Switching power supply device and semiconductor device

1. A switching power supply device, comprising:

an energy conversion circuit to which an input voltage is input, the input voltage being a direct current input voltage;

an output rectifying and smoothening circuit that rectifies and smoothens a voltage output from the energy conversion circuit to output an output voltage to a load;

a switching element that is connected to the energy conversion circuit and switches the input voltage;

a switching control circuit that controls a switching operation of the switching element; and

an output state detection circuit that generates an output state signal indicating a state of the load, the state determining electric power output from the energy conversion circuit,

wherein the switching control circuit:

switches two control modes from one to another in accordance with the output state signal, the two control modes including: (i) a frequency changing control mode in which a turn-on timing at which the switching element is turned on is controlled in accordance with a change of the output state signal to change a first switching frequency that is a switching frequency of the switching element; and (ii) a frequency fixing control mode in which a turn-off timing at which the switching element is turned off is controlled in accordance with the change of the output state signal and the switching frequency of the switching element is set to a second switching frequency that is constant regardless of the change of the output state signal; and

performs, in the frequency changing control mode, first modulation control in which the turn-off timing at which the switching element is turned off is modulated by a first modulation signal so that the first switching frequency is modulated;

performs, in the frequency fixing control mode, second modulation control in which the second switching frequency is modulated by a second modulation signal; and

performs at least one of the following:

when switching the frequency changing control mode to the frequency fixing control mode, continuing the first modulation control even after the frequency changing control mode is switched to the frequency fixing control mode, the first modulation control being performed in the frequency changing mode; and

when switching the frequency fixing control mode to the frequency changing control mode, continuing the second modulation control even after the frequency fixing control mode is switched to the frequency changing control mode, the second modulation control being performed in the frequency fixing control mode,

wherein the switching control circuit includes an oscillator for which the first switching frequency and the second switching frequency are set.

2. The switching power supply device according to claim 1 ,

wherein the switching control circuit performs at least one of the following:

when switching the frequency changing control mode to the frequency fixing control mode, gradually decreasing a modulation amplitude in the first modulation control until the frequency changing control mode is switched to the frequency fixing control mode; and

when switching the frequency fixing control mode to the frequency changing control mode, gradually decreasing a modulation amplitude in the second modulation control until the frequency fixing control mode is switched to the frequency changing control mode.

3. The switching power supply device according to claim 1 ,

wherein the switching control circuit:

compares the turn-on timing at which the switching element is turned on and which has been modulated in accordance with the second modulation signal with a turn-on timing set in the frequency changing control mode; and

turns on the switching element at a later turn-on timing of the turn-on timings.

4. The switching power supply device according to claim 1 ,

wherein the switching control circuit:

compares the turn-off timing at which the switching element is turned off and which has been modulated in accordance with the first modulation signal with a turn-off timing set in the frequency fixing control mode; and

turns off the switching element at a later turn-off timing of the turn-off timings.

5. The switching power supply device according to claim 1 ,

wherein the switching control circuit performs at least one of the following:

when switching the frequency changing control mode to the frequency fixing control mode, gradually decreasing a modulation amplitude in the first modulation control after the frequency changing control mode is switched to the frequency fixing control mode; and

when switching the frequency fixing control mode to the frequency changing control mode, gradually decreasing a modulation amplitude in the second modulation control after the frequency fixing control mode is switched to the frequency changing control mode.

6. The switching power supply device according to claim 1 ,

wherein the switching control circuit modulates the first switching frequency by the second modulation signal across an entire segment in which the switching power supply device operates in the frequency changing control mode.

7. The switching power supply device according to claim 6 ,

wherein in the control which is performed in the frequency changing control mode and in which the first switching frequency is modulated by the second modulation signal, a modulation amplitude is set to a larger value for a higher switching frequency of the switching element.

8. The switching power supply device according to claim 1 ,

wherein the switching control circuit controls the turn-off timing at which the switching element is turned off in the frequency changing control mode to be constant irrespective of the output state signal.

9. The switching power supply device according to claim 1 ,

wherein the switching control circuit changes, in accordance with the output state signal, the turn-off timing at which the switching element is turned off in the frequency changing control mode.

10. The switching power supply device according to claim 9 ,

wherein the switching control circuit delays the turn-off timing at which the switching element is turned off in the frequency changing control mode as the first switching frequency becomes higher.

11. A semiconductor device for switching control, the semiconductor device comprising:

a semiconductor substrate; and

the switching control circuit in the switching power supply device according to claim 1 disposed as an integrated circuit on the semiconductor substrate.