A current control method and circuit for a buck-boost converter

Abstract

The present invention discloses a current control method and circuit for a buck-boost converter, the method comprising: o Calculate the negative current I N ; According to the converter output voltage V in 、V o and I N Calculate the output current I o ; will I o Converted into analog quantity, and then output to the constant current loop circuit and short circuit / overcurrent protection circuit respectively; Based on the set first voltage reference signal, the second voltage reference signal output by the constant current loop circuit and V o , the constant voltage loop circuit outputs the error control signal v er1 , the converter is based on the signal v er1 Adjust the converter output voltage V o , thereby achieving constant current output control. This invention eliminates the need for a current sampling circuit, simplifies circuit implementation, and introduces no additional losses, thereby improving converter efficiency and power density. It also enables accurate output current control with high precision. Furthermore, when the output current is excessive, it effectively limits the converter's output voltage, reducing system losses.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic conversion, and in particular to a current control method and circuit for a buck-boost converter. Background Art

[0002] Figure 1 The four-switch buck-boost converter shown in the figure consists of switch tubes S1 to S4, energy transfer inductor L f and output filter capacitor C o The converter circuit structure is simple and has the ability to convert voltage up and down. o Can be lower, equal to or higher than the input voltage V in , the converter can achieve more efficient power conversion in a wide voltage range and is widely used in power supply fields such as new energy generation and communications. Lf is the energy flowing through the energy transfer inductor L f The current, v A The midpoint voltage of the switch bridge arm composed of switch tubes S1 and S2, v B The midpoint voltage of the switch bridge arm composed of switch tubes S3 and S4. The typical operating waveform of the converter is as follows: Figure 2 As shown in the figure, the converter has four main working stages in one switching cycle, including input energy storage stage, direct power transmission stage, freewheeling stage and current clamping stage. The duration of the four working stages is T1 to T4 respectively. GS1 ~v GS4 They are the driving control signals of the switch tubes S1~S4, t0~t4 are the time, T s is the switching period, I N is a negative current.

[0003] In the input energy storage stage (T1), switches S1 and S4 are turned on, and the input voltage V in Applied to the energy transfer inductor L f The inductor current i Lf Linear rise, slope is V in / L f At time t1, the switch tube S4 is turned off, and the input energy storage phase ends. After that, the switch tube S3 is turned on, and the converter enters the direct power transfer phase (T2). At this time, the input is connected to the inductor L f Energy is continuously transferred to the output, and the difference between the input voltage and the output voltage is applied to the inductor L f The inductor current i Lf May rise linearly (V in >V o ), remain unchanged (V in =V o ) or linearly decrease (V in <V o), the slope of change is (V in -V o ) / L f At time t2, the switch tube S1 is turned off, and the direct power transmission phase ends. After that, the switch tube S2 is turned on, and the converter enters the freewheeling phase (T3). The output voltage V o Applied to the inductor L f Both ends, inductor L f Transfer energy to the load alone, the inductor current i Lf Linear decrease, slope is -V o / L f At t3, the inductor current i Lf When the current drops to zero, the switch tube S3 is turned off, and the freewheeling stage ends. Then the switch tube S4 is turned on, and the converter enters the current clamping stage (T4). The inductor L f The voltage across the two ends is zero, and the inductor current i Lf As the load increases, the converter input energy storage phase time T1 and direct power transfer phase time T2 increase, the current clamping phase time T4 decreases, and the switching period T s The converter continues to work in the fixed frequency working area. At this time, T1+T2+T3+T4=T s ; When T4 decreases to zero, the switching period T s Gradually increases, the converter enters the variable frequency working area, at this time T1+T2+T3=T s . Figure 2 Medium inductance L f Negative current I N After the switch tube S3 is turned off, the junction capacitance of the switch tube S4 is discharged and the inductor L f Resonance occurs.

