阅读说明：本技术 一种大纹波输出电压的DCM Buck PFC变换器 (DCM Buck PFC converter with large ripple output voltage ) 是由 高阳 姚凯 杨坚 刘劲滔 刘乐 王泽松 李家镇 于 2020-04-30 设计创作，主要内容包括：本发明公开了一种大纹波输出电压的DCM Buck PFC变换器包括主功率电路和控制电路,其中控制电路包括分压采样电路、电压调节电路、锯齿波比较及驱动信号生成电路和隔离驱动电路；主功率电路的输出端与分压采样电路的输入端相连,分压采样电路的输出端与电压调节电路的输入端相连,电压调节电路的输出端与锯齿波比较及驱动信号生成电路的输入端相连,锯齿波比较及驱动信号生成电路的输出端与隔离驱动电路的输入端相连,隔离驱动电路的输出端与主功率电路相连；使用分压采样电路采集输出电压数据,并由电压调节电路与基准电压进行调节,电压调节电路的输出信号v<Sub>EA</Sub>经过锯齿波比较及驱动信号生成电路,输出占空比固定的驱动信号,实行定占空比控制。本发明减小了PFC变换器的储能电容容值,使用薄膜电容代替电解电容,提高了功率密度和变换器的工作寿命。(The invention discloses a DCM Buck PFC converter with large ripple output voltage, which comprises a main power circuit and a control circuit, wherein the control circuit comprises a voltage division sampling circuit, a voltage regulating circuit, a sawtooth wave comparison and drive signal generation circuit and an isolation drive circuit; the output end of the main power circuit is connected with the input end of the voltage division sampling circuit, the output end of the voltage division sampling circuit is connected with the input end of the voltage regulation circuit, the output end of the voltage regulation circuit is connected with the input end of the sawtooth wave comparison and drive signal generation circuit, the output end of the sawtooth wave comparison and drive signal generation circuit is connected with the input end of the isolation drive circuit, and the output end of the isolation drive circuit is connected with the main power circuit; the voltage dividing and sampling circuit is used for collecting output voltage data, and the voltage data is regulated by the voltage regulating circuit and the reference voltageOutput signal v EA The drive signal with fixed duty ratio is output through a sawtooth wave comparison and drive signal generation circuit, and the constant duty ratio control is carried out. The invention reduces the capacitance value of the energy storage capacitor of the PFC converter, uses the film capacitor to replace an electrolytic capacitor, and improves the power density and the service life of the converter.)

1. A DCM Buck PFC converter with large ripple output voltage is characterized by comprising a main power circuit (1) and a control circuit, wherein the control circuit comprises a voltage division sampling circuit (2), a voltage regulating circuit (3), a sawtooth wave comparison and drive signal generation circuit (4) and an isolation drive circuit (5); the output end of the main power circuit (1) is connected with the input end of the voltage division sampling circuit (2), the output end of the voltage division sampling circuit (2) is connected with the input end of the voltage regulation circuit (3), the output end of the voltage regulation circuit (3) is connected with the input end of the sawtooth wave comparison and drive signal generation circuit (4), the output end of the sawtooth wave comparison and drive signal generation circuit (4) is connected with the input end of the isolation drive circuit (5), and the output end of the isolation drive circuit (5) is connected with the main power circuit (1); the voltage division sampling circuit (2) is used for collecting output voltage data, the voltage data is regulated by a voltage regulating circuit (3) and a reference voltage, and an output signal v of the voltage regulating circuit_EAThe drive signal with fixed duty ratio is output through a sawtooth wave comparison and drive signal generation circuit, and the constant duty ratio control is carried out.

