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Why cannot power modules be used in parallel?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.15
In high-power applications, a single power module often cannot meet the requirements and usually needs to be used in parallel. However, many power modules cannot be used in parallel, and improper handling can lead to the failure of the entire system. Below is an analysis of why power modules cannot be used in parallel.

The above figure shows the internal equivalent and output load characteristic curve of the power module: VO=f (IO), R is the output impedance of the module (including wire resistance and contact resistance, etc.), and when unloaded, the module output voltage is the maximum value VO (max). When the load current changes △ IO, the change in load voltage is △ VO, △ VO=R * △ IO, and R * △ IO also represents the load adjustment rate of the module. The relationship between load voltage VO and load current IO can be expressed as: VO=VO (max) - R * IO



Internal equivalent diagram of power module and load characteristics of power module: VO=f (IO)

As shown in the above figure: When two modules are connected in parallel with each other, there is:
VO1=VO1 (max) - R1 * IO1
VO2=VO2 (max) - R2 * IO2
IO=IO1+IO2


Schematic diagram of parallel power supply module and load characteristic diagram of parallel power supply module

If the parameters of two modules are exactly the same, that is, VO1 (max)=VO2 (max), R1=R2, then the two load characteristic curves coincide and can achieve uniform distribution of load current. However, in practical applications, two modules with the same capacity, VO1 (max) and VO2 (max), and R1 and R2, cannot have exactly the same parameters. From the graph, it can be seen that due to the small equivalent impedances R1 and R2 outputted to the load RL, even a small difference in output voltage can cause a significant change in output current. For example, when the load RL current increases from IO=IO1+IO2 to IO,=IO1,+IO2, Module 1 with a small slope of the load characteristic curve will bear most of the load current, and Module 1 will operate in a full load or overload current limiting state, affecting the reliability of the module.

In an ideal state, two power modules are used in parallel to supply power to the load. The two power modules work together to evenly share the load power. However, in practical use, they cannot be simply connected in parallel. The main reason is that the output voltage of the two power modules cannot be completely equal. The modules with higher output voltage will provide the majority of the load current, and in severe cases, it can cause one of them to overload, affecting its service life.

Even if the output voltage of two power modules can be adjusted to be completely equal, the different output impedances of the two power modules will cause the load current of the two power modules to be unbalanced. Therefore, simply paralleling the power modules for output will encounter many problems in practical operation.



Taking two power modules in parallel as an example, to ensure stable operation of the power modules after parallel connection, the primary task is to control the maximum output power of each module, and avoid the phenomenon of one module working under overload and the other module working under light load. If such a phenomenon occurs, it will cause damage to the overloaded modules, leading to abnormalities in the entire system.

According to the principle of switch mode power supply, to ensure that the power modules can still accurately limit the output current of each module when used in parallel, an upper end sampling current limiting circuit can be used to achieve this. Considering the actual production of power modules, there will definitely be some differences in output voltage. If two modules are randomly connected in parallel, it is possible for one to work at full load and the other to work at light load. However, since each output is limited within the safe range, even if it works at full load, it will not have a significant impact on the service life of a single circuit and will not affect the entire system design.

The design of parallel circuits for power modules is much more complex than that of series circuits, and requires consideration of issues such as output voltage difference, output impedance matching, and output current balance. Common methods include resistance parallel connection, diode parallel connection, and current sharing parallel connection.
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