CN102684487B - BJT type self-excited Sepic converter equipped with main switching tube with low drive loss - Google Patents

Abstract

The BJT type self-excited Sepic converter with small driving loss of the main switching tube includes the Sepic converter composed of input capacitor Ci, inductor L1, NPN BJT tube Q1, capacitor C, inductor L2, diode D1, diode D and capacitor Co The main circuit also includes a drive unit for the main switching transistor Q1. The driving unit of the main switching tube Q1 is composed of a resistor R1, a resistor R2, a voltage regulator tube Z1 and a PNP type BJT tube Q2, the emitter of the PNP type BJT tube Q2 is connected to one end of the resistor R1, and the other end of the resistor R1 It is connected to one end of the inductor L1 and the positive end of the DC voltage source Vi, the base of the PNP type BJT transistor Q2 is connected to the anode of the voltage regulator tube Z1 and one end of the resistor R2, and the cathode of the voltage regulator tube Z1 is connected to the other end of the inductor L1 and One end of the capacitor C is connected, the other end of the resistor R2 is connected to the negative end of the DC voltage source Vi, and the collector of the PNP BJT transistor Q2 is connected to the base of the NPN BJT transistor Q1. In order to improve the dynamic performance of the circuit, a capacitor C1 can be connected in parallel between the positive terminal of the DC voltage source Vi and the base of the PNP type BJT transistor Q2. The circuit structure of the invention is simple, the number of components is small, the driving loss of the main switching tube is small, and the circuit efficiency is high at light load.

Description

BJT type self-excited Sepic converter with small driving loss of main switching tube

technical field

The invention relates to a self-excited direct current-direct current (DC-DC) converter, which is applied to a switching or constant-current power supply, a high-brightness LED drive circuit, etc., in particular to a self-excited Sepic converter.

Background technique

Compared with linear (stabilized voltage or steady current) regulators and other excited DC-DC converters, self-excited DC-DC converters have significant advantages of high cost performance. Figure 1 shows a BJT (bipolar transistor) type self-excited Sepic converter with simple circuit structure and few components, including input capacitor Ci, inductor L1, diode D1, NPN type BJT tube Q1, The main circuit of the Sepic converter composed of capacitor C, inductor L2, diode D and output capacitor Co, the input capacitor Ci is connected in parallel with the DC voltage source Vi, the voltage across the output capacitor Co is the DC output voltage Vo, and the load Ro is connected in parallel with the output capacitor Co , the negative terminal of the DC voltage source Vi is connected to the negative terminal of the DC output voltage Vo and the emitter of the NPN BJT tube Q1, the positive terminal of the DC voltage source Vi is connected to one end of the inductor L1, and the other end of the inductor L1 is connected to the diode D1 The anode is connected, the cathode of the diode D1 is connected to the collector of the NPN type BJT transistor Q1 and one end of the capacitor C, the other end of the capacitor C is connected to the anode of the diode D and one end of the inductor L2, and the other end of the inductor L2 is connected to the DC voltage source Vi The negative end of the diode is connected, and the cathode of the diode D is connected to the positive end of the output voltage Vo.

