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Posted on 01 December 2019

Dual Input Boost Converter Shares Power from Two Inputs

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Current mode operation ensures balanced current sharing

Multiple power sources are often used by high power applications for load sharing (for high power) or redundancy (for high availability).

By Goran Perica, Linear Technology

 

A diode-OR circuit, as shown in 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 (e.g., 12V and 24V).

Diode-OR circuit

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.

This simple load sharing diode-OR circuit requires current sharing resistors in order to balance the currents of two sources. The added resistors present a heat dissipation problem and related efficiency hit in high power circuits.

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 discussed here is a boost converter, but the underlying design principle can be applied to other topologies—such as buck, boost, flyback and SEPIC—to satisfy various input and output specifications.

130W to 260W Base Station Boost Converter With Dual Power Source

Figure 2 illustrates an active current sharing boost converter that can deliver 130W of output power from a either of two redundant 12V power sources, or 260W by load sharing the two 12V power sources. The circuit can also generate 260W from single 12V power source if inputs A and B are tied together.

Powered from either input A or input B

The centerpiece 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 5V source and 75% from a 12V source.

This 95% efficient, 28V base station power converter can operate from redundant power sources.

The circuit in Figure 2 can be powered from either input A or input B. The only condition is that at least one of the inputs is greater than 10V, 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 controller U1 can also be provided by a separate power source. In that case, theoretically, the circuit could regulate with inputs down to 0V. In practice, the lowest required input voltage depends on the control circuit’s maximum duty cycle and output voltage. The circuit in Figure 2 can produce 28V of output voltage from 2V input. However, the higher input current at 2V input will result in lower available output power.

The converter in Figure 2 peaks at 95% efficiency when operating from two inputs.

The efficiency of this converter (Figure 3) is high enough that it can be built entirely with surface mount components, without the need for heat sinks. In a 130W, redundant supply application, the power dissipation of 8.4W should be relatively easy to manage; but for 260W application, the circuit’s power dissipation of 17W needs more attention. A well laid out large multilayer PCB with some forced airflow should be sufficient to keep the components cool.

High efficiency allows surface mount components, without the need for heat sinks

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.

 

 

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