Operating a flyback converter using a signal indicative of a resonant tank current of the flyback converter

1. A power converter controller comprising:

a control logic circuit configured to control at least two switches in a half bridge configuration on a primary side of a flyback converter, the at least two switches including a high-side switch and a low-side switch, and

an input configured to receive a signal indicative of a resonant tank current of the flyback converter,

wherein the control logic circuit is configured to:

use the resonant tank current indicated by the signal at a time in a phase where there is no reflected current to the primary side of the flyback converter as a measure of an output current of the flyback converter;

calculate the output current based on a peak current of the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current and further based on the peak current of the resonant tank current while the output current of the flyback converter is zero; and

control the low-side switch to keep the on-time of the low-side switch constant,

wherein the phase where there is no reflected current is determined by the controlling of at least one switch of the at least two switches, and

wherein the phase occurs:

when the high-side switch is turned on; or

during a dead time after the low-side switch is turned off and before the high-side switch is turned on.

2. The power converter controller of claim 1 , wherein the control logic circuit is configured to use a peak current of the resonant tank current as indicated by the signal as the measure of the output current.

3. The power converter controller of claim 1 , wherein the control logic circuit is configured to use one of a minimum current of the resonant tank current as indicated by the signal or a current in a middle time between the minimum current and a peak current of the resonant tank current as measure for the output current.

4. The power converter controller of claim 1 , wherein the control logic circuit is configured to control the at least one switch to provide a continuously changing resonant tank current.

5. The power converter controller of claim 1 , wherein the control logic circuit is configured to:

detect an overcurrent condition based on the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current; and

reduce an output power of the flyback converter in response to detecting the overcurrent situation.

6. The power converter controller of claim 1 , wherein the control logic circuit is configured to:

detect a low output power condition of the flyback converter based on the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current, and

change controlling of the at least two switches to a low power mode of operation in response to detecting the low output power condition.

7. The power converter controller of claim 6 , wherein the low power mode comprises a burst mode of operation.

8. The power converter controller of claim 1 , wherein the control logic circuit is further configured to:

determine whether the at least two switches are in the dead time after the low-side switch is turned off and before the high-side switch is turned on;

measure the resonant tank current using the signal in response to determining that the at least two switches are in the dead time after the low-side switch is turned off and before the high-side switch is turned on;

detect an overcurrent situation for the output current using the measured resonant tank current; and

reduce an output power of the flyback converter in response to detecting the overcurrent situation.

9. The power converter controller of claim 1 , wherein the control logic circuit is configured to control the low-side switch to keep the on-time of the low-side switch between 90% of a time of half a resonance period and 120% of the time of half the resonance period.

10. The power converter controller of claim 1 , wherein the control logic circuit is configured to control the high-side switch using peak current control.

11. A flyback converter comprising:

a primary side comprising at least two switches connected in series in a half bridge configuration and a resonant tank coupled to a connection node between the at least two switches, the at least two switches including a high-side switch and a low-side switch,

a secondary side inductively coupled to the primary side,

a current measurement circuit configured to measure a resonant tank current, and

a power converter controller comprising:

a control logic circuit configured to control the at least two switches in the half bridge configuration, and

an input configured to receive a signal indicative of a resonant tank current of the flyback converter,

wherein the control logic circuit is configured to:

use the resonant tank current indicated by the signal at a time in a phase where there is no reflected current to the primary side of the flyback converter as a measure of an output current of the flyback converter;

calculate the output current based on a peak current of the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current and further based on the peak current of the resonant tank current while the output current of the flyback converter is zero; and

control the low-side switch to keep the on-time of the low-side switch constant,

wherein the phase where there is no reflected current is determined by the controlling of at least one switch of the at least two switches,

wherein the phase occurs:

when the high-side switch is turned on; or

during a dead time after the low-side switch is turned off and before the high-side switch is turned on, and

wherein the current measurement circuit is coupled to the input of the power converter controller.

12. The flyback converter of claim 11 , wherein the current measurement circuit comprises a shunt resistor, a capacitive current sensor and/or a magnetic field based current sensor.

13. The flyback converter of claim 11 ,

wherein the resonant tank is coupled to a node between the high-side switch and the low-side switch, and

wherein the current measurement circuit is provided in at least one position of:

between the low-side switch and ground; or

between the resonant tank and ground.

14. A method for operating a flyback converter comprising at least two switches connected in series and a resonant tank coupled to a connection node between the at least two switches, the at least two switches including a high-side switch and a low-side switch, the method comprising:

measuring a resonant tank current of the flyback converter;

using the measured resonant tank current at a time in a phase where there is no reflected current from a secondary side to a primary side of the flyback converter as a measure for the output current of the flyback converter;

calculating the output current based on a peak current of the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current and further based on the peak current of the resonant tank current while the output current of the flyback converter is zero; and

controlling the low-side switch to keep the on-time of the low-side switch constant,

wherein the phase occurs:

when the high-side switch is turned on; or

during a dead time after the low-side switch is turned off and before the high-side switch is turned on.

15. The method of claim 14 , further comprising operating the flyback converter with a continuously changing tank current while measuring the resonant tank current.

16. The method of claim 14 , further comprising:

detecting an overcurrent condition based on the measured resonant tank current as indicated by the signal at the time in the phase where there is no reflected current; and

reducing an output power of the flyback converter in response to detecting the overcurrent situation.

17. The method of claim 14 , further comprising:

detecting a low output power condition based on the resonant tank current as indicated by the signal at the time in the phase where there is no reflected current, and

changing operation of the flyback converter to a low power mode of operation in response to detecting the low output power condition.

18. The method of claim 14 , wherein using the resonant tank current as a measure for the output current comprises using a peak value of the resonant tank current as a measure for the output current.

19. The method of claim 14 , wherein using the resonant tank current as a measure for the output current comprises using one of a minimum of the resonant tank current or a value of the resonant tank current in a middle time between the minimum and a peak value of the resonant tank current as measure for the output current.

20. The method of claim 14 , further comprising:

determining whether the at least two switches are in the dead time after the low-side switch is turned off and before the high-side switch is turned on;

measuring the resonant tank current using the signal in response to determining that the at least two switches are in the dead time after the low-side switch is turned off and before the high-side switch is turned on;

detecting an overcurrent situation for the output current using the measured resonant tank current; and

reducing an output power of the flyback converter in response to detecting the overcurrent situation.