The BJT type self-excited Sepic converter shown in Figure 1 also includes a drive unit of the main switch tube Q1, the drive unit of the main switch tube Q1 is composed of a resistor R1, a resistor R2, a resistor R3, a capacitor C1 and an NPN type BJT tube Q2 Composition, the collector and emitter of the NPN BJT Q2 are respectively connected to the base and emitter of the NPN BJT Q1, and the base of the NPN BJT Q1 is also connected to the positive terminal of the DC voltage source Vi through the resistor R1 , the resistor R3 and the capacitor C1 form a parallel branch, one end of the parallel branch is connected with one end of the inductor L1 and the anode of the diode D1, and the other end of the parallel branch is connected with the base of the NPN type BJT transistor Q2 and the resistor R2 One end of the resistor R2 is connected, and the other end of the resistor R2 is connected with the emitter of the NPN type BJT tube Q2. The BJT type self-excited Sepic converter shown in Figure 1 also includes a voltage feedback branch, the voltage feedback branch is composed of a resistor R4, a resistor R5, a voltage regulator tube Z1 and an NPN type BJT tube Q3, and the voltage regulator tube Z1 The cathode of the voltage regulator is connected to the positive terminal of the DC output voltage Vo, the anode of the regulator tube Z1 is connected to one end of the resistor R5 and the base of the NPN BJT tube Q3, and the collector of the NPN BJT tube Q3 is connected to the NPN BJT tube through the resistor R4 The base of Q1 is connected, the emitter of NPN BJT transistor Q3 and the other end of resistor R5 are connected to the negative end of DC voltage source Vi. The disadvantage of this circuit is that the drive unit of the main switching tube Q1 composed of driving resistor R1, NPN BJT tube Q2, resistor R2, resistor R3 and capacitor C1 still has a large current flow when the main switching tube Q1 is turned off. Overdriving the resistor R1 leads to a large driving loss of Q1, thereby affecting the efficiency of the circuit, especially the light-load efficiency of the circuit.

Contents of the invention

In order to overcome the deficiency of the existing BJT type self-excited Sepic converter with relatively large driving loss of the main switching tube, the present invention provides a BJT type self-excited Sepic converter with small driving loss of the main switching tube.

The technical solution adopted by the present invention to solve the technical problem is: a BJT type self-excited Sepic converter with small driving loss of the main switching tube includes an input capacitor Ci, an inductor L1, an NPN type BJT tube Q1, a capacitor C, and an inductor L2 , diode D1, diode D and capacitor Co constitute the main circuit of the Sepic converter, the input capacitor Ci is connected in parallel with the DC voltage source Vi, the voltage across the output capacitor Co is the DC output voltage Vo, the load Ro is connected in parallel with the output capacitor Co, and the DC voltage The positive end of the source Vi is connected to one end of the inductance L1, the other end of the inductance L1 is connected to the collector of the NPN BJT transistor Q1 and one end of the capacitor C, the emitter of the NPN BJT transistor Q1 is connected to the negative end of the DC voltage source Vi and The negative end of the DC output voltage Vo is connected, the other end of the capacitor C is connected to the anode of the diode D and one end of the inductor L2, the other end of the inductor L2 is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the negative end of the DC voltage source Vi Connected, the cathode of the diode D is connected to the positive end of the output voltage Vo;

The BJT type self-excited Sepic converter with small driving loss of the main switching tube also includes a driving unit of the main switching tube Q1, and the driving unit of the main switching tube Q1 is composed of a resistor R1, a resistor R2, a voltage regulator tube Z1 and a PNP type Composed of BJT tube Q2, the emitter of the PNP type BJT tube Q2 is connected to one end of the resistor R1, the other end of the resistor R1 is connected to one end of the inductor L1 and the positive end of the DC voltage source Vi, and the base of the PNP type BJT tube Q2 It is connected to the anode of the voltage regulator tube Z1 and one end of the resistor R2, the cathode of the voltage regulator tube Z1 is connected to the other end of the inductor L1 and one end of the capacitor C, and the other end of the resistor R2 is connected to the negative end of the DC voltage source Vi, PNP type The collector of the BJT transistor Q2 is connected to the base of the NPN type BJT transistor Q1. In order to improve the dynamic performance of the circuit, a capacitor C1 can be connected in parallel between the positive terminal of the DC voltage source Vi and the base of the PNP type BJT transistor Q2.

Further, as a preferred solution: the BJT type self-excited Sepic converter with small driving loss of the main switching tube also includes a voltage feedback branch, and the voltage feedback branch is composed of a resistor R3, a voltage regulator tube Z2 and an NPN type BJT Composed of tube Q3, the collector of the NPN BJT tube Q3 is connected to the collector of the PNP BJT tube Q2 and the base of the NPN BJT tube Q1, and the emitter of the NPN BJT tube Q3 is connected to the negative terminal of the DC voltage source Vi And one end of the resistor R3 is connected, the base of the NPN type BJT transistor Q3 is connected with the other end of the resistor R3 and the anode of the voltage regulator tube Z2, and the cathode of the voltage regulator tube Z2 is connected with the positive terminal of the DC output voltage Vo. In order to improve the dynamic performance of the circuit, a capacitor C2 can be connected in parallel at both ends of the regulator tube Z2.

