CN108668400B

CN108668400B - LED lamp

Info

Publication number: CN108668400B
Application number: CN201710197944.6A
Authority: CN
Inventors: 丁实宇; 杨旭生
Original assignee: Ledvance GmbH
Current assignee: Ledvance GmbH
Priority date: 2017-03-29
Filing date: 2017-03-29
Publication date: 2021-07-02
Anticipated expiration: 2037-03-29
Also published as: DE102018107486B4; DE102018107486A1; CN108668400A

Abstract

An LED lamp (1) is provided, the LED lamp (1) comprising a first LED (11) and a second LED (12) and a PWM control circuit (20) adapted to provide a pulsed PWM signal having a duty cycle, wherein the connection between the first LED (11) and the second LED (12) is switchable between a parallel connection and a series connection in accordance with a PWM signal (22).

Description

LED lamp

Technical Field

The invention relates to an LED lamp with adjustable current.

Background

As an effective alternative to incandescent light bulbs, fluorescent lamps have become a well-known and widely used lighting device. However, with the advent of LED lamps (LEDs are an abbreviation for light emitting diodes), there is even a more efficient and long-lived lighting device. Furthermore, the material of LED lamps is safer compared to fluorescent lamps, since, for example, mercury is not required. Therefore, there is a need to replace existing fluorescent lamps with LED lamps, preferably without changing the entire luminaire or fixture.

Currently available fluorescent lamp fixtures typically comprise a so-called electronic ballast (also called electronic control means, ECG) for regulating and limiting the current supplied to the fluorescent lamp. Electronic ballasts typically operate lamps at high frequencies of several hundred Hz, up to 50 kHz. The frequency of the electronic ballast is selected to be at the resonant frequency of the fluorescent gas so that the current through the fluorescent lamp can be actively controlled, thereby increasing the efficiency of the fluorescent lamp.

Furthermore, the frequency and/or maximum current provided by the electronic ballast varies with different markets and/or regions having different standardization rules. For example, in the EMEA market (EMEA is an abbreviation for europe, middle east and africa), electronic ballasts currently in use may provide different output currents.

In order to provide LED lamps that are compatible with conventional luminaires having electronic ballasts as described above, currently available LED lamps comprise an electronic driver for adapting the voltage and current provided by the electronic ballast to the requirements of the LED. Otherwise, the electronic and/or optoelectronic components of the LED lamp may be damaged or destroyed by the electronic ballast due to the high voltage or current generated by the ballast.

Compatible drivers for currently available electronic ballasts typically include two types of converters, a single stage active driver and a purely passive driver stage. The single-stage active driver regulates current flowing to an LED of the LED lamp; thus, its output current is independent of the connected electronic ballast. Typically, a single-stage active driver includes a boost converter, a buck converter, and/or a buck-boost converter. A purely passive driver stage does not regulate the current of the LED and its output current depends on the type of electronic ballast connected. Due to this dependency of the electronic driver on the electronic ballast, the LED lamp has to be changed for different countries. Furthermore, the additional stage converters of the LED lamp driver, in addition to the stage converters of the electronic ballast, significantly reduce the efficiency of the LED lamp.

Disclosure of Invention

In view of the above-mentioned drawbacks of the presently known LED lamps, it is an object of the present invention to provide an LED lamp with an efficient electronic driver, which can be connected to various electronic ballasts and maintain the same output current between them.

This object is solved by an LED lamp according to the independent claim. Preferred embodiments are given by the dependent claims, the description and the figures.

Thus, an LED lamp is provided. The LED lamp comprises a first LED, a second LED and a PWM control circuit (PWM is an abbreviation for pulse width modulation) the PWM control circuit is adapted to provide a PWM signal having a duty cycle. The connection between the first LED and the second LED may be switched between the parallel connection and the series connection according to the PWM signal.

