CN109004851A - A kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter and method - Google Patents

Abstract

The invention discloses a space high-voltage, high-frequency, high-power interleaved three-level PFC converter and a method thereof, comprising: four high-power fast recovery diodes, an input filter capacitor C1, two boost inductors, and four power switch tubes, Three freewheeling diodes and two output capacitors; this circuit topology cleverly combines the interleaved parallel PFC technology with the three-level PFC technology, and realizes that the input current ripple ratio after the interleaved parallel connection is higher than that of the three-level PFC with a single branch. The current ripple is smaller, and the branch current is approximately half of the input current, thereby reducing the current stress of the switching device, and the voltage stress of each switching device is also reduced by half, which provides a powerful basis for the research of high-power PFC converters Technical support, while improving the zero-crossing distortion of the input current, minimize the voltage and current stress and inductance volume of the converter switching device.

Description

A space high-voltage high-frequency high-power interleaved three-level PFC converter and method

technical field

The invention belongs to the field of AC/DC conversion, and particularly designs a space high-voltage, high-frequency, high-power interleaved three-level PFC converter and a method.

Background technique

With the increasing difficulty of deep space exploration missions, the design requirements for spacecraft power systems are more stringent. Under the premise of ensuring the specified volume and weight of the power system, it is also necessary to make the power system achieve high reliability and high power density. In order to reduce the bus current and cable power loss of the power system, the distributed power system based on the AC bus architecture has gradually attracted people's attention.

In the high-voltage and high-frequency AC bus power system, there is inevitably large harmonic interference. A large number of harmonic currents will not only bring great difficulty to the design of the power system, but also affect the conversion efficiency of the power system and reduce the power supply. quality. The PFC (Power Factor Correction, power factor correction) converter realizes that the input current waveform strictly tracks the input voltage waveform through the control circuit, so as to achieve the purpose of suppressing the harmonic current and improving the power factor.

As the frequency increases, as shown in Figure 1, the traditional PFC circuit includes: four high-power fast recovery diodes D1, D2, D3, D4, input filter capacitor C1, two boost inductors L1, L2, two Power switch tubes S1, S2, freewheeling diodes D5, D6, output capacitor C3 and load R; high-power fast recovery diodes D1, D2, D3, D4 together form a rectifier bridge. In a traditional boost PFC circuit, distortion occurs when the input current crosses zero, and the higher the frequency, the more severe the distortion, which reduces the power factor of the system; at the same time, the input current ripple is serious, which leads to an increase in loss. With the increase of the power of the power system, the switching devices of the boost PFC converter must bear excessive voltage and current stress.

The interleaved parallel PFC technology can make it easier for the converter to work in the continuous conduction mode of the inductor current, which can effectively reduce the required rising slope of the input current, and can be used to improve the zero-crossing distortion of the high grid frequency input current. Although an inductance branch added by the interleaved parallel topology plays a role of shunting, the voltage stress borne by each branch switching device is still serious. In order to reduce the size of the inductor, the inductance value can be further reduced in theory, but the reduction of the inductance will inevitably increase the current ripple. Moreover, in practical applications, the inductance cannot be excessively reduced, otherwise it will affect the stability of the entire system.

Contents of the invention

The purpose of the present invention is to propose a space high-voltage high-frequency high-power interleaved three-level PFC converter and its method. The circuit topology cleverly combines the interleaved parallel PFC technology with the three-level PFC technology to realize the input after the interleaved parallel connection The current ripple is smaller than that of a three-level PFC with a single branch, and the branch current is approximately half of the input current, thereby reducing the current stress of the switching devices, and the voltage stress of each switching device is also reduced by half , provides strong technical support for the research of high-power PFC converter.

