Abstract:
A light emitting diode retrofit lamp for replacement of fluorescent lamp or tube driven by fluorescent ballast is provided. The retrofit lamp consists of an enhanced circuit that overcomes the compatibility issues with the fluorescent ballasts. The circuit of the retrofit lamp comprises a deflickerizer circuit that prevents the flicker in the lamp when the lamp operates in the frequency range of 0.1 Hz to 60 Hz. The deflickerizer circuit further provides a method to align frequency with the ballast frequency and maintains phase to avoid low frequency interference.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a circuit level enhancement for retrofit light emitting diode (LED) lamps, and more particularly to a drive circuit for LED retrofit lamps to prevent flickering in the LED lamps operating on AC ballasts. 
       BACKGROUND 
       [0002]    With the recent evolution in the lighting industry, the solid state lighting lamps based on LED technology are used to replace existing fluorescent and high intensity discharge (HID) lamps. However, the main concern for replacing the existing lamp with the LED lamp was the considerable labor costs involved in the installation, because people need to open the light fixture to disassemble the existing ballast, either it be an electronic one, or a magnetic one. To overcome this concern, various retrofit LED lamps were proposed and used that were compatible with the existing ballast, thus eliminating the need for a skilled person to remove part of the existing lighting fixtures. These retrofit LED lamps use a circuit with a bridge rectifier that converts the AC waveform generated by the electronic or magnetic ballast to the DC waveform for the LEDs. However there are certain compatibility issues with the LED retrofit lamp and a few models of electronic ballasts, especially with the older models. 
         [0003]    The LED retrofit lamps are designed to reduce energy consumption and to pull less power from the ballast. The ballast is stable up to 30-50% of their designated load. However when the load is further reduced the power factor controller (PFC) circuit is unable to maintain stable DC bus and oscillations are introduced resulting in flickering of the lamp. 
         [0004]    Another issue that arises is the presence of audible noise that comes from the magnetic ballast when the dimming of the LED array is carried out by the lamp circuit that operates at a frequency other than the mains AC frequency of 50 or 60 Hz. 
         [0005]    In order to overcome the aforementioned incompatibility issues, the present invention provides an improved LED retrofit lamp with circuit level enhancement. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In a first aspect of the present invention, a LED (light emitting diode) retrofit lamp with a deflickerizer circuit is provided. The deflickerizer circuit measures the current being delivered to the LED retrofit lamp and uses a FET (field-effect transistor) to short out the array with a PWM signal at approximately 1 kHz. The PWM (pulse-width modulation) circuit is able to adjust its duty cycle in real time in response to changes in ballast current and thus back out the visible flicker that would otherwise be seen by the observer. 
         [0007]    In a second aspect of the present invention, a method for aligning phase and frequency to the ballast frequency is provided. When the deflickerizer circuit is used to dim the LED array, it is necessary to align frequency to the ballast frequency to avoid audible noise coming out of the ballast. Once frequency is aligned it is also necessary to maintain phase to avoid a low frequency interference wave which may show up as flicker to the user. The method comprises: completing an edge detection of the incoming waveform to measure frequency; calculating the required PWM duty cycle as a function of external dimming signal, temperature dimming and deflickerizer dimming; asserting the FET voltage for the required period in order to create the designed output level; repeating with each cycle of the magnetic ballast. 
         [0008]    In a third aspect of the present invention, an enhanced circuit that enables reactive coupling of the LED retrofit lamp with the ballast is provided. For 60 kHz lamp, the enhanced circuit comprises a large inductive element 100s of mH and for 25 kHz lamp, a capacitor at the input end. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The preferred embodiment of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the scope of the invention, wherein like designation denote like element and in which: 
           [0010]      FIG. 1  illustrates a schematic representation of LED retrofit lamp with enhanced circuit component in accordance with an embodiment of the present invention. 
           [0011]      FIG. 2  illustrates a driving circuit for LED retrofit lamp with a deflickerizer circuit in accordance with an embodiment of the present invention. 
           [0012]      FIG. 3  illustrates a LED retrofit lamp circuit with a reactive coupling to the ballast in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a thorough understanding of the embodiment of invention. However, it will be obvious to a person skilled in art that the embodiments of invention may be practiced with or without these specific details. In other instances well known methods, procedures and components have not been described in details so as not to unnecessarily obscure aspects of the embodiments of the invention. 
