System and method for a switched-mode power supply
In accordance with an embodiment, a method of operating a switched-mode power includes turning on an output switch of the switched-mode power converter coupled to a supply output port of the switched-mode power converter, where an output switch current flows to the supply output port through the output switch in a first direction after turning on the output switch. The method further includes turning off the output switch a first period of time after the output switch current changes polarity from the first direction to a second direction opposite the first direction.
1. A method of operating a switched-mode power converter comprising one of a SEPIC and a Zeta converter, the method comprising:
turning on an output switch of the switched-mode power converter coupled to a supply output port of the switched-mode power converter, wherein an output switch current flows to the supply output port through the output switch in a first direction after turning on the output switch; and
establishing a resonant condition by turning off the output switch a first period of time after the output switch current changes polarity from the first direction to a second direction opposite the first direction, wherein the resonant condition decreases a voltage across an input switch of the switched-mode power converter.
2. The method of claim 1 , wherein the output switch current flows through at least one of a body diode and a parallel capacitance of the output switch in the first direction just prior to the turning on of the output switch.
3. The method of claim 1 , further comprising using a timer to track the first period of time.
4. The method of claim 1 , further comprising, after turning off the output switch, turning on input switch of the switched-mode power converter when the voltage across the input switch decreases below a first threshold due to the resonant condition, wherein the input switch is coupled to a supply input port of the switched-mode power converter.
5. The method of claim 4 , further comprising measuring the voltage across the input switch and comparing the measured voltage across the input switch with the first threshold.
6. The method of claim 4 , further comprising turning-off the input switch after turning-on the input switch.
7. The method of claim 6 , further comprising determining an on-time using a power supply controller, wherein turning off the input switch comprises turning off the input switch in response to an expiration of the determined on-time after turning on the input switch.
8. The method of claim 6 , wherein:
turning on the input switch comprises turning on the input switch after an expiration of a first predetermined resonant time after turning off the output switch; and
turning on the output switch comprises turning on the output switch after an expiration of a second predetermined resonant time after turning off the input switch.
9. The method of claim 1 , wherein:
turning on the output switch comprises turning on the output switch at a time when the input switch is off, a first current through a first inductor flows in a first direction, and a current through a second inductor flows in the first direction, wherein the input switch is coupled to a supply input port of the switched-mode power converter, the first inductor coupled in series with a first one of the supply input port and the supply output port, a second inductor coupled in shunt with a second one of the supply input port and the supply output port, and a first capacitor coupled between the first inductor and the second inductor.
10. The method of claim 1 , wherein the switched-mode power converter is a single-ended primary-inductor converter (SEPIC) comprising:
a first circuit loop having a supply input port, a first inductor and the input switch;
a second circuit loop having the input switch, a first capacitor and a second inductor; and
a third circuit loop having the second inductor, the output switch and the supply output port.
11. The method of claim 10 , wherein:
the first capacitor is coupled between a first terminal of the input switch and a first terminal of the second inductor in the second circuit loop; and
the second circuit loop further comprises a second capacitor coupled between a second terminal of the input switch and a second terminal of the second inductor.
12. The method of claim 1 , wherein the switched-mode power converter is a Zeta converter comprising
a first circuit loop having a supply input port, a second inductor and the input switch,
a second circuit loop having the second inductor, a first capacitor and the output switch, and
a third circuit loop having the output switch, a first inductor and the supply output port.
13. The method of claim 12 , wherein:
the first capacitor is coupled between a first terminal of the second inductor and a first terminal of the output switch in the second circuit loop; and
the second circuit loop further comprises a second capacitor coupled between a second terminal of the second inductor and a second terminal of the output switch.
14. The method of claim 1 , further comprising:
coupling a supply input port to a source voltage; and
coupling the supply output port to a load.
15. A power supply comprising:
one of a SEPIC and a Zeta converter comprising an output switch coupled to a supply output port; and
a power supply controller configured to
turn-on the output switch, wherein an output switch current flows to the supply output port through the output switch in a first direction after turning on the output switch, and
establish a resonant condition by turning-off the output switch a first period of time after the output switch current changes polarity from the first direction to a second direction opposite the first direction, wherein the resonant condition decreases a voltage across an input switch of the power supply.
16. The power supply of claim 15 , wherein the output switch current flows through at least one of a body diode and a parallel capacitance of the output switch in the first direction just prior to the power supply controller turning on the output switch.
17. The power supply of claim 15 , wherein:
the power supply controller comprises a state machine; and
the state machine is configured to turn-on the output switch and turn-off the output switch.
