Optimize Flyback Magnetics to Empower the PD

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

Continuous, Discontinuous Flyback Modes

Fig. 4 shows three different sets of primary and secondary waveforms. In both continuous and discontinuous modes, the initial primary current starts at zero. In Fig. 4a, during the off-time the secondary current ramps down to zero because all of the energy stored in the magnetic field during the on-time of the transformer has been delivered to the load before the start of the next cycle. This creates a period of time (t_DEAD) in which no current is flowing in the transformer and the circuit is said to be in discontinuous mode. As can be seen in Fig. 4a at the start of the next cycle, the primary current again ramps up from zero.

In continuous mode, the initial primary waveform starts from zero, but during the off time, the secondary current does not deliver all of the stored energy to the load before the start of the next cycle. As a result, energy is left in the transformer’s magnetic field. At all times, current is flowing in the transformer (t_DEAD = 0) and the circuit is said to be continuous. As can be seen in Fig. 4b, the primary current begins its ramp at a nonzero point, defined as:

I_PRIMIN = I_SECMIN 3 (N_SEC / N_PRI), (Eq. 19)

where I_PRIMIN is the minimum primary-winding current (A) and I_SECMIN is the minimum secondary-winding current. It should be apparent from these waveforms that the two main factors, other than the turns ratio, that influence whether a circuit is continuous or discontinuous are the transformer primary inductance (which controls the slope of the current ramp) and the required output current (which controls how much energy is taken from the transformer). As will be seen later in the article, although it is possible to design a transformer that always operates in discontinuous mode, any continuous-mode transformer will eventually go discontinuous as the load current is decreased below a minimum threshold I_OUTMIN.

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