Resonant converter and driving method thereof

1. A resonant converter, comprising:

a first switch on a primary side and a second switch coupled to the first switch;

a synchronization rectification switch on a secondary side configured to conduct during a conduction period in response to a switching operation of the first switch; and

a switch control circuit configured to determine an operating region of the resonant converter to be below resonance based on a result of a comparison between the conduction period and an on period of the first switch.

2. The resonant converter of claim 1 , wherein the switch control circuit is configured to generate a first clock signal based on a current detection voltage corresponding to a load current and a control voltage corresponding to an output voltage, and generate on-period information about the on period of the first switch by subtracting dead time information of the first and the second switches from maximum on-period information obtained by counting a period in which the first clock signal is enabled.

3. The resonant converter of claim 2 , wherein the switch control circuit is configured to compare conduction period information corresponding to the conduction period with the on-period information and determine the operating region of the resonant converter to be below resonance if a state in which the conduction period information is smaller than the on-period information is maintained during a first debounce time.

4. The resonant converter of claim 2 , wherein the switch control circuit is configured to compare conduction period information corresponding to the conduction period of the synchronization rectification switch with the on-period information and determine the operating region of the resonant converter to not be below resonance if a state in which the conduction period information is greater than or equal to the on-period information is maintained during a second debounce time.

5. The resonant converter of claim 1 , wherein the switch control circuit comprises a below resonance detector configured to generate a below detection signal indicative of below resonance operation based on maximum on-period information corresponding to a period in which a first clock signal for controlling the switching operation of the first switch is enabled, dead time information of the first and the second switches and conduction period information corresponding to the conduction period of the synchronization rectification switch.

6. The resonant converter of claim 5 , wherein:

the below resonance detector comprises:

a digital subtractor configured to generate on-period information about the on period of the first switch by subtracting the dead time information from the maximum on-period information; and

a first comparison unit configured to generate first reference information based on the on-period information and compare the first reference information with the conduction period information to check whether the first reference information is smaller than the conduction period information,

wherein the below resonance detector is configured to enable the below detection signal if output of the first comparison unit maintains a first level during a first debounce time.

7. The resonant converter of claim 6 , wherein:

the below resonance detector further comprises a second comparison unit configured to generate second reference information based on the on-period information and compare the conduction period information with the second reference information to check whether the conduction period information is greater than or equal to the second reference information; and

the below resonance detector is configured to disable the below detection signal if output of the second comparison unit maintains the first level during a second debounce time.

8. The resonant converter of claim 7 , wherein:

the first comparison unit is configured to generate the first reference information based on a value obtained by multiplying the on-period information by a first ratio smaller than 1, and

the second comparison unit is configured to generate the second reference information based on a value obtained by multiplying the on-period information by a second ratio which is smaller than 1 and which is greater than the first ratio.

9. The resonant converter of claim 7 , wherein:

the below resonance detector further comprises:

a first debounce unit configured to count a period in which the output of the first comparison unit maintains the first level and generate a first detection signal when the counted period is maintained during the first debounce time; and

a second debounce configured to count a period in which the output of the second comparison unit maintains the first level and generate a second detection signal when the counted period is maintained during the second debounce time,

wherein the below resonance detector is configured to enable the below detection signal in synchronization with the first detection signal and disable the below detection signal in synchronization with the second detection signal.

10. The resonant converter of claim 9 , wherein the below resonance detector further comprises:

an SR latch comprising a set terminal to which the first detection signal is inputted and a reset terminal to which a signal corresponding to the second detection signal is inputted, wherein the SR latch is configured to enable the below detection signal in response to input to the set terminal and disable the below detection signal in response to input to the reset terminal.

11. The resonant converter of claim 1 , wherein the switch control circuit is configured to increase a current limit level to maintain an output voltage of the resonant converter at least during a predetermined hold-up time when the resonant converter operates below resonance.

12. The resonant converter of claim 11 , wherein the switch control circuit is configured to control the current limit level based on a feedback voltage corresponding to the output voltage.

13. The resonant converter of claim 11 , wherein the switch control circuit is configured to clamp a current detection voltage corresponding to a load current to the current limit level when the current detection voltage is greater than or equal to the current limit level, and control switching operations of the first and the second switches based on a result of a comparison between the clamped current limit level and a control voltage corresponding to the output voltage.

14. The resonant converter of claim 1 , wherein the switch control circuit is configured to detect non-zero voltage switching when the resonant converter operates below resonance.

15. A method of driving a resonant converter comprising first and second switches on a primary side and at least one synchronization rectification switch on a secondary side, the method comprising:

calculating an on period of the first switch;

detecting a conduction period of the synchronization rectification switch;

counting a period in which the conduction period is smaller than the on period; and

determining an operating region of the resonant converter to be below resonance if a result of the count reaches a first debounce time.

16. The method of claim 15 , wherein calculating the on period of the first switch comprises:

generating a first clock signal based on a current detection voltage corresponding to a load current and a control voltage corresponding to an output voltage of the resonant converter;

counting a period in which the first clock signal is enabled; and

subtracting a dead time of the first and the second switches from the counted enable period.

17. The method of claim 15 , wherein counting the period in which the conduction period is smaller than the on period comprises comparing a period corresponding to a first ratio of the on period with the conduction period.

18. The method of claim 15 , further comprising:

counting a period in which the conduction period is greater than or equal to the on period, and

determining the operating region of the resonant converter to not be below resonance if the period in which the conduction period is greater than or equal to the on period reaches a second debounce time.

19. The method of claim 18 , wherein counting the period in which the conduction period is the on period or more comprises comparing a period corresponding to a second ratio of the on period with the conduction period.

20. The method of claim 15 , further comprising:

increasing a current limit level to maintain an output voltage of the resonant converter during at least a predetermined hold-up time when the resonant converter operates in the below resonance.