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

onsemi BU508AFTBTU

Name: BU508AFTBTU
Brand: onsemi
SKU: BU508AFTBTU
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BU508AFTBTU

Manufacturer:: onsemi

Category:: Single Bipolar Transistors

Package:: TO-3P-3 Full Pack

Datasheet:: BU508AFTBTU.pdf

Description:: TRANS NPN 700V 5A TO-3PF

Quantity:: RFQ Add To RFQ list

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Inventory:3,740

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Product details

Overview

BU508AFTBTU from ON Semiconductor (formerly Fairchild) is an NPN triple-diffused planar silicon power transistor in TO-3PF package, rated for 700 V V_CEO, 5 A DC collector current, and 60 W collector dissipation at T_C = 25°C, designed for TV horizontal output stage switching applications.

For engineers reviewing the BU508AFTBTU datasheet, pinout, applications, or equivalent options, key selection criteria include high-voltage blocking capability (V_CEO = 700 V), robust pulsed current handling (I_CP = 15 A), low saturation voltage (V_CE(sat) = 1 V @ I_C = 4.5 A, I_B = 2 A), and thermal performance in TO-3PF mounting configuration.

Technical Context

This device operates as a high-voltage, medium-power NPN switching transistor with triple-diffused planar construction enabling stable breakdown characteristics and rugged safe operating area (SOA). Its design targets flyback energy recovery and line deflection timing in CRT television horizontal output stages.

The BU508AFTBTU features V_CEO(sus) = 700 V under zero-bias conditions, h_FE ≥ 2.25 at I_C = 4.5 A, and low V_BE(sat) = 1.5 V - all critical for efficient switching and minimal drive loss in resonant horizontal scan circuits.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	700 V - maximum continuous collector-emitter voltage before avalanche breakdown; enables use in 110–240 VAC line-synchronized horizontal deflection circuits
I_C (DC)	5 A - steady-state collector current rating; supports sustained flyback current during horizontal retrace
P_C @ T_C=25°C	60 W - thermal power limit at case temperature; requires heatsinking per TO-3PF mechanical footprint
V_CE(sat)	1 V @ I_C=4.5 A, I_B=2 A - low on-state loss reduces conduction heating in high-duty-cycle scan operation
h_FE	≥2.25 @ V_CE=5 V, I_C=4.5 A - minimum DC current gain ensures reliable base drive margin in saturated switch mode
T_J max	150 °C - maximum junction temperature; defines thermal derating slope (see Fig. 6) for continuous operation

Pinout & Package

TO-3PF metal package with isolated collector tab, 3-pin configuration: Pin 1 = Base, Pin 2 = Collector (tab), Pin 3 = Emitter. Mounting requires insulating washer and torque-controlled fastening to maintain thermal and electrical integrity.

Pin/Terminal	Circuit Role	Design Meaning
1 (Base)	Control input	Receives current-limited drive signal to saturate or cut off transistor; requires ~2 A peak for full turn-on per datasheet test condition
2 (Collector)	High-voltage switched node	Connected to flyback transformer primary and horizontal deflection yoke; electrically tied to metal tab requiring isolation from heatsink
3 (Emitter)	Power return path	Serves as common reference for base drive and load current; carries full horizontal scan current (up to 5 A DC / 15 A pulse)

Key Features

Feature	Design Value
Triple-diffused planar structure	Provides controlled avalanche behavior and improved second breakdown immunity for repetitive flyback stress
Isolated collector tab (TO-3PF)	Enables direct heatsink mounting with electrical isolation, critical for high-voltage ground-referenced deflection circuits
V_CEO(sus) = 700 V	Guarantees sustaining voltage under zero-base-current condition, preventing unintended turn-on during flyback voltage spikes
Safe Operating Area (SOA) curve	Validated up to 100 µs pulse width, supporting robust horizontal scan timing without secondary breakdown