A key idea of the invention is to operate two LEDs (which may consist of two groups of LEDs, respectively) in a pulsed manner, wherein the operation is switched between a parallel connection (current is split between the two LEDs) and a series connection (the entire current supplied to the circuit flows through the two LEDs). The current is switched at a duty cycle which may preferably depend on: a difference between the reference voltage and a current through and/or a voltage drop at the first and/or second LED. For a first value of the PWM signal (corresponding to a parallel connection) the current flowing through the LED decreases, whereas for a second value of the PWM signal (corresponding to a series connection) the current increases and in particular increases to a maximum value. Thus, the current flowing through the first and second LEDs oscillates between two smaller independent lower currents in case of a parallel connection and a higher current in case of a series connection.

Due to this switching of the first LED and the second LED between the two different connection modes, the LED lamp may require less electronic components and may in particular be free of a stage converter. Thus, the LED lamp can be provided in a cost-effective manner and the efficiency is improved.

The LED lamp may include a plurality of first LEDs constituting the first LED string and a plurality of second LEDs constituting the second LED string. One LED string may correspond to a group of LEDs. During operation of the LED lamp, the first LED and the second LED may emit white light. The color temperature of the white light emitted by the first LED may be the same as the color temperature of the white light emitted by the second LED.

Preferably, the PWM signal is a rectangular signal oscillating between an ON state (e.g., corresponding to a 5V and/or high voltage signal) and an OFF state (e.g., corresponding to a 0V and/or low voltage signal). For example, if the PWM signal is in an ON state, the first LED and the second LED are connected in series, and if the PWM signal is in an OFF state, the first LED and the second LED are connected in parallel, or vice versa. The amount of time occupied in the ON state is determined by the duty cycle. For example, a duty cycle of 90% means that the PWM signal is in the ON state 90% of the time and in the OFF state 10% of the time. The time taken by one ON-OFF period, i.e. the sum of the time taken in the ON state and the time taken in the OFF state, corresponds to the total period of the PWM signal and thus to the inverse of the oscillation frequency of the PWM. The oscillation frequency may be at least 100Hz up to several tens of kHz. The oscillation frequency of the PWM signal may be a ratio of a pulse width of a single rectangular pulse of the PWM signal and a duty ratio of the PWM signal. For example, a pulse width of 0.1ms and a duty cycle of 10% corresponds to an oscillation frequency of 1 kHz.

Preferably, the oscillation frequency of the PWM signal may exceed a frequency that can be resolved by the human eye. If the first LED and the second LED are connected in series, they may emit light having a first brightness. Also, if two LEDs are connected in parallel, they may emit light having a second brightness. The first brightness is higher than the second brightness because a higher current is provided to the LEDs in the series connection. If the switching between parallel and series connections is provided in a fast manner, i.e. at a high oscillation frequency, the human eye is not able to resolve the change in brightness. Instead, it appears as if a constant brightness is provided by the LED lamp, which is between the first brightness and the second brightness and depends on the duty cycle (hereinafter also referred to as "perceived constant brightness"). Preferably, the oscillation frequency is at least 100 Hz.

The perceived constant brightness may be the same as the brightness emitted by the first LED and/or the second LED at the optimal operating current of the first LED and/or the second LED, respectively. At an optimum operating current, the first LED and/or the second LED may operate very efficiently. Advantageously, the time-averaged current provided to the first LED and the second LED is essentially the same as the optimal operating current for the first LED and the second LED, respectively. The time-averaged current may be obtained from a weighted average of the time (corresponding to the duty cycle of the PWM signal) occupied in the respective current state according to the lower current (i.e. the current connected in parallel) and the higher current (i.e. the current connected in series).

To provide the PWM signal, the PWM control circuit may comprise a microcontroller and/or a plurality of comparators, for example in the form of operational amplifiers. In the latter case, the sawtooth signal may be coupled to a first input port of a comparator to provide the PWM signal. The input port of the further comparator may be coupled to the first LED and/or the second LED via a measurement resistor (e.g. a shunt resistor) for sensing a current flowing through the first LED and/or the second LED. The voltage drop over the measurement resistor (hereinafter also referred to as "sensing voltage") may be proportional to the current flowing through the measurement resistor, and thus proportional to the current flowing through the first LED and/or the second LED. The duty cycle and/or the oscillation frequency of the PWM signal may be adjusted by using an adjustable resistor and/or at least one comparator. Additionally, or alternatively, a port of the microcontroller may be used to adjust the duty cycle and/or oscillation frequency of the PWM signal.

