Seamless transition hybrid buck-boost DC-DC converter and conversion method

Abstract

The invention discloses a seamless transition hybrid buck-boost DC-DC converter and a conversion method, and aims to realize further reduction of inductive current in a power supply voltage range of an all-lithium battery, which is only half of load current, and can reduce large-inductance parasitic resistance loss and switch conduction loss. In the step-down mode and the step-up mode, the inductive current is a constant value (irrelevant to the conversion ratio CR), and the transient response speed can be increased. The transition mode is arranged between the step-down mode and the step-up mode, so that the problem that the duty ratios of the converter in the step-down mode and the step-up mode are very close to 100% and 0% respectively when the conversion ratio is very close to 1 can be solved, the surge current of the flying capacitors CF1 and CF2 is reduced, and the stability of the converter is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a seamlessly transitioned hybrid buck-boost DC-DC converter and a conversion method. Background Art

[0002] In order to meet the needs of electronic devices powered by lithium batteries, a switching power converter that can provide a stable output voltage for the target load within the input voltage range of the lithium battery is widely used, and its common application scenarios include RF power amplifiers, battery chargers, and light-emitting diode drivers. The input voltage range of lithium batteries is usually 2.7V-4.2V, and its output voltage will gradually decrease during use. At present, there is a buck-boost switching power converter composed of a buck switching power converter and a boost switching power converter, which can reduce the higher input voltage to 3.3V; when the input voltage is low, it can be increased to 3.3V.

[0003] Figure 6 The figure shows a conventional buck-boost switching power converter (CBBC), which can achieve a very high conversion ratio range (CR) and efficiency. However, the current development of smart devices is gradually tending towards miniaturization, lightness, and long battery life. Since the size of the inductor is inversely proportional to the DCR (parasitic DC resistance of the inductor), the reduction in the inductor volume leads to a continuous increase in the DCR, resulting in an increase in the conduction loss caused by the inductor DCR and a decrease in the converter efficiency. This situation is even more severe under heavy load conditions (driving high current loads). In order to further improve efficiency, in recent years, research has begun on hybrid inductor-capacitor buck-boost switching power converters, which use capacitors to share the current flowing through the inductor branch, thereby reducing the DCR conduction loss of the inductor.

[0004] Among them, Lin Cheng proposed a hybrid buck-boost converter (Hybrid Buck-Boost Converter) in his academic paper “A Battery-to-3.4V Hybrid Buck-Boost Converter With Always Reduced Conduction Loss” that can simultaneously reduce the inductor current in both the boost mode and the buck mode. Figure 7 As shown in FIG. 1 , the hybrid buck-boost converter has four working phases. In phase 1, the switch device S1 is turned on, S2, S3, and S4 are turned off, and the inductor L is charged to 2V. IN -V OUT , capacitor C F Charge to V OUT -VIN ; At phase two, switch devices S2 and S3 are turned on, S1 and S4 are turned off, and inductor L discharges to V IN -V OUT , and at the same time, capacitor C F discharges to V OUT -V IN ; At phase three, switch devices S1 and S3 are turned on, S2 and S4 are turned off, and inductor L charges to V IN -V OUT , and capacitor C F discharges to V OUT ; At phase four, switch device S4 is turned on, S1, S2, and S3 are turned off, and inductor L discharges to V IN -2V OUT , and at the same time, capacitor C F charges to V OUT .

[0005] The boost mode of the hybrid buck-boost converter is composed of phase one and two, and the buck mode is composed of phase three and four. According to the volt-second balance law of inductor L, the voltage conversion ratio expressions of this structure in the boost mode and the buck mode can be obtained as follows:

[0006]

[0007] From the charge balance law of capacitor C F , C L , the expressions of inductor current and output current can be obtained as follows:

[0008] Boost Mode: I L = I OUT (3)

[0009] Buck Mode: I L = CRI OUT (4)

[0010] Among them, CR is the voltage conversion ratio (the ratio of output voltage to input voltage). From equations (1) and (2), it can be seen that the voltage conversion ratio range of this structure is 0.5 < CR < 2, and it can realize the conversion of the full input voltage range of the lithium battery, 2.7V - 4.2V, to a stable output voltage of 3.3V. And from equations (3) and (4), compared with the traditional buck-boost switch-mode power supply converter (Boost Mode: I L = CRI OUT , Buck Mode: I L = I OUT ), this structure realizes the reduction of inductor current in the full mode, so the conduction loss caused by the inductor parasitic resistance DCR can be reduced.

