WO2016197971A1

WO2016197971A1 - Converter-free led driver with low-frequency flicker reduction

Info

Publication number: WO2016197971A1
Application number: PCT/CN2016/085417
Authority: WO
Inventors: Yuan Gao; Kwok Tai Philip Mok; Lisong LI
Original assignee: The Hong Kong University Of Science And Technology
Priority date: 2015-06-12
Filing date: 2016-06-12
Publication date: 2016-12-15

Abstract

A light-emitting diode (LED) driving system includes: a voltage sensor, configured to sense LED voltage; a current sensor, configured to sense LED current, and a controller, configured to regulate a product of the sensed LED voltage and the sensed LED current to reduce power fluctuations on LEDs driven by the LED driving system.

Description

CONVERTER-FREE LED DRIVER WITH LOW-FREQUENCY FLICKER REDUCTION

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional Patent Application No. 62/174,807, filed June 12, 2015, which is incorporated herein by reference.

FIELD

Exemplary embodiments of the disclosure relate to an alternating current (AC) -powered light-emitting diode (LED) driving system, and to circuitry and methodology that reduce the low-frequency flicker of the LED output light.

BACKGROUND

LEDs, including organic LEDs (OLEDs) , offer many advantages over conventional incandescent and compact fluorescent lighting devices for general lighting applications, such as superior power-to-light conversion efficacy, exceptionally longer life span, and freedom from toxic materials. Usually, an AC LED driver is required for converting power from AC mains to LED devices. The performance of an alternating current-direct current (AC-DC) LED driver is measured in several aspects. Besides high power efficiency and a good power factor (PF) , the driver should also be able to provide small low-frequency flicker (typically at the double-line-frequency, 100Hz or 120Hz) , as flicker may cause health problems such as headaches and eye strain. Meanwhile, it is preferable to rule out bulky and expensive power inductors and electrolytic capacitors (E-Cap) in LED drivers for the considerations of volume, cost and life-time.

Current switching converter-based AC LED driver topologies include Buck, Fly-back or any other switching converter based topologies with inductors or transformers. When designing with a power factor correction (PFC) control strategy, the driver is capable of regulating the input current in phase with the input line wave, thereby achieving high PF. However, the driving circuity may suffer from large ripple at double-line-frequency because PFC control delivers non-uniform power to LEDs at different times in each cycle. In order to filter out this low frequency ripple, bulky aluminum E-Caps are usually employed as the energy storage device in parallel with LEDs. Since an LED may operate as a current-driven device, small voltage ripples on LEDs will result in significant output lighting variation. Sometimes these drivers have flicker ranging from 20％to 50％even with the help of a big E-cap. In some implementations, a switching converter is not included in an LED driver circuit designed to power the LEDs. Instead, switches are utilized to select different number of LEDs to be turned ON or OFF. The main drawback of this type of driver is that it may have 100％flicker at the double-line-frequency.

SUMMARY

In an embodiment, the invention provides a light-emitting diode (LED) driving system. The LED driving system includes: a voltage sensor, configured to sense LED voltage； a current sensor, configured to sense LED current； and a controller, configured to regulate a product of the sensed LED voltage and the sensed LED current to reduce power fluctuations on LEDs driven by the LED driving system.

In another embodiment of the invention, a system including a plurality of LEDs and an LED driver is provided. The LED driver includes: a voltage sensor, configured to sense LED voltage； a current sensor, configured to sense LED current； and a controller, configured to regulate a product of the sensed LED voltage and the sensed LED current to reduce power fluctuations on LEDs driven by the LED driving system.

In yet another embodiment of the invention, an LED driver is provided. The LED driver includes: a rectifier, configured to receive an alternating current (AC) electrical signal and convert the AC electrical signal to a DC electrical signal； two or more switches coupled to two or more LEDs in an LED string； and a multiplier. The DC electrical signal is provided to the LED string and provided as a first input to the multiplier. A current signal from the LED string is used as a second input to the multiplier. Furthermore, an output of the multiplier is used to control the two or more switches to reduce power fluctuations on the LED string.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

FIG. 1 is a system diagram illustrating a structure of a highly-integrated flicker-reduced linear AC LED driver according to an embodiment of the disclosure；

FIG. 2 is a schematic diagram of an AC LED driver according to an embodiment of the disclosure；

