Current regulated LED strobe drive circuit

[0001] The disclosure relates generally to optical element driving circuits, and more particularly to a current regulating drive circuit for LED strobe lights.

[0002] Emergency notification systems, such as fire alarm systems, typically include one or more notification appliances for providing occupants of a building with a prominent visual or auditory indication of a hazardous condition, such as the presence of smoke or fire. A notification appliance circuit (NAC) connects the notification appliances to a central control panel, such as a fire alarm control panel. A primary power source, such as line power from an AC line, may supply power to the control panel. The NAC may thus provide power from the control panel to the notification appliances.

[0003] A notification appliance that is commonly employed in emergency notification systems is a strobe. The most common type of strobe is a Xenon flash tube based strobe. Light emitting diode (LED) strobes have recently been introduced into the marketplace and offer prospects of lower energy consumption. A notification system may include dozens, or even hundreds, of strobes distributed throughout a building. A first important consideration when designing a notification system that employs strobes is the energy efficiency of the strobes. It is generally preferable to maximize the number of strobes connected in series on a single NAC of a notification system in order to minimize wiring requirements and to reduce the overall cost of installing the notification system. It is generally also preferable to minimize the current requirements of a NAC in order to reduce energy consumption and operating costs. Employing strobes that operate more efficiently allows a greater number of strobes to be connected to a NAC at a lower current draw relative to strobes that operate less efficiently.

[0004] A second important consideration when designing a notification system that employs strobes is the ability of the strobes to deliver consistent light output, including consistent color and intensity, during operation. National Fire Protection Association (NFPA) requirements dictate that notification system strobe lights output a minimum total amount of light over a given time period for a given area. A strobe that lacks consistency and produces varying levels of light output from flash to flash may cumulatively project too little light over a given time period and thus fail to meet the NFPA output requirement. Conversely, an inconsistent strobe may produce a cumulative amount of light over a given time period that greatly exceeds the NFPA output requirement, thereby compromising the efficiency of the NAC. Consistent light output is therefore critical for ensuring compliance with NFPA requirements while optimizing system efficiency.

[0005] Referring to FIG. 1, a schematic diagram of a conventional drive circuit 10 for an LED strobe application is shown. The drive circuit 10 includes a buck convertor or boost convertor 12 for stepping down or stepping up a NAC input voltage, respectively, a current limiting resistor 14, an LED 16, and a transistor switch 18 for flashing the LED 16. While generally effective for providing a strobe, this configuration exhibits certain inefficiencies and can cause significant variations in light output among a group of serially-connected LED strobe units. For example, with regard to efficiency, an embodiment of the drive circuit 10 that employs a buck convertor at 12 requires a minimum convertor input voltage that is greater than the convertor's regulated output voltage which is greater than the stack up voltage of the LED 16 and other drive circuit elements. Alternatively, an embodiment of the drive circuit 10 that employs a boost convertor at 12 requires a minimum input voltage for facilitating a defined duty cycle for proper boost operation. The output voltage of the boost convertor 12 must also be greater than the stack up voltage of the LED 16 and other drive circuit elements. Thus, for either embodiment of the drive circuit 10, a substantial amount of energy is wasted during operation.

[0006] With regard to light output, the tolerances of the energy source voltage, the current limiting resistor 14, the LED's forward voltage drop, and the voltage drop across the switch element 18 of the drive circuit 10 can all affect the amount of current passing through the LED 16, thereby diminishing the consistency of the LED's output. While the tolerances of the energy source components, limiting resistor 14, and switch element 18 can be narrowed by implementing components with tighter tolerances, the forward voltage drop across the LED 16 can nonetheless vary by 15% or more from the effects of drive current, duty cycle, thermal resistance, and ambient temperature on the LED's junction. LED output may therefore be highly inconsistent and may vary in color and intensity during strobe operation.

[0007] In view of the forgoing, it would be advantageous to provide an LED strobe that operates efficiently and consistently relative to existing LED strobe appliances. In accordance with the present disclosure, a current regulating LED strobe drive circuit is disclosed for efficiently producing an LED strobe that is consistent in both intensity and color. The drive circuit may include a filter/energy storage element, a current regulator, a flash control element, and an LED.

[0008] The inrush control of the drive circuit may be connected to an input line from a NAC and is provided for limiting the amount of current that is allowed to flow to the drive circuit immediately after activation. The inrush control thereby protects the circuit against overcurrent damage that could otherwise result from current surges during startup.

