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

STMicroelectronics BU508AF

Name: BU508AF
Brand: STMicroelectronics
SKU: BU508AF
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BU508AF

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-3P-3 Full Pack

Datasheet:: BU508AF.pdf

Description:: TRANS NPN 700V 8A TO-3PF

Quantity:: RFQ Add To RFQ list

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Inventory:727

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Product details

Overview

BU508AF from STMicroelectronics is a high-voltage NPN power transistor designed for horizontal deflection output stages in standard-definition CRT TVs and switch-mode power supplies in CRT television systems. It features V_CEO = 700 V, I_C = 8 A, P_TOT = 50 W at T_C = 25°C, V_ins = 2500 V RMS insulation withstand voltage, and h_FE = 10–30 at I_C = 0.1–4.5 A.

For engineers reviewing the BU508AF datasheet, BU508AF pinout, BU508AF application, or BU508AF equivalent, key selection considerations include ruggedness under inductive switching, low base-drive requirement for horizontal deflection linearity, tight h_FE tolerance at operating current, and UL-compliant ISOWATT218FX insulated package for CRT safety compliance.

Technical Context

The BU508AF employs diffused collector planar technology with an enhanced high-voltage structure optimized for horizontal deflection duty cycles. Its design supports pulsed operation with t_P < 5 ms (I_CM = 15 A), storage time t_s = 2.5 µs, and fall time t_f = 0.2 µs under specified inductive load conditions (L_BB = 4.5 µH, f_h = 16 kHz).

It delivers stable performance across -65°C to 150°C junction temperature range, with thermal resistance R_thj-case ≤ 2.5°C/W and safe operating area validated for repetitive inductive switching. The device sustains V_CEO(sus) = 700 V at I_C = 100 mA and exhibits V_CE(sat) = 1 V at I_C = 4.5 A / I_B = 1.6 A.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	700 V - Sustained collector-emitter blocking voltage with base open, enabling use in CRT horizontal output stage without external snubber overload.
I_C	8 A - Continuous collector current rating supporting full-swing deflection drive at 15.625 kHz line frequency.
P_TOT	50 W at T_C = 25°C - Total dissipation capability compatible with heatsinked ISOWATT218FX mounting in compact CRT chassis layouts.
V_ins	2500 V RMS - Insulation withstand voltage from all three leads to external heatsink, satisfying UL safety requirements for CRT primary-side isolation.
h_FE	10–30 - Tight DC current gain spread at I_C = 0.1–4.5 A, ensuring consistent base drive design across production batches.
t_s/t_f	2.5 µs / 0.2 µs - Verified storage and fall times under 16 kHz inductive switching, critical for minimizing switching loss and flyback spike in deflection circuits.

Pinout & Package

The BU508AF is housed in the fully insulated ISOWATT218FX package, compliant with UL insulation standards and featuring integral heatsink isolation. Its three-terminal configuration supports direct mounting to grounded metal chassis while maintaining high-voltage isolation between collector and heatsink.

Pin/Terminal	Circuit Role	Design Meaning
1 (Collector)	Main high-current, high-voltage output node	Internally connected to the insulated metal tab; electrically isolated from heatsink surface per UL 2580 requirements.
2 (Base)	Control input for bipolar conduction	Low-impedance gate for precise timing of horizontal sweep onset and cutoff; requires I_B ≤ 4 A peak.
3 (Emitter)	Current return path and reference node	Common emitter connection referenced to deflection yoke ground; enables standard grounded-emitter switching topology.

Key Features

Feature	Design Value
Diffused collector "Enhanced generation" structure	Improved ruggedness against secondary breakdown during flyback recovery, extending reliability in CRT horizontal deflection applications.
Tight h_FE range at operating I_C	Enables fixed-base-resistor drive design without feedback compensation, reducing BOM count and layout complexity in cost-sensitive TV designs.
Low base-drive requirement	Supports direct drive from small-signal horizontal oscillator transistors (e.g., BC337), eliminating need for intermediate driver stages.
Stable performance vs. temperature	Maintains V_CEO(sus), h_FE, and switching times across -25°C to +85°C ambient, ensuring consistent deflection linearity over operating life.

Applications

Horizontal Deflection Output

Switch-Mode Power Supply

Use Scenario: Horizontal scanning circuit in analog CRT television sets operating at 15.625 kHz line frequency with flyback transformer energy recovery.

IC Role / Device Role / Timing Role: Main switching transistor in self-oscillating or synchronized horizontal output stage; conducts during trace, blocks during retrace.

Use Value: High V_CEO and rugged SOA enable reliable operation under repetitive inductive stress; 2500 V insulation prevents chassis leakage and meets UL Class X/Y safety requirements.

Use Scenario: Primary-side power switch in offline flyback converters supplying CRT filament, EHT, and logic rails.

IC Role / Device Role / Timing Role: High-voltage switching element controlling energy transfer from AC mains to isolated secondary windings.

