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 self-control degree in the industrial field and the increasing high-end of civilian electrical products, higher and higher requirements have been placed on the structure and performance of the regulated power supply. Efficient, precise, integrated, and lightweight have become the trend and direction of development.
Although the traditional linear regulated power supply has the advantages of high stability and small output ripple voltage, it is difficult to overcome the shortcomings of large power consumption, bulky size, and low conversion efficiency. Switching power supplies are favored by people for their remarkable advantages such as low loss, high efficiency, and simple circuit. They are known as high-efficiency energy-saving power supplies. The biggest advantage of switching power supply is the use of tens or even hundreds of high-frequency circuits, which can achieve fast dynamic response and output feedback adjustment. The switching power supply consists of two main parts: the main circuit and the control circuit. The energy of the main circuit is transmitted to the load circuit, and the control circuit controls the working state of the main circuit according to the input and output conditions, and the control circuit is integrated into the switching power supply management control. Switching power supplies have been in development for decades. Advances in integrated circuit design and manufacturing technology and the emergence of new components and materials for switching power supplies have provided the necessary conditions for the vigorous development of switching power supplies. Since the beginning of the century, switching-type power conversion technology has developed rapidly at explosive speeds, both in technical theory and in industrial processes. New technologies and new products are emerging. The integrated switching power supply is continuously developed in two directions: the first direction is the integration of the control unit of the switching power supply core unit; the second direction is the monolithic integration of the medium and small power switching power supplies. The single-chip switching power supply integrated circuit has the advantages of high integration, high cost performance, simple peripheral circuit and excellent performance index. It is the first choice integrated circuit for developing medium and small power switching power supply, precision switching power supply and switching power supply module. The switching power supply composed of it is equivalent in cost to the linear power supply of the same power, and the power supply efficiency is remarkably improved, and the volume and weight are greatly reduced. This has created favorable conditions for the promotion and popularization of new switching power supplies. With the rapid growth of various battery-powered portable electronic products, the demand for power management chips, especially converters, will further expand. The current control mode has better voltage stability and load regulation, and the stability and dynamic characteristics of the system are significantly improved. In particular, its inherent current limiting capability and parallel current sharing capability make the control circuit simple and reliable. Technology gained widespread attention after its publication in the early part of the last century. Currently, small power converters are shifting from voltage control mode to current control mode. Compared with the voltage type, the current type control technology can respond to changes in the load voltage and current on a switching pulse, thereby improving the dynamic characteristics of the circuit.
The PWM comparator will output a high switching transistor open until the induced inductor current is equal to the control voltage. Once this condition is met, the PWM comparator output is low and the switch is turned off. An RS flip-flop is set by a fixed frequency clock signal to initiate the beginning of the next cycle. In this way, the peak current of the inductor is precisely controlled by the control voltage. Intuitively, the current loop causes the inductor to "play" a current source. This configuration has many current-mode control characteristics.
The duty cycle is determined by the inductor current and the output voltage. It is difficult to understand what effect such a structure has on the converter. An intuitive understanding of the important characteristics of current-mode control is best done with small-signal personality analysis.
A small signal block diagram of peak current type control is shown in Figure 1. There are two feedback loops in the figure: the external feedback loop feedback voltage information, and the internal feedback loop (Ti) feedback current information. The voltage loop acts as a voltage type control (a compensation control voltage is generated from the output voltage error).
Figure 1 Block diagram of the step-down current mode PWM switching power supply
The current loop—Ti—is a distinguishing component of the current-mode control structure. The input to the current loop is the control voltage, which is compared to the sensed inductor current and sets the duty cycle. The duty cycle is turned into the power supply state (switching element, inductor, output capacitor), and the corresponding inductor current and output voltage are generated. The inductor current is sensed through Ri and fed back to compare with Vc.
A seemingly ridiculous situation arises when the current loop is turned off: A second-order system with two reactive components (L and COUT) and a first-order system 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 the first order controlled by the ROUT//RLOAD impedance.
However, the effect of the current loop on the power supply state is not just low frequency. Analysis of small signal current disturbances in the current loop shows that it is much like a separate time sampling system. Such a sample and hold system has complex pole pairs at multiple sampling switching frequencies. Accurate results can be obtained for second-order approximations of sampling and maintenance when the frequency can be as high as half the switching frequency. This is the theoretical limit to a power supply bandwidth.
Several performance parameters have been improved in peak current control. The key benefits are excellent linearity adjustment, simple compensation design, fast response to large load changes, and inherent “cycle-by-cycle†current limiting. Disadvantages and problems of current mode: current error and instability - need slope compensation; shallow slope - poor anti-noise ability; DC open-loop load regulation rate difference; loop in multi-output buck line law.
Figure 2 is a schematic diagram of the internal circuit of the chip. Compared with the voltage mode, the current mode increases the inductor current sampling section, compensation slope, and RS flip-flop of 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: switch M1 is off; M2 is open; COMP pin voltage is higher than current sense amplifier output; and current comparator output is low. The rising edge of the oscillator clock signal asserts the RS flip-flop. Its output turns off M2 and turns on M1, which connects the SW pin and inductor to the input supply. The rising inductor current is sensed by RS and amplified by a current sense amplifier. The slope compensation is added to the current sense amplifier output and compared to the error amplifier output by a current comparator. When the sum of the current sense amplifier output and the slope compensated signal exceeds the COMP pin voltage, the RS flip-flop is reset and returns to the initial state where M1 is turned off and M2 is turned on. If the sum of the current sense amplifier output and the slope compensated signal does not exceed the COMP pin voltage, then the falling edge of clock CLK resets the flip flop. 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 at the COMP pin is proportional to the peak current of the inductor, the increase in the voltage at the COMP pin causes the current delivered to the output to increase. The external Schottky diode is a freewheeling inductor when M1 is turned off. The function description of each module is shown in Table 1.
Figure 2 circuit block diagram
Table 1 Internal module function description
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