Design of intelligent management system for lithium battery switching power supply

0 Introduction

Lithium batteries have the characteristics of small size, high specific energy, long life, and good discharge performance. In just a few years, they have been widely used in notebook computers, mobile phones, portable DVDs and VCDs, and with the development of production technology, there is a trend of further optimization. Lithium batteries have so many advantages, and their manufacturing cost is relatively low, so they are the most promising portable batteries in the future.

For portable batteries, people hope to obtain large-capacity power while reducing the weight of the system as much as possible, 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 system.

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

1 Structure of the 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 single-chip microcomputer and a series of peripheral circuits to achieve charge and discharge control and battery protection functions.

By combining a single-chip microcomputer with a switching power supply, we can construct an intelligent lithium-ion/lithium polymer battery management system: the main power circuit of the switching power supply is responsible for converting electrical energy into the form required for battery charging, while trying to improve efficiency and reduce voltage and current ripple; the single-chip microcomputer is responsible for controlling the operation of the entire system, including setting the reference voltage and current values ​​of the charger, shutting down the charger when charging is completed or in protection status, and intelligently monitoring the battery charging status and realizing a series of battery protection functions based on various parameters such as battery voltage, charging current, and temperature.

The entire intelligent management system is divided into two parts: the charger and the battery pack. The charger mainly includes the main power circuit and a part of the power control circuit; while the battery pack includes batteries, detection circuits and single-chip control circuits. The two parts are connected to each other through an 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 battery pack's charge and discharge circuits, and the other part is sent from the battery pack to the charger to control the charger's start-up and shutdown and output constant voltage and constant current values. Figure 1 is a block diagram of the entire system.

In the charger part of Figure 1, the power circuit is the switching power supply, which is responsible for charging the battery. The power control chip receives control signals such as constant voltage and constant current reference from the single-chip microcomputer, and combines the voltage and current feedback output by the power circuit to achieve the correct output of the circuit under different requirements. In the battery pack part, the single-chip microcomputer uses circuits such as voltage detection, current detection and temperature detection to understand the state of the battery in real time, determine whether the battery needs to be charged, whether the charging method is constant current or constant voltage, when the charging process should be ended, whether the battery has overheating, etc. In addition, the single-chip microcomputer must also detect abnormal conditions, such as overvoltage, overcurrent and overtemperature of the battery, take protective measures in time, and send out alarm signals and display the alarm reasons. External control can preset constant voltage and constant current reference values, and can also manually intervene in the charging and discharging process.

2 Circuit Design

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

1) Design of charger

This article designs an AC/DC switching power supply with power frequency input and constant current and constant voltage output. The specific indicators of the charger are as follows: input voltage: 130~265Vac; output voltage Uo range: 0~30Vdc; output constant current Io 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 the circuit efficiency η=80%, the maximum input power is Due to the large power, we adopt the two-stage mode of PFC+DC/DC 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 circuits and waveforms of Boost and PFC are shown in Figures 2 and 3:

As can be seen from the figure, the average value of the inductor current basically follows the change of the sinusoidal input voltage, so that the entire circuit has a good PF effect and a small THD content, reducing 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 selects a dual-tube forward topology with large power capacity and relatively simple control (Figure 4). This topology uses two diodes to reset the excitation current, and the voltage across the two MOSFETs is also clamped at the input voltage. Therefore, we can choose a switch tube with relatively low withstand voltage and 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 fully fed back to the input before the end of a cycle, avoiding possible transformer bias or even saturation.

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 control circuit and the power control chip part of the main power circuit (including the control of PFC and DC/DC). Except for the constant current reference signal, constant voltage reference signal and circuit protection signal sent by the microcontroller of the battery pack part to the charger control circuit, the rest of the control functions are all completed independently by the charger control circuit.

(1) Design of the main power supply control part

Since the circuit is a two-stage PFC+DC/DC, in order to simplify the control circuit, we use the UCC28517 hybrid control chip produced by TI to realize 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 function of triggering the PFC signal on the rising edge and the PWM signal on the falling edge (TEM/LEM), which can significantly reduce the current ripple on the energy storage capacitor. The chip also provides average current mode PFC control, optional PFC and PWM frequency ratio (1:1 or 1:2), undervoltage protection, DC/DC programmable soft start and other functions. The UCC28517 frequency ratio we selected is 1:2. The DC/DC level control starts working when the PFC level output voltage reaches 90% of the rated value. When the line voltage drops or shuts down, the DC/DC control level can be shut down when the PFC output voltage drops to 47% of the rated value, reducing the impact of power grid fluctuations on the circuit.

The power factor correction of the PFC circuit mainly involves the design of the PFC voltage regulation loop and the current regulation loop. The design of the voltage regulation loop is shown in Figure 5. The design of the voltage regulation loop not only needs to provide circuit stability, but also must attenuate the impact of the second harmonic 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 to enable the PFC current to closely follow the changes in the input voltage.

The DC/DC part adopts a current control mode with a current feedforward link. Due to the feedforward effect of the current loop, the current control mode makes the entire system a single-pole system, and the regulation loop is relatively easy to stabilize. The constant current and constant voltage output of the charger is achieved through the output of the DC/DC, so the control loop of the DC/DC part must have two parallel regulation links: current regulation and voltage regulation. The reference values ​​of the two regulation loops are provided by the single-chip microcomputer, and the circuit is shown in Figure 7.

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(2) Design of single chip microcomputer 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; determine which operations are allowed and which are prohibited according to different states, and inform the user through the LCD display; when the battery state is abnormal, it should be able to detect it in time and alert the operator through alarm means.

Figure 8 is a schematic diagram of the battery pack and its control circuit. Since the battery voltage cannot be completely discharged, the microcontroller is powered by the terminal voltage of the battery pack after voltage stabilization. The battery is a series structure, and a current sampling resistor with a very small resistance is connected to the most negative end of the battery. Since the battery pack can be charged or discharged, the voltage on the current sampling resistor can be positive or negative, and an absolute value amplifier circuit is required to amplify the positive and negative voltages. The absolute value amplifier circuit is shown in Figure 9. Each battery has a voltage sampling point at the positive end, and the voltage signal is transmitted to one of the channels of the AD converter through a voltage follower . Using a voltage follower can make the input impedance infinite, thereby reducing the extraction of battery current, reducing unnecessary battery power consumption, and increasing battery life.

When the circuit outputs 30V, the relationship between the overall efficiency and the output load current is shown in Figure 13.

When the charger output voltage is 12.6V and the load is a battery pack, the output voltage ripple of the circuit is shown in Figure 14. It can be seen that the voltage ripple is basically controlled at around 50mV, which can meet the voltage accuracy requirements for charging lithium batteries.

The experimental results show that the battery management system with a single-chip microcomputer as the core can provide a high-performance and high-flexibility solution.