A control circuit and control method of a four-switch buck-boost converter

Abstract

The application discloses a control circuit and a control method of a four-switch buck-boost converter in the field of buck-boost converters. The four-switch buck-boost converter comprises switch tubes S1-S4, an input capacitor CB, an output capacitor Co and an inductor L, wherein the switch tubes S1 and S2 constitute a front bridge arm, the switch tubes S3 and S4 constitute a rear bridge arm, and the inductor L is connected between the front bridge arm and the rear bridge arm. The control method comprises the following steps: sampling to obtain an input voltage Vin and an output voltage Vo of the four-switch buck-boost converter; feeding back the input voltage Vin to a rear PWM control circuit, outputting a drive signal with a duty ratio Da to control the on-off of the switch tube S4, and complementarily turning on the switch tubes S3 and S4; feeding back the input voltage Vin and the output voltage Vo to a front PWM control circuit, outputting a drive signal with a duty ratio D to control the on-off of the switch tube S1, and complementarily turning on the switch tubes S1 and S2; and synchronously working the front PWM control circuit and the rear PWM control circuit. The control method has the characteristics of low cost and high efficiency.

Description

Technical Field

[0001] This invention relates to the field of buck-boost converters, specifically a control circuit and control method for a four-switch buck-boost converter. Background Technology

[0002] Electrical and electronic equipment and systems typically require the conversion of input voltage to output voltage, which may be higher, lower, or approximately equal to the input voltage. For example, in systems powered by fuel cells or DC batteries, the output voltage may vary with the application time or load. Because the output voltage may be higher or lower than the required voltage at any given time, it is usually necessary to maintain a constant output voltage unaffected by variations in the fuel cell or battery output voltage. For this type of power management problem, a same-polarity BOB converter is the optimal solution. It provides the aforementioned functionality and is particularly suitable for fuel cell or battery power systems where isolation is not required and the output voltage does not have polarity reversal relative to the input power supply.

[0003] Same-polarity BOB converters, also known as same-polarity Buck-or-Boost converters or four-switch Buck-Boost converters, are rarely reported in public publications. Traditional Buck-Boost circuits are unsuitable for applications where the output voltage and input source polarity are reversed. Same-polarity BOB converters primarily consist of a switching controller with four switching transistors S1 to S4, capable of operating at input voltages higher than, lower than, or equal to the output voltage in Buck mode, Boost mode, or Buck-Boost mode. In any of these modes, one switching transistor remains off to fully achieve buck or boost regulation.

[0004] The same-polarity BOB converter is based on an H-bridge topology. Similar to other bridge topologies, each pair of switches in each branch (S1 and S2 or S3 and S4) is driven by complementary signals. A common strategy is that the diagonal pairs of switches S1 and S4 are controlled by the same duty cycle D, thus the output-input voltage ratio is given by D / (1-D). Using this strategy, the BOB converter is inefficient, with switching losses potentially twice that of Buck or Boost converters.

[0005] The applicant proposed improvement plans to address the technical issues. Summary of the Invention

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A control method for a four-switch buck-boost converter, the four-switch buck-boost converter including switches S1-S4, input capacitor CB, output capacitor Co, and inductor L, wherein switches S1 and S2 constitute the front bridge arm, switches S3 and S4 constitute the rear bridge arm, and inductor L is connected between the front and rear bridge arms; the control method includes:

[0008] The input voltage Vin and output voltage Vo of the four-switch buck-boost converter are obtained by sampling.

[0009] The input voltage Vin is fed back to the post-PWM control circuit, which outputs a drive signal with a duty cycle of Da to control the on / off state of switch S4. Switches S3 and S4 are complementary in conduction.

[0010] The input voltage Vin and the output voltage Vo are fed back to the front PWM control circuit. The front PWM control circuit outputs a drive signal with a duty cycle of D to control the on and off of the switch S1. The switches S1 and S2 are complementary in conduction.

[0011] The front PWM control circuit and the rear PWM control circuit operate synchronously at the same frequency.

[0012] Furthermore, a preset voltage reference value Vinref is defined. When the sampled input voltage Vin is less than the voltage reference value Vinref, the duty cycle Da of the drive signal output by the subsequent PWM control circuit decreases as the input voltage Vin increases according to the set parameters, until the input voltage Vin is equal to the voltage reference value Vinref. The value of the duty cycle Da remains constant as the input voltage Vin continues to increase.