In one embodiment of the LED lamp, the PWM control circuit includes a first error amplifier and a second error amplifier. The first error amplifier and the second error amplifier may each be provided in the form of a comparator, in particular an operational amplifier. The sensed voltage from a shunt resistor coupled to the first LED or the second LED is fed into the input port of the first error amplifier. The input port may be a so-called positive input of the first error amplifier. Furthermore, the reference voltage is connected to another input port of the first error amplifier, which may be a so-called negative input of the first error amplifier. The output signal of the first error amplifier represents the difference between its two inputs, i.e. the difference between the sense voltage and the reference voltage. When a sensing voltage corresponding to a current flowing through the first LED or the second LED is higher than a reference voltage, an output signal of the first error amplifier increases, resulting in an increase in a duty ratio. Vice versa, for a decrease of the output signal, the duty cycle is decreased. For smaller duty cycles, the contribution from the parallel connection is higher than the contribution from the series connection, thereby reducing the total current flowing through the LED.

The output signal of the first error amplifier is coupled to the input port of the second error amplifier and compared to a sawtooth signal coupled to another input of the second error amplifier. The frequency of the sawtooth signal defines the oscillation frequency of the PWM signal. The output of the second error amplifier is a PWM signal having a particular duty cycle that depends on the output signal of the first error amplifier.

According to at least one embodiment, the LED lamp may be connected to a conventional electronic ballast of a fluorescent lamp fixture. The LED lamp may be an LED tube. Advantageously, the LED lamp may be connected to a variety of electronic ballasts for fluorescent light fixtures. In particular, the LED lamp can be connected to a conventional electronic ballast standardized according to the specifications of at least two of the following regions: europe, middle east, africa, north america (especially the united states) and asia (especially china and/or japan). Preferably, no electronic components need to be changed to switch between the regions.

Preferably, to provide an LED lamp connectable to a conventional electronic ballast, the PWM control circuit is adapted to vary the duty cycle in dependence on the value of the supply current provided by the electronic ballast. For example, electronic ballasts in different markets may provide different current values, particularly different maximum current values. The current flowing through the first LED and/or the second LED may be proportional to the current provided by the electronic ballast. The PWM control circuit may be adapted to compare the current flowing through the first LED and/or the second LED with, for example, an optimal operating current of the first LED and/or the second LED. Based on the deviation from the optimal operating current, the duty cycle of the PWM signal may be adjusted to provide a time-averaged current to the first LED and the second LED. The time-averaged current may be adjusted by comparing the ratio of the constant reference voltage and the sensed voltage at the measurement resistor.

According to at least one embodiment, the LED lamp further comprises at least one switch. The switches are coupled to the output of the PWM control circuit and are connected to the first LED and the second LED such that if the at least one switch is in a first state, the first LED and the second LED are connected in series and if the at least one switch is in a second state, the first LED and the second LED are connected in parallel. The switch may be open (i.e., non-conductive) when it is in the first state and may be closed (i.e., conductive) when it is in the second state, or vice versa. For example, the switch may comprise a transistor, such as a MOSFET. Thus, the LED lamp can achieve oscillation between a higher current and a lower current as described above with simple electronic components.

According to at least one embodiment of the LED lamp, at least one diode is connected between the first LED and the second LED. The diode is connected to the first LED and the second LED such that during operation of the LED lamp, the diode operates in a forward direction if the first LED and the second LED are connected in series and operates in a blocking direction if the first LED and the second LED are connected in parallel, or vice versa. Here, the term "vice versa" means that the diode operates in the blocking direction if the first LED and the second LED are connected in series, and operates in the forward direction if the first LED and the second LED are connected in parallel. Thus, the diode may have the function of a rectifier diode and/or a zener diode. Preferably, the diode is a fast diode. Thereby, a fast switching between the series connection and the parallel connection is possible. Preferably, the switching time and/or the conduction time of the diode is less than the rise time of the rectangular PWM signal. This allows to provide a switchable connection between the first LED and the second LED in a simple manner and by using only a small number of electronic components.