Applications

TV Horizontal Output Stage	SMPS Primary Switch (Legacy)
Use Scenario: Horizontal deflection circuit in CRT televisions generating 15.625 kHz sawtooth current for line scanning. IC Role / Device Role / Timing Role: High-voltage NPN switch controlling flyback transformer primary current and energy recovery timing. Use Value: V_CEO = 700 V and SOA compliance ensure reliable operation under repetitive 1.2 kV flyback transients.	Use Scenario: Primary-side switching element in offline quasi-resonant or RCD-clamped flyback converters (≤100 W). IC Role / Device Role / Timing Role: Main power switch turning on/off mains-referred transformer primary to regulate output voltage. Use Value: Low V_CE(sat) = 1 V minimizes conduction loss; h_FE ≥ 2.25 simplifies base drive design in discrete controller topologies.
Monitor Deflection Circuit	Industrial HV Pulse Generator
Use Scenario: Line scan driver in analog computer monitors operating at 31.5 kHz horizontal frequency. IC Role / Device Role / Timing Role: Sustained high-current switching node synchronized to video timing signals. Use Value: 60 W power dissipation and TO-3PF thermal mass support continuous duty cycle without thermal runaway.	Use Scenario: Repetitive high-voltage pulse generation for plasma ignition, capacitor discharge, or laser triggering. IC Role / Device Role / Timing Role: Fast-switching, high-energy discharge switch triggered by narrow base pulses. Use Value: I_CP = 15 A pulse rating and rugged SOA allow reliable 100–500 µs pulse delivery into capacitive loads.

TV Horizontal Output Stage

SMPS Primary Switch (Legacy)

Use Scenario: Horizontal deflection circuit in CRT televisions generating 15.625 kHz sawtooth current for line scanning.

IC Role / Device Role / Timing Role: High-voltage NPN switch controlling flyback transformer primary current and energy recovery timing.

Use Value: V_CEO = 700 V and SOA compliance ensure reliable operation under repetitive 1.2 kV flyback transients.

Use Scenario: Primary-side switching element in offline quasi-resonant or RCD-clamped flyback converters (≤100 W).

IC Role / Device Role / Timing Role: Main power switch turning on/off mains-referred transformer primary to regulate output voltage.

Use Value: Low V_CE(sat) = 1 V minimizes conduction loss; h_FE ≥ 2.25 simplifies base drive design in discrete controller topologies.

Monitor Deflection Circuit

Industrial HV Pulse Generator

Use Scenario: Line scan driver in analog computer monitors operating at 31.5 kHz horizontal frequency.

IC Role / Device Role / Timing Role: Sustained high-current switching node synchronized to video timing signals.

Use Value: 60 W power dissipation and TO-3PF thermal mass support continuous duty cycle without thermal runaway.

Use Scenario: Repetitive high-voltage pulse generation for plasma ignition, capacitor discharge, or laser triggering.

IC Role / Device Role / Timing Role: Fast-switching, high-energy discharge switch triggered by narrow base pulses.

Use Value: I_CP = 15 A pulse rating and rugged SOA allow reliable 100–500 µs pulse delivery into capacitive loads.

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
BU2520DX	V_CEO = 1000 V, I_C = 10 A, P_C = 125 W, TO-3P package	Higher voltage/current rating; larger thermal mass; requires different mounting hardware	Preferred where higher flyback margin or >5 A peak current is required; not drop-in due to TO-3P vs TO-3PF pin spacing
2SC5027	V_CEO = 700 V, I_C = 5 A, P_C = 60 W, TO-3P package, h_FE min = 3.0	Same voltage/current specs but higher DC gain; TO-3P footprint differs slightly in lead pitch and tab hole location	Offers improved base drive efficiency; mechanical adaptation needed for PCB layout and heatsink interface

Compared with BU508AFTBTU, BU2520DX provides greater voltage headroom and thermal capacity but demands board redesign, while 2SC5027 delivers higher h_FE for reduced drive power yet requires mechanical validation for TO-3P compatibility.

Availability

BU508AFTBTU is available at Aetrix Electronics and suitable for CRT television repair, legacy monitor manufacturing, industrial HV pulse generator builds, and discontinued SMPS refurbishment requiring stable component supply and long-term traceability.

Supply support for BU508AFTBTU 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

ON Semiconductor is a global semiconductor supplier delivering energy-efficient, high-performance silicon solutions for automotive, industrial, cloud computing, and consumer electronics markets.

