A constant quality factor design method based on Buck converter COT charge control

Abstract

This invention proposes a constant quality factor design method based on Buck converter COT charge control. While avoiding subharmonic oscillations, it also designs a threshold voltage V. TH The expression is used to offset the effects of output voltage Vo and duty cycle D on the quality factor Q. Theory and simulation show that the constant Q value design scheme with charge COT can not only improve the transient performance of the system, but also ensure that the system remains stable when the duty cycle changes.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter control technology, specifically relating to a constant quality factor design method based on COT charge control of Buck converter. Background Technology

[0002] To ensure the reliable and efficient operation of portable electronic devices, the power supply modules must have a fast load response speed. Traditional pulse-width modulation (PWM) control techniques for switching converters (such as voltage-mode and current-mode control) are limited by the control loop response speed and cannot achieve a fast load response. Variable frequency drive (VFD) COT control is widely used in industry due to its higher light-load efficiency, higher bandwidth design capability, and better small-signal characteristics. However, traditional ripple-based COT control methods, such as COT based on inductor current ripple control (CMCOT), are limited in dynamic response by the on-time and minimum off-time. COT based on output voltage ripple control (V... 2 -COT (Charge-Obstructed Tunneling) can cause subharmonic oscillations when the ESR of the output capacitor is very small. Therefore, this invention proposes a new scheme based on charge COT control, which can not only effectively avoid the problem of subharmonic oscillations, but also improve the transient response speed of the system by extending the conduction time.

[0003] However, in charge-controlled (COT) circuits, the quality factor Q is affected by the output voltage Vo and the duty cycle D after the circuit design is complete. As can be seen from the Bode plot, when the quality factor Q is small, the complex poles split into two real poles, shifting towards the low-frequency and high-frequency regions respectively. This may lead to additional phase drop near the unity-gain bandwidth, reducing the system's phase margin and causing stability issues. Conversely, when the quality factor Q is large, for high-bandwidth designs, it may cross 0dB twice at the high-frequency gain, resulting in system instability. Therefore, both excessively large and small Q values ​​can be detrimental to the system. However, most COT-controlled buck converters suffer from Q-value issues. Therefore, to avoid this problem, a well-controlled Q value independent of the duty cycle is needed for wide duty cycles. Summary of the Invention

[0004] To address the problems existing in the background technology, this invention proposes a design method for maintaining a constant quality factor based on charge COT control of a Buck converter. This method avoids subharmonic oscillations while also designing a threshold voltage V. TH The expression is used to offset the effects of output voltage Vo and duty cycle D on the quality factor Q. Theory and simulation show that the constant Q value design scheme with charge COT can not only improve the transient performance of the system, but also ensure that the system remains stable when the duty cycle changes.

[0005] To achieve the above objectives, such as Figure 1 The diagram shown is a control structure block diagram. The technical solution of this invention is as follows:

[0006] 1. Parameter Measurement: Sample real-time data from the Buck converter and measure the converter's output voltage and inductor current parameters using an ADC sampling module;

[0007] 2. Control Strategy: In steady state, the sampled output voltage is passed through the outer loop type II compensator to output control voltage Vc. The product of Vc and inductor current iL*Ri is then used as the input of the transconductance amplifier. The output current information is then used to charge and discharge the capacitor to achieve charge control. The output is sent to the COT generator module, which in turn controls the switching transistor to output a stable voltage waveform.

[0008] 3. Threshold Voltage Design: To eliminate the influence of output voltage Vo and duty cycle D on the quality factor Q, the threshold voltage V of the comparator is designed. TH To ensure that the Q value remains constant, taking into account the effects of output voltage Vo and duty cycle D, thereby improving the stability and speed of the system during transient responses;

[0009] The mechanism of this invention is: by designing a threshold voltage V TH The output voltage Vo and duty cycle D are used as functions to offset their effects on the quality factor Q. With a constant Q value, this not only improves system stability but also enhances the system's dynamic response speed.