2. The DCM Buck PFC converter with large ripple output voltage according to claim 1, wherein the main power circuit (1) includes an input voltage source v_inEMI filter, diode rectification circuit RB, LC filter, main circuit inductor L, first switch tube Q, freewheeling diode D, output capacitor C_oAnd a constant power load R_cot(ii) a Said input voltage source v_inThe output port of the EMI filter is connected with the input port of the rectifier bridge RB, the output cathode of the rectifier bridge RB is connected with the input negative port of the LC filter, the output positive port of the rectifier bridge RB is connected with the input positive port of the LC filter, the output positive port of the LC filter is connected with one end of the first switch tube Q, and the LC filter is connected with the input port of the first switch tube QThe output negative port of the LC filter is connected with the anode of a freewheeling diode D, and the negative port of the LC filter is a reference potential zero point; the other end of the first switch tube Q is connected with the cathode of the fly-wheel diode D and one end of a main circuit inductor L, and the other end of the main circuit inductor L is connected with an output capacitor C_oPositive electrode and constant power load R_cotConnecting; output capacitor C_oThe negative electrode of the flywheel diode D is connected with the positive electrode of the fly-wheel diode D; constant power load R_cotIs connected with the input end of the voltage division sampling circuit (2).

3. The DCM Buck PFC converter with large ripple output voltage according to claim 1, wherein the control circuit comprises a voltage division sampling circuit (2), a voltage regulation circuit (3), a sawtooth wave comparison and drive signal generation circuit (4) and an isolation drive circuit (5); the voltage division sampling circuit (2) comprises a first voltage division resistor R₁And a second voltage dividing resistor R₂(ii) a The positive input end of the voltage division sampling circuit (2) passes through a first voltage division resistor R₁And the output voltage v_oIs connected with the positive port, and the reverse input end of the voltage division sampling circuit (2) passes through a second voltage division resistor R₂And the output voltage v_oThe negative port of the voltage division sampling circuit (2), the output port A of the voltage division sampling circuit (2) is connected with the input port B of the voltage regulating circuit (3), and the output port C of the voltage regulating circuit (3) is connected with the input port D of the sawtooth wave comparison and drive signal generating circuit (4); an output port E of the sawtooth wave comparison and drive signal generation circuit (4) is connected with an input port F of the isolation drive circuit (5); an output port G of the isolation driving circuit (5) is connected with the first switch tube Q.

4. The DCM Buck PFC converter with large ripple output voltage according to claim 1, wherein an output voltage signal kv of a voltage dividing resistor is used_oInput into a voltage regulating circuit (3), and obtain an error signal v of a voltage closed loop through the voltage regulating circuit (3)_EAWill error signal v_EAThe PWM signal is directly input into a sawtooth wave comparison and drive signal generation circuit (4) to generate a PWM wave, so that the on and off of a first switching tube Q are controlled:

duty ratio D of the first switch tube Q_QComprises the following steps:

wherein L is the main inductance of the converter, f_sFor the converter switching frequency, P_oTo output power, V_mFor input voltage amplitude, v_oTo output a voltage, [ theta ]₁、θ₂Is the working dead zone angle.

5. The DCM Buck PFC converter with large ripple output voltage according to claim 3, wherein the voltage regulation circuit (3) comprises a first operational amplifier IC1, a resistor R₃A first capacitor C₁And a second capacitor C₂(ii) a The positive input end of the first operational amplifier IC1 is connected with the output end A of the voltage division sampling circuit (2), the negative input end of the first operational amplifier IC1 is connected with a reference voltage, and the positive input end of the first operational amplifier IC1 is connected with the reference voltage through a resistor R₃A first capacitor C₁And a second capacitor C₂Is connected with the output end; second capacitor C₂And a resistance R₃Is connected with the input end of the sawtooth wave comparison and drive signal generation circuit (4).

6. The DCM Buck PFC converter with large ripple output voltage according to claim 3, wherein the sawtooth wave comparison and drive signal generation circuit (4) includes a second operational amplifier IC 2; the positive input end of the second operational amplifier IC2 is connected with the output end of the first operational amplifier IC1 in the voltage regulating circuit (3), and the negative input end of the second operational amplifier IC1 is connected with the sawtooth wave; the output of the second operational amplifier IC2 is connected to the input of the isolation drive circuit (5).

7. The DCM Buck PFC converter with large ripple output voltage according to claim 3, wherein the amplifiers used in the first operational amplifier IC1 and the second operational amplifier IC2 are TL074, TL072, LM358 or LM324 type operational amplifiers.

