Abstract:
A feedback circuit has an optical coupler with a feedback gain control. The feedback gain control includes an active element connected to vary current flow depending on changes in gain of the optical coupler.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an optical coupler circuit, and, in particular, to a gain compensation circuit for an optical coupler. 
   2. Description of the Related Art 
   Optical coupler circuits, also referred to as optically coupled isolator devices or optocouplers, provide isolation between different circuit portions which, for example, operate at vastly different voltages. An optocoupler typically includes a light source, such as an LED, at one side and a light sensor, such as a phototransistor or a photodiode, at the other side. Changing light levels from the LED traverses a gap and are received by the light sensor. The imaginary division between the circuit portions operating at the two different voltages is referred to as an isolation barrier and the optocoupler communicates across this isolation barrier. The current gain of an optical coupler between the input and the output is referred to as the current transfer ratio (CTR) of the optical coupler. The CTR is the ratio of the output current which is the same as the collector current to the input current which is the current exciting the LED. 
   Optocouplers are commonly used in switched-mode power supplies and other analog circuits to provide an analog feedback control signal across the isolation barrier. Optocouplers are used because they are small, simple, easy to use, and reliable. On the other hand, optical couplers have large initial gain variations. The initial gain variation can occur from device to device or from production lot to production lot in typical optical couplers. The CTR range variation introduces undesirable variations in performance and capability of a circuit that must transmit information across the isolation barrier. 
   A potential solution to this problem is to actively trim a gain setting resistor to null out the initial CTR (Current Transfer Ratio) variations. A typical optocoupler feedback circuit has an overall circuit gain that is proportional to the current transfer ratio of the optocoupler divided by the impedance of the setting resistor. Thus, varying the resistor impedance value will adjust for variations in the optocoupler gain. 
   An example of a circuit utilizing such active trimming is shown in  FIG. 1 , including a power supply  10  with an optocoupler feedback circuit  12 . The power supply  10  is illustrated only generally for purposes of simplicity and includes an input  14  for an input voltage V in , a power transformer  16 , and a rectifier and filter  18  at the secondary side of the transformer  16 . A DC output voltage V out  is produced at the output of the rectifier and filter  18  and is available at  20 . 
   The high and low voltage portions of the power supply are separated by an isolation barrier  22 , an imaginary boundary indicated in the drawing by a line. An optocoupler  24  is used to communicate a feedback signal across the isolation barrier  22  so that the output voltage V out  of the power supply can be precisely regulated. In the feedback circuit, the optocoupler  24  is driven from an error amplifier circuit which includes the light emitting diode  26  of the optocoupler  24 , an npn transistor  28 , and a trim resistor  30 , as well as a operational amplifier  32  driving the base of the transistor  28 . The operational amplifier  32  receives the output voltage V out  through a voltage divider of resistors  34  and  36  at the non-inverting input  38 , while a reference voltage V ref  at  40  is fed through a resistor  42  to the inverting input  44 . The inverting input  44  of the operational amplifier  32  also receives the voltage across the trim resistor  30  via a capacitor  46 . A test point  48  is provided at which the voltage across the trim resister  30  is measured. 
   On the input side of the circuit, on the other side of the isolation barrier  22 , a phototransistor or photodiode  50  of the optocoupler  24  is connected to a second trim resistor  52  as well as to the input of an error amplifier, pulse width modulator and driver circuit  54 . The circuit  54 , while having multiple functions, is available on a single chip. The output of the error amplifier, pulse width modulator and driver circuit  54  is fed to the power transformer  16 , potentially through a switch (not shown). An input test point  56  is utilized to measure the voltage across the trim resistor  52 . 
   The error amplifier compares the output voltage  20  against the reference voltage  40  in the operational amplifier  32  and produces an error voltage at  58  to provide closed loop regulation of the power supply. The error voltage  58  is converted to a current by the transistor  28  and the trim resistor  30 . This current is transferred across the isolation barrier  22  by the optocoupler  24 , where it is converted back to a voltage at the trim resistor  52 . 