[0010] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0011] This invention employs a charge control (COT) scheme, which not only effectively avoids subharmonic oscillations but also, by rationally designing the expression for the threshold voltage VTH, offsets the influence of the output voltage Vo and duty cycle D on the system, thereby further improving the system's stability and dynamic performance. Attached Figure Description

[0012] Figure 1 Block diagram of synchronous Buck converter

[0013] Figure 2 Block diagram of a Buck converter based on charge-controlled COT.

[0014] Figure 3 The block diagram of the charge control structure of the proposed Buck converter with constant Q value is shown below.

[0015] Figure 4 The variation of Q value with duty cycle after adopting the constant Q value scheme.

[0016] Figure 5 The simulation waveforms show the results of not using a constant Q-value scheme when the duty cycle changes.

[0017] Figure 6 The simulation waveforms show the constant Q value scheme when the duty cycle changes.

[0018] Figure 7 Simulation waveforms of a scheme without a constant Q value when the load changes abruptly.

[0019] Figure 8 Simulation waveforms of a constant Q-value scheme when the load changes abruptly. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0021] The control method of this invention is designed for Buck converters using charge-controlled charge (COT) circuits, and a simplified circuit diagram of the Buck converter is shown below. Figure 1 As shown, its circuit structure includes an input voltage (Vin), an inductor (Ls), a capacitor (C), switching transistors (Q1, Q2), and an output load (R). The drain of switching transistor Q1 is connected to one end of the inductor, and its source is connected to the positive terminal of the input power supply. The source of switching transistor Q2 is connected to one end of the inductor and the drain of switching transistor Q1, with its drain grounded. The output capacitor C is connected in parallel with the load R. When switching transistor Q1 is on and Q2 is off, the inductor current increases, simultaneously charging the output capacitor. When switching transistor Q2 is on and Q1 is off, the inductor current decreases, and its stored energy is transferred to the load.

[0022] A block diagram of the charge control structure of a Buck converter with a constant Q value is shown below. Figure 2 As shown, it includes the following steps:

[0023] Step 1. Based on the circuit diagram of the proposed control framework, the transfer function of the Buck converter control-output under charge COT is derived using the describing function method as shown in Equation (1).

[0024]

[0025] in,

[0026]

[0027]

[0028] Step 2. Analyze the transfer function obtained in Step 1 and analyze its zeros and poles to obtain the stability condition of the system as shown in Equation (2).

[0029]

[0030] Step 3. As shown in equation (1), Q2 varies with the output voltage and duty cycle, which may lead to stability and dynamic issues in applications with a wide input-output voltage range. In this section, a generalized method is proposed to achieve a constant Q value design independent of the output voltage and duty cycle. The charging capacitor threshold voltage V is designed according to the following design criteria. TH The value of to offset the output voltage V O The influence of duty cycle D on the quality factor Q2 is explained in the following design criteria.

[0031] The expression for Q2 is extracted as shown in equation (3).

[0032]

[0033] To reduce the number of control variables, the variables in equation (3) will be reduced. and Replace them with equations (4) and (5) respectively.

[0034]

[0035]

[0036] The rewritten expression for Q2 is shown in equation (6).

[0037]

[0038] Because constant on-time (COT) control is used, the on-time Ton is a constant value. Also, after the circuit design is completed, the charging capacitor... Both are fixed. Therefore, the change in Q2 is mainly affected by the output voltage Vo and the duty cycle D. In the traditional charge COT control strategy, the threshold voltage V of the charging capacitor is... TH The quality factor Q2 is a fixed value, meaning that when the output voltage and duty cycle change, the quality factor Q2 will change accordingly. When the change reaches a point where the stability condition of equation (2) is no longer satisfied, the system will become unstable. Therefore, in order to solve this problem and improve the system's stability over a wide range of duty cycles, the threshold voltage V can be adjusted. TH A reasonable design should be implemented to offset the effects of output voltage Vo and duty cycle D on the quality factor Q2.