The BU508AFTBTU belongs to Fairchild's legacy high-voltage bipolar transistor family, engineered specifically for reliability in CRT horizontal deflection and flyback converter applications demanding rugged SOA and repeatable avalanche tolerance.

FAQ

What is the maximum safe collector-emitter voltage for BU508AFTBTU in continuous operation?

The BU508AFTBTU has a continuous collector-emitter voltage rating of V_CEO = 700 V at T_C = 25°C. This value decreases with rising case temperature per the derating curve in Figure 6 of the datasheet. Operation above this voltage risks avalanche breakdown unless transient conditions are within the validated Safe Operating Area (SOA) limits.

Does BU508AFTBTU require a heatsink, and what thermal resistance is recommended?

Yes, BU508AFTBTU requires a heatsink due to its 60 W power dissipation rating at T_C = 25°C. For reliable operation at full DC current, a thermal resistance (R_θJC + R_θCS + R_θSA) ≤ 1.5°C/W is recommended to maintain junction temperature below 150°C. The TO-3PF package uses an isolated collector tab, so electrical isolation between tab and heatsink must be maintained.

What is the typical base drive current needed to fully saturate BU508AFTBTU?

The BU508AFTBTU achieves full saturation with I_B = 2 A when I_C = 4.5 A, resulting in V_CE(sat) ≤ 1 V. This corresponds to a forced beta (I_C/I_B) of 2.25, matching its minimum h_FE specification. Drive circuits must deliver this peak current without excessive overshoot to avoid secondary breakdown.

Can BU508AFTBTU be used in modern switch-mode power supplies?

BU508AFTBTU can be used in legacy or repair-focused SMPS designs (e.g., flyback, RCD-clamped) where bipolar switching topology and 700 V rating are appropriate. However, it lacks the speed and efficiency of modern MOSFETs or IGBTs; designers should verify SOA compliance, switching losses, and thermal performance against target operating frequency and load profile before selecting BU508AFTBTU.

Is BU508AFTBTU pin-compatible with BU508DF or BU508AF?

BU508AFTBTU shares identical pinout (Base–Collector–Emitter), package (TO-3PF), and core electrical specifications with BU508AF and BU508DF. All three are functionally interchangeable in circuit design, though minor variations in h_FE distribution and test conditions exist. Mechanical fit and electrical behavior in-circuit remain consistent across this variant group.

BU508AFTBTU Specifications

Product attributes: Attribute value

Manufacturer:: onsemi

Series:: -

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

Packaging:: Tube

Product Status:: Obsolete

Transistor Type:: NPN

Current - Collector (Ic) (Max):: 5 A

Voltage - Collector Emitter Breakdown (Max):: 700 V

Vce Saturation (Max) @ Ib, Ic:: 1V @ 2A, 4.5A

Current - Collector Cutoff (Max):: 1mA

DC Current Gain (hFE) (Min) @ Ic, Vce:: 2.25 @ 4.5A, 5V

Power - Max:: 60 W

Frequency - Transition:: -

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-3PF

BU508AFTBTU FAQ

1.How can I place an order for BU508AFTBTU through Aetrix?

Please submit a Request for Quotation (RFQ) for BU508AFTBTU 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 BU508AFTBTU reliable?

The price and inventory of BU508AFTBTU are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BU508AFTBTU is usually 5 days.

3.What payment methods are accepted for BU508AFTBTU?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BU508AFTBTU transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BU508AFTBTU?

BU508AFTBTU orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BU508AFTBTU 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 BU508AFTBTU?

For technical support, including BU508AFTBTU datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BU508AFTBTU requirements.

6.How does Aetrix verify that BU508AFTBTU is sourced from the original manufacturer or authorized distributors?

All BU508AFTBTU 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 BU508AFTBTU meets industry standards.

7.What is the process for return or replacement of BU508AFTBTU?

All BU508AFTBTU units undergo pre-shipment inspection (PSI). If there is an issue with BU508AFTBTU, 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 BU508AFTBTU part is unused and in its original packaging.

Return procedure for BU508AFTBTU:

1.Submit a request within 90 days.

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

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