Methods and systems for LED driver having constant output current

A control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and producing a first signal, an analog signal processor coupled to the amplifier circuit for receiving the first signal and a second signal from the input of the power supply and a third signal from an output of the power supply. The analog signal processor is configured to produce a fourth signal as a function of the first, the second, and the third signals. An adder circuit is coupled to the fourth signal and a dimmer control signal, and the adder circuit is configured to output a fifth signal. A comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor that controls current flow in the power supply based on comparison of the fifth signal and a reference signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No. 200910260587.9, filed Dec. 21, 2009, by inventors Hongyue Du, et al., commonly assigned and incorporated in its entirety by reference herein for all purposes.

BACKGROUND OF THE INVENTION

Embodiments of the present invention are directed to power supply control circuits and power supply systems and their applications. More particularly, embodiments of the present invention provide methods for systems for controlling a switched mode power supply for providing constant output current in LED light systems.

A DC-DC converter receives a rectified DC voltage and delivers a regulated DC output. DC-DC converters are widely used in white light-emitting diode (LED) drivers or flash LED drivers. Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency, and larger output power capability. On the other hand, they also have the disadvantages of greater noise, especially Electromagnetic Interference at the power transistor's switching frequency or its harmonics.

Conventional power supplies of buck-boost topology use current control mode (CCM) or voltage control mode (VCM) loop control that needs internal or outside compensation, which can often cause circuit instability. Compared with the ordinary structure of CCM or VCM switch controller, the architecture described inFIG. 1tends to be more stable. Such a controller is extensively used in home-lighting, auto-motor, and backlight instruments. In LED lighting systems, the LEDs are often connected in series in the inductor loop.

FIG. 1is a schematic diagram of an LED lighting system100driven by a conventional switching mode power supply. As shown inFIG. 1, lighting system100includes serially connected multiple LEDs104coupled with a load capacitor111. The LEDs are driven by a power supply120, which includes a sense resistor101, an inductor102, and a Schottky diode103. Power supply120also includes a controller130, which includes a transconductance amplifier105, a Dim linear amplifier106, a current adder107, a resistor108, a comparator109, and a power MOSFET110. As shown, transconductance amplifier105receives inputs from both ends of sense resistor101, and power transistor110is connected to a node between inductor102and Schottky diode103.

As shown inFIG. 1, power supply120receives a rectified DC input voltage Vin. During the charging period, the current from Vin flows through resistor101and inductor102through power transistor110to ground. In this period, energy is stored in inductor102. The voltage across resistor101is sensed by transconductance amplifier105, which produces an output current I1. Current I1is fed to resistor108through current adder107, and the resulting voltage is compared with an internal voltage reference Vref. When an internal turn-on reference voltage is reached, the output of comparator109drives power MOSFET110to switch off through a drive block (not shown). In the discharging period, the energy stored in inductor102discharges through diode103which, along with capacitor111, provide a current Iout to LEDs104. When the sense voltage becomes lower than an internal off reference voltage and detected by comparator109, power transistor110is turned on again, and the charging period is repeated. Controller130is capable of boosting input voltage Vin to a higher regulated output voltage Vout.

Even though conventional LED lighting systems, such as system100ofFIG. 1, can be found in many application, they suffer from many limitations. These limitations include, for example, instability in light output, which may result in flickers.

Therefore, it is desirable to have improved methods and devices for controlling the output current in a power supply in LED lighting and other applications.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide methods and systems for a buck-boost pulse width modulation (PWM) power supply. Merely as an example, some embodiments are described in the context of light-emitting diode (LED) driver applications. But it would be recognized that the invention has a much broader range of applicability.

Conventional LED lighting systems such as system100shown inFIG. 1, LEDs104are driven by current Iout. According to embodiments of the invention, current Iout can vary with input voltage Vin if a conventional controller, such as130, is used to control the power supply. This variation may lead to changes and instabilities in LED light output, including flickers.

