Soft start control circuit and switching power supply

Abstract

The utility model provides a kind of soft-start control circuit and switching power supply, wherein soft-start control circuit includes: input side soft-start control unit, in the starting stage of switching power supply, according to the feedback pin current of main controller, control main power switch tube to execute soft-start control strategy, so that output voltage linearly increases with first slope;Output side soft-start control unit, in the starting stage of switching power supply, when identifying that output voltage reaches first set value, for the feedback pin of main controller, inject holding current, control main power switch tube continues to execute soft-start control strategy, so that output voltage linearly increases with lower slope, until output voltage reaches output voltage loop setting reference value, first set value is less than output voltage loop setting reference value;Soft-start control strategy is: control the duty cycle of main power switch tube drive signal linearly increases from small to big;Or switching frequency linearly decreases from big to small.The utility model can achieve linear soft-start effect.

Description

Technical Field

[0001] This utility model relates to the field of power supply technology, and in particular to a soft-start control circuit and a switching power supply. Background Technology

[0002] With the widespread application of ACDC and DCDC switching power supplies, power supplies used in precision equipment, such as server power supplies, have very high requirements for the output voltage set-up timing and output voltage ramp-up process of switching power supplies. In order to fully ensure the power safety of server loads, general server CRPS (Common Redundant Power Supply) power supplies require that the output voltage ramp-up must be linear under any load current, and there should be no dips or severe overshoots during the output voltage ramp-up process. This is mainly because the server application load side is equipped with comprehensive undervoltage and overvoltage protection. If the power supply output voltage suddenly drops below the undervoltage protection point after ramping up to the undervoltage set value during the loading process, the load side will report a fault, affecting the overall matching application of the power supply.

[0003] Based on the above application requirements, many types of CRPS power supplies currently use digital control, specifically intelligent adjustment of the soft-start reference to achieve linear output voltage ramp-up. Analog control schemes, due to their fixed reference voltage, struggle to achieve good linear soft-start performance. Taking the commonly used TEA2017 analog controller (NXP brand) as an example, this controller controls the primary-side LLC switch, and the secondary-side voltage loop samples the output voltage using a 431 microcontroller. An optocoupler feedback error comparison signal is then fed to the TEA2017 for closed-loop control. This controller has built-in primary-side PFM soft-start control, gradually changing the switching frequency from high frequency (e.g., 350kHz) to low frequency during startup, gradually increasing the gain of the main power stage to achieve a gradual increase in output voltage from 0V. However, because it relies solely on the built-in primary-side PFM soft-start control, overshoot is very likely to occur under no-load conditions. Figure 1 The diagram shown is the no-load output voltage start-up waveform of this scheme.

[0004] To improve the overshoot using the aforementioned control scheme for the TEA2017 analog controller, a dummy load needs to be connected in parallel at the output, which unnecessarily increases losses. Figure 2As shown, a generally better method to improve overshoot is to add a soft-start circuit consisting of diode D1 and soft-start capacitor C3 to the cathode of the 431. A bleeder resistor R7 should also be connected in parallel with the soft-start capacitor C3 to release its energy after power-off and before restarting. This allows the soft-start capacitor C3 to gradually charge from 0V to its highest steady-state voltage upon restarting, achieving a soft-start effect again. This solution can optimize no-load output voltage overshoot, but it introduces a new problem: during the output voltage rise process, the primary-side soft-start and secondary-side soft-start compete, leading to nonlinearity issues during the output voltage rise process, such as... Figure 3 As shown.

[0005] To better apply analog control solutions to server power supplies, it is necessary to solve... Figure 1 The output voltage overshoot and insufficient output voltage are presented Figure 3 The resulting output voltage startup nonlinearity problem. Utility Model Content

[0006] Therefore, the technical problem to be solved by this utility model is to provide a soft-start control circuit and a switching power supply, which at least partially solves one of the technical problems existing in the prior art.

[0007] The inventive concept of this utility model is as follows: When the power supply is initially turned on, the duty cycle or switching frequency of the main power switch is slowly adjusted by the input-side soft start control to achieve a first stage of output voltage slowly rising from 0V with a first slope; when the output voltage increases to a certain value, the output-side soft start detects that the output voltage is greater than the set first threshold level, triggering the output-side soft start to switch to the main loop control and enter the second stage of soft start control: at this time, the input-side soft start and the output-side soft start simultaneously control the output voltage to increase, but the output-side soft start control is dominant. As the output voltage increases, the duty cycle of the main power switch is gradually increased or the switching frequency is gradually decreased, and the power stage gain is gradually increased, thereby achieving a closed-loop linear rise of the output voltage with a lower slope, achieving the effect of linear soft start.