[0011] However, when the conversion ratio is very close to 1, the duty cycle of the converter in buck mode and boost mode is very close to 100% and 0% respectively, causing the capacitor C F Charging or discharging in a very limited time will generate a large surge current, affecting the stability of the converter. Therefore, it is necessary to set a transition mode between the buck mode and the boost mode when the conversion ratio is close to 1 to ensure the normal operation of the converter. Although the structure of the hybrid buck-boost converter proposes a transition mode (composed of phases one, two, and four), when the conversion ratio is less than 1, the inductor current has no discharge phase and the converter cannot work normally. Therefore, the structure of the hybrid buck-boost converter can enter the transition mode from the boost mode, but cannot enter the transition mode from the buck mode, resulting in the duty cycle of the buck mode being very close to 100%, and the capacitor C F The charging time is close to 0%, resulting in a large inrush current, which affects the stability of the converter. In addition, although the structure of the hybrid buck-boost converter can reduce the inductor current in both buck mode and boost mode, the inductor current is still large in lithium battery applications, resulting in large inductor parasitic resistance loss and switch conduction loss. In addition, the inductor current of this structure in buck mode is not a constant value (I L =CRI OUT ), which also affects the transient response speed of the converter because the inductor needs time to charge or discharge when the conversion ratio M changes. Summary of the invention

[0012] The object of the present invention is to overcome the deficiencies of the prior art and provide a hybrid buck-boost DC-DC converter with seamless transition.

[0013] To achieve the above purpose, the technical solution provided by the present invention is:

[0014] A seamless transition hybrid buck-boost DC-DC converter includes NMOS switch devices S1, S2, S3, S4, S5, S6, and a flying capacitor C F1 , C F2 , inductance L, load capacitance C L ;

[0015] in,

[0016] The source of the NMOS switch device S1 is connected to one end of the inductor L and the flying capacitor C F1 The drain of NMOS switch device S1 is connected to the flying capacitor C F2 The upper plate and the drain of the NMOS switch device S2;

[0017] The source end of the NMOS switch device S2 is connected to the drain end of the NMOS switch device S5 and the load capacitor CL The upper plate is connected to the voltage output terminal;

[0018] The source of the NMOS switch device S3 is connected to the voltage input terminal and one end of the inductor L, and the drain of the NMOS switch device S3 is connected to the flying capacitor C F1 The lower plate and the drain of the NMOS switch device S4;

[0019] The source of the NMOS switch device S4 is connected to the flying capacitor C F2 The lower plate of the NMOS switch device S5, the source of the NMOS switch device S5 and the drain of the NMOS switch device S6;

[0020] The source of the NMOS switch device S6 is connected to the ground level.

[0021] This technical solution further reduces the inductor current within the full lithium battery power supply voltage range by adding a capacitor branch, thereby reducing the conduction loss caused by the large inductor parasitic resistance and the switch tube conduction loss.

[0022] Furthermore, to achieve the above-mentioned object, the present invention further provides a seamless transition hybrid buck-boost DC-DC conversion method, which is implemented by the above-mentioned seamless transition hybrid buck-boost DC-DC converter, and includes a buck mode, a transition mode, and a boost mode, wherein the transition mode is set between the buck mode and the boost mode;

[0023] The transition mode is coordinated to prevent the duty cycles of the hybrid buck-boost DC-DC converter in the buck mode and the boost mode from being close to 100% and 0% respectively when the conversion ratio is very close to 1.

[0024] Furthermore, in buck mode, the topology of the hybrid buck-boost DC-DC converter consists of two working phases, including:

[0025] Phase 1: The duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, and NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2 (V IN -V OUT ), flying capacitor C F1 Charging, at the same time the flying capacitor C F2 Supply power to the load until the flying capacitor C F2 The voltage across the terminals is discharged to the output voltage V OUT ; V IN is the input voltage;

[0026] Phase 2: The duty cycle is 1-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , flying capacitor CF2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2V OUT -V IN ;

[0027] From the volt-second balance law of inductance L, we get:

[0028] D*2(V IN -V OUT )+(1-D)*(V IN -2V OUT )=0 (5)

[0029] The conversion ratio is obtained from the above formula:

[0030]

[0031] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L :

[0032]

[0033] I OUT is the load current.