[0004] In order to meet the load power supply requirements and ensure reliable operation of the converter, the converter usually needs to have a certain output current control capability. For example, it can achieve constant current output to meet the requirements of specific load conditions, and has protection functions such as output overcurrent and output short circuit to prevent the converter from being damaged by overcurrent and causing power failure to the load. Figure 3 The implementation circuit of the converter current control method based on current sampling is given, where k vo1 is the output voltage sampling coefficient, k io1 and k io2 is the output current sampling coefficient, V CC To control the supply voltage, the short-circuit / overcurrent protection circuit determines whether the output current signal obtained by sampling exceeds the short-circuit / overcurrent threshold and outputs the protection control signal v pt1, used to implement corresponding protection measures; the constant voltage loop and constant current loop generate error control signals according to the sampled output voltage and output current signals respectively, and the final error control signal v is obtained after the diodes D1 and D2 take the smaller value. er This signal is used to control the duration of each converter operating phase, achieving power control. This current sampling-based current control method can achieve accurate constant current control and output overcurrent protection point control, but it requires the addition of a current sampling circuit and incurs additional losses, which is not conducive to improving converter efficiency and power density. Current control methods based on the error control signal output by the constant voltage loop will cause the constant current value or overcurrent protection value to fluctuate widely due to changes in input and output voltages, making accurate current control impossible. Furthermore, some operating points may damage the converter due to excessive current. Summary of the Invention

[0005] The object of the present invention is to provide a current control method and circuit for a buck-boost converter in order to solve the problems in the prior art.

[0006] The technical solution for achieving the object of the present invention is as follows: In a first aspect, a current control method for a buck-boost converter is provided, the method comprising the following steps:

[0007] According to the converter output voltage V o Calculate the negative current I N ;

[0008] According to the converter output voltage V in , converter output voltage V o and negative current I N Calculate the output current I o ;

[0009] The output current I o Converted into analog quantity, and then output to the constant current loop circuit and short circuit / overcurrent protection circuit respectively;

[0010] Based on the set first voltage reference signal, the second voltage reference signal output by the constant current loop circuit and the converter output voltage V o , the constant voltage loop circuit outputs the error control signal v er1 , the converter is based on the signal v er1 Adjust the converter output voltage V o , thereby achieving constant current output control.

[0011] In a second aspect, a current control circuit of a buck-boost converter is provided, the circuit comprising: a negative current I N Calculation unit, output current I o Computational unit, k io3 Conversion unit, k io4Conversion unit, constant current loop circuit, constant voltage loop circuit and short circuit / overcurrent protection circuit;

[0012] The negative current I N The calculation unit is used to calculate the output voltage V o Calculate the negative current I N ;

[0013] The output current I o The calculation unit is used to calculate the output voltage V in , converter output voltage V o and negative current I N Calculate the output current I o ;

[0014] The k io3 Conversion unit, used to output analog quantity k io3 I o , k io3 is a conversion factor greater than zero;

[0015] The k io4 Conversion unit, used to output analog quantity k io4 I o , k io4 is a conversion factor greater than zero;

[0016] The constant current loop circuit is used to convert the analog quantity k io3 I o The value and the first current reference value I ref1 After error amplification, the second voltage reference signal V ref2 ;

[0017] The short circuit / overcurrent protection circuit is used to io4 I o The value and the second current reference value I ref2 Compare and output protection control signal v pt2 To the drive control circuit of the subsequent converter;

[0018] The constant voltage loop circuit is used to output the voltage sampling signal k vo2 ·V o With the voltage reference signal V ref After error amplification, the error control signal v is output er1 To output current I o Calculation unit; the voltage reference signal V ref is the first current reference value I ref1 and the second voltage reference signal V ref2 The result after weighted average, k vo2 is the sampling coefficient.

[0019] Compared with the prior art, the present invention has the following significant advantages:

[0020] (1) No current sampling circuit is required, the circuit is simple to implement, and no additional loss is caused, which is beneficial to improving the efficiency and power density of the converter.

[0021] (2) Accurate output current control can be achieved with high current control precision.

[0022] (3) When the output current is too large, it can effectively limit the output voltage of the converter and reduce system losses.

[0023] The present invention is further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the circuit topology diagram of the four-switch buck-boost converter.

