Let's discuss why a current sharing circuit is necessary. When a single module cannot supply the required current for a load, multiple modules can be connected in parallel to meet the total demand. However, due to variations in output voltage and differing output impedance characteristics among modules, simply connecting them in parallel does not ensure equal current distribution. Some modules may end up operating at full load while others remain underloaded. This is not ideal, as modules perform best within their optimal load range. Prolonged operation outside this range can reduce the system’s overall lifespan. Therefore, additional circuitry is essential to achieve balanced current sharing across all modules.
So, how do we implement current sharing? The core issue lies in the inconsistency of output voltages. One might think adjusting each module’s voltage to match exactly would solve the problem. While theoretically possible, in practice, it’s challenging to maintain identical output voltages across all modules under varying loads. If you have a method to achieve this, I’d love to hear about it. But in most real-world scenarios, this isn’t feasible. Thus, we need an external current sharing circuit that dynamically adjusts the output voltage—lowering it when the current is high and raising it when the current is low, ensuring even current distribution. Sounds simple, right?
**Output Impedance Method (Droop Method)**
One common approach is the output impedance method, also known as the droop method. In this technique, the output voltage of each module decreases as its output current increases. This naturally causes the modules with higher initial voltages to reduce their output, allowing other modules to take on more current. The circuit typically involves amplifying the current signal and combining it with the feedback signal to adjust the voltage loop. As current increases, the output voltage is reduced. It’s important to connect this correctly—stacking it incorrectly could reverse the logic, causing the output voltage to rise with higher current. This method is not suitable for modules requiring high voltage regulation accuracy, as the output voltage can vary significantly with load.
**Master-Slave Configuration**
Another method is the master-slave configuration, where one module is designated as the "master" and others follow its lead. The master operates in voltage mode, while the slaves operate in current mode. This creates a double-loop control system: an outer voltage loop and an inner current loop. The main drawback is that if the master fails, the entire system may crash. It’s a reliable solution but lacks redundancy.
**Average Current Method**
The average current method calculates the total load current and divides it by the number of modules to determine the target current per unit. Each module compares its own current to the average. If it exceeds the average, its output voltage is lowered; if it’s below, the voltage is increased. This ensures balanced current sharing. A current sharing bus is used to measure and compare the average current. For example, if four modules are connected, the bus voltage represents the average of their outputs. This method is flexible and widely used, especially in systems requiring precise current balancing.
**Peak Current Method**
The peak current method allows modules to self-select a "master" based on the highest current. Other modules then adjust their current to match the peak. This is achieved using diodes instead of resistors to create a current-sharing bus. The module with the highest current becomes the reference, and others align with it. This method is called "democratic" because the master is chosen automatically, and if it fails, another takes over. It’s a robust solution for redundant systems.
In practical applications, such as a 12-module system with an output voltage range of 198–286V and a current sharing accuracy of less than 3%, the wiring is critical. Ensuring low-impedance connections between module outputs helps maintain stability and accuracy. Proper grounding and layout are key to achieving effective current sharing.
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