{"id":119802,"date":"2025-11-24T03:15:14","date_gmt":"2025-11-24T03:15:14","guid":{"rendered":"https:\/\/www.seeedstudio.com\/blog\/?p=119802"},"modified":"2025-11-27T10:11:38","modified_gmt":"2025-11-27T10:11:38","slug":"designing-a-xiao-expansion-board-using-a-schottky-diode-to-prevent-power-backfeeding","status":"publish","type":"post","link":"https:\/\/www.seeedstudio.com\/blog\/2025\/11\/24\/designing-a-xiao-expansion-board-using-a-schottky-diode-to-prevent-power-backfeeding\/","title":{"rendered":"Designing a XIAO Expansion Board \u2014 Using a Schottky Diode to Prevent Power Backfeeding"},"content":{"rendered":"\t\t
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Introduction<\/strong><\/span><\/h2>\n

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In many portable projects, devices often need to support both battery power<\/strong> and external charger power<\/strong>.<\/p>\n

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While this dual-power structure is convenient, improper handling can lead to backfeeding<\/strong>\u2014where current flows backward from the XIAO\u2019s USB port into the external power source or even the main control circuitry, potentially causing damage.<\/p>\n

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Problem Scenario<\/strong><\/span><\/h2>\n

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In an (XIAO ESP32S3\/Sense\/XIAO with 5V<\/strong> pin) expansion board, the system supports both USB (XIAO_5V) power input and external 5V (EXT_5V) input.<\/p>\n

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If both sources are connected at the same time without proper isolation<\/strong>, current may flow back from the XIAO USB port into the EXT_5V rail, causing over-current issues or even permanent damage.<\/p>\n

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Solution Approach<\/strong><\/span><\/h2>\n

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Use a low forward-voltage Schottky diode<\/strong> (e.g., SS14 or BAT54) in series with the EXT_5V<\/strong> input to achieve automatic isolation.<\/p>\n

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When only EXT_5V is connected:<\/strong><\/h4>\n

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The diode is forward-biased, supplying: EXT_5V \u2192 XIAO ESP32S3 (or other XIAO) 5V input.<\/p>\n

When USB Type-C is connected: The diode becomes reverse-biased, blocking current from flowing back into EXT_5V.<\/p>\n

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When both USB Type-C and EXT_5V are connected:<\/strong><\/span><\/h2>\n

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1.The two voltages are very close.<\/p>\n

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2.The Schottky diode sees almost no forward voltage and stays reverse-biased or nearly off.<\/p>\n

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3.The system automatically draws power from the XIAO USB Type-C (XIAO_5V)<\/strong>.<\/p>\n

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4.The Schottky diode effectively blocks any backfeeding from the USB 5V rail into the external EXT_5V<\/strong> input.<\/p>\n

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This ensures USB has priority while maintaining safe power-source isolation.<\/strong><\/p>\n

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Design Considerations \/ Risks & Decisions<\/strong><\/span><\/h2>\n

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Design Goal 1 (Common Requirement): USB 5V Priority<\/strong><\/h4>\n

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USB should be the main power source.<\/p>\n

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This is normally achieved if EXT_5V <\/strong>is fed through a diode, causing its voltage to be slightly lower than XIAO_5V<\/strong>.<\/p>\n

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Design Goal 2 (Backfeed Prevention):<\/strong><\/h4>\n

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If you must guarantee that EXT_5V<\/strong> never exceeds<\/strong> the USB 5V<\/strong> rail (e.g., the external adapter might output >5V), you should:<\/p>\n

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1.clamp\/limit the EXT_5V<\/strong> voltage or add isolation on the XIAO_5V<\/strong> side as well.<\/p>\n

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Charging Path Consideration:<\/strong><\/h4>\n

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If the system contains a battery charging IC (with a BAT terminal), then:<\/p>\n

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1.When USB is plugged in, VBUS typically powers the battery charging.<\/p>\n

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2.When USB and EXT_5V coexist, the charger\u2019s input must be correctly tied to XIAO_5V (or a power-path IC)<\/strong> to avoid abnormal charging behavior.<\/p>\n

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Advantages:<\/strong><\/h3>\n

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Low cost and easy to implement<\/p>\n

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Low forward voltage drop, efficient and reliable<\/p>\n

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No additional control logic required<\/p>\n

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\u2699\ufe0f Why Schottky?<\/span><\/strong><\/h2>\n

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We choose a Schottky diode not simply because it can \u201cblock\u201d reverse current\u2014any diode can do that\u2014but because it performs this task more efficiently and with better overall characteristics<\/strong>.<\/p>\n

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Recommended models: SS14 \/ BAT54 \/ MBRS340<\/strong>, all available in the OPL<\/a>.<\/p>\n

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\n\n\n\n\n\n\n
Features<\/strong><\/td>\nStandard Diode<\/strong><\/td>\nSchottky<\/strong><\/td>\n<\/tr>\n
Forward voltage drop<\/strong><\/td>\n\u22480.7V<\/td>\n\u22480.3V<\/td>\n<\/tr>\n
Fast switching response<\/strong><\/td>\nNormal<\/td>\nFast<\/td>\n<\/tr>\n
L<\/strong>ow-voltage systems<\/strong><\/td>\nNormal<\/td>\nSuitable<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

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Extension board Quick Prototyping-Seeed Fusion <\/a><\/strong><\/span><\/h2>\n

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After finalizing your schematic, Seeed Fusion PCBA<\/a> service helps you:<\/p>\n

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1. Get official XIAO design verification;<\/p>\n

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2. Receive prototype boards within 7\u201315 days;<\/p>\n

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