[0008] Based on the above inventive concept, the technical solution of the soft-start control circuit embodiment proposed by this utility model is as follows:

[0009] A soft-start control circuit is applied to a switching power supply. The switching power supply includes a main controller and an output voltage control loop. The output voltage control loop is used to sample the output voltage, compare it with a reference, and generate an error amplification signal. Then, the error amplification signal is isolated and processed to provide feedback current to the feedback pin of the main controller. The main controller controls the switching action of the main power switch transistor according to the feedback current so that the output voltage tends to a preset value. The soft-start control circuit includes: The input-side soft-start control unit is used to control the main power switch to execute a soft-start control strategy based on the magnitude of the current of the feedback pin of the main controller during the startup phase of the switching power supply, so that the output voltage increases linearly with a first slope. The output-side soft-start control unit is used to inject a holding current into the feedback pin of the main controller when the output voltage reaches a first set value during the startup phase of the switching power supply. This controls the main power switch to continue executing the soft-start control strategy, so that the output voltage increases linearly at a lower slope until the output voltage reaches the output voltage loop set reference value, at which point the startup phase ends. The first set value is less than the output voltage loop set reference value. The soft-start control strategy is as follows: the duty cycle of the main power switch drive signal is linearly increased from small to large; or the switching frequency of the main power switch is linearly decreased from large to small.

[0010] Optionally, the main controller is a TEA2017.

[0011] Optionally, the input-side soft-start control unit is integrated into the main controller.

[0012] Optionally, the output voltage control loop includes resistors R1, R2, R3, R4, R5, capacitor C2, 431 device U1, and optocoupler OC1. One end of resistor R1 and one end of resistor R4 are both used to connect to the output voltage. The other end of resistor R1 is simultaneously connected to one end of resistor R2, one end of resistor R3, and the reference terminal of 431 device U1. The other end of resistor R3 is connected to one end of capacitor C2. The other end of resistor R4 is simultaneously connected to one end of resistor R5 and the anode of the primary diode of optocoupler OC1. The other end of resistor R5 is simultaneously connected to the cathode of the primary diode of optocoupler OC1, the other end of capacitor C2, and the cathode of 431 device U1. The other end of resistor R2 and the anode of 431 device U1 are connected together and then used to connect to the secondary ground. The collector of the secondary transistor of optocoupler OC1 is used to connect to the feedback pin of the main controller, and the emitter of the secondary transistor of optocoupler OC1 is used to connect to the primary ground.

[0013] Optionally, the output-side soft-start control unit includes resistor R6, resistor R7, capacitor C3, and Zener diode ZD1. One end of resistor R6 is connected to the cathode of the 431 device U1, and the other end is connected to the cathode of Zener diode ZD1. The anode of Zener diode ZD1 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3 and the other end of resistor R7 are connected together to the secondary ground.

[0014] Optionally, the output-side soft-start control unit includes resistors R6 and R7, capacitor C3, and Zener diode ZD1. The cathode of Zener diode ZD1 is connected to the output voltage, and the anode is connected to one end of resistor R6. The other end of resistor R6 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3, the other end of resistor R7, and the reference terminal of the 431 device U1 are all connected to one end of resistor R2. The other end of resistor R2 is connected to the secondary ground.

[0015] This utility model also provides a switching power supply, the technical solution of which is as follows: A switching power supply, including the soft-start control circuit described in any of the preceding claims.