[0025] Figure 2 This is the typical operating waveform of the four-switch buck-boost converter.

[0026] Figure 3 This is the circuit diagram for implementing the current control method based on current sampling.

[0027] Figure 4 This is a circuit diagram for implementing the current control method proposed in the present invention.

[0028] Figure 5 is the junction capacitance of the switch tube S3 / S4 and the inductor L f Equivalent circuit diagram of the resonance process, where Figure (a) is the equivalent circuit diagram of the resonance stage, and Figure (b) is the equivalent circuit diagram of the switch tube S4 during the body diode conduction stage.

[0029] Figures 6 to 8 Graph showing experimental results of a specific embodiment. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0031] Figure 4 The implementation circuit of the current control method of the present invention is given, which mainly consists of the negative current I N Calculation unit, output current I o The calculation unit, constant current loop, constant voltage loop and short circuit / overcurrent protection circuit are composed. 11 and R 12 is the resistance, k io3 and kio4 is the conversion unit, k vo2 is the output voltage sampling coefficient and is greater than zero, I ref1 and I ref2 are the first and second current reference values, both greater than zero, V ref1 is the first voltage reference signal, greater than zero, v pt2 It is the protection control signal output by the short circuit / overcurrent protection circuit, V ref2 The second voltage reference signal output by the constant current loop, v er1 It is the error control signal output by the constant pressure loop.

[0032] According to the working principle of the four-switch buck-boost converter, we can get:

[0033]

[0034] According to formula (1), we can get

[0035]

[0036] From the volt-second balance, we can get (T1+T2)V in =(T2+T3)V o , ignoring the loss, combined with formula (2), we can get

[0037]

[0038] The durations of T1 and T2 are determined by the drive control circuit of the converter (not provided in this patent) according to the error control signal v er1 And the input and output voltages of the converter are generated, and:

[0039]

[0040] In formula (4), k t1 、k t2 and k t2_min are coefficients, all greater than zero. From formulas (3) and (4), we can see that for the same v er1 When the input and output voltages are different, the durations of T1 and T2 are also different, and the corresponding output current I o are also different, so only according to v er1 The value cannot achieve accurate current control of the converter.

[0041] Figure 4 The current control method of the present invention realizes the negative current I in the circuit N The calculation unit is based on the converter output voltage V o Calculate the negative current I N I Lf_S3off is the inductor current i when the switch tube S3 is turned off LfThe instantaneous value of I Lf_S3off ≥0, after the switch tube S3 is turned off, its body diode remains on, and the inductor current i Lf Linear decrease, slope is -V o / L f , inductor current i Lf After the current decreases to zero, the body diode of the switch tube S3 is turned off with zero current, and then the junction capacitance of the switch tube S4 is discharged, and the inductor L f Resonance, its equivalent circuit is as follows Figure 5 As shown in (a), at this time:

[0042]

[0043] Where, L f is the energy transfer inductor, C oss_34 =C oss_S4 +C oss_S3 , C oss_S3 and C oss_S4 They are the output junction capacitances of the switch tubes S3 and S4 respectively; the initial value i Lf_0- =0 and v DS4_0- =V o Substituting into formula (5), we can get v DS4 =0 when the inductor current i Lf The instantaneous value (ie negative current I N )for:

[0044]

[0045] After that, the body diode of the switch tube S4 is turned on, and the converter equivalent circuit is as follows: Figure 5 As shown in (b), the inductor L f The voltage across the two ends is zero, and the inductor current i Lf Remain unchanged.

[0046] Similar to I Lf_S3off Negative current I<0 N for

[0047]

[0048] The negative current I N The calculation unit calculates the accurate negative current I according to equations (6) and (7): N The value can be calculated in real time by a digital controller or by looking up a table.

[0049] The output current I o The calculation unit is based on equations (3) and (4) and the negative current I N The output of the calculation unit I N Calculate the value and get the accurate output current I oThe output current I o The execution frequency of the calculation unit is higher than I N The execution frequency of the calculation unit is used to improve the stability of the control circuit and ensure I o The calculated value can track the actual value in a timely manner. Preferably, the difference between the execution frequencies of the two values ​​can be 3 to 5 times.

