A self-excited step-down converter based on constant on-time control

Abstract

The application is a self-excitation step-down converter based on constant conduction time control, which comprises a step-down main circuit, a control circuit and a driving circuit; an input power (Vin) is connected to the input end of the control circuit, i.e. the emitter of the second PNP tube (Q2); the output end of the control circuit is connected to the input end of the driving circuit, i.e. the base of the third PNP tube (Q3); and the output end of the driving circuit is connected to the input end of the step-down main circuit, i.e. the gate of the power switch tube (Q1). The structure can make the circuit simpler, solve the problems of high production cost of the circuit and difficult adjustment of the switching frequency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of switching power supply, and in particular to a self-excited step-down converter based on constant on-time control. BACKGROUND

[0002] With the rapid development of the technical field of switching power supply, the topological structure of switching power supply is also increasing. Common topological structures of switching power supply are step-down buck, step-up boost, step-up / down buck-boost, flyback and bridge, etc. Each topological structure of switching power supply has its own characteristics and application occasions, and different topological structures are needed according to different application scenarios. In order to meet the requirements of low voltage and large current, the step-down buck converter is commonly used today. The switching tube and control tube of the traditional self-excited buck converter usually use bipolar transistors. In addition, the step-down buck converter is widely used in life due to its advantages of low power consumption, high efficiency and simple circuit.

[0003] The working mode of the step-down buck converter has two modes of DCM (discontinuous conduction mode) and CCM (continuous conduction mode). In the CCM mode, the inductor current in the step-down buck converter is always greater than zero, and is in a steady state. In the DCM mode, the inductor in the step-down buck converter does not have current flowing through it within a certain time interval. The buck converter in the DCM mode can reduce power consumption and improve conversion efficiency. At the same time, the duty cycle of the converter depends on the load current, and the output voltage of the buck converter working in the DCM mode is affected by the load. In order to control the voltage, a closed-loop control needs to be introduced.

[0004] The control mode of the step-down buck converter is divided into voltage mode control and current mode control. The voltage mode control directly samples the output voltage, then compares it with the reference voltage to generate an error signal, and then compares it with the sawtooth signal to obtain the pulse width modulation signal, which is used to control the switching tube and complete the control logic. The current mode control samples the inductor current signal and the output voltage at the same time, compares them with the reference voltage to generate an error signal, and then compares the current signal with the error signal to generate a control signal to control the switching tube. The current mode control can also be divided into two types of variable frequency control and constant frequency control. The constant frequency control current mode is divided into peak current mode and valley current mode. The variable frequency control current mode is divided into COF (constant off-time peak current mode) and COT (constant on-time valley current mode).

[0005] If the excitation pulse of the switch tube is generated by the positive feedback self-oscillation of the converter circuit and the switch tube, it is called self-excited switching power supply. Since the regulating tube of the self-excited switching power supply serves as the oscillation tube, a separate oscillator is not required. The pulse signal of the self-excited switching power supply is generated by self-oscillation, which is a variable frequency conversion circuit that changes with the input voltage and load power. When the switching frequency is high, intermittent oscillation occurs.

[0006] In order to simplify the circuit structure, the traditional buck converter can be changed to a self-excited buck converter. At the same time, the traditional buck converter has the disadvantages of large system loss, slow system response speed, and poor circuit stability, and the buck converter with COT control mode can be used to make the circuit structure simpler and improve the system response speed. SUMMARY

[0007] TECHNICAL PROBLEM: The purpose of the present application is to address the deficiencies in the prior art, and to provide a self-excited buck converter based on constant on-time control. The structure can make the circuit simpler, solve the problem of high production cost and difficult to adjust the switching frequency.

[0008] TECHNICAL SCHEME: The present application is a self-excited buck converter based on constant on-time control, which comprises a buck main circuit, a control circuit and a driving circuit. The input power is connected to the input end of the control circuit, i.e. the emitter of the second PNP tube. The output end of the control circuit is connected to the input end of the driving circuit, i.e. the base of the third PNP tube. The output end of the driving circuit is connected to the input end of the buck main circuit, i.e. the gate of the power switch tube.

[0009] The buck main circuit comprises a power switch tube, an inductor, a second diode, a third diode, a fourth capacitor, an eighth resistor, a ninth resistor, an output capacitor and a load resistor. The drain of the power switch tube is connected to the input power source, the source is connected to the eighth resistor, the other end of the eighth resistor is connected to the positive electrode of the second diode, the positive electrode of the third diode is connected to the ground, the negative electrodes of the second diode and the third diode are connected to one end of the inductor, the other end of the inductor is connected to the output capacitor and the load resistor, and the fourth capacitor and the ninth resistor are connected in series and then connected in parallel across the third diode.

