An auxiliary circuit for improving transient response of a buck converter

Abstract

The application discloses an auxiliary circuit for improving transient response of a Buck converter, comprising a transient detection module, a resonant auxiliary circuit and a state transition module. The transient detection module is used for comparing an output voltage of the converter with a threshold voltage to determine whether the converter enters light load cutting heavy load, output voltage falling or heavy load cutting light load, output voltage rising. The resonant auxiliary circuit is connected in parallel between input terminals and load terminals of a main topology, and has three working modes of pre-starting, releasing current to the main circuit and absorbing current from the main circuit. The state transition module is used for controlling the working mode of the auxiliary circuit and is realized by using a finite state machine. The transient enhancement auxiliary circuit adopted by the application can effectively improve transient response of the main topology, reduce output voltage peak and response time, reduce influence on load life, and the enhancement effect is not limited by duty cycle saturation, and can be designed flexibly.

Description

Technical Field

[0001] This invention relates to switching power supplies and proposes an auxiliary circuit for Buck converters that can improve their transient response. Background Technology

[0002] A switching DC-DC power supply is a device that converts one DC voltage to another by controlling the switching action of transistors. Compared to linear regulated power supplies, it has advantages such as small size, high reliability, high current capacity, and low power consumption, and is therefore widely used in many fields. Switching power supplies can be classified into buck converters, boost converters, and buck-boost converters according to their topology, with buck converters, which convert high voltage to low voltage, being the most widely used. The main topology of a buck converter includes a pair of transistors, an energy storage inductor, and a filter capacitor. When the load current changes abruptly, because the inductor current cannot change abruptly, the capacitor will charge and discharge, causing an overshoot or undershoot spike in the output voltage, affecting the lifespan of the load. The transient response of a buck converter refers to the change in output voltage when the load current changes abruptly and the time required for the system to recover to a steady state. Improving the transient response of the converter has always been a research hotspot.

[0003] There are generally two approaches to improving the transient response of Buck converters: one is to optimize the system's control loop, such as Time-Optimal Control (TOC); the other is to modify the Buck converter's circuitry. For voltage regulator modules (VRMs) used in low-voltage, high-current scenarios, the conversion ratio between their input and output voltages is very large, and the rate of decrease of the inductor current is much smaller than the rate of increase, as shown in (1). This results in asymmetry between the positive and negative step transitions of the load, with the overshoot voltage being much larger than the undershoot voltage. At this point, regardless of the control method used, the response speed will be limited by duty cycle saturation, reaching a limit that is difficult to overcome. Therefore, improvements need to be made at the topology level.

[0004]

[0005] The proposed topology-level improvement methods can be divided into two categories. One category is to change the inductor current slope of the Buck converter. As can be seen from (1), there are two ways to change it: reducing the equivalent inductance value and increasing the voltage across the inductor. The former is typically achieved by connecting a coupling inductor in parallel at the output terminal. When a transient occurs, this inductor is connected into the circuit, thereby reducing the inductance value. The latter can accelerate the reduction of the inductor current by connecting an auxiliary power supply in series in the inductor circuit when the load jumps downward, thereby improving the overshoot voltage. The other category is to add another path to the original topology for energy storage and discharge during transients. Depending on the location of the auxiliary path, it can be divided into two types: directly connected in parallel at the load terminal and connected in parallel between the input and the load. Compared with changing the inductor current slope, adding a path can be considered to recover energy for secondary use, which has higher efficiency. Summary of the Invention

[0006] Objective: To address the aforementioned technologies and their problems, this invention proposes an auxiliary circuit that improves the transient response of Buck converters, primarily applicable to, but not limited to, low-voltage, high-current scenarios. By activating the auxiliary circuit during load transitions, the response time and overshoot / undershoot voltage spikes of the Buck converter can be reduced, thereby improving the converter's transient performance.

[0007] Technical solution:

[0008] To achieve the above objectives, the present invention employs the following technical solution for specific application in the Buck converter:

[0009] An auxiliary circuit for accelerating the transient response of a Buck converter, the main functional modules of which include a transient detection module, a resonant auxiliary circuit, and a state transition module.

[0010] Transient detection module: This module compares the converter's output voltage with the threshold voltage to determine whether the converter has entered a state of light load switching to heavy load with output voltage undershoot or heavy load switching to light load with output voltage overshoot.