8. The DCM Buck PFC converter with large ripple output voltage according to claim 3, wherein the voltage regulation circuit (3) and the sawtooth wave comparison and driving signal generation circuit (4) can use SG3525 or UC3525 type chips, and the isolation driving circuit (5) uses TLP250 type driving chips.

Technical Field

The invention belongs to the technology of an alternating current-direct current converter of an electric energy conversion device, and particularly relates to a DCM Buck PFC converter with large ripple output voltage.

Background

An energy storage capacitor of a conventional Buck PFC converter generally uses an electrolytic capacitor, and the capacitance value and the volume of the electrolytic capacitor are large, so that the working life and the power density of the converter are low, and the technical requirements of design cannot be met. In order to solve the problem that the capacitance value of an energy storage capacitor of the converter is large, a two-stage Boost-flyback converter is provided, a proper amount of third harmonic is injected into input current, but in order to ensure that the input power factor of the two-stage Boost-flyback converter cannot be lower than 0.9, the content of the third harmonic cannot be too large, and the capacitance value of the energy storage capacitor is still large in the whole 90V-264V AC input voltage range. Although the two-stage Boost-flyback converter which is used for solving the problem of large capacitance value of the energy storage capacitor can reduce the capacitance value of the energy storage capacitor of the converter, a control circuit is complex, the control effect is not good, and the industrial technical requirement cannot be met.

Disclosure of Invention

The invention aims to provide a DCM Buck PFC converter with a large ripple output voltage, which has a simple control circuit and a good control effect, and effectively reduces the capacitance value of an energy storage capacitor in the whole 90V-264 VAC input voltage range, thereby replacing an electrolytic capacitor with a thin-film capacitor.

The technical solution for realizing the purpose of the invention is as follows: a DCM buck PFC converter with large ripple output voltage comprises a main power circuit and a control circuit, wherein the control circuit comprises a voltage division sampling circuit, a voltage regulating circuit, a sawtooth wave comparison circuit and a drive signal generation circuitAn integrated circuit and an isolated driving circuit; the output end of the main power circuit is connected with the input end of the voltage division sampling circuit, the output end of the voltage division sampling circuit is connected with the input end of the voltage regulation circuit, the output end of the voltage regulation circuit is connected with the input end of the sawtooth wave comparison and drive signal generation circuit, the output end of the sawtooth wave comparison and drive signal generation circuit is connected with the input end of the isolation drive circuit, and the output end of the isolation drive circuit is connected with the main power circuit; the voltage dividing and sampling circuit is used for collecting output voltage data, the voltage data is regulated by the voltage regulating circuit and reference voltage, and the output signal v of the voltage regulating circuit_EAThe drive signal with fixed duty ratio is output through a sawtooth wave comparison and drive signal generation circuit, and the constant duty ratio control is carried out.

Further, the main power circuit comprises an input voltage source v_inEMI filter, diode rectification circuit RB, LC filter, main circuit inductor L, first switch tube Q, freewheeling diode D, output capacitor C_oAnd a constant power load R_cot(ii) a Said input voltage source v_inThe output port of the EMI filter is connected with the input port of a rectifier bridge RB, the output cathode of the rectifier bridge RB is connected with the input negative port of an LC filter, the output positive port of the rectifier bridge RB is connected with the input positive port of the LC filter, the output positive port of the LC filter is connected with one end of a first switching tube Q, the output negative port of the LC filter is connected with the anode of a freewheeling diode D, and the negative port of the LC filter is a reference potential zero point; the other end of the first switch tube Q is connected with the cathode of the fly-wheel diode D and one end of a main circuit inductor L, and the other end of the main circuit inductor L is connected with an output capacitor C_oPositive electrode and constant power load R_cotConnecting; output capacitor C_oThe negative electrode of the flywheel diode D is connected with the positive electrode of the fly-wheel diode D; constant power load R_cotIs connected with the input end of the voltage division sampling circuit (2).