   The voltage gain of the optocoupler circuit  12  is determined from the ratio of the test point  48  to the test point  56 . The test point  48  is the feedback error voltage and the point  56  is the pulse width modulation control voltage of the circuit  54 . The voltage gain Av is determined from the equation 
             Av   =       V56   V48     =       CTR   ·   R52     R30         ,         
wherein V 56  is the voltage at test point  56 , V 48  is the voltage at test point  48 , R 52  is the resistance of the resistor  52 , R 30  is the resistance of the resistor  30  and CTR is the current transfer ratio of the optocoupler  24 .
 
   The circuit of  FIG. 1  is used with an active laser trimming process to eliminate performance sensitivity to initial gain (CTR) variations associated with the optocoupler circuit. The active laser trimming process adjusts the resistors  30  and  52  depending on the initial current transfer ratio of the optocoupler  24 . By trimming the resistor  52  to a higher resistance, an adjustment is made to increase the gain for low current transfer ratio optocouplers, and by trimming the resistor  30  to a higher resistance, an adjustment is made in the gain for high current transfer ratio optocouplers. Thus, the voltage gain of the circuit is set to the desired level by compensating for initial variations in the current gain (CTR) of the optocoupler. 
   However, the current transfer ratio of the optocoupler increases or decreases with changes in temperature, and it also suffers a degradation over the life of the device. Over the typical life of an optocoupler, a loss of 30 to 50 percent of the gain can be expected. The drivers for optocouplers permit feedback loop crossover frequencies to vary as the gain of the optocoupler varies. Lower performance from the isolated feedback loop results, along with a degraded performance of the optocoupler. No adjustment is made for changes in the current transfer ratio due to changes resulting from temperature variations or aging. Instead, a margin for error for these changes is built into the circuit. Even in light of this, the gain of the feedback circuit changes with the age and temperature. 
   Referring to  FIG. 4 , an embodiment of an optocoupler feedback circuit is shown without the power supply. The illustrated circuit includes the optical coupler  24  connected to a shunt regulator  100 . The shunt regulator  100  regulates the operating current of the optocoupler  24  as necessary to keep the output voltage V out  constant. As the output voltage V out  increases, the current through the shunt regulator  100  and through the light emitting diode  26  of the optocoupler  24  increases, thereby producing a larger feedback signal at the test point  56 . A capacitor  101  may be connected between the control lead and the cathode of the shunt regulator  100 . 
   A control circuit, such as shown in  FIG. 1 , attached at the test point  56  lowers the power supply duty cycle. This causes a slight decrease in the output voltage V out  and keeps the output voltage in regulation. The output voltage V out  is connected at  102  to a voltage divider made up of resistors  104  and  106 , the midpoint of which is connected to the control lead of the shunt regulator  100 . 
   A resistor  108  is between the optocoupler  24  and the power supply voltage at  110 , which in this case is the output voltage V out . The low voltage test point  48  is between the resistor  108  and the optocoupler  24 . On the other side of the isolation barrier  22 , the light sensor  50  of the optocoupler  24  is connected to the resistor  52  to define the voltage at the test point  56 . 
   The voltage gain in the circuit illustrated in  FIG. 4  from the test point  48  to the test point  56  is 
             Av   =       V56     Vout   -   V48       =       R52   ·   CTR     R108         ,         
wherein Vout is the voltage at  102  and  110 , V 56  is the voltage at the test point  56 , V 48  is the voltage at the test point  48 , R 52  is the resistance of the resistor  52 , R 108  is the resistance of the resistor  108 , and CTR is the current transfer ratio of the optocoupler  24 .
 
   Typically, in feedback loops with a shunt regulator  100  as the voltage regulating device, the current transfer ratio of the optocoupler  24  is not compensated. The current transfer ratio rating of the optical coupler  24  is incorporated into the circuit. However, this results in feedback loop variations that yield a gain variation in the circuit of 10 to 20 dB, and a corresponding feedback loop frequency crossover that varies by as much as a factor of 10. 
