Structural Analysis of Improving Power Supply Redundancy

To increase redundancy, several power supplies using ORing diodes can be connected to the same load. During maintenance, when you remove any power supply, you want to minimize the power disturbance to the load. To compensate for the voltage drop across the ORing diode, you must connect the power supply feedback line at the load after the ORing diode. Therefore, the feedback connection of all participating power supplies is common (Figure 1).

Figure 1. Standard redundant configurations for power modules all use ORing diodes at the output.

Because each power supply has natural variations, only the power supply with the largest VOUT is active. The other power supplies that sense a "high" output attempt to reduce their output, effectively suspending regulation. If you remove the "active" power supply module from a setup similar to Figure 1, you will cause VOUT to drop (Figure 2).

Figure 2 When you remove a power supply from a redundant configuration, it causes a drop in output voltage (a) and a transient spike (b).

Figure 2a applies to a linear power module consisting of two regulators with output voltages of 3.339V and 3.298V respectively. The load of both regulators consists of a 10Ω resistor and a 100μF capacitor in parallel. Figure 2b applies to a boost power module consisting of two regulators with output voltages of 5.08V and 4.99V respectively, where the load of each regulator consists of a 2.5Ω resistor and a 100μF capacitor in parallel. The reason for the drop in output voltage and the transient fluctuation is that there is a delay in the start-up and start of regulation of the two regulated power supplies. Expensive power modules use current sharing technology to solve this problem. Current sharing technology distributes the output current to all power modules approximately equally, so that all power modules are effective. The configuration shown in Figure 3 does not add much cost to the power system. However, the improvement in performance of this configuration is very obvious from Figures 4a and 4b representing two types of redundant power modules.

Figure 3. Adding an instrumentation amplifier and a few passive components can prevent output voltage droop and transient fluctuations in a redundant configuration.

Figure 4 Both the linear regulator (a) and the boost regulator use the circuit shown in Figure 3 to eliminate output voltage drops and transient fluctuations.

Instrumentation amplifier IC1 measures and generates a voltage, VC, that is proportional to the current input to the regulator. VC in turn controls VOUT, which puts the regulator into operation. For most adjustable controllers, VOUT = VREF (1 + RA / RB), where RA and RB are R1A and R1B in module 1. If no current flows through RSENSE, the output of IC1 is close to ground potential, making R1B in parallel with the resistance of D12, R11, and R12, making RB smaller and VOUT1 higher. The increase in VOUT1 only needs to compensate for the variation in VOUT between power modules with the same configuration. This variation is only a few percentage points. If the current flowing into the load increases, VC will also increase, reducing the current flowing through D12, resulting in a decrease in VOUT1. When the output voltage of IC1 rises and the difference from VFB is less than the direct voltage drop across D12, no current flows in D12. Therefore, for any larger current, VOUT remains at the value specified by the above formula. As long as R3 (gain setting resistor of the instrumentation amplifier) ​​is selected properly, R1 and R2 of the other power modules use all power supplies to provide the required current to the load, thus ensuring that all power supplies are in working condition.