         [0014]    Furthermore, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without parting from the spirit and scope of the invention. 
         [0015]    Embodiments of the present invention provide an enhanced circuit for LED retrofit lamp. The LED retrofit lamp is compatibility with all types of existing fluorescent and HID ballast. The enhanced circuit contains a deflickerizer circuit, a means for reactively coupling the LED lamp with the ballast and a means for aligning frequency during the deflickerizer dimming. The deflickerizer prevents the flicker generated in the ballast at 0.1 Hz to 60 Hz from being visually apparent at the LED lamp. The reactive coupling couples the lamp to the reference ballast so as to enable it to draw less power, less current and less voltage using a combination of capacitive and inductive sources. 
         [0016]      FIG. 1  illustrates a schematic representation of LED retrofit lamp with enhanced circuit component in accordance with an embodiment of the present invention. The LED lamp has a plurality of pins F 1 , F 2 , F 3  and F 4  at the terminals of the lamp. The pluralities of pins are connected to the first terminal connection and second terminal connection of the ballast to draw the power from the ballast. The enhanced circuit  100  of the retrofit LED lamp comprises a bridge rectifier  102  connected to the plurality of pins and receives the input from AC (alternating current) ballast to convert the AC current to the DC (direct current) volt supply. In an embodiment, the bridge rectifier is made of schottky diodes. The electronic ballasts are typically made up of a two staged architecture. A front end which converts an incoming AC waveform to a high voltage DC constant voltage bus, then a back end which drives a constant current AC output using this bus as a source. The front end bus has active power factor control (PFC) to enable it to provide a high power factor (typically greater than 0.9) and low THD (total harmonic distortion) (typically &lt;20%) to the AC circuit. However, this PFC circuit has a limited dynamic range. Since the LED retrofit lamp is designed for reduced energy consumption and thus pulling less power form the ballast, therefore the ballast has to provide output less than their designated load. Most ballasts are stable down to 30-50% of their designated load, but below this load, oscillations are introduced into the DC bus which flows through the AC output stage and appears as flicker in the range of 0.1 Hz to 60 Hz. To prevent these flickers a deflickerizer circuit  104  is provide in the LED retrofit lamp circuit. The deflickerizer  104  measures the current being delivered to the lamp and uses a FET to short out the LED array  106  with a PWM signal at approximately 1 kHz. The PWM circuit is able to adjust its duty cycle in real time in response to changes in ballast current and thus back out the visible flicker that would otherwise be seen by the observer. 
         [0017]      FIG. 2  illustrates a circuit for LED retrofit lamp with a deflickerizer in accordance with an embodiment of the present invention. The AC ballast has a first bus having active power factor control (PFC) that enables it to provide a high power factor and low THD to the circuit. However LEDs are designed to consume less power and therefore less power is sourced out from the ballast that makes the PFC circuit unable to maintain stable bus and hence oscillations are introduced resulting in the flicker of the LED lamp. The deflickerizer circuit  104  comprises a plurality of operational amplifiers  202 , a low drop out linear regulator  204  and a MOSFET (metal-oxide-semiconductor field-effect transistor)  206 . A plurality of resistors and capacitors are also integrated in the circuit for providing the functionality to the deflickerizer circuit  104 . The low dropout linear regulator  204  features low input voltage capability and adjustable low output voltage while exhibiting good PSRR and transient response. The LED retrofit lamp comprises a LED array  106  having a multiple string of LEDs arranged in a parallel. The LEDs string is short with the deflickerizer circuit  104  where the input from the shorted LED is feed into the low dropout linear regulator  204 . The plurality of operational amplifier  202  generates a high output potential. Furthermore, the deflickerizer circuit  104  comprises a plurality of resistors and capacitors arranged in series or parallel with the low dropout linear regulator  204  and the pluralities of operational amplifiers  202 . The deflickerizer circuit  104  measures the current being supplied to the array  106  by the ballast using the operational amplifiers  202  and the low dropout linear regulator  204 . The deflickerizer circuit  104  after measuring the current being supplied to LED lamp uses the MOSFET  206  to short out the LED array  106  with a PWM signal at approximately 1 kHz. The PWM circuit is able to adjust its duty cycle in real time in response to changes in ballast current and thus back out the visible flicker that would otherwise be seen by the observer. Although this has been seen on older, programmed start ballasts, however this issue seems to be fundamental to any ballast with active PFC including: all fluorescent and HID electronic ballasts. 