Some embodiments of the invention provide a method and circuit for providing a constant current output in a Buck-Boost topology of power supply system. In a specific embodiment, the input current is sensed at a resistor with an OTA (operation Transconductance Amplifier) converting a voltage drop on the sense resistor to a current. A voltage signal derived from the resistor is compared with a reference voltage in a comparator with hysteresis to drive a switching power MOSFET. In this embodiment, an analog signal processor is used to convert the output of the OTA by a ratio of input voltage plus load voltage of LED over the input voltage

(ratio=(Vin+Vload)Vin).
The output current is substantially insensitive to changes in the input supply voltage. In some embodiments, circuit safety features are also provided, such as over-voltage protection, over-current protection, and over-temperature protection, etc. Thus, accurate output current can be realized using embodiments of the invention.

Various embodiments of the invention provide a stable buck-boost power supply structure that can be used in MR16 LEDs lighting and other applications. The stable output can prevent flicker conditions when LEDs are connected as load elements of an electrical transformer.

In one or more embodiments, an analog signal processor is provided that can perform high speed multiplier/divider operations. In some embodiment, the base-emitter junctions of bipolar transistors are configured for performing the multiplication and division operations of currents and voltages.

According to an embodiment of the present invention, a control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and providing a first signal. An analog signal processor is coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from the input of the power supply and a third signal from an output of the power supply. The analog signal processor is configured to produce a fourth signal as a function of the first, the second, and the third signals. An adder circuit is coupled to the fourth signal and a dimmer control signal, and the adder circuit is configured to output a fifth signal. Moreover, a comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison of the fifth signal and a reference signal.

According to another embodiment of the present invention, an LED lighting system includes one or more light emitting diodes (LEDs) connected in series, and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also has a switched mode power supply having a control circuit described above. An output terminal of the power supply is coupled to the one or more LEDs for providing a drive current.

According to yet another embodiment, a control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and providing a first signal. An analog signal processor is coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from an output of the power supply. The analog signal processor is configured to produce a third signal as a function of the first and the second signals. An adder circuit is coupled to the third signal and a fourth signal related to an input of the power supply. The adder circuit is configured to output a fifth signal. Moreover, a comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison of the fifth signal and a reference signal.

According to still another embodiment of the invention, a control circuit for a switched mode power supply includes an analog signal processor coupled to an input terminal and an output terminal of the power supply. The analog signal processor is configured to receive a first signal related to a current at the input terminal, a second signal related to a voltage at the input terminal, and a third signal related to a voltage at the output terminal. The analog signal processor is also configured to provide a fourth signal related to the first, the second, and the third signal. A comparison circuit is configure for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison between a reference signal with the fourth signal or a fifth signal related to the fourth signal.

In an alternative embodiment of the present invention, an LED lighting system includes one or more light emitting diodes (LEDs) connected in series and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also includes a switched mode power supply having a control circuit as described above. An output terminal of the power supply is coupled to the one or more LEDs for providing a drive current.

According to yet another embodiment of the present invention, a switched mode power supply includes an input terminal for receiving a rectified input voltage, an output terminal for providing a regulated output voltage and a regulated output current, a resistor, an inductor, and a diode coupled in series between the input terminal and the output terminal. The power supply also has a first voltage divider coupled to the input terminal and a second voltage divider coupled to the output terminal. The power supply also includes a control circuit that has a power transistor coupled to the inductor and the diode, an amplifier circuit coupled to the resistor for receiving a voltage signal related to a current from an input of the power supply and producing a first signal. The control circuit also has a first signal processing circuit coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from the input terminal of the power supply and a third signal from an output terminal of the power supply. The first signal processing circuit is configured to produce a fourth signal as a function of the first, the second, and the third signals. The control circuit also has a second signal processing circuit coupled to the first signal processing circuit and configured to output a fifth signal related to the fourth signal. The control circuit further has a comparator circuit coupled to the second signal processing circuit for providing a control signal to the power transistor for controlling current flow in the power supply based on comparison of the fifth signal with a reference signal.