In order to achieve the above object, the present invention provides a space high-voltage high-frequency high-power interleaved three-level PFC converter, comprising:

A number of high-power fast-recovery diodes that jointly form a rectifier bridge connected to an AC power supply with high-voltage and high-frequency output;

an input filter capacitor connected in parallel with the rectifier bridge;

Four power switch tubes, the branch formed by the first power switch tube and the second power switch tube connected in series, and the branch formed by the third power switch tube and the fourth power switch tube connected in parallel;

Two boost inductors, the first side of the first boost inductor is connected to the output terminal of the rectifier bridge, the second side is connected to the first freewheeling diode and the third power switch tube; the first side of the second boost inductor is connected to the output terminal of the rectifier bridge The output terminal is connected to the input filter capacitor, and the second side is connected to the second freewheeling diode and the first power switch tube;

Two output capacitors, the branch formed by the first output capacitor and the second output capacitor in series is connected in parallel with the output load; the first side of the first output capacitor is connected to the first freewheeling diode and the second freewheeling diode, and the second side It is connected with the third power switch tube and the fourth power switch tube; the first side of the second output capacitor is connected with the first power switch tube and the second power switch tube, and the second side is connected with the third freewheeling diode; The third freewheeling diode is also connected to the fourth power switch tube.

Preferably, the cathode of the first freewheeling diode is connected to the first side of the first output capacitor, the anode of the first freewheeling diode is connected to the second side of the first boost inductor; the cathode of the second freewheeling diode is connected to the first side of the first output capacitor connected, and the anode of the second freewheeling diode is connected to the second side of the second boost inductor.

Preferably, the source of the first power switch tube is connected to the drain of the second power switch tube, the drain of the first power switch tube is connected to the second side of the second boost inductor, and the source of the second power switch tube is connected to the common ground; The source of the third power switch tube is connected to the drain of the fourth power switch tube, the drain of the third power switch tube is connected to the second side of the first boost inductor, and the source of the fourth power switch tube is connected to the common ground.

Preferably, the second side of the first output capacitor is connected to the source of the third power switch tube and the drain of the fourth power switch tube; the first side of the second output capacitor is connected to the source of the first power switch tube and the drain of the second power switch tube. The second side of the second output capacitor is connected to the anode of the third freewheeling diode, and the cathode of the third freewheeling diode is connected to the source of the fourth power switch tube.

Preferably, the several high-power fast recovery diodes are set as four high-power fast recovery diodes; the branch formed by the first high-power fast recovery diode and the third high-power fast recovery diode in series, and the second high-power fast recovery diode The branches formed by series connection of the diode and the fourth largest power fast recovery diode are connected in parallel with each other.

Preferably, the first boost inductor and the second boost inductor have the same magnitude.

Preferably, the capacity of the first output capacitor and the second output capacitor are equal; the voltage of the first output capacitor and the second output capacitor are both half of the output voltage.

Preferably, the phase of the drive signal of the third power switch tube lags behind the drive signal of the first power switch tube by T/4, and the phase of the drive signal of the second power switch tube lags behind the drive signal of the third power switch tube signal T/4, the phase of the drive signal of the fourth power switch tube lags behind the drive signal of the second power switch tube by T/4, wherein T represents a switching period.

The present invention also provides a control method using a space high-voltage high-frequency high-power interleaved three-level PFC converter as described above. The control method includes:

When the duty cycle is ≥0.5, the working mode of the converter includes the following process:

In the first time period: the third power switch tube is turned off, the first power switch tube, the second power switch tube, and the fourth power switch tube are all closed, and the rectified current flows through the second boost inductor, the first power switch tube The switch tube and the second power switch tube charge the first boost inductor, and the current of the first boost inductor rises; the current flows through the second boost inductor, the first freewheeling diode, the first output capacitor, and the second output capacitor at the same time and load, the second boost inductor current drops;

In the second time period: the third power switch is turned on, the second power switch is turned off, the current in the first boost inductor branch charges the second output capacitor, the current of the first boost inductor drops, and the second boost The inductor current rises;

In the third time period: the second power switch tube is closed, the fourth power switch tube is turned off, the working condition of the second boost inductor branch is the same as that of the first time period, and the current of the first boost inductor branch has an effect on the first boost inductor branch. The second output capacitor is charged, the current of the first boost inductor rises, and the current of the second boost inductor decreases;

In the fourth time period: the fourth power switch is turned on, the first power switch is turned off, the current in the second boost inductor branch charges the first output capacitor, and the working condition of the first boost inductor branch is the same as described above. In the second time period, the current of the first boost inductor drops, and the current of the second boost inductor rises.

