Switching power supply
View Patent ↗Self-oscillating, current controlled switching power supplies, used for example, as energy storage device charging circuits, and related methods are provided herein. In one implementation, a switching power supply comprises a power transformer having a primary and a secondary, a switching element that switches a voltage source to the primary, and a control circuit that controls the operation of the switching element in response to the measured primary current and secondary current. The switching element disconnects the voltage source from the primary when the primary current reaches a first threshold, causing the secondary current to conduct. The switching element switches back to the primary when the secondary current drops to second threshold. The switching power supply oscillates between charging and discharging the power transformer. In one embodiment, a high value energy storage capacitor is coupled to the secondary and is charged with the secondary current.
1. A switching power supply comprising:
a power transformer having a primary and a secondary; a switching element for coupling a voltage source to the primary;
a current sensor that measures a primary current and a secondary current;
a rectifier coupled to the secondary adapted to conduct current to a load coupled thereto; and
a control circuit coupled to the current sensor and the switching element that controls the operation of the switching element in response to the primary current and the secondary current;
wherein when the primary current in the primary reaches a first threshold, the control circuit causes the switching element to disconnect the voltage source from the primary which stops flow of the primary current in the primary and which causes the secondary current to conduct to the load; and
wherein when the secondary current drops to a second threshold, the control circuit causes the switching element to reconnect the voltage source to the primary which terminates the secondary current flow in the secondary and which causes the primary current to flow in the primary.
2. The supply of claim 1 wherein the voltage source comprises a DC voltage source.
3. The supply of claim 1 wherein the voltage source comprises a rectified AC voltage source.
4. The supply of claim 3 wherein the rectified AC voltage source comprises a full wave rectifier coupled to an AC power line.
5. The supply of claim 1 wherein the switching element comprises a power field effect transistor.
6. The supply of claim 1 wherein the current sensor is coupled to the primary.
7. The supply of claim 1 wherein the current sensor comprises a Hall effect sensor for measuring the primary current and the secondary current.
8. The supply of claim 7 wherein the Hall effect sensor has a turns ratio equal to a turns ratio of the power transformer.
9. The supply of claim 1 wherein the secondary current supplied to the load comprises a substantially constant current.
10. The supply of claim 1 wherein the load comprises an energy storage capacitor, the supply further comprising:
an isolated voltage sensor coupled to the energy storage capacitor for measuring the voltage across the energy storage capacitor, the voltage coupled to the control circuit.
11. The supply of claim 10 , when the voltage reaches a fully charged level, the control circuit preventing the switching element from switching back on when the secondary current reaches the second threshold in order to stop the charging process.
12. The supply of claim 10 wherein the control circuit adjusts the first threshold as the voltage of the energy storage capacitor increases.
13. The supply of claim 10 wherein a full amount of the secondary current directly conducts into the energy storage capacitor when energy storage capacitor is in an initially discharged state.
14. The supply of claim 10 wherein a time duration of each successive cycle of the current flow in the secondary decreases as the energy storage capacitor charges.
15. The supply of claim 10 wherein a ratio of the frequency of the switching element switching between connected and disconnected states with the voltage source when the energy storage capacitor is charged to the frequency during initial charge ranges between about 10:1 and about 20:1.
16. The supply of claim 10 wherein the energy storage capacitor comprises a high energy storage capacitor having a capacitance of at least 100 farads.
17. The supply of claim 1 wherein the control circuit adjusts the first threshold based upon variations in a voltage level of the voltage source.
18. The supply of claim 1 wherein when the secondary current drops to the second threshold, the control circuit immediately causes the switching element to reconnect the voltage source to the primary.
19. A switching power supply comprising:
a power transformer having a primary and a secondary; a switching element for coupling a rectified AC voltage source to the primary;
a current sensor that measures a primary current and a secondary current;
a rectifier coupled to the secondary that conducts current to an energy storage capacitor to be charged by the power supply; and
a control circuit coupled to the current sensor and the switching element that controls the operation of the switching element in response to the primary current reaching a first threshold and the secondary current dropping to a second threshold; and
wherein the control circuit adjusts the first threshold based upon variations of the voltage of voltage source.
20. The supply of claim 19 wherein the rectified AC voltage source comprises a full wave rectifier coupled to an AC power line.
21. The supply of claim 19 wherein the control circuit modulates the first threshold corresponding to varying voltage levels of the rectified AC voltage source.
22. The supply of claim 19 wherein the control circuit sets the first threshold at a minimum value at zero voltage crossings of the rectified AC voltage source.
23. A switching power supply comprising:
a power transformer having a primary and a secondary;
a switching element for coupling a rectified AC voltage to the primary;
a current sensor that measures a primary current and a secondary current; a rectifier coupled to the secondary that conducts current to a load when coupled thereto; and
a control circuit coupled to the current sensor and the switching element that controls the operation of the switching element in response to the primary current and the secondary current.