In order to increase redundancy, many power supplies using OR circuits can be connected to the same load. During maintenance, when you remove any of the power supplies, you want the power supply to be as small as possible. To compensate for the voltage drop across the OR diode, you must connect the power feedback line at the load after the OR circuit. Therefore, all feedback connections to the power supply are common (Figure 1).
Figure 1 The standard redundant configuration of the power module uses an OR circuit at the output.
Because each power supply will naturally change, only the power supply with the largest VOUT is valid. Other power supplies that detect "high-potential" outputs attempt to reduce their output, effectively stopping the regulation function. If the "active" power module is removed from a setup similar to Figure 1, VOUT will drop (Figure 2).
Figure 2 When you remove a power supply from a redundant configuration, it causes a drop in output voltage (a) and instantaneous fluctuations (b).
Figure 2a applies to a linear power module consisting of two regulators with output voltages of 3.339V and 3.298V each. The load of both regulators consists of a 10Ω resistor and a 100μF capacitor in parallel. Figure 2b is for a boost power supply module consisting of two regulators with output voltages of 5.08V and 4.99V, each of which is composed of a 2.5Ω resistor and a 100μF capacitor in parallel. The reason why the output voltage drops and instantaneous fluctuations is that there is a delay in the start-up and start-up of the two regulated power supplies. Expensive power modules use current sharing techniques to solve this problem. The current sharing technique distributes the output current approximately equally to all of the power modules so that all power modules are active. The configuration shown in Figure 3 adds little to the power system. However, the performance improvement of this configuration is apparent from Figures 4a and 4b, which represent two types of redundant power modules.
Figure 3 Adding an instrumentation amplifier and several passive components prevents redundant configuration output voltage drops and transient fluctuations.
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 tests and produces a voltage VC proportional to the current of the input 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, respectively. If no current flows through RSENSE, the output of IC1 is close to the ground potential, so that R1B is connected in parallel with the resistances of D12, R11 and R12, so that RB is smaller, VOUT1 is higher, and VOUT1 is increased only by compensating for the same power supply. VOUT changes between modules. This change is only a few percentage points. If the current flowing into the load increases, the VC also increases, thereby 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 through D12. Therefore, for any larger current, VOUT maintains the value specified by the above formula. As long as R3 (the gain setting resistor of the instrumentation amplifier) ​​is properly selected, R1 and R2 of the other power modules use all the power supplies to supply the required current to the load, thus ensuring that all power supplies are in operation.
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