The Flyback topology is a popular topology in off-line charger applications due to its capability to handle wide input voltage range. And multiple outputs are easily generated by using extra windings on the main transformer. Figure.1 illustrates the Flyback converter with multiple outputs.

Fig.1 Flyback converter with multiple outputs

However, with higher output power requirements, the voltage ripple of the Flyback on its output capacitors increases. Traditional way to solve the problem is that more multiple capacitors in parallel or single capacitor with big capacitance is necessary to meet the design specification, which adds extra cost and increases the size of the adapter.

Alternatively, adding one more output stage of LC filtering circuit can significantly reduce the output ripple. The figure 2 illustrates the Flyback using 2^{nd} stage LC filters.

Fig. 2 the Flyback using 2^{nd} stage LC filters

Here is an application circuit as shown in Figure 3. We have a Flyback converter running at 5V with 8A load. The input is 200V. The output has two stages. The first one is the traditional filtering caps stage with 5x1500uF capacitance. And the 2^{nd} stage is the additional LC filter stage. For comparison, suppose L2=0uH and C2=1500uF – the output ripple is shown in Figure 4. In another instance, suppose L2=1.35uH (with C2=1500uF) – the output ripple waveforms is shown in Figure 5. With a 2^{nd} stage LC filter, we can see that the output voltage ripple of the Flyback converter can be greatly reduced. Adding the L with the C reduces the need to parallel so many capacitors to achieve lower output voltage ripple.

Fig. 3, Application Circuit with 2^{nd} Stage LC filter

Fig. 4, Output Ripple without 2nd stage LC filter.

Output voltage ripples (VPK_PK=180mV) and load current (IOUT=8A)

(VIN=200V, CO=5×1500µF, RC=2.6mΩ, L2=0µH, C2=1500µF, RC2=13mΩ, Vo=5V@8A

Fig. 5, Output Ripple without 2nd stage LC filter.

Output voltage ripples (VPK_PK=29mV) and load current (IOUT=8A)

(VIN=200V, CO=5×1500µF, RC=2.6mΩ, L2=1.35µH, C2=1500µF, RC2=13mΩ, Vo=5V@8A

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