According to at least one embodiment of the LED lamp, the LED lamp comprises a first LED string and a second LED string. The first LED string includes a first LED and the second LED string includes a second LED. Preferably, the first LED string comprises a plurality of first LEDs connected in series, and the second LED string comprises a plurality of second LEDs connected in series. The first LED string and the second LED string may have the functions of the first LED and the second LED, respectively, as described above. The first LED string and the second LED string are identical and/or have the same resistive effect. For example, the first LED string and the second LED string may have the same number of the same LEDs. In other words, in this case, the first LED and the second LED may be the same. Alternatively, the first LED string may have a first number of first LEDs and the second LED string may have a second number of second LEDs, wherein the first and second numbers and the first and second LEDs are different such that the resistance values of the first and second LED strings are the same. Since the first LED string and the second LED string are identical and/or have the same resistance value, the higher current flowing through the first LED string and through the second LED string in case of a series connection is essentially twice the lower current flowing through the first LED string and the second LED string in case of a parallel connection. Furthermore, this same configuration may allow for accurate current measurement at only one of the two LED strings, for example by using a measurement resistor.

According to at least one embodiment of the LED lamp, the current flowing through the first LED and/or (if applicable) the first LED string is monitored with a PWM control circuit. In addition, or as an alternative, the current flowing through the second LED and/or (if applicable) the second LED string is monitored with a PWM control circuit. In particular, the switching characteristics of the PWM signal provided by the PWM control circuit may depend on the current measured by the PWM control circuit. Depending on the measured current, the oscillation frequency and/or the duty cycle may be varied by the PWM control circuit. Thus, the current flowing through the first LED and/or the second LED may be adjusted depending on the measured current. If, as an example, the time-averaged current flowing through the first LED and/or the second LED is too high, the duty cycle may be adjusted such that the parallel connection occupies a time that is higher than the time occupied by the series connection,

according to at least one embodiment of the LED lamp, the PWM control circuit includes a comparator. The input port of the comparator is connected to the current measurement output of the first LED and/or, if applicable, the first LED string. That is, the PWM control circuit provides a rectangular PWM signal using a comparator, wherein the comparator signal is dependent on the current flowing through the first LED. For example, the current measurement output may be the output of a measurement resistor connected to the first LED and/or (if applicable) the first LED string.

According to at least one embodiment, the LED lamp comprises a first switch and a second switch. The first switch is connected in series to the first LED and/or (if applicable) the first LED string. The second switch is connected in series to the second LED and/or (if applicable) the second LED string. Alternatively, the first LED string may include a first switch, and the second LED string may include a second switch. The first switch and the second switch may have the same function as the at least one switch described above.

The PWM signal provided by the PWM control circuit may be used to control the switches. For example, a first switch is connected in series to a first LED string and a second switch is connected in series to a second LED string. If the PWM signal is in the ON state, the first switch and the second switch may be simultaneously closed, thereby switching the connection between the first LED and the second LED to the parallel connection. Similarly, if the PWM signal is in the OFF state, the first switch and the second switch may be simultaneously turned OFF, thereby switching the connection between the first LED and the second LED to the series connection.

Drawings

Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. This is shown in:

fig. 1, 2, 3A and 3B show exemplary embodiments of LED lamps according to the present invention.

Fig. 4 shows an exemplary embodiment of an LED lamp according to an alternative embodiment.

Fig. 5A, 5B and 5C show PWM signals and currents, respectively, of an exemplary embodiment of an LED lamp according to the present invention.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Here, elements that are identical, similar or have the same or similar effect are provided with the same reference numerals in the figures. To prevent redundant description, repeated description of these elements may be omitted.

The drawings and the dimensional relationships between elements shown in the drawings to one another should not be considered to be drawn to scale. Rather, individual elements may be shown in exaggerated dimensions to enable better illustration and/or better understanding.