[0013] Furthermore, the duty cycle D of the drive signal output by the pre-PWM control circuit changes according to the following relationship:

[0014]

[0015] A control circuit for implementing the above control method includes a front PWM control circuit, a rear PWM control circuit, a synchronization control circuit, an input voltage acquisition circuit, an output voltage acquisition circuit, a front bridge drive circuit, and a rear bridge drive circuit. The input voltage acquisition circuit and the output voltage acquisition circuit are respectively used to acquire the input voltage Vin and the output voltage Vo. The output end of the input voltage acquisition circuit is connected to the inputs of the front PWM control circuit and the rear PWM control circuit. The output end of the output voltage acquisition circuit is connected to the input end of the front PWM control circuit. The output end of the rear PWM control circuit is connected to the controlled ends of two switching tubes in the rear bridge arm of the four-switch buck-boost converter through the rear bridge drive circuit. The output end of the front PWM control circuit is connected to the controlled ends of two switching tubes in the front bridge arm of the four-switch buck-boost converter through the front bridge drive circuit. The front PWM control circuit and the rear PWM control circuit are both connected to the synchronization control circuit and achieve synchronous and same-frequency control through the synchronization control circuit.

[0016] Beneficial effects: The present invention proposes a novel control method and a control circuit, which can be easily implemented by using a conventional PWM control circuit. Using this control strategy, when the input voltage changes from the maximum value to the minimum value, the four-switch buck-boost converter can adaptively operate in the buck mode (Vin>Vo), the boost mode (Vin<Vo), or the buck-boost mode (Vin = Vo). Without special timing control, the continuous conversion between the above working modes can be easily achieved according to the set duty ratios D and Da, which is very suitable for applications where the output voltage is within the input voltage range. According to the proposed control method, all switching tubes operate in the fixed-frequency PWM mode. The above control method ensures the continuity of the transfer function, which makes the four-switch buck-boost converter more suitable for applications. Brief Description of the Drawings

[0017] Figure 1 is the circuit topology diagram of the four-switch buck-boost converter;

[0018] Figure 2 is the control timing diagram of the switching tubes S1~S4 of the present invention in the buck, buck-boost, and boost modes;

[0019] Figure 3 is the relationship curve of D, Da, and Vin of the present invention;

[0020] Figure 4 is the system control architecture diagram of the present invention;

[0021] Figure 5 is the relationship curve of D&Da and Vin when Da is approximately modified to a non-linear relationship with the input voltage;

[0022] Figure 6a Inductance current and driving signal variation diagrams of switching transistors S1 and S4 under buck mode (Vin > Vo) at full load conditions;

[0023] Figure 6b Inductance current and driving signal variation diagrams of switching transistors S1 and S4 under boost mode (Vin < Vo) at full load conditions;

[0024] Figure 7a Vds waveform diagrams of switching transistors S1 and S2 at full load conditions;

[0025] Figure 7b Vds waveform diagrams of switching transistors S3 and S4 at full load conditions. Specific implementation manners

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] See Figure 1 , the four-switch buck-boost converter includes switching transistors S1 to S4, an input capacitor CB, an output capacitor Co, and an inductor L. Among them, switching transistors S1 and S2 form the front bridge arm, switching transistors S3 and S4 form the rear bridge arm, and the inductor L is connected between the front bridge arm and the rear bridge arm. The input capacitor CB is connected to the input side of the four-switch buck-boost converter, and the output capacitor Co is connected to the output side of the four-switch buck-boost converter.

[0028] A control method for a four-switch buck-boost converter includes:

[0029] The input voltage Vin is fed back to the rear PWM control circuit. The rear PWM control circuit outputs a driving signal with a duty cycle of Da to control the on and off of the switching transistor S4, and the switching transistors S3 and S4 conduct complementary.

[0030] The input voltage Vin and the output voltage Vo are fed back to the front PWM control circuit. The front PWM control circuit outputs a driving signal with a duty cycle of D to control the on and off of the switching transistor S1, and the switching transistors S1 and S2 conduct complementary.