[0034] Furthermore, in boost mode, the topology of the hybrid buck-boost DC-DC converter consists of three working phases, including:

[0035] Phase 1: Duty cycle is D, NMOS switch devices S4 and S6 are turned on, and NMOS switch devices S1, S2, S3, and S5 are turned off; at this time, the inductor L is charged to 3V IN -2V OUT , while the flying capacitor C F1 Charge;

[0036] Phase 2: The duty cycle is 0.2, NMOS switch devices S2, S3, S4 are turned on, and NMOS switch devices S1, S5, S6 are turned off; at this time, the inductor L is discharged and the flying capacitor C F1 Continue to charge, at the same time the flying capacitor C F2 The voltage across the terminals is discharged to V OUT -V IN ; V IN is the input voltage, V OUT is the output voltage;

[0037] Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L continues to discharge to 2 (V IN -VOUT ), flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2(V IN -V OUT );

[0038] From the volt-second balance law of inductance L, we get:

[0039]

[0040] The conversion ratio is obtained from the above formula:

[0041]

[0042] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L :

[0043]

[0044] I OUT is the load current.

[0045] Further, the transition mode includes:

[0046] Phase 1: Duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2V IN -V OUT , while the flying capacitor C F1 Charging, flying capacitor C F2 The voltage across the two ends is discharged to the output voltage V OUT ;

[0047] Phase 2: The duty cycle is 0.2, NMOS switch devices S1, S2, S3, S6 are turned on, and NMOS switch devices S4 and S5 are turned off; at this time, the inductor L continues to charge or discharge to V IN -V OUT , flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the terminals is discharged to V OUT -V IN ;

[0048] Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1 and S5 are turned on, and NMOS switch devices S2, S3, S4, and S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , while the flying capacitor C F2 Charge;

[0049] From the volt-second balance law of inductance L, we get:

[0050]

[0051] The conversion ratio is obtained from the above formula:

[0052]

[0053] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L :

[0054]

[0055] Compared with the prior art, the principles and advantages of this technical solution are as follows:

[0056] 1. By adding a capacitor branch, the inductor current can be further reduced within the full lithium battery power supply voltage range to only half of the load current, thereby reducing the conduction loss caused by the large inductor parasitic resistance and the switch tube conduction loss.

[0057] 2. In both buck mode and boost mode, the inductor current is a constant value (regardless of the conversion ratio CR), which can improve the speed of transient response.

[0058] 3. Setting a transition mode between buck mode and boost mode can solve the problem that when the conversion ratio is very close to 1, the duty cycle of the converter in buck mode and boost mode is very close to 100% and 0% respectively, thus reducing the flying capacitor C. F1 , C F2 The surge current is reduced and the stability of the converter is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0060] Figure 1 A structural diagram of a seamlessly transitioned hybrid buck-boost DC-DC converter according to the present invention;

[0061] Figure 2 A working state diagram of a seamlessly transitioned hybrid buck-boost DC-DC converter in buck mode according to the present invention;

[0062] Figure 3 A working state diagram of a seamlessly transitioned hybrid buck-boost DC-DC converter in boost mode of the present invention;

[0063] Figure 4 This is a working state diagram of a seamlessly transitioned hybrid buck-boost DC-DC converter in a transition mode of the present invention;

[0064] Figure 5 This is a comparison chart of the inductor current of each solution structure at the same lithium battery voltage;

[0065] Figure 6 is a block diagram of a conventional buck-boost switching power converter;

[0066] Figure 7 This is the block diagram of the hybrid boost-buck switching power converter proposed by Lin Cheng. DETAILED DESCRIPTION

[0067] The present invention will be further described below in conjunction with specific embodiments:

[0068] like Figure 1 As shown, a seamless transition hybrid buck-boost DC-DC converter described in this embodiment includes NMOS switch devices S1, S2, S3, S4, S5, S6, and a flying capacitor C F1 , C F2 , inductance L, load capacitance C L ;

[0069] in,

[0070] The source of the NMOS switch device S1 is connected to one end of the inductor L and the flying capacitor C F1 The drain of NMOS switch device S1 is connected to the flying capacitor C F2 The upper plate and the drain of the NMOS switch device S2;

[0071] The source end of the NMOS switch device S2 is connected to the drain end of the NMOS switch device S5 and the load capacitor C L The upper plate is connected to the voltage output terminal;

[0072] The source of the NMOS switch device S3 is connected to the voltage input terminal and one end of the inductor L, and the drain of the NMOS switch device S3 is connected to the flying capacitor C F1 The lower plate and the drain of the NMOS switch device S4;

[0073] The source of the NMOS switch device S4 is connected to the flying capacitor C F2 The lower plate of the NMOS switch device S5, the source of the NMOS switch device S5 and the drain of the NMOS switch device S6;

[0074] The source of the NMOS switch device S6 is connected to the ground level.

