Abstract:
A linear light-emitting diode (LED)-based solid-state lamp comprises an LED driving circuit, LED arrays, at least one rectifier, and an operation monitoring module. The LED driving circuit comprises a current sensing device that is originally used to precisely control an electric current to flow into the LED arrays. The operation monitoring module uses the same current sensing device in a way that it detects an electric shock current and determines if the linear LED-based solid-state lamp is operated in a normal mode or in an electric shock hazard mode. When an electric shock hazard is identified, the operation monitoring module shut off a return current flow from the LED arrays to reach the at least one rectifier, thus eliminating an overall through-lamp electric shock current. The scheme can effectively reduce a risk of electric shock hazard to users during relamping or maintenance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is part of a continuation-in-part (CIP) application of U.S. patent application Ser. No. 15/362,772, filed 28 Nov. 2016 and currently pending, which is a CIP application of U.S. patent application Ser. No. 15/225,748, filed 1 Aug. 2016 and currently pending, which is a CIP application of U.S. patent application Ser. No. 14/818,041, filed 4 Aug. 2015 and issued as U.S. Pat. No. 9,420,663 on 16 Aug. 2016, which is a CIP application of U.S. patent application Ser. No. 14/688,841, filed 16 Apr. 2015 and issued as U.S. Pat. No. 9,288,867 on 15 Mar. 2016, which is a CIP application of U.S. patent application Ser. No. 14/465,174, filed 21 Aug. 2014 and issued as U.S. Pat. No. 9,277,603 on 1 Mar. 2016, which is a CIP application of U.S. patent application Ser. No. 14/135,116, filed 19 Dec. 2013 and issued as U.S. Pat. No. 9,163,818 on 20 Oct. 2015, which is a CIP application of U.S. patent application Ser. No. 13/525,249, filed 15 Jun. 2012 and issued as U.S. Pat. No. 8,749,167 on 10 Jun. 2014. The above-identified applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to linear light-emitting diode (LED) lamps and more particularly to a linear LED lamp with electric shock current sensing, configured to shut off an accidental LED current to reach ground through a person&#39;s body. 
     Description of the Related Art 
     Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (no hazardous materials used), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. As LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock become especially important and need to be well addressed. 
     In today&#39;s retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, consumers may choose either to adopt a ballast-compatible LLT lamp with an existing ballast used to operate the fluorescent tube or to employ an AC mains-operable LED lamp by removing/bypassing the ballast. Either application has its advantages and disadvantages. In the former case, although the ballast consumes extra power, it is straightforward to replace the fluorescent tube without rewiring, which consumers may have a first impression that it is the best alternative to fluorescent tube lamps. But the fact is that total cost of ownership for this approach is high regardless of very low initial cost. For example, the ballast-compatible LLT lamps work only with particular types of ballasts. If the existing ballast is not compatible with the ballast-compatible LLT lamp, the consumers will have to replace the ballast. Some facilities built long time ago incorporate different types of fixtures, which requires extensive labor for both identifying ballasts and replacing incompatible ones. Moreover, a ballast-compatible LLT lamp can operate longer than the ballast. When an old ballast fails, a new ballast will be needed to replace in order to keep the ballast-compatible LLT lamps working. Maintenance will be complicated, sometimes for lamps and sometimes for ballasts. The incurred cost will preponderate over the initial cost savings by changeover to the ballast-compatible LLT lamps for hundreds of fixtures throughout a facility. When the ballast in a fixture dies, all the ballast-compatible tube lamps in the fixture go out until the ballast is replaced. In addition, replacing a failed ballast requires a certified electrician. The labor costs and long-term maintenance costs will be unacceptable to end users. From energy saving point of view, a ballast constantly draws power, even when the ballast-compatible LLT lamps are dead or not installed. In this sense, any energy saved while using the ballast-compatible LLT lamps becomes meaningless with the constant energy use by the ballast. In the long run, ballast-compatible LLT lamps are more expensive and less efficient than self-sustaining AC mains-operable LLT lamps. 