[0016] This invention aims to overcome the problems in existing soft-start technology, such as input soft-start no-load output overshoot, and the simultaneous intervention and mutual competition between input and output soft-start, leading to nonlinear output voltage rise. The advantages compared to existing technologies are as follows: During the initial startup phase, since the output voltage has not reached the first set value, the output-side soft-start control unit does not inject holding current into the feedback pin of the main controller. The soft-start control is primarily driven by the input-side soft-start control unit. Only after the output voltage increases to the first set value does the soft-start control function of the output-side soft-start control unit begin to operate. Because the main controller and the output voltage control loop form a negative feedback control system, a sampling resistor is connected to the feedback pin inside the main controller to generate the control quantity for the main power switch. This control quantity decreases as the output voltage increases. When the output voltage reaches the first set value, the control effect of the input-side soft-start control unit on the gain of the main power stage gradually weakens, leading to an excessively rapid voltage rise after reaching a certain value. Furthermore, the linearity of the output voltage is easily affected by sudden load loading. In this case, the output-side soft-start control unit switches to soft-start control and injects holding current into the feedback pin of the main controller. With proper circuit parameter design, the rise rate of the output voltage can be controlled, resulting in a lower linear rise slope. At this time, the main power gain control effect is mainly achieved through the soft-start control of the output-side soft-start control unit, which differs from the power gain control of the input-side soft-start control unit. The main control effect of the output-side soft-start control unit is the gain control of the output voltage, which ensures that the output voltage rises at a lower slope during the soft-start control process of the output-side soft-start control unit, maintaining absolute linearity. This effectively improves the problem of nonlinearity in the output voltage rise waveform caused by the competition and interference between input and output soft-start in the existing technology. Attached Figure Description

[0017] Figure 1 The waveform diagram of the no-load output voltage start-up when the existing analog control scheme only has primary-side soft-start control is shown. Figure 2 This is a circuit diagram for an existing analog control scheme that includes both input-side soft-start control and output-side soft-start control based on the soft-start control. Figure 3 for Figure 2 Output voltage startup waveform diagram; Figure 4 This is a schematic diagram of the first soft-start control circuit of this utility model; Figure 5 for Figure 4 The output voltage startup waveform diagram; Figure 6 This is a schematic diagram of the second type of soft-start control circuit of this utility model; Figure 7 for Figure 6 The output voltage startup waveform diagram. Detailed Implementation

[0018] To make the above-mentioned objectives, features, and advantages of this utility model more apparent and understandable, the specific embodiments of this utility model will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this application.

[0019] It should be noted that the terms "comprising" and "having" and any variations thereof described in the specification and claims of this application are intended to cover non-exclusive inclusion. For example, including a series of components, unit circuits or control timings is not necessarily limited to those components, unit circuits or control timings that are explicitly listed, but may include components, unit circuits or control timings that are not explicitly listed or that are inherent to these circuits.

[0020] Furthermore, unless otherwise specified, the embodiments and features described in this application may be combined with each other.

[0021] It should be understood that, in the specification and claims, when an element is described as being "connected" to another element, that element may be "directly connected" to that other element or "connected" to that other element through a third element; when a step is described as being connected to another step, that step may be connected directly to that other step or connected to that other step through a third step.

[0022] This utility model provides a soft-start control circuit for a switching power supply. The switching power supply includes a main controller and an output voltage control loop. The output voltage control loop samples the output voltage and, after isolation, provides feedback current to the feedback pin of the main controller. The main controller controls the switching action of the main power switching transistor based on the feedback current, causing the output voltage to tend towards a preset value. The soft-start control circuit includes: The input-side soft-start control unit is used to control the main power switch to execute a soft-start control strategy based on the magnitude of the current at the feedback pin of the main controller during the startup phase of the switching power supply, thereby achieving a linear increase in the output voltage. The output-side soft-start control unit is used to inject holding current into the feedback pin of the main controller when the output voltage reaches the first set value during the startup phase of the switching power supply. It continues to control the main power switch to execute the soft-start control strategy until the output voltage reaches the second set value, at which point the startup phase ends. The first set value is less than the second set value. The soft-start control strategy is as follows: the duty cycle of the main power switch drive signal is linearly increased from small to large; or the switching frequency of the main power switch is linearly decreased from large to small.

[0023] As a specific embodiment, the first setting value is 60% of the rated output voltage.

[0024] As a specific example, the main controller is the TEA2017. The TEA2017 is a power management chip launched by NXP, which is mainly used in the design of high-efficiency switching power supplies. Its main functions are to integrate PFC (Power Factor Correction) and LLC resonant conversion control functions, support DCM / QR, CCM fixed frequency and mixed mode operation, and are suitable for PFC+LLC power topology switching power supplies. The controller integrates the input side soft start control unit into the controller.

[0025] It should be noted that while the TEA2017 controller has limitations on the applicable switching power supplies, the soft-start control circuit of this invention has no such limitations; that is, it is applicable to both ACDC and DCDC switching power supplies, and there are no power topologies restrictions.