Preferably, the control method of the space high-voltage high-frequency high-power interleaved three-level PFC converter further includes:

When the duty cycle is <0.5, the working mode of the converter includes the following process:

In the first period of time: only the first power switch tube is turned on, the second power switch tube, the third power switch tube, and the fourth power switch tube are all turned off, and the current of the second boost inductor branch has an effect on the second output Capacitor charging; the voltage of the second boost inductor V _L2 =V _in -1/2V _o , the current of the second boost inductor rises; the voltage of the first boost inductor V _L1 =V _in -V _o , the first boost inductor The current drops; among them, V _in is the rectified voltage, V _o is the output DC voltage;

In the second time period: only the third power switch is turned on, the voltage of the second boost inductor is V _L2 =V _in -V _o , the current of the second boost inductor drops; the branch current of the first boost inductor The second output capacitor is charged, the voltage of the first boost inductor is V _L1 =V _in -1/2V _o , and the current of the first boost inductor rises;

In the third time period: only the second power switch is turned on, the current of the second boost inductor branch charges the first output capacitor, and the working condition of the first boost inductor branch is the same as the first time period; The current of the first boost inductor decreases, and the current of the second boost inductor rises;

In the fourth time period: only the fourth power switch is turned on, the working condition of the second boost inductor branch is the same as the second time period, the first boost inductor branch current charges the first output capacitor, and the second boost inductor branch current charges the first output capacitor. The current of the first boost inductor increases, and the current of the second boost inductor decreases.

Compared with the prior art, the beneficial effect of the present invention is: the circuit topology of the space high-voltage high-frequency high-power interleaved three-level PFC converter of the present invention skillfully combines the interleaved parallel PFC technology and the three-level PFC technology, The input current ripple after interleaved parallel connection is smaller than the current ripple of a single branch three-level PFC, and the branch current is approximately half of the input current, thereby reducing the current stress of the switching device, and each switching device The voltage stress is also reduced by half, which provides a strong technical support for the research of high-power PFC converters. The invention improves the zero-crossing distortion of the input current and at the same time reduces the voltage and current stress and the inductance volume of the switch device of the converter as much as possible.

Description of drawings

Fig. 1 structure diagram of interleaved parallel PFC converter in the prior art;

Fig. 2 structure diagram of space high voltage high frequency high power interleaved three-level PFC converter of the present invention;

Fig. 3 interleaved three-level PFC waveform when the duty ratio D≥0.5 in the present invention;

Fig. 4 is an interleaved three-level PFC waveform when the duty cycle D<0.5 in the present invention.

Detailed ways

The present invention provides a space high-voltage, high-frequency, high-power interleaved three-level PFC converter and method. In order to make the present invention more obvious and understandable, the present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods.

As shown in Figure 2, the space high-voltage high-frequency high-power interleaved three-level PFC converter of the present invention includes: four high-power fast recovery diodes D1, D2, D3, D4, input filter capacitor C1, two boost inductors L1, L2, four power switch tubes S1, S2, S3, S4, freewheeling diodes D5, D6, D7 and two output capacitors C2, C3.

The high-power fast recovery diodes D1, D2, D3, and D4 together form a rectifier bridge. The high-power fast-recovery diode D1 and the high-power fast-recovery diode D3 are connected in series to form a branch, and the high-power fast-recovery diode D2 and the high-power fast-recovery diode D4 The branches formed in series are connected in parallel with each other. A rectifier bridge composed of high-power fast recovery diodes D1, D2, D3, and D4 is connected to the high-voltage and high-frequency output AC power supply AC, and the rectifier bridge is connected in parallel with the input filter capacitor C1.

Boost inductors L1 and L2 have the same magnitude.

The branch circuit formed by the series connection of the output capacitor C2 and the output capacitor C3 is connected in parallel with the output load R, the output capacitors C2 and C3 are large-capacity capacitors, the capacity of the output capacitors C2 and C3 is equal, and the voltage is half of the output voltage.

The phase of the drive signal of the power switch tube S3 lags behind the phase of the drive signal of the power switch tube S1 by T/4, the phase of the drive signal of the power switch tube S2 lags behind the phase of the drive signal of the power switch tube S3 by T/4, and the phase of the drive signal of the power switch tube S4 lags behind the power The driving signal of the switching tube S2 is T/4, where T represents a switching cycle.

