CN109004840A - A kind of control method improving Switching Power Supply output accuracy - Google Patents

Abstract

A control method for improving the output accuracy of a switching power supply, based on a control system composed of a sampling module, an accuracy control module, an error calculation module, a PID module and a PWM module, the control system is connected with a controlled switching power supply to form a closed loop, through Detect the output voltage control mode switching. When the output voltage exceeds the upper limit voltage, the circuit enters the DCM mode through mode switching to reduce the input energy and stabilize the output voltage within the adjustment range. When the output voltage is lower than the lower limit voltage, through mode switching Make the circuit enter CCM mode, increase the input energy, and quickly restore the output voltage to the regulation range. In the normal regulation process, the change of the output voltage is limited between the upper limit voltage and the lower limit voltage, the voltage output ripple is reduced, and the accuracy is improved.

Description

A Control Method for Improving Output Precision of Switching Power Supply

technical field

The invention relates to a switching power supply, in particular to a control method for improving the output precision of the switching power supply, which can reduce the output ripple of the switching power supply and improve the output precision of the switching power supply.

Background technique

Switching power supplies are usually used as power supplies for various types of electrical equipment to convert unadjusted AC or DC input voltages into adjusted AC or DC output voltages. Since the switching power supply needs to adapt to different working conditions, the output accuracy performance requirements of the power supply are getting higher and higher. As the size of the process gradually decreases, the withstand voltage of the devices in the chip also decreases. If the power supply voltage is too large at this time, it is easy to cause damage to the chip device due to excessive voltage or excessive power consumption, while the output of the power supply voltage is too low. , it will degrade the performance of some devices, or even fail to work. When the accuracy of the output voltage is not high and the ripple is large, it will affect the performance of the chip. For example, a digital voltmeter requires an extremely accurate and stable power supply inside to ensure the accuracy of voltage/digital conversion; another example is that the power supply in the oscilloscope must be stable to ensure the accuracy of the deflection sensitivity of the light spot and the scanning time.

In today's power management, high power efficiency has become a necessary performance. In order to achieve high efficiency, many high-voltage power supplies (such as flyback converters whose input is the mains voltage) adopt soft switching technology to conduct when the drain terminal voltage of the primary side power tube drops to zero, that is, valley conduction technology. Although this technology achieves high efficiency, due to the deviation between the actual turn-on time and the turn-on time calculated by the system theory, the output voltage ripple brought by this circuit is larger than that of the non-valley turn-on circuit. At the same time, in a SOC (System On Chip) system, different modules require different current capabilities. Even if the same module operates in different modes at different times, its required current is also different. This results in that the current supplied by the power supply chip to these modules is not fixed. When the current provided by the power supply just makes the power supply work in the critical area of continuous conduction mode (CCM) and discontinuous conduction mode (DCM), and the power supply is intermittent If the circuit works in CCM, there is a hysteresis between the output energy it obtains and the increase in inductor current, and in DCM, because the current cycle transfers all the energy to the output terminal, the output energy obtained and the increase in inductor current There is no hysteresis between quantities. If the power supply operates multiple CCMs and enters DCM, and then enters CCM after multiple DCMs, the output voltage ripple will be large and the output accuracy will be very low.

In addition, for power chips with low adjustment accuracy, when the chip works at the boundary between DCM and CCM, there will also be a load area, large output voltage ripple, and low accuracy.

Therefore, due to the higher and higher requirements for output accuracy, the problem of low output accuracy caused by the soft switching control method, a control method to improve the accuracy of the output voltage is proposed. It has a good effect on reducing voltage overshoot and undervoltage and reducing output ripple, and it is necessary to improve the output accuracy performance of the circuit.

The applicant's previous application "A Control Method for Improving the Dynamic Response of Switching Power Supply" aims at switching the output load of the system (such as switching from heavy load to light load or switching from light load to heavy load), the output voltage will produce a large overshoot. For the problem of overshoot or undershoot, a dynamic fast recovery algorithm is proposed, so that the output voltage can return to the normal regulation range in the shortest possible time, thereby reducing the output voltage overshoot or undershoot, which belongs to dynamic regulation, and the system adopts valley bottom The conduction mode makes the difference between the actual switching period and the theoretically calculated switching period possibly a maximum of half the resonant period, so the current on the inductor will be unstable compared with the power system with non-valley switch conduction, and the power supply with this structure belongs to the output current cut-off The continuous structure, which has the zero point on the right plane, will bring the phase delay of the compensation loop, and finally lead to the problem of large output voltage ripple under the condition of stable output load.

