System designers often consider using DC power supplies in parallel for various reasons. These can range from cost and logistics considerations to meeting system current, performance, or reliability requirements. From a non-design perspective, the ability to connect power supplies in parallel also allows a single unit to be used independently or combined across a wide product range. This simplifies procurement, increases the volume of a single power supply, and streamlines inventory management.
However, the technical reasons for considering parallel configurations are more complex. Sometimes, it's due to the unavailability of components with lower power consumption or because the market demands new features that require higher current than initially planned. In such cases, using multiple power supplies in parallel can act as an "insurance" measure. Additionally, parallel power supplies can support N+1 or even N+2 redundancy, preventing single points of failure and enabling hot swapping of faulty units without disrupting the system. They also allow the use of proven power supplies with known features and form factors, reducing design risk. Finally, if a high-power unit generates excessive heat in a confined space, distributing the load can help manage thermal issues.
While parallel power supplies offer flexibility and benefits, they also present challenges. Not all power supplies can be safely connected in parallel. It depends on their internal design, the technology used to interconnect them, and the purpose of the parallel configuration.
The simplest way to connect power supplies in parallel is to join their outputs directly. However, this usually doesn't work well because each power supply has its own voltage regulation. When one supply adjusts its output based on load changes, it can interfere with others. In systems with conventional error amplifiers, differences in parameters may cause one power supply—typically the one with the highest reference voltage—to carry the entire load, leaving others idle. If the load exceeds the capacity of this "leading" supply, it might enter current limit mode or shut down, potentially causing a cascading failure.
If one supply is in constant voltage (CV) mode and another in constant current (CC), a direct connection might work under certain conditions, but not all power supplies allow mode selection. The higher-voltage supply would then provide current until the voltage matches the CV supply. However, this setup makes the power supplies unequal, reducing some of the advantages of parallel operation.
When a power supply is designed for parallel operation or includes a control loop that shares the load, direct connections can be viable. However, this often requires a "current sharing bus" for communication between master and slave units.
Another approach involves adding small ballast resistors at each power supply output to balance the current distribution. While effective, these resistors introduce losses and reduce efficiency. Diode ORing is another method, where diodes prevent reverse current flow. However, it doesn’t fully solve current-sharing issues due to the steep diode curve and can lead to imbalances.
Diode ORing is essential for dual-quadrant power supplies, which can both source and sink current. Without it, direct parallel connections could result in dangerous circulating currents. Using Schottky diodes or active ORing solutions can mitigate this, though at the cost of efficiency and complexity.
In some cases, diode ORing improves system reliability by isolating a faulty power supply quickly, protecting the rest of the system.
To achieve reliable parallel operation, power supplies must be specifically designed for it. Startup synchronization, fault protection coordination, and control loop stability are crucial. A common control strategy involves a single error amplifier regulating the system, with a shared feedback signal distributed to all units. While this ensures good voltage regulation, it introduces a single point of failure.
For isolated DC-DC converters, transmitting the error signal across isolation boundaries can add cost and complexity. An alternative is the "droop-share" method, where each power supply reduces its output voltage slightly as the load increases, helping to distribute current evenly.
Modern solutions like Vicor’s DCM converters use built-in negative-slope load lines, allowing direct parallel connections without external components. This approach eliminates heat loss from resistors and provides fast dynamic response. The DCM’s digital/analog modulator calculates the reference voltage based on output current, simulating a resistor without physical components.
These converters maintain balanced current distribution when all units are identical, making them behave like a single, higher-current power supply. Their temperature-stable output voltage helps maintain equilibrium, even under varying loads.
While this technique is ideal for large converters, it can also apply to smaller ICs like the LT3083, which supports parallel operation with ballast resistors.
In summary, parallel power supplies offer many benefits, including scalability, redundancy, and cost savings. However, proper design and understanding of the topology are essential to ensure stable and reliable operation.
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