0.6 µs and robust RBSOA, preventing failure during lamp end-of-life open-circuit events.

Use Scenario: Primary-side switch in 15–35 W flyback converters for LED drivers and appliance auxiliary supplies.

IC Role / Device Role / Timing Role: Discrete power switch replacing integrated controllers where cost sensitivity and thermal margin outweigh integration benefits.

Use Value: V_CES = 700 V allows direct rectified 230 VAC operation without voltage clamping, reducing BOM count and layout complexity.

Inductive Load Snubberless Switching

DC Motor Commutation Driver

Use Scenario: Unclamped inductive switching in relay drivers or solenoid controls with V_clamp = 200 V and L = 25 mH.

IC Role / Device Role / Timing Role: Fast-turn-off power switch managing stored inductive energy via controlled avalanche and low t_f.

Use Value: t_f ≤ 0.2 µs minimizes voltage overshoot during turn-off, eliminating need for external snubbers in cost-constrained designs.

Use Scenario: H-bridge low-side switch in 24–48 V brushed DC motor control for HVAC actuators and industrial valves.

IC Role / Device Role / Timing Role: Medium-power NPN switch delivering 4 A continuous current with low V_CE(sat) to reduce heat generation.

Use Value: R_thj-case = 1.78 °C/W enables operation at 70 W dissipation with modest heatsinking-ideal for space-limited enclosures.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
STBUL128D	Same die, TO-220FP package (full-pack, insulated); identical electrical specs but lower R_thj-amb (50 °C/W vs 62.5 °C/W).	Preferred when board-level isolation is required and ambient cooling dominates over case conduction.	Select for improved thermal performance in non-heatsinked or convection-cooled layouts.
BU2520DX	V_CES = 1000 V, higher V_CE(sat) (2.0 V @ 4 A), slower t_f (0.5 µs), same TO-220 footprint.	Suitable for 400 V+ DC link applications requiring wider voltage margin but tolerating lower efficiency.	Choose only if system overvoltage risk exceeds 700 V or legacy BU2520DX footprint compatibility is mandatory.

Compared with STBUL128D, the BUL128-K offers identical switching performance but requires external insulation for heatsink mounting; versus BU2520DX, it delivers faster switching and lower conduction loss at the expense of 300 V lower V_CES, making it optimal for cost-sensitive 230 VAC-input ballasts.

Availability

BUL128-K is available at Aetrix Electronics and suitable for electronic ballasts, low-cost switch-mode power supplies, inductive load drivers, and DC motor commutation circuits requiring stable component supply and long-term manufacturability.

Supply support for BUL128-K 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, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

The BUL128-K belongs to ST's high-voltage bipolar transistor product line, engineered specifically for lighting and power conversion applications demanding ruggedness, consistency, and fast switching at voltages up to 700 V.

FAQ

Is the BUL128-K pin-compatible with standard TO-220 NPN transistors like the 2N3055?

No. While both use TO-220 packages, the BUL128-K has collector-connected tab (pin 2), whereas 2N3055 uses emitter-connected tab. Pin 1 (emitter) and pin 3 (base) positions match, but mechanical mounting and electrical isolation requirements differ significantly due to internal tab connectivity.

What is the significance of "Group A" and "Group B" h_FE labeling on BUL128-K units?

ST pre-sorts BUL128-K devices into two DC current gain bands: Group A (h_FE = 10–25) and Group B (h_FE = 14–40) at I_C = 10 mA. This enables consistent base drive design across production lots; users must verify gain range in their circuit and may request specific groups via order notes.

Can the BUL128-K operate safely without a heatsink in low-duty-cycle applications?

No. Even at 25% duty cycle, P_tot derating begins immediately above 25 °C case temperature. With R_thj-amb = 62.5 °C/W, junction temperature exceeds 150 °C within seconds at full current. A minimum heatsink (R_th ≤ 10 °C/W) is mandatory for any sustained conduction above 500 mA.

Does the BUL128-K support avalanche energy rating for unclamped inductive switching?

Yes. The device is characterized for inductive switching with V_clamp = 200 V and L = 25 mH per datasheet Figure 1. Its planar edge termination and cellular emitter provide controlled single-pulse avalanche capability, but repetitive unclamped operation requires verification against the published RBSOA curve at T_j = 150 °C.

BUL128-K 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):: 200µA

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-K FAQ

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

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

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

3.What payment methods are accepted for BUL128-K?

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

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BUL128-K?

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

Once your BUL128-K 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-K?

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

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

All BUL128-K 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-K meets industry standards.

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

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

Return procedure for BUL128-K:

1.Submit a request within 90 days.

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

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