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

Abstract

The application provides a FCCM soft start control method, a control device and a switching power supply. The control method comprises: identifying whether the switching power supply is started without output power or with output power; and performing a corresponding control strategy according to the starting type, wherein: when the switching power supply is started without output power, a first control strategy is performed, which comprises: the synchronous switch and the main switch are directly and complementarily turned on; the switch in the secondary side rectifier circuit and the switch in the primary side switching circuit are directly and complementarily turned on; when the switching power supply is started with output power, a second control strategy is performed, which comprises: the synchronous switch and the main switch are directly and complementarily turned on; the switch in the secondary side rectifier circuit is first soft open controlled and complementarily turned on with the switch in the primary side switching circuit after a set time; and the switching power supply detects the inductance negative current peak value when the output is reversed to the input in real time and limits the inductance negative current peak value in the negative current threshold. The application can ensure the monotonicity of the output voltage starting and the reliable work of the power supply.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronic converters, and particularly relates to an FCCM soft-start control method, a control device and a switching power supply. BACKGROUND

[0002] Switching power supplies have become an integral part of various industries. For a switching power supply, when the output load is light or even empty, the converter is usually controlled to operate in a forced continuous mode (FCCM for short) to reduce the output voltage ripple. In addition, in application scenarios where the dynamic response speed of the switching power supply is required to be high, such as the communication power supply industry, in order to ensure that the load can respond in time when it jumps greatly, avoid the output voltage overshoots out of the range, and affects the normal work of the subsequent system, FCCM mode is usually used.

[0003] Generally, the input end and the output end of a switching power supply are configured with large-capacity capacitors. If the output capacitor has a very high voltage residual after shutdown in a light or empty load condition, when the output capacitor 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 at this time. If the backflow current is large, it may damage the device, and during the backflow process, the output voltage will drop rapidly until the input energy is greater than the output energy, and then the output voltage will rise. This output voltage drop phenomenon does not meet the monotonicity requirement of starting, and in severe cases, it directly affects the normal work of the subsequent system, such as Figure 1 .

[0004] Please refer to Figure 2 , a cascaded circuit and a control method thereof are disclosed in Chinese patent application No. CN114221549A. Specifically, it relates to a boost-buck cascaded circuit, which includes a front-stage boost-buck circuit and a rear-stage isolated switching power supply circuit, and realizes boost-buck mode control cascaded circuit at least including a switching tube S1, a switching tube S2, a power inductor L and an isolated switching power supply circuit. Specifically, the isolated switching power supply circuit needs to include a primary side switching circuit, a transformer T1 and a secondary side rectifier circuit. The cascaded circuit adopts a current feeding mode, and the primary side switching tube in the rear-stage isolated switching circuit has a common conduction part, which plays a role of excitation in the boost mode and provides a freewheeling path for the inductor current in the buck mode.

[0005] In the industry, for the problem of starting with a pre-biased voltage output, when a conventional digital MCU control scheme is used, the inductor excitation PWM duty cycle to be sent is calculated according to the sampled output capacitor residual voltage. However, for Figure 2The cascade circuit, the main control is located in the primary side of the transformer in the low-cost application, cannot directly sample the output voltage to calculate the duty cycle information, therefore, in the case that the output has a pre-biased voltage, how to ensure the reliable single monotone starting of the switching power supply product becomes a difficult problem to be solved urgently under the low-cost non-digital MCU control scheme. SUMMARY

[0006] Therefore, the technical problem to be solved by the present application is to provide an FCCM soft start control method, a control device and a switching power supply, so as to solve the problem of starting the switching power supply in FCCM mode with pre-biased voltage output, meet the single monotonicity requirement, and avoid damage caused by current backflow, so that the switching power supply product is more safe and reliable, and has good economic and social benefits.

