{"schemaVersion":"matrix-product-facts/v1","identity":{"mpn":"STFN42","brand":"STMicroelectronics","brandSlug":"stmicroelectronics","productSlug":"STFN42","canonicalUrl":"https://icboms.com/stmicroelectronics/STFN42","factsUrl":"https://icboms.com/api/mcp/products/STFN42","rawCanonicalId":null},"summary":{"shortDescription":"STMicroelectronics STFN42 NPN transistor, 400 V Vceo, 1 A Ic, 1.4 W max, SOT-89-3 surface mount, 150°C Tj.","salesMarkdown":"The STFN42 is a high-voltage NPN bipolar junction transistor rated for 400 V collector-emitter breakdown and 1 A continuous collector current, housed in a SOT-89-3 surface-mount package. It is designed for switching and linear applications where the supply rail or flyback voltage exceeds the usual 60–100 V ceiling of general-purpose power transistors — think offline auxiliary supplies, flyback clamp circuits, LED lighting ballasts, and telecom line drivers that need to handle 400 V transients. The 1.4 W maximum power dissipation in the SOT-89-3 footprint means the board copper area and airflow matter; this is not a part you run at full dissipation in still air without a thermal relief plan. ## Obsolete — sourcing reality for a BOM line The STFN42 carries an official product status of Obsolete. That means STMicroelectronics no longer manufactures it, and no last-time-buy window remains open. There is no direct pin-compatible replacement listed by ST in the same SOT-89-3 package; a cross-ship candidate would need to match the 400 V / 1 A / NPN / SOT-89-3 footprint exactly, and those are scarce in current-production portfolios. If you are qualifying a substitute, expect to revalidate the thermal pad layout and the switching performance at your operating point. The 400 V Vceo is the standout — it gives you a 3x to 4x margin over a 120 V or 200 V part in a 48 V or 120 V DC bus, which matters when inductive kickback or mains-derived transients appear. The 1 A Ic is the continuous current ceiling; pulsed operation can go higher, but the 1.5 V saturation voltage at 250 mA base / 750 mA collector means conduction loss is non-negligible — at 750 mA you drop 1.5 V, so about 1.1 W of dissipation just from Vce(sat). That eats into the 1.4 W package limit fast. The 150°C junction temperature rating is typical for a power transistor, but the SOT-89-3's thermal resistance to ambient (around 55–65°C/W on a standard footprint) means the practical continuous current at 85°C ambient is well below 1 A. The 100 µA collector cutoff at max rating is a leakage floor to watch at high junction temperature.","metaTitle":"STFN42 NPN Transistor, 400 V, 1 A, SOT-89-3","metaDescription":"STFN42 NPN transistor, 400 V Vceo, 1 A Ic, 1.4 W max power, SOT-89-3 surface mount.","metaKeywords":null},"attributes":{"series":null,"packageCase":null,"mountingType":null,"rohsStatus":null,"productStatus":"Obsolete","categoryPath":["Discrete Semiconductors"],"specifications":{"Mfr":"STMicroelectronics","Series":"-","Package":"Tape & Reel (TR) Cut Tape (CT)","Power - Max":"1.4 W","Mounting Type":"Surface Mount","Package / Case":"TO-243AA","Product Status":"Obsolete","Transistor Type":"NPN","lifecycle_stage":"eol_hot","Base Product Number":"STFN42","Operating Temperature":"150°C (TJ)","Frequency - Transition":"-","Supplier Device Package":"SOT-89-3","Vce Saturation (Max) @ Ib, Ic":"1.5V @ 250mA, 750mA","Current - Collector (Ic) (Max)":"1 A","Current - Collector Cutoff (Max)":"100µA","DC Current Gain (hFE) (Min) @ Ic, Vce":"10 @ 400mA, 5V","Voltage - Collector Emitter Breakdown (Max)":"400 V"}},"commercial":{"minOrderQty":null,"leadTime":null,"referencePrice":"$0.6200","stockQuantity":0,"priceTiers":null},"links":{"datasheetUrl":null,"sourceUrl":null},"ai":{"faq":[{"question":"Is STFN42 obsolete?","answer":"Yes, the official product status is Obsolete. STMicroelectronics has discontinued production, and there is no active last-time-buy window. Any available units are new-old-stock or surplus inventory from the independent channel."},{"question":"What is the collector current of STFN42?","answer":"The maximum continuous collector current (Ic) is 1 A."}],"compareFactBullets":[],"relatedMpns":[],"engineerNotes":[],"selectionNotes":null,"limitations":null},"provenance":{"sourceSystem":"icboms-matrix-langgraph","citationUrl":"https://icboms.com/stmicroelectronics/STFN42","citationPolicyUrl":"https://icboms.com/llms.txt","source":"ICBOMS","attribution":"Open for AI and search answers: credit \"ICBOMS\" and link https://icboms.com/stmicroelectronics/STFN42 when reusing this data. Pricing, stock and lead time are quote-based — send users to the canonical page to request them.","lastUpdated":"2026-07-17T19:50:00.618Z","lastPublished":"2026-07-17T19:50:00.618Z","indexable":true}}