[0050] Figure 4 Middle K io3 and k io4 The conversion unit is realized by the ADC module of the digital controller, and the output current I o The digital value I output by the calculation unit o The value is converted into analog quantity for output and sent to the constant current loop and short circuit / overcurrent protection circuit respectively. The conversion coefficients are k io3 and k io4 , are both greater than zero.

[0051] The short circuit / overcurrent protection circuit will k io4 The analog value k output by the conversion unit io4 I o The value and the second current reference value I ref2 Compare, when k io4 I o Greater than I ref2 , comparator CP1 output signal v pt2 is high level, the signal is sent to the drive control circuit of the converter (not shown in the present invention), enabling the protection action and blocking the drive control signal; when k io4 I o Less than I ref2 , comparator CP1 output signal v pt2 If it is low level, the protection will not be triggered.

[0052] The constant current loop circuit will k io3 The analog value k output by the conversion unit io3 I o The value and the first current reference value I ref1 After error amplification, the second voltage reference signal V ref2 , and there are

[0053]

[0054] Where G io (s) is the transfer function of the constant current loop compensation network, Figure 4 The compensation network is single-pole compensation, and double-pole-single-zero compensation or triple-pole-double-zero compensation can also be used.

[0055] The first and second voltage reference signals (V ref1 and V ref2 ) is weighted averaged to generate the final voltage reference signal V ref , and there is:

[0056]

[0057] The constant voltage loop circuit outputs a voltage sampling signal k vo2 ·V o With the voltage reference signal V ref After error amplification, the error control signal v is output er1 , and there are

[0058] v er1 =G vo (s)(V ref -k vo2 V o ) (10)

[0059] Where G vo (s) is the transfer function of the constant voltage loop compensation network, Figure 4 The compensation network is single-pole compensation, and double-pole-single-zero compensation or triple-pole-double-zero compensation can also be used.

[0060] From equations (8) and (9), we can see that when the output current I o Increase, and k io3 I o Greater than I ref1 When the constant current loop controls the second voltage reference signal V ref2 Decreases and is less than zero, so that the voltage reference signal V ref Reduced, so that the error control signal v output by the constant pressure loop er1 Decreases, and finally the converter output voltage decreases, thereby limiting the output current from continuing to increase, and finally achieving constant current output control.

[0061] In order to further verify the effectiveness of the control method and circuit of the present invention, Figures 6 to 8 The experimental results of specific embodiments are given. Figure 6 As shown, when the converter load is large, the output current I o After reaching the constant current limit, the converter output voltage begins to decrease gradually to maintain the output current I o The constant current output is unchanged, and accurate constant current output control is achieved. When the output voltage decreases to a certain value, the converter triggers the output undervoltage protection, and the output is pulled low. After restarting, the protection will be triggered again because the load is still large. Figure 7The dynamic experimental waveforms of the converter's constant voltage output and constant current output switching are given. Initially, the converter operates in constant voltage mode and the output voltage is stably controlled at 48V. As the load is suddenly added, the output current is stably controlled at 2.5A (constant current threshold) and the output voltage decreases to about 42V. Figure 8 The experimental waveforms of a short circuit during converter operation are given. It can be seen that the control method of the present invention can effectively limit the output voltage and current after short circuit restart, thereby reducing system power consumption.

