ICE3BR0365 Datasheet, PDF (5/16 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
Figure 2B: Normalized RMS Current Ratio for Trapezoidal/Triangular Waveforms
Figure 2B shows the relationship between trapezoidal and triangular current waveforms on the primary side of the
Flyback. I_A is the nominal starting point of the waveform, which will be zero for a triangular waveform and some
higher value determined by the current still flowing in the primary winding during CCM operation when the switch
turns back on. I_B is the end point for the current level during the Ton interval. The IRMS normalized current value as
a function of the K factor (I_A/I_B) is shown on the Y axis; this is the multiplier that should be used for estimating
resistive losses for different wave shapes in comparison to a flat top trapezoidal waveform, and highlights the
additional DC conduction losses inherent to the transformer windings and semiconductors as a function of the
current waveform. This can give an 8-12% conduction loss advantage to a well designed CCM converter; this is
something to consider in applications where higher RMS currents are required, and also if optimizing efficiency is a
key goal. The additional copper losses can be overcome, but that in itself may require a larger core to
accommodate an increased winding window, compared with core requirements alone.
Because the DCM mode may allow a smaller transformer and provide fast transient response and lower turn-on
losses, it is the usually the best choice for lower power or a Flyback with a high output voltage and low output
current requirement. While a Flyback converter can be designed to operate in both modes, it is important to take
note that when the system shifts from DCM to CCM operation, its transfer function is changed to a two pole system
with low output impedance; thus, additional design rules have to be taken into account including different loop and
slope compensation for the inner current loop. In practice, what this means is designing for CCM and allowing the
converter to work in DCM at light loads.
Also, more advanced transformer techniques make it possible to extend CCM mode and clean light load regulation
and high cross regulation over a wide load range by using a stepped gap transformer. In this type of design, a small
portion of the core gap is very small or bypassed with solid material to provide high initial inductance, and CCM
mode operation at light load. A discussion of the stepped gap transformer technique is outside the scope of this
paper.
Given these characteristics of DCM mode, it is the preferred choice for a simple, easy to design for low power
SMPS. The following is a step-by-step design guide on designing a DCM operation Flyback converter.
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