[0039] V TH Designed as the target variable, its relationship with the duty cycle D and output voltage Vo can be examined. It's easy to see that the denominator of the Q2 expression is the threshold voltage V. THDivide by a function of the output voltage Vo, then subtract the constant multiplied by the reciprocal of the duty cycle D. Therefore, to keep Q2 constant, V TH It should also change at the same rate as the duty cycle and the output voltage, i.e., V TH It should be a function of the duty cycle D and the output voltage Vo, to compensate for the effects of Vo and D on Q2. Therefore, V can be... TH The design is as shown in equation (7).

[0040]

[0041] Substituting equation (7) into equation (6), we can obtain a new expression for Q2 as shown in equation (8).

[0042]

[0043] in,

[0044]

[0045] in This determines the precise Q value. This helps to achieve a Q value that is independent of the duty cycle. Therefore, to achieve independence from the duty cycle D, it is only necessary to set a1 = a2, that is, β should be designed as:

[0046]

[0047] And by changing The value of Q2 can be easily designed to the value we need, and the gain... The lower the value, the higher the Q2 value. In this way, a constant Q2 value can be achieved across the entire duty cycle range, and the control is excellent. The exact value. Its exact value expression is as follows:

[0048]

[0049] In summary, by calculating equation (9) The value of can offset the effect of the duty cycle on the system, and is given in advance by equation (10). The quality factor Q can be set to the desired value. Figure 5 It gives in , The Q value changes with the duty cycle under the condition that the charging capacitor C T =1nF, conduction time Ton=1us, current gain R i =0.1, transconductance amplifier gain g m =0.01A / V, inductance value Ls=4.7uH. Through Figure 5It can be seen that no matter how the duty cycle changes, the Q value remains constant around 0.707.

[0050] To verify the above conclusions, the SIMPLIS simulation software was used. The simulation results are as follows: Figure 5 , Figure 6 , Figure 7 , Figure 8 As shown.

[0051] The simulation parameters are designed as follows: input voltage Vin = 12V, output voltage Vo = 5V, inductance L = 4.7uH, output capacitor Co = 470uF, and charging capacitor C. T =1nF, conduction time Ton=1us, current gain R i =0.1, transconductance amplifier gain g m =0.01A / V. Figure 5 and Figure 6 The waveforms are compared and analyzed when the duty cycle changes, with and without a constant Q-value scheme. Figure 7 and Figure 8 The waveforms are compared when the load changes abruptly, with and without a constant Q-value scheme.

[0052] From simulation Figure 5 and Figure 6 As can be seen, when the duty cycle changes, if a constant Q value scheme is not adopted, the inductor current and output voltage will oscillate significantly until they become unstable. However, with a constant Q value scheme, the inductor current and output voltage can reach a new steady state in a very short time when the duty cycle changes, and there will be no instability. This shows that using a constant Q value scheme can suppress the adverse effects of duty cycle changes on the system.

[0053] From simulation Figure 7 and Figure 8 As can be seen, when the load changes abruptly, the dynamic response time of the system without the constant Q-value scheme is approximately 7.92 μs; however, by adopting the constant Q-value scheme, the dynamic response time of the system can be shortened to 6.48 μs. Figure 8 The comparison shows that the voltage drop under the solid line (using the constant Q value scheme) is lower than that under the dashed line (not using the constant Q value scheme), indicating that using the constant Q value scheme can not only improve the dynamic response speed of the system, but also reduce the voltage undershoot.

[0054] In summary, this invention proposes a constant Q-value scheme based on Buck converter charge COT control, by designing a threshold voltage V. THThe constant Q value is a function of duty cycle and output voltage, used to counteract the adverse effects of changes in duty cycle or output voltage on the system. Adopting a constant Q value not only improves system stability but also enhances dynamic response speed and reduces voltage undershoot.

[0055] The above description is merely a specific implementation method of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar alternative features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features or steps.

