Power supply for a load control device

• US 8,067,926 B2
• Filed: 12/05/2008
• Issued: 11/29/2011
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: Lutron Technology Company LLC
• Original Assignee: Lutron Electronics Company Incorporated (Lutron Electronics)
• Inventors: Carmen, Daniel F.
• Primary Examiner(s): NGUYEN, VINCENT Q
• Assistant Examiner(s): NGUYEN, VINCENT Q
• Application Number:
  US12/328,906
• Publication Number:
  US 20090160409A1
• Time in Patent Office: 1,089 Days
• Field of Search: 320/166
• US Class Current: 323/282
• CPC Class Codes: H02M 1/0006 Arrangements for supplying an adequate voltage to the control circuit of converters
• CPC Class Codes: H02M 5/257 using semiconductor devices only
• CPC Class Codes: H05B 39/044 continuously H05B39/042 takes precedence
• CPC Class Codes: H05B 47/185 via power line carrier transmission
• CPC Class Codes: Y02B 20/00 Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps

Abstract

A power supply for a load control device generates a DC voltage and provides an asymmetrical output current, while drawing a substantially symmetrical input current. The power supply comprises a controllably conductive switching circuit for controllably charging an energy storage capacitor across which the DC voltage is produced. The energy storage capacitor begins charging at the beginning of a half-cycle and stops charging after a charging time in response to the magnitude of the DC voltage and the amount of time that the energy storage capacitor has been charging during the present half-cycle. The charging time is maintained substantially constant from one half-cycle to the next. The power supply is particularly beneficial for preventing asymmetrical current from flowing in a multiple location load control system having a master load control device supplying power to a plurality of remote load control devices all located on either the line-side or the load-side of the system.

Claims

1. A power supply for a load control device for controlling the amount of power delivered to an electrical load from an AC power source, the load control device operable to receive an AC voltage waveform of the AC power source, the power supply operable to generate a DC voltage, the power supply comprising:

an energy storage element;

a controllably conductive switching circuit coupled in series electrical connection with the energy storage element for selectively charging the energy storage element to produce the DC voltage, the controllably conductive switching circuit switching to a conductive state after the magnitude of the AC voltage waveform exceeds approximately the magnitude of the DC voltage waveform during a half-cycle of the AC voltage waveform;

a latching circuit coupled to the controllably conductive switching circuit, the latching circuit causing the controllably conductive switching circuit to become non-conductive in response to the magnitude of the DC voltage and in response to the amount of time that the energy storage element has been charging during the half-cycle;

a first voltage-responsive current source that conducts a first current having a magnitude representative of the magnitude of the DC voltage; and

a second time-responsive current source that conducts a second current having a magnitude representative of the amount of time that the energy storage element has been charging during the half-cycle, wherein the energy storage device charges for the length of a charging time period during the half-cycle, and the first and second current sources operate to maintain the length of the charging time period substantially the same from one half-cycle to the next.

2. The power supply of claim 1, wherein the length of the charging time period changes by approximately 20% or less from one half-cycle to the next.

3. The power supply of claim 1, wherein the length of the charging time period is approximately 1 msec+/−

0.2 msec.

4. The power supply of claim 1, further comprising:

a current threshold detect circuit operable to sum the first and second currents to produce a total current.

5. The power supply of claim 4, wherein the latching circuit is responsive to the current threshold detect circuit to cause the controllably conductive switching circuit to become non-conductive when the total current exceeds a predetermined current threshold.

6. The power supply of claim 1, wherein the controllably conductive switching circuit comprises a semiconductor switch.

7. The power supply of claim 6, wherein the semiconductor switch comprises a field-effect transistor.

8. The power supply of claim 1, wherein the energy storage element comprises a capacitor.

9. A power supply for a load control device for controlling the amount of power delivered to an electrical load from an AC power source, the load control device operable to receive an AC voltage waveform of the AC power source, the power supply comprising:

an output for providing the DC voltage that conducts an asymmetrical output current;

an energy storage element, the DC voltage generated across the energy storage element;

a controllably conductive switching circuit coupled in series electrical connection with the energy storage element for selectively charging the energy storage element to produce the DC voltage, such that the power supply draws an input current from the AC power source;

a latching circuit coupled to the controllably conductive switching circuit, and;

wherein the controllably conductive switching circuit becomes conductive after the magnitude of the AC voltage waveform exceeds approximately the magnitude of the DC voltage, and the latching circuit causes the controllably conductive switching circuit to become non-conductive, such that the energy storage element stops charging after the energy storage element has been charging for a charging time period; and

wherein the controllably conductive switching circuit is controlled such that the input current is substantially symmetrical and the length of the charging time period is substantially the same from one half-cycle to the next.

10. The power supply of claim 9, wherein the length of the charging time period changes by approximately 20% or less from one half-cycle to the next.

11. The power supply of claim 9, wherein the length of the charging time period is approximately 1 msec+/−

0.2 msec.

12. The power supply of claim 9, wherein the input current has a DC component of less than approximately 0.3 V.

13. The power supply of claim 9, wherein the latching circuit causes the controllably conductive switching circuit to become non-conductive in response to the magnitude of the DC voltage and the amount of time that the energy storage element has been charging during the half-cycle.

14. The power supply of claim 9, wherein the energy storage element comprises a capacitor.

15. A method for generating a DC power supply voltage for use in a load control device for controlling the amount of power delivered to an electrical load from an AC power source, the method comprising the steps of:

beginning to charge an energy storage element for generating the DC power supply voltage during a half-cycle of the AC power source;

generating a first control signal representative of the magnitude of the DC power supply voltage;

generating a second control signal representative of the amount of time that the energy storage element has been charging during the half-cycle; and

causing the energy storage element to stop charging in response to both the first and second control signals.

16. The method of claim 15, wherein the step of generating a first control signal comprises conducting a first current having a magnitude representative of the magnitude of the DC power supply voltage, and the step of generating a second control signal comprises conducting a second current having a magnitude representative of the amount of time that the energy storage element has been charging during the half-cycle.

17. The method of claim 16, further comprising the steps of:

summing the first and second currents to produce a total current; and

determining if the total current exceeds a predetermined current threshold.

18. The method of claim 17, wherein the step of causing the energy storage element to stop charging further comprises causing the energy storage element to stop charging if the total current exceeds the predetermined current threshold.

19. The method of claim 15, wherein the energy storage device charges for the length of a charging time period during the half-cycle, and the length of the charging time period is substantially the same from one half-cycle to the next.

20. The method of claim 19, wherein the length of the charging time period changes by approximately 20% or less from one half-cycle to the next.

21. The method of claim 19, wherein the length of the charging time period is approximately 1 msec+/−

0.2 msec.