FCCM soft start control method, control device and switching power supply

Abstract

The invention provides an FCCM soft start control method, a control device and a switching power supply. The control method comprises the following steps: identifying that the switching power supply is started to output no power or be started with power; a corresponding control strategy is executed according to the starting type, and when starting is output non-electric starting, a first control strategy is executed, and the first control strategy comprises the steps that a synchronous switch tube and a main switch tube are directly and complementarily conducted; a switching tube in the secondary side rectifying circuit and a switching tube in the primary side switching circuit are directly and complementarily conducted; when the starting is output electrification starting, a second control strategy is executed, and the second control strategy comprises the steps that the synchronous switch tube and the main switch tube are directly and complementarily conducted; a switching tube in the secondary side rectifying circuit is in first soft opening control and is in complementary conduction with a switching tube in the primary side switching circuit after set time; the switching power supply detects an inductance negative current peak value in real time when output flows backward to input from starting, and limits the inductance negative current peak value within a negative current threshold value. According to the invention, the monotonicity of output voltage starting and the reliable work of the power supply can be ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic converters, and in particular relates to an FCCM soft start control method, a control device and a switching power supply. Background Art

[0002] Switching power supplies have become an inseparable part of various industries. For switching power supplies, when the output load is light or even no-load, the converter is often controlled to operate in forced continuous mode (FCCM for short) to reduce the output voltage ripple. In addition, in applications where the dynamic response speed of the switching power supply is high, such as the communication power supply industry, in order to ensure timely response to large dynamic jumps in the load and avoid the output voltage overshoot or undershoot beyond the range, which affects the normal operation of the subsequent system, the FCCM mode is usually also used.

[0003] Usually, both the input and output ends of the switching power supply are equipped with large-capacity capacitors. If the power supply is shut down under light no-load conditions, a high voltage will remain on the output capacitor. When the power supply is restarted, if the residual voltage on the output capacitor is higher than the input voltage, the energy on the output capacitor will flow back to the input side. If the backflow current is large, the device may be damaged, and the output voltage will drop rapidly during the backflow process. The output voltage will not rise until the input energy is greater than the output energy. This output voltage drop phenomenon does not meet the startup monotonicity requirements. In severe cases, it will directly affect the normal operation of the subsequent system. Figure 1 shown.

[0004] Please refer to Figure 2 , a Chinese patent application with publication number CN114221549A discloses a cascade circuit and a control method thereof, specifically involving a buck-boost cascade circuit, the buck-boost cascade circuit includes a front-stage buck-boost circuit and a rear-stage isolated switch power supply circuit, and the buck-boost mode control cascade circuit at least includes a switch tube S1, a switch tube S2, a power inductor L and an isolated switch power supply circuit, and specifically the isolated switch power supply circuit needs to include a primary switch circuit, a transformer T1 and a secondary rectifier circuit. The cascade circuit adopts a current feeding method, and the primary switch tube in the rear-stage isolated switch circuit has a common conduction part, which plays an excitation role in the boost mode and provides a freewheeling channel for the inductor current in the buck mode.

[0005] In the industry, for the problem of output pre-bias voltage starting, the conventional digital MCU control solution will calculate the inductor excitation PWM duty cycle that should be issued based on the sampled output capacitor residual voltage. Figure 2In this kind of cascade circuit, the main control is located on the primary side of the transformer in low-cost applications, and it is impossible to directly sample the output voltage to calculate the duty cycle information. Therefore, when the output has a pre-bias voltage, how a low-cost non-digital MCU control solution can ensure reliable monotonic starting of the switching power supply product has become a difficult problem that needs to be overcome urgently. Summary of the invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide an FCCM soft start control method, a control device and a switching power supply to solve the problem of starting with a pre-bias voltage output in the FCCM mode of the buck-boost cascade switching power supply, which can meet the monotonicity requirements of the startup and has no damage caused by current backflow, making the switching power supply product safer and more reliable, and having good economic and social benefits.