An embodiment of an LED lamp 1 according to the invention is explained in detail with reference to the schematic circuit diagram of fig. 1. The LED lamp 1 includes a first LED string 110 having a plurality of first LEDs 11 connected in series, a second LED string 120 having a plurality of second LEDs 12 connected in series, and a PWM control circuit 20. The first LED string 110 is connected to the second LED string 120 via a switchable parallel connection and via a series connection through the diode 14. For filtering purposes, the first LED 11 of the first LED string 110 and the second LED 12 of the second LED string 120 are connected in parallel to the respective smoothing capacitor 42. Further, the first LED string 110 is connected to ground 40 via a measurement resistor 41, which measurement resistor 41 may be a shunt. The measurement resistor 41 provides a voltage at the current measurement output port 21 that is proportional to the current flowing through the first LED string 110

The first and

second switches

131 and 132 are connected to the first and second LED strings 110 and 120, respectively. If the first switch 131 and the second switch 132 are closed, i.e. in a conducting state, the first LED string 110 and the second LED string 120 are connected in parallel, since the diode 14 is now operating in the blocking direction.

The first switch 131 and the second switch 132 are operated by the PWM signal 22 provided by the PWM control circuit 20. To stabilize and/or control the PWM signal 22 depending on the current flowing through the first LED string 110, a current measurement output 21 is connected to the PWM control circuit 20.

The LED lamp 1 may further comprise a rectifier circuit 43. At the output port of the rectifier circuit 43, a supply current and/or a supply voltage is provided. The supply current and/or supply voltage may be, for example, a direct current and/or voltage and have a sawtooth shape. The input port of the rectifier circuit 43, which may be considered a bus port, is coupled to a ballast resistor 46 and an inductance 44. At the ballast resistor 46, the LED lamp 1 may be connected to the electronic ballast 10. Preferably, no filter capacitor is coupled between rectifier circuit 43 and electronic ballast 10.

An embodiment of the LED lamp 1 according to the invention is explained in detail with reference to the schematic circuit diagrams of fig. 2, 3A and 3B. Here, fig. 2 shows a schematic circuit diagram of the embodiment as a whole, and fig. 3A and 3B show schematic circuit diagrams of different PWM signals 22 provided by the PWM control circuit 20.

A more detailed structure of the PWM control circuit 20 is shown in the exemplary embodiment of fig. 2, 3A, and 3B, as compared to fig. 1. The PWM control circuit 20 comprises a PWM generator 23 which provides a control signal 22 at an output port. The PWM generator 23 is depicted as a comparator in the figure. However, contrary to this depiction, the PWM generator 23 may comprise other electronic components, in particular other comparators, and/or may be a microcontroller.

One input port of the PWM generator 23 is coupled to the current measurement output 21, wherein the high frequency signal is filtered out via a filter capacitor 25. Therefore, a voltage signal proportional to the current flowing through the first LED string 110 is connected to the input. In order to adjust the value of the voltage signal at the current measurement output 21, a further resistor 45 is coupled between the first LED string 110 and the PWM generator 23.

The other input port of the PWM generator 23 is coupled to the rectifier circuit 43 via a first resistor 241 and thus to the supply current. Accordingly, a voltage signal proportional to the current flowing through the first LED string 110 is compared with the supply voltage provided by the electronic ballast 10, thereby generating a PWM signal 22 having a duty ratio adapted to the value of the supply voltage at the output of the PWM generator 23. The supply port of the PWM generator 23 is coupled to the rectifying circuit 43 through a second resistor 242.

Fig. 3A shows a schematic circuit diagram of fig. 2 when both the first switch 131 and the second switch 132 are in the off-state 51, i.e. they are non-conductive. Fig. 3B shows a schematic circuit diagram of fig. 2 when both the first switch 131 and the second switch 132 are in the closed state 53, i.e. they are both conducting. For example, when the PWM signal 22 is in the OFF state (corresponding to 5V), both switches 131,132 are in the closed state 53, and when the PWM signal 22 is in the ON state (corresponding to 0V), both switches 131,132 are in the open state 51.

In fig. 3A, the first LED string 110 and the second LED string 120 are connected in series. Thus, a single higher current 52 (which may be equal to the supply current) flows through the first LED string 110 and the second LED string 120. In fig. 3B, the first LED string 110 and the second LED string 120 are connected in parallel. Thus, the supply current is split between the first LED string 110 and the second LED string 120, and the lower current 54 flows independently through each of the first LED string 110 and the second LED string 120. The lower current 54 is less than the higher current 52.