[0010] The control circuit comprises a first resistor, a second resistor, a third resistor, a fourth resistor, a fifth resistor, a voltage stabilizing tube, a first capacitor, a second capacitor, a third capacitor, a second PNP tube and an inverter; wherein one end of the first resistor is connected with a positive pole of an input power supply, the other end of the first resistor is connected with the voltage stabilizing tube and the first capacitor, the input power supply charges the first capacitor to a reference voltage through the first resistor, and the reference voltage provides a reference voltage Vref for the control circuit; an output voltage Vout is connected with a base of the second PNP tube of the control circuit, the reference voltage is connected with an emitter of the second PNP tube through the fifth resistor, a collector of the second PNP tube is connected with the second capacitor and a positive pole of the inverter; the inverter is connected with a first diode and the third resistor in parallel, the first diode and the third resistor are connected in series, and the inverter, the first diode, the second capacitor and the third resistor constitute a reverse pulse square wave generator to provide a trigger signal for a power switch tube; an output end of the inverter is connected with the fourth resistor and the third capacitor, the other end of the third capacitor is connected with the second resistor, the other end of the second resistor is connected with the positive pole of the input power supply, and one end of the third capacitor connected with the second resistor is connected with a base of a third PNP tube to provide a driving signal for a driving circuit.

[0011] The driving circuit comprises a fourth NPN tube, a third PNP tube, a sixth resistor and a seventh resistor; wherein the base of the third PNP tube is connected with the third capacitor, the emitter of the third PNP tube is connected with a drain of the power switch tube, the collector of the third PNP tube is connected with the collector of the fourth NPN tube, the base of the fourth NPN tube is connected with the sixth resistor, the other end of the sixth resistor is connected with a source of the power switch tube, the emitter of the fourth NPN tube is connected with a positive pole of a second diode, and the seventh resistor is connected with the gate and the source of the power switch tube respectively, and the driving circuit controls the turn-on and turn-off of the power switch tube by driving the gate of the power switch tube.

[0012] The output end of the step-down main circuit is connected with the base of the second PNP tube of the control circuit to generate an error signal compared with the reference voltage, the control circuit generates a reverse pulse signal to the driving circuit to control the power switch tube.

[0013] When the power switch tube is turned on, the second diode is turned on and the third diode is turned off at this time, the current through the inductor linearly increases, at this time, part of the energy of the input power supply is transmitted to the inductor through the power switch tube, and part of the energy is directly transmitted to the output end; when the power switch tube is turned off, the second diode is turned off and the third diode is turned on at this time, the current through the inductor linearly decreases, at this time, there is no energy from the input power supply, the energy stored in the inductor is transmitted to the output end, if the power switch tube is turned off for too long, the current in the inductor linearly decreases to 0, at this time, the inductor will oscillate with the fourth capacitor and the ninth resistor.

[0014] The voltage reduction main circuit, the power switch tube controls the inductance energy storage, the output end accesses the control circuit and further constitutes a loop, controls the conduction and cut-off of the power switch tube;The second diode and the third diode interact to constitute a judgment, when the power switch tube is turned on, the second diode input voltage is approximately input power supply, then the third diode is cut off;When the power switch tube is turned off, the third diode is turned on;The fourth capacitor and the ninth resistor and the inductor can constitute an oscillation circuit.

[0015] Advantages: the application adopts the above technical scheme, and has the following advantages:

[0016] (1) the buck converter in the DCM working state is different from the CCM, BCM and other working states, and has lower circuit power consumption and higher circuit conversion efficiency when in the DCM working state.

[0017] (2) the unique COT self-excited buck converter of the application is different from the traditional COT buck converter, can effectively realize the stable work of the COT control self-excited buck converter, make the circuit more simple, and the production cost is low;At the same time, the pulse signal of the switch tube is generated by self-oscillation, the frequency changes with the input voltage and load, the switching frequency is higher or intermittent oscillation at light load, and the frequency automatically reduces at full load. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The COT self-excited buck converter proposed in the application is shown in the schematic diagram.

[0019] Figure 2 The COT self-excited buck converter proposed in the application is shown in the simulation waveform diagram. DETAILED DESCRIPTION

[0020] The technical scheme of the application will be described in more detail below with reference to the drawings.

