Patent Application
20100253245
The present invention relates generally to the field of light illumination. More particularly, the present invention relates to providing a method, system and electronic circuit for substantially preventing excess initial output current surges when connecting a load (e.g., one or more light sources, such as LEDs (Light Emitting Diodes)), to a driver, such as a constant current driver.
In recent years, the usage of LED illumination instead of other kinds of illumination (such as the fluorescent illumination, incandescent bulb illumination, and the like), has significantly increased due to the increasing luminosity of LED devices and due to their continuously decreasing costs. Although most people around the world still use fluorescent and incandescent bulb lighting, development of low-cost and efficient LED illuminating devices has recently accelerated rapidly.
However, modern light emitting diodes have relatively stringent current requirements. If excess currents are passed through them, they may be damaged by the associated heat. Most LED driver circuits, which generate a constant current to be provided to a LED load, have a capacitor at the output, which in turn is used to smooth out high frequency fluctuations. When the LED load is not connected, the output voltage goes up to its compliance voltage limit, which is usually set at 60V (Volts) to meet UL (Underwriters Laboratories®) safety requirements. On the other hand, when the LED load is connected, the voltage on the LED load can vary, for example, from 40V (e.g., for twelve LEDs) down to 12V (e.g., for three LEDs). With a difference between the above 60V and 12V (60V−12V=48V), the result is that a relatively large current flows through the LEDs as the capacitor at the output stage of the LED driver discharges from 60V down to 12V. This is normally a one time event, except that some systems involve repeatedly switching the output ON and OFF, such as for precision light exposure purposes in industrial equipment.
Problems related to limiting output surges have been recognized in the prior art, and various methods have been proposed to provide a solution. It should be noted that according to the prior art, there are two main kinds of limiters: those which sense the fall in the output voltage caused by the relatively large surge (so called “voltage sensing limiters”) and those which sense the passing current, regardless of the voltage, and react to the current (so called “current sensing limiters”).
U.S. Pat. No. 5,374,887 discloses an inrush current limiting circuit that contains a FET (Field Effect Transistor) as an active component, which is controlled by a network of passive components. The network includes a gate control circuit for controlling the operation of the FET and a negative feedback circuit, which responds to the load voltage during the transient state. Thus, the circuit of U.S. Pat. No. 5,374,887 is actually a voltage sensing circuit, in which a FET is placed in series with the load, and its gate is biased through a resistor connected across the power rails. According to U.S. Pat. No. 5,374,887, the normal operation involves permanent connection of the load, and connecting the power supply. The FET is initially switched OFF, and when the power supply is connected, it is slowly switched ON, thereby connecting the power supply to the load. Then, the power rail is pulled down and a capacitor connected between the power rail and the gate of the FET transfers a downward pulse to the gate, and thus turns OFF the FET.
U.S. Pat. No. 7,262,559 presents a power supply that provides power to a LED light source having a variable number of LEDs wired in series and/or in parallel. The power supply uses current and voltage feedback to adjust power provided to the LED light sources, and as a result to protect them. A feedback controller compares the sensed current and sensed voltage to reference signals and generates feedback signals, which are processed by a power factor corrector to adjust the current flow through the transformer supplying current to the LED light sources. Thus, the circuit of U.S. Pat. No. 7,262,559 is actually a current sensing circuit, in which a FET is provided in series with the load, and the gate of the FET is normally pulled up to a positive rail potential. In addition, another resistor is connected in series with the FET, with a n-p-n transistor connected across the resistor (the collector of the n-p-n transistor is connected to the gate of the FET). When the current becomes sufficient, then the voltage generated across the resistor is sufficient enough to turn ON the n-p-n transistor, so that the gate of the FET is pulled down, thus turning OFF the FET and as a result, limiting the current.
The prior art limitations are well known and there is a continuous need to provide a current limiting circuit, which can limit the excess current surge to a relatively low level, such as approximately a hundred millamperes. In addition, there is a need to provide a current limiting circuit that reacts relatively fast to a current surge of substantially any level. Further, there is a need in the prior art to provide a current limiting circuit, which does not involve continuous power dissipation and eliminates the need in providing one or more resistors in series with a load, such as a LED load.