[0062] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only illustrative of the principles of the present invention. Without departing from the spirit and scope of the present invention, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A current control method for a buck-boost converter, characterized in that: The method comprises the following steps: According to the converter output voltage V o Calculate the negative current I N ; According to the converter input voltage V in , converter output voltage V o and negative current I N Calculate the output current I o ; The output current I o Converted into analog quantity, and then output to the constant current loop circuit and short circuit / overcurrent protection circuit respectively; Based on the set first voltage reference signal, the second voltage reference signal output by the constant current loop circuit and the converter output voltage V o , the constant voltage loop circuit outputs the error control signal v er1 , the converter is based on the signal v er1 Adjust the converter output voltage V o , thereby achieving constant current output control; A current control circuit for a buck-boost converter based on the current control method, the circuit comprising: a negative current I N Calculation unit, output current I o Computational unit, k io3 Conversion unit, k io4 Conversion unit, constant current loop circuit, constant voltage loop circuit and short circuit / overcurrent protection circuit; The negative current I N The calculation unit is used to calculate the output voltage V o Calculate the negative current I N ; The output current I o The calculation unit is used to calculate the converter input voltage V in , converter output voltage V o and negative current I N Calculate the output current I o ; The k io3 Conversion unit, used to output analog quantity k io3 I o , k io3 is a conversion factor greater than zero; The k io4 Conversion unit, used to output analog quantity k io4 I o , k io4 is a conversion factor greater than zero; The constant current loop circuit is used to convert the analog quantity k io3 I o The value and the first current reference value I ref1 After error amplification, the second voltage reference signal V ref2 ; The short circuit / overcurrent protection circuit is used to io4 I o The value and the second current reference value I ref2 Compare and output protection control signal v pt2 To the drive control circuit of the subsequent converter; The constant voltage loop circuit is used to output the voltage sampling signal k vo2 ·V o With the voltage reference signal V ref After error amplification, the error control signal v is output er1 To output current I o Calculation unit; the voltage reference signal V ref is the first voltage reference value V ref1 and the second voltage reference signal V ref2 The result after weighted average, k vo2 is the sampling coefficient; The negative current I N The calculation unit calculates and obtains the negative current I N The formula is: The voltage across the drain and source of the switch tube S4 is v DS4 =0, the inductor current i Lf The instantaneous value of the negative current I N ; The inductor current i when the switch tube S3 is turned off Lf The instantaneous value I Lf_S3off ≥0, the negative current I N for: Where, L f is the energy transfer inductor, C oss_34 =C oss_S4 +C oss_S3 , C oss_S3 and C oss_S4 They are the output junction capacitances of the switches S3 and S4 respectively; The inductor current i when the switch tube S3 is turned off Lf The instantaneous value of When the negative current I N for: The output current I o The calculation unit calculates the output current I o The formula is: Where t1 and t3 are time, T s is the switching period, and the duration of T1 and T2 is determined by the converter drive control circuit according to the error control signal v er1 And the input and output voltages of the converter are generated, and: Where k t1 、k t2 and k t2_min are coefficients, all greater than zero.

2. The current control method of the buck-boost converter according to claim 1, wherein: The short circuit / overcurrent protection circuit is used to io4 I o The value and the second current reference value I ref2 Compare and output protection control signal v pt2 , specifically: When k io4 I o Greater than I ref2 , comparator CP1 output signal v pt2 When k is high, the signal is output to the drive control circuit of the converter, enabling the protection action and blocking the drive control signal; io4 I o Less than I ref2 , comparator CP1 output signal v pt2 If it is low level, the protection will not be triggered.

3. The current control method of the buck-boost converter according to claim 2, wherein: The second voltage reference signal V output by the constant current loop circuit ref2 for: Where G io (s) is the transfer function of the constant current loop compensation network.

4. The current control method of the buck-boost converter according to claim 2, wherein: The error control signal v output by the constant voltage loop circuit er1 for: v er1 =G vo (s)(V ref -k vo2 V o ) Among them, V ref for: Where G vo (s) is the transfer function of the constant voltage loop compensation network, R 11 、R 12 All are resistors.

5. The current control method of the buck-boost converter according to claim 3 or 4, characterized in that: The compensation network is single-pole compensation, double-pole-single-zero compensation, or triple-pole-double-zero compensation.

6. The current control method of the buck-boost converter according to claim 1, wherein: The negative current I N Calculation unit, output current I o The calculation of the calculation unit is realized by real-time calculation or table lookup by the digital controller.

7. The current control method of the buck-boost converter according to claim 6, wherein: The output current I o The execution frequency of the calculation unit is higher than the negative current I N The execution frequency of the computational unit.

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