BD678AS PNP Darlington Transistor Equivalent & Substitute Parts

Part Overview

The BD678AS is a PNP Darlington bipolar junction transistor manufactured by onsemi, rated for 60 V collector-emitter breakdown voltage and 4 A maximum collector current. This component is classified as obsolete, which necessitates identification of functionally equivalent alternatives for ongoing design support and procurement continuity. The BD678AS is packaged in TO-126-3 (TO-225AA) through-hole configuration and is suitable for applications requiring moderate power dissipation up to 14 W.

Substiute Parts

BD678AS

onsemiIn Stock: 873BD678AS Datasheet

Current PartRFQ

BD678

STMicroelectronicsIn Stock: 1259BD678 Datasheet

DirectRFQ

Key Parameters

Substitute Part Grouping Explanation

Substitution of the BD678AS is determined by electrical parameter equivalence across the following critical specifications:

• Transistor Configuration: PNP Darlington topology
• Voltage Rating: 60 V collector-emitter breakdown voltage
• Current Rating: 4 A maximum collector current
• DC Current Gain: Minimum 750 hFE
• Operating Temperature: 150°C junction temperature capability
• Compliance: ROHS3 compliance and REACH unaffected status
• Mounting: Through-hole technology

The BD678 manufactured by STMicroelectronics meets all electrical parameter requirements for direct substitution. While packaging differs between the onsemi TO-126-3 and STMicroelectronics SOT-32-3 variants, both are through-hole configurations compatible with standard PCB layouts. The STMicroelectronics variant offers enhanced power dissipation capability (40 W versus 14 W), providing additional thermal margin in applications.

Parameter Comparison

Engineering Selection Recommendations

The BD678 manufactured by STMicroelectronics is the direct electrical equivalent to the obsolete BD678AS. Selection of the STMicroelectronics variant is supported by the following factors:

Product Status: The STMicroelectronics BD678 maintains active product status, ensuring long-term availability and supply chain continuity compared to the obsolete onsemi BD678AS.

Compliance Certifications: Both components maintain ROHS3 compliance and REACH unaffected status, satisfying regulatory requirements for new designs and procurement.

Electrical Equivalence: Matching collector current (4 A), breakdown voltage (60 V), DC current gain (750 hFE minimum), and junction temperature rating (150°C) confirm functional interchangeability in circuit applications.

Thermal Performance: The STMicroelectronics variant provides 40 W maximum power dissipation compared to 14 W for the onsemi version, offering improved thermal headroom without circuit modification.

Through-Hole Compatibility: Both components utilize through-hole mounting technology, compatible with existing PCB designs and assembly processes.

Frequently Asked Questions (FAQ)

Q: Can the BD678 (STMicroelectronics) directly replace the BD678AS (onsemi) in existing designs?

A: Yes. Both components share identical electrical specifications for collector current (4 A), breakdown voltage (60 V), DC current gain (750 hFE), and operating temperature (150°C). Through-hole mounting compatibility permits direct PCB substitution.

Q: What is the difference between TO-126-3 and SOT-32-3 packaging?

A: Both are through-hole package variants of the TO-225AA family. TO-126-3 and SOT-32-3 differ in physical dimensions and lead configuration but maintain electrical equivalence. PCB layout verification is required to confirm mechanical fit.

Q: Why does the STMicroelectronics BD678 have higher power dissipation (40 W vs. 14 W)?

A: Power dissipation rating reflects the thermal design of the specific package variant. The higher rating provides additional thermal margin but does not alter electrical performance or circuit behavior.

Q: Are there any compliance differences between the two parts?

A: No. Both BD678AS and BD678 maintain ROHS3 compliance and REACH unaffected status, satisfying identical regulatory requirements.

Q: What is the significance of the obsolete product status for BD678AS?

A: Obsolete status indicates the onsemi BD678AS is no longer manufactured or supported. The active STMicroelectronics BD678 provides equivalent functionality with assured long-term availability.

Q: How do saturation voltage specifications differ between the two parts?

A: The BD678AS specifies 2.8 V saturation at 40 mA base current and 2 A collector current, while the STMicroelectronics BD678 specifies 2.5 V at 30 mA base current and 1.5 A collector current. Both values remain within acceptable operating ranges for typical Darlington applications.