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

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

[0009] A starting type identification step for identifying whether the starting of the switching power supply is output without electricity or output with electricity;

[0010] A starting control strategy execution step for executing a corresponding control strategy according to the starting type, wherein:

[0011] When the starting is output without electricity, a first control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary to each other; the switch tube in the secondary side rectifier circuit and the switch tube in the primary side switching circuit are directly complementary to each other;

[0012] When the starting is output with electricity, a second control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary to each other; the switch tube in the secondary side rectifier circuit is first soft open controlled, and is complementary to the switch tube in the primary side switching circuit after a set time;

[0013] The switching power supply starts from the starting, detects the inductive negative current peak value when the output is backflowed to the input in real time, and limits the negative current peak value within a negative current threshold value.

[0014] Further, the starting type identifying step comprises: judging whether the output feedback voltage of the isolation feedback circuit is less than the open-loop soft starting voltage during the open-loop soft starting voltage in the controller is rising from zero to a first set threshold, and if yes, determining that the output has power starting, otherwise, determining that the output has no power starting.

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

[0016] Preferably, the first soft-on control is front edge modulation, back edge modulation or double edge modulation to increase the pulse width until the pulse width of the switch tube in the secondary rectifier circuit is increased to be complementary to the switch tube in the primary switch circuit.

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

[0018] Further, when the starting of the switching power supply is determined to be output power starting, the method further comprises the following steps:

[0019] judging whether the input voltage of the switching power supply is less than a second set threshold, and executing corresponding control strategy according to the judgment result, comprising:

[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, executing a third control strategy, comprising: the synchronous switch tube is a second soft-on control and is complementary to the main switch tube after a set time; and the switch tube in the secondary rectifier circuit is directly complementary to the switch tube in the primary switch circuit.

[0022] Preferably, the second set threshold is less than the output voltage of the front-stage boost-buck circuit.

[0023] Preferably, the second soft-on control is front edge modulation, back edge modulation or double edge modulation to increase the pulse width until the pulse width of the synchronous switch tube is increased to be complementary to the main switch tube.

[0024] Further, the second soft-on control is to make the duty cycle of the pulse width less than 100% after the set time.

[0025] Preferably, from the starting control, the absolute value of the inductive negative current peak value when the output is backfed to the input is detected in real time, and 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, so as to limit the negative current peak value within the negative current threshold.

[0026] As a second aspect of the present application, the embodiment of the FCCM soft start control device provided by the present application has the technical solutions as follows:

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

[0028] A start type identification unit is configured to identify that start of the switching power supply is output no-power start or output power start.

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

[0030] When the start is output no-power start, a first control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary on.

[0031] When the start is output power start, a second control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary on.

[0032] The switching power supply detects an inductive negative current peak value when output is reversed to input in real time from start, and limits the negative current peak value within a negative current threshold.

[0033] As a third aspect of the present application, the embodiment of the switching power supply provided by the present application has the technical solutions as follows:

[0034] The switching power supply comprises a front-stage boost-buck circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, the front-stage boost-buck circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamp circuit, the rear-stage isolated switching power supply circuit comprises a primary side switching circuit, a transformer and a secondary side rectifier circuit, and the switching power supply comprises the FCCM soft start control device of the second aspect.

[0035] Term explanation:

[0036] Front edge modulation: specifically, a falling edge of a high-level pulse width signal sent by the controller is fixed in position in each switching period, and a rising edge position can be adjusted and changed, so that front edge modulation soft on control of a switch tube driving signal can be realized.

[0037] Late edge modulation: specifically, the rising edge of the high level pulse width signal sent by the controller is fixed in position within each switching cycle, and the falling edge position can be adjusted and changed, thereby realizing late edge modulation soft on control of the switch tube driving signal.

[0038] Double edge modulation: specifically, the rising edge and the falling edge of the high level pulse width signal sent by the controller can be adjusted and changed within each switching cycle, thereby realizing double edge modulation of the switch tube driving signal.

[0039] The present application has the following beneficial effects compared with the conventional FCCM control scheme:

[0040] (1) The two FCCM soft start control methods provided by the present application can ensure that the output voltage completely monotonically rises under the condition of having a pre-biased voltage, the control is simple, and the product has high reliability.