[0031] The front PWM control circuit and the rear PWM control circuit operate synchronously and at the same frequency.

[0032] The above method is implemented through the control circuit of a four-switch buck-boost converter, including a front PWM control circuit, a rear PWM control circuit, a synchronization control circuit, an input voltage acquisition circuit, an output voltage acquisition circuit, a front bridge drive circuit, and a rear bridge drive circuit. The input voltage acquisition circuit and the output voltage acquisition circuit are used to acquire the input voltage Vin and the output voltage Vo, respectively. The output terminal of the input voltage acquisition circuit is connected to the input of the front PWM control circuit and the rear PWM control circuit. The output terminal of the output voltage acquisition circuit is connected to the input terminal of the front PWM control circuit. The output terminal of the rear PWM control circuit is connected to the controlled terminals of two switches in the rear bridge arm of the four-switch buck-boost converter through the rear bridge drive circuit. The output terminal of the front PWM control circuit is connected to the controlled terminals of two switches in the front bridge arm of the four-switch buck-boost converter through the front bridge drive circuit. Both the front PWM control circuit and the rear PWM control circuit are connected to the synchronization control circuit, and synchronous frequency control is achieved through the synchronization control circuit. The PWM control circuit can be implemented using a conventional PWM control chip, such as the UC3843, while the synchronization control circuit can be implemented using a timer circuit to achieve synchronous frequency control of the two PWM control circuits.

[0033] like Figure 2 As shown, Figures (a), (b), and (c) respectively illustrate a proposed control timing sequence for each switch in a four-switch buck-boost converter operating in buck mode, buck-boost mode, and boost mode under the aforementioned control method. Figure 2 As shown, regardless of the operating mode of the four-switch buck-boost converter, the switching transistors of the four-switch buck-boost converter have the same timing control sequence. The control circuit automatically changes the duty cycle D and Da according to the change of input voltage, thereby adaptively changing the operating mode.

[0034] In buck pattern (e.g.) Figure 2 Taking (a) as an example, for ease of explanation, switches S1 and S4 are shown as simultaneously on. However, in reality, switch S4 can turn on after switch S1, with almost no impact on system operation. Before analysis, assume the four-switch buck-boost converter has a certain heavy load, which always results in a positive current in inductor L. The operation of the four-switch buck-boost converter is as follows:

[0035] [t0~t1]: Input phase. At t=t0, switches S1 and S4 are simultaneously turned on. The four-switch buck-boost converter begins the "input phase". During this input phase, the input voltage Vin increases across the inductor L through switches S1 and S4, and the inductor current iL increases linearly with a slope equal to Vin / L.

[0036]

[0037] [t1~t3]: Input / output phase. At t=t1, switch S4 is off. The body diode of switch S3 begins to conduct, and then switch S3 naturally turns on under zero-voltage conditions at t=t2. This phase continues until switch S1 is off at t=t3. During the time interval between t1 and t3, the input voltage Vin and the energy stored in inductor L together transfer energy to the load. The inductor current iL increases linearly with a slope equal to (Vin-Vo) / L.

[0038]

[0039] [t3~t6]: Freewheeling stage. At t=t3, switch S1 is off. The body diode of switch S2 conducts first, and then switch S2 naturally conducts at t=t4 under zero voltage conditions. During this stage, the output voltage Vo increases across inductor L, and the inductor current iL decreases with a slope equal to Vo / L.

[0040]

[0041] At t = t5, switch S2 is off. Since the inductor current is positive under the assumed heavy load condition, the inductor current iL is conducted by the body diode of switch S2 before switch S1 turns on, continuing until t = t6. After t6, a new power conversion cycle begins. In a four-switch buck-boost converter under light load or no-load conditions, the inductor current iL is negative after switch S2 turns on or before the "input phase" begins, and is conducted by the body diode of switch S1.

[0042] Tables 1 and 2 show the switching states of all the switching transistors, such as some ZVS states of the converter.

[0043] Table 1 Switch Status under Heavy Load

[0044]

[0045] Table 2 Switching Status under Light Load

[0046]

[0047] Under heavy load conditions, only switching transistors S2 and S3 can achieve zero-voltage switching. Under light load conditions, all switching transistors can only achieve zero-voltage turn-on, not zero-voltage turn-off.