[0011] Resonant Auxiliary Circuit: This module consists of a switching transistor, an inductor, and a capacitor, connected in parallel across the input and load terminals of the main topology. It has three operating modes: pre-start, current release to the main circuit, and current absorption from the main circuit. When the main topology is powered on and just beginning to stabilize, the state machine controls the auxiliary circuit to operate for several cycles, causing the voltage across the capacitor to resonate from zero to a negative voltage, completing the pre-start of the auxiliary circuit. When the load shifts from light to heavy, the main topology output voltage decreases, which is detected by the transient detection module. The state machine receives the signal and causes the auxiliary circuit to operate in the state of releasing current to the main circuit. When the load shifts from heavy to light, the main topology output voltage increases, which is detected by the transient detection module. The state machine receives the signal and causes the auxiliary circuit to operate in the state of absorbing current from the main circuit. By absorbing and releasing current from the main topology during transients, this module achieves the purpose of accelerating transient response and is not limited by the duty cycle.

[0012] State Transition Module: This module controls the operating mode of the auxiliary circuit and is implemented using a finite state machine. Upon system power-on, it counts to determine the pre-start time of the auxiliary circuit, controlling the start and end of this pre-start. After power-on, it receives a detection signal from the transient detection module to determine whether the system is in a steady state or a transient state. If the system is in a steady state, the auxiliary circuit is turned off and not connected to the main topology; if the system is in a transient state, the auxiliary circuit is connected to the main topology and enters the corresponding operating mode to accelerate transient response.

[0013] Preferably, the resonant auxiliary circuit includes six switching transistors Q1 to Q6, two inductors L1 and L2, and a capacitor C. The sources of switching transistors Q2 and Q3 are connected, the drain of Q2 is connected to the input terminal of the main circuit, and the drain of Q3 is connected to inductor L1. The sources of switching transistors Q5 and Q6 are connected, the drain of Q6 is connected to the load terminal of the main circuit, and the drain of Q5 is connected to inductor L2. The source of switching transistor Q4 is grounded, and its drain is connected to capacitor C. The source of switching transistor Q1 is connected to the drain of switching transistor Q4, and its drain is connected to the drain of switching transistor Q2. The ends of inductors L1 and L2 and capacitor C that are not connected to the switching transistors are connected. The switching on and off of the switching transistors are determined by the control signal output by the state transition module.

[0014] This invention also provides a method for improving the transient response of a Buck converter based on the aforementioned auxiliary circuit, comprising the following steps: when the main circuit is powered on and begins to operate stably, the state transition module controls the auxiliary circuit to operate for several cycles, causing the voltage on its capacitor to resonate from zero voltage to a negative voltage, thus completing the pre-start-up of the auxiliary circuit; when the load is switched from light to heavy, the output voltage of the main circuit decreases, which is detected by the transient detection module, and the state transition module receives a signal to make the auxiliary circuit operate in a state of releasing current to the main circuit; when the load is switched from heavy to light, the output voltage of the main circuit increases, which is detected by the transient detection module, and the state transition module receives a signal to make the auxiliary circuit operate in a state of absorbing current from the main circuit.

[0015] Preferably, when the main circuit is powered on, the pre-start time of the auxiliary circuit is determined by counting, and the start and end of the pre-start of the auxiliary circuit are controlled.

[0016] The present invention also provides a Buck converter, the circuit of which includes a main circuit and the auxiliary circuit, one end of the auxiliary circuit being connected to the Vin terminal of the main circuit and the other end being connected to the Vout terminal of the main circuit.

[0017] Beneficial effects:

[0018] 1. The transient enhancement auxiliary circuit used in this invention can effectively improve the transient response of the main topology, reduce its output voltage spikes and response time, reduce the impact on load life, and its enhancement effect is not limited by duty cycle saturation, allowing for flexible design.

[0019] 2. The transient enhancement auxiliary circuit used in this invention is achieved by adding an energy path to the main topology. The energy absorbed during the transient enhancement is fed back to the input, realizing energy recovery and improving the conversion efficiency of the converter.

[0020] 3. The transient enhancement auxiliary circuit used in this invention utilizes inductor-capacitor resonance, which has a relatively low di / dt value. Therefore, it can reduce potential electromagnetic interference and make the circuit more compact by reducing the resonant inductance.

[0021] 4. The transient enhancement auxiliary circuit used in this invention controls its working mode through a finite state machine, and operates in different modes when the system is in steady state and transient state, so as to avoid its impact on the steady-state performance of the system.