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

[0042] Figure 1 Fig. 1 is a schematic diagram of a drop pit of an output voltage with power-on;

[0043] Figure 2 Fig. 2 is a schematic diagram of a cascaded circuit;

[0044] Figure 3 Fig. 3 is a typical schematic diagram of a switching power supply suitable for the present application;

[0045] Figure 4 Fig. 4 is a first flowchart of the FCCM soft start control method of the first embodiment of the present application;

[0046] Figure 5 Fig. 5 is a second flowchart of the FCCM soft start control method of the first embodiment of the present application. DETAILED DESCRIPTION

[0047] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0048] In order to enable the personnel in the technical field to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the personnel in the field without creative labor should belong to the scope of protection of the present application.

[0049] It should be noted that the terms "first", "second", and the like in the description and in the claims of the present application and above accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a particular sequential or chronological order. It should be understood that the data thus used can be interchanged, where appropriate, to describe the embodiments of the present application herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or apparatus that includes a list of steps or units is not necessarily limited to those steps or units that are clearly listed, but can include other steps or units that are not clearly listed or inherent to such processes, methods, products, or apparatuses.

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

[0051] In addition, the accompanying drawings of the present disclosure are only schematic and not necessarily drawn to scale. The same reference numbers in the drawings represent the same or similar elements / functionality, and thus repeated description thereof will be omitted. Some of the blocks in the drawings are functional entities that do not necessarily have to correspond to physically or logically independent entities. These functional entities can be implemented by software, or in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

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

[0053] The one end of the switch tube S1 and the one end of the input capacitor Cin are connected to the input power Vin, that is, the input positive of the buck-boost circuit, and also the input positive of the cascade circuit, the other end of the switch tube S1 and the one end of the switch tube S2 are connected to the one end of the inductor L, the other end of the inductor L, the one end of the active clamping capacitor C1 and the center tap of the transformer in the isolated switching power supply are connected to the bus Vbus, that is, the output positive of the buck-boost circuit, and also the input positive of the isolated switching power supply circuit, the same end of the upper winding of the transformer primary side is connected to the one end of the primary side switch tube S5, the different end of the lower winding of the transformer primary side is connected to the one end of the primary side switch tube S6, the other end of the active clamping capacitor C1 is connected to the one end of the active clamping 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 side switch tubes S5 and S6 and the other end of the active clamping switch tube S4 are connected to the primary side input power GND, the same end of the upper winding of the transformer secondary side is connected to the one end of the secondary side rectifier S7, the different end of the lower winding of the transformer secondary side is connected to the one end of the secondary side rectifier S8, the center tap of the transformer secondary side and the one end of the output capacitor Co are connected to the output positive of the cascade circuit, and the other ends of the secondary side rectifiers S7 and S8 and the other end of the output capacitor Co are connected to the output negative 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 the resistor R1, the resistor R2, the controllable voltage regulator 431, the resistor R3, the optocoupler and the resistor R4. The one end of the resistor R1 and the one end of the secondary side of the optocoupler are connected to the output voltage Vo, the other end of the resistor R1 and the one end of the resistor R2 are connected to the 1 pin of the controllable voltage regulator 431, the 2 pin of the controllable voltage regulator 431 is connected to the other end of the secondary side of the optocoupler, the 3 pin of the controllable voltage regulator 431 and the other end of the resistor R2 are connected to the secondary side reference ground of the cascade circuit; the one end of the primary side of the optocoupler and the one end of the resistor R4 are connected to the 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 voltage VCC, and the other end of the primary side of the optocoupler is connected to the primary side input power GND.

[0055] The controller is used for controlling the switch tubes in the cascade circuit to work at a fixed frequency; the controller samples the input voltage, and outputs the driving signals S1, S2, S3, S4, S5, S6, S7 and S8 according to the FCCM soft start control method of the application, so as to control 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, which is used for identifying the output no-electric start and the output electric start working conditions when starting, so as to perform corresponding control logic processing; from the start control, the controller detects the inductive negative current when the output is reversed 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 isolation feedback circuit with the open-loop soft start voltage, and when starting, the open-loop soft start voltage starts to rise from zero to a first set threshold value, and if the output voltage feedback signal is less than the open-loop soft start voltage in the first set threshold value interval, it is determined that the output has power start; otherwise, if the output voltage feedback signal is not less than the open-loop soft start voltage in the set threshold value interval, it is determined that the output has no power start.