Claims

1. A digital V-type Buck converter 2 The structural block diagram of COT control is shown in Figure 2, which includes the following steps: Step 1. Based on the circuit diagram of the proposed control framework, the transfer function of the Buck converter under charge COT is derived using the describing function method as shown in equation (1); ； in, ； ； Step 2. Analyze the transfer function obtained in Step 1 and analyze its zeros and poles to obtain the stability condition of the system as shown in Equation (2); ； Step 3. As can be seen from equation (1), Q2 varies with the output voltage and duty cycle, which may lead to stability and dynamic problems in applications with a wide input-output voltage range. In this section, a generalized method is proposed to achieve a constant Q value design independent of the output voltage and duty cycle. The charging capacitor threshold voltage V is designed according to the following design criteria. TH The value of to offset the output voltage V O The influence of duty cycle D on the quality factor Q2 is determined by the following design criteria. Extract the expression for Q2, as shown in equation (3); ； To reduce the number of control variables, the variables in equation (3) will be reduced. and Replace them with equations (4) and (5) respectively; ； ； The rewritten expression for Q2 is shown in equation (6); ； Because constant on-time (COT) control is used, the on-time Ton is a constant value; at the same time, after the circuit design is completed, the charging capacitor... Both are fixed; therefore, the change in Q2 is mainly affected by the output voltage Vo and the duty cycle D. In the traditional charge COT control strategy, the threshold voltage V of the charging capacitor is... TH The quality factor Q2 is a fixed value, meaning that when the output voltage and duty cycle change, the quality factor Q2 will change accordingly. When the change reaches a point where the stability condition of equation (2) is no longer satisfied, the system will become unstable. Therefore, in order to solve this problem and improve the system's stability over a wide range of duty cycles, the threshold voltage V can be adjusted. TH Make reasonable designs to offset the effects of output voltage Vo and duty cycle D on the quality factor Q2; V TH Designed as the target variable, its relationship with the duty cycle D and output voltage Vo can be examined; it is not difficult to see that the denominator of the Q2 expression is the threshold voltage V. TH Divide by a function of the output voltage Vo, then subtract the constant multiplied by the reciprocal of the duty cycle D; therefore, to keep Q2 constant, V TH It should also change at the same rate as the duty cycle and the output voltage, i.e., V TH It should be a function of the duty cycle D and the output voltage Vo, to compensate for the effects of Vo and D on Q2; therefore, V can be... TH The design is as shown in equation (7); ； Substituting equation (7) into equation (6), we can obtain a new expression for Q2 as shown in equation (8); ； in, ； in This determines the precise Q value. This helps to achieve a Q value that is independent of the duty cycle; therefore, to achieve independence from the duty cycle D, it is only necessary to let a1 = a2, that is, β should be designed as: ； And by changing The value of Q2 can be easily designed to the value we need, and the gain... The lower the value, the higher the value of Q2; in this way, a constant Q2 value can be achieved throughout the entire duty cycle range, and the control can be optimized. The exact value of ; its exact value expression is as follows: ； In summary, by calculating equation (9) The value of can offset the effect of the duty cycle on the system, and is given in advance by equation (10). The quality factor Q can be set to the desired value.

2. The charge COT control method with constant Q-value optimization scheme as described in claim 1, characterized in that, The circuit structure of a synchronous Buck converter is adopted. The drain of the switching transistor Q1 is connected to one end of the inductor, and the source is connected to the positive terminal of the input power supply. The source of the switching transistor Q2 is connected to one end of the inductor and the drain of the switching transistor Q1, and the drain is grounded. The output capacitor C is connected in parallel with the load R. This circuit structure is more efficient than the asynchronous Buck converter.

3. The charge COT control method with constant Q-value optimization scheme as described in claim 1, characterized in that, The Buck converter operates in a continuous conduction mode during steady state.

4. The charge COT control method with constant Q-value optimization scheme as described in claim 1, characterized in that, The Buck converter is more efficient under light load conditions.

5. The charge COT control method with constant Q-value optimization scheme as described in claim 1, characterized in that, The system is more stable under dynamic conditions and is not affected by the duty cycle.

6. The charge COT control method with constant Q-value optimization scheme as described in claim 1, characterized in that, The system output voltage drops less and responds faster when the load changes abruptly.

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