One side of the inductor L1 is connected to the output terminal of the rectifier bridge, and the other side is connected to the anode of the freewheeling diode D5 and the drain of the power switch tube S3. One side of the inductor L2 is connected to the output terminal of the rectifier bridge and the input filter capacitor C1, and the other side is connected to the anode of the freewheeling diode D6 and the drain of the power switch tube S1.

The branch formed by the series connection of the power switch S1 and the power switch S2 is connected in parallel with the branch formed by the series connection of the power switch S3 and the power S4.

Wherein, the source of the power switch S1 is connected to the drain of the power switch S2, and the source of the power switch S2 is connected to the common ground. The source of the power switch S3 is connected to the drain of the power switch S4, and the source of the power switch S4 is connected to the common ground. The branch circuit formed by the output capacitor C2 and the output capacitor C3 in series is connected in parallel with the output load R. The cathodes of the freewheeling diode D5 and the freewheeling diode D6 are both connected to one side of the output capacitor C2, and the other side of the output capacitor C2 is connected to the source of the power switch S3 and the drain of the power switch S4. One side of the output capacitor C3 is connected to the source of the switching tube S1 and the drain of the switching tube S2, the other side of the output capacitor C3 is connected to the anode of the freewheeling diode D7, and the cathode of the freewheeling diode D7 is connected to the source of the power switching tube S4 connected.

The freewheeling diode D7 of the present invention is used for freewheeling when the power switch tube S3 and the power switch tube S4 are disconnected, and charges the output capacitors C2 and C3.

As shown in Figure 3, when the duty ratio D≥0.5, the converter works as follows:

(1) Section t ₀ -t ₁ : the power switch tube S3 is turned off, and the power switch tubes S1, S2, and S4 are all closed. The rectified current flows through the inductor L2, the power switch tube S1, and the power switch tube S2 to charge the inductor L1, and the current of the inductor L1 rises. In addition, the current flows through the inductor L2, the freewheeling diode D5, the output capacitor C2, the output capacitor C3 and the load R at the same time, and the current of the inductor L2 drops.

(2) Stage t ₁ -t ₂ : the power switch S3 is closed, the power switch S2 is turned off, the branch current of the inductor L1 charges the output capacitor C3, the current of the inductor L1 drops, and the current of the inductor L2 rises.

(3) t ₂ -t ₃ section: the power switch S2 is closed, the power switch S4 is turned off, the working condition of the inductor L2 branch is the same as the t ₀ -t ₁ section, the current of the inductor L1 branch charges the output capacitor C3, and the current drops The amount is the same as that of t ₀ -t ₁ .

(4) t ₃ -t ₄ section: power switch S4 is closed, power switch S1 is turned off, the current in the inductor L2 branch charges the output capacitor C2, the working condition of the inductor L1 branch is the same as the t ₁ -t ₂ section, and the current drops The amount is the same as that of t ₁ -t ₂ .

As shown in Figure 4, when the duty cycle D<0.5, the converter works as follows:

(1) Stage t ₀ -t ₁ : only the power switch S1 is turned on, and the power switches S2, S3, and S4 are all turned off, and the branch current of the inductor L2 charges the output capacitor C3; the voltage of the inductor L2 V _L2 =V _in - 1/2V _o , the current of the inductor L2 increases; the voltage of the inductor L1 V _L1 =V _in −V _o , the current of the inductor L1 decreases. Among them, V _in is the voltage after rectification, and V _o is the output DC voltage.

(2) t ₁ -t ₂ section: only the switch tube S3 is turned on, the voltage of the inductor L2 V _L2 =V _in -V _o , the current of the inductor L2 drops; the branch current of the inductor L1 charges the output capacitor C3, and the voltage of the inductor L1 V _L1 =V _in -1/2V _o , the current of the inductor L1 rises.

(3) Section t ₂ -t ₃ : Only the power switch tube S2 is turned on, the current of the inductor L2 branch charges the output capacitor C2, and the current rise of the inductor L2 is the same as that of the t ₀ -t ₁ section; the working condition of the inductor L1 branch is the same as t ₀ -t ₁ segment.

(4) Section t ₃ -t ₄ : only the power switch tube S4 is turned on, the working condition of the inductor L2 branch is the same as that of the t ₁ -t ₂ section; the current of the inductor L1 branch charges the output capacitor C2, and the current rise of the inductor L1 is the same as t ₁ -t ₂ paragraphs.