The control circuit comprises a voltage division sampling circuit, a voltage regulating circuit, a sawtooth wave comparison and drive signal generation circuit and an isolation drive circuit; the voltage division sampling circuit comprises a first voltage division resistor R₁And a second voltage dividing resistor R₂(ii) a The positive input end of the voltage division sampling circuit passes through a first voltage division resistor R₁And the output voltage v_oThe positive port of the voltage division sampling circuit is connected, and the reverse input end of the voltage division sampling circuit passes through a second voltage division resistor R₂And the output voltage v_oThe output port A of the voltage division sampling circuit is connected with the input port B of the voltage regulating circuit, and the output port C of the voltage regulating circuit is connected with the input port D of the sawtooth wave comparison and drive signal generating circuit; the output port E of the sawtooth wave comparison and drive signal generation circuit is connected with the input port F of the isolation drive circuit; the output port G of the isolation driving circuit is connected with the first switch tube Q.

Output voltage signal kv of voltage dividing resistor_oInputting the error signal into a voltage regulating circuit, and obtaining an error signal v of a voltage closed loop through the voltage regulating circuit_EAWill error signal v_EAThe PWM signal is directly input to a sawtooth wave comparison and drive signal generation circuit to generate PWM waves, so that the on and off of a first switching tube Q are controlled:

duty ratio D of the first switch tube Q_QComprises the following steps:

wherein L is the main inductance of the converter, f_sFor the converter switching frequency, P_oTo output power, V_mFor input voltage amplitude, v_oTo output a voltage, [ theta ]₁、θ₂Is the working dead zone angle.

The voltage regulating circuit comprises a first operational amplifier IC1, a resistor R₃A first capacitor C₁And a second capacitor C₂(ii) a The positive input end of the first operational amplifier IC1 is connected with the output end A of the voltage division sampling circuit, the negative input end of the first operational amplifier IC1 is connected with the reference voltage, and the positive input end of the first operational amplifier IC1 is connected with the reference voltage through a resistor R₃A first capacitor C₁And a second capacitor C₂Is connected with the output end; second capacitor C₂And a resistance R₃And the input end of the sawtooth wave comparison and drive signal generation circuit is connected with the input end of the sawtooth wave comparison and drive signal generation circuit.

The sawtooth wave comparison and drive signal generation circuit comprises a second operational amplifier IC 2; the positive input end of the second operational amplifier IC2 is connected with the output end of the first operational amplifier IC1 in the voltage regulating circuit, and the negative input end of the second operational amplifier IC1 is connected with the sawtooth wave; the output of the second operational amplifier IC2 is connected to the input of the isolation drive circuit.

The amplifiers used in the first operational amplifier IC1 and the second operational amplifier IC2 are TL074, TL072, LM358 or LM324 type operational amplifiers.

The voltage regulating circuit and the sawtooth wave comparing and driving signal generating circuit can use SG3525 or UC3525 type chips, and the isolation driving circuit uses TLP250 type driving chips.

Compared with the prior art, the invention has the remarkable advantages that: 1) by using a constant duty ratio control mode, the capacitance value of the energy storage capacitor of the converter can be reduced in the whole 90V-264 VAC input voltage range; 2) the control circuit is simplified, and the control stability is improved; 3) the thin film capacitor is used to replace electrolytic capacitor, power density of power supply is increased, and service life of converter is prolonged

Drawings

Fig. 1 is a block diagram of a two-stage Buck PFC converter according to an embodiment of the present invention.

FIG. 2 is a main circuit diagram of a DCM Buck PFC converter in an embodiment of the invention.

FIG. 3 is a waveform diagram of the inductor current and the switching tube current of the DCM Buck PFC converter in one switching cycle in accordance with one embodiment of the present invention.

FIG. 4 is a schematic diagram of the operating range of the DCM Buck PFC converter in the embodiment of the invention.

FIG. 5 shows example a of the present invention₁And V_mGraph of the relationship of (c).

FIG. 6 is a graph showing the relationship between a and C in the embodiment of the present invention.

FIG. 7 is a graph showing the relationship between b and C in the embodiment of the present invention.

FIG. 8 is a diagram of critical inductance values in an embodiment of the present invention.