The present invention relates to providing a method, system and electronic circuit for substantially preventing excess initial output current surges when connecting a load (e.g., one or more light sources, such as LEDs (Light Emitting Diodes)), to a driver, such as a constant current driver.
According to an embodiment of the present invention, a current limiting circuit is configured to limit the excess output current passing through a load, said current limiting circuit comprising a resistor connected in series with said load and in parallel with a switch, which is initially turned OFF, wherein said switch is turned ON, thereby shorting said resistor, when the output voltage applied to said load is decreased by a predetermined level.
According to another embodiment of the present invention, the current limiting circuit is integrated within a driver that is operatively coupled to the load.
According to a particular embodiment of the present invention, the driver is a constant current driver.
According to another particular embodiment of the present invention, the driver is a Light Emitting Diode (LED) driver.
According to still another embodiment of the present invention, the load is a light source.
According to still another embodiment of the present invention, the light source is at least one Light Emitting Diode (LED).
According to a further embodiment of the present invention, the switch is a transistor.
According to still a further embodiment of the present invention, the transistor is partially operated in a linear mode.
According to still a further embodiment of the present invention, the transistor is operated so as to control the rate of decrease of the output voltage.
According to still a further embodiment of the present invention, the switch and the resistor are operatively coupled to one or more additional resistors and to one or more additional switches configured to control the rate of decrease of the output voltage to the substantially equilibrium level,
According to another embodiment of the present invention, a current limiting circuit is configured to limit the excess output current passing through a load, said current limiting circuit comprising a resistor connected in series with said load and in parallel with a switch, which is initially turned OFF, wherein said switch is configured to be turned ON, thereby shorting said resistor, when the output voltage is decreased by a predetermined level configured to allow a substantially brief surge of said excess output current to be passed through said load, and said switch configured to be turned OFF again when said output voltage is further decreased by an additional predetermined level, thereby continuously turning said switch ON and OFF until said output voltage is decreased to a substantially equilibrium level, at which said switch is left turned ON.
According to still another embodiment of the present invention, the output current passes through the load in brief pulses in excess of the predetermined current level until said predefined current level is substantially achieved.
According to an embodiment of the present invention, a method of limiting the excess output current passing through a load comprises:
According to another embodiment of the present invention, a method of limiting the excess output current passing through a load comprises:
According to an embodiment of the present invention, a system is configured to limit the excess output current passing through a load, said system comprising:
According to another embodiment of the present invention, a system is configured to limit the excess output current passing through a load, said system comprising:
In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, systems, procedures, components, units, circuits and the like have not been described in detail so as not to obscure the present invention.
Hereinafter, whenever the term “LED” (“Light Emitting Diode”) is mentioned, it should be understood that it refers to any type of a light illumination source, such as a LED-based source, an incandescent source (a filament lamp, a halogen lamp, etc.), a high-intensity discharge source (sodium vapor, mercury vapor, a metal halide lamp and the like), a fluorescent source, a phosphorescent source, laser, an electroluminescent source, a pyro-luminescent source, a cathode-luminescent source using electronic satiation, a galvano-luminescent source, a crystallo-luminescent source, a kine-luminescent source, a candle-luminescent source (a gas mantle, a carbon arc radiation source, and the like), a radio-luminescent source, a luminescent polymer, a thermo-luminescent source, a tribo-luminescent source, a sono-luminescent source, an organic LED-based source and any other type of light illumination source.