[0022] 5. The transient enhancement auxiliary circuit used in this invention can be used not only in single-phase Buck converters but also in multi-phase Buck converters; not only in current-mode controlled converters but also in voltage-mode controlled converters; and not only in digital power supplies but also in analog power supplies. It is suitable for a variety of application scenarios and has high flexibility. Attached Figure Description

[0023] Figure 1 This is a system block diagram showing the application of the transient enhancement auxiliary circuit of the present invention in a Buck converter;

[0024] Figure 2 This is a schematic diagram of the transient enhancement auxiliary circuit of the present invention absorbing current from the main circuit;

[0025] Figure 3 This is a simulation waveform diagram of the transient enhancement auxiliary circuit of the present invention absorbing current from the main circuit;

[0026] Figure 4 This is a schematic diagram of the transient enhancement auxiliary circuit of the present invention releasing current to the main circuit;

[0027] Figure 5 This is a simulation waveform diagram of the transient enhancement auxiliary circuit of the present invention releasing current to the main circuit;

[0028] Figure 6 This is a schematic diagram of the state machine of the transient enhancement auxiliary circuit of the present invention;

[0029] Figure 7 This is a SIMPLIS simulation waveform diagram of the transient enhancement auxiliary circuit of the present invention applied in a Buck converter. Detailed Implementation

[0030] To illustrate the present invention more clearly, the technical solution of the present invention will be further explained below with reference to the accompanying drawings.

[0031] Figure 1 This is a system block diagram showing the application of the transient enhancement auxiliary circuit of this invention in a Buck converter. The Buck power system is the main circuit, Vin is the input voltage of the main circuit, and Vout is the output voltage of the main circuit. In low-voltage, high-current applications, Vout is significantly lower than Vin. A load can be connected to both ends of Vout. The Turbo module, which is the auxiliary circuit, is connected to the Vin terminal of the main circuit on one end and the Vout terminal of the main circuit on the other end. The auxiliary circuit consists of six switching transistors Q1 to Q6, two inductors L1 and L2, and one capacitor C connected as shown in the figure. The switching on and off of the transistors are determined by the control signals [Q1, Q2, Q3, Q4, Q5, Q6] output by the digital power chip.

[0032] Figure 2This is a schematic diagram of the transient enhancement auxiliary circuit of the present invention, which absorbs current from the main circuit. In this mode, one cycle can be divided into three stages, S1 to S3. The initial voltage on capacitor C is a negative voltage Vx. In stage S1, switches Q4, Q5, and Q6 are turned on. Inductor L2 and capacitor C resonate in series for half a cycle and then turn off, ending S1. In this stage, the current flows from the output terminal of the main circuit to capacitor C, that is, it absorbs current from the main circuit, and the voltage on capacitor C becomes 2Vout - Vx. In stage S2, switches Q2, Q3, and Q4 are turned on. Inductor L1 and capacitor C resonate in series for half a cycle and then turn off, ending S2. In this stage, the current flows from the capacitor to the input terminal of the main circuit, that is, a part of the absorbed energy is fed back to the main circuit, realizing energy recovery, and the voltage on capacitor C becomes 2Vin - 2Vout + Vx. In stage S3, switches Q1, Q2, and Q3 are turned on. Inductor L1 and capacitor C resonate in series for half a cycle and then turn off, ending S3. The purpose of this stage is to balance the voltage across capacitor C, which becomes -2Vin + 2Vout - Vx. When Vx = Vout - Vin, -2Vin + 2Vout - Vx = Vx. In other words, after three stages of resonance, the voltage across capacitor C returns to its initial voltage, achieving balance.

[0033] Figure 3 This is a Simplis simulation waveform diagram of the transient enhancement auxiliary circuit of this invention absorbing current from the main circuit, including the control signals of Q1 to Q6, the voltage across the resonant capacitor, the current flowing through the resonant capacitor, the current flowing from the output terminal to the auxiliary circuit, and the current flowing from the input terminal to the auxiliary circuit. At the beginning of stage S1, Q4, Q5, and Q6 are high. At the end of stage S1, Q5 and Q6 are low, with Q5 turning off slightly earlier than Q6. This is because if Q5 and Q6 are low simultaneously, the resonant circuit will be forcibly disconnected after half a cycle of resonance. Since the current in the inductor cannot change abruptly, a large positive electromotive force will be generated at the left end of L2, potentially damaging the switching transistor. Therefore, if Q5 turns off before Q6, this voltage can be clamped by the body diode of Q5, preventing damage to the switching transistor. At the beginning of stage S2, Q2 and Q3 are high, and Q4 remains high. At the end of stage S2, Q2 and Q4 are low, with Q2 turning off slightly earlier than Q4. At the beginning of stage S3, Q1 and Q2 are high, and Q3 remains high. At the end of stage S3, Q1, Q2, and Q3 are all low, with Q1 slightly earlier than Q2 and Q3. The reason why the control signals are not low at the end of stages S2 and S3 is that the body diode of the switching transistor is used to clamp the voltage and prevent the induced electromotive force of the inductor from breaking down the switching transistor. In stage S1, the auxiliary circuit absorbs a sinusoidal current with a peak value of approximately 45A from the main circuit to accelerate the transient response, and in stage S2, the absorbed energy is fed back to the input terminal to achieve energy recovery. After stages S1 to S3, the voltage on capacitor C returns to its initial value, preparing for the start of the next cycle.

