SLUSD12A October 2017 – February 2018 UCC28780
There are two resonance approaches for an active clamp flyback (ACF) converter, primary resonance and secondary resonance, which affect the design guide on the clamp capacitor (CCLAMP). Referring to Figure 36, if CO1 serves as the energy-storage capacitor at the output with larger capacitance and CO2 is a high-frequency decoupling capacitor, leakage inductance of transformer (LK) mainly resonates with CCLAMP during the demagnetization time of the magnetizing inductance (LM). This configuration is called the primary-resonance ACF converter. On the other hand, if CO2 serves as the energy-storage capacitor at the output with larger capacitance and CO1 is much smaller than the equivalent capacitance of CCLAMP reflected to the secondary side (CCLAMP/NPS2), LK mainly resonates with CO1. This configuration is called the secondary-resonance ACF converter.
For primary-resonance ACF, the design tradeoff between conduction loss and turn-off switching loss of QH needs to be considered. Higher CCLAMP results in less RMS current flowing through the transformer windings and switching devices, so the conduction loss can be reduced. However, a higher CCLAMP design results in QH turning-off before the clamp current returns to 0 A. The condition of non zero current switching (ZCS) increases the turn-off switching loss of QH. This is aggravated if the turn-off speed of QH is not fast enough. Therefore, CCLAMP needs to be fine-tuned based on the loss attribution. If the resonance between LK and CCLAMP is designed to be completed by the time QH is turned-off, the clamp current should reach close to 0 A around three quarters of the resonant period. The following equation can be used to design CCLAMP for obtaining ZCS at VBULK(MIN) and full load. This design results in a non-ZCS condition at VBULK(MAX), since the switching frequency at VBULK(MAX) is higher in transition-mode operation. A low-ESR clamp capacitor is recommended to minimize the conduction loss. If a ceramic capacitor is used as the low-ESR capacitor, the DC bias effect on the capacitance reduction also needs to be considered.
For secondary-resonance ACF, CO1 is used to adjust the resonance time with LK to fulfill the ZCS condition, so a large CCLAMP will not compromise ZCS. Besides, during the on-time of low-side switch (QL), the small CO1 is partially discharged by the load current at the same time. After QL turns off and the resonance begins, the discharged CO1 makes the initial resonance voltage lower than the reflected clamp capacitor voltage across CCLAMP, which forces more magnetizing current delivered to output, so the conduction loss is reduced with less RMS current flowing through QH and the primary winding.