In a specific embodiment of the above power supply, the control signal is configured to enable the power supply to provide a constant current output. In some embodiments, an LED lighting system one or more light emitting diodes (LEDs) connected in series and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also has a switched mode power supply as described above, and an output terminal of the power supply being coupled to the one or more LEDs for providing a drive current.

According to another alternative embodiment of the present invention, a signal processing circuit has first, second, third, and fourth bipolar transistors connected in such a way that a sum of the first transistor's base-emitter voltage and the second transistor's base-emitter voltage is equal to a sum of the third transistor's base-emitter voltage and the fourth transistor's base-emitter voltage. The first, second, third, and fourth bipolar transistors are coupled to a first current I1, a second current I2, a third current I3, and a fourth current I4, respectively. The signal processing circuit also has a current mirror for providing an output current that mirrors the fourth current. In a specific embodiment, the first, second, third, and fourth current satisfy the following relationship:

These and other features and advantages of embodiments of the present invention will be more fully understood and appreciated upon consideration of the detailed description of the preferred implementations of the embodiments, in conjunction with the following drawings.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the output current in conventional LED lighting systems, such as system100inFIG. 1can vary with input voltage Vin, This variation may lead to variations in LED light output, including flickers. Therefore, it is desirable to have improved methods and devices for controlling drive current in a power supply in LED lighting and other applications.

As described in detail below, embodiments of the present invention provide methods and devices for power supplies that can be used as constant current drivers for white light LEDs and other applications.

FIG. 2is a simplified schematic diagram illustrating an LED lighting system200driven by a switching mode power supply220according to an embodiment of the present invention. As shown inFIG. 2, lighting system200includes serially connected multiple LEDs204connected with a load capacitor214. The LEDs are driven by a power supply220, which includes a sense resistor201, an inductor202, and a Schottky diode203. Power supply220also includes a controller230, which includes a transconductance amplifier205, a Dim linear amplifier206, a current adder207, a resistor208, a comparator209, and a power MOSFET210. As shown, transconductance amplifier205receives input from both ends of sense resistor201, and power transistor210is connected to a node between inductor202and Schottky diode203.

As described above, lighting system200and power supply220have a number of similar components as do light system100and power supply120, respectively. The functions of these common components are not repeated here. It is noted, however, that controller230has an analog signal processor213, which is coupled between transconductance amplifier205and current adder207. Analog signal processor213is also coupled to input voltage Vin and output voltage Vout. As described below, analog signal processor213is configured to enable the power supply to provide an output current that is substantially independent of Vin.

As shown inFIG. 2, analog signal processor213is configured to receive three inputs: I1, I2, and I3, and to produce an output I4as a function of I1, I2, and I3. I1is the output from transconductance amplifier205, I2is related to Vin through voltage divider R1/R2, and I3is related to Vin and also related to Vout (also referred to as Vload) through voltage divider R3/R4. As described in more detail below, in some embodiments, I4can be expressed as a function of I1, I2, and I3:

In an embodiment, output current Iout of system200can be written as an equation of Vin, Vdim, Vload (or Vout), and efficiency η, as follows:

Iout=K*Vdim*VinRsense*(Vin+Vload)*η(2)
where K is a proportionality constant, Rsense is the resistance of resistor201inFIG. 2, and Vdim is a voltage at a light adjustment pin DIM which is used to linearly adjust the output current through LEDs. Alternatively, the DIM pin can receive an external DC voltage or a Pulse Width Modulation (PWM) dimming signal for dimming control. As shown, Iout is affected by changes in Vin and Vload. In Eq. 2, efficiency η, may be related to the on-resistance of the power switch, parasitic resistance in the inductor or Schotty diode, or deterioration of various components in the power supply.