Contents of the invention

In order to overcome the limitations and deficiencies of the prior art, the present invention proposes a control method for improving the output precision of the switching power supply, which belongs to the steady-state regulation. Aiming at the problem of large output ripple and low precision of the system under stable output load, corresponding solutions are proposed. The algorithm can limit the overshoot and undervoltage of the output voltage within a certain range, reduce the output ripple, and improve the output accuracy. It will not cause system instability in multi-mode control, making the output accuracy of the circuit better. .

In order to achieve the above object, the technical solution adopted by the present invention is: a control method for improving the output precision of switching power supply, characterized in that: based on a control system composed of a sampling module, an accuracy control module, an error calculation module, a PID module and a PWM module , the control system is connected with the controlled switching power supply to form a closed loop;

The sampling module includes a sampling circuit and a sampling calculation module. The sampling circuit obtains output voltage information through the output voltage division of the switching power supply. The sampling calculation module calculates the sampling voltage Vo corresponding to the output voltage information according to the output voltage information and outputs it to the error calculation module at the same time. and precision control module;

The precision control module includes a voltage monitoring module and a mode switching module. The voltage monitoring module receives the sampling voltage Vo output by the sampling module and compares it with the set sampling voltage Vo upper limit Vomax and sampling voltage Vo lower limit Vomin according to the size of the sampling voltage Vo. The relationship between the size of the model, determine whether to use mode switching, choose whether to use CCM mode or DCM mode; the voltage monitoring module contains two comparators and a logic unit, one of which is used to compare the sampling voltage Vo and the setting of the sampling voltage Vo The size between the limit value Vomax, another comparator is used to compare the size between the sampling voltage Vo and the set lower limit value Vomin of the sampling voltage Vo, the outputs of the two comparators are respectively connected to the logic unit, and the logic unit is based on the two The comparison result of the comparator, the output mode selection result mode_F is given to the mode switching module. When Vo<Vomin, the logic unit outputs mode_F=1, which is the CCM mode; when Vo>Vomax, the logic unit outputs mode_F=0, which is the DCM mode; when Vo Between Vomin and Vomax, the mode selection result mode_F keeps the currently selected mode unchanged; the mode switching module receives the mode selection result mode_F output by the voltage detection module and the output voltage information Vsense obtained by output voltage division, and combines the output voltage information Vsense with The zero-level signal GND is compared to generate a signal ZCMP that reflects the drop of the inductor current to zero. The signal ZCMP is a signal that determines the conduction of the power transistor in DCM mode, and the internal clock Fclk is a signal that determines the conduction of the power transistor in CCM mode. The shutdown signal of the power transistor in DCM mode and CCM mode is determined by the output signal V _PI of the PID module; the ZCMP signal and the internal clock Fclk pass through the two-to-one logic combination gate circuit, according to the different values of the mode selection result mode_F, resulting in mode switching As a result, the control signal mode_ctl is given to the PWM module;

The error calculation module receives the sampling voltage Vo output by the sampling module, and subtracts the difference of the sampling voltage Vo from the reference voltage Vref to obtain the current sampling voltage error e1 and outputs it to the PID module. The PID module passes the proportional parameter Kp and the integral parameter according to the sampling voltage error e1 K _i , the differential parameter K _d performs PID operation, and the compensation result V _PI is obtained and output to the PWM module, which is used to determine the peak current value of the next cycle;

The PWM module includes a PWM unit and a driving unit. The input of the PWM unit is the mode switching result control signal mode_ctl output by the mode switching module and the compensation result V _PI output by the PID module. The switching period Ts and the peak current Ipeak are obtained through calculation, and are output by the driving unit The duty cycle waveform realizes loop control on the gate of the power tube of the switching power supply, the mode switching result mode_ctl determines the conduction of the power tube, and the compensation result V _PI determines the shutdown of the power tube;

Repeat the above process, sample the output voltage of the switching power supply again, and control the switching on and off of the power tube of the switching power supply cyclically, so as to make the system more stable and obtain higher output accuracy.