   SUMMARY OF THE INVENTION 
   The present invention provides gain compensation in the feedback loop of an optical coupler. The optical coupler circuit varies the gain to give the same feedback loop crossover frequency in an isolated feedback loop. The current transfer ratio is actively compensated for increases and decreases due to temperature and aging. The present invention also compensates for initial gain variations from device-to-device in the optocoupler gain. 
   A dynamic load is provided in parallel to the gain setting resistor of the optocoupler&#39;s drive circuit. The dynamic load varies the effective impedance of the gain setting resistor based on the gain of the optocoupler. 
   In the present invention, the gain setting resistor has an effective value that is raised when the current transfer ratio increases and is decreased as the current transfer ratio decreases. This results in the overall gain of the circuit being held constant and independent of the current transfer gain. 
   The present invention accomplishes the advantages set forth above, in the preferred embodiment, by a small number of parts added to the feedback circuit to establish a constant crossover frequency feedback loop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a power supply with an optocoupler and feedback loop according to a prior development; 
       FIG. 2  is a circuit diagram of an optocoupler feedback circuit according the principles of the present invention; 
       FIG. 3  is a circuit diagram of an alternative embodiment of the feedback circuit of the invention; 
       FIG. 4  is a circuit diagram of a feedback circuit for a power supply, for example, which uses a shunt regulator; 
       FIG. 5  is a circuit diagram of a feedback circuit using a shunt regulator as in  FIG. 4  and including an active regulation according to the present invention and  FIG. 6  is a circuit diagram of a further embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 2 , only the feedback circuit is shown. The feedback circuit may be used with a power supply as shown in  FIG. 1 , or may be used with any other circuit for which a fixed gain feedback would benefit. The elements which are common to  FIG. 1  are shown with the same reference characters. The voltage at the test point  56  to the control circuit is recognized as a fairly constant DC value. Using this assumption, a compensation is made for the variations in the current transfer ratio of the optocoupler  24 . A circuit  60  is added to replace the trim resistor  30  with a resistor  62  and a resistor  64  to form a voltage divider that senses the current through the light emitting diode  26  of the optocoupler  24 . The voltage at the midpoint (the physical midpoint, but not necessarily electrical midpoint) of the voltage divider  62  and  64  is connected to a non-inverting input  66  of a operational amplifier  68 , the inverting input  70  of which is connected to a reference voltage  72 . A resistor  73  is provided between the reference input  72  and the op-amp input  70  The operational amplifier  68  drives an active element  74 , such as an NFET, that is coupled to a resistor  76  and to the test point  48 . A capacitor  78  is added across the operational amplifier  68 . Also, a capacitor  65  may be added across the resistor  64  in an alternative embodiment to roll off the gain of the circuit. 
   The circuit  60  operates to keep the voltage at the test point  48  constant by sinking (drawing off) any extra current through the active load (which is a variable resistance) formed by the FET  74  and the resistor  76 . The current through the divider resistors  62  and  64  is held constant, as is the voltage across the resistor  64 . Should the voltage across the resistor  64  increase, the operational amplifier  68  generates a higher output to cause the FET  74  to draw off more current. The active load, or variable resistance, of the operational amplifier  68  and the FET  74  sinks the extra load current from the optocoupler  24  and the transistor  28 . The voltage gain Av from the primary to the secondary is thus constant, since Av=V 56 /V 48 . If a pulse width modulation of the control circuit of an opposite polarity is used, this gain formula would be different. The gain of the feedback circuit is therefore held constant. 
   A capacitor  78  is used to roll off the frequency response of the gain compensation circuit so that the circuit does not interfere with the feedback loop normal operation. In particular, the capacitor causes a low frequency operation for the circuit so that the operational amplifier  68  operates slower than the operational amplifier  32 . This results in a faster change in the operating point and a slower change in the gain point. In one example, the operational amplifier  68  operates with a response time in the seconds whereas the operational amplifier  32  operates with a response time in the microseconds. Alternatively, the gain compensation circuit may be made fast to improve the power supply reaction to changes in output loads. 
   The resistor  76  and the voltage drop across the FET  74  are sufficiently low that the lowest current transfer ratio of an optocoupler is accommodated. 
