Richard Markell, Linear Technology Corp, Milpitas, CA -- 6/22/2000
You often need to know when a telecomm power supply's output drops below its nominal value. The dropout generally indicates a failure and may dictate replacement of the supply or some other form of system maintenance. The circuit in Figure 1 uses an isolated comparator to monitor two 48V telecomm supplies (of either polarity). The comparison occurs on the 48V side of the isolation barrier. The data travels across the barrier inside the package to the output of the comparator, where a µP or another system monitor can check it. IC1, an LTC1531 self-powered, isolated comparator, performs the isolated-comparator function. The IC has an internal capacitive barrier that provides 3000V rms of isolation between the comparator's inputs and outputs. The part provides UL-rated comparisons without an isolated supply or cumbersome optoisolators. The comparator's power and output data traverse the capacitive barrier.
The two power supplies to be monitored connect to the -48 A and -48 B points; the 48V returns connect to the "Common" input (not to isolated ground). Resistor dividers attenuate the -48V inputs; the attenuated voltages connect to the dual comparator at V1 and V2. The VREG pin of IC1 provides a 2.5V regulated output, and the voltage divider consisting of the 11.2- and 8.8-kW resistors provides approximately 1.1V to the Common point for the 48V supplies. Connecting V3 and V4 to isolated ground makes the trip point a negative voltage set by the voltage divider at approximately -1.1V. The series-connected 1N4148 diodes act as crude clamps on inputs V1 and V2. Clamping the inputs is necessary because the comparator function is V1+V2>V3+V4. If the inputs were not clamped, a high voltage on one input would allow a low voltage on the other input to go undetected. The 866- and 22-kW resistors provide a small amount of hysteresis to stabilize the output for slow-moving inputs.
When the inequality V1+V23+V4 is false (that is, the sum of the power-supply voltages when attenuated is greater than the sum of the reference voltages), the comparator sends a signal across the isolation barrier such that the Data output goes low and the LED turns on. (Note that the sense of the inequality is reversed, because you are sensing negative voltages.) The voltages in the circuit are such that when the sum of the two voltages at -48 A and -48 B are approximately -72V, the inequality is false, and the "Supplies OK" LED turns on. Thus, if one supply is "good" at -48V, the other supply is considered "bad" if it falls below approximately -24V. (DI #2541)