[0009] The filter/energy storage element may be electrically connected in series with the inrush control. A storage component of the filter/energy storage element is provided for storing energy supplied by the inrush control and outputting such energy to flash the LED. A filter component of the filter/energy storage element is provided for preventing signal noise generated by components of the drive circuit from being transmitted into the NAC.

[0010] The current regulator of the drive circuit may be electrically connected in series with the filter/energy storage element and is provided for measuring and supplementing the steady state operating current that is supplied to the LED in a predefined manner. Particularly, the current regulator may include a current sensing component for sensing an input current that is delivered to the LED and control device for supplementing the input current in a predefined manner in response to the sensed current. The current regulator thereby facilitates consistent light output from the LED during strobe operation.

[0011] The flash control element of the drive circuit may be electrically connected intermediate a control input from the NAC and the current regulator. The flash control element is provided for outputting, in response to an activation signal from the NAC, a pulsed flash signal that dictates a correspondingly-pulsed current pattern that is allowed to pass from the current regulator to the LED, thereby causing the LED to generate a strobe pattern with predefined duration and frequency characteristics.

[0012] An embodiment of the device disclosed herein can thus include an energy storage component, a current regulator electrically coupled to the energy storage component for sensing and regulating a current supplied by the energy storage element, an LED electrically coupled to the current regulator, and a flash control element electrically coupled to the current regulator for communicating a flash pulse signal that dictates a pulsed pattern with which current is allowed to flow from the current regulator to the LED.

[0013] An embodiment of the method disclosed herein can thus include the steps of charging an energy storage element with an input current, generating a pulsed electrical signal, allowing energy to flow from the energy storage element to an LED in accordance with the pulsed electrical signal, and sensing the input current and providing a corresponding regulated current to the LED.

[0021] Referring to FIG. 2, an embodiment of an LED strobe drive circuit 100 in accordance with the present disclosure is shown. The drive circuit 100 may generally include a current regulator 102, a sense element 104, an LED 106 and a switch 108. The drive circuit 100 may be electrically connected to a NAC input line 110. The NAC may be a component of an emergency notification system, such as a fire alarm system installed in a building, and may be electrically connected to one or more additional LED strobes or other notification appliances, such as in a series configuration. It will be appreciated by those of ordinary skill in the art that the particular drive circuit configuration shown in FIG. 2 is provided by way of example only, and that the drive circuit 100 can be implemented using a variety of alternative circuit configurations without departing from the present disclosure.

[0022] The current regulator 102 of the drive circuit 100 may be implemented using a control device such as a microcontroller, application specific integrated circuit (ASIC), or other suitable control device. In general, the current regulator 102 receives as input a sensed input current from the sense element 104 and, in response to the sensed current, delivers a regulated amount of current from the NAC wiring 110 to the LED 106. The switch element 108 is provided for turning the LED on and off in a predefined pattern (i.e., with predefined duration and frequency characteristics), thereby causing the LED 106 to flash in a corresponding pattern.

[0023] Referring to FIG. 3, a schematic diagram of an embodiment of an LED strobe drive circuit 200 in accordance with the present disclosure is shown. The drive circuit 200 may generally include an inrush control 202, a filter/energy storage element 204, a current regulator 206, and a flash control element 208, connected to LED 210. The drive circuit may be electrically connected to a NAC input line 212 at input terminals 214.

[0024] The inrush control 202 of the drive circuit 200 may be electrically connected to the input terminals 214 and is provided for limiting the amount of current that is allowed to flow to the drive circuit 200 immediately after activation. The inrush control 202 thereby mitigates the possibility of damage to the other components of the drive circuit 200 that could otherwise result from startup current surges. It is contemplated that virtually any type of passive or active current limiting component can be implemented for effectuating the inrush control 202, including, but not limited to, negative temperature coefficient (NTC) thermistors (“surge limiters”), triacs, resistors, thyristors, and various combinations thereof, along with appropriate circuitry for driving such elements as will be understood by those of ordinary skill in the art. For example, a possible embodiment of the inrush control 202 is presented in FIG. 4 at 302 and will be discussed in greater detail below. Alternative embodiments of the drive circuit 200 are contemplated in which the inrush control 202 is omitted.