Or, as another preferred solution: the BJT type self-excited Sepic converter with small driving loss of the main switching tube also includes a current feedback branch, and the current feedback branch is composed of a resistor R3, a resistor R4, a diode D2 and an NPN Type BJT tube Q3, the collector of the NPN type BJT tube Q3 is connected to the collector of the PNP type BJT tube Q2 and the base of the NPN type BJT tube Q1, and the emitter of the NPN type BJT tube Q3 is connected to the DC voltage source Vi The negative terminal, the emitter of the NPN BJT tube Q1, one end of the capacitor Co, and one end of the resistor R4 are connected, and the other end of the resistor R4 is connected to the negative terminal of the DC output voltage Vo and the cathode of the diode D2. The base of the NPN BJT tube Q3 The pole is connected to the anode of the diode D2 and one end of the resistor R3, and the other end of the resistor R3 is connected to the positive end of the DC voltage source Vi. In order to improve the dynamic performance of the circuit, a capacitor C2 can be connected in parallel to both ends of the resistor R3.

The technical idea of the present invention is: on the basis of the existing BJT type self-excited Sepic converter shown in Figure 1, replace the original main switch tube drive unit with small loss (as shown in Figure 2 and Figure 3). The main switching tube drive unit with low loss is composed of resistor R1, resistor R2, PNP type BJT tube Q2 and regulator tube Z1. Its features are as follows: the emitter of the PNP type BJT tube Q2 is connected to one end of the resistor R1, the other end of the resistor R1 is connected to one end of the inductor L1 and the positive end of the DC voltage source Vi, the base of the PNP type BJT tube Q2 is connected to the regulator The anode of the tube Z1 is connected to one end of the resistor R2, the cathode of the regulator tube Z1 is connected to the other end of the inductor L1 and one end of the capacitor C, the other end of the resistor R2 is connected to the negative end of the DC voltage source Vi, and the PNP type BJT tube Q2 The collector of the NPN type BJT tube Q1 is connected to the base. In order to improve the dynamic performance of the circuit, a capacitor C1 can be connected in parallel between the positive terminal of the DC voltage source Vi and the base of the PNP type BJT transistor Q2.

In order to obtain a stable DC output voltage, a voltage feedback branch can be added between the output end of the main circuit of the Sepic converter and the main switch tube drive unit, which is composed of NPN type BJT tube Q3, resistor R3 and voltage regulator tube Z2 (such as Figure 2). In order to improve the dynamic performance of the circuit, a capacitor C2 can be connected in parallel at both ends of the regulator tube Z2.

In order to obtain a stable DC output current, a current feedback branch can be added between the output end of the main circuit of the Sepic converter and the main switch tube drive unit, which is composed of NPN type BJT tube Q3, resistor R3, resistor R4 and diode D2, etc. (As shown in Figure 3). In order to improve the dynamic performance of the circuit, a capacitor C2 can be connected in parallel to both ends of the resistor R3.

The beneficial effects of the present invention are mainly manifested in: the BJT type self-excited Sepic converter proposed by the present invention not only has the advantages of simple circuit structure and fewer components, but also has the advantages of small driving loss of the main switch tube and high light-load efficiency , very suitable for low-power (less than a few watts) buck-boost switching or constant-current power supplies, high-brightness LED drive circuits and other applications.

Description of drawings

Fig. 1 is a circuit diagram of an existing BJT type self-excited Sepic converter.

FIG. 2 is a circuit diagram of Embodiment 1 of a BJT type self-excited Sepic converter with low driving loss of the main switching tube.

Fig. 3 is a circuit diagram of Embodiment 2 of a BJT type self-excited Sepic converter with small driving loss of the main switching tube.

