Application and development of digital technology in switching power supply control

　　1. Introduction

　　With the rapid development of power electronics technology, switching power supplies have been widely used, and the ever-changing high-tech products have also put forward higher requirements for switching power supplies. The analog control technology of switching power supplies has also been developed for many years and is relatively mature in all aspects, but it cannot overcome the inherent shortcomings: the control circuit is complex, there are many components, which is not conducive to the development of miniaturization; once the control circuit is formed, it is difficult to modify and debug; the control is not flexible, and complex control methods are difficult to implement with analog methods.

　　2. Current status of digital control technology development

　　There are two main methods for realizing digital control of switching power supplies [1]. The first method is to use an external A/D conversion chip to sample the data, calculate and adjust the data after sampling, and then transfer the result to the PWM chip after D/A conversion, so as to realize indirect control of the switching power supply by the single-chip microcomputer (as shown in the figure below). As shown in the figure below, the technology of this method is relatively mature, the design method is easy to master, and the requirements for the single-chip microcomputer are not high, and the cost is relatively low. However, since the control circuit uses multiple chips, the circuit is relatively complex; the single chip has a relatively large delay after A/D and D/A conversion, which will inevitably affect the dynamic performance and voltage regulation accuracy of the power supply. There are also single-chip microcomputers that integrate PWM output, but the switching power supply is developing towards high frequency. The clock frequency of the general single-chip microcomputer is limited, and the generated PWM output frequency is inversely proportional to the accuracy, and it is impossible to generate a PWM output signal with sufficient frequency and accuracy. The second method is to directly control the power supply through a high-performance digital chip such as DSP. The digital chip completes the signal sampling AD conversion and PWM output. Since the output digital PWM signal power is not enough to drive the switch tube, a driver chip is required to drive the switch tube. This can simplify the design of the control circuit. Since these chips have relatively high sampling speed (the 10-bit AD converter inside TMS320LF2407 only needs 500ns to complete an AD conversion, and the fastest 8-bit microcontroller also needs several microseconds at the fastest) and computing speed, they can quickly and effectively implement various complex control algorithms, realize effective control of the power supply, and have high dynamic performance and voltage stabilization accuracy. However, the DSP chip structure is complex and the cost is relatively high; and the DSP control technology is difficult to master, and the requirements for designers are relatively high, so it is difficult to be widely used in the field of switching power supplies. At present, DSP technology has begun to be used in switching power supplies, but it is mainly limited to the field with high requirements for power supply performance and relatively expensive prices.

　　3. A novel power control technology

　　The digitally controlled switching power supply inevitably has the following problems: the speed and accuracy of the AD converter are inversely proportional. In order to ensure that the switching power supply has a higher voltage regulation accuracy, the AD converter must have a relatively high-precision sampling, but high-precision sampling requires a longer AD conversion time. As part of the feedback loop, a long AD conversion time will inevitably cause additional phase delay time. In addition to the phase delay existing in analog control, the delay time of the conversion process will inevitably cause additional phase lag, making the loop responsiveness worse. Just like the method of using RC compensation for PI regulation in analog chips, the method of introducing PI regulation in the control loop to improve the responsiveness of the control loop requires a large number of system resources for digital chips, because digital control is different from analog control. Signal sampling is not continuous, but discrete. There is an interval between two samples, and the value during this period cannot be obtained. To achieve precise control, the time interval between each sampling cannot be too long, that is, the sampling frequency cannot be too low. As a digital chip, after each AD conversion, the result will be sent to the central processing unit of the system, and then the processor will calculate and PI-regulate the sampled value. When the sampling frequency is high, this method consumes more system resources and places higher requirements on digital chips. Since there are relatively few digital chips dedicated to power control, DSP chips are generally used in situations with higher requirements. They have fast calculation and sampling speeds and powerful functions, but are relatively expensive. Moreover, DSP chips are not dedicated power control chips, and general power applications do not make high use of their chip resources.

　　With the development of digital chip and power supply technology, a control processor developed for power supply control has emerged. It is different from the central processing unit of digital chip. The control processor mainly consists of three parts: high-speed AD converter, digital PID compensator and digital PWM output. The control of the feedback loop is completed by it, and the central processing unit is used in the power supply as a management module. Its schematic diagram is shown in Figure 3:

　　Figure 1 Control loop processor schematic

　　The control processor consists of a high-speed A/D converter, a digital PID compensator and a digital DPWM output. The external memory records the relevant programs of the control processor. The high-speed A/D converter is based on the principle that the transmission delay time td of CMOS affects the input voltage VDD. The VDD voltage and the transmission time are approximately inversely proportional, that is, the larger the VDD, the smaller the signal transmission delay time. As shown in Figure 2, the CMOS input voltage VDD is used as the input port of the bit sampling voltage, and the transmission time delay td between each signal is affected by the sampling voltage VDD. After the fourth third of the sampling cycle, the sampling end bit generates a high level, and the output of q1 to q8 begins to be recorded. The result is sent to the encoder to obtain the digital output e, completing the A/D conversion. As shown in Figure 2b, the digital sampling value is 11111100. The larger the VDD, the smaller the td, and the larger the sampling value obtained.

　　High-speed AD converter schematic and waveform

　　The traditional ADC converter uses active devices to establish the sampling signal, which requires a signal establishment time, and high-precision sampling requires a longer signal establishment time. The use of new technology greatly reduces the time required for AD conversion, and can reach a MHz-level sampling frequency. The high sampling frequency can make the update speed of the DPWM signal reach hundreds of nanoseconds, and realize voltage regulation by continuously updating the PWM signal similar to analog control. It is not necessary to improve the voltage regulation accuracy by improving the AD conversion accuracy and PWM resolution within the limited sampling frequency and reducing the switching frequency as in traditional ADC sampling. The DPWM clock is multiplied by the processor system clock through the phase-locked logic loop (PLL) and the frequency can reach 200MHz. Through this DPWM control signal with a resolution of up to 5ns, the power supply switching frequency can reach 1MHz. The digital compensator provides great flexibility for power supply design. The control parameters are set through the program of the external memory. The control strategy can be changed through programming, and debugging is more convenient. Since the chip is specially designed for power supply switches, the structure is simplified and the cost is reduced. It is believed that this control processor specially developed for power supply design will be widely used.

　　Currently, there are relatively few chips that use this control technology. Silicon Labs' Si8250 is one of them [3]. Si8250 adopts a dual-processor approach. All communication and management tasks are completed by the system management processor, while the control processor is responsible for feedback loop control. The system control loop is a 6-bit AD converter with a sampling frequency of 10MHz. It can update the digital PWM output signal every 100ns to achieve a better voltage regulation effect. In the digital PID compensator, registers are provided for the coefficients KP, KI and KD of P, I and D respectively. The PID control strategy can be changed by changing the values ​​of these coefficients. The PID value is set through the register, which is easy for engineers who are accustomed to designing analog control chips to master. It provides six digital PWM outputs with different phases, and can use a relatively simple method to implement multiple control methods such as phase shift control. The clock frequency of the digital PWM can be selected from 25MHz, 50MHz and 200MHz, with a resolution of up to 5ns. The switching frequency can reach 100MHz.

　　5 Conclusion

　　Compared with analog control, digital control has obvious advantages. However, since most digital chips cannot fully meet the requirements of switching power supplies, and the expensive DSP chips that can meet the requirements are too expensive, digital control technology is not widely used in the power supply field. With the introduction of control processor technology and the emergence of digital chips for power supply control, digital control technology will surely be more widely used in switching power supplies.