Multiple power sources are often used in high-power applications for load sharing to deliver greater power, or redundancy for high availability. A diode-OR circuit, Figure 1 , is often the accepted solution to combining the inputs to a single output. If the two power sources are identical, a diode-OR circuit is not a bad solution, but it may not be the best option if the two power sources have different voltages, such as 12 V and 24 V.
Figure 1: This simple load-sharing diode-OR circuit requires current-sharing resistors to balance the currents of two sources.
However, if increased power capability is the goal via power sharing of two different sources, a diode-OR circuit will have hard time balancing the currents of the two sources. Slightly different voltage drops in diode-OR circuit can produce large current imbalance between the two power sources, which may overload one power source while preventing the second source from providing any power to the load. To remedy the imbalance, current-balancing resistors may be required, which results in additional power loss and decreased system efficiency.
An efficient and space-conscious solution is to use an active current-sharing circuit in the form of a power converter. A power converter can provide both load-current balancing and regulation, precluding the need for another regulator.
The circuit of Figure 2 is a 130W to 260 watt base station boost converter with dual power source, but its underlying design principle can be applied to other topologies, including buck, boost, flyback and SEPIC, to satisfy various input and output specifications.
Figure 2: This 95% efficient, 28 V base station power converter can operate from redundant power sources.
(Click on image to enlarge)
It is an active current-sharing boost converter that can deliver 130 W of output power from a either of two redundant 12 V power sources, or 260 W by load sharing the two 12 V power sources. The circuit can also generate 260 W from single 12 V power source if inputs A and B are tied together.
The core of the design is the LT3782 current-mode PWM controller. Current-mode operation ensures balanced current sharing between the two power sources, even if the sources have different voltages. Current balancing improves efficiency of the entire system by allowing each power source to operate at lower power level, where the efficiency is typically higher. The current balance is achieved by choosing appropriate values of the two current-sense resistors, RCS1 and RCS2 in Figure 2, to provide relatively more power from the supply with higher output-power rating. For example, the currents can be programmed to provide 25% of output power from a 5 V source and 75% from a 12 V source.
The circuit can be powered from either input A or input B. The only condition is that at least one of the inputs is greater than 10 V, which is required for biasing of PWM controller circuit U1 . Diodes D3 and D4 provide the diode-OR function for biasing of controller U1 . The bias power for the controller can also be provided by a separate power source. In that case, theoretically, the circuit could regulate with inputs down to 0 V. In practice, the lowest required input voltage depends on the control circuit’s maximum duty cycle and output voltage. The circuit can produce 28 V of output voltage from 2 V input. However, the higher input current at 2 V input will result in lower available output power.
The efficiency of the converter, Figure 3 , is high enough that it can be built entirely with surface-mount components, without the need for heat sinks.
Figure 3: The converter in Figure 2 peaks at 95% efficiency when operating from two inputs.
In a 130 W, redundant-supply application, the power dissipation of 8.4 W should be relatively easy to manage; but for the 260 W application, the 17 W power dissipation needs more attention. A large, well laid-out, multilayer PCB with some forced airflow should be sufficient to keep the components cool.
The simple switching-power converter shown here can be used to boost one of two redundant supplies, or it can be used to combine the supplies for high-power output. Either way, the result is an efficient and compact circuit, better than a diode-OR circuit, which would dissipate additional power.
About the author
Goran Perica is an applications engineer with the power group atLinear Technology Corp, San Jose, CA.