[0058] It should be noted that, Figure 3 is only a typical schematic diagram of the switching power supply to which the present application is applied, and should not constitute a limitation on the protection scope of the present application. The specific circuit design of the switching power supply to which the present application is applied can be selected according to actual needs by those skilled in the art, and the primary side switching circuit therein can be a push-pull circuit, a half-bridge circuit, a full-bridge circuit or an LLC circuit, etc. in addition to the Figure 3 in the description; and the secondary side rectifier circuit therein can be a half-wave rectifier circuit, a full-bridge rectifier circuit or a voltage doubler rectifier circuit, etc. in addition to the full-wave rectifier circuit in the description. Figure 3

[0059] First embodiment

[0060] The present embodiment provides a FCCM soft start control method, which is applied to start control of a switching power supply. The switching power supply comprises a front-stage boost-buck circuit, a rear-stage isolated switching power supply circuit, an isolation feedback circuit and a controller. The front-stage boost-buck circuit comprises a main switch tube, a synchronous switch tube, an inductor and an active clamp circuit. The rear-stage isolated switching power supply circuit comprises a primary side switching circuit, a transformer and a secondary side rectifier circuit, Figure 4 The first flow chart of the FCCM soft start control method of the first embodiment of the present application is shown in Figure 4 The FCCM soft start control method comprises the following steps:

[0061] A start type identification step is provided to identify whether the start of the switching power supply is output power start or output no power start.

[0062] A start control strategy execution step is provided to execute a corresponding control strategy according to the start type, wherein:

[0063] When the start is output no power start, a first control strategy is executed, which comprises: the synchronous switch tube and the main switch tube are directly complementary to each other; and the switch tube in the secondary side rectifier circuit and the switch tube in the primary side switching circuit are directly complementary to each other.

[0064] When the start is output power start, a second control strategy is executed, which comprises: the synchronous switch tube and the main switch tube are directly complementary to each other; and the switch tube in the secondary side rectifier circuit is controlled by a first soft start control, and is complementary to the switch tube in the primary side switching circuit after a set time. ​

[0065] The switch power supply detects the negative inductance current peak value when the output is reversed to the input in real time from starting, and limits the negative current peak value within the negative current threshold.

[0066] Please continue to see Figure 4 Wherein the starting type identification step comprises: during the open loop soft starting voltage in the controller rising from zero to a first set threshold, judging whether the output feedback voltage of the isolation feedback circuit is less than the open loop soft starting voltage, if yes, determining that the output is powered starting, otherwise, determining that the output is unpowered starting.

[0067] As a specific embodiment, 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 is unpowered, the output feedback voltage transmitted through the optocoupler is always pulled up to the supply VCC voltage, and when the output is powered, the output feedback voltage transmitted through the optocoupler is always pulled down to the saturation conduction voltage drop of the optocoupler, so the first set threshold can be designed according to the saturation conduction voltage drop of the optocoupler to identify the output unpowered and powered, so that the corresponding starting control strategy can be executed to ensure the output monotonically rising.

[0068] As a specific embodiment, the first soft opening control is front edge modulation, rear 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 complementary conduction with the switch tube in the primary side switching circuit.

[0069] As a specific embodiment, the first soft opening control is that 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 switch power supply.

