Basic principles, implementation methods and simulation applications of current sharing in power modules

In many situations with high current output, in order to improve the reliability of the system, a commonly used method is to use hot backup - multiple power modules are used in parallel. Each power module also has the function of online plug-in and unplug, which is convenient for disassembly, repair and maintenance.

But we know that the internal resistance of each power module is slightly different, and the output voltage cannot be completely consistent. Therefore, the voltage source with regulated output cannot be directly connected in parallel, or even if it is connected in parallel, the output power of each module is different. There may be a phenomenon that some modules are idle and some are busy. Some modules are overloaded, and the loss and heat are relatively severe, which will reduce their lifespan. Some modules work under light load and do not even enter a good working state (for example, the phase-shifted full bridge is not easy to achieve soft switching under light load), which is also not good for the health of the power supply.

At this time, we need a method to make the output power of each module basically the same. This method of evenly distributing the load to each module is called current sharing .

There are many ways to share current, such as:

1. Output impedance method, also known as droop method, tilt method, voltage regulation method. It is achieved by adjusting the output internal resistance of the power supply. The characteristic of this method is simplicity. But the biggest disadvantage is the poor voltage regulation rate.

2. Master-slave setting method, artificially select a master module from the parallel modules, and the output of other modules will be close to this module. The biggest problem is that if the master module fails, the entire power system will not work.

3. Average current automatic current sharing method: After the current sampling of each module is amplified, it is connected to the common current sharing bus through a resistor. Everyone adjusts and completes the current sharing according to the average voltage on the current sharing bus. The average current automatic current sharing method can achieve accurate current sharing, but if the current sharing bus is short-circuited or a module fails, the bus voltage drops, which will cause the voltage of each module to drop.

4. The maximum current automatic current balancing method is also called automatic master-slave current balancing or democratic current balancing. Among all the parallel modules, the module with the largest output current automatically becomes the master module, and the outputs of other modules are close to this module.

5. There are many other methods, such as automatic current sharing due to thermal stress, current sharing with an external current sharing controller, etc.

The most widely used method is the maximum current automatic current equalization method, and there are ICs designed specifically for this purpose, such as UC3907. However, here, I will not use a dedicated IC, but only use an ordinary op amp to try to implement this function. Use ORCAD for simulation.

The specific working principle is actually very simple, which is to amplify the error between the current sampling value and the current sharing value of this module, and then use the value of the error amplifier to adjust the value of the voltage feedback loop to change the output voltage, so as to adjust the output current of this module and make the current feedback value the same as the value of the current sharing bus, thereby realizing the maximum current automatic sharing.

The following figure is the current sharing circuit inside a single module. U1A is a voltage error amplifier, U2A is a voltage follower for voltage sampling, U3A is a current sampling amplifier, which reversely amplifies the collected current signal 100 times as a positive voltage signal, U4A is a current sharing error amplifier, and U5A is a voltage follower, which outputs the sampling signal of the output current of this module to the current sharing bus. However, this voltage follower has a slight change, that is, if the voltage on the current sharing bus is higher than the voltage of the current sampling signal of this module, then the signal of this module will not be output to the bus. Therefore, the voltage signal on the bus is always the module with the largest output current. In addition, there is a model E, which is a module that amplifies the output voltage. It is used here as a power converter. The output signal of the voltage error amplifier is amplified as the output, and the gain is set to 10. For the load, I used a 9A current source to simulate the constant current load.

OK, let's copy the part of the circuit with the op amp in the middle of the diagram twice and paste them in the same circuit diagram. Then, connect a voltage source in reverse series to the output of one of the modules, and use the method of slowly increasing the voltage source to simulate the process of this power module failing, to try to observe whether other modules can continue to maintain current sharing. Of course, if you have a better simulation method, you can also propose it for joint research. After copying the circuit, select windows->xxx.opj in the menu to enter the OPJ management window, then select the Hierarchy tab, click schematic1, and then select the menu TOOLS->annotate. The Annotate dialog box pops up, select Unconditional reference update in the action item, and click OK.

Then return to the circuit diagram window from the menu window, perform a DC sweep simulation, and set our reverse voltage source to sweep from 0V to 5V with a step size of 0.01V.

Perform simulation and place current probe

View simulation results

It can be seen that in the process of the reverse voltage source voltage changing from 0 to 3V, due to the regulation function of the circuit, the output of the module can still maintain the current balance. Then, because the voltage of the simulated reverse voltage source increases, it exceeds the regulation ability of the circuit, and the output current of the module begins to decrease, while the output of the other two modules begins to increase. For the two normal modules, the current is still balanced. When the faulty module completely stops outputting current, the load current is completely provided by the two normal modules on average.

Changing the resistance value of R20 in Figure 1 can change the ability of the current-sharing error amplifier to regulate the output voltage. If we change the resistance of each module to 3K, re-simulate, and look at the current waveform, we can see that all modules can still output current in the entire range of the reverse series voltage source from 0 to 5V.