[0021] In order to achieve the above-mentioned application purposes, a self-excited step-down converter based on constant on-time control is adopted, and the working mode of the converter is as follows:

[0022] As Figure 1As shown, the error signal is generated by comparing the output end Vout of the step-down main circuit in the application with the reference voltage Vref, and the control circuit generates a reverse pulse signal to the driving circuit, and then controls the power switch tube Q1. In the step-down main circuit, when the power switch tube Q1 is turned on, the second diode D2 is turned on at this time, the third diode D3 is cut off, and the current through the inductor L1 is linearly increased. At this time, part of the energy of the input power is transmitted to the inductor L1 through the power switch tube Q1, and part of the energy is directly transmitted to the output end Vout. When the power switch tube Q1 is cut off, the second diode D2 is cut off at this time, the third diode D3 is turned on, and the current through the inductor L1 is linearly reduced. At this time, there is no energy from the input power Vin, and the energy stored in the inductor L1 is transmitted to the output end Vout. If the power switch tube Q1 is cut off for too long, the current in the inductor L1 will linearly decrease to 0, and at this time the inductor L1 will oscillate with the fourth capacitor C4 and the ninth resistor R9.

[0023] In the steady state stage of the application, the inductor L1 satisfies the volt-second conservation theorem in a period. When the power switch tube Q1 is turned on, the inductor L1 voltage is V on = V in -V Q1 -V R8 -V D2 -V out , and when the power switch tube Q1 is cut off, the inductor L1 voltage is V off = V out + V D3 , then the duty ratio and the output voltage Vout are obtained by using the volt-second balance:

[0024]

[0025] V otu = D(V in -V Q1 )

[0026] Wherein: VQ1 is the source-drain voltage of the power switch tube Q1 when turned on, VR8 is the voltage drop on the eighth resistor R8, and VD2 and VD3 are the conduction voltages of the second diode D2 and the third diode D3.

[0027] As Figure 1 shown, the application adopts a self-excited step-down converter based on constant on-time control, which can make the circuit simpler, solve the problem of high production cost of the circuit, and adjust the switching frequency. The COT self-excited converter is composed of a main power circuit, a control circuit and a driving circuit.

[0028] The output end Vout of the voltage reduction main circuit of the application is connected with the second PNP tube Q2, the input power Vin charges the first capacitor C1 through the first resistor R1, and because of the existence of the stabilizing tube D4, when the first capacitor C1 is charged to Vref, the voltage of the stabilizing tube D4 is stabilized at Vref, when the converter is in steady state, the emitter voltage of the second PNP tube Q2 is Vref, the base voltage is Vout, and the error signal is generated at the collector of the second PNP tube (Q2) by comparing the two. In the control circuit of the application, the Schmitt comparator U1, the first diode D1, the second capacitor C2 and the third resistor R3 constitute a reverse pulse signal generator: when the second capacitor C2 is in low level state, the output end of the Schmitt inverter U1 is in high level, at this time the first diode D1 is reverse-biased and cut off; when the second capacitor C2 is charged to high level, the input end of the Schmitt inverter U1 becomes low level, at this time the first diode D1 is forward-biased and turned on, the second capacitor C2 is discharged rapidly through the first diode D1 and the third resistor R3 loop, and because the discharge time constant is very small, when the capacitor is discharged to low level, the output end of the Schmitt inverter U1 becomes high level state immediately, so the time of the output end of the Schmitt inverter U1 being in low level is very short, the reverse pulse signal is generated at the output end, and the sawtooth wave signal is generated on the second capacitor C2. The input power Vin charges the capacitor through the second resistor R2 and the third capacitor C3, and when in steady state, the voltage across the third capacitor C3 is Vin, because the reverse pulse is generated at the output end of the Schmitt inverter U1, the base voltage of the third PNP tube Q3 is reduced reversely, the emitter voltage of the third PNP tube Q3 is greater than the base voltage, so that the third PNP tube Q3 changes from cut-off state to on state, at this time the input power Vin charges the gate capacitor of the power switch tube Q1 through the third PNP tube Q3, so that the power switch tube Q1 is turned on, when the power switch tube Q1 is turned on for a period of time, the fourth NPN tube Q4 is turned on, the gate capacitor of the power switch tube Q1 is discharged through the fourth NPN tube Q4, so that the power switch tube Q1 changes from on state to cut-off state.

