News Column

Researchers Submit Patent Application, "Dimmable LED Driver and Method for Controlling the Same", for Approval

May 28, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Bordin, Luca (Munchen, DE); Chen, Shaoping (Shenzhen, CN); Chen, Yuli (Shenzhen, CN); Nie, Shijun (Shenzhen, CN); Tan, Wei (Shenzhen, CN), filed on May 3, 2012, was made available online on May 15, 2014.

No assignee for this patent application has been made.

News editors obtained the following quote from the background information supplied by the inventors: "The LED lighting system is used more and more in current lighting devices. With the market demands and energy level regulation, dimmable LED drives with a high PF and high efficiency emerge. But the dimmable LED driving apparatus with a high performance on the market have the following problems more or less: a) a lot of control chips and complex external circuits are used to satisfy design requirements of LED driving; b) some dimmable drivers use a single stage PFC control chip, but flicker may appear thereby, and the LED will bear a significant amount of low frequency (100 Hz/120 Hz) ripple current, then, a big output capacitor is needed in order to re-duce the influence of the ripple current, which again increases the volume and cost of the entire driver and occupies a large structure space; c) the traditional BOOST PFC+DC/DC structure applied to the LED driving does not have a high efficiency, because an output therefrom is changed from a very high voltage (an output voltage from boost PFC is usually 400V) to a very low voltage. In addition, both PFC and second DC-DC need high voltage rated components, which increases the cost; d) the traditional averaging dimming will affect the optical effect and causes color temperature shift, and influences the LED luminescence quality; and e) an extensibility is lacked, and increasing new market demands, such as intelligent control and color mixing, can hardly be satisfied.

"At present, there are a lot of dimmable LED driving systems on the market for solving related problems. For instance, the dimmable LED driving chip IW3610 of IWATT solves the problems of dimmer matching and frequent flicker using quite a few parts. This driving chip uses a"

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "According to one aspect of the present invention, a dimmable LED driver is adapted to be operated with a dimmer configured to generate a predetermined conductive angle, wherein the dimmable LED driver comprises a rectifier configured to convert an alternating current output by the dimmer to a direct current, a buck PFC block configured to adjust an output voltage of the direct current so as to obtain a stable output voltage, a second buck DC/DC block configured to realize output of a constant current after the stable output voltage is realized, a dimming block configured to, after realizing output of the constant current, accomplish a dimming function jointly with the second buck DC/DC block, and an MCU configured to control the buck PFC block, the second buck DC/DC block and the dimming block.

"The dimmable LED driver according to an embodiment of the present invention uses a double buck structure, an output voltage is reduced twice, and a higher efficiency is obtained. A current of the LED is controlled by the buck DC/DC block, a working frequency is high (>100 Khz), no low frequency ripple current flows through the LED, and there is no flicker problem due to a significant amount of low frequency ripple; moreover, a capacitor connected in parallel with the LED is quite small, which prominently reduces the cost and the volume of the entire driver. In addition, as the buck PFC block converts the AC voltage to a stable DC voltage with a quite low voltage, for the second buck DC/DC block, there is no need to use a power component with a quite high voltage, capable of reducing the cost and increasing the efficiency. Besides, a PWM dimming manner is used in the present invention, a peak value current flowing through the LED is unchanged, and the optical effect will not affected and the color temperature shift will not be produced. Further, in the dimmable LED driver according to an embodiment of the present invention, only a single control block is used to control all blocks, greatly simplifying the circuits and increasing the flexibility, and intelligence and flexibility of the control block makes the function extension become quite easy.

"Preferably, the MCU adjusts a duty cycle of a PWM PFC signal that is output according to an error between a sampling value of a first sampling voltage of an output voltage of the buck PFC block and a set reference value so as to realize the output voltage (V_buck) that is stable and conforms to the reference value. As the buck PFC block converts the AC voltage to a stable DC voltage with a quite low voltage, for the second buck DC/DC block, there is no need to use a power component with a quite high voltage, capable of reducing the cost and increasing the efficiency.