According to an embodiment of the present invention, the n-p-n transistor T1 is turned ON only when the output voltage (on LED load 110) is up at its “limiting high voltage”, which is the maximum output voltage of LED driver 101 (that is usually predefined by safety requirements), while using the 47V zener diode ZD1 to sense the voltage reaching the corresponding high level. Transistor T1 turns OFF transistor Q1, when the output is open-circuited (when switch S1 is open). It should be noted that conventional LED driver 101 is generally bound by the requirements of SELV (Safety Extra-Low Voltage) standard in Europe or “Class 2” standard in the United States, which require that the output voltage should not exceed 60V DC (Direct Current). So even though conventional LED driver 101 (
According to an embodiment of the present invention, when switch S1 is open, then the output voltage of LED driver 101 (
In addition, according to an embodiment of the present invention, transistor Q1 substantially does not get “turned hard ON” during the current limiting circuit 105 operation. Instead, it turns ON partially in a “linear mode” of operation, and relatively briefly dissipates energy from the discharging output capacitor (not shown) of LED driver 101.
Also, it should be noted that according to an embodiment of the present invention, it is assumed that LED driver 101 is a constant current driver. In addition, FET transistor Q1 is initially turned OFF.
According to an embodiment of the present invention, diode D1 is provided because otherwise capacitor C1 could also turn ON the FET Q1 and the current limiting circuit 105 could oscillate. Also, due to providing diode D1, the capacitor C1 can only turn OFF the FET Q1, and as a result, a substantially stable operation of said circuit 105 can be achieved. In addition, resistor R3 across diode D1 is used to reset the capacitor C1 voltage after each operation (i.e., after each event, in which LED load 110 is connected to the output and the current surge through said LED load 110 is limited by current limiting circuit 105). In addition, resistor R2 is used to absorb voltage leakage through zener diode ZD1, which might otherwise cause n-p-n transistor T1 to turn ON, when this is not intended. Such, resistor R2 can have a value of 1MΩ, for example. Further, 6.8V zener diode ZD2 enables limiting the voltage on the gate of transistor Q1 to a predefined level (the level that is considered to be a “safe” level, such as 5V to 20V).
According to another embodiment of the present invention, a terminal of resistor R4 is connected to the predefined power rail (e.g., 59 Volts rail), which feeds the LED driver 101. This ensures that transistor Q1 is turned ON in the final equilibrium state (level), substantially preventing any power dissipation in resistor R5, which in turn can be, for example, a 100Ω resistor. According to another embodiment of the present invention, said above terminal of resistor R4 is connected to a terminal of switch S1 instead of said 59V rail. It should be noted that according to this embodiment, the current limiting circuit 105 may not be used with a single LED as load 110, because there may be not enough voltage to properly turn ON the gate of transistor Q1.
It should be noted that according to an embodiment of the present invention, current limiting circuit 105 reacts relatively fast to substantially any current surge, and does not involve continuous power dissipation. In addition, it should be noted that the current passes through LED 110 load in relatively small and brief pulses, in excess of the normal current, until the normal current is achieved (in a substantially steady way). Also, when LED load 110 is connected, then the excess current that passes to said LED load 110 for an initial period of time, before commencing the normal current level (such as 0.5 Amperes), is substantially low and can be, for example, no more than twice said normal current level. Finally, after a series of pulses, which can last for example, two milliseconds, transistor Q1 is permanently switched ON, because the rail voltage has become substantially steady at a voltage below 47V. The maximum excess current surge can be as low as a hundred milliAmperes. Further, it should be noted that according to an embodiment of the present invention at least one resistor (such as resistor R5) is connected in series with an output port of current limiting circuit 105 to limit the initial current flow out of LED driver 101 and into LED load 110. The value of said resistor R5 is chosen so that the initial current which flows approximates the intended LED driver 101 current. It is this current through R5 that starts discharging the output capacitor of the LED driver 101 until it gets down to a voltage below 47V, after which point transistor Q1 starts turning ON. In the steady state, resistor R5 is permanently shorted by a switch (such as transistor Q1), when LED load 110 is connected to said current limiting circuit 105. As a result, the power dissipation of system 100 (
Below is presented a table (Table 1) with sample characteristics of electronic components of the current limiting circuit, according to an embodiment of the present invention.
It should be noted that according to an embodiment of the present invention, transistors Q1 and Q2 can be, for example, IGBT (Insulated Gate Bipolar Transistor), MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or any other bipolar or field effect transistors.
While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be put into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.