When the sampling voltage Vo is smaller than the lower limit voltage Vomin, the output signal mode_F of the voltage detection module controls the mode switching module to make it a constant frequency CCM mode. In this mode, the conduction of the power transistor is determined by the rising edge of the internal clock, and the power transistor Shutdown is determined by the output signal V _PI of the PID module. The system inputs high power so that the sampling voltage Vo quickly rises to the reference voltage Vref, and keeps working in this mode until the sampling voltage Vo is greater than the upper limit voltage Vomax, the system will switch to DCM mode ;

When the sampling voltage Vo is greater than the upper limit voltage Vomax, the output signal mode_F of the voltage detection module controls the mode switching module to change to the DCM mode. In this mode, the power transistor needs to be turned on after the inductor current becomes zero. The next valley of the source voltage Vds is turned on, and the power transistor is turned off is determined by the output signal V _PI of the PID module. In this mode, the system inputs a small power to make the sampling voltage Vo quickly drop to the reference voltage Vref, and the mode remains in operation until The system switches to the CCM mode only when the sampling voltage Vo is smaller than the lower limit voltage Vomin.

The two comparators in the voltage monitoring module are COMP1 and COMP3, the positive terminal of comparator COMP1 is connected to Vomax, the negative terminal is connected to Vo, the positive terminal of comparator COMP3 is connected to Vo, and the negative terminal is connected to Vomin, the output signal of comparator COMP1 Both SMAX and the output signal SMIN of the comparator COMP3 are connected to a logic unit, and the logic unit outputs a mode selection result mode_F.

The mode switching module includes a comparator COMP, an inverter INV, an AND gate AND and two NOR gates NOR1 and NOR2, the positive input of the comparator COMP is connected to the zero-level signal GND, and the negative input of the comparator COMP is connected to The output voltage information Vsense obtained by voltage division, the output of the comparator COMP is connected to one input terminal of the NOR gate NOR1, and the other input terminal of the NOR gate NOR1 is connected to the mode selection result mode_F and one input terminal of the AND gate AND, and the AND gate AND The other input terminal of the inverter is connected to the output of the inverter INV, the input of the inverter INV is connected to the internal clock Fclk, the output of the AND gate AND is connected to one input terminal of the NOR gate NOR2, and the other input terminal of the NOR gate NOR2 is connected to the OR The output of the NOR gate NOR1, and the NOR gate NOR2 output the mode switching result control signal mode_ctl.

Advantage of the present invention and remarkable effect:

1. The control method for improving the output precision of the switching power supply of the present invention can make the circuit immediately enter the DCM mode through mode switching when the output voltage exceeds the upper limit voltage, and reduce the input energy, thereby stabilizing the output within the adjustment range. When the output voltage is low At the lower limit voltage, the circuit immediately enters the CCM mode through mode switching, increasing the input energy, and the output voltage quickly returns to the regulation range. In the normal adjustment process, the change of the output voltage is limited between the upper limit voltage and the lower limit voltage, the voltage output ripple is reduced, and the accuracy is improved.

2. The control method for improving the output precision of the switching power supply of the present invention can compensate for the output ripple caused by the low adjustment precision and the large single-step energy change by detecting the output voltage control mode switch when the circuit adjustment accuracy is low Too big a problem.

3. The present invention belongs to the steady-state adjustment, which adds a new algorithm on the basis of the dynamic adjustment, and the control method for improving the dynamic response is still retained. A circuit for implementing the algorithm is shown in Figure 1c.

4. The present invention is applicable to various switching power supply circuit structures with intermittent output current and small load output, that is, small load means that the average inductor current in CCM mode and the average inductor current in DCM mode cannot be too different.

Description of drawings

Fig. 1a is a system structural block diagram of the control method of the present invention; Fig. 1b is a structural block diagram of a voltage monitoring module in Fig. 1a; Fig. 1c is an implementation structure of a mode switching module in Fig. 1a;

Fig. 2 is the application schematic diagram of judging switching mode according to Vo;

Fig. 3 is the embodiment of the structure diagram of the closed-loop circuit with the single tube resonant converter of the present invention;

Fig. 4 is a simulation diagram of the relationship between the output voltage of the structure of Fig. 3 in a steady state, the resonant voltage and the primary current;

Fig. 5 is a test diagram of the relationship between output voltage ripple, resonant voltage and primary current of the structure in Fig. 3 in a steady state;

Figure 6 is the output voltage ripple in the steady state when the patent algorithm is not used in the dynamic adjustment patent;

Fig. 7 is the output voltage ripple in the steady state when the algorithm of this patent is adopted in the dynamic adjustment patent.

