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 load-step 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 42.

where t_RESP is the response time delay from the moment ΔI_O is applied to the moment when I_FB falls below 10 μA. At ~10 μA, full power is available to prevent further drop of V_O and to recharge C_O. The response delay time consists of the time for the secondary regulator to stop driving the opto-coupler input plus the time for the opto-coupler output transistor to turn off. R_Co is the equivalent series resistance (ESR) of the output capacitor C_O.

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 resonant 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 resonant current to flow through C_O1 only. L_O is chosen with minimal ESR to achieve minimal conduction loss.

Equation 43.

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 resonant 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 much damping 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 availability of small-size chip inductors with high winding resistance can provide "free" R_DAMP. For the 65-W 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 44.

Equation 45.

The equation for R_DAMP assumes that C_O2 >> C_O1. Select a standard component available with parameter values that satisfy both of these two equations. It is usually not necessary to use two separate components.