For redundancy purposes, a number of power supplies, using ORing diodes, can work into the same load. During maintenance, when you can remove any power supply, the minimum possible power perturbation at the load is desirable. To compensate for the voltage drop across the ORing diodes, you must connect the power-supply feedback lines after the diodes, at the load. Thus, the feedback connection is common for all participating power supplies (Figure 1). Because of natural variations in each power supply, only the one with the highest V_OUT is active. The others, sensing the "higher" output voltage, try to reduce their own outputs, effectively turning off their regulation. If you remove the "active" module from the setup similar to Figure 1, it causes V_OUT to dip (Figure 2). Figure 2a applies to a linear module that comprises two regulators that have independent 3.339 and 3.298V output voltages. Both have loads of approximately 10Ω in parallel with 100 µF. Figure 2b applies to a boost configuration that comprises two regulators with 5.08 and 4.99V outputs, each loaded with approximately 2.5Ω in parallel with 100 µF. The sags and glitches in the voltages arise from the inevitable delay for another power supply to step in and to start the regulation. Costly power-supply bricks deal with this problem by using current-sharing techniques. The techniques provide roughly equal output-current distribution among all power modules, thus keeping all modules active. The configuration in Figure 3 adds little cost to a power system. The improvements in performance are evident in Figures 4a and 4b, representing the two types of redundant power-supply modules.

An instrumentation amplifier, IC₁, measures and produces a voltage, V_C, proportional to the current going into the regulator. V_C in turn controls V_OUT, pushing the regulator into active mode. For most adjustable controllers, V_OUT=V_REF(1+R_A/R_B), where R_A is R_1A, and R_B is R_1B for module 1. If no current flows through R_SENSE, IC₁'s output is close to ground, paralleling R_1B with the resistances of D_1.2, R₁₁, and R₁₂, thereby making R_B smaller and V_OUT1 consequently higher. The increase needs only to compensate the V_OUT variation between same-configuration power supplies. This variation is only a few percentage points. If the current into the load rises, V_C also rises, reducing the current through D_1.2 and consequently reducing V_OUT1. When IC₁'s output rises and differs from V_FB by less than the direct voltage drop across D_1.2, no current flows through D_1.2. Thus, V_OUT1, for any higher current, stays at the value the above equation defines. With the proper selection of R₃ (the instrumentation amplifier's gain-setting resistor), R₂ and R₁ from other modules provide the required current into the load from all power supplies, guaranteeing that they stay in active condition.

Is this the best Design Idea in this issue? Select at www.edn.com.