FIG. 9 isExample V of the present invention_{m_rms}110V, input current waveform at different capacitances.

FIG. 10 shows a view of V in an embodiment of the present invention_{m_rms}220V, input current waveform at different capacitances.

FIG. 11 shows a view of V in an embodiment of the present invention_{m_rms}110V, output voltage waveform diagram under different capacitance.

FIG. 12 shows a view of V in an embodiment of the present invention_{m_rms}220V, output voltage waveform diagram under different capacitances.

FIG. 13 is a graph of C vs. PF in an example of the present invention.

Fig. 14 is a schematic diagram of a main power circuit structure and a control structure of the DCM Buck PFC converter with large ripple output voltage according to the embodiment of the present invention.

Main symbol names in the above figures: v. of_inAnd a power supply voltage. i.e. i_inAnd inputting the current. RB, a rectifier bridge. v. of_gAnd the rectified output voltage. i.e. i_LAnd an inductor current. L, inductance. Q, a switching tube. D. And a diode. C_oAnd an output filter capacitor. R_cotAnd a constant power load. v. of_oAnd outputting the voltage. V_refAnd outputting the reference voltage of voltage feedback control. v. of_EAAnd outputting the error voltage signal controlled by the output voltage feedback. t, time. ω, input voltage angular frequency. V_mInput voltage peak. v. of_gsAnd the driving voltage of the switching tube Q. D_QAnd the duty cycle of the Buck converter. D_DAnd the follow current duty ratio of the follow current diode in the Buck working stage. T is_sAnd the switching period of the converter. f. of_sAnd the converter switching frequency. PF, power factor. I is_{in_rms}And an input current effective value.

Detailed Description

The present invention is described in further detail below with reference to the attached drawing figures.

Working principle of 1DCM Buck PFC converter

Fig. 1 is a block diagram of a two-stage Buck PFC converter. In the two-stage PFC converter, the rear-stage DC/DC converter is a constant power load of the front-stage PFC converter, so the two-stage DCM Buck PFC converter can be simplified as shown in fig. 2.

Fig. 2 is a DCM Buck PFC converter main circuit.

Setting: 1. all devices are ideal elements; 2. the output voltage ripple is large compared to its direct current amount; 3. the switching frequency is much higher than the input voltage frequency.

Figure 3 shows the switching tube current and inductor current waveforms during one switching cycle in DCM mode of operation,

when the input voltage v_gLess than the output voltage v_oWhen the converter is in a dead zone, the first switching tube Q is cut off, the freewheeling diode D is cut off, and the converter does not work. When the input voltage v_gGreater than the output voltage v_oWhen the first switch tube Q is turned on, the freewheeling diode D is turned off, and the voltage across the inductor L is the rectified output voltage v_g-v_oIts inductive current i_LStarting from zero with (v)_g-v_o) Slope of/L rises linearly, rectified output voltage v_gTo an output filter capacitor C_oAnd load supply. When the first switch tube Q is turned off, the inductive current i_LFreewheeling via freewheeling diode D when the voltage across inductor L is-v_oInductor current i_LWith v_oThe slope of/L decreases and the inductor current i_LCan be reduced to zero before a new period begins, and the output filter capacitor C_oPower is supplied to the load.

Without loss of generality, define the input AC voltage v_inThe expression of (a) is:

v_in＝V_msin ωt (1)

wherein V_mAnd ω is the amplitude and angular frequency of the input ac voltage, respectively.

The rectified voltage of the input voltage is:

v_g＝V_m|sin ωt| (2)

peak value i of inductor current in one switching period_L__pkComprises the following steps:

wherein D_QFor the on-duty ratio of the switching tube, T_sIs a switching period, v_oIs the output voltage.

The volt-second area across L is balanced during each switching cycle, i.e.

(v_g-v_o)D_QT_s＝v_oD_DT_s(4)

Wherein D is_DThe follow current duty ratio of the fly-wheel diode in the Buck working stage.