     On the contrary, an AC mains-operable LLT lamp does not require a ballast to operate. Before use of an AC mains-operable LLT lamp, the ballast in a fixture must be removed or bypassed. Removing or bypassing the ballast does not require an electrician and can be replaced by end users. Each AC mains-operable LLT lamp is self-sustaining. If one AC mains-operable tube lamp in a fixture goes out, other lamps in the fixture are not affected. Once installed, the AC mains-operable LLT lamps will only need to be replaced after 50,000 hours. In view of above advantages and disadvantages of both ballast-compatible LLT lamps and AC mains-operable LLT lamps, it seems that market needs a most cost-effective solution by using a universal LLT lamp that can be used with the AC mains and is compatible with an electronic ballast so that LLT lamp users can save an initial cost by changeover to such a universal LLT lamp followed by retrofitting the lamp fixture to be used with the AC mains when the ballast dies. 
     In the U.S. patent application Ser. No. 14/688,841, filed Apr. 16, 2015, two shock prevention switches and an all-in-one driving circuit are adopted in an LLT lamp such that AC power from either an electronic ballast or the AC mains can operate the lamp without operational uncertainty and electric shock hazards. In other words, no matter what a lamp fixture is configured as the AC mains or an electronic ballast compatible fashion, the LLT lamp automatically detects configurations and works for either one. All of such LLT lamps, no matter whether AC mains-operable or ballast compatible, are electrically wired as double-ended and have one construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps, if no shock prevention scheme is adopted in, always fails a safety test, which measures a through-lamp electric shock current. Because an AC-mains voltage applies to both opposite ends of the tube when connected to a power source, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present a risk of an electric shock during re-lamping. Due to this potential shock risk to the person who replaces the LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL) uses its safety standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do a current leakage test and to determine if the LLT lamps meet the consumer safety requirement. Although the LLT lamps used with an electronic ballast can pass the current leakage test, some kinds of electric shock hazards do exist. Experimental results show that the skin of the person who touches an exposed bi-pin may be burned due to such an electric shock. Fortunately, a mechanism of double shock prevention switches used in applications with the AC mains is also effective in applications with the ballasts to prevent the electric shock from occurring, thus protecting consumers from such a hazard, no matter whether input voltage is from the AC mains or the electronic ballast. Therefore, a universal LLT lamp that can work with either the AC mains or the electronic ballast makes sense. The effectiveness of using double shock prevention switches for applications in the AC mains has been well addressed in U.S. Pat. No. 8,147,091, issued on Apr. 3, 2012. However, a conventional shock prevention switch has an inherent issue related to an electric arc when operated with an electronic ballast. Unlike an AC voltage of 120 or 277 V/50-60 Hz from the AC mains, the output AC voltage and current from the electronic ballast presents a negative resistance characteristic. The feature that originally supports a fluorescent tube to function properly becomes extremely detrimental to the conventional shock prevention switch due to the electric arc likely occurring between two electrical contacts that have a high electric potential difference with a high frequency, such as 600 V/50 kHz. Once a consumer fails to follow installation instructions to install or uninstall linear LED tube lamps such that one of two ends of the tube lamp is in the fixture socket connected to a powered electronic ballast, and the other end is tweaked to connect to or disconnect from the associated socket, an internal arcing may occur between the electrical contacts in the associated switch. The arcing even in a short period such as several seconds can generate high heat, burning and melting electrical contacts and neighboring plastic enclosures, creating a fire hazard. The AC voltage of 120 or 277 V/50˜60 Hz from the AC mains does not have such an issue because its voltage is relatively low compared with the ballast output voltage of 600 V. Moreover, the AC frequency of 60 Hz automatically extinguishes an arc every 1/60 seconds, if existed. That is why a utility switch can be used in an electrical appliance to turn power on and off without any problem. However when used with the electronic ballast, the electrical contacts used in the conventional shock prevention switch can easily be burned out due to the high-voltage and high-frequency arcing introduced between each gap of each pair of the electrical contacts in the conventional shock prevention switch when someone tries to abusively tweak to remove the tube lamp from the fixture with the ballast that has a power on it. Although such a situation is rare, an internal arcing, if occurred, does cause burning and even welding of the electrical contacts and melting of the plastic enclosure, so called internal fire, creating consumer safety issues. 