[0034] Figure 4This is a schematic diagram of the transient enhancement auxiliary circuit of this invention releasing current to the main circuit. In this mode, one cycle can also be divided into three stages S1 to S3. The initial voltage on capacitor C is a negative voltage Vx. In stage S1, switches Q1, Q2, and Q3 are turned on, and inductor L1 and capacitor C resonate in series for half a cycle before being turned off, ending S1. The purpose of this stage is to make the voltage on capacitor C positive, preparing for the next two stages. The voltage on the capacitor becomes -Vx. In stage S2, switches Q2, Q3, and Q4 are turned on, and inductor L1 and capacitor C resonate in series for half a cycle before being turned off, ending S2. In this stage, current flows from the input terminal of the main circuit to the capacitor, that is, it absorbs some energy from the main circuit, and the voltage on capacitor C becomes 2Vin + Vx. In stage S3, switches Q4, Q5, and Q6 are turned on, and inductor L2 and capacitor C resonate in series for half a cycle before being turned off, ending S3. During this stage, current flows from capacitor C to the output of the main circuit, i.e., current is released into the main circuit. The voltage across capacitor C becomes 2Vout - 2Vin - Vx. When Vx = Vout - Vin, 2Vout - 2Vin - Vx = Vx. In other words, after three stages of resonance, the voltage across capacitor C returns to its initial voltage, reaching equilibrium. Furthermore, the initial voltage across capacitor C in this operating mode is the same as the initial voltage required to absorb current from the main circuit. This means that the two operating modes can be seamlessly switched.

[0035] Figure 5 This is a Simplis simulation waveform diagram of the transient enhancement auxiliary circuit of this invention releasing current to the main circuit. It also includes the control signals of Q1 to Q6, the voltage across the resonant capacitor, the current flowing through the resonant capacitor, the current flowing from the output terminal to the auxiliary circuit, and the current flowing from the input terminal to the auxiliary circuit. At the beginning of stage S1, Q1, Q2, and Q3 are high. At the end of stage S1, Q2 is low. At the beginning of stage S2, Q4 is high, and Q2 and Q3 remain high. At the end of stage S2, Q2 and Q3 are low, with Q2 slightly earlier than Q3. At the beginning of stage S3, Q5 and Q6 are high, and Q4 remains high. At the end of stage S3, Q4, Q5, and Q6 are all low, with Q6 slightly earlier than Q5. The reason why the control signals are not simultaneously low at the end of stages S2 and S3 is that the body diode of the switching transistor is used to clamp the voltage, preventing the induced electromotive force of the inductor from breaking down the switching transistor. In phase S2, the auxiliary circuit absorbs energy from the input terminal and releases a sinusoidal current with a peak value of approximately 48A to the main circuit in phase S3 to accelerate the transient response. After phases S1 to S3, the voltage across capacitor C returns to its initial value, preparing for the start of the next cycle.

[0036] Figure 6This is a schematic diagram of the state machine controlling the transient enhancement auxiliary circuit of this invention, which can be divided into four parts: pre-start, idle, auxiliary circuit charging the main circuit, and main circuit discharging the auxiliary circuit. First, in the pre-start state, after power-on, a counter counts, and after a certain time, the auxiliary circuit is pre-started. The counting time is equal to the time required for the main circuit to go from power-on to a basically stable output. The operations performed in the pre-start state are the same as those performed when discharging the main circuit. After more than ten cycles of resonance, the voltage on the resonant capacitor C changes from zero voltage to a negative voltage, i.e., the initial voltage, and the pre-start ends, entering the idle state. In the idle state, the output result of the transient detection circuit is continuously checked. If the transient detection circuit shows that the output voltage exceeds the lower limit of the threshold voltage (i.e., the voltage is too low), the state machine jumps to the state where the auxiliary circuit charges the main circuit; if the transient detection circuit shows that the output voltage exceeds the upper limit of the threshold voltage (i.e., the voltage is too high), the state machine jumps to the state where the main circuit discharges the auxiliary circuit. After entering the charging and discharging state, the corresponding switching transistors are turned on in a certain order, completing stages S1 to S3. The order and duration of the opening are as follows: Figure 3 and Figure 5 After each cycle is completed, the result of the transient detection circuit is checked again to determine whether the next state should be to return to idle or continue charging and discharging.

