Designing Isolated Flyback Converter Circuits


Points of this article

・Calculation of part constants and important points in selecting parts will be explained using this circuit as an example.

From this point on, we start the design of an actual isolated flyback converter. First, we show a circuit diagram using the [PWM controller IC for AC-DC: BM1P011FJ] that was chosen for a case study.

In this sections and sections that follow, we will explain how to select the parts necessary for the circuit and a method for calculating the constants used. Therefore, the other sections also need to reference this circuit, by means of links that are provided for your convenience.


This circuit is designed to satisfy the [Power Supply Specifications as a Case Study] that were established previously.

Power Supply Specifications as a Case Study

  • Input voltage: 85 to 264 VAC
  • Output: 12 VDC±5% / 3A 36W
  • Output ripple voltage: 200mVp-p
  • Isolation voltage: 3kVAC between primary and secondary
  • Operating temperature range: 0 to 50℃
  • Efficiency: 80% or higher
  • No load input power: 0.1W or less

Through a filter, the input voltage undergoes rectification and smoothing in the full-wave rectifiers DA1 and C1. The rectified DC high voltage is input into the transformer T1 and switched by the switching transistor Q1. The output on the secondary side of the transformer T1 is isolated by an optocoupler, and it is fed back to the IC1 for the AC-DC converter IC. The feedback controls the switching transistor Q1 and regulates the output. Such is the overall structure of the circuit.

The other parts and circuits will be described in the succeeding sections.

【Download Documents】Basic of AC-DC Conversion

Basic studies to understand AC-DC converters and to go designing.

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Basic of AC-DC Conversion

Basic studies to understand AC-DC converters and to go designing.