[0026] Figure 4 Please refer to the schematic diagram of the first soft-start control circuit of this utility model. Figure 4 ,in: The output voltage control loop includes resistors R1, R2, R3, R4, R5, capacitor C2, 431 device U1, and optocoupler OC1. One end of resistor R1 and one end of resistor R4 are used to connect to the output voltage. The other end of resistor R1 is connected to one end of resistor R2, one end of resistor R3, and the reference terminal of 431 device U1. The other end of resistor R3 is connected to one end of capacitor C2. The other end of resistor R4 is connected to one end of resistor R5 and the anode of the primary diode of optocoupler OC1. The other end of resistor R5 is connected to the cathode of the primary diode of optocoupler OC1, the other end of capacitor C2, and the cathode of 431 device U1. The other end of resistor R2 and the anode of 431 device U1 are connected together and then used to connect to the secondary ground. The collector of the secondary transistor of optocoupler OC1 is used to connect to the feedback pin of the main controller, and the emitter of the secondary transistor of optocoupler OC1 is used to connect to the primary ground. The output-side soft-start control unit includes resistors R6 and R7, capacitor C3, and Zener diode ZD1. One end of resistor R6 is connected to the cathode of device U1 (431), and the other end is connected to the cathode of Zener diode ZD1. The anode of Zener diode ZD1 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3 and the other end of resistor R7 are connected together to the secondary ground.

[0027] The following combination Figure 4 The working principle of the soft-start control circuit of this utility model embodiment is analyzed: like Figure 4 As shown, the output-side soft-start control unit switches in via an added Zener diode ZD1. When the switching power supply starts up, the output voltage controlled by the input-side soft-start control unit reaches the rated voltage value of the Zener diode ZD1. Figure 4 When the output-side soft-start control unit (with the dashed box) is activated, the output voltage charges the soft-start capacitor C3 via resistor R4, optocoupler LED, resistor R6, and Zener diode ZD1. Since the voltage of capacitor C3 is 0 before the output-side soft-start control unit is activated, the charging current of the soft-start capacitor C3 is fed back to the feedback pin of the main controller via the optocoupler after the output-side soft-start control unit is activated. This further controls the duty cycle or frequency of the primary-side switching transistor. At this time, the loop is mainly controlled by the output-side soft-start control unit.

[0028] Unlike the power gain control of the input-side soft-start control unit, the main control effect of the output-side soft-start control unit lies in the two-stage gain control of the output voltage: at the beginning of startup, the output voltage rises at the first slope, and after the output voltage reaches a certain value, the output voltage rises at a lower slope. In this way, the output-side soft-start control unit controls the output voltage to gradually reach the set steady-state value, ensuring a smooth transition of the power supply from soft start to steady state. This ensures the monotonic linear rise of the output voltage, effectively improving the problem of nonlinearity in the output voltage rise waveform caused by the competition and interference between input soft start and output soft start in the background technology. At the same time, it can also improve the output voltage sag caused by the sudden application of a high-rate load current at the output terminal after the output voltage has risen to a certain level.

[0029] Figure 5 The image shows Figure 4 The startup effect diagram shows that the output voltage rises linearly from 0V to the rated voltage value without any dips or overshoots.

[0030] Figure 6 Please refer to the schematic diagram of the second type of soft-start control circuit of this utility model. Figure 6 ,and Figure 4 The difference is that the output-side soft-start control unit includes resistor R6, resistor R7, capacitor C3, and Zener diode ZD1. The cathode of Zener diode ZD1 is connected to the output voltage, and the anode is connected to one end of resistor R6. The other end of resistor R6 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3, the other end of resistor R7, and the reference terminal of device U1 of 431 are all connected to one end of resistor R2. The other end of resistor R2 is used to connect to the secondary ground.

[0031] Figure 6 The location of the output-side soft-start control unit has been slightly changed; it is achieved by adding an output-side soft-start control unit to the reference terminal of the 431 device U1. Figure 6 Core control logic and Figure 4 Both are similar, working together with the input-side soft-start control unit to achieve soft-start control during the startup phase. When the output voltage controlled by the input-side soft-start control unit reaches the rated voltage of the Zener diode ZD1, the output-side soft-start control unit switches to control. The output voltage charges the soft-start capacitor C3 through the Zener diode ZD1, resistor R6, resistor R2, and output ground. Because the voltage of C3 was 0 before the output-side soft-start control unit switched to control, the charging current of the soft-start capacitor C3 quickly charges the voltage of resistor R2 to 2.5V after the output-side soft-start control unit switches to control. At this time, current begins to flow through the cathode and anode of the 431 device U1. This current is fed back to the feedback pin of the main controller through the optocoupler, further controlling the duty cycle or frequency of the primary-side switching transistor. At this time, the loop is mainly controlled by the output-side soft-start control unit.