     Today, such LLT lamps are mostly used in a ceiling light fixture with a wall-mount power switch. The ceiling light fixture could be an existing one used with fluorescent tubes but retrofitted for LLT lamps or a specific LLT lamp fixture. The drivers that provide a proper voltage and current to LED arrays could be internal or external ones. Not like LLT lamps with an external driver that is inherently electric-shock free if the driver can pass a dielectric withstand test used in the industry, LLT lamps with an internal driver could have a shock hazard during relamping or maintenance, when the substantial through-lamp electric shock current flows from any one of AC voltage inputs through the internal driver connecting to LED arrays to the earth ground. Despite this disadvantage, LLT lamps with the internal driver still receive wide acceptance because they provide a stand-alone functionality and an easy retrofit for an LLT lamp fixture. As consumerism develops, consumer product safety becomes extremely important. Any products with electric shock hazards and risk of injuries or deaths are absolutely not acceptable for consumers. However, commercially available LLT lamps with internal drivers, single-ended or double-ended, fail to provide effective solutions to the problems of possible electric shock and internal arcing and fire. 
     In the prior art mentioned above, the double shock protection switches with mechanical actuation mechanisms protruding outwards from both ends of the LLT lamp are proposed to be used in the LLT lamp. However, a length control of the LLT lamp becomes critical to operate the LLT lamp because sometimes the double shock protection switches may not be actuated by the mechanical actuation mechanisms. Also, the conventional LLT lamp is so vulnerable because it may cause internal fire if consumers abusively tweak the mechanical actuation mechanisms at both ends of the LLT lamp operable with an electronic ballast during relamping. It is therefore the purpose of the present disclosure to disclose an electric shock current sensing approach to be used in the LLT lamp to eliminate above-mentioned electric shock and internal fire hazards and to work more reliably to protect consumers. 
     SUMMARY 
     A linear light-emitting diode (LED)-based solid-state lamp comprising two lamp bases respectively connected to two ends of a housing, each lamp base comprising at least one electrical conductor connecting to a lamp fixture socket; at least one rectifier; an LED driving circuit; LED arrays; and an operation monitoring module, is used to replace a fluorescent tube or a conventional LED tube lamp without the operation monitoring module in an existing lamp fixture. The LED driving circuit comprises a current sensing device that is originally used to precisely control an electric current to flow into the LED arrays. The operation monitoring module uses the same current sensing device in a way that it detects an electric shock current and determines if the LED-based solid-state lamp is operated in a normal mode or in an electric shock hazard mode. When an installer touches an exposed at least one electrical conductor on a lamp base, and when an electric shock hazard is identified, the operation monitoring module shut off a return current flow from the LED arrays to reach the at least one rectifier, thus eliminating an overall through-lamp electric shock current. 
     The operation monitoring module comprises an error amplifier, a power up control, a logic control, a switch control section, and at least one switch configured to connect or disconnect the electric current return from the LED arrays. The at least one switch is connected between the LED arrays and the at least one rectifier. When the current sensing device detects an electric shock current that appears at an exposed at least one electrical conductor, the operation monitoring module controls the at least one switch to disconnect the electric current flow on the at least one switch, thus turning off the power delivering to the LED arrays. The logic control in the operation monitoring module maintains the at least one switch in “off” state until the exposed at least one electrical conductor is removed from the installer and normally installed in the lamp fixture socket receiving a normal AC voltage. When the current sensing device detects no electric shock current, the operation monitoring module controls the at least one switch to continue “on”, thus the electric current being able to continue to flow out from the LED arrays. The scheme can effectively reduce a risk of electric shock hazard to users during relamping or maintenance. 
     The LED driving circuit further comprises a Buck control circuit comprising an inductor, a diode, and a switch. The current sensing device can be connected in front of or after the LED arrays and the inductor, in which the current sensing device is respectively at high electric potential side and a low electric potential side along an LED current path. In one embodiment, the operation monitoring module receives a signal from the current sensing device connected at the low electric potential side along the LED current path in the LED driving circuit, in which a low electric potential terminal of the current sensing device is directly connected to the at least one rectifier through the at least one switch in the operation monitoring module. 
     In another embodiment, the operation monitoring module receives a signal from the current sensing device connected at the high electric potential side along the LED current path in the LED driving circuit, in which the low electric potential terminal of the current sensing device is indirectly connected to the at least one rectifier, rather through the inductor, LED arrays, and the at least one switch. Although configurations of the Buck control circuit in two embodiments are different, the current sensing device originally working with the Buck control circuit can effectively provide a detection signal of the electric shock current for the operation monitoring module to process and to shut off the electric shock current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present disclosure are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
         FIG. 1  is an embodiment of an LLT lamp installed in lamp fixture sockets connected with AC power sources according to the present disclosure. 