[0075] The operation of this embodiment includes a buck mode, a transition mode, and a boost mode, and the transition mode is set between the buck mode and the boost mode; through the coordination of the transition mode, it is prevented that when the conversion ratio is very close to 1, the duty cycles in the buck mode and the boost mode of the hybrid buck-boost DC-DC converter are close to 100% and 0% respectively.

[0076] in,

[0077] In buck mode, the topology of the hybrid buck-boost DC-DC converter is as follows: Figure 2 The two working phases shown consist of:

[0078] Phase 1: The duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, and NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2 (V IN -V OUT ), flying capacitor C F1 Charging, at the same time the flying capacitor C F2 Supply power to the load until the flying capacitor C F2 The voltage across the terminals is discharged to the output voltage V OUT ; V IN is the input voltage;

[0079] Phase 2: The duty cycle is 1-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2V OUT -V IN ;

[0080] From the volt-second balance law of inductance L, we get:

[0081] D*2(V IN -V OUT )+(1-D)*(V IN -2V OUT )=0 (5)

[0082] The conversion ratio is obtained from the above formula:

[0083]

[0084] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current IL :

[0085]

[0086] I OUT is the load current.

[0087] In boost mode, the topology of the hybrid buck-boost DC-DC converter is as follows: Figure 3 The three working phases shown are composed of:

[0088] Phase 1: Duty cycle is D, NMOS switch devices S4 and S6 are turned on, and NMOS switch devices S1, S2, S3, and S5 are turned off; at this time, the inductor L is charged to 3V IN -2V OUT , while the flying capacitor C F1 Charge;

[0089] Phase 2: The duty cycle is 0.2, NMOS switch devices S2, S3, S4 are turned on, and NMOS switch devices S1, S5, S6 are turned off; at this time, the inductor L is discharged and the flying capacitor C F1 Continue to charge, at the same time the flying capacitor C F2 The voltage across the terminals is discharged to V OUT -V IN ; V IN is the input voltage, V OUT is the output voltage;

[0090] Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L continues to discharge to 2 (V IN -V OUT ), flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2(V IN -V OUT );

[0091] From the volt-second balance law of inductance L, we get:

[0092]

[0093] The conversion ratio is obtained from the above formula:

[0094]

[0095] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L :

[0096]

[0097] When the conversion ratio is very close to 1, the transition mode is used to prevent the duty cycle D from being too close to 100% or 0%. The working phase of the transition mode is as follows: Figure 4 shown.

[0098] Phase 1: Duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2V IN -V OUT , while the flying capacitor C F1 Charging, flying capacitor C F2 The voltage across the two ends is discharged to the output voltage V OUT ;

[0099] Phase 2: The duty cycle is 0.2, NMOS switch devices S1, S2, S3, S6 are turned on, and NMOS switch devices S4 and S5 are turned off; at this time, the inductor L continues to charge or discharge to V IN -V OUT , flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the terminals is discharged to V OUT -V IN ;

[0100] Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1 and S5 are turned on, and NMOS switch devices S2, S3, S4, and S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , while the flying capacitor C F2 Charge;

[0101] From the volt-second balance law of inductance L, we get:

[0102]

[0103] The conversion ratio is obtained from the above formula:

[0104]

[0105] By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L :

[0106]

[0107] In order to highlight the advantages of the solution of the present invention, the inductor current is compared with the structure proposed by CBBC and Lin Cheng. The comparison results of the inductor current are as follows: Figure 5 shown.

[0108] from Figure 5 It can be seen that compared with CBBC, the structure proposed by Lin Cheng can reduce the inductor current in both boost mode and buck mode, but the inductor current is constant only in boost mode and is equal to the load current. In contrast, the scheme of the present invention continues to reduce the inductor current, achieving a constant inductor current within the full lithium battery supply voltage range, and is half of the load current, which can effectively reduce the inductor DCR conduction loss and switch conduction loss, thereby improving efficiency. At the same time, the constant inductor current can also improve the speed of transient response.

