[66] Flyback Development Part III - Rectifier and Transformer Design

Flyback input rectifier and transformer design.

Introduction

This article is the third of a series in which Dr. Ridley shows the steps involved in designing and building an offline flyback converter. The input filter rectifier is simulated to find the design range of the converter, and the transformer design is completed and measured.

 

Input Voltage Range

The specifications for the power supply were as follows:

  1. Input Voltage: 185 – 265 AC
  2. Output 1: 15 VDC @ 1.1 A
  3. Output 2: 15 VDC @ 50 mA nonisolated (bias and regulation supply)
  4. Maximum power: 17.25 W (was 22 W)

Notice that between the first article of this series, and this third part, the load on the main output dropped from 1.4 A to 1.1 A. This is common during a power supply development; the end user is rarely finished with design and measurements before the power supply design starts. Being flexible in your design to accommodate changes both up and down in power is just a fact of life for power supply engineers.

The first step in the design process is to determine the actual range of the dc input bus after the rectifier. There is no simple equation to calculate this, and the fastest way to determine the range is to run a simulation, using a constant power load on a capacitor and rectifier. As the design progresses, and input filter and protection circuits are added to the design, the simulation can be modified to accommodate the additional components. Filter inductors, surge resistors and fuses all reduce the actual input voltage to the converter. Trying to incorporate all of this in a calculation is probably not the best use of design time, and simulation is a powerful aid here. Just about any circuit simulator will be able to do this part of the circuit for you, but POWER 4-5-6 was used to generate the waveforms in Figure 1 for maximum speed.

fig 1

Figure 1: Input line voltage POWER 4-5-6 simulation, showing rectified voltage on a 10 µF input capacitor.

If you were to do true worst-case analysis design, it is necessary to introduce variation in the frequency range of the AC supply, and include the anticipated variations of the input capacitor with initial tolerance and aging effects, but that is beyond the scope of this article.

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