Patent Publication Number: US-2010109545-A1

Title: Automatic Compensation For Degradation Of Optocoupler Light Emitting Diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/111,062, filed on Nov. 4, 2008. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to optocouplers. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     In electronics, an optocoupler is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated. Typically, an electrical signal is transformed to an optical signal before being transmitted through the transmission path. The transmitted optical signal is then transformed back to an electrical signal. In this way, electrical contact along the path is broken. 
     Referring now to  FIG. 1 , a simplified schematic of a typical optocoupler  100  is presented. The optocoupler  100  has a transmitting element, such as a Light Emitting Diode (LED)  105 , and a receiving element, such as a phototransistor  106 . The LED  105  and the phototransistor  106  are separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to an input  101  of the optocoupler, which is coupled to an anode of the LED  105 , the LED  105  lights. Light from the LED  105  then activates the phototransistor  106 , which in turn generates a corresponding electrical signal. The output current from the phototransistor  106  is typically proportional to the amount of incident light supplied by the LED  105 . 
     The optical path between LED  105  and phototransistor  106  can be air or a dielectric waveguide. LED  105  and phototransistor  106  can be contained within a single compact module, for mounting, for example, on a circuit board. 
     When using optocouplers to isolate a system with high frequency signals, propagation delay across the optocoupler is directly related to the maximum frequency of the signals that can be reliably transmitted across the optocoupler. A LED as a light source degrades (that is, its light output decreases) over time, mainly based on the total time that it has been lit. LED&#39;s of optocouplers are typically driven with a fixed amount of drive current. Consequently, as the LED of an optocoupler degrades, the propagation delay across the optocoupler increases. An increased propagation delay can reduce the maximum communication speed of a system employing an optocoupler. To compensate for this degradation, the systems are over designed, or the maximum communication speed of the system is reduced. Over designed in this context is setting the fixed LED drive current higher than required for correct operation during the early life of the optocoupler to compensate for the higher current required to drive the LED of the optocoupler later in the life of the optocoupler due to the degradation of the LED over time. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     A system for compensating for degradation of a light emitting diode of an optocoupler includes a controller that accumulates time that the light emitting diode has been lit. The controller determines an amount of additional drive current to be provided to the light emitting diode based on the accumulated time that the light emitting diode has been lit and when the light emitting diode is lit, causes the determined amount of additional drive current to be provided to the light emitting diode. 
     In as aspect, the system generates the determined additional drive current and outputs the determined additional drive current to an anode of the light emitting diode. 
     In aspect, the system in outputting the determined additional drive current outputs a pulse width modulated signal and the controller adjusts the duty cycle of the pulse width modulated signal to provide the determined additional drive current. 
     In an aspect, the controller provides a feedback signal indicative of the determined amount of additional drive current at a feedback output. 
     In an aspect, the controller determines the amount of additional drive current based on a predictive model of degradation of the light emitting diode over time as well as the accumulated time that the light emitting diode has been lit. 
     In an aspect, an optocoupler system has an optocoupler and a controller. The optocoupler has a phototransistor and a plurality of light emitting diodes. The controller drives the light emitting diodes sequentially with each light emitting diode being driven until it degrades to a certain point and a next one of the light emitting diode is then driven. After each of the plurality of light emitting diodes have degraded to a certain point, the controller drives the plurality of light emitting diodes in parallel. 
     In an aspect, the controller determines an amount that each light emitting diode has degraded based upon an accumulated amount of time that the light emitting diode has been lit. 
     In an aspect, the controller determines the amount that each light emitting diode has degraded based upon a predictive model of degradation of that light emitting diode as well as the accumulated time that the light emitting diode has been lit. 
     In an aspect, the controller determines an additional amount of drive current to be provided to each light emitting diode based upon the amount that each light emitting diode has degraded and causing the determined additional amount of drive current for each light emitting diode to be provided to that light emitting diode when that light emitting diode is lit. 
     In an aspect, the system generates the determined additional drive currents and outputs the determined additional drive current to anodes of the light emitting diodes. 
     In an aspect, the system in outputting the determined additional drive currents outputs pulse width modulated signals and the controller adjusts the duty cycle of the pulse width modulated signals to provide the determined additional drive currents. 
     In an aspect, the system provides feedback signals indicative of the determined amount of additional drive currents at a feedback output. 
     In an aspect, a method of compensating for degradation of a light emitting diode of an optocoupler includes accumulating time that the light emitting diode has been lit, determining an amount of additional drive current to provide to the light emitting diode based on the accumulated amount of time that the light emitting diode has been lit, and providing the determined amount of additional drive current to the light emitting diode when the light emitting diode is lit. 
