Transformers for resonant converters need to be designed to have enough magnetizing current to discharge output capacitance of the switches on both the sides. There are two conditions to be met. Make the energy stored in magnetizing and resonant inductors bigger than energy stored in output capacitance of switches on both sides. Also, the magnetizing current needs to be big enough to discharge capacitance of switches within a given dead time.

where

• I_{M_PEAK} is maximum peak current in transformer
• L_M is the magnetizing inductance of the transformer
• L_R is the resonant tank inductance
• V_IN is the input voltage of the CLLLC converter
• C_EQ is the equivalent capacitance of the switching node

where

• T_DT is the duration of the dead time
• C_EQ is the equivalent capacitance of the switching node
• L_M is the magnetizing inductance of the transformer
• f_SW is the switching frequency of the CLLLC converter

The first condition (Equation 4) is the energy requirement and the second condition (Equation 5) is the slew rate requirement.

The peak magnetizing current is a half of the peak-to-peak value of magnetizing current.

The equivalent capacitance is the sum of capacitance on the LV side plus capacitance of the HV side reflected to the LV side:

where

• C_EQ is the equivalent capacitance of the switching node
• C_P is the capacitance of primary side switches
• C'_S is the capacitance of secondary side switches reflected to primary side

The reflected capacitance C'_S can be calculated using transformer ratio:

where

• C_S is the capacitance of secondary side switches
• N_S is the number of turns of the secondary winding
• N_P is the number of turns of the primary winding

The output capacitance of switches limit the maximum magnetizing inductance. Select switches as a trade-off between R_DS(on) and C_OSS.