With reference to the schematic diagram of fig. 4, an exemplary embodiment of an alternative LED lamp 1' is described in detail. The replacement LED lamp 1' may be a conventional LED lamp. The replacement LED lamp 1' is connected to a conventional electronic ballast 10. The electronic ballast 10 is also connected to a common power supply having a line port (L), a neutral port (N) and a potential ground (PE). The electronic ballast 10 may comprise at least one boost converter and/or at least one half-bridge converter for converting the current and/or voltage provided by the common power source.

The alternative LED lamp 1' comprises a first driver stage 11 ' and a second driver stage 12 ', which may be a light engine. The first driver stage 11' may be a single stage active driver. The second driver stage 12 'may be a passive driver stage depending on the type of electronic ballast 10 connected to the replacement LED lamp 1'.

The alternative LED lamp 1' comprises at least three-level converters, i.e. at least two converters in the electronic ballast 10 and at least one converter in the second driver stage 12 ' and/or in the first driver stage 11 '. Therefore, the efficiency of the alternative LED lamp 1' is significantly reduced compared to the LED lamp 1 according to the invention.

With reference to the measurement data of fig. 5A, 5B and 5C, an exemplary embodiment of the LED lamp 1 according to the present invention is explained in detail. Each of fig. 5A, 5B, and 5C shows the first current 71, the second current 72, and the PWM signal 22 as a function of time (in arbitrary units, a.u.).

A first current 71 flows through the first LED 11 during operation of the LED lamp 1, wherein a first zero current 710 shows a zero crossing position of the first current 71. A second current 72 flows through the second LED 12 during operation of the LED lamp 1, wherein a second zero current 720 shows a zero crossing position of the second current 72. The PWM zero signal 220 shows the zero crossing position of the PWM signal 22.

The data according to fig. 5A, 5B and 5C are acquired with an LED lamp 1 having an electronic circuit similar to that shown with reference to the embodiment of fig. 2, 3A and 3B. The supply current of the LED lamp 1 is the same for the different embodiments of fig. 5A, 5B and 5C to allow a comparison between the results of the different duty cycles.

For example only, the first LED string 110 and the second LED string 120 have the same design. That is, the first LED string 110 has a plurality of first LEDs 11 and the second LED string 120 has a plurality of second LEDs 12, wherein the number of

LEDs

11, 12 is the same in the first LED string 110 and the second LED string 120, and wherein the first LEDs 11 are the same as the second LEDs 12. Due to this equality, the first current 71 and the second current 72 have essentially the same value.

The first current 71 and the second current 72 in each of the exemplary embodiments of fig. 5A, 5B and 5C oscillate between a high value and a low value. The high current value corresponds to the series connection of the first LED string 110 and the second LED string 120. The low current value corresponds to the parallel connection of the first LED string 110 and the second LED string 120. In particular, the low current value may be half of the high current value. In this example, the average low current value is 200mA and the average high current value is 400mA, where the average value is taken to account for noise. The parallel connection is achieved when the PWM signal 22 is in the ON state (i.e., in the high voltage state) and the series connection is achieved when the PWM signal 22 is in the OFF state (i.e., in the low voltage state).

The measurement data shown in fig. 5A, 5B, and 5C are different as follows. In the exemplary embodiment of fig. 5A, the duty cycle of the PWM signal 22 is 10%. The average value (i.e., time-averaged current) of each of the first current 71 and the second current 72 in fig. 5A fluctuates between at least 370mA and at most 385mA, particularly between at least 374mA and at most 379 mA. In the embodiment of fig. 5B, the duty cycle of the PWM signal 22 is 50%. The respective average values of the first current 71 and the second current 72 in fig. 5B fluctuate between at least 295mA and at most 310mA, in particular between at least 301mA and at most 304 mA. In the embodiment of fig. 5C, the duty cycle of the PWM signal 22 is 100%. The respective average values of the first current 71 and the second current 72 in fig. 5C fluctuate between at least 195mA and at most 205mA, in particular between at least 200mA and at most 202 mA. Each of the first current 71 and the second current 72 corresponds to a low current value for a duty ratio of 100%.