Detailed ways

Referring to Fig. 1a, the present invention is based on a control system comprising a sampling module, an accuracy control module, an error calculation module, a PID module and a PWM module, and the control system is connected with a controlled switching power supply to form a closed loop.

The sampling module includes a sampling circuit and a sampling calculation module. The sampling circuit obtains the information of the output voltage through the output voltage division. The sampling calculation module obtains the sampling voltage Vo reflecting the current output voltage through the sampling algorithm and outputs it to the precision control module. Unlike the error calculation module, the sampling here can be direct sampling or indirect sampling, and the sampling result can be analog or digital.

The precision control module includes a voltage monitoring module and a mode switching module, and the voltage monitoring module judges which mode to use according to the sampling voltage Vo. The voltage monitoring module receives the sampling voltage Vo output by the sampling module, and judges whether to use mode switching according to the size of Vo and the set Vo upper limit Vomax and Vo lower limit Vomin respectively. Vomin and Vomax are more acceptable than Vo The range should be narrow, that is, a certain margin is left. Mode switching means that when the sampling voltage Vo changes beyond the acceptable range, the single-cycle energy transmission is changed through mode conversion, so that the sampling voltage Vo can be adjusted within the set range to achieve high-precision output. The mode switching here refers to switching between the CCM and DCM modes, and the output result of the mode switching module is mode_ctl.

As shown in Figure 1b, the voltage monitoring module includes two comparators COMP1 and COMP3 and a logic unit for judging which mode to use. The two comparators respectively judge the sampling voltage Vo and the lower limit voltage Vomin, the sampling voltage Vo and the upper limit voltage Vomax, and the logic unit outputs a mode selection result mode_F according to the comparator results. mode_F is used as the input of the mode switching module. When mode_F=1, it is CCM mode. When mode_F=0, it is DCM mode. When Vo is greater than Vomax, output mode_F=0, and the mode switching module starts DCM mode. When Vo is less than Vomin, Output mode_F=1, the mode switch module starts CCM mode, when Vo is between Vomin and Vomax, mode_F keeps the current mode unchanged.

As shown in FIG. 1c, the mode switching module receives the output signal mode_F of the voltage detection module and the signal Vsense reflecting the magnitude of the output voltage. The Vsense signal is compared with the zero-level signal GND by the comparator COMP to generate a signal ZCMP that reflects the inductor current to drop to zero. This signal and mode_F pass through the NOR gate NOR1 to obtain the DCM_ON signal. When mode_F=0, DCM_ON=ZCMP_B (ZCMP inverse signal ); when mode_F=1, ZCMP is shielded, and DCM_ON=0. The internal clock signal Fclk gets FclkB through the inverter INV, and the signal and mode_F output the CCM_ON signal through the AND gate AND. When mode_F=1, CCM_ON=FclkB; when mode_F=0, FclkB is shielded, CCM_ON=0. CCM_ON and DCM_ON pass through the NOR gate NOR2 to obtain the signal mode_ctl that finally controls the power tube to be turned off. The mode_ctl signal is passed to the PWM module. In both modes (CCM and DCM), the shutdown signal of the power transistor is determined by the output signal V _PI of the PID module. In short, when Mode_F=1, the internal clock Fclk determines Mode_ctl, and the system works in CCM mode at this time, the system turn-on is determined by the internal clock, and the system turn-off is determined by the output of the PID. When Mode_F=0, Mode_ctl is determined by the comparison result of Vsense and zero level, where Vsense is the voltage reflecting the inductor current, which can be equal to IL*Rsense, IL is the inductor current, Rsense is the sampling resistor, and the comparator COMP is a zero current comparison When the inductor current drops to zero, the system can be turned on in the next cycle, that is, the system turn-on is determined by the current zero-crossing comparator, and the turn-off is determined by the PID output signal.

When Vo<Vomin, the logic unit outputs mode_F=1, and the logic unit output mode switches in the constant frequency CCM mode. In this mode, the power tube is turned on by the rising edge of the internal clock, and the power tube is turned off by the output signal of the PID module. It is determined that the input power of the system makes Vo rapidly rise to the reference voltage Vref, and keep working in this mode until Vo is greater than the upper limit voltage Vomax. When Vo is smaller than the lower limit voltage Vomin, the logic unit outputs mode_F=1, and the logic unit output mode switches to the constant frequency CCM mode. In this mode, the power tube is turned on by the rising edge of the internal clock, and the power tube is turned off by the PID The output signal of the module is determined, and the system inputs a large power to make Vo quickly rise to the reference voltage Vref, and keep working in this mode until Vo is greater than the upper limit voltage Vomax, and the system switches to DCM mode.