[0029] The output voltage Vout of the application is connected to the base of the second PNP tube Q2 to form a loop, which can ensure that the circuit is more stable, and at the same time, the fifth resistor R5 is associated in front of the emitter of the second PNP tube Q2, which can better protect the circuit and prevent the circuit current from being too large to damage the device. In the starting stage of the converter, the input power Vin charges the first capacitor C1 through the first resistor R1, and the voltage across the first capacitor C1 rises slowly, at this time the second PNP tube Q2 is in cut-off state; when the converter is in steady state, the voltage across the first capacitor C1 is Vref, and when the current of the second PNP tube Q2 is too large, the fifth resistor R5 clamps the voltage. When the current of the second PNP tube Q2 is too small, the second PNP tube Q2 clamps the voltage, and the first capacitor C1 and the second capacitor C2, the fifth resistor R5 and the second PNP tube Q2 constitute a stable sawtooth wave signal output.

[0030] In the transformer starting stage, the input power Vin charges the third capacitor C3 to Vin, because the Schmidt inverter U1, the first diode D1 and the third resistor R3 output the reverse pulse square wave signal, the third PNP tube Q3 base voltage decreases, the third PNP tube Q3 changes from off to on state, the input power Vin charges the power switch tube Q1 gate capacitor, and provides the ON signal of the power switch tube Q1. When the power switch tube Q1 is on, the eighth resistor R8 generates a forward voltage drop, because the current through the inductor L1 increases linearly, when the current increases to a certain value, the voltage across the eighth resistor R8 reaches the turn-on voltage of the fourth NPN tube Q4, at this time the fourth NPN tube Q4 is on, the power switch tube Q1 is discharged through the fourth NPN tube Q4, the sixth resistor R6 and the eighth resistor R8, so that the converter provides the OFF signal of the power switch tube Q1 by detecting the peak current Ipk flowing through the inductor L1, and the RC discharge time is determined by the sixth resistor R6 and the eighth resistor R8, and the seventh resistor R7 is connected across the gate and source of the power switch tube Q1 for protecting the power switch tube Q1, wherein the peak current Ipk can be obtained:

[0031]

[0032] Wherein: UBE is the turn-on voltage of the NPN tube Q4, Uon is the voltage on the inductor L1 when the power switch tube Q1 is on.

[0033] Figure 2 The simulation waveform diagram is shown in the figure, which is the simulation waveform of the COT self-excited buck converter of the present application, and is divided into a starting stage and a steady state stage. The starting stage is mainly to meet the normal starting of the present application. In the steady state stage, IL and VC2 are the current simulation waveform in the inductor L1 and the voltage simulation waveform across the second capacitor C2. It can be seen that the voltage across the second capacitor C2 is a sawtooth wave, the inductor L1 current is in DCM mode, the Schmidt inverter U1 output U1-out is a reverse pulse square wave signal, VDS and VGS are the drain-source voltage and gate-source voltage of the power switch tube Q1, Vin, Vref and Vout are the input power voltage, reference voltage and output voltage of the present application respectively, the third diode D3 output voltage VD3 is approximately Vin when the power switch tube Q1 is on, and when the power switch tube Q1 is off, if the inductor L1 current decreases linearly, the third diode D3 output voltage is 0; if the inductor L1 current decreases to 0, at this time the inductor L1 resonates with the fourth capacitor C4 and the ninth resistor R9, generating a sinusoidal signal.