In general, for a constant current supplied to the LED lamp 1, only the duty ratio is changed, and the average value of the first current 71 and/or the second current 72 is typically:

I_1/2＝D*I_L+(1-D)*I_H,

wherein I_1/2Is the first/second current 71, 72, D is the duty cycle, I_HIs a high current value, I_LIs a low current value. The measurement data shown in fig. 5A, 5B and 5C are consistent with this equation.

The present invention is not limited to the description based on the embodiment. Rather, the invention encompasses any novel feature and any combination of features, in particular any combination of features in the claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

List of reference numerals

1 LED lamp

1' substituted LED lamp

10 electronic ballast

11' first driver stage

12' second driver stage

11 first LED

110 first LED string

12 second LED

120 second LED string

20 PWM control circuit

21 current measurement output

22 PWM signal

220 PWM zero signal

23 PWM generator

241 first resistor

242 second resistor

25 filter capacitor

40 floor

41 measuring resistor

42 smoothing capacitor

43 rectification circuit

44 inductance

45 another resistor

46 ballast resistor

50 lower current

51 off state

52 higher current

53 closed state

54 lower current

71 first current

710 first zero current

72 second current

720 second zero current

Claims

1. An LED lamp (1) comprising a first LED (11) and a second LED (12) and a PWM control circuit (20) adapted to provide a pulsed PWM signal having a duty cycle, wherein the connection between the first LED (11) and the second LED (12) is switchable between a parallel connection and a series connection in dependence of a PWM signal (22), wherein the PWM control circuit (20) is adapted to vary the duty cycle in dependence of a value of a supply current provided by an electronic ballast (10).

2. The LED lamp (1) according to claim 1, wherein the LED lamp (1) is connectable to a conventional electronic ballast (10) of a fluorescent lamp fixture.

3. The LED lamp (1) according to claim 1 or 2, further comprising at least one switch (131, 132) coupled to an output of the PWM control circuit (20) and connected to the first LED (11) and the second LED (12) such that if the at least one switch (131, 132) is in a first state, the first LED (11) and the second LED (12) are connected in series, and if the at least one switch (131, 132) is in a second state, the first LED (11) and the second LED (12) are connected in parallel.

4. The LED lamp (1) according to claim 1 or 2, wherein a diode (14) is connected between the first LED (11) and the second LED (12) such that during operation of the LED lamp (1), the diode (14) operates in a forward direction if the first LED (11) and the second LED (12) are connected in series, and the diode (14) operates in a blocking direction if the first LED (11) and the second LED (12) are connected in parallel, or vice versa.

5. The LED lamp (1) according to claim 1 or 2, wherein the LED lamp (1) comprises a first LED string (110) and a second LED string (120), the first LED string (110) comprising the first LED (11) and the second LED string (120) comprising the second LED (12), wherein the first LED string (110) and the second LED string (120) are identical and/or have the same effective resistance.

6. The LED lamp (1) according to claim 1 or 2, wherein the current through the first LED (11) and/or the current through the second LED (12) is monitored with a PWM control circuit (20).

7. The LED lamp (1) according to claim 5, wherein the current through the first LED string (110) and/or the current through the second LED string (120) is monitored with a PWM control circuit (20).

8. The LED lamp (1) according to claim 1 or 2, wherein the PWM control circuit (20) comprises a comparator, wherein an input port of the comparator is connected to a current measurement output (21) of the first LED (11).

9. The LED lamp (1) according to claim 5, wherein the PWM control circuit (20) comprises a comparator, wherein an input port of the comparator is connected to a current measurement output (21) of the first LED string (110).

10. The LED lamp (1) according to claim 1 or 2, comprising a first switch (131) and a second switch (132), the first switch (131) being connected in series with the first LED (11), the second switch (132) being connected in series with the second LED (12), wherein the PWM signal (22) is coupled to the first switch (131) and the second switch (132) such that the first switch (131) and the second switch (132) have the same switching characteristics.

11. The LED lamp (1) according to claim 5, comprising a first switch (131) and a second switch (132), the first switch (131) being connected in series with the first LED string (110), the second switch (132) being connected in series with the second LED string (120), wherein the PWM signal (22) is coupled to the first switch (131) and the second switch (132) such that the first switch (131) and the second switch (132) have the same switching characteristics.