Basic knowledge of module power supply paralle
ion of current stress and thermal stress between modules, and one or more modules are kept in the current limit state. Because the characteristics of the modules running in parallel are not consistent, good external characteristics may bear more current, or even overload; while those with poor external characteristics run at light load or even no load. This uneven current makes the thermal stress large and reduces the reliability. Tests have proved that when the temperature rise of electronic components rises from 25 degrees to 50 degrees, their life is only 1/6 of that at 25 degrees.
Therefore, for a power system with several switching converter modules in parallel, the requirements are:
1) The current carried by each module can be automatically balanced to achieve current sharing
2) In order to improve the adjustability of the system, do not add external current sharing control measures as much as possible, and combine current sharing with redundant technology
3) When the input voltage and/or load current change, the output voltage should be kept stable, and the transient state of the current sharing should be reconciled
Common current sharing methods are:
1. Output impedance method (sag method, voltage adjustment rate method)
The external characteristics of each module connected in parallel show droop characteristics, the heavier the load, the lower the output voltage. When connected in parallel, modules with hard external characteristics (small internal resistance) have a large output current; modules with soft external characteristics have a small output current. The idea of the output impedance method is to try to adjust the slope of the external characteristic with hard external characteristics (small internal resistance and low slope) to be close to the module with soft external characteristics, so that the current distribution of the two modules is close to average.
2. Master-slave setting method
The master-slave setting method is that one module is selected as the master module (MasterModule), and the remaining modules are used as the slave modules (SlaveModule). The voltage regulator of the main module is used to control the voltage adjustment values of the remaining parallel modules, and all parallel modules have current-type inner loop control. Since the current of each slave module is modulated according to the same reference current (the reference current converted from the voltage error of the master module), it is consistent with the current of the master module to achieve current sharing.
The main disadvantages of the master-slave setting method:
1) There must be a communication link between the master and slave modules, which makes the system complicated
2) If the main module fails, the entire system will not work, not applicable to redundant parallel systems
3) The voltage loop has a large bandwidth and is easily affected by external interference
3. Automatic current sharing method for uniform current
A current-sharing bus is used to connect the output terminals of all power modules to output current sampling voltage. The voltage on the current-sharing bus is supplied by the current-sharing resistance of each power module by the sampling voltage of all parallel power modules. In layman's terms, the voltage of the current-sharing bus is the uniform value of the current signal of each module (presented in voltage), and then the current signal (presented in voltage) of each module is compared with the current-sharing signal to obtain the compensation amount for control .
The uniform current automatic current sharing method can accurately share current. However, when a certain module connected to the bus bar does not work, it will cause the uniform value of the bus bar to decrease, the voltage will drop, and the failure will occur when it reaches the offline.
4. Automatic current sharing by maximum current method
Also known as "democratic current sharing method", this method is similar to the master-slave setting method, the difference is that the master module is not fixed, and the module with the highest current in the system automatically works as the master module.
5. Thermal stress automatic current sharing method
This method automatically equalizes the current according to the current and temperature (ie thermal stress) of each module. In the system, the current uniform value of each module is still used as a comparison reference, and the current signal of each module is compared with the current sharing bus to obtain the error, and then the control is compensated. (I don’t understand the difference with the previous uniform current method at present)
6. Additional current sharing controller
When using this method, a special current sharing controller must be added to the control circuit of each module to test the current imbalance of each module in parallel and adjust the control signal to achieve current sharing. However, the introduction of the current-sharing controller adds to the complexity of the system. If the design is not correct, the system may become unstable