FIG. 3A is a first state in a circuit operation principle of a flicker cancellation controller according to an embodiment of the disclosure；

FIG. 3B is a second state of the circuit operation principle of the flicker cancellation controller according to an embodiment of the disclosure；

FIG. 3C is a third state of the circuit operation principle of the flicker cancellation controller according to an embodiment of the disclosure；

FIG. 4 illustrates a set of waveform diagrams of the AC LED driver of FIG. 2,

FIG. 5 illustrates a circuit diagram of a multiplier that may be used in some embodiments of the disclosure；

FIG. 6 illustrates a working principle of the example multiplier in FIG. 5.

DETAILED DESCRIPTION

Embodiments of the disclosure provide an AC mains-powered highly-integrated LED driver for general lighting applications. Using a quasi-constant power control scheme, the driver is able to regulate total LED power instead of only the LED current, thereby reducing the light output flicker of LEDs. The driver accomplishes this task without the need for bulky magnetics or electrolytic capacitors. Additionally, the driver maintains high efficiency and a good power factor at the same time.

FIG. 1 is a system diagram illustrating the structure of a highly-integrated flicker-reduced linear AC LED driver according to an embodiment of the disclosure. The driver includes a rectifier 104, which may be a full bridge rectifier, a power factor correction (PFC) circuit 106 which may be a passive valley fill PFC (VF-PFC) circuit, a string of LEDs in series denoted as LED String 108, and a flicker cancellation controller 110. The flicker cancellation controller 110 may be divided into two parts. One part includes a multiplier 112 for processing sensed voltage and current signal, while the other part includes multiple switch and current regulators 114 for regulating the total power of the LED String 108. In some embodiments, the rectifier 104 is a full bridge rectifier which is directly connected to the input voltage provided by AC Line 102 for the conversion from an AC input voltage to a DC voltage. The PFC circuit 106 may be placed after the rectifier to offer enough voltage for driving the LEDs in the LED String 108 and to achieve a good PF. To reduce variation in the collective LED string light output, the flicker cancellation controller 110 may be designed to regulate the total LED power instead of the LED current. Therefore, the flicker cancellation controller 110 senses the voltage applied to the LED string (V_LED) and input current of the LEDs (I_LED) , and utilizes the multiplier 112 to obtain the product of the voltage V_LED and current I_LED, which is approximately proportional to both the total LED power and intensity of light output. In some embodiments, as V_LED changes, a variable number of LEDs are selected to be lit accordingly to ensure efficient power delivery at any time of each cycle. At the same time, the multiple regulators in item 114 will push the LED current (I_LED) in the opposite direction to keep the total LED power almost constant. In other words, a higher input voltage will power up more LEDs on the LED string 108, but decrease the current on each LED, and vice versa. Thus, a small light output variation is achieved even with significant input voltage ripple.

FIG. 2 is system diagram illustrating a detailed view of an exemplary embodiment of an AC LED driver with a similar structure as that in FIG. 1. The string of LEDs 208 can be divided into N segments by N power switches M₁ to M_N. The turn on voltage of each segment of LED string 208 are V_LED1, V_LED2 … and V_LEDN, separately. R₁ and R₂ form a voltage divider for sensing the input voltage V_VF, and R_S is a current sense resistor for collecting the LED current information. N operational amplifiers (OTAs) and N power switches with a shared sense resistor R_S form N current regulators (in an exemplary implementation, N may be, for example, ten current regulators or some other number of current regulators) . A voltage regulator 218 is also included in the flicker cancellation controller 210 for providing supply V_DD for all active circuits. N reference voltages as provided in item 220 are also utilized in the controller. For all these reference voltages, a later one V_REF, N is larger than a previous one V_REF, N-1 by a small offset voltage V_OS which may be about 10 mV. In some implementations, the sum of the turn on voltage of all segments may be larger than the peak input voltage, but this is not a requirement.

