This application note describes a UWR flyback converter that can be used to supply a DSL Multiplexer. The advantage in using a flyback converter in this application is the moderate variation of the duty cycle with the input voltage. The main drawback is the high value of the rms current in the secondary winding, in the output diode and in the output capacitor.
This document details the design of a 27 W power supply unit (PSU) capable of handling ultra wide range input voltages, from 36 V DC to 265 V AC. The design is specifically intended for use in telecommunications equipment, such as a DSL Multiplexer (DSLM), where a single power supply must manage varying input conditions efficiently. The topology chosen is a flyback converter, selected for its moderate duty cycle variation compared to other topologies like the forward converter.
The design utilizes the L5991 current mode controller and incorporates specific circuits for active start-up, slope compensation, and a synchronous buck post-regulator to achieve two stable outputs of 5 V and 3.3 V. The document provides schematics, electrical specifications, and explanations for key design choices, including the use of separate connectors for AC and DC inputs to optimize efficiency and inrush current limiting.
| Total output power | 27 W |
|---|---|
| Isolated | Yes |
| Minimum DC input voltage | 36 V |
| Maximum DC input voltage | 72 V |
| Minimum AC input voltage | 88 V |
| Maximum AC input voltage | 265 V |
| Output voltage 2 | 3.3 V |
| Maximum output current 1 | 2 A |
| Maximum output current 2 | 5 A |
| Maximum precision error on output 1 | 3% |
| Maximum precision error on output 2 | 3% |
| Switching frequency | 70 kHz |
| Reflected voltage | 70 V |
The total output power of the converter is 27 W.
The controller used in this application is the L5991.
The switching frequency was selected to be equal to 70 kHz.
The post regulator is used to obtain a second output of 3.3 V able to provide up to 5 A to its load.