[0070] It needs to be specially pointed out that the cascade circuit has the characteristics of step-up and step-down, and in general, the input voltage Vin can be greater than or less than the bus voltage Vbus, and in Figure 4 Under the FCCM soft starting control method, the cascade circuit starts in buck mode first when starting, and when it is determined that the output is powered starting, the synchronous switch tube in the front-stage step-up and step-down circuit and the main switch tube are directly complementary, in order to ensure the monotonicity of the output voltage starting, the soft opening time of the secondary rectifier tube is greater than the soft starting time of the output voltage, so the bus voltage is refracted according to the output voltage of the transformer turns ratio when the secondary rectifier tube is turned on, and the bus voltage is zero when the secondary rectifier tube is turned off, so the average value of the bus voltage slowly rises from low to high, at this time the front-stage step-up and step-down circuit will enter the FCCM mode from the DCM mode, and the negative inductance current will gradually increase, the DCM loop responds slowly and cannot quickly adjust the duty cycle of the main switch tube, which further leads to the phenomenon of output voltage drop, if the soft opening speed of the secondary rectifier tube is slowed down, the response speed of the DCM loop can adjust the duty cycle of the main switch tube so that the negative inductance current does not continuously increase during the whole starting period, which leads to the output voltage drop. Figure 4The FCCM soft start control method is applicable to the communication power supply application scene with extremely strict single monotonicity requirement of starting machine, and the output voltage soft start is not immediately switched to large dynamic load after the soft start, so that the soft start of the secondary rectifier tube is ensured to be in the FCCM mode, and the over-undervoltage indicators of the large dynamic load output voltage are not affected.

[0071] In combination with the cascade circuit shown in FIG. 1, the FCCM soft start control method is further described as follows: Figure 3 Figure 4 The flow chart of the FCCM soft start control method is further described as follows:

[0072] If the heavy load shutdown condition is in the heavy load shutdown condition, there is no residual voltage on the output capacitor, the COMP voltage is pulled to the highest VCC voltage through the isolated feedback circuit negative feedback regulation, at this time, the machine is started, the open loop soft start SS voltage in the controller gradually rises, at this time, the controller directly outputs the synchronous switch tube S2 and the main switch tube S1 (ignoring the dead time), the secondary rectifier tube S8 and the primary switch tube S5 (ignoring the dead time), the secondary rectifier tube S7 and the primary switch tube S6 (ignoring the dead time), at this time, the inductor current is directly started in the FCCM mode, the output voltage is monotonously increased, and when the 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 then the closed loop takes over the output voltage regulation;

[0073] If the light load shutdown condition is in the light load shutdown condition, the residual voltage on the output capacitor is relatively high, the COMP voltage is pulled to the typical value 0.3V through the isolated feedback circuit negative feedback regulation, at this time, the machine is started, the open loop soft start voltage in the controller rises from zero to greater than the first set threshold voltage 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 electricity, at this time, the direct output voltage feedback COMP voltage takes over the output voltage regulation, and in this process, the comp voltage is maintained at the typical value 0.3V for a long time, and in this process, the output voltage continues to drop, and the closed loop regulation comp starts to rise to about 0.5V, the synchronous switch tube S2 and the main switch tube S1 are directly complementary to each other (ignoring the dead time), the secondary rectifier tubes S7 and S8 gradually increase the pulse width, and the first set time 100ms is released to 50% duty cycle at most until the primary switch tubes S5 and S6 are complementary to each other, in this process, the inductor current gradually transits from DCM to FCCM mode, and the output voltage monotonously increases;

[0074] During the whole working process, the controller needs to detect the inductor negative current, if the inductor negative current reaches 40A in the extreme conditions such as static electricity and lightning, the synchronous switch tube S2 and the secondary rectifier tubes S7 and S8 are turned off periodically. Figure 5 For the second flow chart of the FCCM soft start control method of the first embodiment of the application, please refer to FIG. 2. Figure 5 , and Figure 5 ​The difference is that the following steps are further included when it is identified that the switching power supply is started with output power:

[0075] The input voltage of the switching power supply is judged whether it is less than a second set threshold, and a corresponding control strategy is executed according to the judgment result, including:

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

[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 that the synchronous switch is controlled by the second soft opening, and the synchronous switch and the main switch are complementary on for a set time; the switch in the secondary rectifier circuit and the switch in the primary switch circuit are directly complementary on.

