Multiple PSUs share load

August 26, 2013 // By Vardan Antonyan
 Multiple PSUs share load
Vardan Antonyan explains how to use steering Schottky diodes to provide load balancing to deliver more power than a single power supply.

In some projects, we need to deliver more power than a single power supply can provide, and in this situation we can use steering Schottky diodes to provide load balancing ( Figure 1) . In this schematic, we combine the output currents to provide some simple load sharing. Note that this is different from power redundancy, but instead the case where total output power cannot be delivered by a single power supply. This circuit is simple enough, and will work in ideal conditions where VPS2 = VPS1. What happens on the production floor is much more interesting, and invalidates this approach.

Figure 1:
Two power supplies of equal voltage drive the load in current-share mode.

To evaluate the circuit, we can use the formula for Schottky diode forward voltage calculation at different currents to analyze the circuit performance at different loads and power supply deviations.

The problem is that this formula is a good approximation only, and you need to use appropriate n to get similar results to diode manufacturers’ V-I graph (in this case, n was selected to be 10). The analysis proved a little harder than expected since we have to consider two different power supplies, and calculate currents in an iterative manner. To solve this, we used multiple iterations using C (download available below) to calculate currents and voltages for this circuit.

The results were disappointing, since they show that in the case of ±1% voltage deviation, 90% of the power is supplied by a single supply. Basically, this circuit is not a good solution for power supplies with more than a few tens of millivolts difference. The problem is that not all off-the-shelf power supplies have output voltage adjustments – especially not the sealed ones. To solve this issue, a circuit was developed to ensure load sharing using off-the-shelf power supplies and components ( Figure 2 ).
 

Figure 2:
High-side current monitors U1 and U2
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