         FIG. 2  is another embodiment of an LLT lamp installed in lamp fixture sockets connected with AC power sources according to the present disclosure. 
         FIG. 3  is an embodiment of an LED driving circuit configured to detect electric shock current according to the present disclosure. 
         FIG. 4  is another embodiment of an LED driving circuit configured to detect electric shock current according to the present disclosure. 
         FIG. 5  shows two waveforms of a voltage measured across an inductor used in an LED driving circuit when an AC voltage from 285 V AC mains is used to operate an LLT lamp according to the present disclosure. 
         FIG. 6  shows two waveforms of a voltage measured across an inductor used in an LED driving circuit when an AC voltage from 120 V AC mains is used to operate an LLT lamp according to the present disclosure. 
         FIG. 7  shows two waveforms of a voltage measured across a current sensing device used in an LED driving circuit when an AC voltage from 285 V AC mains is used to operate an LLT lamp according to the present disclosure. 
         FIG. 8  shows two waveforms of a voltage measured across a current sensing device used in an LED driving circuit when an AC voltage from 120 V AC mains is used to operate an LLT lamp according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an embodiment of an LLT lamp installed in lamp fixture sockets connected with alternate current (AC) sources according to the present disclosure. The LLT lamp  500  comprises a housing having two ends; two lamp bases  660  and  760  each having at least one electrical conductor  250  and  350  at each end of the housing; an operation monitoring module  700 ; a pair of electrical contacts  410  and  420  of at least one switch  400  controlled by the operation monitoring module  700 ; at least one rectifier  603  comprising diodes  611 ,  612 ,  613 , and  614  interconnected at ports  402 ,  404 ,  503 , and  504 ; an LED driving circuit  100  having a first and a second inputs  503  and  504 ; and LED arrays  214  disposed between the two ends of the housing with the LED arrays  214  connected to the LED driving circuit  100 . The LLT lamp may further comprise an interface module  251  and  351  for each lamp base configured to work with an electronic ballast for maximum compatibility. The interface module may comprise a resistor, a resistor in parallel with capacitor, a jumper, or simply a passing-through connection such as a direct connection between a connection point  401  and the interconnection port  402  for the interface module  251  and a direct connection between a connection point  405  and the interconnection port  404  for the interface module  351 . In the following description, such direct connections will be used for simplicity. The LED driving circuit  100  comprises a Buck control circuit  101  and a current sensing device  107  connected to the Buck control circuit  101 , which is connected to the LED arrays  214 . When the at least one electrical conductor  250  and the at least one electrical conductor  350  in each lamp base are inserted into the lamp fixture sockets  810  and  820 , the at least one rectifier  603  receives AC power through the at least one electrical conductors  250  and  350  at each end of the housing and converts into a DC (direct current) voltage to supply the LED driving circuit  100 . An LED current will flow into the LED arrays  214  and return to the current sensing device  107 , passing through it with a current sensing signal sent through an electrical connection  109  to the operation monitoring module  700 . Because the at least one electrical conductor  250  and the at least one electrical conductor  350  in each lamp base are inserted into the lamp fixture sockets  810  and  820 , the at least one rectifier  603  receives a normal input AC voltage and converts into a DC voltage without a compromise. The Buck control circuit  101  delivers a current equal to a preset value to the LED arrays  214  by using current sensing device  107 . At the same time, and the current sensing device  107  senses a correct current passing through and sends a current sensing signal through the electrical connection  109  to the operation monitoring module  700 . The operation monitoring module  700  then controls the at least one switch  400  through a control link  110  so that the electrical contacts  410  and  420  of the at least one switch  400  are electrically connected. Consequently, the electric current returned from the LED arrays  214  can flow back to the at least one rectifier to complete a power transfer. 