"Preferably, the MCU, after obtaining the stable output voltage, generates a PWM dimming signal and a PWM buck signal, controls the second buck DC/DC block according to the PWM buck signal to realize output of a constant current, controls simultaneously the dimming block according to the PWM dimming signal, and realizes a dimming function jointly with the second buck DC/DC block. In such a PWM dimming manner, the peak value current flowing through the LED is unchanged, the optical effect will not be affected and the color temperature shift will not be produced.

"According to an embodiment of the present invention, the MCU comprises an ADC, a CPU, a PWM PFC unit, a PWM buck unit, a PWM dimming unit and a comparator unit, wherein the ADC is connected to an input end of the CPU, and output ends of the CPU are connected with input ends of the PWM PFC unit, the PWM buck unit and the PWM dimming unit, while the other input end of the PWM buck unit is connected with an output end of the comparator unit. By controlling all blocks with only a single control block, the circuits are greatly simplified and the flexibility is increased; moreover, intelligence and flexibility of the control block makes the function extension become quite easy.

"According to an embodiment of the present invention, the buck PFC block comprises a first MOSFET, a first MOSFET driver, a first filter inductor, a second diode, a first energy storage capacitor, a third resistor and a fourth resistor, wherein the first MOSFET driver has an input end connected to the PWM PFC unit and an output end connected to a gate of the first MOSFET, a drain electrode of the first MOSFET is connected to a live wire output end of the rectifier through the first diode, and wherein the first diode has an anode connected to the live wire output end of the rectifier and a cathode connected to a drain electrode of the first MOSFET, one end of the first filter inductor and a cathode of the second diode are connected to a source electrode of the first MOSFET, the other end of the first filter inductor is connected with one end of the first energy storage capacitor and one end of the third resistor to be connected with an anode of the LED, wherein the other end of the third resistor is connected in series with the fourth resistor, and a first pin that is connected to the ADC is provided between the third resistor and the fourth resistor, and wherein the anode of the second diode is connected with the other end of the first energy storage capacitor and the other end of the fourth resistor to be grounded together. The MCU controls on and off of the first MOSFET through the first MOSFET driver using the PWM PFC signal so as to chop an input voltage, and the MCU receives a first sampling voltage fed back from the first pin. The first sampling voltage, after divided by the third and fourth resistors, is fed back to the ADC of the MCU. A stable output voltage is obtained through this buck PFC block.

"According to an embodiment of the present invention, the MCU only adjusts the duty cycle of the PWM PFC signal at a time of each zero-crossing of an AC voltage so as to make sure that the duty cycle keeps constant in each half AC cycle. It can be known from the formula

"I Lpk = ( v in - v 0 ) T on L ##EQU00001##

"that, as an output voltage Vo and an inductance quantity L are constant, a peak value current ILpk on the inductor will be approximately proportional to an input voltage Vin as long as the on-time Ton of the MOSFET keeps constant, so to as make the input current follow the input voltage to realize PFC and to obtain a high power factor.

"According to an embodiment of the present invention, the second buck DC/DC block comprises a third diode, a second MOSFET, a second MOSFET driver, a second filter inductor, a fifth resistor and a sixth resistor, wherein the second MOSFET driver has an input end connected to the PWM buck unit through the sixth resistor and an output end connected to a gate of the second MOSFET, the second MOSFET has a drain electrode connected to the anode of the third diode and a cathode connected to an anode of the LED, through the second filter inductor, a source electrode of the second MOSFET is connected with one end of the fifth resistor and an in-phase input end of the comparator unit, respectively, a reversed-phase input end of the comparator unit is connected with a reference voltage, and the other end of the fifth resistor is grounded, and wherein the second buck DC/DC block works in a peak current mode. A constant output current is obtained through this second buck DC/DC block.

"According to an embodiment of the present invention, the MCU controls the PWM buck signal to output a high level and controls the second MOSFET to be turned on, a state of the comparator unit turns over when the second sampling voltage on the fifth resistor reaches the reference voltage, and the PWM buck signal is triggered to output a low level. Thus, a linkage between the comparator unit and the second buck DC/DC block enables the peak value of a current flowing through the LED to be controlled at a predetermined value.