The operation principle of the highly-integrated AC LED driver can be explained with the help of the schematic diagrams of FIGS. 2 and 3A-C and the waveform diagram of FIG. 4. The passive VF-PFC 206 charges energy storage capacitors C₁ and C₂ in series when the input line is high in every cycle, and discharges them in parallel to power the LEDs as the AC line falls below V_C1 (V_C2) , as shown in FIG. 4. Benefits to the series charging and parallel discharging include reshaping the input current from the AC line to the system to improve the PF of the system and providing enough voltage to turn on the first branch of LEDs when the input voltage is low. Besides the charge and discharge phases, C₁ and C₂ are in a floating state when the input voltage is larger than V_C1 (V_C2) and smaller than the sum of V_C1 and V_C2. In this period, the power from the AC line will be directly transferred to the LEDs instead of through the passive VF-PFC. Since the capacitors in the passive VF-PFC 206 are not in parallel with the LEDs in the LED string 208, the LED string 208 may afford a larger voltage ripple and a smaller capacitance. In addition, because the LED driver configuration of the current invention can handle large voltage variation on V_VF, the requirement for the capacitors C₁ and C₂ will be further alleviated, making it possible to replace E-Cap with compact ceramic capacitor for a several-watt LED driver.

The larger voltage ripple on the LED string 208 can reduce the total capacitance requirement for C₁ and C₂. In the prior art, this larger voltage ripple would render the driver susceptible to flicker. However, in embodiments of the disclosure, a small flicker may be achieved even when the voltage ripple is large. In some embodiments, when designing the circuit in FIG. 2, the capacitors C₁ and C₂ should be the same value. The values for C₁ and C₂ depend on the voltage ripple on the capacitors. Their capacitance is still much lower than that of an E-Cap. For example, the E-caps used in the conventional switching converter based single stage drivers are in the range of several hundreds of microfarads (μF) . In order to achieve the same output power and flicker, C₁ and C₂ utilized in some embodiments of the disclosure may result in a 90％capacitance reduction compared to conventional single stage solutions.

In FIG. 2, the estimated information of the LED power V_PWR is obtained by multiplying the sensed LED voltage V_{VF_S} and sensed LED current V_{ILED_S}. V_PWR is then fed back to the OTAs for power control. No matter how the input voltage varies, only one of these loops will be enabled at one time. The input voltage V_VF determines which switch (M₁ through M_N) current will flow through. For example, if V_VF is smaller than V_LED1, no current will go through M₁. If V_VF is much larger than the sum of V_LED1 and V_LED2, M₁ will be forced to turn OFF and no current will flow through it and M₂ will turn ON. For the intermediate situation where V_VF is larger than V_LED1 but less than the sum of V_LED1 and V_LED2, the current through M₁ depends on the M₁ loop, that is, the loop formed by OTA ₁ 214, multiplier 212, R_S and M ₁ 216.

In this intermediate situation, since V_REF1 < V_REF2, as V_VF increases V_PWR will reach V_REF1 before V_REF2, and the loop formed by the OTA ₁ 214, multiplier 212, and M ₁ 216 will force V_PWR to clamp to V_REF1. When this happens, since V_REF2 > V_PWR (V_REF1) , the M₂ transistor is fully turned on by the OTA₂. However, if the V_VF is not high enough to turn on both V_LED1 and V_LED2, no current will go through M₂. Therefore, the LED current I_LED is controlled by the M₁ loop.

As V_VF is further increased to a voltage high enough to turn on both V_LED1 and V_LED2, some current will flow through M₂. In this case, the M₂ transistor is still fully turned ON, and the current I_LED2 is not controlled by the M₂ loop (the loop formed by OTA₂, multiplier 212, R_S, and M₂) . The total current in this case, I_LED is still regulated by the M₁ loop since there is no regulation through the M₂ loop. The M₁ loop will push to reduce the I_LED1 to compensate for the increased I_LED2. The aforementioned states may be extended to discuss the dynamics between adjacent segments. So, for example, when M₁ is forced to turn OFF and M2 is ON due to V_VF being much larger than V_LED1+V_LED2. The intermediate steps between turning on a third segment LED segment VLED3 will depend on the dynamics between the M₂ loop, the M3 loop, and V_VF being larger than V_LED1+V_LED2+V_LED3.