     When either one of the at least one electrical conductor  250  and the at least one electrical conductor  350  in each lamp base is inserted into the lamp fixture sockets  810  or  820  that is connected with “L” of AC mains, the LLT lamp does not light up but is live and energized, meaning that there is an electric shock hazard. If an installer touches the exposed at least one electrical conductor  250  or at least one electrical conductor  350  in each lamp base, an electric shock current can flow from the LED arrays through the electric current sensing device  107 , and the at least one switch  400  to reach the at least one rectifier  603 , further flowing to earth ground through the installer&#39;s body, creating an electric shock hazard. However, when such a situation exists, the at least one rectifier  603  receives a compromised AC voltage according to a divided voltage because a human body is analogous to a 500 ohm-resistor. When a DC voltage provided by the at least one rectifier  603  is not as high as expected, an electric current provided to drive the LED arrays  214  by the Buck control circuit  101  is lower than a preset value, the same as the electric current return from the LED arrays  214 . The sensing device  107  senses a current decrease and sends a signal through the electrical connection  109  to the operation monitoring module  700 , which then controls the at least one switch  400  through the control link  110  to turn off an electrical connection between the electrical contacts  410  and  420  of the at least one switch  400 . Thus the electric shock current is blocked, no substantial leakage current flowing out to the exposed at least one conductor on either lamp base. As can be seen in  FIG. 1 , two sockets in each of the external fixture lamp sockets  810  and  820  are shunted, meaning that as long as the at least one electrical conductor  250  in the lamp base  660  and the at least one electrical conductor  350  in the lamp base  760  connect to the AC power sources, the LLT lamp can operate with an acceptable through-lamp electric shock current, which is deemed safe for users. 
       FIG. 2  is another embodiment of an LLT lamp installed in lamp fixture sockets connected with AC power sources according to the present disclosure.  FIG. 2  is almost the same as  FIG. 1  except that the current sensing device  107  at a low electric potential side depicted in  FIG. 1  is arranged at a high electric potential side in  FIG. 2 . In this case, a DC current supplied by the at least one rectifier  603  passes through the current sensing device  107  before going into the LED arrays  214 . Same as in  FIG. 1 , when the DC voltage provided by the at least one rectifier  603  is not as high as expected due to a compromised input voltage in the electric shock current hazard, the electric current provided to drive the LED arrays  214  by the Buck control circuit  101  is lower than a preset value, the same as the electric current return from the LED arrays  214 . The current sensing device  107  senses a current decrease and sends a signal through the electrical connection  109  to the operation monitoring module  700 , which then controls the at least one switch  400  through the control link  110  to turn off an electrical connection between the electrical contacts  410  and  420  of the at least one switch  400 . Thus the electric shock current is blocked, no substantial leakage current flowing out to the exposed at least one conductor on either lamp base. 
       FIG. 3  is an embodiment of an LED driving circuit configured to detect electric shock current according to the present disclosure. The at least one rectifier  603  connecting to an AC power source, either the AC mains or an electronic ballast, converts an AC into a DC voltage. The LED driving circuit  100  connecting to the at least one rectifier  603  comprises an input filter  102  used to filter the input voltage and to suppress EMI noise created in the LED driving circuit  100 , a power factor correction (PFC) and control device  103 , a Buck converter  200  in communicating with the PFC and control device  103 , a switch  201  controlled by the PFC and control device  103 , an output capacitor  105  in parallel with a resistor  106  connected to the Buck converter  200  to build up an output voltage and to power the LED arrays  214 , a current sensing device  107 , and a voltage feedback module  300  extracting partial energy from the output voltage to sustain the PFC and control device  103 . The at least one rectifier  603  has four input/output ports, among which a high electric potential appears at the input/output port  503 , and a low electric potential appears at the input/output port  504  respectively connecting to the high side and the low side of the input filter  102  with the low electric potential port  504  as a common ground. 