[0007] As a first aspect of the present invention, the embodiment and technical solution of the FCCM soft start control method provided by the present invention is as follows:

[0008] An FCCM soft start control method is applied to the startup control of a switching power supply, wherein the switching power supply comprises a front-stage buck-boost circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switching power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, wherein the FCCM soft start control method comprises:

[0009] The starting type identification step is to identify whether the switch power supply starting is an output no-power starting or an output power starting;

[0010] The startup control strategy execution steps are to execute the corresponding control strategy according to the startup type, where:

[0011] When the starter is a starter without output power, a first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly and complementaryly turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly and complementaryly turned on;

[0012] When the starter is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time;

[0013] The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

[0014] Furthermore, the start type identification step includes: when the open-loop soft-start voltage in the controller rises from zero to a first set threshold, determining whether the output feedback voltage of the isolation feedback circuit is less than the open-loop soft-start voltage; if so, determining that the output is an electric starter; otherwise, determining that the output is a non-electric starter.

[0015] Preferably, the first set threshold is greater than a saturation conduction voltage drop of an optocoupler in the isolation feedback circuit.

[0016] Preferably, the first soft-open control is leading edge modulation, trailing edge modulation or double edge modulation to increase the pulse width until the pulse width of the switch tube in the secondary rectifier circuit increases to complement the switch tube in the primary switch circuit.

[0017] Furthermore, in the first soft-open control, the duty cycle of the pulse width after the set time is less than or equal to 50%.

[0018] Furthermore, when it is identified that the switch power supply startup is an output power startup, the following steps are also included:

[0019] Determine whether the input voltage of the switching power supply is less than a second set threshold value, and execute a corresponding control strategy according to the determination result, including:

[0020] When the input voltage of the switching power supply is less than the second set threshold, executing the second control strategy;

[0021] When the input voltage of the switching power supply is greater than or equal to the second set threshold, a third control strategy is executed, including: the synchronous switch tube is a second soft-open control, and is complementarily turned on with the main switch tube after a set time; the switch tube in the secondary rectifier circuit is directly complementarily turned on with the switch tube in the primary switch circuit.

[0022] Preferably, the second set threshold is smaller than the output voltage of the preceding step-up / step-down circuit.

[0023] Preferably, the second soft-open control is a leading edge modulation, a trailing edge modulation or a double edge modulation to increase the pulse width until the pulse width of the synchronous switch tube increases to complement the main switch tube.

[0024] Furthermore, in the second soft-open control, the duty cycle of the pulse width is less than 100% after the set time.

[0025] Preferably, starting from the start-up control, the absolute value of the negative current peak of the inductor when the output flows back to the input is detected in real time. When the absolute value is greater than the absolute value of the negative current threshold, the synchronous switch tube and the switch tube in the secondary rectifier circuit are turned off cycle by cycle, thereby limiting the negative current peak within the negative current threshold.

[0026] As a second aspect of the present invention, the embodiment and technical solution of the FCCM soft start control device provided by the present invention is as follows:

[0027] An FCCM soft start control device is applied to the startup control of a switching power supply, wherein the switching power supply comprises a front-stage buck-boost circuit and a rear-stage isolated switching power supply circuit, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switching power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, wherein the FCCM soft start control device comprises:

[0028] A starter type identification unit is configured to identify whether the switch power supply starter is an output non-powered starter or an output powered starter;

[0029] The startup control strategy execution unit is configured to execute a corresponding control strategy according to the startup type, wherein:

[0030] When the starter is a starter without output power, a first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly and complementaryly turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly and complementaryly turned on;

[0031] When the starter is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time;

[0032] The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

[0033] As a third aspect of the present invention, the technical solution of the embodiment of the switching power supply provided by the present invention is as follows:

[0034] A switching power supply, comprising a front-stage buck-boost circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switching power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, wherein: the switching power supply comprises the FCCM soft start control device described in the second aspect above.

[0035] Terminology Note:

[0036] Leading-edge modulation: Specifically, the falling edge of the high-level pulse width signal emitted by the controller remains fixed in each switching cycle, while the rising edge position can be adjusted, thereby realizing leading-edge modulation soft-opening control of the switch tube drive signal.

[0037] Trailing edge modulation: Specifically, the rising edge of the high-level pulse width signal emitted by the controller remains fixed in each switching cycle, while the falling edge position can be adjusted, thereby realizing the trailing edge modulation soft-opening control of the switch tube drive signal.

[0038] Double-edge modulation: Specifically, the rising edge and falling edge of the high-level pulse width signal issued by the controller can be adjusted and changed in each switching cycle, thereby realizing double-edge modulation of the switch tube drive signal.