From formulas (2) and (4):

according to equation (3) and fig. 3, the average value of the current flowing through the switching tube in one switching period can be obtained as:

buck PFC converter only has input voltage v after rectification_gGreater than the output voltage v_oWhen the input current is larger than the input current, the switch tube Q is triggered to conduct only when the switch tube Q starts to bear the forward voltage, that is, the input current i_inThere is a dead zone, the size of which is determined by the input voltage and the output voltage. FIG. 4 is a schematic diagram of the operating interval of the Buck converter, which is (theta)₁，θ₂)。

The input current i of the DCM Buck PFC converter can be obtained according to the formula (6) and the graph of FIG. 4_inThe expression is as follows:

the average value P of the input power of the converter in a half power frequency period can be obtained according to the formula (1) and the formula (7)_inComprises the following steps:

setting the converter efficiency to 100%, i.e. P_in＝P_o. The duty ratio D is obtained from equation (8)_QComprises the following steps:

from fig. 3, the expression of the inductor current in one switching cycle can be obtained as follows:

the expression of the energy storage capacitor current in one switching period can be obtained according to the formula (10):

the output voltage ripple expression obtained from equation (11) is:

in the above formula, t_on＝D_QT_sThe conduction time of the switching tube is set; t is t_off＝D_DT_sThe inductor current fall time. Because the power frequency period is far greater than the switching period (T)_line>>T_s) Can be regarded as v_g、v_oAnd i_oA constant value in one switching period, so that it can be obtained according to equation (12):

as can be seen from equation (13), when the output voltage ripple is increased, the capacitance of the storage capacitor can be reduced.

2 method of increasing voltage ripple

2.1 instantaneous expression of output Voltage

The energy stored by the output capacitor can be expressed as:

from formulas (1), (7), (9) and (14), it is possible to obtain:

from equation (15), the relationship of the demarcation point time can be obtained:

from equations (15) and (16), the output voltage expression is given as:

according to the contents of the integral median generalization form (first theorem): if the functions f (x), g (x) are continuous over a closed interval [ a, b ] and g (x) does not change sign over [ a, b ], then there is at least one point over the integration interval (a, b) such that:

according to the above theorem, v_o(ω t), sin (ω t) in [ θ [ ]₁，θ₂]Is continuous and sin (ω t) is in [ θ ]₁，θ₂]No sign change, in the integration interval (theta)₁，θ₂) At least one time x exists, such that:

to make v_{o_x}And (3) satisfying the condition of integral median theorem, and approximating:

by substituting formula (19) or formula (20) for formula (17):

at this time v_oOnly two dead zone points θ in (ω t)₁And theta₂Is an unknown parameter, but according to the formula (16), when θ is₁When determined, theta₂Is also determined, so only the solution of theta is needed₁To find out theta₁And V_mC, making V_{m_rms}Saber simulation of 123-984 uF 90-264V, C. Specific data are shown in table 1. For ease of analysis, θ₁＝a₁∙π。

TABLE 1V_{m_rms}A of 123 to 984uF (90 to 264V, C ═ 123 to 984 uF)₁Value of

According to Table 1, a is obtained for different values of capacity₁And V_mThe relationship of (2) is shown in FIG. 5. At different capacitances, a₁And V_mThe general trend of the relationship of (a) is the same and is close to the trend of the power function. And then fit it using a type of power function. The fitted expression is:

both a and b in the fitting expression are related to C, and a and b are fitted twice according to Table 2.

TABLE 2C relations with a, b

The curve fit of a to C is shown in FIG. 6, and the curve fit of b to C is shown in FIG. 7. The expression of a, b with respect to C can be found as:

coefficient value A of a in formula (23)₀～A₅As shown in Table 3, coefficient value B of B₀～B₆As shown in table 4.

Coefficient values A of Table 3a₀～A₅

Coefficient values B of Table 4B₀～B₆

According to the formula (24), θ₁Having been solved, the output voltage transient expression can be expressed by equation (21).