[0025] The filter/energy storage element 204 of the drive circuit 200 may be electrically connected in series with the inrush control 202, opposite the input terminals 214. A filter component 211 of the filter/energy storage element 204 is provided for preventing electrical signal noise generated by the current regulator 206 (described below) from being transmitted into the NAC wiring 212. The filter component 211 may include virtually any type of suitable signal filtering component, device, or arrangement, including, but not limited to, LC filters, chokes, ferrite beads, and simple inductors such as the inductor 312 shown in FIG. 4.

[0026] A storage component of the filter/energy storage element 204 is provided for storing and supplying energy to flash the LED 210 as further described below. The storage component may be implemented using virtually any type of suitable electrical energy storage component, device, or arrangement that can be controllably discharged, including, but not limited to, inductors and capacitors coupled in a variety of configurations, such as the pair of capacitors 314 and 316 coupled in parallel as shown in FIG. 4. Although the filter and storage components 211 and 213 define a single circuit element 204 in FIG. 3, it is contemplated that the filter and storage components 211 and 213 can alternatively be implemented in separate, independent elements of the drive circuit 200. It is further contemplated that the filter component 211 of the filter/energy storage element 204 can be entirely omitted from the drive circuit 200.

[0027] The current regulator 206 of the drive circuit 200 may be electrically connected in series with the filter/energy storage element 204, opposite the inrush control 202. The current regulator 206 is provided for measuring and supplementing the steady state operating current that is supplied to the LED 210 in a predefined manner. Particularly, the current regulator 206 delivers a consistent level of operating current to the LED 210 during strobe operation regardless of current fluctuations elsewhere in the drive circuit 200, such as may be caused by forward voltage drops across other circuit components. The current regulator 206 thereby facilitates consistent light output from the LED 210 from flash to flash as further described below. The specific steady state operating current will generally depend on the particular type of LED used.

[0028] The current regulator 206 may be implemented using virtually any suitable current regulating component, device, or arrangement that is capable of managing the total amount of current that is delivered to the LED 210. For example, referring to the exemplary drive circuit 300 shown in FIG. 4, the current regulator 306 may be implemented using a pair of resistors 318 and 320 coupled in parallel for sensing an amount of current that is output by the capacitors 314 and 316, and an LED driver 328 configured to deliver an amount of current to the LED 330 in response to the sensed current as described in greater detail below. The LED driver 328 may be implemented using any type of suitable control device, including, but not limited to, a microcontroller, ASIC, or other control device, such as the integrated circuit shown in FIG. 4.

[0029] The LED 210 may be a conventional light emitting diode having an anode terminal 216 and a cathode terminal 218 electrically connected to the current regulator 206, opposite the filter/energy storage element 204. The LED 210 is provided for emitting light when supplied with current by the current regulator 206, such as during strobe operation, as further described below. The LED 210 may be capable of emitting at least about 15 candela of light to meet minimum NFPA requirements. It is contemplated that the LED 210 can be any color (or can be provided with a lens of any color) that is appropriate for a particular application. For example, white notification lights are typically used for fire applications, while amber notification lights are typically used for mass notification applications. It is further contemplated that the drive circuit 200 can include more than one LED 210 of the same or different color, and that such plurality of LEDs 210 can be flashed simultaneously or in a predefined pattern.

[0030] The flash control element 208 of the drive circuit 200 may be electrically connected intermediate a control input line from the NAC (such as may be integral with the input line 212) and the current regulator 206. The flash control element 208 is provided for outputting, in response to an activation signal from the NAC, a pulsed flash signal that dictates a correspondingly-pulsed current pattern that is allowed to pass from the current regulator 206 to the LED 210, thereby causing the LED 210 to generate a strobe pattern with predefined duration and frequency characteristics. For example, the flash control element 208 may generate a flash signal with 100 ms pulses at a frequency of ˜1.5 Hz for causing the LED 210 to produce 100 ms light flashes at a frequency of ˜1.5 Hz. The flash control element 208 may be implemented using virtually any suitable component, device, or arrangement that is capable of producing, in response to an input control signal, a pulsed flash signal having predefined duration and frequency characteristics. For example, the current regulator 206 may be implemented using an appropriately configured microcontroller or application specific integrated circuit (ASIC) as further described below.

[0031] Typical operation of the LED strobe drive circuit 200 will now be described with respect to the exemplary drive circuit 300 shown in FIG. 4 and the flow diagram shown in FIG. 6. However, it will be appreciated that many other circuit configurations and component arrangements may be employed for implementing the drive circuit 200 in a similar manner without departing from the spirit and the scope of the present disclosure.

