1. Field of the Invention
The present invention generally relates to the use of couplers as a means for generating a feedback signal in an input circuit that is galvanically isolated from an output circuit, and more particularly to non-linearly adjusting the output of the transmitter in the coupler to compensate for variations in the gain of the coupler.
2. Description of the Art
Power supplies for converting an input AC or DC power source into one or more regulated output voltages, which are coupled to associated loads, typically comprise a power stage and a feedback loop between the input and output stages of the power stage. The feedback loop controls the power stage so that it compensates for fluctuations in the load in order to deliver a constant amplitude output voltage. Such AC to DC or DC to DC converter circuits typically require separate ground references for the primary and secondary sides (i.e., the input and output stages) of the power supply circuit so that they are galvanically isolated from each other. This requires that an isolation barrier be created which prevents any electrical connection between the load being powered by the DC output of the power supply and the power supply's input power source.
Galvanic isolation is typically accomplished by use of a transformer connected between the primary and secondary sides of the power stage of a power supply. The DC to DC converter works by converting the input DC input to an alternating current. The transformer functions to convert the alternating current in a primary winding to an alternating magnetic field and then back to an alternating current in a secondary winding. This secondary current is then rectified to achieve a DC output. The primary and secondary sides of the power supply are thus separated by an "isolation barrier" across which no direct current is allowed to flow.
With the power stage including such an isolation barrier, it is also necessary to galvanically isolate the primary and secondary sides of the feedback loop such that the feedback signal is also coupled across an isolation barrier from the secondary side back to the primary side. Optocouplers are commonly used for this purpose. Optocouplers consist of a light-emitting diode (LED) and a corresponding photodiode (or phototransistor), and are typically integrated in a single dual in-line plastic package. The LED is connected to the secondary side of the power supply and the photodiode is connected to the primary side to enable coupling of the feedback control signal back to the primary side. A current flowing in the LED, typically made of gallium arsenide (GaAs), results in the emission of photons of infrared light. The photodiode is typically a silicon based semiconductor which is designed to be especially receptive to infrared light. The photodiode is positioned with respect to the LED to maximize the number of photons that reach the active area of the photodiode. The photons received by the photodiode generate electron carriers within the semiconductor, resulting in a leakage current proportional to, but having a current that is several orders of magnitude less than, the LED drive current. To amplify this current to a more useable value, it is common practice to use a phototransistor (i.e., where the collector-base junction of an npn transistor is the photodiode). As such, the leakage or photocurrent (I.sub.D) of the photodiode serves as a base current (I.sub.B) for the bipolar transistor (I.sub.B =I.sub.D). The inherent current gain (beta=I.sub.C /I.sub.B) of the transistor, typically on the order of several hundred to above 10,000 for Darlington transistors, causes the collector current (I.sub.C) to be of a magnitude comparable to the current flowing in the LED (I.sub.LED) The ratio between the collector current and the LED drive current is referred to as the current transfer ratio (CTR=I.sub.C /I.sub.LED) of the optocoupler.
Ideally, the CTR should be unity in order to provide a stable feedback signal. However, there are several inherent characteristics of optocouplers that affect their performance and CTRs. First, the CTR is not predictable due to variations in the fabrication and packaging of optocouplers. For example, normal tolerances in the processing of silicon semiconductors will produce phototransistors having a beta that varies over a 4 to 1 range. In addition, the emissivity of LEDs varies considerably within normal semiconductor processing tolerances. Variations in the mounting and packaging of the LED and phototransistor affects an optocoupler's CTR in several ways. Typically, the LED and photodiode of an optocoupler are mounted facing each other on separate metal leadframes and sealed in an optically transparent plastic. The properties of the plastic are optimized to withstand very intense electric fields. This optically transparent plastic is sometimes coated with a reflective coating to further increase the amount of light directed at the photodiode. The assembly is then encapsulated in a layer of opaque plastic which is optimized for its ability to protect the semiconductors inside from moisture and contamination. Thus, variations in the transparency of the inner plastic and the reflectivity of the interface between the two plastics can vary the number of photons received by the photodiode thus affecting the CTR. Additionally, the distance between the LED and the photodiode can effect the CTR, as well as the incidence angle at which the photons from the LED impinge upon the photodiode.
Another inherent characteristic that effects the performance of the optocoupler is the large junction area of the photodiode. This large junction area creates a parasitic capacitance between the base and emitter junctions of the phototransistor comprising the photodiode. This limits the bandwidth of the feedback signal being coupled through the optocoupler. That is, the gain of the phototransistor rolls off at higher frequencies.
A third inherent problem with optocouplers is that their gain is nonlinear. This not only limits the useful range of the coupler, but also causes the small signal AC gain to be higher than the DC gain, further adding to the unpredictability of the CTR.
Finally, the CTR decreases with age. This is due primarily to a reduction in the emissivity of the LED and to a degradation over time, particularly at high temperatures, in the optical properties of the transparent plastic separating the LED and the photodiode.
To compensate for these characteristic deficiencies in optocoupler performance, prior art circuits have been developed that attempt to improve on the portion of the feedback circuit which drives the LED. More specifically, they attempt to provide an LED drive circuit that has a gain independently controllable from that of the optocoupler. An example of such a circuit, illustrated in FIG. 2, is the Unitrode UC39431 opto driver, which employs an amplifier which feeds a control voltage to a voltage-to-current converter. The voltage-to-current converter converts the control voltage to a ground-referenced current signal which is a linear function of the control voltage and which determines the LED current. With this arrangement, the gain of the LED drive circuit is controlled independently of the optocoupler gain. However, as the current signal which determines the LED current is a linear function of the control voltage, the LED current is varied linearly with respect to the control voltage. The linear variation of the LED current does not compensate for variations in the optocoupler gain in a way that maintains the gain of the overall circuit substantially constant, i.e., substantially independent of the optocoupler's gain. Therefore, the gain of the overall circuit still varies with variations in the gain of the optocoupler despite the linear adjustment of the LED current.
Therefore, there is a need for an adjustable gain block within the drive portion of the optocoupler circuit that compensates for the unpredictable gain fluctuations of the optocoupler, as well as for thermal drift and aging of the circuit, such that the gain of the overall circuit remains constant despite variations in the gain of the optocoupler.