AN849| Application Note
Maxim/Dallas > App Notes > POWER-SUPPLY CIRCUITS Keywords: SLIC, flyback, inverting supply, inverter, DC-DC, PWM, reverse recovery, ringer, talk battery, subscriber line interface card
Nov 12, 2001
APPLICATION NOTE 849
Selection of Ultra-Fast Recovery Diodes Used in Flyback Circuits
This application note describes the diode selection process and snubber design for high voltage inverting flyback converter for subscriber-line interface card (SLIC) applications. Critical diode parameters that affect the switching transients in the circuit, the design of the snubber circuit at the output diode are discussed. The recent developments in the PC and Telecom market now stretch power electronics switching frequencies from the line frequencies to the MHz range. This trend has led to a corresponding growth in technologies for electronic switching components, such as power rectifiers and power switches. Ultra-fast power rectifier performance is important at these switching frequencies. It requires the diode to have low recovery charge with soft recovery characteristics and a low forward voltage drop with fast turn-on. The purpose of this application note is to discuss the diode parameters affecting the circuit, in order to design a reliable power supply. In order to enumerate the effect of diode par
ameters on the circuit performance, the example of a flyback circuit using the MAX1856 is considered in this application note. The first section briefly describes the flyback circuit used as an example here. The second section discusses the important diode parameters that affect the switching transients in the circuit, the design of the snubber circuit at the output diode and the contributions of conduction, switching and reverse blocking of the rectifier to the overall power dissipation. The manufacturers of fast rectifiers may list all or only some of the parameters discussed in the second section. The third and last section discusses the performance of four different diodes in this circuit. This indicates a way to evaluate the performance of the different diodes in the application circuit. The article concludes with future expectation of improvement in performance brought by further technological development.
MAX1856 Flyback Circuit
The MAX1856 is used here (Figure 1) in a flyback configuration to generate power for a subscriber line interface card (SLIC) from a 12V input. The -90V@0.32A output is for the ringer function and the -30V@0.15A output for the talk battery.
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