[0032] Operation of the LED strobe drive circuit 300 may be initiated by the application of electrical power to the drive circuit 300 by an NAC (not shown), such as may be provided upon the detection of a hazardous condition by sensing elements within an emergency notification system (step 500 in FIG. 6). A control device (not shown) may receive the electrical power from the NAC and apply at least a portion of the power on a slow charge control line 322 of a slow charge circuit 321 to slowly charge the capacitors 314 and 316 (step 510 in FIG. 6) via slow charge circuit 321. This avoids placing a large current draw on the NAC upon circuit activation (such as may be caused by an initial spike in current) that could otherwise cause the NAC to reach an overcurrent condition when a plurality of serially-connected drive circuits are activated simultaneously. As the capacitors 314 and 316 approach full charge, the control device applies current to a bypass control line 324 of the energizing circuit 323, thereby bypassing the slow charge circuit 321 to charge the capacitors 314 and 316 to a level substantially equal to the input voltage from the NAC (step 520 in FIG. 6). Completing the charging of the capacitors 314 and 316 in this manner overcomes the voltage drop associated with the slow charge circuit.

[0033] A control device (not shown), which may be the same control device discussed above, may receive an activation signal from the NAC. In response to the activation signal, the control device applies a pulsed flash signal on a strobe trigger line 326 of driver interface circuit 325 (step 530 in FIG. 6). As described above, the flash signal has a predefined duration and frequency characteristics, such as may be configured in the control device. It is contemplated that the characteristics of the flash signal can be manually adjusted as will be described below.

[0034] The flash signal output from the driver interface circuit 325 is applied to the LED driver 328, which also receives an input current from the current sensing resistors 318 and 320. The LED driver 328 is configured to supply current to the LED 330 in a pulsed pattern that substantially mirrors the duration and frequency characteristics of the flash signal on the strobe trigger line 326 (step 540 in FIG. 6). The LED 330 is thereby caused to emit a strobe pattern that also substantially mirrors the duration and frequency characteristics of the predefined flash signal.

[0035] To achieve current regulation, the LED driver 328 senses the input current at the resistors 318 and 320 and provides current to the LED 330 by cycling the switch 332as necessary to intermittently establish a path for current to flow from the LED driver 328 to the LED 330 to account for voltage drops in the circuit 300 (step 550 in FIG. 6). The LED 330 is thereby provided with a highly regulated level of current that is consistent from pulse to pulse, thus resulting in strobe flashes that are consistent in both intensity and color. The inductor 312 is provided for preventing electrical signal noise generated by the current regulating components of the drive circuit 300 from being transmitted into the NAC wiring.

[0036] Referring to FIG. 5, an alternative drive circuit 400 in accordance with the present disclosure is shown that includes an output control 402 for allowing an operator to selectively regulate the quantity of light that is output by the LED 404 over a period of time, such as may be desirable for ensuring compliance with NFPA guidelines. The output control 402 may be implemented using a control device, and includes an operator interface, such as a dial, knob, lever, or buttons, for allowing an operator to specify a desired light output level. The output control can be implemented in a variety of different ways for regulating the quantity of light produced by the LED 404 in accordance with the output level specified by the operator. For example, it is contemplated that the output control 402 can be coupled to the current regulator 406 for selectively varying the degree to which the input current is supplemented thereby as described above. The amount of current supplied to the LED 404 each time it is flashed can thereby be increased or decreased, thus increasing or decreasing the total candela output of the LED 404 over time. Additionally or alternatively, it is contemplated that the output control 402 can be coupled to the flash control element 408 for selectively varying the duration and/or frequency characteristics of the flash pulse generated by the flash control element 408. The duration of each flash and/or the frequency of the flashes produced by LED 404 can thereby be increased or decreased, thus increasing or decreasing the total candela output of the LED 404 over time.

[0037] In view of the forgoing, it is apparent that by regulating the current through the LED 210 the effects of forward voltage drop in the drive circuit 200 on light output from the LED 210 can be minimized while reducing other tolerance effects to only those presented by the current sensing element (e.g. resistors 318 and 320 in FIG. 4) of the current regulator 206. Current regulation facilitates consistency of light output color among a group of strobe appliances that are connected in series, especially at higher drive currents. Still further, current regulation facilitates a broader operating voltage range when compared to conventional strobe appliances that employ buck or boost converters because current levels can be regulated down to the limits of circuit component voltage stack up.

[0038] As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0039] While certain embodiments of the disclosure have been described herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.