FIG. 4 is an ideal waveform diagram of the embodiment 1 of the BJT type self-excited Sepic converter with small driving loss of the main switching tube under the critical continuous operation mode of the inductor current iL1 and the intermittent operation mode of the inductor current iL2.

FIG. 5 is an ideal waveform diagram of the BJT type self-excited Sepic converter embodiment 2 with low driving loss of the main switch tube under the critical continuous operation mode of the inductor current iL1 and the intermittent operation mode of the inductor current iL2.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

Example 1

Referring to Fig. 2 and Fig. 4, a BJT type self-excited Sepic converter with low driving loss of the main switching tube includes input capacitor Ci, inductor L1, NPN type BJT tube Q1, capacitor C, inductor L2, diode D1, diode D The main circuit of the Sepic converter composed of capacitor Co, the input capacitor Ci is connected in parallel with the DC voltage source Vi, the voltage across the output capacitor Co is the DC output voltage Vo, the load Ro is connected in parallel with the output capacitor Co, the positive terminal of the DC voltage source Vi is connected to One end of the inductor L1 is connected, the other end of the inductor L1 is connected to the collector of the NPN type BJT tube Q1 and one end of the capacitor C, the emitter of the NPN type BJT tube Q1 is connected to the negative end of the DC voltage source Vi and the negative end of the DC output voltage Vo The other end of the capacitor C is connected to the anode of the diode D and one end of the inductor L2, the other end of the inductor L2 is connected to the cathode of the diode D1, the anode of the diode D1 is connected to the negative end of the DC voltage source Vi, and the cathode of the diode D It is connected with the positive end of the output voltage Vo.

The BJT type self-excited Sepic converter with small driving loss of the main switching tube also includes a driving unit of the main switching tube Q1, and the driving unit of the main switching tube Q1 is composed of a resistor R1, a resistor R2, a capacitor C1, and a voltage regulator tube Z1 Composed of a PNP type BJT tube Q2, the emitter of the PNP type BJT tube Q2 is connected to one end of the resistor R1, the other end of the resistor R1 is connected to one end of the inductor L1 and the positive end of the DC voltage source Vi, and the PNP type BJT tube Q2 The base of the voltage regulator tube Z1 is connected to the anode of the voltage regulator tube Z1 and one end of the resistor R2, the cathode of the voltage regulator tube Z1 is connected to the other end of the inductor L1 and one end of the capacitor C, and the other end of the resistor R2 is connected to the negative end of the DC voltage source Vi , the collector of the PNP-type BJT transistor Q2 is connected to the base of the NPN-type BJT transistor Q1. In order to improve the dynamic performance of the circuit, a capacitor C1 is connected in parallel between the positive end of the DC voltage source Vi and the base of the PNP BJT transistor Q2.

Fig. 2 is a circuit diagram of Embodiment 1 of the BJT type self-excited Sepic converter with small driving loss of the main switching tube, which adopts a voltage feedback branch. The voltage feedback branch includes an NPN type BJT tube Q3, a resistor R3, a voltage regulator tube Z2 and a capacitor C2, the collector of the NPN type BJT tube Q3, the collector of the PNP type BJT tube Q2, and the base of the NPN type BJT tube Q1 The emitter of the NPN BJT tube Q3 is connected to the negative terminal of the DC voltage source Vi and one end of the resistor R3, the base of the NPN BJT tube Q3 is connected to the other end of the resistor R3 and the anode of the voltage regulator tube Z2, and the voltage regulator The cathode of the tube Z2 is connected to the positive end of the DC output voltage Vo. In order to improve the dynamic performance of the circuit, a capacitor C2 is connected in parallel at both ends of the regulator tube Z2.