FIGS. 3A-C utilize exemplary stages to graphically illustrate the effects of increasing V_VF. In FIG. 3A, when V_VF is high enough to turn on the first segment of LEDs (V_LED1) , but not high enough to turn on other LEDs (V_LED2 to V_LEDN) , there will be no current going through M₂ to M_N. In FIG. 3A, the grayed out section signifies that these circuits are in an inactive state. For example, in the M₂ transistor 316, I_LED2 is zero because V_VF < (V_LED1+V_LED2) . The sizes for transistors M₁ through M_N may be optimized to follow a gradual reduction in size starting from M₁ as the largest and M_N as the smallest. This sizing criteria may achieve high system efficiency as well as an optimized chip area when building the LED driver on-chip. In other cases, M₁ through M_N may be sized the same to achieve a similar performance and system efficiency, but with a less optimal area when implemented on-chip.

When V_VF increases high enough to turn on two segments of LEDs (V_LED1+V_LED2) , as shown in FIG. 3B, current will flow through both M₁ and M₂ at first. However, because there is no regulation for the loop of M₂ as previously explained, sensed current as well as V_PWR will increase very fast, making the current through M₁ smaller. Finally, after the amplification of OTA₁ and OTA₂, I_LED2 will be much larger than I_LED1 and M₁ transistor will be fully turned off. As a result, the controller is dominated by the loop formed by the M₂ transistor and the OTA₂, just as shown in FIG. 3C. In FIG. 3C, owing to the high gain of the OTA₂, V_PWR will be regulated to V_REF2. The operation principle is similar when V_VF further increases to a higher voltage or decreases from a high voltage to a low voltage, as shown in FIG. 4. As the reference voltages only have a small variation (e.g. V_REF1 ＝ 2V, V_REF10 ＝ 2.09V) , the LED power can be regulated as a quasi-constant value in each cycle. Therefore, a very small light flicker may be achieved.

FIG. 4 illustrates some waveform diagrams of the system shown in FIG. 2 in a half of an AC line cycle. V_IN and I_IN are the input voltage and input current from the AC line to the system (See item 202 in FIG. 2) . With the help of the passive VF-PFC circuit, V_IN and I_IN together shows a good power factor. Additionally, the passive VF-PFC circuit also increases the minimum voltage of V_VF to ensure that the first segment of LEDs can always be turned on in every cycle. (V_VF) _MIN>V_LED1.

In some embodiments, when implementating FIG. 2, V_VF is designed to be close to the effective LED voltage. Some consideration may be taken to account for the small voltage difference on the power switch and current sense resistor. For simplifying the design, the V_VF rather than the effective LED voltage is sensed here. Some embodiments may utilize the effective LED voltage instead of sensing V_VF. V_VF, I_LED and P_LED (almost proportional to the product of V_VF and I_LED) together illustrates the key idea of flicker reduction. The controller can automatically adjust LED current based on the variation on V_VF, to keep the product of V_VF and I_LED constant. I_LED1, …N are currents flowing through the switches M₁ to M_N, separately. It shows that the LED current will flow through different paths according to the input voltage V_VF.

FIG. 5 shows an example of a trans-conductance multiplier that may be used in some embodiments of the disclosure, while FIG. 6 shows the working principle of the multiplier. Four matched transistors, M_M1 to M_M4, compose the core part of the trans-conductance multiplier by using the square law relation of the MOSFET. A level shifter is used for ensuring a proper DC working point for these transistors. CMOS current mirrors can be used for subtraction and duplication operations in the current domain, and V_PWR can be obtained with a resistor R_PWR.

Devices in the disclosure may be discrete components or be integrated in one or more chips or packages. The circuit and method for flicker cancellation of the LED driver in this disclosure may apply to any AC input applications, for example, 110 V_AC or 220 V_AC, if suitable devices are chosen.

In an exemplary implementation, an LED driver according to the principles described herein was fabricated using a 0.35-μm 120V HV CMOS process on a chip area of 1.32mm² (on a 1.2mm x 1.1mm chip) , where the off-chip components include a diode bridge and a valley fill circuit having 3 diodes and 2 15μF ceramic capacitors. Under testing conditions of 110 V_AC, 60Hz input, and 3.5W output power, an 82％reduction of flicker relative to a conventional converter-free LED driver was shown.

