Apparatus and method to provide a hybrid linear/switching current source, such as for high-efficiency, wide dimming range light emitting diode (LED) backlighting

• US 7,535,183 B2
• Filed: 04/27/2007
• Issued: 05/19/2009
• Est. Priority Date: 04/27/2007
• Status: Active Grant

• Current Assignee: Korry Electronics Co. (Transdigm Group Incorporated)
• Original Assignee: Korry Electronics Co. (Transdigm Group Incorporated)
• Inventors: Gurr, Walter P. M.
• Primary Examiner(s): Owens; Douglas W.
• Assistant Examiner(s): Le; Tung X
• Application Number:
  US11/741,335
• Publication Number:
  US 20080265793A1
• Time in Patent Office: 753 Days
• Field of Search: 315/209.R, 315224-226, 315246-247, 315/291, 315/299, 315307-308, 315/360, 345/84, 345/102, 345/204, 323282-283, 323/901
• US Class Current: 315/247
• CPC Class Codes: H02M 1/0045 Converters combining the concepts of switch-mode regulation and linear regulation, e.g. linear pre-regulator to switching converter, linear and switching converter in parallel, same converter or same transistor operating either in linear or switching mode
• CPC Class Codes: H02M 3/1584 with a plurality of power processing stages connected in parallel
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/3725 Switched mode power supply [SMPS]
• CPC Class Codes: H05B 45/395 Linear regulators
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

A hybrid power supply provides powers to a load, such as a light emitting diode (LED) backlight display, using pulse wave modulation (PWM) enable signal to control activation of a linear power supply and a switching power supply. During an initial part of an active portion of a duty cycle of the PWM enable signal, the linear power supply is activated to supply the load with constant current. During the remainder of the active portion of the duty cycle of the PWM enable signal, the linear power supply is deactivated and the switching power supply is activated to provide current to the load. A bias current and voltage may be provided to the LED during an inactive portion of the duty cycle of the PWM enable signal. The hybrid power supply combines the high efficiency of the switching power supply with smooth dimming control at low luminance of the linear power supply.

Claims

1. A method to provide power to a load, the method comprising:

generating a PWM enable signal that has a duty cycle that includes a first portion and a second portion;

during an initial part of said first portion, supplying current to said load from a linear power supply to cause said load to generate light;

during a remainder part of said first portion that is subsequent to said initial part of said first portion, deactivating said linear power supply and supplying a current to said load from a switching power supply to cause said load to continue to generate light; and

deactivating said switching power supply during said second portion of said duty cycle.

2. The method of claim 1 wherein supplying said current from said linear power supply and said current from said switched power supply to said load includes supplying said currents to at least one light emitting diode (LED) of a backlight display.

3. The method of claim 1 wherein supplying said current to said load from the linear power supply during the initial part of the first portion includes supplying the current from the linear power supply at a scaled current value less than a commanded current value for said load, and wherein supplying said current to said load from the switching power supply includes supplying said current from the switching power supply nominally at said commanded current value.

4. The method of claim 3, further comprising:

obtaining a first feedback current value from said load;

comparing the obtained first feedback current value with said scaled current value of said current from the linear power supply, and providing said current having said scaled current value from the linear power supply to said load;

obtaining a second feedback current value from said load; and

comparing the obtained second feedback current value with said commanded current value, and deactivating the linear power supply and providing said current from said switching power supply nominally at said commanded current value to said load.

5. The method of claim 1 wherein deactivating said linear power supply and supplying current from said switching power supply to said load is performed after expiration of a delay, wherein a command signal to activate said switching power supply is provided to said switching power supply after expiration of said delay.

6. The method of claim 1, further comprising adjusting said duty cycle, wherein only said linear power supply is used to provide said current to said load during an adjusted duty cycle having an adjusted first portion substantially shorter in duration relative to an adjusted second portion.

7. The method of claim 1 wherein said remainder part of said first portion is longer in duration than said initial part of said first portion.