     In  FIG. 3 , when the power is on, an input current enters the input filter  102  and then the PFC and control device  103 , turning on the switch  201 . Whereas the diode  202  is reverse-biased, the input current goes from the resistor  106  and the LED arrays  214 , a primary winding of the transformer  206 , the switch  201 , and the current sensing device  107  to the common ground  504 . The primary winding of the transformer  206  serves as an inductor. When the input current goes into the primary winding of the transformer  206 , energy is stored in it. The PFC and control device  103  detects the input voltage level and control the switch  201  on and off in a way that a desired or otherwise predetermined output voltage V o  across the LED arrays  214  is reached to light up the LED arrays  214 . When the switch  201  is off, the diode  202  is forward-biased, and the primary winding of the transformer  206  releases the energy stored, resulting in a loop current flowing from the diode  202  and the LED arrays  214 , back to the primary winding of the transformer  206 , completing the energy transfer to the LED arrays  214 . When the switch  201  is on, the input current flows into the LED arrays  214 , the primary winding of the transformer  206 , the switch  201 , and the current sensing device  107 , creating a voltage drop across the current sensing device  107 . The voltage appearing at the port  204  of the current sensing device  107  inputs to the PFC and control device  103  to control the off-time of the switch  201 . The voltage feedback module  300  has two connection ports  301  and  302 , with the first connection port  301  connecting to a high side of a secondary winding  207  in the transformer  206  and with the second connection port  302  connecting to the PFC and control device  103 . The voltage feedback module  300  continuously monitors the output voltage by using the secondary winding  207  in the transformer  206 . When the voltage at the high side of the secondary winding  207  is higher than a becoming lower operating voltage in the PFC and control device  103  due to increased internal operations, the diode (not shown) in the voltage feedback module  300  conducts to supply energy in time through the second connection port  302  to sustain the operating voltage in the PFC and control device  103 . 
     In  FIG. 3 , the LED driving circuit  100  is further connected to the operation monitoring module  700  for electric shock current detection. The operation monitoring module  700  comprises a peak detector  505 , an error amplifier  506  with an output connected to an output capacitor  507 , a logic control  508 , a power up control  509 , a switch control section  510 , and at least one switch  400  configured to connect or disconnect the electric current return from the LED arrays  214 . The peak detector  505  receives a signal from the first port  204  of the current sensing device  107  and feeds the error amplifier  506 . The error amplifier  506  then generates an error signal associated with a measured voltage from the peak detector  505  and a preset voltage V t  and sends the error signal to the logic control  508 , subsequently controlling the switch control section  510  to control the at least one switch  400  to switch on when an electric shock current is not detected or to switch off when an electric shock current is detected. The power up control dictates the switch control  510  to turn on the at least one switch  400  only in a short period when power is on no matter whether an input voltage is normal or compromised due to the electric shock current. After the short power-up period, the logic control  508  takes over the switch control  510  to turn the at least one switch  400  on or off based on the error signal generated. The logic control comprises a one-bit memory to latch the at least one switch  400  in a way that the at least one switch  400  will remain on if the electric shock current is not detected and remain off if the electric shock current is detected in the short power-up period. This function ensures that the LLT lamp can operate more reliably without flickering when an input voltage accidentally becomes lower than a standard line voltage due to possible power grid fluctuations in a long time. 
       FIG. 4  is another embodiment of an LED driving circuit configured to detect electric shock current according to the present disclosure.  FIG. 4  has all the components as in  FIG. 3 , except that interconnections are different, that the current sensing device  107  is at a high electric potential side rather than at the low electric potential side as in  FIG. 3 , and that a center-tapped inductor  203  in  FIG. 4  replaces the transformer  206  in  FIG. 3 . In  FIG. 4 , the same numerals are used for the same components as in  FIG. 3 . In  FIG. 4 , the Buck converter  200  comprises a switch  201  controlled by the PFC and control device  103 , a diode  202 , and an inductor  203  with its current charging and discharging controlled by the switch  201 . The PFC and control device  103  detects zero current in the inductor  203  within an AC cycle of an input voltage generating a zero current detection signal and controls the switch  201  on and off with a constant on-time and a varied off-time controlled by the zero current detection signal. By adapting switching frequencies for a high frequency associated with a ballast and a low frequency associated with the AC mains, the PFC and control device  103  controls the switch  201  on and off in a way that the inductor  203  is charged during on-time and discharged during off-time, and that a desired or otherwise predetermined output voltage V o  across the LED arrays  214  is reached to light up the LED arrays  214 . The average inductor current is thus equal to the output current that flows into the LED array  214 . When the switch  201  is on, the diode  202  is reverse-biased, and an input current flows from an output port  108  in the input filter  102 , the switch  201 , the first port  204  of the current sensing device  107 , the current sensing device  107  itself, and the second port  205  of the current sensing device  107 , into the inductor  203 . When the current flowing into the inductor  203  increases, the voltage across the current sensing device  107  increases. The first port  204  of the current sensing device  107  also connects with the PFC and control device  103 , which continuously receives signals and adjusts the off-time such that the output voltage and current to the LED arrays  214  are regulated to meet the output requirements. The output capacitor  105  in parallel with the resistor  106  connects to the inductor  203 , receiving energy to build up an output voltage and to power the LED arrays  214 . 