[0039] Compared with the conventional FCCM control scheme, the present invention has the following beneficial effects:

[0040] (1) The two FCCM soft start control methods provided by the present invention can ensure that the output voltage rises completely monotonically when the starter output has a pre-bias voltage, and the control is simple and the product reliability is high;

[0041] (2) The FCCM soft start control method provided by the present invention is applied to low-cost product development, and is particularly suitable for primary-side master control of cascade circuits, which can effectively reduce the cost and volume of the power supply system and has good economic and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a schematic diagram of the output voltage drop of the electric starter;

[0043] Figure 2 It is the schematic diagram of cascade circuit;

[0044] Figure 3 A typical schematic diagram of a switching power supply applicable to the present invention;

[0045] Figure 4 A first flow chart of the FCCM soft start control method according to the first embodiment of the present invention;

[0046] Figure 5 This is a second flow chart of the FCCM soft start control method according to the first embodiment of the present invention. DETAILED DESCRIPTION

[0047] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0048] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present application.

[0049] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application are described here. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, the process, method, system, product or equipment comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or equipment.

[0050] It should be understood that, in the specification, claims and drawings, when a step is described as being connected to another step, the step may be directly connected to the other step, or be connected to the other step through a third step; when an element / unit is described as being "connected" to another element / unit, the element / unit may be "directly connected" to the other element / unit, or be "connected" to the other element / unit through a third element / unit.

[0051] In addition, the drawings of the present disclosure are only schematic diagrams of the present disclosure and are not necessarily drawn to scale. The same symbols in the drawings represent the same or similar parts, and thus repeated descriptions thereof will be omitted. Some of the block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented using software, or implemented in one or more hardware modules or integrated circuits, or implemented in different networks and / or processor devices and / or microcontrollers.

[0052] Figure 3 A typical schematic diagram of a switching power supply applicable to the present invention includes a cascade circuit, an isolated feedback circuit and a controller; the front-stage buck-boost circuit of the cascade circuit includes an input power positive, an input power ground, an input capacitor Cin, a main switch tube S1, a synchronous switch tube S2, an inductor L, an active clamping capacitor C1 and a clamping switch tube S4; the rear-stage isolated switching power supply circuit of the cascade circuit includes a primary push-pull circuit, a secondary full-wave rectifier circuit and a transformer connecting the primary push-pull circuit and the secondary full-wave rectifier circuit.

[0053] Among them, one end of the switch tube S1 and one end of the input capacitor Cin are connected to the input power supply Vin, that is, the positive input of the buck-boost circuit, and also the positive input of the cascade circuit. The other end of the switch tube S1 and one end of the switch tube S2 are connected to one end of the inductor L. The other end of the inductor L, one end of the active clamping capacitor C1 and the center tap of the transformer in the subsequent isolation switch power supply are connected to the bus Vbus, that is, the positive output of the buck-boost circuit, and also the positive input of the isolation switch power supply circuit. The same-name end of the primary upper winding of the transformer is connected to one end of the primary switch tube S5, and the opposite-name end of the primary lower winding of the transformer is connected to one end of the primary switch tube S6. The other end of the active clamp capacitor C1 is connected to one end of the active clamp switch tube S4, the other end of the switch tube S2, the other end of the input capacitor Cin, the other ends of the primary switch tubes S5 and S6, and the other end of the active clamp switch tube S4 are connected to the primary input power supply GND, the same-name end of the secondary winding of the transformer is connected to one end of the secondary rectifier tube S7, the opposite-name end of the secondary winding of the transformer is connected to one end of the secondary rectifier tube S8, the center tap of the secondary side of the transformer and one end of the output capacitor Co are connected to the positive output of the cascade circuit, and the other ends of the secondary rectifier tubes S7 and S8 and the other end of the output capacitor Co are connected to the negative output of the cascade circuit;

[0054] The isolated feedback circuit is mainly used for isolated feedback and closed-loop control of the output voltage, and mainly includes resistor R1, resistor R2, controllable voltage regulator 431, resistor R3, optocoupler and resistor R4. One end of resistor R1 and one end of the secondary side of the optocoupler are connected to the output voltage Vo, the other end of resistor R1 and one end of resistor R2 are connected to pin 1 of the controllable voltage regulator 431, pin 2 of the controllable voltage regulator 431 is connected to the other end of the secondary side of the optocoupler, and pin 3 of the controllable voltage regulator 431 and the other end of resistor R2 are connected to the secondary side reference ground of the cascade circuit together; one end of the primary side of the optocoupler and one end of the resistor R4 are connected to one end of the controller as the output voltage feedback signal COMP, the other end of the resistor R4 is connected to the optocoupler power supply voltage VCC, and the other end of the primary side of the optocoupler is connected to the primary input power supply GND.