FIG. 4 is an ideal waveform diagram of the embodiment 1 of the BJT type self-excited Sepic converter with small driving loss of the main switching tube under the critical continuous operation mode of the inductor current iL1 and the intermittent operation mode of the inductor current iL2. The working principle of the circuit is as follows:

(1) Circuit power-on start-up stage: at time t=t0, the circuit is powered on, and the DC voltage source Vi (vi) starts to rise from 0. At the beginning, Q1, Q2, Q3, D and D1 are all cut off, the collector voltage vc1 of Q1 changes with vi, and the DC output voltage Vo (vo) is 0. At the moment t=t1, that is, when the DC voltage source vi rises to the forward conduction voltage drop of the diode D, D is turned on, and vi charges C and Co through L1 and D, and the capacitor voltage vc and the DC output voltage vo both start to rise. At t=t2, that is, when the DC voltage source vi rises to a certain value, Q2 is turned on, Z1 is forward-conducting, and Q1 is turned on. At time t=t3, that is, when vc1 rises to a certain value, Q2 is turned off, Z1 is turned off, and Q1 is turned off. After Q1 is turned off, vc1 continues to rise. Subsequently, Z1 conducts in reverse, because vc1 is greater than the sum of vi, the emitter-base conduction voltage drop of Q2, and the reverse conduction voltage drop of Z1, Q2 and Q1 are still turned off. At t=t4, the inductor current iL1 drops to 0, and D cuts off. After D cuts off, vc1 drops, and Q2 and Q1 are turned on. After Q1 is turned on, vi, L1, Q1 form a loop, C, Q1, D1, L2 form another loop, L1 and L2 are charged, and iL1 and iL2 increase. At this point, Z1 is conducting forward. As iL1 increases, Q1's collector current ic1 and collector voltage vc1 also increase. At time t=t5, that is, when the collector voltage vc1 of Q1 rises to a certain value, Q2 is turned off, Z1 is turned off, and Q1 is turned off. After Q1 is turned off, D is turned on, vi, L1, C, D, Co and Ro form a loop, D1, L2, D, Co and Ro form another loop, L1 and L2 discharge, and iL1 and iL2 decrease. At this time, Z1 conducts reversely. At t=t6, the inductor current iL2 drops to 0, and D1 ends. At t=t7, the inductor current iL1 drops to 0, and D cuts off. After D cuts off, vc1 drops again, Q2 and Q1 are turned on, and the circuit enters the next self-excited working cycle. After several cycles, when the output voltage of the circuit reaches the set value Vo, the circuit completes the power-on start-up process and enters the steady-state working stage.

(2) Circuit steady-state working stage: When the output voltage of the circuit reaches the set value Vo, the voltage feedback branch of the circuit starts to work. When the output voltage is higher than the set value Vo, Q3 turns on, causing Q1 to turn off. By shortening the turn-on time of Q1 (ie t9-t8) and prolonging the off-time of Q1 (ie t11-t9), the reduction of the output voltage is realized. When the output voltage is lower than the set value Vo, Q3 is turned off, and the turn-on and turn-off times of Q1 are restored to the original state to realize the increase of the output voltage. Thus, the circuit can achieve output regulation. In Figure 4, at time t=t10, iL2 drops to 0 and D1 ends.

Example 2

Referring to Fig. 3 and Fig. 5, this embodiment includes the main circuit of the Sepic converter composed of input capacitor Ci, inductor L1, NPN type BJT tube Q1, capacitor C, inductor L2, diode D1, diode D and capacitor Co, and consists of resistors The driving unit of the main switching transistor Q1 composed of R1, resistor R2, capacitor C1, voltage regulator transistor Z1 and PNP type BJT transistor Q2 also includes a current feedback branch. The current feedback branch includes NPN type BJT tube Q3, resistor R3, resistor R4, diode D2 and capacitor C2, the collector of NPN type BJT tube Q3, the collector of PNP type BJT tube Q2 and the base of NPN type BJT tube Q1 The emitter of the NPN BJT tube Q3 is connected to the negative terminal of the DC voltage source Vi, the emitter of the NPN BJT tube Q1, one end of the capacitor Co, and one end of the resistor R4, and the other end of the resistor R4 is connected to the DC output voltage Vo The negative end of the diode D2 is connected to the cathode, the base of the NPN BJT transistor Q3 is connected to the anode of the diode D2 and one end of the resistor R3, and the other end of the resistor R3 is connected to the positive end of the DC voltage source Vi. In order to improve the dynamic performance of the circuit, a capacitor C2 is connected in parallel with both ends of the resistor R3.