         [0018]    UL™ (Underwriters Laboratory™ a certification agency)) certifies LED replacement lamps as equivalent to the ANSI™ (standards agency) standard if the LED replacement lamp can operate within the power, current and voltage envelop of the lamp they are replacing. The lighting industry developed a series of reference ballasts to represent the performance of magnetic (60 Hz) and electronic (25 Hz) ballast. The reference ballast consists of a constant voltage source with a series resistance and reactance to attempt to replicate the performance of a more complex and varied system. The ANSI standard lamp can operate within the power, current and voltage envelop of the lamp they are replacing. In order to satisfy this requirement the lamp must be reactively coupled to the reference ballast to enable it to draw less power, less current and less voltage using a combination of capacitive and inductive sources. This is done by providing a reactive coupling to the input circuit of LED retrofit lamp. 
         [0019]      FIG. 3  illustrates a LED retrofit lamp circuit with a reactive coupling to the ballast in accordance with an embodiment of the present invention. The reactive coupling comprises a TRIAC  302  driven by a SIDAC (silicon diode for alternating current)  306  and a current limiting resistor, wherein the main terminal of the TRIAC (triode for alternating current)  302  is connected to the bridge rectifier  102  and to the SIDAC  306 . The SIDAC  306  is receiving the input voltage from the first terminal connection  314  of the ballast and the voltage of SIDAC  306  is set to approximately 220V DC. The gate terminal of TRIAC  302  is connected to a Zener diode  312  that regulates the flow of current across the TRIAC  302 . The Zener diode  312  receives the current from second input terminal connection  316  of the ballast and conducts the current to the gate terminal of the TRIAC  302  when the current reaches a threshold value. For low frequency ballast, i.e. ballast operating at 60 Hz frequency, the SIDAC  306  triggers at a threshold frequency which causes the TRIAC  302  to be open circuit for approximately 25% of the half cycle reducing current flow in the ANSI reference ballast with its 60 Hz sinusoidal waveform @ 220V rms. In an actual magnetic ballast, voltage increases much more rapidly when the TRIAC is open circuit allowing it to exceed 220V within about 15 micro second (0.1% of the half cycle) making its impact inconsequential. 
         [0020]    For high frequency ballast that operates at frequency of 25 kHz, the Zener  312  voltage is set to a level that the 220V rms ANSI ballast will not achieve (i.e. &gt;315V) and put a capacitor  304  in parallel to the TRIAC  302  such that the TRIAC remains open circuit and the conducting current can be controlled with the parallel capacitor  304 ; in a real ballast, the input voltage exceeds 315V, the TRIAC is turned on and a separate conduction path is enabled. A capacitor  308  is placed in series with each of the input terminal connection of the lamp and a resistor  310  is used to short the two pins of single phase. 
         [0021]    In another embodiment of the present invention a method to align frequency and phase to avoid audible noise and flicker is provided. When working with magnetic ballasts to enable dimming using the deflickerizer circuit, it is necessary to align frequency to the ballast frequency (typically 50 or 60 Hz) to avoid audible noise coming out of the ballast. Once frequency is aligned it is also necessary to maintain phase to avoid a low frequency interference wave which may show up as flicker to the user. The method for aligning the frequency and phase using deflickerizer circuit comprises: completing an edge detection of the incoming waveform to measure frequency; calculating the required PWM duty cycle as a function of external diming signal, temperature diming and deflickerizing dimming; asserting the FET voltage for the required period in order to create the designed output level; repeating with each cycle of the magnetic ballast. 
         [0022]    The foregoing merely illustrates the principles of the present invention. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used advantageously. Any reference signs in the claims should not be construed as limiting the scope of the claims. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous techniques which, although not explicitly described herein, embody the principles of the present invention and are thus within the spirit and scope of the present invention. All references cited herein are incorporated herein by reference in their entireties.