[0055] The controller is used to control the switch tubes in the cascade circuit to work at a fixed frequency; the controller samples the input voltage, and outputs drive signals S1, S2, S3, S4, S5, S6, S7, and S8 according to the FCCM soft start control method of the present invention, and controls the switch tubes S1, S2, S3, S4, S5, S6, S7, and S8 respectively;

[0056] In specific implementation, the controller can compare the set open-loop soft-start voltage with the output voltage feedback COMP to identify the starting conditions of the machine without output power and the starting conditions of the machine with output power, so as to perform corresponding control logic processing; starting from the start-up control, the controller detects the negative inductor current when the output flows back to the input in real time, limits it within a certain range, and ensures that the switching power supply will not be damaged;

[0057] The controller compares the output voltage feedback signal obtained from the isolated feedback circuit with the open-loop soft-start voltage. When starting the machine, the switch soft-start voltage rises from zero to a first set threshold value. If the output voltage feedback signal is less than the open-loop soft-start voltage within this first set threshold value interval, it is determined that the output has powered the machine. Conversely, if the output voltage feedback signal is not less than the open-loop soft-start voltage within this set threshold value interval, it is determined that the output has no power to start the machine.

[0058] It should be noted that Figure 3 It is only a typical schematic diagram of the switching power supply applicable to the present invention, and should not constitute a limitation on the protection scope of the present invention. The specific circuit design of the switching power supply applicable to the present invention can be selected by technicians in this field according to actual needs. Figure 3 The push-pull circuit in the circuit can also be a half-bridge circuit, a full-bridge circuit or an LLC circuit, etc.; the secondary rectifier circuit is Figure 3 The full-wave rectifier circuit in can also be a half-wave rectifier circuit, a full-bridge rectifier circuit or a voltage doubler rectifier circuit.

[0059] First embodiment

[0060] The present embodiment provides an FCCM soft start control method, which is applied to the startup control of a switching power supply. The switching power supply includes a front-stage buck-boost circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller. The front-stage buck-boost circuit includes a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit. The rear-stage isolated switching power supply circuit includes a primary switch circuit, a transformer and a secondary rectifier circuit. Figure 4 This is a first flow chart of the FCCM soft start control method of the first embodiment of the present invention, see Figure 4 , wherein the FCCM soft start control method includes:

[0061] The starting type identification step is to identify whether the switch power supply starting is an output no-power starting or an output power starting;

[0062] The startup control strategy execution steps are to execute the corresponding control strategy according to the startup type, where:

[0063] When the starter is started without output power, the first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly complementary turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly complementary turned on;

[0064] When the machine is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time;

[0065] The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

[0066] Please continue to see Figure 4 , wherein the start type identification step includes: during the period when the open-loop soft-start voltage in the controller rises from zero to a first set threshold, determining whether the output feedback voltage of the isolation feedback circuit is less than the open-loop soft-start voltage, if so, determining that the output is an electric starter, otherwise determining that the output is a non-electric starter.

[0067] As a specific implementation method, the first set threshold is greater than the saturation conduction voltage drop of the optocoupler in the isolation feedback circuit. The main reason is that when the output has no power, the output feedback voltage transmitted by the optocoupler is always pulled up to the power supply VCC voltage, and when the output has power, the output feedback voltage transmitted by the optocoupler is always pulled down to the saturation conduction voltage drop of the optocoupler. Therefore, the first set threshold can be designed according to the saturation conduction voltage drop of the optocoupler to identify whether the output has no power or power, so that the corresponding startup control strategy can be executed to ensure that the output rises monotonically.

[0068] As a specific implementation, the first soft-open control increases the pulse width by leading edge modulation, trailing edge modulation or double edge modulation until the pulse width of the switch tube in the secondary rectifier circuit increases to complement the switch tube in the primary switch circuit.

