Design of power supply current sharing circuit based on UCC29002

In a distributed power supply system, the characteristics of each power module working in parallel may not be completely consistent. If no measures are taken, a certain module may be subjected to a large current pressure, causing the failure of the module or even the entire system [1-2]. Therefore, an effective current sharing control strategy must be introduced in a multi-module parallel operation system so that each module can even bear the load power and improve the reliability of the system.

At present, the commonly used current sharing methods for parallel switching power supply systems include: output impedance method, master-slave current sharing method, average current automatic current sharing method, maximum current automatic current sharing method and external current sharing controller method [3]. Among them, the maximum current automatic current sharing method is widely used due to its advantages of high current sharing accuracy, high load regulation rate, good dynamic response and easy redundancy. Its working principle is: among n parallel modules, the module with the largest output current will automatically become the master module, and the remaining modules will be slave modules. The voltage error of each slave module is adjusted in turn to correct the imbalance of load current distribution. This method can better achieve redundancy and will not affect the operation of the entire system due to the failure of a certain module [4].

1 Introduction to the current sharing control chip UCC29002

UCC29002 is an 8-pin current sharing controller developed based on the maximum current automatic current sharing method. It provides all the functions required for current sharing of multiple independent power supplies or DC/DC modules in parallel. Its main features are [5]:
(1) High accuracy: the current sharing error is less than 1% in the full load range;
(2) High-end and low-end current detection;
(3) Current detection amplifier with ultra-low offset voltage;
(4) Full range adjustable;
(5) Short circuit protection of the current sharing bus to ground or to the positive pole of the power supply;
(6) Small size 8-pin MSOP package with few peripheral components;
(7) Operating temperature range: -40°~+105°.

Figure 1 is the internal block diagram of UCC29002. Its current sharing process is as follows: the current detection resistor detects a signal proportional to the output current of the module power supply at the output end of the module power supply, which is sent to the current detection amplifier, and the output of the current detection amplifier is proportional to the output current of the module power supply, and is used as the input signal to supply the positive input end of the current sharing drive amplifier. Since the gain of the current sharing driver amplifier is 1, the output voltage of the current sharing driver amplifier is equal to that of the current detection amplifier. When this voltage is the highest relative to the potential of all module power supplies, the power module is called the master module, and the output of the master module's current sharing driver amplifier determines the voltage of the current sharing bus. The power supplies of other modules become slave modules, and due to the effect of the diode connected in series at the output end of the current sharing driver amplifier, the output of the slave module's current sharing driver amplifier is not connected to the current sharing bus.

The output voltage of the error amplifier in steady state is a function of the output voltage difference between the current detection amplifier and the output voltage of the current sharing detection amplifier. When working in the master module state, the voltage difference is zero. To ensure the correct output state of the error amplifier, a 25 mV bias voltage is connected in series to its reverse input terminal to increase the conversion margin between the master and slave modules. At the same time, it will ensure that the output of the error amplifier working in the master module state is zero, but all slave modules generate non-zero error voltages, which are proportional to the difference between the output of the current detection amplifier of each module power supply and the bus voltage. The error

voltage is used to adjust the output voltage of the module power supply to balance the load current of all parallel module power supplies. This is achieved by adjusting the amplifier and the buffer transistor. The error signal output by the adjustment amplifier is used to drive the buffer transistor. The error signal is defined as iADJ. The output voltage of the module power supply is adjusted by changing the voltage on RADJUST through iADJ, thereby achieving current sharing between the module power supplies.

The malfunction protection unit prevents the current sharing bus from being short-circuited to the ground or short-circuited to the positive pole of the power supply by comparing the values ​​of CSO and LS. When the control chip fails, the control amplifier starts and adjusts the logic unit to prevent the output of the wrong adjustment signal and realize the protection of the system.

2 Automatic current sharing control circuit design

2.1 Circuit design

Take two module power supplies with an output DC voltage of 28 V and a rated output current of 20 A in parallel as an example. The application circuit of UCC29002 current sharing controller is shown in Figure 2. In the design, the working voltage of UCC29002 is provided by the module power supply.

2.2.4 Determine the error amplifier compensation

CEAO and REAO are the compensation elements of the error amplifier, and their values ​​are related to the stability of the system. In order to avoid mutual interference between the current control loop and the voltage control loop and ensure the stability of the system, the crossover frequency of the current control loop should not exceed one tenth of the crossover frequency of the voltage control loop. CEAO and REAO can be calculated by the following formula:

It meets the national standard requirements.

The maximum current automatic current sharing method achieves load balancing between modules by fine-tuning the output voltage, which actually sacrifices the constant voltage accuracy to obtain the current distribution error index. In addition, to prevent current backflow when the power module in the parallel system fails or is short-circuited, the parallel power modules need to be connected to OR-ing tubes. The power consumption and conduction voltage drop of the OR-ing tube will also affect the power supply performance. How to balance the output voltage accuracy and the current sharing error accuracy still needs further research and improvement.

References
[1] Wei Congying, Zhang Bo. Parallel operation of switching power supplies and their current sharing technology [J]. Electrical Automation, 2004, 26 (2): 3-5.
[2] HSIN HH, HORNG CK. Improved current sharing performance by dynamic droop scaling technique in multiple power systems [J]. IEEE Transaction. On Power Electronics Specialists Conference, 2007, 17 (6): 189-190.
[3] Shi Sanbao. Overview of distributed parallel current sharing technology for switching power supplies [J]. Marine Electrical Technology, 2006, 6 (2): 19-23.
[4] Wang Yufei. Research on parallel system of DC/DC converters based on maximum current method [D]. Chinese Academy of Sciences, 2004.
[5] Texas Instruments. Application note. UCC29002 advanced 8-pin load-share controller [M]. 2008.