"According to an embodiment of the present invention, the dimming block comprises the first and second resistors, and the fourth diode. The first and second resistors are connected in series between the live wire output end and a zero line output end of the rectifier, the other end of the second resistor is grounded jointly with the zero line output end, a second pin that is connected to the ADC is provided between the first and second resistors, and the fourth diode has a cathode connected to the PWM dimming unit and an anode connected between the sixth resistor and the second MOSFET driver. The AC volt-age is rectified by the rectifier and is guided into the MCU through the second pin, and a conductive angle of the dimmer is calculated by the MCU. The MCU generates one channel of PWM dimming signal through the PWM dimming unit and adjusts a duty cycle of the PWM dimming signal according to the conductive angle. The PWM dimming signal is output to the second MOSFET driver through the fourth diode so as to control on and off of the second MOSFET. When the PWM dimming signal has a high level, the fourth diode is not turned on, the signal does not affect the second MOSFET driver, and the second buck DC/DC block outputs a current normally. When the PWM dimming signal has a low level, the fourth diode is turned on, a level of the second MOSFET driver is drawn low, the second buck DC/DC block stops working, and an output current is zero.

"Preferably, the duty cycle of the PWM dimming signal is calculated from a function D=f (.THETA.). Optionally, the duty cycle of the PWM dimming signal is obtained in a manner of looking for a preset comparison table of conductive angel with duty cycle. When the conductive angle changes, the PWM dimming signal changes correspondingly, and the time when the fourth diode is turned off also changes correspondingly, further causing light and shade of a beam output from the LED changes so as to realize dimming.

"According to another aspect of the present invention, a method for controlling an LED dimmer of the above type includes the following steps: a) initializing a system and activating all function blocks of the LED dimmer; b) controlling a duty cycle of a PWM PFC signal of a buck PFC block through an MCU so as to realize a stable output voltage; and c) controlling a second buck DC/DC block through the MCU so as to realize control to output of a constant current, and simultaneously, controlling a dimming block and the second buck DC/DC block through the MCU so as to realize dimming. With application of the method, the LED is enabled not be affected by the ripple current as much as possible and the flicker phenomenon is eliminated from an output beam thereof, while the LED is dimmed. Moreover, the LED driver is enabled to have a high efficiency and power factor.

"According to an embodiment of the method in the present invention, in step b), a first sampling voltage of the output voltage fed back is analyzed through the MCU. If the sampling value of the first sampling voltage conforms to a set reference value, carry out step c); otherwise, adjust the duty cycle of the PWM PFC signal that is output until a stable output voltage is obtained.

"Further in step c), a second sampling voltage and a reference voltage are compared through the MCU to enable a peak value current flowing through the LED to be controlled at a predetermined value.

"And further, in step c) a voltage, after rectified by a rectifier, is divided and sampled by the MCU to calculate a conductive angle of the dimmer and to send a PWM dimming signal to dim the LED.

BRIEF DESCRIPTION OF THE DRAWINGS

"The drawings constitute a portion of the Description for further understanding of the present invention. These drawings illustrate the embodiments of the present invention and explain the embodiments together with the Description. In the drawings,

"FIG. 1 is a schematic block of a dimmable LED driver according to an embodiment of the present invention;

"FIG. 2 is a circuit diagram of a dimmable LED driver according to an embodiment of the present invention;

"FIG. 3 is a flowchart of a controlling method according to an embodiment of the present invention;

"FIG. 4 is a time sequence diagram of dimming of a dimmable LED driver according to an embodiment of the present invention;

"FIG. 5 is a waveform diagram of a voltage divided by a first and a second resistors; and

"FIG. 6 is an operating waveform diagram of a second buck DC/DC block."

For additional information on this patent application, see: Bordin, Luca; Chen, Shaoping; Chen, Yuli; Nie, Shijun; Tan, Wei. Dimmable LED Driver and Method for Controlling the Same. Filed May 3, 2012 and posted May 15, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4983&p=100&f=G&l=50&d=PG01&S1=20140508.PD.&OS=PD/20140508&RS=PD/20140508

Keywords for this news article include: Patents, Electronics, High Voltage.

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Source: Electronics Newsweekly


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