     In an aspect, the method includes determining the amount of additional drive current based on a predictive model of degradation of the light emitting diode over time as well as the accumulated time that the light emitting diode has been lit. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a simplified schematic of a prior art optocoupler; 
         FIG. 2  is a simplified schematic of a system for compensating for degradation of a light emitting diode of an optocoupler in accordance with an aspect of the present disclosure; 
         FIG. 3  is a simplified schematic of a monitoring system of the system of  FIG. 2 ; 
         FIG. 4  is a simplified schematic of the system of  FIG. 2  in which the optocoupler and monitoring system are integrated together; 
         FIG. 5  is a variation of the system of  FIG. 4 ; 
         FIG. 6  is a graph showing brightness degradation vs. forward current and time of a LED; and 
         FIG. 7  is a flow chart of a program executed by the monitoring system of  FIGS. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     In accordance with an illustrative aspect of the disclosure, a system monitors the total amount of time that the LED of an optocoupler has been lit. Based on the total amount of time that the LED has been lit and information about the degradation over time of the LED, the system determines an amount by which the drive current used to drive the LED should be increased to compensate for the degradation of the LED to provide a constant propagation delay as the LED ages. 
     Referring now to  FIG. 2 , an illustrative embodiment of the disclosure is now described. An optocoupler  200  includes LED  205  and phototransistor  206 . An input terminal  201  of optocoupler  200  is coupled to an anode of LED  205 . A terminal  202  of the optocoupler  200 , which is coupled to the cathode of the LED  205 , is coupled to common. An output terminal  203  of optocoupler  200  is coupled to an output of phototransistor  206 , such as by way of example and not of limitation, to the collector of phototransistor  206 . Another terminal  204  of the optocoupler  200  is illustratively coupled to the emitter of the phototransistor  206 , and may be coupled to common or to another terminal of a destination system (not shown). 
     A monitoring system  210  monitors optocoupler  200  to keep track of the accumulated time that LED  205  has been lit. Illustratively, monitoring system  210  is coupled to any part of the system having optocoupler  200  from which it can be determined that LED  205  is turned on. For example, monitoring system  210  can be coupled to any or all of input terminal  201  of optocoupler  200 , output terminal  203  of optocoupler  200 , or a part of an upstream system upstream of input terminal  201  that provides the drive signal to input terminal  201  of optocoupler  200 , shown representatively by element  214 . 
     Monitoring system  210  determines whether and how much additional drive current should be added to drive LED  205  when LED  205  is lit. In an aspect, it does so based on the accumulated time that LED  205  has been lit and a predictive model of the degradation of LED  205  over time. In this way, the monitoring system  210  can monitor the input or drive current to the optocoupler  200  and adjust the drive current according to need. In an aspect, the predictive model is a degradation curve of LED  205  obtained from a manufacturer of LED  205  or determined heuristically. In an aspect, the predictive model is a degradation function in the form of an equation, that may be determined heuristically. 
     In the illustrative embodiment of  FIG. 2 , an input  211  of the monitoring system  210  is coupled to input terminal  201  of the optocoupler  200  and an output  212  of monitoring system  210  is coupled to the anode of LED  205 . In an aspect, a resistor  218  is coupled between the input terminal  201  of the optocoupler  200  and the anode of LED  205  and across input  211  and output  212  of monitoring system  210 . It should be understood that resistor  218  could be external to optocoupler  200 . Alternatively, the input terminal  201  of optocoupler  200  is coupled through monitoring system  210  to the anode of LED  205  and monitoring system  210  includes a resistor disposed therebetween. Monitoring system  210  determines the compensation for LED  205 , which illustratively is how much additional drive current to add to the drive signal driving LED  205  and provides that additional drive current at output  212 . The monitoring system  210  may illustratively have another output  213  that provides a drive compensation status of the monitoring system  210 . Generally speaking, the drive compensation status could be an On/Off signal indicating that a limit has been reached (e.g., 80% of the compensation range has been reached), or could be a unit of measure indicating the compensation value. In the latter case, the output  212  could be a serial data output on which the compensation value is transmitted. For example, drive compensation status can be the amount by which the drive current to LED  205  has been increased. 
     In an aspect, output  212  of monitoring system  210  may be a pulse width modulated output and monitoring system  210  sets the duty cycle of the pulse width modulated output to provide the desired amount of additional drive current. 
     As discussed, monitoring system  210  may itself provide the additional drive current to input terminal  201  of optocoupler  200 . Alternatively, as shown by the dashed lines in  FIG. 2 , monitoring system  210  can provide a feedback signal at a feedback output  216  to upstream system  214  generating the drive signal for LED  205  and that upstream system  214  then increases the drive current of the drive signal based on the feedback signal provided by monitoring system  210  so that the drive signal includes the determined additional drive current. “Feedback signal” as that term is used herein can mean an analog signal, a digital signal, and/or data. 
