BF420-AP Equivalent & Substitute Parts

Part Overview

The BF420-AP is an NPN bipolar junction transistor manufactured by STMicroelectronics, designed for general-purpose switching and amplification applications. It features a 300 V collector-emitter breakdown voltage rating and 500 mA maximum collector current in a TO-92-3 through-hole package. The device is classified as obsolete, indicating discontinued production and limited availability from original sources. Identification of equivalent substitute parts is necessary to maintain design continuity and ensure project completion when original inventory is depleted.

Substiute Parts

BF420-AP

STMicroelectronicsIn Stock: 2124BF420-AP Datasheet

Current PartRFQ

KSP42BU

onsemiIn Stock: 105225KSP42BU Datasheet

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Key Parameters

Substitute Part Grouping Explanation

Substitution of the BF420-AP is determined by strict equivalence across the following critical parameters:

Mandatory Matching Criteria:

• Transistor polarity: NPN
• Collector-emitter breakdown voltage: 300 V (maximum rating)
• Maximum collector current: 500 mA
• Through-hole mounting configuration
• TO-92-3 package format

Acceptable Parameter Variations: Substitute parts may exhibit lower maximum power dissipation, lower transition frequency, or lower DC current gain (hFE) values, provided all mandatory criteria are met. These variations do not prevent functional substitution in applications designed within the BF420-AP specifications.

The KSP42BU from onsemi satisfies all mandatory matching criteria and is therefore classified as a direct substitute.

Parameter Comparison

Engineering Selection Recommendations

Primary Substitute: KSP42BU

The KSP42BU is the qualified substitute for the BF420-AP based on the following factors:

1. Electrical Equivalence: Both devices share identical maximum ratings for collector current (500 mA) and collector-emitter breakdown voltage (300 V), ensuring pin-compatible operation in existing circuit designs.

2. Package Compatibility: Both transistors utilize the TO-92-3 through-hole package with identical lead configuration, permitting direct physical replacement without PCB modification.

3. Compliance Status: The KSP42BU maintains ROHS3 compliance and REACH unaffected status, matching the regulatory profile of the original BF420-AP.

4. Product Availability: The KSP42BU is classified as active product status with substantial inventory availability (105,200 pieces), providing reliable long-term sourcing compared to the obsolete BF420-AP.

5. Performance Margin: The KSP42BU exhibits lower maximum power dissipation (625 mW versus 830 mW) and lower transition frequency (50 MHz versus 60 MHz). These reductions do not compromise substitution validity in applications designed to the BF420-AP specification envelope.

Frequently Asked Questions (FAQ)

Q: Can the KSP42BU directly replace the BF420-AP without circuit modification?

A: Yes. Both transistors share identical collector current and collector-emitter breakdown voltage ratings, identical TO-92-3 package format, and identical lead pinout. Direct substitution is supported without circuit redesign.

Q: Why does the KSP42BU have lower maximum power dissipation than the BF420-AP?

A: The KSP42BU is rated at 625 mW maximum power dissipation compared to the BF420-AP at 830 mW. This difference reflects manufacturing process variations between onsemi and STMicroelectronics. Substitution remains valid provided the application power dissipation does not exceed the KSP42BU rating of 625 mW.

Q: Are there differences in saturation voltage between these devices?

A: Yes. The KSP42BU exhibits 500 mV saturation voltage at 2 mA base current and 20 mA collector current, while the BF420-AP exhibits 600 mV at 5 mA base current and 30 mA collector current. These measurements are taken at different operating points and do not prevent substitution in standard switching applications.

Q: What is the significance of the higher collector cutoff current in the KSP42BU?

A: The KSP42BU specifies 100 nA maximum collector cutoff current (ICBO) compared to 10 nA for the BF420-AP. This represents a tenfold increase in leakage current. In applications requiring extremely low leakage, this difference may be significant; however, for general-purpose switching and amplification, this leakage level remains negligible.

Q: Is the lower transition frequency of the KSP42BU a limitation?

A: The KSP42BU transition frequency is 50 MHz compared to 60 MHz for the BF420-AP. This 10 MHz reduction does not prevent substitution in applications designed to the BF420-AP specification. Applications requiring frequencies approaching or exceeding 60 MHz should be evaluated individually.

Q: Are both devices RoHS compliant?

A: Yes. Both the BF420-AP and KSP42BU are ROHS3 compliant and REACH unaffected, meeting current environmental and regulatory requirements for electronic component procurement.

Q: What is the difference in DC current gain between these transistors?

A: The BF420-AP specifies minimum hFE of 50 at 25 mA collector current and 20 V collector-emitter voltage. The KSP42BU specifies minimum hFE of 40 at 30 mA collector current and 10 V collector-emitter voltage. These measurements are taken at different operating points. The lower hFE value of the KSP42BU does not prevent substitution in applications designed to the BF420-AP specification.