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

STMicroelectronics BUL128FP

Name: BUL128FP
Brand: STMicroelectronics
SKU: BUL128FP
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BUL128FP

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-220-3 Full Pack

Datasheet:: BUL128FP.pdf

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

Quantity:: RFQ Add To RFQ list

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Inventory:8,805

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Product details

Overview

BUL128FP from STMicroelectronics is a high-voltage fast-switching NPN power transistor in TO-220FP package, rated for V_CEO = 400 V, I_C = 4 A, P_tot = 31 W at T_c = 25 °C, with t_s/t_f = 0.6/0.1 µs (inductive load), designed for electronic ballasts and low-cost SMPS.

For engineers reviewing the BUL128FP datasheet, BUL128FP pinout, BUL128FP application, or BUL128FP equivalent, key selection criteria include switching speed under inductive load, RBSOA robustness, V_CEO(sus) = 400 V sustain capability, and thermal resistance R_thj-case = 4.1 °C/W for reliable thermal design in lighting and power conversion.

Technical Context

The BUL128FP employs Multi Epitaxial Planar technology with Cellular Emitter structure and planar edge termination to simultaneously achieve high switching speed and wide Reverse Biased Safe Operating Area (RBSOA). Its optimized base drive sensitivity (h_FE = 15–45 at I_C = 1–2 A) supports efficient gate-driving in self-oscillating ballast topologies.

It delivers pulsed switching performance with t_s = 0.6 µs and t_f = 0.1 µs under inductive load (V_clamp = 200 V, I_C = 2 A), and maintains low V_CE(sat) = 1.5 V at I_C = 4 A / I_B = 1 A - critical for conduction loss minimization in 20–100 kHz ballast circuits.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	400 V - Sustains 400 V collector-emitter voltage with zero base current, enabling use in 230 VAC-fed electronic ballasts without snubber overdesign.
I_C	4 A continuous - Supports lamp currents up to 3.5 A in 40–65 W fluorescent ballasts with margin for surge.
t_s/t_f (inductive)	0.6 / 0.1 µs - Enables clean turn-off in resonant LC ballast circuits operating above 40 kHz, reducing switching losses and EMI.
R_thj-case	4.1 °C/W - Allows 25 °C case temperature rise at full 31 W dissipation, simplifying heatsink selection for enclosed fixtures.
V_CE(sat)	1.5 V @ I_C = 4 A, I_B = 1 A - Limits conduction loss to ≤6 W at full load, improving efficiency vs. slower alternatives.
h_FE	15–45 @ I_C = 1–2 A - Provides stable current gain across production lots, ensuring consistent oscillator frequency in self-driven ballast designs.

Pinout & Package

TO-220FP package: insulated plastic case with metal tab (collector-connected), 3-pin vertical mounting, creepage distance optimized for 400 V operation. Pin 1 = Base, Pin 2 = Collector (tab), Pin 3 = Emitter.

Pin/Terminal	Circuit Role	Design Meaning
Pin 1	Base	Low-impedance control input; requires 1 A peak drive for full saturation at 4 A collector current.
Pin 2	Collector (Tab)	Internally connected to metal tab; electrically active and thermally critical - must be mounted to heatsink with insulating washer.
Pin 3	Emitter	Reference node for base drive; low-inductance layout essential to preserve fast switching and prevent parasitic turn-on.

Key Features

Feature	Design Value
Cellular Emitter Structure	Enables uniform current distribution and reduces localized heating during high di/dt switching in ballast resonant tanks.
Planar Edge Termination	Extends RBSOA by suppressing premature edge breakdown, allowing safe operation near V_CEO limit under transient overvoltage.
Low Lot-to-Lot Parameter Spread	Guarantees <±10% variation in h_FE and V_CE(sat), eliminating need for per-unit bias trimming in mass-produced ballasts.
High dV/dt Immunity	Rated for >1000 V/µs - prevents false triggering during rapid collector voltage transitions in half-bridge configurations.

Applications

Fluorescent Lamp Electronic Ballast

Low-Cost AC-DC SMPS

Use Scenario: High-frequency (40–70 kHz) series-resonant driver for T8/T5 fluorescent tubes in commercial lighting fixtures.

IC Role / Device Role / Timing Role: Main switching transistor in self-oscillating or IC-controlled half-bridge, handling lamp current and resonant tank voltage reversal.

Use Value: Fast t_f = 0.1 µs minimizes overlap loss during zero-voltage switching transitions, improving system efficiency by ≥3% vs. standard BUL128 variants.

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

IC Role / Device Role / Timing Role: Power switch modulating energy transfer into transformer primary; operates in discontinuous conduction mode (DCM) at 60–100 kHz.