18. The power supply of claim 15 , wherein:
the one of the SEPIC and a Zeta converter further comprises the input switch coupled to a supply input port; and
the power supply controller is further configured to, after the output switch is turned-off, turn-on the input switch when a voltage across the input switch decreases below a first threshold due to the resonant condition.
19. The power supply of claim 18 , further comprising a first voltage measurement circuit coupled to the input switch, the first voltage measurement circuit configured to measure the voltage across the input switch and compare the measured voltage across the input switch with the first threshold.
20. The power supply of claim 18 , further comprising a second voltage measurement circuit coupled to the output switch, the second voltage measurement circuit configured to measure the voltage across the output switch and compare the measured voltage across the input switch with a second threshold.
21. The power supply of claim 18 , wherein the power supply controller is further configured to turn-off the input switch after turning-on the input switch.
22. The power supply of claim 21 , wherein:
the power supply controller comprises a control circuit configured to determine an on-time of the power supply; and
the power supply controller is configured to turn off the input switch in response to an expiration of the determined on-time.
23. The power supply of claim 22 , wherein the control circuit configured to determine the on-time of the power supply comprises a PID controller.
24. The power supply of claim 21 , wherein the power supply controller is further configured to:
turn-on the input switch after an expiration of a first predetermined resonant time; and
turn-on the output switch after an expiration of a second predetermined resonant time.
25. The power supply of claim 21 , wherein:
the power supply further comprises a first inductor coupled in series with a first one of the supply input port and the supply output port, a second inductor coupled in shunt with a second one of the supply input port and the supply output port, and a first capacitor coupled between the first inductor and the second inductor; and
the power supply controller is further configured to
turn-on the output switch by turning on the output switch at a time when the input switch is off, a first current flows through the first inductor in the first direction, and a current through the second inductor flows in the first direction.
26. The power supply of claim 18 , wherein:
the power supply controller further comprises a first timer circuit configured to determine a first predetermined resonant time from when the input switch turns off, and determine a second predetermined resonant time from when the output switch turns off; and
the power supply controller is configured to turn-on the input switch after an expiration of a first predetermined resonant time, and turn-on the output switch after an expiration of a second predetermined resonant time.
27. The power supply of claim 15 , wherein:
the power supply controller further comprises
a current measurement circuit coupled to the output switch, the current measurement circuit configured to measure the output switch current, and
a timer circuit configured to determine when a first predetermined period of time expires after the output switch current changes polarity from the first direction to the second direction; and
the power supply controller is further configured to turn off the output switch only after the first predetermined period of time expires.
28. The power supply of claim 15 , wherein the power supply is a single-ended primary-inductor converter (SEPIC) comprising:
a first circuit loop having a supply input port, a first inductor and the input switch
a second circuit loop having the input switch, a first capacitor and a second inductor, and
a third circuit loop having the second inductor, the output switch and the supply output port.
29. The power supply of claim 28 , wherein:
the first capacitor is coupled between a first terminal of the input switch and a first terminal of the second inductor in the second circuit loop; and
the second circuit loop further comprises a second capacitor coupled between a second terminal of the input switch and a second terminal of the second inductor.
30. The power supply of claim 15 , wherein the power supply is a Zeta converter comprising
a first circuit loop having a supply input port, a second inductor and the input switch,
a second circuit loop having the second inductor, a first capacitor and the output switch, and
a third loop having the output switch, a first inductor and the supply output port.
31. The power supply of claim 30 , wherein:
the first capacitor is coupled between a first terminal of the second inductor and a first terminal of the output switch in the second circuit loop; and
the second circuit loop further comprises a second capacitor coupled between a second terminal of the second inductor and a second terminal of the output switch.
32. The power supply of claim 30 , wherein:
the supply input port is coupled to a source voltage; and
the supply output port is coupled to a load.
33. A power supply comprising:
one of a SEPIC and a Zeta power supply converter having an input switch coupled to a supply input port and an output switch coupled to a supply output port; and
a controller having a first output coupled to a control terminal of the input switch and a second output coupled to a control terminal of the output switch, wherein the controller is configured to:
establish a first resonant condition on the input switch by allowing a current through the output switch to reverse polarity for a first period of time, wherein the first resonant condition reduces a voltage across the input switch,
turn-on the input switch when a voltage across the input switch is below a first threshold,
establish a second resonant condition on the output switch after turning off the input switch, wherein the second resonant condition reduces a voltage across the output switch, and
turn-on the output switch when a voltage across the output switch is below a second threshold.
34. The power supply of claim 33 , wherein the first threshold and the second threshold is zero volts.