WO2015038026A1

WO2015038026A1 - Led light

Info

Publication number: WO2015038026A1
Application number: PCT/RU2013/000799
Authority: WO
Inventors: Valery Nikolayevich MOLODTSOV; Ilya Olegovich GRIGORIEV
Original assignee: Zakrytoye Aktsionernoye Obshchestvo "Kb "Sveta-Led"
Priority date: 2013-09-16
Filing date: 2013-09-16
Publication date: 2015-03-19

Abstract

The invention relates to a LED light. The led light of the invention comprises a set of light-emitting diodes and a driver circuit, the driver circuit comprising a switching mode power converter with power factor correction, wherein the switching mode power converter has a power input from an AC electrical power source, a voltage feedback input, and an output connected to the set of light-emitting diodes. The driver circuit further comprises a current monitor circuit adapted to convert a current through the set of light-emitting diodes into a voltage feedback signal supplied to the voltage feedback input of the switching mode power converter. According to the invention, the switching mode power converter is a boost converter, comprising a controller having a power supply input, and the controller power supply input is connected to a tap in the set of light-emitting diodes. The invention can be used in a light bulb, led strip, led bar, led rope, etc.

Description

LED LIGHT

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to light-emitting diode (LED) lights, and more particularly, to LED lights formed by a set of LEDs and a driver circuit having an improved power factor and efficiency.

Brief Description of the Prior Art

Although white LEDs are more effective light sources than incandescent bulbs, their application is more complex. LED based bulbs must have high efficiency when powered by AC mains, both in terms of low internal power loss and high power factor for low loss in mains wiring. Also they should provide a stable light intensity over ambient temperature and AC voltage variations.

To reach high efficiency a switching mode power supply (SMPS) is used in a typical LED driver circuit. High power factor requirement can be met if a power factor correction (PFC) functionality is implemented in SMPS controller. Typical SMPS with PFC consumes a near sinusoidal current from AC mains, thus keeping its power factor high and produces a direct current (DC) output voltage with high efficiency.

Since LEDs have near exponential current versus voltage function i(v), they have a near constant voltage drop across them for their supply current varying within decade. However, LED voltage drop for a fixed current varies with its temperature. This means that feeding a LED by a low-impedance source requires tight control of its output voltage according to required LED current and its temperature. Even if such control is implemented, small ripple in LED supply voltage will cause much higher percentage of ripples in its supply current. Thus, feeding LED by a voltage source is impractical, and LEDs are preferably fed by power supplies close to ideal current source, that is with high output impedance.

A block diagram of a typical multi-stage LED light comprising a set of light-emitting diodes and a driver circuit is shown on Fig. 1. A driver circuit for such led light consists of four cascaded stages^

-AC rectifier;

-power factor corrector;

-DC/DC converter of PFC output voltage into LED module supply voltage; -voltage to current converter.

At each of these stages there are energy losses that reduce the total efficiency of the driver circuit.

Output impedance can be increased by adding a series resistor to voltage source, but this means significant power loss in the resistor, and is not acceptable for LED lighting. A more effective solution is described in US 8,390,214 (Van Laanen et al.). Here, a driver circuit for powering a set of light-emitting diodes uses boost PFC as voltage source but adds an analog constant current sink in series with LED. This eliminates ripple in LED current and makes it stable but adds some unavoidable power loss in the analog current sink. To keep the loss as small as possible, voltage drop across the sink is kept at the lowest allowed level. Further, in Van Laanen et al. the driver circuit is higher in cost due to additional components, that leads to higher cost of the whole led light.

Keeping a power factor close to unity for AC mains is another goal for a LED driver. Classical way of power factor correction is a multi-stage converter where the first stage is a boost switching mode converter tracking rectified AC voltage by its input current and having a large value electrolytic capacitor on its output. The second stage is also switching mode converter taking DC voltage from the first stage and providing required output DC voltage level. This two-stage solution has high cost for LED lighting application. The power loss is higher than can be achieved in a single SMPS stage.

Another possibility to implement PFC is modulating LED current to make it proportional to the rectified AC voltage at SMPS input. This greatly simplifies the LED driver. For example in US 8,188,671 and US 8,461,767 (Canter et al.) LED current is sensed to control the same, but the HV9910 LED driver reference voltage is varied by means of its linear dimming input according to rectified AC waveform. This has the effect that LED current follows the same rectified AC waveform. LED current is not constant in this application, and this solution limits LED current by its peak value, thus its average value is less than that which can be achieved for clean DC supply. The controller circuit is powered by the full rectified AC voltage, which leads to additional power loss. Since the LED current ripples at twice mains frequency, LED blinking can limit application of such driver circuit if a stroboscopic effect can be observed on moving or rotating parts illuminated by LED.

The SMPS controller needs its own power supply for proper operation. During power-on time, when SMPS output is zero, the controller is powered from rectified AC voltage through a resistor. During normal operation when the controller consumes higher current it is usually powered through a separate winding on SMPS transformer. For instance in the Datasheet of Transition- mode PFC controller L6562A, STMicroelectronics, 2007, standard L6562A application uses current sense coil also for powering the chip by using one diode and one Zener diode, one resistor and one capacitor in pulse rectifying circuit. This leads to some power loss in Zener diode since it needs its minimum operating current for the whole range of AC input voltage. Also the additional winding and circuit components increase the LED driver cost.

