Abstract:
An LED driver is provided with transient protection circuitry to satisfy dielectric testing requirements. An embodiment of the driver includes a fuse coupled between an AC input and a DC power source, and first and second electrolytic capacitors coupled across the DC power source. Respective circuits including a transient voltage suppressor and a current limiting resistor are coupled in series across each of the electrolytic capacitors. The transient voltage suppressors have breakdown voltage thresholds slightly below a full rated output for the DC source, wherein a sensed short condition across one of the first and second electrolytic capacitors causes a short circuit across the DC source and thereby disabling of the LED driver prior to failure of the other electrolytic capacitor. The current limiting resistors further are configured to avoid causing the LED driver to be disabled for transient overvoltage conditions in which neither of the electrolytic capacitors is shorted.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 62/145,045, filed Apr. 9, 2015, and which is hereby incorporated by reference. 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to LED power supplies. More particularly, the present invention relates to an LED driver having transient protection circuitry designed to satisfy dielectric testing requirements. 
     A conventional LED power supply  10 , an example of which is represented in  FIG. 1 , may typically include a line fuse  12  and electromagnetic interference (hereinafter “EMI”) filtering circuitry  13  provided between a sinusoidal AC source  11  and an input rectifier  14 . The rectifier and a subsequent power factor correction (hereinafter “PFC”) stage  15  convert an AC input to a high voltage DC output, further regulating the power factor and total harmonic distortion. A DC-DC converter stage  16  receives the high voltage DC output from the PFC stage, and is further configured to regulate the output voltage and current to a load  17 . 
     Energy storage elements are conventionally used in DC-DC power converters for filtering, or “smoothing,” pulsating DC input from a rectifier or PFC stage by absorbing peak currents and ripple currents while providing a relatively constant DC voltage output. For a 120-277V input driver, the output voltage of the PFC stage is typically around 470V DC. For a 347V input driver, the output voltage of the PFC stage is around 600V. Because of the relatively high voltages involved, first and second electrolytic capacitors C 1 , C 2  are often provided as energy storage elements at the output of the PFC stage. In the particular case of a 470V output, two 250V electrolytic capacitors may be used, whereas for the 600V output a pair of 350V electrolytic capacitors may be implemented. These electrolytic capacitors are connected in series to satisfy the voltage rating requirement. 
     Such conventional configurations are potentially susceptible to failures that may occur where one of the electrolytic capacitors is shorted. 
     In one particular and contemporary example, all LED power supplies must be designed to pass an abnormal component fault test as administered according to the UL 8750 standard. One part of this abnormal component fault test involves shorting one of the output electrolytic capacitors, after which the respective power supply must pass the UL dielectric test (i.e., a leakage current test between power source and earth ground). 
     However, when one of the two electrolytic capacitors is shorted, all of the DC output voltage from the PFC stage will be applied across a single electrolytic capacitor. As a result, the voltage rating of the single electrolytic capacitor would be greatly exceeded, assuming a practical voltage rating as previously noted. Further assuming that the increased voltage across the remaining electrolytic capacitor does not exceed a rating for the fuse  12 , the fuse will not open or otherwise prevent subsequent electrolytic capacitor failure, which in turn causes the electrolytic capacitor to blow up and/or substantially discharge the liquid electrolyte included therein. The electrolyte is conductive and may subsequently short the circuit on an associated PCB, which could further form a short circuit to earth ground or the device enclosure. The power supply would accordingly fail the UL leakage test due to this short circuit, as caused by the exploded electrolytic capacitor. 
     It would therefore be desirable to provide an LED driver with circuitry that prevents total electrolytic capacitor failure in the event that a short condition is sensed in a corresponding electrolytic capacitor. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with embodiments of a LED driver having transient protection circuitry as disclosed herein, one solution for the aforementioned problem is to force the fuse to blow up immediately after one of the two electrolytic capacitors is shorted. Such a solution as offered herein may preferably allow the LED driver configuration to pass the UL 8750 abnormal component fault test. 
     In one embodiment, an LED driver as disclosed herein includes a fuse coupled between an AC input and a DC power source, and first and second energy storage elements coupled across the DC power source. A DC-DC converter is further coupled across the DC power source, between the energy storage elements and a load. Transient protection circuits are coupled across each of the energy storage elements and configured upon sensing a short condition across one of the first and second energy storage elements to disable the LED driver prior to failure of the other energy storage element. 