[0078] As a specific embodiment, the second set threshold is less than the output voltage of the front-stage boost-buck circuit, when the input voltage is less than the second set threshold, i.e. less than the bus voltage, the output voltage is higher than the input voltage through the turns ratio refraction of the transformer when the output is powered on, and the output is easy to be back-feeding to the input power supply, at this time, the second control strategy is executed, and the soft opening of the switch in the secondary rectifier circuit can effectively control the back-feeding, and can basically ensure the monotone rising of the output voltage; when the input voltage is greater than the second set threshold, i.e. greater than the bus voltage, the output voltage is less than the input voltage through the turns ratio refraction of the transformer when the output is powered on, and the output will not back-feeding to the input power supply, but will back-feeding through the synchronous switch, at this time, the third control strategy is executed, and the soft opening of the synchronous switch can effectively control the back-feeding, and can basically ensure the monotone rising of the output voltage.

[0079] As a specific embodiment, the second soft opening control is the front edge modulation, the rear edge modulation or the double edge modulation to increase the pulse width until the pulse width of the synchronous switch is increased to the complementary on with the main switch.

[0080] As a specific embodiment, the second soft opening control is that 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 embodiment, from the start of the control, the absolute value of the inductive negative current peak value when the output is back-feeding 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 and the switch in the secondary rectifier circuit are turned off cycle by cycle, so as to realize the real-time detection of the negative current peak value when the output is back-feeding to the input, and limit the negative current peak value within the negative current threshold.

[0082] It needs to be particularly pointed out that, Figure 5In FCCM soft start control method, the cascade circuit starts with buck mode when starting, and when the output has power, the switch tube in the secondary rectifier circuit directly conducts with the switch tube in the primary switch circuit when the input voltage is low. The residual voltage of the output capacitor is higher than the input voltage through the transformer turns ratio refraction, and thus directly back-feeding to the input power. Therefore, the switch tube in the secondary rectifier circuit is controlled to soft start in the low input voltage section, and the average value of the bus voltage slowly rises from low to high, which is not easy to back-feeding to the input power. When the input voltage is high, the switch tube in the secondary rectifier circuit directly conducts with the switch tube in the primary switch circuit, and the bus voltage directly outputs the voltage according to the transformer turns ratio refraction, but is still lower than the input voltage, and thus does not directly back-feeding to the input power. However, if the synchronous switch tube in the front-stage boost-buck circuit directly conducts with the main switch tube, the synchronous switch tube has a large duty ratio, and the midpoint SW voltage of the main switch tube and the synchronous switch tube is higher than the input voltage due to the reverse boost process of the buck circuit, which is also easy to back-feeding to the input power. Therefore, the synchronous switch tube in the front-stage boost-buck circuit is controlled to soft start in the high input voltage section. The control method can ensure that the starting monotonicity requirement is met, and the soft start process is completed before the soft start ends, and thus does not affect the large dynamic load switching.

[0083] In combination with the cascade circuit shown in FIG. 1, the FCCM soft start control method shown in FIG. 2 is further described as follows. Figure 3 In combination with the cascade circuit shown in FIG. 1, the FCCM soft start control method shown in FIG. 2 is further described as follows. Figure 5 The flow chart of the FCCM soft start control method shown in FIG. 2 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, the output voltage Vo is 12V, and the second set threshold voltage is 20V.

[0085] If it is in the light idle shutdown working condition, the residual voltage on the output capacitor is high, and the COMP voltage is pulled to the typical value 0.3V through the negative feedback adjustment of the isolation feedback circuit. When the controller starts, the open-loop soft start voltage from zero rises to greater than the first set threshold voltage 0.4V. At this time, the output voltage feedback COMP voltage is less than the open-loop soft start voltage, and thus it is determined that the output has power. At this time, the output voltage feedback COMP voltage directly takes over the output voltage regulation.

[0086] When the sampled input voltage Vin is less than 20V, the synchronous switch tube S2 directly conducts with the main switch tube S1 (ignoring the dead zone), and the secondary rectifier tubes S8 and S7 gradually soft start to increase the pulse width. Specifically, the second set time 1ms is released to 50% duty ratio at most until it is complementary with the primary switch tubes S5 and S6. In this process, the inductor current gradually transitions from DCM to FCCM mode, the output voltage monotonically rises, and the soft start process of the secondary rectifier tubes S7 and S8 is completed before the output voltage is established.

