Design of switching power supply intelligent management system for 3500mah 18650 battery

　　3500mah 18650 battery have the characteristics of small size, large specific energy, long life, and good discharge performance. In just a few years, it has been widely used in laptops, mobile phones, portable DVDs, VCDs and other devices, and with the development of production technology, there is a trend of further optimization. 3500mah 18650 battery have so many advantages and their manufacturing costs are relatively low, so they are the most promising portable batteries in the future.

　　For portable batteries, people hope to obtain large-capacity power while minimizing the weight of the system and increasing the efficiency and life of the battery. In addition, since the heat dissipation conditions of portable devices are generally poor, higher requirements are also placed on the efficiency of the entire power supply system.

　　The biggest feature of switching power supply is high efficiency. The use of switching power supplies can effectively reduce the power loss of large-capacity battery charging systems, thereby greatly reducing the heat generation of the entire system. This article analyzes and designs a lithium-ion/lithium-polymer intelligent management system based on switch-mode power supply in detail.

　　1Structure of intelligent management system

　　In this article, we use the most widely used constant current and constant voltage charging method. We use a switch-mode power supply to provide the voltage and current required for battery charging, and use a microcontroller and a series of peripheral circuits to achieve charge and discharge control and control. Battery protection function.

　　Using the combination of microcontroller and switching power supply, we can construct an intelligent lithium-ion/lithium polymer battery intelligent management system: the main power loop of the switching power supply is responsible for converting electrical energy into the form required for battery charging, and at the same time, the efficiency should be improved as much as possible , reducing the voltage and current ripple; the single-chip microcomputer is responsible for controlling the operation of the entire system, including the setting of the charger's reference voltage and current value, the shutdown of the charger when charging is completed or in the protection state, according to various parameters such as battery voltage, charging current, temperature, etc. To intelligently monitor the battery charging status and implement a series of battery protection functions.

　　The entire intelligent management system is divided into two parts: charger and battery pack. The charger mainly includes the main power circuit and a part of the power control circuit; while the battery pack part includes the battery, detection circuit and microcontroller control circuit. The two parts are connected to each other through the interface, and energy is transmitted from the charger to the battery pack. One part of the control signal controls the opening and closing of the charge and discharge circuit of the battery pack, and the other part is sent from the battery pack to the charger to control the start-up of the charger. Turn off and output constant voltage and constant current values. Figure 1 is a block diagram of the entire system.

　　2Circuit design

　　The circuit design includes two parts: the charger and the battery pack.

　　1)Charger design

　　This article designs an AC/DC switching power supply with power frequency input that can achieve constant current and constant voltage output. The specific index requirements of the charger are as follows: input voltage: 130~265Vac; output voltage Uo change range: 0~30Vdc; output constant current Io change range: 0~10A; output voltage ripple △Uppm: <100mV.

　　According to the above charger indicators, the maximum output power of the switching power supply reaches pOMAX=UOMAX·IOMAX=30V×10A=300W. Assuming that the circuit efficiency η=80%, the maximum input power is DC two-stage mode to increase the power factor of the power supply and reduce the adverse impact on the power grid.

　　The pFC part uses the most commonly used Boost circuit combined with the CCM average current mode to achieve the pFC effect. The basic circuit and waveforms of Boost and pFC are shown in Figure 2 and Figure 3:

　　It can be seen from the figure that the average value of the inductor current basically follows the sinusoidal input voltage, so that the entire circuit has a good pF effect and a small THD content, which reduces the adverse impact of the converter on the power grid.

　　Since the maximum output power of the circuit is pOMAX=300W, the design of the DC/DC part chooses a two-tube forward topology with large power capacity and relatively simple control (Figure 4). This topology uses two diodes to reset the excitation current. At the same time, the voltage across the two MOSFETs is also clamped at the input voltage. Therefore, we can choose a switching tube with a relatively low withstand voltage and a relatively small on-resistance, which effectively reduces the conduction loss of the circuit. However, since the excitation current must drop to zero before the start of a new switching cycle, the duty cycle of the circuit must be limited to less than 0.5, so that the excitation energy is completely fed back to the input before the end of a cycle, avoiding possible transformer Magnetic bias or even saturation phenomenon.

　　2) Control circuit design

　　The control circuit of the charger part mainly includes the auxiliary power supply part responsible for supplying power to the charger part control circuit and the power control chip part of the main power circuit (including the control of pFC and DC/DC parts). In addition to the constant current reference signal and constant voltage The reference signal and circuit protection signal are sent out of the charger control circuit by the microcontroller of the battery pack, and all other control functions are completed independently by the charger control circuit.

　　(1) Design of main power supply control part

　　Since the circuit is a two-stage form of pFC+DC/DC, in order to simplify the control circuit, we use the UCC28517 hybrid control chip produced by TI to implement the control function. UCC28517 is one of the UCC2851x series. An important feature of this series of hybrid control chips is that it can provide the functions of rising edge triggering pFC signal and falling edge triggering pWM signal (TEM/LEM), which can significantly reduce energy storage. Current ripple on the capacitor. The chip also provides average current mode pFC control, optional pFC to pWM frequency ratio (1:1 or 1:2), undervoltage protection, DC/DC level programmable soft start and other functions. The UCC28517 frequency ratio we chose is 1:2. The DC/DC stage control starts to work when the output voltage of the pFC stage reaches 90% of the rated value. When the line voltage drops or is turned off, the DC/DC control stage can It turns off only when the output voltage drops to 47% of the rated value, which reduces the impact of power grid fluctuations on the circuit.

　　The power factor correction of pFC circuit is mainly the design of pFC voltage adjustment loop and current adjustment loop. The design of voltage adjustment loop is shown in Figure 5. The design of voltage adjustment loop not only needs to provide the stability of the circuit, but also must attenuate the second harmonic. Effect on THD. The design of the current regulation loop is shown in Figure 6. Unlike the voltage regulation loop, the bandwidth of the current regulation loop must be large enough so that the pFC current can closely follow changes in the input voltage.

　　The DC/DC part adopts the current control mode with current feedforward link. In the current control mode, due to the feedforward effect of the current loop, the entire system becomes a single-pole system, and the adjustment loop is relatively easy to stabilize. The constant current and constant voltage output of the charger is realized through the DC/DC output, so the control loop of the DC/DC part must have two parallel adjustment links: current adjustment and voltage adjustment. The reference values of the two adjustment rings are provided by the microcontroller, and the circuit is shown in Figure 7.

　　(2) Design of microcontroller part and protection circuit

　　In the entire intelligent management system, the microcontroller plays a very important role as the controller of the entire system. It must be able to determine the current state of the battery based on voltage and current sampling; for different states, decide which operations are allowed and which operations are prohibited, and inform the user through the LCD display; when the battery status is abnormal, it should be able to detect in time And alert the operator's attention through alarm means.

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