[0069] As a specific implementation manner, in the first soft-open control, the duty cycle of the pulse width is less than or equal to 50% after a set time, wherein the set time can be determined according to the actual working condition of the switching power supply.

[0070] It should be noted that the cascade circuit has a buck-boost characteristic. In general, the input voltage Vin can be greater than the bus voltage Vbus or less than the bus voltage Vbus. Figure 4 Under the FCCM soft-start control method, the cascade circuit starts in buck mode first. When it is determined that there is power at the output, the synchronous switch tube and the main switch tube in the front-stage buck-boost circuit are directly complementary. In order to ensure the monotonicity of the output voltage startup, the soft-opening time of the secondary rectifier tube is greater than the output voltage soft-starting time. When the secondary rectifier tube is turned on, the bus voltage refracts the output voltage according to the transformer turns ratio. When the secondary rectifier tube is turned off, the bus voltage drops to zero, and the average bus voltage slowly climbs from low to high. At this time, the front-stage buck-boost circuit will enter the FCCM mode from the DCM mode, and the negative inductor current will gradually increase. The DCM loop responds slowly and cannot quickly adjust the duty cycle of the main switch tube, which leads to an output voltage drop. If the soft-opening speed of the secondary rectifier tube is slowed down, the DCM loop response speed can adjust the duty cycle of the main switch tube so that the negative inductor current will not continue to increase during the entire startup period, resulting in an output drop. Figure 4The FCCM soft-start control method is suitable for communication power supply application scenarios with extremely strict requirements on startup monotonicity. After the output voltage soft-start is completed, large dynamic load switching will not be performed immediately, ensuring that the secondary side rectifier tube has entered the FCCM mode after the soft start is completed. In this way, the overshoot and undershoot indicators of the output voltage under large dynamic loads are not affected.

[0071] Combination Figure 3 The cascade circuit shown is Figure 4 The flowchart of the FCCM soft start control method shown is further described as follows:

[0072] If the machine is in a heavy load shutdown condition, there is no residual voltage on the output capacitor. The COMP voltage is pulled to the highest VCC voltage through negative feedback regulation of the isolation feedback circuit. When the machine is started, the open-loop soft-start SS voltage in the controller gradually climbs. At this time, the controller sends out a synchronous switch tube S2 to directly complement the main switch tube S1 (ignoring the dead time), the secondary rectifier tube S8 to directly complement the primary switch tube S5 (ignoring the dead time), and the secondary rectifier tube S7 to directly complement the primary switch tube S6 (ignoring the dead time). At this time, the inductor current is positive and negative, and the machine is started directly in FCCM mode. The output voltage rises monotonically. When the direct output voltage is quickly established, the output voltage feedback COMP voltage gradually decreases until it is less than the open-loop soft-start voltage SS, and the closed-loop takes over the output voltage regulation.

[0073] If the machine is in a light no-load shutdown condition, the residual voltage on the output capacitor is high. After negative feedback regulation by the isolated feedback circuit, the COMP voltage is pulled to a typical value of 0.3V. When the machine is started, the open-loop soft-start voltage in the controller rises from zero to a value greater than the first set threshold voltage of 0.4V. At this time, the output voltage feedback COMP voltage is less than the open-loop soft-start voltage, and it is determined that the output has power to start the machine. At this time, the output voltage feedback COMP voltage closes and takes over the output voltage regulation. During this process, the comp voltage is maintained at a typical value of 0.3V for a long time. During this process, the output voltage continues to drop. When the closed-loop regulation comp begins to rise to about 0.5V, the synchronous switch tube S2 and the main switch tube S1 are directly complementary and turned on (ignoring the dead zone), and the secondary rectifier tubes S7 and S8 are gradually soft-opened to increase the pulse width. Specifically, the first set time is 100ms and the maximum duty cycle is released to 50% until it complements the primary switch tubes S5 and S6. During this process, the inductor current gradually transitions from DCM to FCCM mode, and the output voltage rises monotonically.