[0087] When the sampling input voltage Vin is greater than 20V, the secondary rectifier S7 and S8 are directly and complementarily conducted with the primary switch S5 and S6, and the synchronous switch S2 gradually increases the pulse width in soft opening, and specifically, the third set time 2ms is released to 100% duty ratio until it is complementarily conducted with the main switch S1, in which the inductor current gradually transits from DCM to FCCM, and the output voltage monotonously rises, and the soft opening process of the synchronous switch S2 is completed before the output voltage is established.

[0088] During the whole working process, the controller needs to detect the inductor negative current, and if the extreme working condition such as static electricity or lightning strike occurs when the inductor negative current reaches 40A, the synchronous switch S2 and the secondary rectifier S7 and S8 are gradually turned off cycle by cycle.

[0089] Second embodiment

[0090] The embodiment provides an FCCM soft start control device applied to start control of a switching power supply, the switching power supply comprising a front-stage boost-buck circuit and a rear-stage isolated switching power supply circuit, the front-stage boost-buck circuit comprising a main switch, a synchronous switch, an inductor and an active clamp circuit, and the rear-stage isolated switching power supply circuit comprising a primary-side switching circuit, a transformer and a secondary-side rectifier circuit, wherein the FCCM soft start control device comprises:

[0091] A start type identification unit is configured to identify that the switching power supply start is output no-power start or output power start.

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

[0093] When the start is the output no-power start, the first control strategy is executed, comprising that the synchronous switch and the main switch are directly and complementarily conducted, and the switch in the secondary-side rectifier circuit and the switch in the primary-side switching circuit are directly and complementarily conducted.

[0094] When the start is the output power start, the second control strategy is executed, comprising that the synchronous switch and the main switch are directly and complementarily conducted, and the switch in the secondary-side rectifier circuit is controlled in the first soft opening, and is complementarily conducted with the switch in the primary-side switching circuit after a set time.

[0095] The switching power supply starts, and the inductor negative current peak value when the output is inversely flooded to the input is detected in real time, and the negative current peak value is limited in the negative current threshold.

[0096] The technical means adopted by the control device of the embodiment is consistent with the control method of the first embodiment, and has the same beneficial effects, and thus is not described in detail.

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

[0098] Third embodiment

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

[0100] The switching power supply of the present embodiment can ensure that the output voltage completely monotonically rises under the condition that the start-up output has a pre-bias voltage, without current backflow damage, and the control is simple and the product reliability is high, due to the FCCM soft start control device in the second embodiment.

[0101] The above is only the preferred embodiment of the present application, and it should be noted that the above preferred embodiment should not be regarded as limiting the present application. For those skilled in the art, without departing from the spirit and scope of the present application, a number of equivalent replacements, improvements and refinements can also be made, which should also be regarded as the protection scope of the present application, and the protection scope of the present application should be limited by the scope defined by the claims.

Claims

1. A FCCM soft start control method applied to start-up control of a switching power supply, the switching power supply comprising a front-stage boost-buck circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, the front-stage boost-buck circuit comprising a main switch tube, a synchronous switch tube, an inductor and an active clamp circuit, the rear-stage isolated switching power supply circuit comprising a primary-side switching circuit, a transformer and a secondary-side rectifier circuit, characterized in that, The FCCM soft start control method comprises: A start-up type identification step of identifying whether the start-up of the switching power supply is output no-power start-up or output power start-up; A start-up control strategy execution step of executing a corresponding control strategy according to the start-up type, wherein: When the start-up is output no-power start-up, a first control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary conduction; the switch tube in the secondary side rectifier circuit and the switch tube in the primary side switching circuit are directly complementary conduction; When the start-up is output power start-up, a second control strategy is executed, comprising: the synchronous switch tube and the main switch tube are directly complementary conduction; the switch tube in the secondary side rectifier circuit is first soft open control, and is complementary conduction with the switch tube in the primary side switching circuit after a set time; The switching power supply detects the inductance negative current peak value when the output is reversed to the input in real time from the start-up, and limits the negative current peak value within a negative current threshold value.