     In one aspect, the transient protection circuitry may be configured to disable the LED driver by causing the fuse or other circuit interrupter to open. In one embodiment for implementing this aspect, the LED driver may be disabled by causing a short condition across the DC power source, thereby opening the fuse. 
     In another aspect, the energy storage elements may include electrolytic capacitors coupled in series across an output end of the DC source. 
     In another aspect, the respective transient protection circuits may include a transient voltage suppressor and a current limiting resistor coupled in series across each of the electrolytic capacitors. The transient voltage suppressors may have breakdown voltage thresholds slightly below a full rated output for the DC source, wherein a sensed short condition across one of the first and second electrolytic capacitors causes a short circuit across the DC source, thereby disabling the LED driver prior to failure of the other electrolytic capacitor. The current limiting resistors may be configured to avoid causing the LED driver to be disabled for transient overvoltage conditions in which neither of the electrolytic capacitors is shorted. 
     In another aspect, the DC power source may include a bridge rectifier coupled to the AC input and a PFC circuit coupled between the bridge rectifier and the energy storage elements. The voltage suppressors may be Zener diodes having respective breakdown voltage thresholds of &gt;50% and &lt;100% for a full rated output voltage value of the PFC circuit. 
     In another aspect, the transient protection circuitry may further have first and second current limiting resistances being respectively coupled in series with the first and second transient voltage suppressors. 
     In various embodiments, the transient protection circuitry may further or in the alternative be configured to avoid causing the LED driver to be disabled for transient overvoltage conditions in which neither of the first and second energy storage elements is shorted. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram representing a conventional LED driver configuration. 
         FIG. 2  is a circuit diagram representing an embodiment of an LED driver according to the invention disclosed herein. 
         FIG. 3  is a circuit diagram representing another embodiment of an LED driver as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring generally to  FIGS. 2 and 3 , exemplary embodiments of an invention may now be described in detail. Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Beginning with  FIG. 2 , an embodiment of an LED driver  20  as disclosed herein may be configured for coupling to an AC input  21 . A circuit interrupter such as a line fuse  22  is connected in series with the input  21 , so that in the event of certain abnormal situations the circuit interrupter would become non-conductive (e.g., fuse  22  would open) and disconnect the power supply from the input power source. An EMI filtering circuit  23  is configured to suppress conductive emission and in the illustrated example is formed of a common mode inductor L 1 , differential mode inductor L 2 , a first capacitor C 3  coupled across the AC line and neutral inputs, and a second capacitor C 4  coupled between the neutral and earth ground  28  to bypass common mode noise. 
     An input diode rectifier bridge  24  includes diodes D 1 -D 4 . Capacitor C 5  is provided as a filtering capacitor between the rectifier and the PFC stage  25 . A PFC controller integrated circuit (IC) provides driver signals (terminal DRV) to a switching element Q 1  to perform PFC regulation. The switching element Q 1  (coupled to earth ground through resistor R 4 ), diode D 5  and boost inductor L 3  collectively define a typical boost-type converter. The PFC IC may be coupled to boost inductor L 3  through resistor R 6  at terminal ICD, to a rectified input voltage at the node between resistors R 1 , R 2  and capacitor C 6  applied at terminal MULI, and to a voltage source V 2  across terminals VCC and GND. Current and voltage sensing signals are coupled to terminals I_sense (through resistor R 3 ) and V_sense respectively, and to comparator terminal COMP through capacitor C 7 . 
     Two electrolytic capacitors C 1  and C 2  are coupled across the PFC output to buffer the energy for the next DC-DC converter stage  26 . The DC-DC converter stage  26  may in various embodiments include an isolated or non-isolated power converter, effective to regulate output power from the LED driver to a load  27 , generally an array of one or more LED elements. 
     In an embodiment as shown, transient protection circuitry  30  is provided across each electrolytic capacitor to immediately sense an electrolytic short condition. Such protection circuitry may include transient voltage suppressors TVS 1  and TVS 2  connected in parallel with capacitors C 1  and C 2 , respectively. The transient protection circuitry may in an embodiment be formed of Zener diodes. It may be understood generally that the voltage across a single electrolytic capacitor is half of the PFC output voltage. The voltage protection rating for each transient voltage suppressor can therefore be set slightly below (but substantially more than half of) the output voltage rating of the PFC stage. For example, with respect to a 470V output case, the breakdown voltage threshold for a respective transient voltage suppressor can be set around 400V, so that in steady state the voltage across each transient voltage suppressor TVS 1 , TVS 2 , is well below the threshold voltage and the respective transient voltage suppressor acts like an open circuit. 