Other circuit structures of this embodiment are the same as those of Embodiment 1.

FIG. 5 is an ideal waveform diagram of the BJT type self-excited Sepic converter embodiment 2 with low driving loss of the main switch tube under the critical continuous operation mode of the inductor current iL1 and the intermittent operation mode of the inductor current iL2. The working principle of the circuit is as follows:

(1) Circuit power-on start-up stage: same as embodiment 1, after several cycles, when the output current of the circuit reaches the set value Io, the circuit completes the power-on start-up process and enters the steady-state working stage.

(2) Circuit steady-state working stage: When the output current of the circuit reaches the set value Io, the current feedback branch of the circuit starts to work. When the output current is higher than the set value Io, Q3 turns on, causing Q1 to turn off. By shortening the turn-on time of Q1 (ie t9-t8) and prolonging the off-time of Q1 (ie t11-t9), the output current is reduced. When the output current is lower than the set value Io, Q3 is turned off, and the turn-on and turn-off time of Q1 is restored to the original state to realize the increase of the output current. Thus, the circuit can realize output steady current. In Figure 5, at time t=t10, iL2 drops to 0 and D1 ends.

The content described in the embodiments of this specification is only an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. The protection scope of the present invention also extends to the field Equivalent technical means that the skilled person can think of based on the concept of the present invention.

Claims (6)

1. a main switch drives the little BJT type self-excitation Sepic converter of loss, comprise by input capacitance Ci, inductance L 1, NPN type BJT manages Q1, capacitor C, inductance L 2, diode D1, the major loop of the Sepic converter of diode D and capacitor C o composition, input capacitance Ci is in parallel with direct voltage source Vi, output capacitance Co both end voltage is VD Vo, load Ro is in parallel with output capacitance Co, the anode of direct voltage source Vi is connected with one end of inductance L 1, the other end of inductance L 1 is connected with the NPN type BJT pipe collector electrode of Q1 and one end of capacitor C, the negative terminal of the NPN type BJT pipe emitter of Q1 and the negative terminal of direct voltage source Vi and VD Vo is connected, the other end of capacitor C is connected with the anode of diode D and one end of inductance L 2, the other end of inductance L 2 is connected with the negative electrode of diode D1, the anode of diode D1 is connected with the negative terminal of direct voltage source Vi, the negative electrode of diode D is connected with the anode of output voltage V o, it is characterized in that: described main switch drives the little BJT type self-excitation Sepic converter of loss also to comprise the driver element of NPN type BJT pipe Q1, the driver element of described NPN type BJT pipe Q1 is by resistance R 1, resistance R 2, voltage-stabiliser tube Z1 and positive-negative-positive BJT pipe Q2 composition, the emitter of described positive-negative-positive BJT pipe Q2 is connected with one end of resistance R 1, the other end of resistance R 1 is connected with one end of inductance L 1 and the anode of direct voltage source Vi, one end of the positive-negative-positive BJT pipe base stage of Q2 and the anode of voltage-stabiliser tube Z1 and resistance R 2 is connected, the negative electrode of voltage-stabiliser tube Z1 is connected with the other end of inductance L 1 and one end of capacitor C, the other end of resistance R 2 is connected with the negative terminal of direct voltage source Vi, the collector electrode of positive-negative-positive BJT pipe Q2 is connected with the base stage of NPN type BJT pipe Q1,