Another way of increasing SMPS output impedance is arranging its overall feedback loop on load current rather than on its output voltage. In US 6,091,614 (Malenfant) a LED light is disclosed comprising a set of light-emitting diodes and a driver circuit. The driver circuit comprises a switching mode power converter with power factor correction, the switching mode power converter having a power input from an AC electrical power source, a voltage feedback input, and an output connected to the set of light-emitting diodes. The driver circuit further comprises a current monitor circuit adapted to convert a Current through the. set of light-emitting diodes into a voltage feedback signal supplied to the voltage feedback input of the switching mode power converter. In US 6,091,614 (Malenfant) the feedback explicitly monitors LED current and converts it into feedback voltage for transformer coupled flyback converter. It uses RC low-pass filter for smoothing the feedback voltage. Due to the transformer coupled flyback converter SMPS used in this example, it has three windings in its pulse transformer. This solution solves a main problem of keeping the driver circuit fully functional during mains brownout, which is a key requirement for railway signaling system it is used in. Its component count and cost are of lower priority for this application and therefore are not optimized out.

OBJECTS OF THE INVENTION

An objective of the present invention is therefore to provide a LED light comprising a set of light-emitting diodes with a low cost LED driver circuit comprising a switching mode power converter with power factor correction, operating in current source mode with high output impedance, high power factor and high efficiency.

SUMMARY OF THE INVENTION

The objective of the invention can be attained by providing a LED light comprising a set of light-emitting diodes and a driver circuit, the driver circuit comprising a switching mode power converter with power factor correction, the switching mode power converter having a power input from an AC electrical power source, a voltage feedback input; and an output connected to the set of light-emitting diodes, the driver circuit further comprising a current monitor circuit adapted to convert a current through the set of light-emitting diodes into a voltage feedback signal supplied to the voltage feedback input of the switching mode power converter; wherein the switching mode power converter is a boost converter, comprising a controller having a power supply input, and the controller power supply input is connected to a tap in the set of light-emitting diodes.

Supplying the voltage feedback input of the switching mode power converter with its load current converted to voltage by means of the current monitor circuit turns the power converter into a constant current source mode with high output impedance. The switching mode power converter with power factor correction of boost type provides high power factor and high efficiency at low cost due to its single-winding inductor, while feeding its controller from the tap of the set of light-emitting diodes eliminates additional power loss and reduces the component count and the total cost of the LED light.

The LED light can be a light bulb. However, the invention can also be used in other led lights, e.g. led strips, bars and ropes.

The light-emitting diodes connected between the output of the power converter and the tap can have a bigger active area of p-n junction to provide the same brightness at higher current.

The current monitor circuit can be a Hall Effect current sensor.

Each light-emitting diode in the set of light-emitting diodes can be shunted by a protective component, e.g. a Zener diode.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example only and with reference to accompanying drawings, in which:

Fig. 1 shows a general block diagram of one prior art set of LEDs and a driver having a power factor correction;

Fig. 2 shows a block diagram of one embodiment of a set of LEDs with driver according to the present invention; and

Fig. 3 shows a detailed diagram of the embodiment of Fig. 2.

It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the preferred embodiments. The figures do not illustrate every aspect of the described embodiments and do not limit the scope of the present disclosure. DETAILED DESCRIPTION

FIG. 2 is a block diagram illustrating one exemplary LED light comprising a set of light-emitting diodes (also referred here as LED set) and a driver circuit. The driver circuit comprises an AC rectifier 1, a switching mode power converter 2 of boost type with power factor correction (PFC), a set 5 of LEDs and a LED current monitor implemented as a current sense resistor 6. Rectifier 1 receives mains AC voltage onto its input terminals 20, 21 and converts it to rectified AC voltage on its positive output terminal 7 and negative output terminal 8. Positive output terminal 7 of the AC rectifier 1 is connected to the input terminal 9 of the switching mode power converter 2, negative output terminal of the AC rectifier 1 is connected to the ground terminal 10 of the switching mode power converter 2 and to the first terminal 19 of the current sense resistor 6. A positive terminal 14 of the LED set 5 is connected to the output terminal 11 of the switching mode power converter 2, the tap terminal 15 of the LED set 5 is connected to the controller supply input 12 of the switching mode power converter 2, the negative terminal 16 of the LED set 5 is connected to the second terminal 18 of the LED set current sense resistor 6 and to the voltage feedback input 13 of the switching mode power converter 2. The switching mode power converter 2 maintains a constant voltage on its voltage feedback input 13 by varying voltage on its output 11, thus providing a stable current through the LED set 5. The total voltage drop on the current sense resistor 6 and a part of LED set 5 connected between its negative terminal 16 and positive terminal 15 is used as supply voltage for the controller supply input 12 of the switching mode power converter 2. This voltage has little variation over temperature and the tap position in the LED set can be calculated from known voltage drop on each LED of the LED set and the value of reference voltage of switching mode power converter 2 maintained on its voltage feedback input 13.