In embodiments of the present invention, the DIM pin is a multi-function On/Off and brightness control pin. In some embodiments, when the Vdim is within a first voltage range, the DIM pin can be used to adjust the brightness of the lighting device. When the Vdim is within a second voltage range, Vdim is not used for the dimming function, and the DIM pin can be coupled to the input (as shown inFIG. 3) and used in controlling the output current in the power supply. Additionally, the DIM pin can also be used in a soft start function.

In the embodiment shown inFIG. 2, where I2is related to Vin and I3is related to Vin+Vload, controller230is configured such that Iout can be expressed as follows:

Iout=K⁢⁢1*K⁢⁢2*VdimRsense*η(3)
where K1and K2are constants. It can be seen from Eq. (3) that Iout is not a function of Vin or Vload, when the current relationship described in Eq. (1) is implemented. Thus, a constant output current Iout can be obtained.

In an alternative embodiment, as described below in connection withFIG. 3, when I2is a constant internal current related to the voltage at DIM through a voltage divider as shown inFIG. 3, Iout can be expressed as follows:

In equations (3) and (4), η represents the transformation efficiency, I1represents the transconductance amplifier current, I2and I3are related to Vin and Vload (Vout) converter current as shown inFIG. 3. As shown in equations (3) and (4), embodiments of the present invention provides constant output current Iout, which is substantially independent of Vin.

As shown inFIG. 3, another embodiment of the present invention provides an LED driver circuit. As shown, a current adjustment linear amplifier306is coupled between resistors R1and R2at the input, and signal processor313is coupled between resistors R3and R4which couple load capacitor314to ground. Moreover, signal processor313is coupled between transconductance amplifier305and current adder307. That is, current flows from load capacitor314and divider resistor R3and enters signal processor313, and current from transconductance amplifier305also enters signal processor313. As shown inFIG. 3, Vdimadjusts the voltage between two terminals of sampling resistor301. Therefore, V301=K1×Vdim.

The operations of LED driver circuits inFIGS. 2 and 3can be briefly analyzed as follows. In the output of the power supply,
L×I=Vin×D×T
where I is the current through inductor202, and D is the duty cycle for charging and discharging the inductor202. Moreover,
L×I=Vout×T×(1−D),
where I is the inductor current and D is the duty cycle of the charging circuit. Then

Iout=η*Vin*IinVout*D
where η is the efficiency of the driver and

Iin=V⁢⁢201R⁢⁢201.
Substituting in the expression for D, Iout can be expressed as

Iout=η×Vin×V⁢⁢101(Vin+Vout)×R⁢⁢101.
As can be seen, Iout can be kept constant, if

Vsense=Vin×V⁢⁢101(Vin+Vout)
is kept constant. InFIG. 2, signal processor213is configured to provide such a function.

InFIG. 3, the following relationship holds:
V301=K×Vdim=K1×K2×Vin.
Signal processor313is configured such that its output current can be expressed as

Iout∝VinVin+Vout.
Here, the input to signal processor313can be expressed as

Vin+VoutVin.
Signal processor313is configured to receive V301and produce an output that is proportional to

VinVin+Vout,
then the input to comparator309Vsample is also proportional to

Vin+VoutVin.
Thus, by maintaining Vsample at a reference voltage using the comparator circuit, a constant output current can be achieved.

In another embodiment, a diode function block212is coupled in parallel with power MOS transistor210to provide over voltage protection. Although shown as a diode inFIG. 2, diode function block212can include a rectifying device and other support circuitry. A detection circuit213is coupled to diode block212. When detection circuit213detects an over voltage condition at transistor210, diode block can shut down transistor210. Similar features are also included inFIG. 3.

FIGS. 4A-4Care simplified schematic diagrams illustrating an embodiment of analog signal processor213in the power controller ofFIG. 2. In some embodiments, signal processor213includes first, second, third, and fourth bipolar transistors connected in such a way that a sum of the first transistor's base-emitter voltage and the second transistor's base-emitter voltage is equal to a sum of the third transistor's base-emitter voltage and the fourth transistor's base-emitter voltage. The first, second, third, and fourth bipolar transistors are coupled to a first current I1, a second current I2, a third current I3, and a fourth current I4, respectively. A current mirror for providing an output current that mirrors the fourth current. In one or more embodiments, the currents satisfy the following relationship:

I4=I1*I2I3.
Several specific embodiments are described below.