[0074] The controller needs to detect the negative current of the inductor during the entire working process. If the negative current of the inductor reaches 40A due to extreme working conditions such as static electricity and lightning strike, the synchronous switch tube S2 and the secondary rectifier tubes S7 and S8 will be turned off cycle by cycle. Figure 5 This is a second flow chart of the FCCM soft start control method of the first embodiment of the present invention, see Figure 5 ,and Figure 5The difference is that when it is identified that the switch power supply startup is an output power startup, the following steps are also included:

[0075] Determine whether the input voltage of the switching power supply is less than a second set threshold value, and execute a corresponding control strategy according to the determination result, including:

[0076] When the input voltage of the switching power supply is less than a second set threshold, executing a second control strategy;

[0077] When the input voltage of the switching power supply is greater than or equal to the second set threshold, a third control strategy is executed, including: the synchronous switch tube is controlled by the second soft opening, and is complementarily turned on with the main switch tube after a set time; the switch tube in the secondary rectifier circuit is directly complementarily turned on with the switch tube in the primary switch circuit.

[0078] As a specific implementation method, the second set threshold is less than the output voltage of the previous step-up and step-down circuit. When the input voltage is less than the second set threshold, that is, less than the bus voltage, when the output is powered on, the output voltage is higher than the input voltage after refraction through the transformer turns ratio, and the output is easily fed back to the input power supply. At this time, the second control strategy is executed, and the soft opening of the switch tube in the secondary rectifier circuit can be controlled to effectively control the backflow, which can basically ensure that the output voltage rises monotonically; when the input voltage is greater than the second set threshold, that is, greater than the bus voltage, when the output is powered on, the output voltage is less than the input voltage after refraction through the transformer turns ratio, and the output will not feed back to the input power supply, but will feed back through the synchronous switch tube. At this time, the third control strategy is executed, and the soft opening of the synchronous switch tube can be controlled to effectively control the backflow, which can basically ensure that the output voltage rises monotonically.

[0079] As a specific implementation, the second soft-open control increases the pulse width by leading edge modulation, trailing edge modulation or double edge modulation until the pulse width of the synchronous switch tube increases to complement the main switch tube.

[0080] As a specific implementation manner, in the second soft-open control, the duty cycle of the pulse width is less than 100% after a set time, wherein the set time can be determined according to the actual working condition of the switching power supply.

[0081] As a specific implementation method, starting from the startup control, the absolute value of the negative current peak of the inductor when the output is backflowing to the input is detected in real time. When the absolute value is greater than the absolute value of the negative current threshold, the synchronous switch tube and the switch tube in the secondary rectifier circuit are turned off cycle by cycle, thereby realizing real-time detection of the negative current peak when the output is backflowing to the input, and limiting the negative current peak within the negative current threshold.

[0082] It should be noted that Figure 5Under the FCCM soft start control method, the cascade circuit starts in buck mode first. When it is determined that the output has power, when the input voltage is low, if the switch tube in the secondary rectifier circuit is directly turned on complementary to the switch tube in the primary switch circuit, the residual voltage of the output capacitor is higher than the input voltage after being refracted by the transformer turns ratio, and it will directly feed back to the input power supply. Therefore, in the low input voltage segment, the switch tube in the secondary rectifier circuit is controlled to be soft-opened, and the average bus voltage slowly climbs from low to high, which is not easy to feed back to the input power supply. When the input voltage is high, the switch tube in the secondary rectifier circuit is directly turned on complementary to the switch tube in the primary switch circuit. When the bus voltage is directly refracted according to the transformer turns ratio, the output voltage is still lower than the input voltage, and it will not directly feed back to the input power supply. However, if the synchronous switch tube in the previous step-up and step-down circuit is directly and complementary to the main switch tube and the synchronous switch tube is turned on, the duty cycle of the synchronous switch tube is relatively large. Due to the reverse boost process of the buck circuit, the midpoint SW voltage between the main switch tube and the synchronous switch tube is higher than the input voltage, which is also easy to feed back to the input power supply. Therefore, the synchronous switch tube in the previous step-down circuit is controlled to be soft-opened in the high input voltage segment. This control method can ensure that the startup monotonicity requirements are met, and can also ensure that the soft-opening process is completed before the end of the soft start, and will not affect the switching of large dynamic loads.

[0083] Combination Figure 3 The cascade circuit shown is Figure 5 The flowchart of the FCCM soft start control method shown is further described as follows:

[0084] If the input voltage Vin of the cascade power supply circuit is 9-36V, the bus voltage Vbus is 24V, and the output voltage Vo is 12V, the second set threshold voltage is 20V.