When Vo>Vmax, the logic unit outputs mode_F=0, and the logic unit output mode is switched to the DCM mode. In this mode, the power transistor needs to be turned on after the inductor current becomes zero, at Vds (the main power transistor drain-source voltage) The next valley is turned on; the power tube is turned off by the output signal of the PID module. In this mode, the system inputs low power so that Vo drops rapidly to the reference voltage Vref, and keeps working in this mode until Vo is lower than the lower limit voltage Vomin, the system switches to CCM mode. When Vo is greater than the upper limit voltage Vomax, the output mode of the logic unit is switched to the DCM mode. In this mode, the power tube is turned on after the inductor current becomes zero, and then it is turned on at the next valley of Vds; the power tube is turned off by PID The module output signal V _PI decides. By inputting a small power, the output quickly drops to the reference voltage Vref, and keeps working in this mode until Vo is smaller than the lower limit voltage Vomin.

The error calculation module calculates the current voltage error. The input of the error calculation module is the output Vo of the sampling module. According to the calculated reference voltage Vref minus the difference of the sampling voltage Vo, it is the current sampling error, recorded as e1, and output to the PID module; PID The module performs compensation calculation according to the input error e1 signal, and performs PID calculation through the proportional parameter Kp, integral parameter K _i and differential parameter K _d , and the compensation result V _PI is input to the PWM module to determine the peak current value of the next cycle.

The PWM module includes a PWM unit and a drive unit. The input of the PWM unit is the mode switching result mode_ctl output by the mode switching module and the compensation result V _PI output by the PID module. After calculating the switching period Ts and peak current Ipeak information during control, the drive unit Output the duty ratio waveform to realize loop control on the gate of the power tube of the switching power supply. The switching result mode_ctl controls the switch tube to be turned on, and the V _PI signal determines the switch tube to be turned off. The drive unit should choose a circuit with a small delay time as much as possible. Then sample the output voltage of the switching power supply again, and repeat the above process to control the switching on and off of the power tube of the switching power supply in a cycle, so as to make the system more stable and obtain higher output accuracy.

Referring to Figure 2, when the load is large, for a switching power supply with intermittent output current, due to the existence of the right half plane zero point, when the circuit works in CCM mode, the change of the input current has a phase with the change of the energy received by the output Hysteresis, even with the PID compensation scheme, there is still a large ripple. Therefore, when the sampling voltage Vo is greater than Vomax, the system works in DCM mode through mode switching, so that the transmitted energy is reduced, and Vo can drop as soon as possible. When the sampling voltage Vo is lower than Vomin, the system works in CCM mode through mode switching, so that the transmission energy increases rapidly, and Vo is adjusted to the preset range. It can also be seen from the figure that the sampling voltage Vo exceeds Vomax and Vomin. This is because the energy obtained by the switching power supply structure with intermittent output and the energy stored in the inductor current are separated when the power tube is turned on, so the output energy is obtained in the same way as There is a certain hysteresis in the change of the inductor current, which eventually brings the output voltage out of the set range. Therefore, the setting range of the output voltage is smaller than the allowable adjustment range, leaving a certain margin.

FIG. 3 is an embodiment of a flyback circuit as an object. The method and system used in the present invention can also be used in other types of switching power supply circuit configurations. Here, the single-tube quasi-resonant circuit fed back by the primary side is taken as an example. The input of the single-tube quasi-resonant converter is 90-265V, the output is 20V, the maximum current is 4A, the inductance is 1.6mH, the transformer turns ratio is 104/24, and the output is constant voltage. An example and the corresponding test waveform are given below to increase the working method of optimizing the output accuracy performance in this example.