     The monitoring system  210  can employ a computational mechanism to determine the compensation. The compensation can be alternatively determined based on statistical analysis. Further, the aforementioned computational mechanism and statistical analysis can be integrated in a functional module. 
     In an aspect, the monitoring system  210  can employ a predictive model to determine the compensation for the degradation of the LED  205 . The predictive model may illustratively be a predictive model of the Current Transfer Ratio (CTR) degradation in the optocoupler. CTR is the ratio between a current change is the output transistor of the optocoupler and the current change in the LED of the optocoupler. This predictive model is dependent on the specific LED used in the optocoupler. It may illustratively be provided by the manufacturer of the optocoupler, or of the LED used in the optocoupler, or may be determined heuristically.  FIG. 6  is an example of a degradation curve for the LED of a Fairchild AN-3001 optocoupler (and is FIG. 6 from Application Note AN-3001 published by Fairchild Semiconductor). The predictive models may also be determined as described in W. J. Stapor and P. T. McDonald, “Simple and Practical Optocoupler CTR Degradation Predictive Model,” or as described in T. F. Miyahira and A. H. Johnston, “Trends in Optocoupler Radiation Degradation.” 
     Referring to  FIG. 3 , an illustrative example of monitoring system  210  is now described. Monitoring system  210  has a control module  302 . Control module  302  may be a processor (shared, dedicated, or group) and memory, such as a microcontroller, that execute one or more software or firmware programs, an Application Specific Integrated Circuit (ASIC), an electronic circuit, a combinational logic circuit, a field programmable gate array, and/or other suitable components that provide the described functionality. Monitoring system  210  also includes a persistent memory  304 , which may be nonvolatile memory. Memory  304  may be a separate module or it may be part of control module  302 . Control module  302  monitors optocoupler  200  and accumulates the time that LED  205  has been lit and stores this time in persistent memory  304 . Based on the accumulated time that LED  205  has been lit and information about the degradation of LED  205  over time, which is illustratively stored in persistent memory  304 , control module  302  determines the compensation for optocoupler  200 , which is the amount by which the drive current to LED  205  should be increased when LED  205  is turned on to maintain a constant propagation delay. 
     The monitoring system  210  and the optocoupler  200  can be coupled in any fashion. For example, the monitoring system and the optocoupler can be integrated together as shown in  FIG. 4 , such as in an integrated circuit  400 . Alternatively, monitoring system  210  and optocoupler  200  may be separate modules. 
       FIG. 7  is a flow chart of an illustrative program executed by monitoring system  210  to compensate for degradation over time of LED  205  of optocoupler  200 . At  700 , monitoring system  210  accumulates the time that LED  205  has been lit which is stored in memory, such as persistent memory  304 . At  702 , monitoring system determines the amount of additional drive current to be provided to LED  205  when LED  205  is lit. It does so based on the accumulated time that LED  205  has been lit. In an aspect, it does so based on the accumulated time that LED  205  has been lit and a predictive model of the degradation of LED  205  over time. As discussed above, in an aspect, the predictive model is a degradation curve of LED  205  obtained from a manufacturer of LED  205  or determined heuristically. Also as discussed above, in an aspect, the predictive model is a degradation function in the form of an equation, that may be determined heuristically. The predictive model is stored (such as in the case of a degradation curve) or programmed (such as in the case of an equation) in memory used by controller  302 , such as persistent memory  304 . In a simple aspect, monitoring system  210  adds an incremental amount of drive current for each incremental amount of time that LED  205  has been lit. 
     At  704 , monitoring system  210  adds the determined additional drive current to the drive current driving LED  205  of optocoupler  200 . 
     With reference to  FIG. 5 , a variation of the embodiment of  FIG. 4  in which the monitoring system  210  and optocoupler  200  are integrated in a single module is described. In the embodiment of  FIG. 5 , an optocoupler system  500  includes optocoupler  501  having a second LED  502  driven by monitoring system  510  via output  504 . Illustratively, LED&#39;s  205  and  502  are first driven sequentially and then in parallel. That is, after LED  205  has degraded to a certain point, LED  502  is driven instead of LED  205 . After LED  502  has degraded to a certain point, both LED  205  and LED  502  are driven in parallel to provide additional useful life. In an aspect, monitoring system  510  also increases the drive current of the signal driving LED  502  to compensate for degradation of LED  502  over time, in a like manner to that described above with respect to increasing the drive current for LED  205  to compensate for the degradation of LED  205  over time. The provision of second LED  502  prolongs the life of optocoupler  501  compared with optocoupler  200 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.