How Do Power Supplies Share Current?

Those who use power supplies to run different devices often face a peculiar problem. The load may demand more power from a single power supply that it can safely provide continuously. Since the voltage to the load has to remain constant, the situation calls for using additional power supplies to supply the excess current, and inevitably, users must connect them in parallel. However, simply connecting power supplies in parallel does not guarantee they will share the load current between them in an acceptable manner to operate normally.

Although designers do design some power supplies with dedicated circuits within them to ensure proper sharing of load current when connected in parallel, this is not a generalized practice. Moreover, even if power supplies of one manufacturer can optimally share current when connected in parallel, they may not do so when operating in parallel with power supplies from another manufacturer. In fact, power supplies from the same manufacturer but different models may also not work satisfactorily in parallel.

Theoretically, an ideal voltage source will supply unlimited amounts of power all the while maintaining a constant voltage level. Real power supplies have a limit to the amount of current they can supply to the load. If a load wants to draw power beyond the capacity of the supply, it will reduce its output voltage such that the power delivered remains within its capacity. Should the demand for current increase further, the output voltage reduces further until it reaches zero, and the power supply shuts down. Recovering automatically or through an external reset from an over-current situation is a design feature.

In reality, all voltage sources come with a positive and non-zero internal impedance. This drops the output voltage at the terminals as the load current increases. Power supply specifications call this change in output voltage with load current as the load regulation, and this is specific to each power supply. As the requirement is to have the output voltage change as little as possible with increasing current, some power supply designers prefer to design for low output impedances. Some power supplies have remote voltage sensing to boost the output voltage by the amount it has drooped. However, this is not desirable when sharing current.

One of the problems in connecting power supplies in parallel to supply higher current than either can supply is the current balance characteristics of the units may not match. In case the error in the initial voltage settings between the units is bigger than the depression in the output voltage at maximum load, the first unit may supply its entire share before shutting down. This leads to the second unit attempting to deliver the load current, and since it cannot do so, it shuts down as well.

One of the methods to enable proper current sharing is to enhance the output impedance of each unit so that their individual output voltage droops at full load is far more than the no-load voltage difference between the units. Although the voltage regulation of the system degrades significantly due to the intentional voltage droop, the current sharing between the power supplies is more successful.