The flyback converter obtains the sampling voltage Vo by sampling the output voltage, uses the precision control module to compare the sampling voltage Vo with the set Vomax and Vomin, when Vo is greater than Vomax, output mode_F=0, and the mode switching module starts the DCM mode , when Vo is less than Vomin, output mode_F=1, the mode switching module starts CCM mode, when Vo is between Vomin and Vomax, mode_F keeps the current mode unchanged. And when Vsense is 0, ZCMP=1, which means that the inductor current energy is released completely. This point is the current zero current point. Compared with the zero-level signal, Vo generates a valley turn-on signal, which determines the turn-on time of the switch tube again in DCM mode. , while in the CCM mode, the ZCMP signal is shielded by mode_F=1, and the conduction of the switch is determined by the internal clock. At the same time, the error detection module detects the error e1, and sends it to the PID module, and obtains a suitable V _PI through the PID algorithm. This value determines the peak current value of the next cycle, and finally the compensation result V _PI of the PID module and the mode switching module are given. After the control signal mode_ctrl of the control signal is calculated and the switching cycle and peak current information are obtained, the duty cycle waveform is output by the drive unit to realize loop control on the gate of the switching power supply power tube; then the output voltage of the switching power supply is sampled again, and Repeat the above process to control the switching on and off of the power tube of the switching power supply in a cycle to make the system more stable and obtain higher output accuracy. The corresponding simulation waveform is shown in Figure 4.

FIG. 4 is the simulated output waveform of the flyback circuit in FIG. 3 under different loads according to the present invention. Vo, Vcr, and Ip refer to the output voltage respectively (the sampling circuit samples and holds the output voltage, and the sampling voltage Vo reflects the output voltage. Without considering the sampling delay, the sampling voltage Vo is equal to the output voltage, so here Vo represents the output voltage. The same below), resonant voltage and primary current. It is set here that once Vo is greater than 20V, the system will work in DCM or even frequency modulation state. When the output voltage is lower than 20V, the system will work in CCM mode, so that the output voltage will rise as soon as possible, and keep this mode until Vo is higher than 20V. Ideally, CCM Evenly cross-distributed with DCM, the output voltage ripple is the smallest, on the contrary, without any control, it is completely regulated by the system itself, then the CCM and DCM work unevenly, and the output ripple will be very large.

Figure 5 is a test waveform using a high-precision low-ripple control algorithm, the output voltage is 12V, and the corresponding load is 1.2A. As shown in the figure, the system works in the switching state of CCM and DCM, and its output voltage ripple is within 100mV.

Figure 6 shows the output voltage ripple in the steady state when the algorithm of this patent is not used in the dynamic adjustment patent, the average value is 17.3V, and the output ripple reaches 2V.

Figure 7 shows the steady-state output voltage ripple waveform when using the patented algorithm under the same structure. The peak-to-peak value of the output ripple is 500mV. Through comparison, it can be seen that the effect is obvious, and the ripple is reduced to 1/4 of the previous one.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. The present invention described here can have many changes, and this change cannot artificially deviate from the scope of the present invention. spirit and scope. Therefore, all changes obvious to those skilled in the art should be included within the scope of the claims.

Claims (4)