[0109] The embodiments described above are only preferred embodiments of the present invention and are not intended to limit the scope of implementation of the present invention. Therefore, all changes made according to the shape and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A seamless transition hybrid buck-boost DC-DC converter, characterized in that: Including NMOS switch devices S1, S2, S3, S4, S5, S6, flying capacitor C F1 , C F2 , inductance L, load capacitance C L ; in, The source of the NMOS switch device S1 is connected to one end of the inductor L and the flying capacitor C F1 The drain of NMOS switch device S1 is connected to the flying capacitor C F2 The upper plate and the drain of the NMOS switch device S2; The source end of the NMOS switch device S2 is connected to the drain end of the NMOS switch device S5 and the load capacitor C L The upper plate is connected to the voltage output terminal; The source of the NMOS switch device S3 is connected to the voltage input terminal and one end of the inductor L, and the drain of the NMOS switch device S3 is connected to the flying capacitor C F1 The lower plate and the drain of the NMOS switch device S4; The source of the NMOS switch device S4 is connected to the flying capacitor C F2 The lower plate of the NMOS switch device S5, the source of the NMOS switch device S5 and the drain of the NMOS switch device S6; The source of the NMOS switch device S6 is connected to the ground level.

2. A seamless transition hybrid buck-boost DC-DC conversion method, characterized in that: The hybrid buck-boost DC-DC converter with seamless transition as claimed in claim 1 is implemented, comprising a buck mode, a transition mode, and a boost mode, wherein the transition mode is arranged between the buck mode and the boost mode; The transition mode is coordinated to prevent the duty cycles of the hybrid buck-boost DC-DC converter in the buck mode and the boost mode from being close to 100% and 0% respectively when the conversion ratio is very close to 1.

3. The seamless transition hybrid buck-boost DC-DC conversion method according to claim 2, characterized in that: In buck mode, the hybrid buck-boost DC-DC converter topology consists of two operating phases, including: Phase 1: The duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, and NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2 (V IN -V OUT ), flying capacitor C F1 Charging, at the same time the flying capacitor C F2 Supply power to the load until the flying capacitor C F2 The voltage across the terminals is discharged to the output voltage V OUT ; V IN is the input voltage; Phase 2: The duty cycle is 1-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2V OUT -V IN ; From the volt-second balance law of inductance L, we get: D*2(V IN -V OUT )+(1-D)*(V IN -2V OUT )=0 (5) The conversion ratio is obtained from the above formula: By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L : I OUT is the load current.

4. The seamless transition hybrid buck-boost DC-DC conversion method according to claim 2, characterized in that: In boost mode, the topology of the hybrid buck-boost DC-DC converter consists of three operating phases, including: Phase 1: Duty cycle is D, NMOS switch devices S4 and S6 are turned on, and NMOS switch devices S1, S2, S3, and S5 are turned off; at this time, the inductor L is charged to 3V IN -2V OUT , while the flying capacitor C F1 Charge; Phase 2: The duty cycle is 0.2, NMOS switch devices S2, S3, S4 are turned on, and NMOS switch devices S1, S5, S6 are turned off; at this time, the inductor L is discharged and the flying capacitor C F1 Continue to charge, at the same time the flying capacitor C F2 The voltage across the terminals is discharged to V OUT -V IN ; V IN is the input voltage, V OUT is the output voltage; Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1, S3, S5 are turned on, and NMOS switch devices S2, S4, S6 are turned off; at this time, the inductor L continues to discharge to 2 (V IN -V OUT ), flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the two ends is discharged to 2(V IN -V OUT ); From the volt-second balance law of inductance L, we get: The conversion ratio is obtained from the above formula: By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L : I OUT is the load current.

5. The seamless transition hybrid buck-boost DC-DC conversion method according to claim 2, characterized in that: Transition modes include: Phase 1: Duty cycle is D, NMOS switch devices S2, S4, S6 are turned on, NMOS switch devices S1, S3, S5 are turned off; at this time, the inductor L is charged to 2V IN -V OUT , while the flying capacitor C F1 Charging, flying capacitor C F2 The voltage across the two ends is discharged to the output voltage V OUT ; Phase 2: The duty cycle is 0.2, NMOS switch devices S1, S2, S3, S6 are turned on, and NMOS switch devices S4 and S5 are turned off; at this time, the inductor L continues to charge or discharge to V IN -V OUT , flying capacitor C F2 Charging, at the same time the flying capacitor C F1 The voltage across the terminals is discharged to V OUT -V IN ; Phase 3: The duty cycle is 0.8-D, NMOS switch devices S1 and S5 are turned on, and NMOS switch devices S2, S3, S4, and S6 are turned off; at this time, the inductor L is discharged to V IN -2V OUT , while the flying capacitor C F2 Charge; From the volt-second balance law of inductance L, we get: The conversion ratio is obtained from the above formula: By flying capacitor C F1 , C F2 and load capacitance C L The charge balance law gives the inductor current I L : I OUT is the load current.

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