Dc-dc converter and power supply device having dc-dc converter

A DC-DC converter, comprising:

• a step-down converter that outputs a first output voltage having a first voltage value that is the same as an input voltage when a step-down operation stops and that outputs the first output voltage having a second voltage value that is lower than the input voltage when the step-down operation is performed;
• a resonant converter communicating with the step-down converter, a voltage transfer factor of the resonant converter being adjusted by a switching frequency of a switching element through which a resonance current flows;
• an output voltage detection circuit communicating with one of the step-down converter and the resonant converter, the output voltage detection circuit that detects one of the first output voltage and a second output voltage that is output from the resonant converter as a detected voltage;
• a command voltage generation circuit communicating with one of the step-down converter and the resonant converter, the command voltage generation circuit that outputs a target value for one of the first output voltage and the second output voltage;
• and
• a control circuit communicating with the step-down converter and the resonant converter, the control circuit that has first and second controllers and that controls the step-down converter and the resonant converter based on the target value and the one of the first and second voltages detected by the output voltage detection circuit, wherein
• the first controller adjusts the voltage transfer factor of the resonant converter by changing the switching frequency between a first predetermined frequency and a second predetermined frequency that is larger than the first predetermined frequency,
• the second controller performs the step-down operation and adjusts a step-down ratio of the step-down operation when the target value is equal to or less than a predetermined value, and
• the step-down ratio is determined based on the target value.

A DC-DC converter, comprising:

• a step-down converter in which a time ratio of a switching operation is controlled by pulse width modulation control, the step-down converter outputting a first output voltage;
• a resonant converter communicating with the step-down converter, the switching frequency of the step-down converter being controlled by the pulse width modulation control, the resonant converter outputting a second output voltage;
• an operation variable generator communicating with one of the step-down converter and the resonant converter, the operation variable generator that generates an operation variable based on one of the first and second output voltages and a target value of the one of the first and second output voltages;
• a first command value generator communicating with the operation variable generator, the first command value generator that generates a first command value based on a first calculation value to adjust the time ratio, the first calculation value being obtained by multiplying the operation variable with a first constant;
• and
• a second command value generator communicating with the operation variable generator, the second command value generator that generates a second command value based on a second calculation value to adjust the switching frequency, the second calculation value being obtained by subtracting a second constant from the operation variable to generate a subtracted value, multiplying the subtracted value with a third constant to generate a multiplied value, and adding a fourth constant to the multiplied value, wherein
• the first command value generator restricts the first calculation value to be equal to or less than a first restriction value that corresponds to a maximum time ratio in the pulse width modulation control, the first command value generator generates the first command value based on the restricted first calculation value,
• the second constant is set to be equal to or less than a value obtained by dividing the first restriction value by the first constant,
• the third constant is set to a second restriction value that is obtained based on a maximum switching frequency in the pulse width modulation control, and
• the second command value generator restricts the second calculation value to be equal to or more than the second restriction value, the second command value generator generates the second command value based on the restricted second calculation value.