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Primary Side Regulated Switching Mode Power Supply Controller

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The AP3781 is a high performance AC/DC power supply controller for battery charger and adapter applications. The controller regulates the output voltage and current in the primary side by Pulse Frequency Modulation in Discontinuous Conduction Mode (DCM).

The AP3781 operates in Pulse Frequency Modulation (PFM) mode and peak current Amplitude Modulation (AM) mode to form a fine tune frequency curve within the whole power range. Therefore, AP3781 can achieve high average efficiency and improve audible noise.

The AP3781 provides comprehensive protections without additional circuitry. It contains VCC over voltage protection, output over voltage protection, output under voltage protection, output short circuit protection, cycle-by-cycle current limit, open loop protection, external OTP and internal OTP, etc.

The AP3781 is packaged in SO-8.


  • Routers
  • Power Tools
  • Set-Top Box (STB) Power Supply
  • Smart Speakers
  • Network Adaptors


  • Primary Side Control for Eliminating Opto-Coupler
  • 75mW No-Load Input Power
  • Flyback Topology in DCM Operation
  • External Adjustable Over Temperature Protection
  • Fixed Internal Cable Compensation: 3%
  • External Cable Compensation: 3% to 8%
  • Multiple Segment AM/PFM Control Mode to Improve Audio Noise and Efficiency
  • Ultra Low Under Voltage Protection, Support Wide Operating Voltage for Power Tool Motor Drive
  • Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)
  • Halogen and Antimony Free. “Green” Device (Note 3)
  • For automotive applications requiring specific change control (i.e. parts qualified to AEC-Q100/101/200, PPAP capable, and manufactured in IATF 16949 certified facilities), please contact us or your local Diodes representative. https://www.diodes.com/quality/product-definitions/

Product Specifications

Product Parameters

VCC Max 37 V
Startup Current 1 µA
UVLO Threshold on/off 6.8/16 V
Operating Current 550 µA
Standby Power 75 mW
Power Rating 45 W
Cable Compensation ADJ

Technical Documents

Evaluation Board User Guides

Recommended Soldering Techniques



Orderable Part Number Buy from Distributor / Contact Sales Request Samples


Authorized Distributor Quantity Inventory Date Countries/Regions Buy Online
Digi-Key Electronics 3957 1/21/2021 Europe, Asia, North America Buy Now
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