8.2.2.3 Clamp Capacitor Calculation

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 (C_CLAMP). Referring to Figure 36, if C_O1 serves as the energy-storage capacitor at the output with larger capacitance and C_O2 is a high-frequency decoupling capacitor, leakage inductance of transformer (L_K) mainly resonates with C_CLAMP during the demagnetization time of the magnetizing inductance (L_M). This configuration is called the primary-resonance ACF converter. On the other hand, if C_O2 serves as the energy-storage capacitor at the output with larger capacitance and C_O1 is much smaller than the equivalent capacitance of C_CLAMP reflected to the secondary side (C_CLAMP/N_PS²), L_K mainly resonates with C_O1. 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 Q_H needs to be considered. Higher C_CLAMP results in less RMS current flowing through the transformer windings and switching devices, so the conduction loss can be reduced. However, a higher C_CLAMP design results in Q_H 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 Q_H. This is aggravated if the turn-off speed of Q_H is not fast enough. Therefore, C_CLAMP needs to be fine-tuned based on the loss attribution. If the resonance between L_K and C_CLAMP is designed to be completed by the time Q_H 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 C_CLAMP for obtaining ZCS at V_BULK(MIN) and full load. This design results in a non-ZCS condition at V_BULK(MAX), since the switching frequency at V_BULK(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, C_O1 is used to adjust the resonance time with L_K to fulfill the ZCS condition, so a large C_CLAMP will not compromise ZCS. Besides, during the on-time of low-side switch (Q_L), the small C_O1 is partially discharged by the load current at the same time. After Q_L turns off and the resonance begins, the discharged C_O1 makes the initial resonance voltage lower than the reflected clamp capacitor voltage across C_CLAMP, which forces more magnetizing current delivered to output, so the conduction loss is reduced with less RMS current flowing through Q_H and the primary winding.