From equations (21) and (7), an input current expression can be obtained:

the power factor expression can be derived from equations (1) and (7):

the expression of the change in PF with respect to C value can be found from equation (26), equation (25), equation (24), and equation (1):

2.2 control Circuit

By observing it, D can be found_QIs about V_mAs a function of, in control D_QWhen the value of (A) is less than the threshold value, we only need to guarantee the output voltagev_oThe average value of (a) is a fixed value, and the duty ratio theoretically calculated by the user can be obtained by automatically regulating the voltage in a closed loop mode, and is specifically represented by the formula (9). From the duty ratio shown in equation (9), a control circuit diagram as shown in fig. 14 can be designed. The collected output voltage signal kv_oInputting the error signal into a voltage regulating circuit, and obtaining an error signal v of a voltage closed loop through the voltage regulating circuit_EAWill error signal v_EAThe drive signal is directly input into the sawtooth wave comparison circuit and the drive signal generation circuit to obtain the drive signal, and the drive signal controls the on and off of the switch tube through the isolation drive circuit.

With reference to fig. 14, the main power circuit (1) comprises an input voltage source v_inEMI filter, diode rectification circuit RB, LC filter, main circuit inductor L, first switch tube Q, freewheeling diode D, output capacitor C_oAnd a constant power load R_cot(ii) a Said input voltage source v_inThe output port of the EMI filter is connected with the input port of the rectifier bridge RB, the output cathode of the rectifier bridge RB is connected with the input negative port of the LC filter, the output positive port of the rectifier bridge RB is connected with the input positive port of the LC filter, the output positive port of the LC filter is connected with one end of the first switch tube Q, the output negative port of the LC filter is connected with the positive port of the freewheeling diode D, and the negative port of the LC filter is a reference potential zero point; one end of a main circuit inductor L is connected with the other end of the first switching tube Q and is connected with the negative port of the fly-wheel diode D, and the other end of the main circuit inductor L is connected with the output capacitor C_oPositive electrode and constant power load R_cotConnecting; output capacitor C_oThe negative electrode of the flywheel diode D is connected with the positive electrode of the fly-wheel diode D; constant power load R_cotAnd is connected with the input end of the voltage division sampling circuit.

Furthermore, the control circuit comprises a voltage division sampling circuit, a sawtooth wave comparison and drive signal generation circuit and an isolation drive circuit; the voltage division sampling circuit comprises a first voltage division resistor R₁And a second voltage dividing resistor R₂(ii) a The positive input end of the voltage division sampling circuit passes through a first voltage division resistor R₁And the output voltage v_oPositive port phase ofThe reverse input end of the voltage division sampling circuit passes through a second voltage division resistor R₂And the output voltage v_oThe output port A of the voltage division sampling circuit is connected with the input port B of the voltage regulating circuit; the output port C of the voltage regulating circuit is connected with the input port D of the sawtooth wave comparison and drive signal generation circuit; an output port E of the sawtooth wave comparison and drive signal generation circuit is connected with an input port F of the isolation drive circuit, and an output port G of the isolation drive circuit is connected with a first switch tube Q.

Further, the voltage regulating circuit comprises a first operational amplifier IC1 and a resistor R₃A first capacitor C₁And a second capacitor C₂(ii) a The positive input end of the first operational amplifier IC1 is connected with the output end A of the voltage division sampling circuit, the negative input end of the first operational amplifier IC1 is connected with the reference voltage, and the positive input end of the first operational amplifier IC1 is connected with the reference voltage through a resistor R₃A first capacitor C₁And a second capacitor C₂Is connected with the output end; second capacitor C₂And a resistance R₃And the input end of the sawtooth wave comparison and drive signal generation circuit is connected with the input end of the sawtooth wave comparison and drive signal generation circuit.

Further, the sawtooth wave comparison and driving signal generation circuit comprises a second operational amplifier IC 2; the positive input end of the second operational amplifier IC2 is connected with the output end of the first operational amplifier IC1 in the voltage regulating circuit, and the negative input end of the second operational amplifier IC1 is connected with the sawtooth wave; the output of the second operational amplifier IC2 is connected to the input of the isolation drive circuit.