[0085] If the machine is in a light no-load shutdown condition, the residual voltage on the output capacitor is relatively high. After negative feedback regulation by the isolated feedback circuit, the COMP voltage is pulled to a typical value of 0.3V. When the machine is started, the open-loop soft-start voltage in the controller rises from zero to a value greater than the first set threshold voltage of 0.4V. At this time, if the output voltage feedback COMP voltage is less than the open-loop soft-start voltage, it is determined that the output has power to start the machine. At this time, the direct output voltage feedback COMP voltage closed-loop takes over the output voltage regulation.

[0086] When the sampled input voltage Vin is less than 20V, the synchronous switch tube S2 and the main switch tube S1 are directly complementary and turned on (ignoring the dead zone), and the secondary rectifier tubes S8 and S7 are gradually soft-opened to increase the pulse width. Specifically, the duty cycle is released to 50% at the maximum within the second setting time of 1ms until it complements the primary switch tubes S5 and S6. During this process, the inductor current gradually transitions from DCM to FCCM mode, and the output voltage rises monotonically. The soft-opening process of the secondary rectifier tubes S7 and S8 is completed before the output voltage is established;

[0087] When the sampled input voltage Vin>20V, the secondary rectifier tubes S7 and S8 are directly turned on complementary to the primary switch tubes S5 and S6, and the synchronous switch tube S2 is gradually soft-opened to increase the pulse width. Specifically, the duty cycle is released to 100% at the maximum within the third setting time of 2ms until it is complementary to the main switch tube S1. During this process, the inductor current gradually transitions from DCM to FCCM, and the output voltage rises monotonically. The soft-open process of the synchronous switch tube S2 is completed before the output voltage is established.

[0088] The controller needs to detect the negative current of the inductor during the entire working process. If the negative current of the inductor reaches 40A due to extreme working conditions such as static electricity and lightning strike, the synchronous switch tube S2 and the secondary rectifier tubes S7 and S8 will be turned off cycle by cycle.

[0089] Second embodiment

[0090] The present embodiment provides an FCCM soft start control device, which is applied to the startup control of a switching power supply. The switching power supply includes a front-stage buck-boost circuit and a rear-stage isolated switching power supply circuit. The front-stage buck-boost circuit includes a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit. The rear-stage isolated switching power supply circuit includes a primary switch circuit, a transformer and a secondary rectifier circuit. The FCCM soft start control device includes:

[0091] A starter type identification unit is configured to identify whether the switch power supply starter is an output non-powered starter or an output powered starter;

[0092] The startup control strategy execution unit is configured to execute a corresponding control strategy according to the startup type, wherein:

[0093] When the starter is started without output power, the first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly complementary turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly complementary turned on;

[0094] When the machine is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time;

[0095] The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

[0096] The technical means adopted by the control device of this embodiment correspond to and are consistent with the control method of the first embodiment, and have the same beneficial effects, so they will not be described in detail.

[0097] In addition, the preferred technical means or further improved means of each step in the control method of the first embodiment can be extended to the corresponding units of this embodiment, and this embodiment will not describe them one by one.

[0098] Third embodiment

[0099] The present embodiment provides a switching power supply, which includes a front-stage buck-boost circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller. The front-stage buck-boost circuit includes a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit. The rear-stage isolated switching power supply circuit includes a primary switch circuit, a transformer and a secondary rectifier circuit, wherein: the switching power supply includes any FCCM soft start control device in the second embodiment.

[0100] The switching power supply of this embodiment includes any FCCM soft start control device in the second embodiment, which can ensure that the output voltage rises completely monotonically when the startup output has a pre-bias voltage, without current backflow damage, and has simple control and high product reliability.

[0101] The above are only preferred embodiments of the present invention, and it should be noted that the above preferred embodiments should not be regarded as limiting the present invention. For those skilled in the art, several equivalent substitutions, improvements and modifications can be made without departing from the spirit and scope of the present invention, and these equivalent substitutions, improvements and modifications should also be regarded as the protection scope of the present invention. The embodiments will not be repeated here, and the protection scope of the present invention shall be based on the scope defined by the claims.