[0048] The only difference between buck mode and the other two modes is the voltage and current waveforms of the inductor. In boost mode, after switch S3 is turned on, the voltage across inductor L is negative, and the inductor current iL decreases linearly with a slope equal to (Vo-Vin) / L. In buck-boost mode, after switch S3 is turned on, the voltage across inductor L is zero, while the inductor current iL remains constant.

[0049] Based on the above control method, if the dead time factor is ignored, and the inductor current iL is continuous, the gain expression of the four-switch buck-boost converter when it enters steady state is:

[0050]

[0051] For ease of discussion, let Vin = 18~40VDC and Vo = 24VDC. Since the subsequent PWM control circuit controls the switching of switch S4 based solely on the feedback input voltage Vin and the output duty cycle Da, the duty cycle Da can be set as a function of the input voltage Vin.

[0052]

[0053] At this point, the gain expression for the four-switch buck-boost converter when it enters steady state can be derived as follows:

[0054]

[0055] Based on the above expression, we can conclude that... Figure 3 The graph shows the relationship between D, Da, and Vin. Figure 4 As shown, a corresponding relationship exists between the duty cycle Da and the duty cycle D. Controlling the input voltage Vin through feedforward control will cause a change in the duty cycle Da, which in turn affects the output voltage Vo, leading to a change in the duty cycle D, thereby adjusting the duty cycle of the switching transistor S1 and the output voltage. Therefore, the PWM control circuit can adjust the duty cycle D through feedback control based on the feedback input voltage Vin and output voltage Vo, thus regulating the output voltage Vo.

[0056] This application employs a dual duty cycle control method, increasing the control flexibility of the four-switch buck-boost converter. The matching degree of duty cycles Da and D in different operating modes will affect the efficiency of the four-switch buck-boost converter. For example, in buck mode, if the duty cycle Da is small, or switch S4 is frequently off, then the duty cycle of switch S3 will be large, or switch S3 will be frequently on. In this case, the energy from the input power supply will be directly transferred to the load, and the four-switch buck-boost converter can achieve higher efficiency in buck mode. Simultaneously, if the duty cycle Da decreases, the duty cycle D will increase, and the on-time of switch S2 will decrease. In this case, the four-switch buck-boost converter can also achieve higher efficiency in boost mode.

[0057] Therefore, a lower duty cycle Da helps improve the overall system efficiency across the input range. However, if the duty cycle Da is too small, the duty cycle D will be too large. For example, assuming Vin = 18V and Da = 0.2, D will be greater than 1; otherwise, the four-switch buck-boost converter will be unable to provide a normal output voltage under low input voltage. Furthermore, an excessively large steady-state D is detrimental to the dynamic performance of the four-switch buck-boost converter, thus requiring a trade-off between high efficiency and good performance.

[0058] In view of this, in some preferred embodiments, such as Figure 5 As shown, the duty cycle Da approximates the nonlinear relationship with the input voltage Vin to improve system efficiency. A voltage reference value Vinref can be preset. When the sampled input voltage Vin is less than the voltage reference value Vinref, the duty cycle Da of the drive signal output by the subsequent PWM control circuit decreases as the input voltage Vin increases, until the input voltage Vin equals the voltage reference value Vinref. The value of the duty cycle Da then remains constant as the input voltage Vin continues to increase. Figure 5 In this case, Vinref = 30V. If the input voltage Vin is greater than 30V, the duty cycle Da will remain at a constant low value. At this time, the duty cycle D will change with the input voltage Vin. Figure 5 and Figure 3 By comparison, it can be concluded that a decrease in duty cycle Da will lead to an increase in duty cycle D.

[0059] The above control method is verified using a 250W (18V~40Vin, 24Vo / 10.5A) fuel cell power supply.

[0060] Figure 6a , Figure 6bSeparate diagrams showing the inductor current and drive signal variations of switching transistors S1 and S4 under full-load conditions in buck mode (Vin > Vo) and boost mode (Vin < Vo) are presented, verifying Figure 2 the control timing shown. In this embodiment, switching transistor S4 conducts after a delay relative to S1.

