SHUNTING TYPE PWM DIMMING CIRCUIT FOR INDIVIDUALLY CONTROLLING BRIGHTNESS OF SERIES CONNECTED LEDS OPERATED AT CONSTANT CURRENT AND METHOD THEREFOR

• US 20070262724A1
• Filed: 05/14/2007
• Published: 11/15/2007
• Est. Priority Date: 05/15/2006
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Supertex Incorporated (Microchip Technology Incorporated)
• Inventors: Mednik, Alexander, Tirumala, Rohit
• Granted Patent: US 7,723,926 B2
• Time in Patent Office: Days
• US Class Current: 315/125
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/3725 Switched mode power supply [SMPS]
• CPC Class Codes: H05B 45/375 using buck topology
• CPC Class Codes: H05B 45/385 using flyback topology
• CPC Class Codes: H05B 45/48 having LEDs organised in strings and incorporating parallel shunting devices

Abstract

A dimming circuit for driving a string of LEDs at constant current has a power converter. A control circuit is coupled to the power converter. A plurality of shunt switches is provided. An individual shut switch is coupled to each LED. Each LED can be shunted individually by the individual shunt switch. The control circuit corrects an internal DC state based on a feedback signal V_O so that the output current of the power converter remains unchanged when at least one LED is shunted.

Claims

1. A dimming circuit for individual controlling brightness of series-connected LEDs driven at constant current comprising:

a first plurality of switching devices, a signal switching device of the first plurality coupled to an individual LED of series connected LEDS to control a brightness of the individual LED by periodically shunting the individual LED;

a plurality of smoothing capacitors, a single smoothing capacitor coupled to each single switching device of the first plurality;

a second plurality of switching devices, a single switching device of the second plurality coupled in series with a single smoothing capacitor for disconnecting the single smoothing capacitor; and

a switching power converter for supplying a constant output current to the series connected LEDs;

wherein individual smoothing capacitors become disconnected when a corresponding LED is shunted.

2. A dimming circuit for driving a string of LEDs at constant current comprising:

a power converter;

a control circuit coupled to the power converter; and

a plurality of shunt switches, an individual shut switch coupled to each LED, wherein each LED can be shunted individually by the individual shunt switch, and wherein the control circuit corrects an internal DC state based on a feedback signal V_O so that the output current of the power converter remains unchanged when at least one LED is shunted.

3. A power supply of claim 2 wherein the power converter comprises:

a buck type converter;

an inductor coupled to the string of LEDS; and

a controlled switch coupled to the inductor;

wherein the control circuit turns the controlled switch off when the inductor current exceeds a reference level, and

wherein the control circuit turns the controlled switch back on after a time period inverse proportional to the voltage at the string of LEDs.

4. A power supply of claim 2 wherein the power converter comprises:

a buck type converter;

an inductor coupled to the string of LEDS; and

a controlled switch coupled to the inductor,

wherein the control circuit turns the controlled switch off when the inductor current exceeds a reference level, and wherein the control circuit turns the controlled switch back on after a time period inverse proportional to the voltage at the string of LEDs.

5. A power supply of claim 2 wherein the power converter comprises:

a buck type power converter;

an inductor coupled to the string of LEDs; and

a controlled switch coupled to the inductor;

wherein the control circuit turns the controlled switch off when the inductor current exceeds a first reference level, and wherein the control circuit turns the controlled switch back on when the inductor current falls below a second reference level.

6. A power supply of claim 2 wherein the power converter comprises:

a buck type power converter;

an inductor coupled to the string of LEDs; and

a controlled switch coupled to the inductor;

wherein the control circuit turns the controlled switch off after a first delay following when a current of the inductor exceeds a reference level, and wherein the control circuit turns the controlled switch back on after a second time delay following when the current of the inductor falls below the same reference level, and wherein both delays are inverse-proportional to a voltage across the inductor.

7. A power supply of claim 2 wherein the power converter is of a flyback type operating in discontinuous conduction mode, and wherein the switching frequency of the power converter is made proportional to the voltage across the string of LEDs.

8. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light. Emitting Diodes (LEDS) operated at constant current comprising:

a first plurality of switching devices, a signal switching device of the first plurality coupled to an individual LED of series connected LEDS to control a brightness of the individual LED by periodically shunting the individual LED;

a power converter for supplying a constant output current to the series connected LEDs; and

control circuitry coupled to the power converter, wherein the control circuit corrects an internal DC state based on a feedback signal V_O so that the output current of the power supply remains unchanged when at least one LEDs is shunted.

9. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 8 further comprising:

a plurality of smoothing capacitors, an individual smoothing capacitor coupled to each single switching device of the first plurality; and

a second plurality of switching devices, a single switching device of the second plurality coupled in series with a single smoothing capacitor for disconnecting the single smoothing capacitor;

wherein individual smoothing capacitors become disconnected when a corresponding LED is shunted.

10. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 8 wherein the switching power converter comprises:

a step-down DC-DC converter of a buck type that receives input voltage V_IN from the input power supply;

a filter inductor coupled to the first plurality of switching devices and the series connected LEDs;

a catch diode coupled to the filter inductor;

a control switch coupled to the filter inductor and the catch diode; and

a control circuit coupled to the control switch for controlling the control switch, wherein the control circuit turns the control switch off when the inductor current exceeds a reference level, and wherein the control circuit turns the controlled switch back on after a time period inverse proportional to a voltage at the string of LEDs.

11. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 10 wherein the control circuit comprises:

a current sensing device coupled to the control switch;

a peak current comparator having a first input coupled to the current sensing device and a second input coupled to a reference current;

a flip-flop circuit having a first input coupled to the output of the peak current comparator and a first output coupled to the control switch; and

a controlled delay circuit coupled to a second input and a second output of the flip-flop circuit.

12. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 8 further comprising:

a plurality of smoothing capacitors, a single smoothing capacitor coupled to each single switching device of the first plurality; and

a second plurality of switching devices, a single switching device of the second plurality coupled in series with a single smoothing capacitor for disconnecting the single smoothing capacitor;

wherein the switching power converter comprises;

a step-down DC-DC converter of a buck type that receives input voltage V_IN from the input power supply;

a filter inductor coupled to the first plurality of switching devices and the series connected LEDs;

a catch diode coupled to the filter inductor;

a control switch coupled to the filter inductor and the catch diode; and

a control circuit coupled to the control switch for controlling the control switch, wherein the control circuit wherein the control circuit turns the control switch off when the inductor current exceeds a reference level, and wherein the control circuit turns the controlled switch back on after a time period inverse proportional to the voltage at the string of LEDs.

13. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 9 wherein the switching power converter comprises:

an input power supply;

a step-down DC-DC converter of a buck type that receives input voltage V_IN from the input power supply;

a filter inductor coupled to the first plurality of switching devices and the series connected LEDs;

a current sensing device coupled to the filter inductor;

a control switch coupled to the filter inductor and current sensing device;

a catch diode coupled to the control switch, filter inductor and current sensing device; and

a current sense comparator having a first input coupled to the current sensing device and a second input coupled to a reference current; and

a controlled delay circuit coupled to the output of the current sense comparator and the control switch;

wherein the controlled switch is turned off when the inductor current exceeds a first reference level, and wherein the control circuit is turned on when the controlled switch back on when the inductor current falls below a second reference level.

14. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 9 wherein the switching power converter comprises:

a buck type power converter;

an inductor coupled to the series connected LEDs; and

a controlled switch coupled to the inductor, wherein the control circuit turns the controlled switch off after a first delay following the moment when the inductor current exceeds a reference level, and wherein the control circuit turns the controlled switch back on after a second time delay following the moment when the inductor current falls below the same reference level, and wherein both delays are inverse-proportional to the voltage across the inductor.

15. A Pulse Width Modulation (PWM) dimming circuit for individually controlling brightness of series connected Light Emitting Diodes (LEDS) operated at constant current in accordance with claim 9 wherein the switching power converter comprises a flyback type power converter operating in discontinuous conduction mode, and wherein the switching frequency of the flyback type power converter is made proportional to a voltage across the string of LEDs.