Claims

1. A FCCM soft start control method is applied to the startup control of a switching power supply, wherein the switching power supply comprises a front-stage buck-boost circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switching power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, characterized in that: The FCCM soft start control method comprises: The starting type identification step is to identify whether the switch power supply starting is an output no-power starting or an output power starting; The startup control strategy execution steps are to execute the corresponding control strategy according to the startup type, where: When the starter is a starter without output power, a first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly and complementaryly turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly and complementaryly turned on; When the starter is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time; The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

2. The FCCM soft start control method according to claim 1, characterized in that: The start type identification step includes: when the open-loop soft-start voltage in the controller starts to rise from zero to a first set threshold, determining whether the output feedback voltage of the isolation feedback circuit is less than the open-loop soft-start voltage, if so, determining that the output is a starter with power on, otherwise determining that the output is a starter without power on.

3. The FCCM soft start control method according to claim 2, characterized in that: The first set threshold is greater than the saturation conduction voltage drop of the optocoupler in the isolation feedback circuit.

4. The FCCM soft start control method according to claim 1, characterized in that: The first soft-open control is to increase the pulse width by leading edge modulation, trailing edge modulation or double edge modulation until the pulse width of the switch tube in the secondary rectifier circuit increases to complement the switch tube in the primary switch circuit.

5. The FCCM soft start control method according to claim 1, characterized in that: In the first soft-open control, the duty ratio of the pulse width after the set time is less than or equal to 50%.

6. The FCCM soft start control method according to claim 1, characterized in that: When it is identified that the switch power supply startup is an output power startup, the following steps are also included: Determine whether the input voltage of the switching power supply is less than a second set threshold value, and execute a corresponding control strategy according to the determination result, including: When the input voltage of the switching power supply is less than the second set threshold, executing the second control strategy; When the input voltage of the switching power supply is greater than or equal to the second set threshold, a third control strategy is executed, including: the synchronous switch tube is a second soft-open control, and is complementarily turned on with the main switch tube after a set time; the switch tube in the secondary rectifier circuit is directly complementarily turned on with the switch tube in the primary switch circuit.

7. The FCCM soft start control method according to claim 6, characterized in that: The second set threshold is smaller than the output voltage of the preceding step-up / step-down circuit.

8. The FCCM soft start control method according to claim 6, characterized in that: The second soft-open control is to increase the pulse width by leading edge modulation, trailing edge modulation or double edge modulation until the pulse width of the synchronous switch tube increases to be complementary to the main switch tube.

9. The FCCM soft start control method according to claim 6, characterized in that: In the second soft-open control, the duty ratio of the pulse width is less than 100% after the set time.

10. The FCCM soft start control method according to claim 1, characterized in that: Starting from the start-up control, the absolute value of the negative current peak of the inductor when the output flows back to the input is detected in real time. When the absolute value is greater than the absolute value of the negative current threshold, the synchronous switch tube and the switch tube in the secondary rectifier circuit are turned off cycle by cycle, thereby limiting the negative current peak within the negative current threshold.

11. An FCCM soft start control device, applied to the startup control of a switching power supply, wherein the switching power supply comprises a front-stage buck-boost circuit and a rear-stage isolated switching power supply circuit, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switching power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, characterized in that: The FCCM soft start control device comprises: A starter type identification unit is configured to identify whether the switch power supply starter is an output non-powered starter or an output powered starter; The startup control strategy execution unit is configured to execute a corresponding control strategy according to the startup type, wherein: When the starter is a starter without output power, a first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly and complementaryly turned on; the switch tube in the secondary rectifier circuit and the switch tube in the primary switch circuit are directly and complementaryly turned on; When the starter is started with output power, the second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly turned on in a complementary manner; the switch tube in the secondary rectifier circuit is a first soft-open control, and is turned on in a complementary manner with the switch tube in the primary switch circuit after a set time; The switching power supply detects the peak value of the negative current of the inductor when the output flows back to the input in real time from the start-up, and limits the peak value of the negative current to within the negative current threshold.

12. A switching power supply, comprising a front-stage buck-boost circuit, a rear-stage isolated switch power supply circuit, an isolated feedback circuit and a controller, wherein the front-stage buck-boost circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamping circuit, and the rear-stage isolated switch power supply circuit comprises a primary switch circuit, a transformer and a secondary rectifier circuit, characterized in that: The switching power supply includes the FCCM soft start control device according to claim 11.

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