   While the current sink of the circuit of  FIG. 2  is through the active element, which is shown at an NFET  74 , it is possible to utilize an operational amplifier which has a current carrying capacity at its output adequate to serve as the current sink. In such a circuit, the FET  74  would be eliminated and the current path would proceed through the operational amplifier  68 . The active element according to the invention which serves as the current path is thus in the output circuitry of the operational amplifier. 
   An alternate embodiment is shown in  FIG. 3 , wherein the operational amplifier  68  and FET  74  of  FIG. 2  are replaced by a shunt regulator  80 . The shunt regulator  80  may be a part number TL431 in one example. The shunt regulator is controlled by a voltage obtained from a voltage divider made up of resistors  82  and  84 . The voltage divider is connected to receive some of the current which passes through the light emitting diode  26  of the optocoupler  24 . 
   In the embodiment of  FIG. 3 , the shunt regulator  80  draws off, or sinks, a variable amount of current until the voltage across the resistor  84  of the voltage divider reaches an internal reference voltage level of the regulator  80 . This results in the voltage at the voltage divider  82  and  84  being held constant by shunting the additional current through the shunt regulator  80 . The voltage at the test point  48  is held constant and the gain of the optocoupler is thereby held constant. A resistor  86  is used to bias the shunt regulator  80 , which is the case as long as the supply voltage Vdd at  88  across the circuit is sufficient. A frequency roll off capacitor  90  is provided as well. 
   As an alternative embodiment to the circuit of  FIG. 3 , the upper end of the resistor  86  may be connected below the transistor  28  at the test point  48 , instead of the illustrated connection above the transistor  28 . 
   As an improvement on the circuit of  FIG. 4 , the circuit of  FIG. 5  provides a feedback control loop using the shunt regulator  100 , such as a TL431, to drive the optocoupler  24 . The capacitor  101  maybe connected at the shunt regulator  100 , as shown. A gain compensation circuit similar to that of  FIG. 2  may be provided. The circuitry of the present embodiment is translated to the upper voltage rail from the lower voltage rail as in the previous embodiments. 
   A reference voltage device  120  is connected to define a reference voltage from current flowing from the output voltage V out  through the reference device  120  and through a resistor  122  to the light emitting diode  26  of the optocoupler  24 . The reference voltage derived from the output voltage V out  is compared to a voltage drop across a resistor  124  of a voltage divider  124  and  126  in a operational amplifier  128 . As the current through the resistor  124  increases, a non-inverting input  130  of the operational amplifier  128  becomes more negative than an inverting input  132 . The output of the operational amplifier  128  goes low to turn on harder a PFET  134  at the output  136  thereof. With the PFET  134  on harder, more current is drawn through a resistor  138  and made available to the light emitting diode  26  of the optocoupler  24 . This added current is also supplied to the shunt regulator  100 . The result is that the control voltage for the shunt regulator  100  and the gain of the optocoupler section of the feedback loop is compensated. A capacitor  140  provides feedback compensation for the operational amplifier  128 . 
   An alternative embodiment is shown in  FIG. 6 . The reference  120 , the operational amplifier  130  and the active element  134  of  FIG. 5  are replaced with a shunt regulator  202  and a biasing resistor  201 . The circuit operation of the circuit of  FIG. 6  provides that as the current through the resistor  126  increases, the shunt regulator  202  sinks additional current so that the voltage across the resistor  126  is held constant. This in turn means that the voltage between the output V out  and the test point  48  is constant and that the gain of the optocoupler section of the feedback loop is compensated. 
   The present invention therefore provides a gain compensation to optocoupler isolated feedback loop circuits. The use of a dynamically varying load in parallel with the optocoupler gain setting resistor provides the gain compensation without resort to trimming of the gain setting resistor and without resort to sorting the optocouplers for gain to compensate for different initial optocoupler gains. 
   The compensation in gain variation prevents changes in the crossover frequency of the power supply or analog circuit feedback loop, even due to the effects of temperature changes and aging of the circuit elements. Improved performance of the circuit is realized. 
   Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.