Embodiments of the disclosure provide a highly-integrated AC powered LED driver with reduced flicker. The driver is capable of achieving high efficiency and good power factor without using any magnetics or electrolytic capacitors. A full bridge rectifier may be used for conversion of the AC input to a DC voltage. A passive valley fill circuit may be placed after the rectifier to offer enough voltage for driving the LEDs and to achieve a good power factor. A flicker cancellation controller with a string of LEDs may be configured to reduce both the variation of total LED power and light flicker by controlling LED power instead of LED current. The flicker cancellation may be achieved by 1) sensing both the input voltage and input current information, 2) estimating the total LED power with the product of the voltage and current information, 3) regulating the total LED power by controlling of number of the LEDs turned on and the current of LEDs in the string.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B” ) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B) , unless otherwise indicated herein or clearly contradicted by context. The terms “comprising, ” “having, ” “including, ” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to, ” ) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as” ) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

A light-emitting diode (LED) driving system, comprising:

a voltage sensor, configured to sense LED voltage；

a current sensor, configured to sense LED current； and

a controller, configured to regulate a product of the sensed LED voltage and the sensed LED current to reduce power fluctuations on LEDs driven by the LED driving system.
The LED driving system according to claim 1, further comprising:

a rectifier, configured to convert an input alternating current (AC) voltage to direct current (DC) voltage； and

a power factor correction (PFC) circuit, configured to provide voltage for driving the LEDs driven by the LED driving system.
The LED driving system according to claim 1, wherein the controller further comprises a multiplier, configured to obtain the product of the sensed LED voltage and the sensed LED current.
The LED driving system according to claim 1, wherein the controller is further configured to:

drive a variable number of LEDs based on changes in the sensed LED voltage； and

push LED current in an opposite direction to keep total LED power substantially constant.
The LED according to claim 4, wherein the controller further comprises a plurality of regulators for pushing LED current in an opposite direction to keep total LED power substantially constant.
The LED driving system according to claim 1, wherein the controller is further configured to drive a variable number of LEDs based on changes in the sensed LED voltage.
The LED driving system according to claim 1, wherein the controller is configured to decrease current corresponding to each LED in response to a higher input voltage powering up the LEDs driven by the LED driving system.
A system, comprising:

a plurality of light-emitting diodes； and

a light-emitting diode (LED) driver, comprising:

a voltage sensor, configured to sense LED voltage；

a current sensor, configured to sense LED current； and

a controller, configured to regulate a product of the sensed LED voltage and the sensed LED current to reduce power fluctuations on LEDs driven by the LED driving system.
The system according to claim 8, wherein the plurality of light-emitting diodes is divided into a plurality of segments by corresponding power switches.
The system according to claim 9, wherein a sum of turn-on voltages corresponding to the plurality of segments is larger than a peak input voltage of the system.
The system according to claim 9, wherein each segment corresponds to a current regulator, each current regulator comprising an operational amplifier and a power switch and utilizing a shared current sense resistor.
The system according to claim 11, wherein the controller further includes a reference voltage source, configured to provide reference voltages for each of the operational amplifiers of the current regulators.
The system according to claim 8, wherein the voltage sensor comprises a voltage divider comprising multiple resistors, and wherein the current sensor comprises a current sense resistor.
The system according to claim 8, wherein the controller further includes a voltage regulator, configured to provide a supply voltage for active circuits.
A light emitting diode (LED) driver, comprising:

a rectifier, configured to receive an alternating current (AC) electrical signal and convert the AC electrical signal to a DC electrical signal；

two or more switches coupled to two or more LEDs in an LED string； and

a multiplier；

wherein the DC electrical signal is provided to the LED string and provided as a first input to the multiplier；

wherein a current signal from the LED string is used as a second input to the multiplier； and

wherein an output of the multiplier is used to control the two or more switches to reduce power fluctuations on the LED string.
The LED driver according to claim 15, wherein the rectifier is a full wave rectifier.
The LED driver according to claim 15, further comprising a power factor correction (PFC) circuit to enhance the DC electrical signal that is provided to the LED string.
The LED driver according to claim 17, wherein the PFC is a passive valley fill PFC circuit comprising a first capacitor and a second capacitor such that when charging, the first capacitor and the second capacitor are charged in a series configuration and discharged in a parallel configuration.
The LED driver according to claim 15, further comprising:

two or more operational transconductance amplifiers (OTAs) , each OTA being configured to turn ON and turn OFF a switch in the two or more switches； and

a voltage reference generator, configured to generate two or more voltage references to serve as first inputs for the two or more OTAs, wherein the output of the multiplier serves as second inputs for the two or more OTAs.