Described main switch drives the little BJT type self-excitation Sepic converter of loss under the interrupted mode of operation of the current i L2 of the current i L1 of inductance L 1 critical continuous mode, inductance L 2, after positive-negative-positive BJT pipe Q2 and NPN type BJT pipe Q1 conducting, direct voltage source Vi, inductance L 1, NPN type BJT pipe Q1 form loop, capacitor C, NPN type BJT pipe Q1, diode D1, inductance L 2 form another loop, voltage-stabiliser tube Z1 forward conduction, the collector voltage vc1 of NPN type BJT pipe Q1 increases, in the time that the collector voltage vc1 of NPN type BJT pipe Q1 rises to certain value, positive-negative-positive BJT pipe Q2 turn-offs, voltage-stabiliser tube Z1 cut-off, and NPN type BJT pipe Q1 turn-offs, NPN type BJT pipe Q1 closes and has no progeny, diode D conducting, direct voltage source Vi, inductance L 1, capacitor C, diode D, output capacitance Co and load Ro form loop, diode D1, inductance L 2, diode D, output capacitance Co and load Ro form another loop, inductive current iL1 and inductive current iL2 reduce, voltage-stabiliser tube Z1 reverse-conducting, because the collector voltage vc1 of NPN type BJT pipe Q1 is greater than direct voltage source Vi, emitter-base stage conduction voltage drop of positive-negative-positive BJT pipe Q2, voltage-stabiliser tube Z1 reverse-conducting pressure drop three sum, positive-negative-positive BJT pipe Q2 and NPN type BJT pipe Q1 still turn-off, in the time that inductive current iL2 drops to 0, diode D1 cut-off, in the time that inductive current iL1 drops to 0, diode D cut-off, after diode D cut-off, the collector voltage vc1 of NPN type BJT pipe Q1 falls, positive-negative-positive BJT pipe Q2 and NPN type BJT pipe Q1 conducting again.

2. main switch as claimed in claim 1 drives the little BJT type self-excitation Sepic converter of loss, it is characterized in that: shunt capacitance C1 between the anode of described direct voltage source Vi and the base stage of positive-negative-positive BJT pipe Q2.

3. the main switch as described in one of claim 1 and 2 drives the little BJT type self-excitation Sepic converter of loss, it is characterized in that: described self-excitation Sepic converter also comprises Voltage Feedback branch road, described Voltage Feedback props up route resistance R 3, voltage-stabiliser tube Z2 and NPN type BJT pipe Q3 composition, the collector electrode of described NPN type BJT pipe Q3 is connected with the collector electrode of positive-negative-positive BJT pipe Q2 and the base stage of NPN type BJT pipe Q1, one end of the NPN type BJT pipe emitter of Q3 and the negative terminal of direct voltage source Vi and resistance R 3 is connected, the anode of the NPN type BJT pipe base stage of Q3 and the other end of resistance R 3 and voltage-stabiliser tube Z2 is connected, the negative electrode of voltage-stabiliser tube Z2 is connected with the anode of VD Vo.

4. main switch as claimed in claim 3 drives the little BJT type self-excitation Sepic converter of loss, it is characterized in that: described voltage-stabiliser tube Z2 two ends shunt capacitance C2.

5. the main switch as described in one of claim 1 and 2 drives the little BJT type self-excitation Sepic converter of loss, it is characterized in that: described self-excitation Sepic converter also comprises current feedback branch road, described current feedback is propped up route resistance R 3, resistance R 4, diode D2 and NPN type BJT pipe Q3 composition, the collector electrode of described NPN type BJT pipe Q3 is connected with the collector electrode of positive-negative-positive BJT pipe Q2 and the base stage of NPN type BJT pipe Q1, the NPN type BJT pipe emitter of Q3 and the negative terminal of direct voltage source Vi, the emitter of NPN type BJT pipe Q1, one end of one end of capacitor C o and resistance R 4 is connected, the other end of resistance R 4 is connected with the negative terminal of VD Vo and the negative electrode of diode D2, one end of the NPN type BJT pipe base stage of Q3 and the anode of diode D2 and resistance R 3 is connected, the other end of resistance R 3 is connected with the anode of direct voltage source Vi.

6. main switch as claimed in claim 5 drives the little BJT type self-excitation Sepic converter of loss, it is characterized in that: described resistance R 3 two ends shunt capacitance C2.