onsemi SA40A

Overview

SA40A from Fairchild Semiconductor is a unidirectional 500 W transient voltage suppressor (TVS) diode in DO-15 package, designed for clamping ESD and surge transients in power rails and I/O lines. It features a 40 V reverse stand-off voltage (VRWM), 44.4–49.1 V breakdown voltage (VBR) at 1 mA, 64.5 V clamping voltage (VC) at 7.8 A peak pulse current (IPPM), and operates up to +175 °C junction temperature.

For engineers reviewing the SA40A datasheet, pinout, applications, or equivalent options, this device serves as a robust overvoltage protection component in DC power supplies, automotive electronics, industrial control inputs, and telecom interface circuits where fast response (<1.0 ps), low leakage (<1 µA above 10 V), and high surge capability are required.

Technical Context

The SA40A employs a glass-passivated PN junction optimized for transient suppression on the standardized 10/1000 µs waveform. Its clamping behavior is defined by a low incremental surge resistance and tight VBR tolerance (min 44.4 V, max 49.1 V), enabling predictable voltage limiting during fast-rising surges.

It operates unidirectionally with cathode band marking, supports non-repetitive forward surge currents up to 70 A (8.3 ms half-sine), and maintains stable performance across -65 °C to +175 °C storage and operating junction temperatures.

Key Specifications

Pinout & Package

SA40A is housed in a DO-15 axial-leaded package with cathode indicated by a color band. The device has two terminals: anode and cathode, with unidirectional conduction from anode to cathode.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar transient voltage suppression applications.

Compared with SA40A, SMAJ40A offers automotive qualification without electrical trade-offs, while SMA6J40A provides identical protection in a smaller footprint at the cost of through-hole compatibility and different thermal dissipation characteristics.

Availability

SA40A is available at Aetrix Electronics and suitable for DC power input protection, automotive body control module inputs, industrial PLC digital input protection, and telecom line interface surge suppression requiring stable component supply and long-term obsolescence management.

Supply support for SA40A 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

Fairchild Semiconductor was a leading designer and manufacturer of high-performance analog and discrete semiconductors, acquired by ON Semiconductor in 2016; its legacy TVS portfolio remains widely deployed and supported.

The SAxxA series was engineered specifically for industrial and automotive transient suppression, emphasizing ruggedness, repeatable clamping, and high-temperature operation in harsh electromagnetic environments.

FAQ

What is the clamping voltage of SA40A and how is it measured?

The SA40A has a maximum clamping voltage (VC) of 64.5 V, measured at 7.8 A peak pulse current (IPPM) using the standardized 10/1000 µs double-exponential waveform. This value represents the highest voltage that will appear across the SA40A terminals during a surge event of that magnitude, ensuring downstream components see no more than 64.5 V. The SA40A achieves this clamping level consistently due to its low incremental surge resistance and tightly controlled breakdown voltage range of 44.4–49.1 V.

Is SA40A bidirectional or unidirectional?

The SA40A is a unidirectional transient voltage suppressor, indicated by the absence of the "C" suffix (which denotes bidirectional variants like SA40CA). It conducts only in reverse breakdown - from cathode to anode - and must be installed with the cathode connected to the protected line and the anode to ground. Its datasheet specifies parameters exclusively for reverse-biased operation, and it includes a visible cathode band for correct orientation. The SA40A does not provide symmetrical clamping in both directions.

What is the maximum operating temperature for SA40A?

The SA40A has a maximum operating junction temperature (TJ) of +175 °C and a storage temperature range of -65 °C to +175 °C. This high-temperature rating allows reliable deployment in under-hood automotive applications, industrial motor drives, and enclosed power supplies where ambient temperatures exceed 100 °C. Thermal performance depends on PCB copper area and lead length; the datasheet specifies 1.0 W steady-state power dissipation with 0.375" lead length at TA = 75 °C.

Can SA40A replace SA40CA in a circuit?

No, SA40A cannot directly replace SA40CA because they differ fundamentally in conduction behavior: SA40A is unidirectional and only clamps negative transients relative to ground, while SA40CA is bidirectional and clamps both polarities symmetrically. Substituting SA40A for SA40CA would leave positive transients unprotected on AC-coupled or floating lines. Circuit redesign - including polarity-aware placement and verification of signal reference integrity - is required before substitution. Always confirm the SA40A's unidirectional behavior matches the protection architecture of the target application.

What packaging format is used for SA40A?

The SA40A is supplied in a DO-15 axial-leaded package with leads spaced 0.4" (10.16 mm) apart, conforming to industry-standard through-hole mounting. The cathode is identified by a colored band near one lead end. This package supports manual or wave-solder assembly and provides mechanical robustness for high-vibration environments. The DO-15 outline is documented in Fairchild's package drawings and aligns with JEDEC DO-204-AC standards, ensuring compatibility with legacy tooling and inspection systems used for SA40A integration.