8. The method of claim 1, further comprising providing a bias current and voltage to said load during the second portion wherein said bias current and voltage does not cause said load to generate light, by clamping said bias voltage below a turn-ON voltage of said load.

9. The method of claim 8 wherein clamping said bias voltage below the turn ON voltage of said load includes using a zener diode to clamp the bias voltage below a turn ON voltage of an LED.

10. An apparatus to provide power to a load, the apparatus comprising:

a pulse wave modulation (PWM) subsystem to generate a PWM enable signal that has a duty cycle that includes a first portion and a second portion;

a first control unit coupled to said PWM subsystem to control a linear power supply to supply current to said load to cause said load to generate light during an initial part of said first portion; and

a second control unit coupled to said PWM subsystem to control a switching power supply to supply a current to said load to cause said load to continue to generate light during a remainder part of said first portion that is subsequent to said initial part of said first portion, said first control unit being adapted to deactivate said linear power supply during said remainder part of said first portion, said second control unit being adapted to deactivate said switching power supply during said second portion of said duty cycle.

11. The apparatus of claim 10 wherein said load includes at least one light emitting diode (LED) of a backlight display.

12. The apparatus of claim 10 wherein said first control unit is a first error amplifier having:

a first input terminal coupled to said PWM subsystem to receive a switching command signal provided in response to expiration of a delay of said PWM enable signal, said switching command signal being nominally at a commanded current value for said load;

a second input terminal coupled to said load to receive a feedback current value from said load; and

an output terminal coupled to a control terminal of said switching power supply to control said switching power supply to generate the current supplied during the remainder part of the first portion and having a nominal amplitude corresponding to the commanded current value, if said first error amplifier determines that said feedback current value is different than said switching command signal nominally at the commanded current value.

13. The apparatus of claim 10 wherein said second control unit is a second error amplifier having:

a first input terminal coupled to said PWM subsystem to receive a linear command signal in response to transition to said first portion of said duty cycle of said PWM enable signal, said linear command signal being at a scaled value less than a commanded current value for said load;

a second input terminal coupled to said load to receive a feedback current value from said load; and

an output terminal coupled to a control terminal of said linear power supply to control said linear power supply to generate the current, having an amplitude less than the commanded current value, if said second error amplifier determines that said feedback current value is different than said linear command signal at the scaled value, and to deactivate said linear power supply if said second error amplifier determines that said feedback current value is greater than said linear command signal at the scaled value.

14. The apparatus of claim 10 wherein said PWM subsystem includes:

an amplitude command unit to generate a linear command signal and a switching command signal;

a PWM command unit to generate said PWM enable signal, the apparatus further comprising;

a first switch coupled between said amplitude command unit and said second control unit, said first switch being actuable in response to said PWM enable signal;

a scale unit coupled to an output terminal of said amplitude command unit to scale down said linear command signal;

a second switch coupled between said scale unit and said first control unit, said second switch being actuable in response to said PWM enable signal; and

a turn on delay unit coupled to an output terminal of said PWM command unit to receive said PWM enable signal and to provide a delay, wherein said scale unit is adapted to provide said scaled down linear command signal to said first control unit in response to actuation of said second switch by the PWM enable signal and wherein said turn on delay unit is adapted to provide said PWM enable signal to said first switch after expiration of said delay so as to actuate said first switch to cause said amplitude command unit to provide said switching command signal, un-scaled, to said second control unit.

15. The apparatus of claim 10, further comprising:

a bias current command unit coupled to said first control unit to provide a bias current and voltage to said load during the second portion; and

a clamping zener diode coupled to said load to prevent said bias voltage from causing said load to generate light during the second portion.