     When one electrolytic capacitor is shorted, all of the output voltage will be applied across the other of the two electrolytic capacitors, as well as the associated transient voltage suppressor TVS 1  or TVS 2  connecting in parallel with the non-shorted capacitor C 1  or C 2 . In this situation the output voltage from the PFC stage will be greater than the breakdown threshold for the transient voltage suppressor TVS, such that the transient voltage suppressor TVS will be overpowered and a short condition ensues. All of the output energy from the PFC stage will accordingly be bypassed by the transient protection circuit  30  because there is nothing to limit the current going through the respective transient voltage suppressors TVS 1 , TVS 2 . As soon as the transient voltage suppressor TVS fails, a short circuit will accordingly be formed with respect to the power supply  20  and the fuse  22  will be forced open. 
     In accordance with the aforementioned process, the fuse  22  will open or other circuit interrupter will be disrupted before failure of the (non-shorted) electrolytic capacitor. As neither of the electrolytic capacitors has failed, the power supply will avoid catastrophic failure, and further may be considered to have successfully passed the UL leakage test after the electrolytic short. 
     One of skill in the art may appreciate, however, that other transient and abnormal conditions may occur during operation with respect to the input power source, such as for example an input voltage surge. During an input surge the input power supply voltage could ring up to, e.g., 2.5 kV, as described in the ANSI 2.5 KV ring-wave surge test. This input energy surge will temporarily increase the PFC output voltage to a relatively high level, which could be more than the two combined breakdown threshold voltages of the transient voltage suppressors TVS 1  and TVS 2 . In such an instance, there is nothing to limit the surge current and therefore the transient voltage suppressor TVS will bypass all the surge current after the electrolytic capacitors and may itself fail. If the transient voltage suppressor TVS fails the fuse will also fail, and cause the power supply to be irretrievably damaged. Obviously, it would further be desirable to provide a power supply configuration that can reliably survive this type of abnormal input surge condition. 
     Referring now to  FIG. 3 , another embodiment of an LED driver as disclosed herein will effectively solve the shorted electrolytic capacitor problem as described above, and further can help the power supply survive input surge conditions as otherwise may result from the aforementioned configuration. An embodiment of a transient protection circuit  30  as represented in  FIG. 3  includes a first current limiting resistor R 8  coupled in series with the first transient voltage suppressor TVS 1  and a second current limiting resistor R 9  coupled in series with the second transient voltage suppressor TVS 2 . Each series connection of a current limiting resistor and a transient voltage suppressor is further connected in parallel with a respective electrolytic capacitor. 
     The resistors R 8  and R 9  will limit the current going through the associated transient voltage suppressor TVS 1 , TVS 2 , during an input surge transient and thereby help the transient voltage suppressor TVS survive the input surge abnormal condition. 
     In the case where one of the electrolytic capacitors is shorted in the configuration as represented in  FIG. 3 , there will be a steady high voltage across the resistor and the transient voltage suppressor as well as the electrolytic capacitor. This steady high voltage will cause the transient voltage suppressor to fail, and then the associated resistor. As a result, the source will be shorted, the fuse will open, and the power supply will still avoid electrolytic capacitor failure, and pass the UL dielectric test after the electrolytic short. 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. 
     The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. Terms such as “wire,” “wiring,” “line,” “signal,” “conductor,” and “bus” may be used to refer to any known structure, construction, arrangement, technique, method and/or process for physically transferring a signal from one point in a circuit to another. 
     The term “circuit interrupter” may include a fuse, a fusible link, a circuit breaker, or other component, device or circuit capable of interrupting current flow in a circuit in response to an overload or threshold current or voltage condition in the circuit. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, IGFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge or various other forms of power conversion or inversion as known to one of skill in the art. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     The terms “controller,” “control circuit” and “control circuitry” as used herein may refer to, be embodied by or otherwise included within a machine, such as a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed and programmed to perform or cause the performance of the functions described herein. A general purpose processor can be a microprocessor, but in the alternative, the processor can be a microcontroller, or state machine, combinations of the same, or the like. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     Depending on the embodiment, certain acts, events, or functions of any of the algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the algorithm). 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of a new and useful invention, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.