[0061] Figure 7a 、 Figure 7b Separate diagrams showing the Vds voltage waveforms of switching transistors S1 and S2, S3 and S4 under full-load conditions are presented. From Figure 7a these diagrams, it can be found that the peak voltage between the switches of switching transistors S1 and S2 is relatively high. This peak is related to a poor layout that introduces stray inductance into the front bridge arm formed by switching transistors S1 and S2.

[0062] The present invention proposes a novel and simple control method for a BOB converter, which has the following characteristics.

[0063] First, the present invention adopts a dual-duty-cycle control scheme, feedback control, and feed-forward control methods, increasing the control flexibility of the system. Second, both the front PWM control circuit and the rear control circuit can use general control circuits to achieve the timing control of all switching transistors. The control circuit is simple and requires almost no additional components except for a controller, a driver, and some passive components. Third, when the input voltage varies from its maximum value to its minimum value, the control method proposed by the present invention can ensure that the two duty cycles change continuously, even during the time from one operating mode to another. In addition to the above advantages, the inherent ability of the switching part to achieve zero-voltage conditions and the selection of low-voltage-rated switches make the system have the potential to achieve a high-efficiency design. In summary, the control method proposed by the present invention has the characteristics of low cost and high efficiency, making the four-switch buck-boost converter more suitable for applications in fuel cells or any other power system.

[0064] Although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0065] Therefore, the above description is only a preferred embodiment of the present application and is not used to limit the scope of implementation of the present application; that is, all equivalent transformations made according to the scope of the claims of the present application are within the protection scope of the claims of the present application.

Claims

1. A control method for a four-switch buck-boost converter, the four-switch buck-boost converter comprising switching transistors S1~S4, an input capacitor CB, an output capacitor Co, and an inductor L, wherein switching transistors S1 and S2 constitute the front bridge arm, switching transistors S3 and S4 constitute the rear bridge arm, and the inductor L is connected between the front bridge arm and the rear bridge arm; characterized in that, Control methods include: The input voltage Vin and output voltage Vo of the four-switch buck-boost converter are obtained by sampling. The input voltage Vin is fed back to the post-PWM control circuit, which outputs a drive signal with a duty cycle of Da to control the on / off state of switch S4. Switches S3 and S4 are complementary in conduction. A preset voltage reference value Vinref is set. When the sampled input voltage Vin is less than the voltage reference value Vinref, the duty cycle Da of the drive signal output by the subsequent PWM control circuit decreases as the input voltage Vin increases until the input voltage Vin is equal to the voltage reference value Vinref. The value of the duty cycle Da remains constant as the input voltage Vin continues to increase. The input voltage Vin and the output voltage Vo are fed back to the front PWM control circuit. The front PWM control circuit outputs a drive signal with a duty cycle of D to control the on and off of the switch S1. The switches S1 and S2 are complementary in conduction. The front PWM control circuit and the rear PWM control circuit operate synchronously at the same frequency.

2. The control method for a four-switch buck-boost converter according to claim 1, characterized in that, The duty cycle D of the drive signal output by the previous PWM control circuit changes according to the following relationship: 。 3. A control circuit for implementing the control method of claim 1, characterized in that, The system includes a front PWM control circuit, a rear PWM control circuit, a synchronization control circuit, an input voltage acquisition circuit, an output voltage acquisition circuit, a front bridge drive circuit, and a rear bridge drive circuit. The input voltage acquisition circuit and the output voltage acquisition circuit are used to acquire the input voltage Vin and the output voltage Vo, respectively. The output terminal of the input voltage acquisition circuit is connected to the input of the front PWM control circuit and the rear PWM control circuit. The output terminal of the output voltage acquisition circuit is connected to the input terminal of the front PWM control circuit. The output terminal of the rear PWM control circuit is connected to the controlled terminals of two switches in the rear bridge arm of the four-switch buck-boost converter through the rear bridge drive circuit. The output terminal of the front PWM control circuit is connected to the controlled terminals of two switches in the front bridge arm of the four-switch buck-boost converter through the front bridge drive circuit. Both the front PWM control circuit and the rear PWM control circuit are connected to the synchronization control circuit, and synchronous frequency control is achieved through the synchronization control circuit.

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