[0037] Figure 7 This is a SIMPLIS simulation waveform diagram of the transient enhancement auxiliary circuit of this invention applied in a Buck converter. From top to bottom, it shows the resonant voltage, auxiliary current, and output voltage. After power-on, when the output reaches stability, the auxiliary circuit is activated for pre-startup. After several cycles, the pre-startup ends, and the auxiliary circuit is deactivated. When the load shifts from light to heavy, the output voltage drops, triggering the auxiliary circuit to release current to the main circuit; when the load shifts from heavy to light, the output voltage surges, triggering the auxiliary circuit to draw current from the main circuit. After adding the auxiliary circuit, the overshoot voltage can be reduced by about 10mV, significantly improving the transient response of the low-voltage, high-current Buck circuit.

[0038] The above description, in conjunction with the accompanying drawings, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions; the above descriptions are merely preferred embodiments of the present invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An auxiliary circuit for improving the transient response of a Buck converter, characterized in that, Includes a transient detection module, a resonant auxiliary circuit, and a state transition module; The transient detection module is used to compare the output voltage of the Buck converter with the threshold voltage to determine whether the Buck converter has entered a light load switching to heavy load and the output voltage is undershooting or a heavy load switching to light load and the output voltage is overshooting. The resonant auxiliary circuit includes six switching transistors Q1 to Q6, two inductors L1 and L2, and one capacitor C. The sources of transistors Q2 and Q3 are connected, the drain of Q2 is connected to the input of the main circuit, and the drain of Q3 is connected to inductor L1. The sources of transistors Q5 and Q6 are connected, the drain of Q6 is connected to the load of the main circuit, and the drain of Q5 is connected to inductor L2. The source of transistor Q4 is grounded, and its drain is connected to capacitor C. The source of transistor Q1 is connected to the drain of transistor Q4, and its drain is connected to the drain of transistor Q2. The ends of inductors L1 and L2 and capacitor C that are not connected to the switching transistors are connected. The switching on and off of the transistors are determined by the control signal output by the state transition module. The resonant auxiliary circuit has three operating modes: pre-start, releasing current to the main circuit, and absorbing current from the main circuit. The state transition module is used to control the operating mode of the resonant auxiliary circuit; When the main circuit is powered on and begins to operate stably, the state transition module controls the auxiliary circuit to operate for several cycles, causing the voltage on its capacitor to resonate from zero voltage to a negative voltage, thus completing the pre-start of the auxiliary circuit. When the load is switched from light to heavy, the output voltage of the main circuit decreases, which is detected by the transient detection module. The state transition module receives the signal and causes the auxiliary circuit to operate in a state of releasing current to the main circuit. When the load is switched from heavy to light, the output voltage of the main circuit increases, which is detected by the transient detection module. The state transition module receives the signal and causes the auxiliary circuit to operate in a state of absorbing current from the main circuit.

2. The auxiliary circuit for improving the transient response of a Buck converter according to claim 1, characterized in that, The state transition module is a finite state machine.

3. A method for improving the transient response of a Buck converter based on the auxiliary circuit described in any one of claims 1-2, characterized in that, The process includes the following steps: When the main circuit is powered on and begins to operate stably, the state transition module controls the auxiliary circuit to operate for several cycles, causing the voltage on its capacitor to resonate from zero voltage to a negative voltage, thus completing the pre-start of the auxiliary circuit; when the load is switched from light to heavy, the output voltage of the main circuit decreases, which is detected by the transient detection module. The state transition module receives the signal and causes the auxiliary circuit to operate in a state of releasing current to the main circuit; when the load is switched from heavy to light, the output voltage of the main circuit increases, which is detected by the transient detection module. The state transition module receives the signal and causes the auxiliary circuit to operate in a state of absorbing current from the main circuit.

4. The method according to claim 3, characterized in that, When the main circuit is powered on, the timing of the auxiliary circuit's pre-start is determined by counting, and the start and end of the auxiliary circuit's pre-start are controlled.

5. A Buck converter, characterized in that, Its circuit includes a main circuit and an auxiliary circuit as described in any one of claims 1-2, with one end of the auxiliary circuit connected to the Vin terminal of the main circuit and the other end connected to the Vout terminal of the main circuit.

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