     The inductor  203  configured as an autotransformer has a center-tapped port connecting to the voltage feedback module  300  comprising a diode. The voltage feedback module  300  has two connection ports  301  and  302 , with the first connection port  301  connecting to the center-tapped port of center-tapped inductor  203  and with the second connection port  302  connecting to the PFC and control device  103 . The PFC and control device  103  has an input capacitor (not shown) with a voltage built up to supply an internal logic control circuit (not shown) in the PFC and control device  103 . When the voltage decreases due to its increased internal operations and controls, and when the voltage at the center-tapped port of the inductor  203  is higher than the supplying voltage, the diode in the voltage feedback module  300  conducts to supply a current to the PFC and control device  103  and sustain its operations. The function of the voltage feedback module  300  is essential because the LED driving circuit  100  has a wide range of operating voltages not only 110 and 277 VAC for AC mains but also 350˜600 VAC for an electronic ballast. In the PFC and control device  103 , a start-up resistor (not shown) is so designed to operate a LLT lamp at the lowest input voltage 110 VAC. When the highest voltage 600 VAC from the electronic ballast comes in, a higher proportional voltage appears at an input of the internal logic control circuit. Therefore an operating voltage for the internal logic control circuit must be in a wide range such as 11˜35 VDC rather than 5˜15 VDC as in a conventional logic control device. To meet requirements of start-up time and current without turn-on failure or flickering occurred at the lamp start-up, the input capacitor in the PFC and control device  103  with a minimum capacitance is designed and used at the input of the internal logic control circuit. The voltage feedback module  300  is thus needed to pump in energy in time and to sustain the operating voltage and ensure no flickering occurred when operating the LLT lamp. 
     When the switch  201  is off, the diode  202  is forward-biased, and the inductor  203  discharges with a loop current flowing from the LED arrays  214 , the diode  202 , the current sensing resistor  107 , back to the inductor  203 . The current sensing resistor  107  keeps track of the output current and feedbacks to the PFC and control device  103  to further control the switch  201  on and off. The closed loop operation in both on-time and off-time of the switch  201  ensures the output current to be accurately controlled within 4%. 
     In  FIG. 4 , the LED driving circuit  100  is also connected to the operation monitoring module  700  for electric shock current detection as in  FIG. 3 . Similarly, the operation monitoring module  700  detects if the electric shock current exists for a short period when power is on. If the electric shock current is detected, the operation monitoring module  700  controls the at least one switch  400  to turn off, thus blocking the electric shock current to flow to the earth ground through the installer&#39;s body. On the other hand, if the electric shock current is not detected for a short period when power is on, the operation monitoring module  700  controls the at least one at least one switch  400  to turn on, thus allowing the current return from the LED arrays  214  to reach the earth ground and completing an energy transfer to the LED arrays  214  for lighting. 
       FIG. 5  shows two waveforms of a voltage measured across an inductor used in an LED driving circuit  100  when an AC voltage from 285 V AC mains is used to operate an LLT lamp according to the present disclosure. As mentioned above, when the installer touches an exposed at least one conductor on one end of the LLT lamp with the at least one conductor on the other end of the LLT lamp installed in a fixture socket with “L” of the AC mains, the at least one rectifier  603  (in  FIG. 1 ) receives a compromised AC voltage according to a divided voltage because a human body is analogous to a 500 ohm-resistor. When a DC voltage provided by the at least one rectifier  603  is not as high as a normal voltage, the electric current provided to drive the LED arrays  214  by the Buck control circuit  101  is lower than a preset value, the same as the electric current return from the LED arrays  214 .  FIG. 5  shows two inductor voltage waveforms measured when the Buck control circuit  101  (in  FIG. 1 ) is operated in a normal voltage mode and in an electric shock hazard mode for a line voltage of 285 V from the AC mains. The peak-to-peak inductor voltage swing in  FIG. 5  represents a rectified DC voltage. As can be seen, the waveform  801  in the normal voltage mode shows a higher inductor voltage than that of the waveform  802  in the electric shock hazard mode. In addition, the Buck control circuit  101  adjusts the switch off-time  803  to be shorter than the off-time  804  in the normal voltage mode to cope with such a lower input voltage in an electric shock hazard mode. 