16. A system, comprising:

a backlight light emitting diode (LED) display device;

a linear power supply coupled to said LED display device;

a switching power supply coupled to said LED display device;

a pulse wave modulation (PWM) subsystem to generate a PWM enable signal that has a duty cycle that includes an ON portion and an OFF portion;

a first control unit coupled to said PWM subsystem to control said linear power supply to supply current to said LED display device during an initial part of said ON portion of the duty cycle; and

a second control unit coupled to said PWM subsystem to control said switching power supply to supply a current to said LED display device during a remainder part of said ON portion that is subsequent to said initial part of said ON portion, said first control unit being adapted to deactivate said linear power supply during said remainder part of said ON portion, said second control unit being adapted to deactivate said switching power supply during said OFF portion of said duty cycle.

17. The system of claim 16 wherein the LED display device includes a plurality of LEDs and power supplies that generate different colors, said currents from the linear and switching power supplies being provided at levels to maintain consistency of color of said display device.

18. The system of claim 16 wherein said PWM subsystem includes:

an amplitude command unit to generate a linear command signal and a switching command signal;

a PWM command unit to generate said PWM enable signal, the system further comprising;

a first switch coupled between said amplitude command unit and said second control unit, said first switch being actuable in response to said PWM enable signal;

a scale unit coupled to an output terminal of said amplitude command unit to scale down said linear command signal;

a second switch coupled between said scale unit and said first current unit, said second switch being actuable in response to said PWM enable signal; and

a turn on delay unit coupled to an output terminal of said PWM command unit to receive said PWM enable signal and to provide a delay, wherein said scale unit is adapted to provide said scaled down linear command signal to said first control unit in response to actuation of said second switch by the PWM enable signal and wherein said turn on delay unit is adapted to provide said PWM enable signal to said first switch after expiration of said delay so as to actuate said first switch to cause said amplitude command unit to provide said switching command signal, un-scaled, to said second control unit.

19. The system of claim 16 wherein said first and second control units include current control error amplifiers responsive to a feedback current from said LED display device to determine whether to turn ON or turn OFF said linear and switching power supplies.

20. The system of claim 16, further comprising:

a bias current command unit coupled to said first control unit to provide a bias current and voltage to said load during the OFF portion; and

a clamping zener diode coupled to said load to prevent said bias voltage from causing said load to generate light during the OFF portion.

21. A method to provide power to loads, the method comprising:

producing a drive signal indicative of an amplitude;

scaling down the drive signal to produce a scaled down drive signal;

applying the scaled down drive signal to a linear power supply error circuit at a first time;

producing a linear power supply error signal indicative of a difference between the scaled down drive signal and a current feedback signal indicative of a magnitude of a current being supplied to a load;

applying the linear power supply error signal to drive a linear power supply to supply current to the load;

applying the drive signal to a switching power supply error circuit at a second time, the second time delayed from the first time;

producing a switching power supply error signal indicative of a difference between the drive signal and the current feed back signal indicative of the magnitude of the current being supplied to the load; and

applying the switching power supply error signal to drive a switching power supply to supply current to the load.

22. The method of claim 21, further comprising:

supplying a pulse width modulated enable signal to selectively control application of the scaled drive signal to the linear power supply error circuit at the first time and to selectively control application of the drive signal to the drive signal to a switching power supply error circuit at the second time.

23. The method of claim 22, further comprising:

from time to time, adjusting a duty cycle of the pulse width modulated signal.

24. The method of claim 21 wherein supplying a pulse width modulated signal to selectively control application of the scaled drive signal to the linear power supply error circuit at the first time includes supplying the pulse width modulated signal to a first switch to couple a scale unit to the linear power supply error circuit.

25. The method of claim 24 wherein supplying a pulse width modulated signal to selectively control application of the drive signal to a switching power supply error circuit at the second time includes supplying the pulse width modulated signal to a second switch via a turn on delay unit to couple the drive signal to the switching power supply error circuit.

26. The method of claim 21, further comprising providing a bias current and voltage to said load during a third time when said switching power supply does not supply said current to the load, wherein said bias voltage does not cause said load to generate light, by clamping said bias voltage below a turn-ON voltage of said load.