LED-based lighting power supplies with power factor correction and dimming control
View Patent ↗A power supply for powering one or more loads includes a boost circuit with power factor correction (PFC) that provides an operating voltage from an electrical power source, and a dimmer detection circuit that determines a dimming level applied to the electrical power source, and generates a pulse width modulated (PWM) signal based upon the dimming level. The power supply also includes one or more current control circuits, each current control circuit being associated with each of the one or more loads, and coupled in series with the operating voltage, its associated load, and a ground of the power supply, so as to control a current through its associated load in response to the PWM signal.
1. A power supply for powering a load, comprising:
a boost circuit with power factor correction (PFC) that derives an operating voltage from an electrical power source;
a current control circuit that controls a current supplied by the operating voltage through the load; and
a voltage control circuit that generates a feedback voltage supplied to the boost circuit to control the operating voltage, wherein the feedback voltage is substantially equal to the sum of (a) a voltage required across the load to drive the current through the load, (b) half of a maximum peak-to-peak voltage of a ripple on the operating voltage, and (c) a minimum voltage drop across the current control circuit;
wherein the current control circuit operates to control the current through the load, with minimal heat loss from the current control circuit and without ripple on the current.
2. The power supply of claim 1 , the current control circuit comprising:
a sense resistor, connected in series with the load, that generates a first voltage representative of the current through the load;
a current sink transistor, connected in series with the load and the sense resistor, that controls the current through the load such that the first voltage is substantially equal to a second voltage representative of a desired current through the load.
3. The power supply of claim 2 , the second voltage being predetermined and fixed.
4. The power supply of claim 2 , the minimum voltage drop across the current control circuit being the voltage drop across the sense resistor plus a voltage drop across the current sink transistor.
5. The power supply of claim 2 , the current control circuit further comprising a current control input responsive to a current control signal such that the current control circuit controls the current through the load based upon the current control signal.
6. The power supply of claim 5 ,
the current control signal being a pulse width modulated (PWM) control signal that modifies the second voltage between substantially zero and a predefined maximum voltage,
the current control circuit modifying the current through the load based upon the second voltage, and
the voltage control circuit varying the feedback voltage based upon a sensed voltage drop across the load, such that the boost circuit modifies the operating voltage based upon the feedback voltage.
7. The power supply of claim 5 , the current through the load being substantially continuous.
8. The power supply of claim 1 , the current control circuit allowing the boost circuit to attain the operating voltage upon start-up of the power supply with the load connected.
9. The power supply of claim 1 , the operating voltage being substantially 2.5V greater than a voltage drop across the load.
10. The power supply of claim 1 , the load comprising one or more light emitting diodes.
11. The power supply of claim 1 , the power supply substantially attaining the current through the load within 100 mS of application of power from the electrical power source.
12. The power supply of claim 1 , wherein size of one or more capacitors of the boost circuit is reduced based upon the maximum peak-to-peak voltage of the ripple on the operating voltage in comparison to the size of capacitors used in boost circuits with less ripple.
13. A method for driving a load using a boost circuit with power factor correction (PFC), a current control circuit, and a voltage feedback circuit, comprising:
receiving electrical power at the boost circuit;
generating an operating voltage, from the electrical power and based upon a voltage feedback signal, within the boost circuit;
controlling a current through the load using a current sink of the current control circuit that is connected in series with the load, the current sink being controlled based upon a difference between (a) a first voltage across a sense resistor of the current control circuit connected in series with the load, and (b) a reference voltage representative of a desired current through the load; and
generating the voltage feedback signal based upon a second voltage sensed at the current sink such that the operating voltage is substantially equal to the sum of (a) a voltage drop produced across the load by the current through the load, (b) half of a maximum peak-to-peak voltage of a ripple on the operating voltage, and (c) a minimum voltage drop across the current control circuit;
wherein current through the load is substantially continuous.
14. The method of claim 13 , wherein the current through the load is controlled without effect from ripple on the operating voltage.
15. The method of claim 14 , wherein allowing the ripple on the operating voltage allows capacitor size within the boost circuit to be reduced, as compared to a capacitor size required for an equivalent boost circuit having substantially no ripple.
16. The method of claim 13 , further comprising:
controlling the reference voltage based upon an input current control signal indicative of a desired current through the load, and
repeating the steps of receiving, controlling, and generating such that the current through the load is substantially equal to the desired current through the load and the power loss is minimized.