In summary, the circuit topology of the space high-voltage, high-frequency, high-power interleaved three-level PFC converter of the present invention cleverly combines the interleaved parallel PFC technology with the three-level PFC technology, and realizes the input current ripple after the interleaved parallel connection. The wave is smaller than the current ripple of the three-level PFC of a single branch, and the branch current is approximately half of the input current, thereby reducing the current stress of the switching device, and the voltage stress of each switching device is also reduced by half, The research on high-power PFC converters provides strong technical support, while improving the zero-crossing distortion of the input current, it also minimizes the voltage and current stress and inductance volume of the converter switching devices.

Although the content of the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description should not be considered as limiting the present invention. Various modifications and alterations to the present invention will become apparent to those skilled in the art upon reading the above disclosure. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims (10)

1. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter, characterized by comprising:

Collectively constitute several high-power fast recovery diodes of rectifier bridge, the alternating current of the rectifier bridge and high voltagehigh frequency output Source (AC) is connected；

Input filter capacitor (C1), it is in parallel with the rectifier bridge；

The branch that four power switch tubes, the first power switch tube (S1) and the second power switch tube (S2) series connection are formed, with the The branch that three power switch tubes (S3) and the series connection of the 4th power switch tube (S4) are formed is parallel with one another；

Two boost inductances, the first side of the first boost inductance (L1) are connect with rectification bridge output end, second side and the first afterflow two Pole pipe (D5), third power switch tube (S3) connection；The output end of the first side of second boost inductance (L2) and rectifier bridge, input filter Wave capacitor (C1) connection, second side are connect with the second freewheeling diode (D6), the first power switch tube (S1)；

Two output capacitances, the first output capacitance (C2) connect with the second output capacitance (C3) formed branch and output loading (R) in parallel；First output capacitance (C2), first side and the first freewheeling diode (D5), the second freewheeling diode (D6) are even It connects, second side is connect with third power switch tube (S3), the 4th power switch tube (S4)；Second output capacitance (C3) first Side is connect with the first power switch tube (S1), the second power switch tube (S2), and second side is connect with third freewheeling diode (D7)； The third freewheeling diode (D7) also connect with the 4th power switch tube (S4).

2. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as described in claim 1, feature exist In,

First freewheeling diode (D5) cathode is connect with the first side of the first output capacitance (C2), the first freewheeling diode (D5) anode It is connect with the first boost inductance (L1) second side；

Second freewheeling diode (D6) cathode is connect with the first side of the first output capacitance (C2), the second freewheeling diode (D6) anode It is connect with the second boost inductance (L2) second side.

3. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as claimed in claim 2, feature exist In,

First power switch tube (S1) source electrode is connected with the second power switch tube (S2) drain electrode, the first power switch tube (S1) drain electrode Connect with the second boost inductance (L2) second side, the second power switch tube (S2) source electrode with publicly connect；

Third power switch tube (S3) source electrode is connected with the drain electrode of the 4th power switch tube (S4), third power switch tube (S3) drain electrode Connect with described first boost inductance (L1) second side, the 4th power switch tube (S4) source electrode with publicly connect.

4. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as claimed in claim 1 or 3, feature It is,

First output capacitance (C2) second side connects with third power switch tube (S3) source electrode, the drain electrode of the 4th power switch tube (S4) It connects；

The first side of second output capacitance (C3) connects with the first power switch tube (S1) source electrode, the second power switch tube (S2) drain electrode It connects, the second output capacitance (C3) second side is connected with third freewheeling diode (D7) anode, third freewheeling diode (D7) cathode It is connect with the 4th power switch tube (S4) source electrode.

5. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as described in claim 1, feature exist In,

Several described high-power fast recovery diodes are set as four high-power fast recovery diodes；

The branch that first high-power fast recovery diode (D1) and the high-power fast recovery diode of third (D3) series connection are formed, Connect with the second high-power fast recovery diode (D2) and the fourth-largest power fast recovery diode (D4) formed branch phase It is mutually in parallel.

6. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as described in claims 1 or 2 or 3, special Sign is,

First boost inductance (L1) is identical with the second boost inductance (L2) magnitude.

7. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as described in claim 1, feature exist In,

First output capacitance (C2) is equal with the capacity of second output capacitance (C3)；

First output capacitance (C2) and the second output capacitance (C3) voltage are the half of output voltage.