1. A control method for improving switching power supply output precision, characterized in that: based on a control system comprising a sampling module, an accuracy control module, an error calculation module, a PID module and a PWM module, the control system is connected with a controlled switching power supply together to form a closed loop; The sampling module includes a sampling circuit and a sampling calculation module. The sampling circuit obtains output voltage information through the output voltage division of the switching power supply. The sampling calculation module calculates the sampling voltage Vo corresponding to the output voltage information according to the output voltage information and outputs it to the error calculation module at the same time. and precision control module; The precision control module includes a voltage monitoring module and a mode switching module. The voltage monitoring module receives the sampling voltage Vo output by the sampling module and compares it with the set sampling voltage Vo upper limit Vomax and sampling voltage Vo lower limit Vomin according to the size of the sampling voltage Vo. The relationship between the size of the model, determine whether to use mode switching, choose whether to use CCM mode or DCM mode; the voltage monitoring module contains two comparators and a logic unit, one of which is used to compare the sampling voltage Vo and the setting of the sampling voltage Vo The size between the limit value Vomax, another comparator is used to compare the size between the sampling voltage Vo and the set lower limit value Vomin of the sampling voltage Vo, the outputs of the two comparators are respectively connected to the logic unit, and the logic unit is based on the two The comparison result of the comparator, the output mode selection result mode_F is given to the mode switching module. When Vo<Vomin, the logic unit outputs mode_F=1, which is the CCM mode; when Vo>Vomax, the logic unit outputs mode_F=0, which is the DCM mode; when Vo Between Vomin and Vomax, the mode selection result mode_F keeps the currently selected mode unchanged; the mode switching module receives the mode selection result mode_F output by the voltage detection module and the output voltage information Vsense obtained by output voltage division, and combines the output voltage information Vsense with The zero-level signal GND is compared to generate a signal ZCMP that reflects the drop of the inductor current to zero. The signal ZCMP is a signal that determines the conduction of the power transistor in DCM mode, and the internal clock Fclk is a signal that determines the conduction of the power transistor in CCM mode. The shutdown signal of the power transistor in DCM mode and CCM mode is determined by the output signal V _PI of the PID module; the ZCMP signal and the internal clock Fclk pass through the two-to-one logic combination gate circuit, according to the different values of the mode selection result mode_F, resulting in mode switching As a result, the control signal mode_ctl is given to the PWM module; The error calculation module receives the sampling voltage Vo output by the sampling module, and subtracts the difference of the sampling voltage Vo from the reference voltage Vref to obtain the current sampling voltage error e1 and outputs it to the PID module. The PID module passes the proportional parameter Kp and the integral parameter according to the sampling voltage error e1 K _i , the differential parameter K _d performs PID operation, and the compensation result V _PI is obtained and output to the PWM module, which is used to determine the peak current value of the next cycle; The PWM module includes a PWM unit and a driving unit. The input of the PWM unit is the mode switching result control signal mode_ctl output by the mode switching module and the compensation result V _PI output by the PID module. The switching period Ts and the peak current Ipeak are obtained through calculation, and are output by the driving unit The duty cycle waveform realizes loop control on the gate of the power tube of the switching power supply, the mode switching result mode_ctl determines the conduction of the power tube, and the compensation result V _PI determines the shutdown of the power tube; Repeat the above process, sample the output voltage of the switching power supply again, and control the switching on and off of the power tube of the switching power supply cyclically, so as to make the system more stable and obtain higher output accuracy. 2. The control method for improving switching power supply output accuracy according to claim 1, characterized in that: when the sampling voltage Vo is smaller than the lower limit voltage Vomin, the output signal mode_F of the voltage detection module controls the mode switching module so that it becomes constant frequency In CCM mode, in this mode, the turn-on of the power tube is determined by the rising edge of the internal clock, and the turn-off of the power tube is determined by the output signal V _PI of the PID module. The system inputs a large power so that the sampling voltage Vo quickly rises to the reference voltage Vref, and Keep working in this mode until the sampling voltage Vo is greater than the upper limit voltage Vomax, the system will switch to the DCM mode; When the sampling voltage Vo is greater than the upper limit voltage Vomax, the output signal mode_F of the voltage detection module controls the mode switching module to change to the DCM mode. In this mode, the power transistor needs to be turned on after the inductor current becomes zero. The next valley of the source voltage Vds is turned on, and the power transistor is turned off is determined by the output signal V _PI of the PID module. In this mode, the system inputs a small power to make the sampling voltage Vo quickly drop to the reference voltage Vref, and the mode remains in operation until The system switches to the CCM mode only when the sampling voltage Vo is smaller than the lower limit voltage Vomin. 3. The control method for improving switching power supply output accuracy according to claim 1, characterized in that: the two comparators in the voltage monitoring module are COMP1 and COMP3, the positive terminal of comparator COMP1 is connected to Vomax, and the negative terminal is connected to Vo, the positive terminal of the comparator COMP3 is connected to Vo, the negative terminal is connected to Vomin, the output signal SMAX of the comparator COMP1 and the output signal SMIN of the comparator COMP3 are both connected to the logic unit, and the logic unit outputs the mode selection result mode_F. 4. The control method for improving the output precision of the switching power supply according to claim 1, wherein the mode switching module includes a comparator COMP, an inverter INV, an AND gate AND and two NOR gates NOR1 and NOR2, The positive input terminal of the comparator COMP is connected to the zero-level signal GND, the negative input terminal of the comparator COMP is connected to the output voltage information Vsense obtained by voltage division, and the output of the comparator COMP is connected to an input terminal of the NOR gate NOR1, or the NOR gate NOR1 The other input of the mode selection result mode_F and one input of the AND gate AND, the other input of the AND gate AND is connected to the output of the inverter INV, the input of the inverter INV is connected to the internal clock Fclk, and the AND gate AND The output is connected to one input terminal of the NOR gate NOR2, and the other input terminal of the NOR gate NOR2 is connected to the output of the NOR gate NOR1, and the NOR gate NOR2 outputs the mode switching result control signal mode_ctl.