ICE3BR0365 Datasheet, PDF (4/16 Pages) Infineon Technologies AG – Design Guide for Off-line Fixed Frequency DCM Flyback Converter

DCM Flyback
Design Note DN 2013-01
V1.0 January 2013
I. Introduction
Flyback is the most widely used SMPS topology for low power application from 100W down to under 1W, whenever
the output needs to be isolated from the input. Its best features are low system cost, simplicity, and relative ease of
implementation. For low current output and power levels below 50W, DCM flyback is the usually the preferred
operating mode, due to it’s simpler control loop implementation and lower turn on loss. The objective of this paper is
to develop a comprehensive, practical and easy to follow approach in designing an off line DCM Flyback power
supply. This includes component selection guide, design knowledge and practical tips for a fast and well optimized
design.
II. Fixes Frequency Flyback Modes of Operation: DCM vs CCM
Figure 1 shows the basic circuit diagram of a Flyback converter. Its main parts are the transformer, the primary
switching MOSFET Q1, secondary rectifier D1, output capacitor C1 and the PWM controller IC. Depending on the
design of T1, the Flyback can operate either in CCM (Continuous
Conduction Mode) or DCM (Discontinuous Conduction Mode).
In DCM, all the energy stored in the core is delivered to the
secondary during the turn off phase (Flyback period), and the
primary current falls back to zero before the Q1 switch turns on
again. For CCM, the energy stored in the transformer is not
completely transferred to the secondary; that is, the Flyback
current (ILPK and ISEC) does not reach zero before the next
switching cycle. Figure 2 shows the difference between DCM and
DCM mode in terms of Flyback primary and secondary current
waveforms.
Figure 1: Flyback Schematic
Table 1: DCM vs CCM
Table 1 highlights the main points of advantage
Figure 2: CCM and DCM Flyback Current Waveforms
for either DCM or CCM mode operation. DCM
operation requires a higher peak currents to
deliver the required output power compared to CCM operation. This translates to a higher RMS current rating on the
primary MOSFET and ouput capacitor, and greater conduction losses in the transformer windings. When these
higher peak and RMS current limits the fulfillment of design requirements, (e.g. larger output capacitor required or
very high conduction loss on the MOSFET and transformer), switching to CCM mode is advised. This condition
usually occurs for designs wherein the output voltage is low and output current is relatively high (> 6A), typically for
output power over 50W, though for 5V or lower outputs this is a problem at power below 50W.
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