Claims

1. A self-oscillating buck converter based on constant on-time control, characterized in that The converter comprises a step-down main circuit, a control circuit and a driving circuit; The control circuit comprises a first resistor (R1), a second resistor (R2), a third resistor (R3), a fourth resistor (R4), a fifth resistor (R5), a voltage stabilizing tube (D4), a first capacitor (C1), a second capacitor (C2), a third capacitor (C3), a second PNP tube (Q2) and an inverter (U1); one end of the first resistor (R1) is connected with a positive electrode of an input power supply (Vin), the other end of the first resistor (R1) is connected with the voltage stabilizing tube (D4) and the first capacitor (C1), the input power supply (Vin) charges the first capacitor (C1) to a reference voltage Vref through the first resistor (R1), and the reference voltage Vref provides a reference voltage for the control circuit; an output voltage Vout is connected with a base of the second PNP tube (Q2) in the control circuit, the reference voltage (Vref) is connected with an emitter of the second PNP tube (Q2) through the fifth resistor (R5), a collector of the second PNP tube (Q2) is connected with the second capacitor (C2) and a positive electrode of the inverter (U1); the inverter (U1) is connected in series with a first diode (D1) and the third resistor (R3) and then is connected in parallel, the inverter (U1), the first diode (D1), the second capacitor (C2) and the third resistor (R3) constitute a reverse pulse square wave generator, and the reverse pulse square wave generator provides a trigger signal for a power switch tube (Q1); an output end of the inverter (U1) is connected with one end of the fourth resistor (R4) and one end of the third capacitor (C3), the other end of the fourth resistor (R4) is grounded; the other end of the third capacitor (C3) is connected with the second resistor (R2), the other end of the second resistor (R2) is connected with the positive electrode of the input power supply (Vin), and one end of the third capacitor (C3) and the second resistor (R2) is connected with a base of a third PNP tube (Q3), and the third PNP tube (Q3) provides a driving signal for the driving circuit; The driving circuit comprises a fourth NPN tube (Q4), the third PNP tube (Q3), a sixth resistor (R6) and a seventh resistor (R7); the base of the third PNP tube (Q3) is connected with one end of the third capacitor (C3), the emitter of the third PNP tube (Q3) is connected with a drain of the power switch tube (Q1), the collector of the third PNP tube (Q3) is connected with a collector of the fourth NPN tube (Q4), the base of the fourth NPN tube (Q4) is connected with one end of the sixth resistor (R6), the other end of the sixth resistor (R6) is connected with a source of the power switch tube (Q1), the emitter of the fourth NPN tube (Q4) is connected with a positive electrode of a second diode (D2), and the seventh resistor (R7) is connected with the gate and the source of the power switch tube (Q1) respectively, and the driving circuit controls the conduction and the turn-off of the power switch tube (Q1) through the driving of the gate of the power switch tube (Q1).

2. The constant on-time control based self-oscillating buck converter according to claim 1, wherein, The buck converter main circuit includes a power switch (Q1), an inductor (L1), a second diode (D2), a third diode (D3), a fourth capacitor (C4), an eighth resistor (R8), a ninth resistor (R9), an output capacitor (C5), and a load resistor (RL). The drain of the power switch (Q1) is connected to the input power supply (Vin), and its source is connected to the eighth resistor (R8). The other end of the eighth resistor (R8) is connected to the anode of the second diode (D2). The anode of the third diode (D3) is connected to ground. The cathodes of the second diode (D2) and the third diode (D3) are connected to one end of the inductor (L1). The other end of the inductor (L1) is connected to the output capacitor (C5) and the load resistor (RL). The fourth capacitor (C4) and the ninth resistor (R9) are connected in series and then in parallel across the third diode (D3).

3. A constant on-time control based self-oscillating buck converter according to claim 2, characterized in that, The output terminal (Vout) of the buck main circuit is connected to the base of the second PNP transistor (Q2) of the control circuit. It is compared with the reference voltage (Vref) to generate an error signal. The control circuit generates a reverse pulse signal and transmits it to the drive circuit, thereby controlling the power switch transistor (Q1).

4. A constant on-time control based self-oscillating buck converter according to claim 1 or 2, characterized in that, When the power switch (Q1) is turned on, the second diode (D2) is turned on and the third diode (D3) is turned off. The current through the inductor (L1) increases linearly. At this time, part of the energy from the input power supply is transferred to the inductor (L1) through the power switch (Q1), and part of the energy is directly transferred to the output terminal (Vout). When the power switch (Q1) is turned off, the second diode (D2) is turned off and the third diode (D3) is turned on. The current through the inductor (L1) decreases linearly. At this time, there is no energy from the input power supply (Vin), and the energy stored in the inductor (L1) is transferred to the output terminal (Vout). If the power switch (Q1) is turned off for too long, the current in the inductor (L1) will decrease linearly to 0. At this time, the inductor (L1) will oscillate with the fourth capacitor (C4) and the ninth resistor (R9).

5. A constant on-time control based self-oscillating buck converter according to claim 1 or 2, characterized in that, In the buck converter main circuit, the power switch (Q1) controls the energy storage of the inductor (L1), and the output terminal (Vout) is connected to the control circuit to form a loop, controlling the conduction and turn-off of the power switch (Q1). When the power switch (Q1) is on, the input voltage of the second diode (D2) is approximately the input power supply (Vin), so the third diode (D3) is off. When the power switch (Q1) is off, the third diode (D3) is on. The fourth capacitor (C4) and the ninth resistor (R9) together with the inductor (L1) can form an oscillation circuit.

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