8. a kind of space high-voltage high-frequency high-power alternating expression three-level PFC converter as claimed in claim 1 or 3, feature It is,

The driving signal of first power switch tube (S1) described in the driving signal delayed phase of the third power switch tube (S3) T/4, the driving signal of third power switch tube (S3) described in the driving signal delayed phase of second power switch tube (S2) T/4, the driving signal of the second power switch tube (S2) described in the driving signal delayed phase of the 4th power switch tube (S4) T/4, wherein T represents a switch periods.

9. a kind of a kind of three level of space high-voltage high-frequency high-power alternating expression using as described in claim 1-8 any one The control method of pfc converter, which is characterized in that the control method includes:

As duty ratio (D) >=0.5, which includes following procedure:

In first time period (t₀-t₁) when: third power switch tube (S3) shutdown, the first power switch tube (S1), the second power are opened Pass pipe (S2), the 4th power switch tube (S4) are closed, and the electric current after rectification flows through the second boost inductance (L2), the first power is opened Pipe (S1), the second power switch tube (S2) are closed, is charged for the first boost inductance (L1), the first boost inductance (L1) electric current rises； Electric current simultaneously flows through the second boost inductance (L2), the first freewheeling diode (D5), the first output capacitance (C2), the second output capacitance (C3) decline with load (R), the second boost inductance (L2) electric current；

In second time period (t₁-t₂) when: enable third power switch tube (S3) to be closed, the second power switch tube (S2) shutdown, first Boost inductance (L1) branch current charges to the second output capacitance (C3), the decline of the first boost inductance (L1) electric current, the second boosting Inductance (L2) electric current rises；

In third period (t₂-t₃) when: enable the second power switch tube (S2) to be closed, the shutdown of the 4th power switch tube (S4), second Boost inductance (L2) branch working condition is with the first time period, and the first boost inductance (L1) branch current is to the second output electricity Hold (C3) charging, the first boost inductance (L1) electric current rises, the decline of the second boost inductance (L2) electric current；

In the 4th period (t₃-t₄) when: enable the 4th power switch tube (S4) be closed, the first power switch tube (S1) shutdown, second Boost inductance (L2) branch current charges to the first output capacitance (C2), and the first boost inductance (L1) branch working condition is the same as described Second time period, the decline of the first boost inductance (L1) electric current, the second boost inductance (L2) electric current rise.

10. the control method of space high-voltage high-frequency high-power alternating expression three-level PFC converter as claimed in claim 9, It is characterized in that, which further includes:

As duty ratio (D) < 0.5, which includes following procedure:

In first time period (t₀-t₁) when: only the first power switch tube (S1) conducting, the second power switch tube (S2), third function Rate switching tube (S3), the 4th power switch tube (S4) are turned off, and the second boost inductance (L2) branch current is to the second output capacitance (C3) it charges；The voltage V of second boost inductance (L2)_L2=V_in-1/2V_o, the rising of the second boost inductance (L2) electric current；First boosting The voltage V of inductance (L1)_L1=V_in-V_o, the decline of the first boost inductance (L1) electric current；Wherein, V_inFor rectified voltage, V_oFor output DC voltage；

In second time period (t₁-t₂) when: only third power switch tube (S3) conducting, the voltage V of the second boost inductance (L2)_L2 =V_in-V_o, the decline of the second boost inductance (L2) electric current；First boost inductance (L1) branch current fills the second output capacitance (C3) Electricity, the voltage V of the first boost inductance (L1)_L1=V_in-1/2V_o, the rising of the first boost inductance (L1) electric current；

In third period (t₂-t₃) when: only the second power switch tube (S2) conducting, the second boost inductance (L2) branch current It charges to the first output capacitance (C2), the first boost inductance (L1) branch working condition is the same as the first time period；First boosting The decline of inductance (L1) electric current, the second boost inductance (L2) electric current rise；

In the 4th period (t₃-t₄) when: the conducting of only the 4th power switch tube (S4), the work of the second boost inductance (L2) branch Situation charges to the first output capacitance (C2) with the second time period, the first boost inductance (L1) branch current, the first boosting Inductance (L1) electric current rises, the decline of the second boost inductance (L2) electric current.