Use Value: V_CEO(sus) = 400 V ensures margin against reflected voltage spikes in universal-input (85–265 VAC) designs without clamping circuitry.

Induction Heating Half-Bridge

DC Motor Commutation Driver

Use Scenario: Low-side switch in 20–50 kHz half-bridge for small-appliance induction cooktops (≤1.2 kW).

IC Role / Device Role / Timing Role: Fast-turning switch controlling resonant tank current direction; paired with complementary device on high side.

Use Value: RBSOA robustness allows safe operation at V_CE > 300 V during dead-time voltage overshoot, preventing secondary breakdown.

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

IC Role / Device Role / Timing Role: Medium-voltage, medium-current switching element handling PWM commutation at ≤20 kHz.

Use Value: Low V_CE(sat) = 1.5 V at 4 A reduces conduction loss to 6 W, enabling compact heatsink design in space-constrained enclosures.

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-220 (non-FP) package; R_thj-case = 3.5 °C/W, no electrical isolation between tab and collector.	Requires isolated mounting hardware; unsuitable where creepage/clearance >4 mm is mandated.	Select when thermal performance is prioritized over insulation requirements and board-level isolation is handled externally.
BU508AF	Higher V_CEO = 1000 V, slower t_f = 0.5 µs, higher V_CE(sat) = 2.0 V @ 4 A.	Better for 380 VAC industrial SMPS; less efficient in high-frequency ballasts due to increased switching loss.	Select only when absolute overvoltage robustness (>600 V transients) outweighs efficiency and EMI targets.

Compared with STBUL128D and BU508AF, the BUL128FP uniquely balances 400 V ruggedness, sub-microsecond switching, and insulated TO-220FP packaging - making it optimal for cost-sensitive, safety-certified fluorescent ballasts where thermal, electrical, and regulatory constraints intersect.

Availability

BUL128FP is available at Aetrix Electronics and suitable for electronic ballasts, low-cost AC-DC SMPS, induction heating modules, and DC motor commutation drivers requiring stable component supply and long-term manufacturing continuity.

Supply support for BUL128FP 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 BUL128FP belongs to ST's high-voltage bipolar power transistor family, engineered specifically for lighting and energy-efficient power conversion where reliability, parameter consistency, and fast switching are non-negotiable.

FAQ

Is BUL128FP pin-compatible with BUL128D?

Yes - both share identical TO-220FP mechanical footprint and pinout (Base–Collector–Emitter). However, BUL128FP features an electrically insulated tab, while BUL128D's tab is internally connected to collector and requires external isolation. Layout must account for this difference in grounding and heatsink mounting.

What is the maximum recommended case temperature for continuous operation?

The maximum case temperature is 100 °C for reliable long-term operation. At T_c = 100 °C, derated power dissipation is 18.6 W (calculated via linear derating from 31 W at 25 °C using R_thj-case = 4.1 °C/W and T_j(max) = 150 °C), ensuring junction temperature remains within specification.

Can BUL128FP be used in avalanche mode?

No - the BUL128FP is not rated for repetitive avalanche operation. Its V_CEO(sus) = 400 V is a sustaining voltage under controlled inductive switching, not an avalanche energy rating. Operation beyond V_CEO without clamping risks irreversible second breakdown.

Does BUL128FP require a base resistor in self-oscillating ballast circuits?

Yes - a base resistor (typically 1–2.2 kΩ, 0.5 W) is required to limit peak base current during startup and ensure stable oscillation. The device's h_FE spread and low V_BE(sat) make uncontrolled base drive prone to thermal runaway or erratic frequency drift.

BUL128FP Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

Package/Case:: TO-220-3 Full Pack

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:: 31 W

Frequency - Transition:: -

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-220FP

BUL128FP FAQ

1.How can I place an order for BUL128FP through Aetrix?

Please submit a Request for Quotation (RFQ) for BUL128FP 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 BUL128FP reliable?

The price and inventory of BUL128FP are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BUL128FP is usually 5 days.

3.What payment methods are accepted for BUL128FP?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BUL128FP transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BUL128FP?

BUL128FP orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BUL128FP 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 BUL128FP?

For technical support, including BUL128FP datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BUL128FP requirements.

6.How does Aetrix verify that BUL128FP is sourced from the original manufacturer or authorized distributors?

All BUL128FP 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 BUL128FP meets industry standards.

7.What is the process for return or replacement of BUL128FP?

All BUL128FP units undergo pre-shipment inspection (PSI). If there is an issue with BUL128FP, 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 BUL128FP part is unused and in its original packaging.

Return procedure for BUL128FP:

1.Submit a request within 90 days.

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

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