[0032] Figure 6 Output-side soft-start control unit and Figure 4 The output-side soft-start control unit achieves the same control effect. Figure 7 The image shows Figure 6 The startup effect diagram shows that the output voltage rises linearly from 0V to the rated voltage value without any dips or overshoots.

[0033] As a specific embodiment, this utility model provides a switching power supply, including any of the above-mentioned soft-start control circuits.

[0034] The above are merely embodiments of this utility model. It should be particularly noted that the above embodiments should not be regarded as limitations on this utility model. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of this utility model, and these improvements and modifications should also be regarded as protection scope of this utility model.

Claims

1. A soft-start control circuit applied to a switching power supply, the switching power supply including a main controller and an output voltage control loop, the output voltage control loop being used to sample the output voltage and compare it with a reference to generate an error amplification signal, and then the error amplification signal being isolated and processed to provide feedback current to the feedback pin of the main controller, the main controller controlling the switching action of the main power switching transistor according to the feedback current so that the output voltage tends to a preset value; The soft-start control circuit includes: An input-side soft-start control unit, integrated in the main controller, is used to control the main power switch to execute a soft-start control strategy based on the magnitude of the current at the feedback pin of the main controller during the power-on phase of the switching power supply, so that the output voltage increases linearly with a first slope; wherein, the soft-start control strategy is: controlling the duty cycle of the main power switch drive signal to increase linearly from small to large; or controlling the switching frequency of the main power switch to decrease linearly from large to small; The feature is that the soft-start control circuit further includes an output-side soft-start control unit, which is used to inject a holding current into the feedback pin of the main controller when the output voltage reaches a first set value during the startup phase of the switching power supply, and control the main power switch to continue to execute the soft-start control strategy, so that the output voltage increases linearly at a lower slope until the output voltage reaches the output voltage loop set reference value before ending the startup phase, wherein the first set value is less than the output voltage loop set reference value.

2. The soft-start control circuit according to claim 1, characterized in that: The main controller is a TEA2017.

3. The soft-start control circuit according to claim 1, characterized in that: The output voltage control loop includes resistors R1, R2, R3, R4, and R5, capacitor C2, 431 device U1, and optocoupler OC1. One end of resistor R1 and one end of resistor R4 are both used to connect to the output voltage. The other end of resistor R1 is simultaneously connected to one end of resistor R2, one end of resistor R3, and the reference terminal of 431 device U1. The other end of resistor R3 is connected to one end of capacitor C2. The other end of resistor R4 is simultaneously connected to one end of resistor R5 and the anode of the primary diode of optocoupler OC1. The other end of resistor R5 is simultaneously connected to the cathode of the primary diode of optocoupler OC1, the other end of capacitor C2, and the cathode of 431 device U1. The other end of resistor R2 and the anode of 431 device U1 are connected together and then used to connect to the secondary ground. The collector of the secondary transistor of optocoupler OC1 is used to connect to the feedback pin of the main controller, and the emitter of the secondary transistor of optocoupler OC1 is used to connect to the primary ground.

4. The soft-start control circuit according to claim 3, characterized in that: The output-side soft-start control unit includes resistor R6, resistor R7, capacitor C3, and Zener diode ZD1. One end of resistor R6 is connected to the cathode of the 431 device U1, and the other end is connected to the cathode of Zener diode ZD1. The anode of Zener diode ZD1 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3 and the other end of resistor R7 are connected together to the secondary ground.

5. The soft-start control circuit according to claim 3, characterized in that: The output-side soft-start control unit includes resistors R6 and R7, capacitor C3, and Zener diode ZD1. The cathode of Zener diode ZD1 is connected to the output voltage, and the anode is connected to one end of resistor R6. The other end of resistor R6 is connected to one end of capacitor C3 and one end of resistor R7. The other end of capacitor C3, the other end of resistor R7, and the reference terminal of the 431 device U1 are all connected to one end of resistor R2. The other end of resistor R2 is used to connect to the secondary ground.

6. A switching power supply, characterized in that: Includes the soft-start control circuit as described in any one of claims 1 to 5.

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