SDG #293 AC-DC Converter using the Power Integrations LNK3204 SMPSU IC

This is the second entry in a mains power supply design series, replacing an earlier capacitive dropper with a proper switching regulator built around the Power Integrations LNK3204P LinkSwitch-TN2. The goal is modest but practical: a 5V rail at roughly 150mA from 85 to 265VAC, with 3.3V derived through a linear regulator, intended for a low power wireless sensor node.

The design walkthrough is genuinely useful for anyone who has never used Power Integrations’ tooling. PI Expert is not supported for this part, so the spreadsheet based PIXls tool does the heavy lifting, and the reference design documentation supplies the schematic that the spreadsheet omits. One notable engineering decision is deliberately overriding the tool’s inductance suggestion: the efficiency optimised value produced impractically short switching times, so 2.2mH was chosen instead, at the cost of 3.2 ohms of DC resistance.

Testing is done properly rather than casually. Because this is a non-isolated buck topology with the output referenced to the rectified mains, differential probes are mandatory, and the live conductor is wound five times through the current probe to convert a 10A range into a more usable 2A range. Measurements across load steps show clean regulation from around 85V upward, roughly a quarter of a watt of no load draw, and peak efficiency near 67 percent, lower than hoped and partly attributed to inductor resistance, freewheeling diode losses and the indicator LEDs.

At roughly $3.83 in components, with the controller itself under 40 cents, it undercuts the capacitive dropper approach while behaving far better at light loads. The obvious caveat remains: nothing on the output can ever be touched, which sets up the isolated flyback design promised next.