Optimize Flyback Magnetics to Empower the PD

Mar 23, 2007 2:58 PM
By John Gallagher, Field Applications Engineer, Pulse Engineering, San Diego

Flyback Topology Waveforms

Before starting any flyback-transformer design it is necessary to understand how the previously discussed current waveforms relate to the output current I_OUT,_RMS(which affects heating in the transformer) and I_PRIPK (which affects transformer saturation). Once this is understood, it is easy to derive the required design parameters. Figs. 5 and 6 show a close-up of the primary and secondary waveforms for a discontinuous- and continuous-mode flyback transformer. Note that the dc value of any waveform is simply the average value of the waveform over the fundamental period of the switching cycle (T) and that, by definition, the root-mean-square (rms) current is:

(Eq. 20)

where T₁ and T₂ define the start and stop times of the period, respectively. Evaluating the integral and calculating teh average (dc) value leads to the following equations:

Discontinuous mode:

(Eq. 21)

(Eq. 22)

and continuous mode:

(Eq. 23)

(Eq. 24)

Discontinuous mode:

(Eq. 25)

(Eq. 26)

and continuous mode:

(Eq. 27)

(Eq. 28)

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