The LED set 5 in this embodiment is a series connection of single LEDs 17. Another embodiment is LED modules connected in series, each module comprising a series connection of individual LED chips. Since the boost type switching mode converter is used, the total voltage drop on the LED set 5 must exceed the peak value of mains voltage. This can be achieved by using multichip LED modules as described in RU 2,465,688 (Grigorev et al.), Light Diode Lamp.

FIG. 3 is a diagram illustrating the embodiment of Fig.2 in more detail, namely the implementation of its switching mode power converter 2. The switching mode power converter 2 receives the rectified AC voltage on its input terminal 9 and converts it into DC voltage connected to the positive terminal 14 of the LED set 5. The switching mode power converter comprises an input filter capacitor 29, an inductor 35, a switch 36, a diode 38, an output filter capacitor 39 and an SMPS controller 22. The voltage drop on the LED set current sense resistor 6 is applied to the voltage feedback input pin 41 of the SMPS controller 22 through resistor 40 and is compared inside the controller against a predefined value of its internal reference voltage. Capacitors 33, 34 and resistor 32 with resistor 40 adjust a frequency response of SMPS controller error amplifier to provide control loop stability. Resistors 23, 24 and capacitor 25 form a scaled rectified AC voltage waveform for SMPS controller 22. Resistor 30 and capacitor 31 pass a negative going edge on inductor 35 to the SMPS controller which is used for keeping it in transition mode. Switch 36 is connected to ground through its own current sense resistor 37, which has a voltage drop proportional to inductor 35 current during on state of the switch 36. This signal is used inside the SMPS controller 22 with its error amplifier output multiplied by scaled AC rectified voltage for power factor correction.

The supply voltage input 28 of the SMPS controller 22 is fed by two sources, namely AC rectified voltage and LEDs set tap. The AC rectified voltage is connected through resistor 26 which feeds the controller during power-on time, when output voltage is not established yet. During steady state operation the controller is supplied by LED set tap 15 directly, controller supply current goes through part of LED set between its positive terminal 14 and tap 15. This creates some difference in operating currents between these LEDs and the LEDs between tap and negative LED set terminal 16, which is not significant in most of LED driver applications. One advantage of this solution is no additional loss, since current consumed by the SMPS controller flows through the LED set which is useful load, another advantage is stability of controller supply voltage over mains voltage and temperature variations.

The LED set in its simplest form can be implemented as a series connection of LEDs, in other form it may combine parallel connection of series LED groups or series connection of parallel groups of identical LEDs. To prevent a fault of the whole string of LEDs connected in series due to an open fault of a single LED, each LED can be shunted by a Zener diode with nominal voltage slightly above the maximum forward voltage drop of the LED.

Error amplifier of the SMPS controller has high gain on DC, thus maintaining voltage drop at LED set current sense resistor at constant level it maintains a constant LED set current and the whole driver circuit operates as current source on DC.

One exemplary embodiment of the above mentioned solution allowed raising efficiency of the driver circuit up to 96% at power consumption of about 12 watts.

The current monitor circuit can be alternatively implemented as a Hall Effect sensor. For this implementation the LED set current can be sensed in its positive terminal 14 as well in any point of the set. It may use a typical current shunt monitor integrated circuit like AD8217 with appropriate shunt resistor connected inside the LED set where a common mode voltage is appropriate for its operation. Frequency response of the current monitor circuit is part of the current control open loop gain, thus it may contain active or passive filter stages.

The LED light can be a light bulb. It is commonly known how can a set of light-emitting diodes and a driver be integrated in a light bulb, and therefore no particular embodiment of such configuration is presented here. Alternatively, the LED light can be a LED strip, LED bar or LED rope.

Claims

What is claimed is-

1. A LED light comprising a set of light-emitting diodes and. a driver circuit, the driver circuit comprising:

a switching mode power converter with power factor correction, the switching mode power converter having a power input from an AC electrical power source, a voltage feedback input, and an output connected to the set of light-emitting diodes, the driver circuit further comprising

a current monitor circuit adapted to convert a current through the set of light-emitting diodes into a voltage feedback signal supplied to the voltage feedback input of the switching mode power converter;

wherein

the switching mode power converter is a boost converter, comprising a controller having a power supply input,

and

the controller power supply input is connected to a tap in the set of light-emitting diodes.

2. The LED light of claim 1, wherein the LED light is a light bulb.

3. The LED light of claim 1, wherein the LED light is any of a led strip, led bar and led rope.

4. The LED light of claim 1, wherein the light -emitting diodes connected between the output of the power converter and the tap have a bigger active area of p-n junction.

5. The LED light of claim 1, wherein the current monitor circuit is a Hall Effect current sensor.

6. The LED light of claim 1, wherein each light-emitting diode in the set of light-emitting diodes is shunted by a protective component.

7. The LED light of claim 1, wherein the protective component is a Zener diode.

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