Analysis on the application of automatic control technology in power management system

Power supply is an indispensable component of various electronic devices, and its performance is directly related to the technical indicators and reliability indicators of electronic devices. In recent years, with the continuous improvement of the degree of automatic control in the industrial field and the increasing high-end of civil electrical products, higher and higher requirements have been put forward for the structure and performance of voltage-stabilized power supplies. High efficiency, precision, integration and lightness have become the trend and direction of development.

传统线性稳压电源虽具稳定度高,输出纹波电压小等优点, 但很难克服其功耗大、体积笨重、转换效率低的不足。而开关电源则以其损耗低、效率高、电路简洁等显著优点受到人们的青睐, 被誉为高效节能电源。开关电源的最大优势在于采用几十甚至几百的高频电路, 这种高频模式可以做到快速的动态响应和输出反馈调节。开关电源由主电路与控制电路两大部分组成。主电路的能量传递给负载电路, 控制电路则按照输入、输出条件控制主电路工作状态, 将控制电路集成化即成为开关电源管理控制。开关电源己有几十年的发展历史。集成电路设计与制造技术的进步以及供开关电源使用的新型元器件和材料的出现, 为开关电源的蓬勃发展提供了必要条件。进入世纪以来, 开关式电能变换技术无论是技术理论还是产业进程, 都以爆炸式的速度飞速发展, 新技术、新产品不断涌现。集成开关电源沿两个方向不断发展: 第一个方向是对开关电源的核心单元——控制电路实现集成化; 第二个方向则是对中、小功率开关电源实现单片集成化。单片开关电源集成电路具有集成度高、性价比高、外围电路简单、性能指标优良等优点, 是开发中小功率开关电源、精密开关电源及开关电源模块的首选集成电路。由它构成的开关电源, 在成本上与同等功率的线性稳压电源相当, 而电源效率显著提高, 体积和重量则大为减小。这就为新型开关电源的推广与普及, 创造了良好的条件。随着各种电池供电便携式电子产品的快速增长, 对电源管理芯片, 特别是变换器的需求将进一步扩大。而电流控制模式由于其具有更好的电压调整率和负载调整率, 系统的稳定性和动态特性得以明显改善, 特别是其内在的限流能力和并联均流能力可以使控制电路简单可靠, 该技术在上世纪年代初公开以后就受到广泛的重视。目前, 小功率变换器正从电压控制模式向电流控制模式方向转化。与电压型相比, 电流型控制技术可以在逐个开关脉冲上响应负载电压的和电流的变化, 从而改善电路的动态特性。

The PWM comparator outputs a high switch to turn on the switch until the sensed inductor current equals the control voltage. Once this condition is met, the PWM comparator output is low, turning off the switch. A fixed frequency clock signal sets an RS flip-flop to initiate the next cycle. In this way, the peak current of the inductor is precisely controlled by the control voltage. Intuitively, the current loop makes the inductor "act" as a current source, and this structure has many characteristics of current-mode control.

The duty cycle is determined by the inductor current and the output voltage. It is difficult to understand what this structure does to the converter. To have an intuitive understanding of the important characteristics of current-mode control, it is best to start with small signal characterization.

A small signal block diagram of a peak current mode control is shown in Figure 1. There are two feedback loops in the figure: the outer feedback loop feeds back voltage information, while the inner feedback loop (Ti) feeds back current information. The voltage loop acts as a voltage mode control (generating a compensation control voltage from the output voltage error).

Figure 1. Schematic diagram of step-down current mode PWM switching power supply

[page]The current loop—Ti—is the distinguishing element of the current-mode control structure. The input to the current loop is the control voltage, which is compared with the sensed inductor current to set the duty cycle. The duty cycle is transferred to the power supply state (switching element, inductor, output capacitor), generating the corresponding inductor current and output voltage. The inductor current is sensed through Ri and fed back to be compared with Vc.

When the current loop is closed, a seemingly absurd situation occurs: a second-order system with two reactive components (L and COUT) becomes a first-order system. Feedback theory provides an explanation for this. In fact, the feedback loop controls the inductor current much like a current source that feeds back the output inductance and load value. Therefore, when the frequency is lower than the current loop bandwidth, the current-type power supply state is only first-order controlled by the ROUT//RLOAD impedance.

However, the current loop affects the power supply state at more than just low frequencies. Analysis of small signal current disturbances in the current loop shows that it behaves like a split-time sampling system. Such a sample-and-hold system has complex pole pairs at multiple sampling switching frequencies. A second-order approximation to the sample-and-hold yields accurate results up to half the switching frequency. This is the theoretical limit to a power supply's bandwidth.

Several performance parameters are improved in peak current control. Key benefits are excellent linear regulation, simple compensation design, fast response to large load changes, inherent "cycle-by-cycle" current limiting. Current mode disadvantages and problems: Current error and instability - slope compensation required; shallow slopes - poor noise immunity; poor DC open loop load regulation; loop irregularities in multi-output buck circuits.

Figure 2 is the schematic diagram of the internal circuit of the chip. Compared with the voltage mode, the current mode adds the inductor current sampling link, compensation slope, RS trigger and other modules in the current inner loop. Working principle: The voltage of the COMP pin is proportional to the peak current of the inductor. At the beginning of a cycle: the switch tube M1 is off; M2 is on; the voltage of the COMP pin is higher than the output of the current sense amplifier; and the output of the current comparator is low. The rising edge of the oscillator clock signal sets the RS trigger. Its output turns off M2 and turns on M1, so that the SW pin and the inductor are connected to the input power supply. The rising inductor current is sensed by RS and amplified by the current sense amplifier. The slope compensation is added to the output of the current sense amplifier and compared with the output of the error amplifier through the current comparator. When the sum of the output of the current sense amplifier and the slope compensation signal exceeds the voltage of the COMP pin, the RS trigger is reset and returns to the initial state where M1 is off and M2 is on. If the sum of the output of the current sense amplifier and the slope compensation signal does not exceed the voltage of the COMP pin, the falling edge of the clock CLK resets the trigger. The output of the error amplifier reflects the difference between the feedback voltage and the bandgap reference voltage of 0.9V. Its polarity:
When the FB pin voltage is lower than 0.9V, the COMP pin voltage increases. Since the voltage of the COMP pin is proportional to the peak current of the inductor, the increase in the voltage of the COMP pin increases the current delivered to the output. The external Schottky diode is the inductor freewheeling when M1 is turned off. The functional description of each module is shown in Table 1.

Figure 2 Circuit block diagram

Table 1 Internal module function description

References

1. 景欣 降压型开关稳压电源芯片电路的研究与设计 沈阳工业大学报 2009年2月

2. Liu Fucai Research and Design of Buck Pulse Width Modulation Power Management Chip January 2009

3. Sun Yi, A Step-Down PWMDC_DC Power Management Chip Design, January 2009

4. Lu Yuan, Design of a Resonant Mode Power Management Chip, January 2009