2. The FCCM soft start control method of claim 1, wherein, The start-up type identification step comprises: during the process that the open loop soft start voltage in the controller starts to rise from zero to a first set threshold value, it is judged whether the output feedback voltage of the isolation feedback circuit is less than the open loop soft start voltage, if yes, it is determined that the start-up is output power start-up, otherwise, it is determined that the start-up is output no-power start-up.

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

4. The FCCM soft start control method of claim 1, wherein: The first soft open control is front edge modulation, rear edge modulation or double edge modulation to increase the pulse width, until the pulse width of the switch tube in the secondary side rectifier circuit is increased to be complementary conduction with the switch tube in the primary side switching circuit.

5. The FCCM soft start control method of claim 1, wherein: The first soft open control is that the duty cycle of the pulse width is less than or equal to 50% after the set time.

6. The FCCM soft start control method of claim 1, wherein, When it is identified that the start-up of the switching power supply is output power start-up, the following steps are further included: It is judged whether the input voltage of the switching power supply is less than a second set threshold value, and corresponding control strategies are executed according to the judgment result, comprising: When the input voltage of the switching power supply is less than the second set threshold value, the second control strategy is executed; When the input voltage of the switching power supply is greater than or equal to the second set threshold value, a third control strategy is executed, comprising: the synchronous switch tube is second soft open control, and is complementary conduction with the main switch tube after a set time; the switch tube in the secondary side rectifier circuit and the switch tube in the primary side switching circuit are directly complementary conduction.

7. The FCCM soft start control method of claim 6, wherein: The second set threshold value is less than the output voltage of the front-stage boost-buck circuit.

8. The FCCM soft start control method of claim 6, wherein: The second soft open control is front edge modulation, rear edge modulation or double edge modulation to increase the pulse width, until the pulse width of the synchronous switch tube is increased to be complementary conduction with the main switch tube.

9. The FCCM soft start control method of claim 6, wherein: The second soft open control is that the duty cycle of the pulse width is less than 100% after the set time.

10. The FCCM soft start control method of claim 1, wherein: From the start-up control, the absolute value of the inductance negative current peak value when the output is reversed to the input is detected in real time, and when the absolute value is greater than the absolute value of the negative current threshold value, the synchronous switch tube and the switch tube in the secondary side rectifier circuit are turned off cycle by cycle, so that the negative current peak value is limited within the negative current threshold value.

11. A FCCM soft start control device applied to start control of a switching power supply, the switching power supply comprising a front-stage boost-buck circuit and a rear-stage isolated switching power supply circuit, the front-stage boost-buck circuit comprising a main switch tube, a synchronous switch tube, an inductor and an active clamp circuit, the rear-stage isolated switching power supply circuit comprising a primary-side switching circuit, a transformer and a secondary-side rectifier circuit, characterized in that, The FCCM soft start control device comprises: The starting type identification unit is configured to identify the starting of the switching power supply as output no-power starting or output power starting. The starting control strategy execution unit is configured to execute a corresponding control strategy according to the starting type, wherein: When the starting is output no-power starting, a first control strategy is executed, including: the synchronous switch tube and the main switch tube are directly complementary conduction; the switch tube in the secondary side rectifier circuit and the switch tube in the primary side switch circuit are directly complementary conduction; When the starting is output power starting, a second control strategy is executed, including: the synchronous switch tube and the main switch tube are directly complementary conduction; the switch tube in the secondary side rectifier circuit is first soft open control, and is complementary conduction with the switch tube in the primary side switch circuit after a set time; The switching power supply detects the inductance negative current peak value when the output is reversed to the input in real time from the starting, and limits the negative current peak value in the negative current threshold.

12. A switching power supply comprising a front-stage step-up / down voltage conversion circuit, a rear-stage isolated switching power supply circuit, an isolated feedback circuit and a controller, the front-stage step-up / down voltage conversion circuit comprising a main switch, a synchronous switch, an inductor and an active clamp circuit, the rear-stage isolated switching power supply circuit comprising a primary-side switching circuit, a transformer and a secondary-side rectification circuit, characterized in that: The switching power supply comprises the FCCM soft starting control device in claim 11.

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