Use Value: 700 V V_CEO and 15 A I_CM support universal input (90–264 VAC) operation with margin; low V_CE(sat) reduces conduction loss at 50–100 kHz switching.

Deflection Yoke Driver

High-Voltage Flyback Switch

Use Scenario: Direct drive of CRT deflection yoke with sawtooth current waveform and rapid retrace collapse.

IC Role / Device Role / Timing Role: Current-controlled switch shaping horizontal deflection current; handles high di/dt during flyback phase.

Use Value: Verified t_s = 2.5 µs and t_f = 0.2 µs ensure minimal distortion in linear trace segment; tight h_FE maintains consistent yoke current amplitude across units.

Use Scenario: High-voltage switch generating EHT (15–30 kV) via flyback transformer in CRT display systems.

IC Role / Device Role / Timing Role: Primary-side switch initiating magnetic energy storage and controlled release into HV secondary winding.

Use Value: 1500 V V_CES rating provides margin against voltage spikes during no-load or fault conditions; insulated package prevents arcing to grounded chassis.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
BU2520DX	V_CEO = 1000 V, I_C = 10 A, R_thj-case = 2.0°C/W, same ISOWATT218FX package	Higher voltage/current headroom; used in larger CRT monitors and projection TVs	Select when >700 V blocking or >8 A current is required; may require revised base drive due to higher h_FE.
2SC5148	V_CEO = 1500 V, I_C = 7 A, TO-3P package, non-insulated	Higher V_CES but lacks integrated insulation; requires external isolation bushing	Choose only if mechanical redesign allows non-insulated mounting and UL certification permits external insulation measures.

Compared with BU2520DX and 2SC5148, the BU508AF offers optimal balance of 700 V/8 A rating, UL-certified insulation, and proven thermal behavior in mass-produced CRT TV horizontal stages-making it the preferred choice where safety compliance and footprint compatibility are mandatory.

Availability

BU508AF is available at Aetrix Electronics and suitable for CRT television horizontal deflection stages, flyback SMPS designs, and high-voltage industrial switching applications requiring stable component supply and long-term lifecycle continuity.

Supply support for BU508AF 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 automotive ICs.

The BU508AF belongs to ST's high-voltage bipolar power transistor product line, engineered specifically for CRT deflection and flyback converter applications demanding robustness, insulation integrity, and thermal stability in consumer display systems.

FAQ

Is BU508AF RoHS-compliant and lead-free?

Yes. The BU508AF is manufactured in ST's ECOPACK® package with lead-free second-level interconnect, compliant with JEDEC Standard JESD97. The inner box label indicates the lead-free category, and soldering profiles follow ST's published ECOPACK specifications.

Can BU508AF replace BU2508DF in existing CRT designs?

No. BU2508DF has V_CEO = 1500 V and different h_FE and switching characteristics. Substituting BU508AF (V_CEO = 700 V) risks premature breakdown in circuits relying on higher blocking voltage; validation of SOA and flyback voltage margins is required before any replacement.

What is the maximum recommended case temperature for continuous operation?

The BU508AF is rated for T_J ≤ 150°C maximum. With R_thj-case ≤ 2.5°C/W, sustained operation at full 50 W dissipation requires T_C ≤ 25°C. For practical CRT applications, derating to ≤35 W at T_C = 70°C ensures reliable long-term performance per Figure 3 in the datasheet.

Does BU508AF require a base resistor in horizontal deflection circuits?

Yes. A series base resistor is mandatory to limit I_B within 4 A peak and control switching speed. Typical values range from 10 Ω to 47 Ω depending on driver strength and desired t_s/t_f; omission risks excessive base current and thermal runaway during flyback recovery.

BU508AF Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

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

Packaging:: Tube

Product Status:: Active

Transistor Type:: NPN

Current - Collector (Ic) (Max):: 8 A

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

Vce Saturation (Max) @ Ib, Ic:: 1V @ 1.6A, 4.5A

Current - Collector Cutoff (Max):: 200µA

DC Current Gain (hFE) (Min) @ Ic, Vce:: 10 @ 100mA, 5V

Power - Max:: 50 W

Frequency - Transition:: -

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-3PF

BU508AF FAQ

1.How can I place an order for BU508AF through Aetrix?

Please submit a Request for Quotation (RFQ) for BU508AF 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 BU508AF reliable?

The price and inventory of BU508AF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BU508AF is usually 5 days.

3.What payment methods are accepted for BU508AF?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BU508AF transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BU508AF?

BU508AF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BU508AF 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 BU508AF?

For technical support, including BU508AF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BU508AF requirements.

6.How does Aetrix verify that BU508AF is sourced from the original manufacturer or authorized distributors?

All BU508AF 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 BU508AF meets industry standards.

7.What is the process for return or replacement of BU508AF?

All BU508AF units undergo pre-shipment inspection (PSI). If there is an issue with BU508AF, 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 BU508AF part is unused and in its original packaging.

Return procedure for BU508AF:

1.Submit a request within 90 days.

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

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