8.2.2.5 Output Filter Calculation

The bulk output capacitor of active clamp flyback (ACF) converters, C_O1 of the primary-resonance ACF or C_O2 of the secondary-resonance ACF, is often determined by the transient-response requirement from no load to full load transition. For a target output voltage undershoot (ΔV_O) with the load step-up transient of ΔI_O, the minimum bulk output capacitance (C_O(MIN)) can be expressed as

Equation 38.

where t_RESP is the time delay from the moment ΔI_O is applied to the moment when I_FB falls below 1 μA.

The output filter inductor (L_O) is an essential component for the secondary-resonance ACF, not only to filter the large switching voltage ripple across C_O1 but also to decouple the effect of C_O2 on the resonance period. The sum of L_O impedance, ESR of C_O2 (R_Co2), and C_O2 impedance at minimum switching frequency (f_SW(MIN)) must be much higher than C_O1 impedance at the same frequency to force most of switching resonance current to flow through C_O1.

Equation 39.

One benefit of lowering the ESR on C_O1 (R_Co1) is to help to reduce the switching ripple on the output voltage. Another benefit is reducing the conduction loss of C_O1 for the secondary-resonance ACF converter. However, the issue is that the damping between L_O and C_O1 is weakened. Without proper damping, the magnitude of low-frequency resonance ripple between L_O and C_O1 enlarges output ripple, affects the loop stability, and affects the operation of synchronous rectifier (Q_SEC). The secondary-resonance ACF converter is the most vulnerable since C_O1 with low capacitance significantly weakens the damping. To resolve this issue, it is found that a serial damping network formed by L_DAMP and R_DAMP is a very effective way to minimize the impact. However, too strong of a damping design results in noticeable conduction loss increase and full load efficiency drop. Therefore, it is recommended that L_DAMP and R_DAMP should be higher than the theoretical strong damping value as the following equations suggest. Even though the damping network is an additional component, the physical size or the footprint is much smaller than L_O, not only because of the small value but also the wide selection of a small-size chip inductor which winding resistance can be a free R_DAMP. For the 45W secondary-resonance ACF design with primary GaN FETs and a polymer-type C_O2, when a 0.68-µH chip inductor is in parallel with a 1-µH output filter inductor, there is only 0.15% full-load efficiency drop at 90-V AC input, and there is a negligible efficiency difference at 230-V AC input.

Equation 40.

Equation 41.