FIG. 4Ais a simplified circuit diagram of an embodiment of the analog signal processor shown inFIG. 2. In this embodiment, NPN transistors402,403,404, and405are interconnected as shown inFIG. 4A. With reference toFIG. 2, I1is the sense current on sense resistor201inFIG. 2, and I4through transistor408is the output current to resistor208inFIG. 2. I4also designates the current flowing through transistor405by way of a current mirror.

As configured inFIG. 4A, the base terminals of transistors402and404are connected, and the emitters of transistors403and405are connected. It follows that:
VBE405=VBE402+VBE403−VBE404(5)
Based on the current-voltage relationship of the base-emitter junction:

IC=Is*e⁢q⁢⁢VBEKVT(6)
the following current relationship is derived: I402*I403=I404*I405. With reference numerals inFIG. 4A, the relationship expressed in Eq. (1) can be obtained.

FIG. 4Bis a simplified circuit diagram illustrating an embodiment of circuit block406inFIG. 4Afor voltage to current conversion. As shown, circuit block406converts the voltage divider1input to current I2inFIG. 4A, and it also converts voltage divider2input to current I3inFIG. 4A. InFIG. 4B, resistors805and806are matched to ensure the relationships described in the above equations holds true.

FIG. 4Cis a simplified circuit diagram illustrating the connection of part of circuit block406inFIG. 4Ato a voltage divider. Voltage divider resistors702and703are used to scale the input voltage at701to meet internal voltage requirement. InFIG. 4C, operational amplifier704, PMOS transistor707, and resistor706form a voltage regulator that maintains the voltage across resistor706to be equal to the input voltage to operational amplifier704. Matching PMOS transistors708and707provide an output current of the regulator. In the embodiment inFIG. 4B, which includes two voltage-to-current converters described inFIG. 4C, resistors805and806are matched to ensure proper current relationship. Mismatch of these resistors can cause errors in converting the voltage signals. Additionally, mismatch of transistors707and708, as well as offset in operational amplifier704, can also lead to signal errors. According to embodiments of the invention, these potential errors can be corrected by using cascode MOS transistors and careful design. Further, another transfer of current can be applied, when a sink current needed. It is also noted that the circuits inFIGS. 4B and 4Ccan be implemented using MOS transistors provided in a CMOS process.

FIG. 5is a simplified schematic diagram illustrating an alternative embodiment of the analog signal processor in the power controller ofFIGS. 2 and 3. In this embodiment, substrate PNP transistors are used, which is compatible with standard CMOS processes. Here, substrate PNP transistor502,503,504,505forms a signal processing circuit, substantially similar to the signal processing circuit inFIG. 4A. Operational amplifier507and PMOSFET508form a current regulator that maintains equal voltages at the two input terminals of507. As a result, the following relationship is established:
VBE504=VBE502+VBE503−VBE505(7)
An output current is provided by PMOSFET509by matching PMOS509with PMOS508. Alternatively, the output circuit can also be configured using PNP transistors.

FIG. 6is a simplified schematic diagram illustrating yet another embodiment of the analog signal processor in the power controller ofFIGS. 2 and 3. As shown, signal processor600includes both NPN and PNP transistors, including NPN transistors602and604and PNP transistors603and605. The operation of signal processor600is substantially similar to that of signal processor400inFIG. 4Aand signal processor500inFIG. 5. The following relationship is established.
VBE604=VBE602+VBE605−VBE603(8)
InFIG. 6, MOSFET or bipolar transistor607is used as a current source and serves to raise the source or emitter voltage and improve the headroom of block606. Block606is receives the divider currents of Vin and Vin plus Vload, similar to the signal processors described above in connection withFIGS. 4A and 4B.