Designing with droop
Droop current sharing is a technique used in power supplies to distribute the load evenly among multiple parallel-connected power modules. In a parallel power supply configuration, multiple power modules are connected together to provide higher output power and/or improve system redundancy.
In contrast to other methods, droop sharing ensures that each power module shares the load proportionally based on its’ capacity without employing a dedicated current sharing bus. The goal is to achieve balanced current sharing, minimizing any difference in output current to a common load. This allows for better utilization of resources and prevents overload situations.
Virtual Resistance
Assume a typical power module with a power train (P(s)) and a controller C(s) as shown below:
When operating within the bandwidth of the system and within the rated output current of the module, the output voltage is related to the reference voltage according to:
Each module adjusts it’s output voltage in response to changes in it’s own load current. When the load current increases, the output voltage drops slightly, emulating a virtual resistance. The reduction in output voltage limits the current supplied by that module, ensuring that the load is shared among all modules. When introducing this droop via a virtual resistance, it is important to make sure the voltage range of the drop is smaller than the operational range of the end load to ensure overall system stability.
In a large population of power modules, the tolerance of voltage set points will vary. Depending on the design and manufacturing capabilities of the manufacturer, typical tolerances can range from +/- 0.1% to 2%. When 2 or more module are operated in parallel, the steady state output current of either module are determined by the load line of each module as a function of the module voltage tolerance and virtual resistance R.
The relationship between the difference in output current, voltage tolerance, and virtual resistance R is:
In order to minimize the difference in current between multiple modules, either the tolerance needs to decrease, the virtual resistance needs to increase, or both. The maximum virtual resistance is limited by the maximum current rating and the minimum system voltage.
For example, if the desired delta I is 8.75A/+/-4.375A and the maximum virtual resistance is 90 mOhm, then the required voltage tolerance needs to be 0.7% or lower. Conversely, if using a power module with +/- 1% tolerance, and the maximum virtual resistance is 90 mOhm, then the maximum difference in current is 12.25A or +/-6.125A. The figure below shows how the different design parameters relate to each other for a hypothetical 54V system. The contours are lines of constant voltage tolerance, 0.2%, 0.4%, 0.6%, etc…
Constant Current Operation
A key requirement when using droop sharing in a larger system is constant current operation. As overall system load increases, one of the modules will reach it’s maximum rated current first. It’s important that each module support constant current operation down to the under-voltage threshold of the overall system. Once any single module reaches the constant current line, it should transition to operating in constant current mode and acts primarily as a constant current source. Voltage regulation is maintained by the module in the system which have not reached their constant current limit. The difference in current between modules will narrow until all modules reach the constant current limit.
Takeaways
Droop current sharing offers several advantages. It enhances system reliability by preventing a single module from carrying an excessive load, reducing the risk of module failure. It also facilitates the hot-swapping of power modules, as the remaining modules can compensate for the removed module without causing disruptions. Additionally, droop current sharing allows for scalability, where additional power modules can be added or removed easily, adapting to changing power requirements. Droop sharing offers all the advantages listed above without adding a single point of failure to the system, which other active current sharing method requires.
Overall, droop current sharing is a technique used in power supplies to ensure balanced current distribution among parallel-connected power modules. It promotes efficient load sharing, improves system reliability, and enables scalability in power supply configurations.