       FIG. 6  shows two waveforms of a voltage measured across an inductor used in an LED driving circuit  100  when an AC voltage from 120 V AC mains is used to operate an LLT lamp according to the present disclosure.  FIG. 6  shows two inductor voltage waveforms measured when the Buck control circuit  101  (in  FIG. 1 ) is operated in a normal voltage mode and in an electric shock hazard mode for a line voltage of 120 V from the AC mains. As can be seen, the waveform  901  in the normal voltage mode shows a higher inductor voltage than that of the waveform  902  in the electric shock hazard mode. Similarly, the Buck control circuit  101  adjusts the switch on-time  903  to be longer than the one-time  904  in the normal voltage mode and adjusts the switch off-time  905  shorter than the off-time  906  in the normal voltage mode to cope with such a lower input voltage in an electric shock hazard mode. 
       FIG. 7  shows two waveforms of a voltage measured across an electric current sensing device (e.g., current sensing device  107 ) used in an LED driving circuit  100  when an AC voltage from 285 V AC mains is used to operate a universal LLT lamp according to the present disclosure. The voltage across the current sensing device  107  corresponds to an inductor charging current also representing a peak LED current. As can be seen, two voltage waveforms measured are different when the Buck control circuit  101  (in  FIG. 1 ) is operated in a normal voltage mode and in an electric shock hazard mode for a line voltage of 285 V from the AC mains. The peak sensing voltage  805  in the normal voltage mode is higher than the peak sensing voltage  806  in the electric shock hazard mode. To cope with such a lower input voltage in an electric shock hazard mode, the Buck control circuit  101  adjusts the inductor discharging time  807  to be shorter than the inductor discharging time  808  in the normal voltage mode. 
       FIG. 8  shows two waveforms of a voltage measured across a current sensing device (e.g., current sensing device  107 ) used in an LED driving circuit  100  when an AC voltage from 120 V AC mains is used to operate an LLT lamp according to the present disclosure. As can be seen, two voltage waveforms measured are different when the Buck control circuit  101  (in  FIG. 1 ) is operated in a normal voltage mode and in an electric shock hazard mode for a line voltage of 120 V from the AC mains. The peak of a sensing voltage  907  in the normal voltage mode is higher than the peak of a sensing voltage  908  in the electric shock hazard mode. To cope with such a lower input voltage in an electric shock hazard mode, the Buck control circuit  101  adjusts not only the inductor charging time  909  to be longer than the inductor charging time  910  in the normal voltage mode but also discharging time  911  to be shorter than the inductor discharging time  912  in the normal voltage mode. Therefore, the voltage across an electric current sensing device can be used to detect the electric shock current and feed through the operation monitoring module to further eliminate the electric shock current. 
     In  FIGS. 1 and 2 , the electrical contacts  410  and  420  of the at least one switch  400  may be an electrical, an electronic, an electro-mechanical, or a mechanical switch such as one in a solid-state relay, an electronic relay, an electro-mechanical relay, a pair of mechanical contacts, or other bidirectional and unidirectional current control devices such as a triac, a back-to-back thyristor, a silicon-controlled rectifier (SCR), a transistor, a metal-oxide-semiconductor field-effect transistor (MOSFET), a field-effect transistor (FET), a transistor, or various combinations thereof. Also such devices may be connected with some snubber circuits to maintain their functionality under voltage spikes. 
     In  FIGS. 3 ˜ 4 , although the preset voltage V t  to the error amplifier is assumed to be independent of the input voltages, in some cases, there may be an additional voltage detection circuit configured to determine if an input voltage is in a range of 100-120 V or 270-285 V from AC mains, or in a range of 350˜600 V from an electronic ballast. The voltage detection circuit thus provides a desired preset voltage to the error amplifier. 
     Whereas preferred embodiments of the present disclosure have been shown and described, it will be realized that alterations, modifications, and improvements may be made thereto without departing from the scope of the following claims. Another kind of the shock prevention schemes in an LED-based lamp using various kinds of combinations to accomplish the same or different objectives could be easily adapted for use from the present disclosure. Accordingly, the foregoing descriptions and attached drawings are by way of example only, and are not intended to be limiting.