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.
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| 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).
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| Figure 2. | High-side current monitors U1 and U2 sense each supply’s current, and cross-coupled control of Q1 and Q2 equalizes the current sourced by each supply. |
| Table 1. | Two power supplies of equal voltage drive the load in current- share mode. |
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J1 and J2 are connected to input power supplies, and the load is connected to VIN. As shown, in addition to the original power steering diodes, we now also have Q1 and Q2 MOSFETs shunting the diodes to regulate load sharing. The MOSFETs are driven by U3B and U3A op-amps, each configured to compare the current of its own supply with that of the other. The circuit does not have any stringent requirements, but R1, R11, R2, and R12 should have 1% tolerance. The op-amps are driven from simple RC low pass filters to smooth out any transient response. We use U1 and U2 to measure the current at each power supply output, and use the RC filter-amplifier-MOSFET combination to equalize the currents. This solution has been proven to work for 12 V-19 V input voltages (common laptop power supplies), and used to provide 10 A current to the load. Load sharing efficiency is good enough to allow cascading of these circuits to combine four power supplies. See the Tables 1 and 2 for circuit performance results.
| Table 2. | With Load Sharing (Figure 2) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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