Further, the collected output voltage signal kv_oInputting the voltage signal into a voltage regulating circuit, comparing the output voltage signal with a reference voltage to obtain the v of a voltage closed loop_EAV is to be_EAThe signal is input into a sawtooth wave comparison and drive signal generation circuit to obtain a drive signal, and the drive signal directly controls the on and off of the first switching tube Q through an isolation drive circuit.

Furthermore, the amplifiers used in the first operational amplifier IC1 and the second operational amplifier IC2 are operational amplifiers of models TL074, TL072, LM358, or LM 324.

Furthermore, the voltage regulation circuit and the sawtooth wave comparison and signal generation circuit can use drive chips of SG3525 or UC3525, and the isolation drive circuit can use drive chips of TLP250 and the like.

3 analysis of Properties

Design of 3.1 inductance value

In order for the Buck PFC converter to operate in DCM completely, the inductor current must drop to 0 before the switching tube is turned on in the next switching cycle, i.e. it must satisfy:

D_Q+D_D＜1 (28)

wherein D_QFor the on-duty ratio of the switching tube, D_DThe duty cycle is turned on for the freewheeling diode.

The minimum inductance value in the critical continuous mode of the Buck operation phase is expressed by the following expressions (5) and (9):

when the capacitance value of the energy storage capacitor is large, the output voltage ripple is small and can be ignored relative to the direct current component, and then

The critical inductance value can be plotted against the input voltage according to the converter parameters and equations (28) - (29), as shown in fig. 8. The minimum value of the critical continuous inductance is 34uH, and an inductance value of 25uH is selected in consideration of the margin.

3.2 input Current waveform

The waveform of the input current in a half power frequency period under different output capacitances can be made by the equation (25), as shown in fig. 9 and 10.

FIG. 9 is V_{m_rms}110V, input current waveform at different capacitances; FIG. 10 is V_{m_rms}220V, input current waveform at different capacitances. Under the same input voltage, when the capacitance value of the energy storage capacitor is smaller, the waveform of the input current is smallerThe forward tilt phenomenon occurs, and the forward tilt phenomenon of the input current is alleviated when the input voltage increases. As can be seen by comparing fig. 10 and 11, the dead zone of the input current decreases as the input voltage increases, and the magnitude of the input current decreases as the input voltage increases.

3.3 output Voltage waveform

According to the formula (21), v can be expressed_oFig. 11 and 12 show theoretical approximate curves of (a). Under the same input voltage, the output voltage ripple increases along with the decrease of the capacitance value of the energy storage capacitor, and conforms to the theoretical change rule of the formula (13). From fig. 11 and 12, it can be concluded that the change in the input voltage has less influence on the output voltage ripple. The variation in capacitance is a major factor affecting the output voltage ripple.

3.4 power factor

According to the equation (27), the relationship between the power factor and the capacitance value of the capacitor can be made, as shown in fig. 13. As can be seen from fig. 13, the power factor increases with an increase in capacitance value of the capacitor, and increases with an increase in input voltage.

3.5 selection principle of capacitance value of energy storage capacitor

Reducing the capacitance of the energy storage capacitor can extend the service life of the power supply and reduce the volume of the capacitor, thereby increasing the power density, but at the cost of reducing the PF, fig. 13 has analyzed the relationship between PF and C, so the PF has to be considered when selecting the capacitor. The normal working condition of the Buck converter is that the rectified voltage v_gGreater than the output voltage v_oWhen the capacitance is small, the voltage ripple is large, and it is difficult to satisfy the normal operating condition. Assuming that the ripple voltage is symmetric about the average value of the output voltage, the ripple voltage peak-to-peak value is:

ΔV_o＜2(V_m-V_o) (31)

therefore, the capacitance is selected according to two principles: 1. a power factor higher than 0.9; 2.Δ V_o<2(V_m-V_o). The capacitance value of the energy storage capacitor selected in the method is 120uF, and then the electrolytic capacitor of 120uF can be replaced by a film capacitor of 120uF, so that the service life and the power density of the converter are improved.

In summary, the DCM Buck PFC converter with large ripple output voltage of the present invention adopts a method of increasing voltage ripple, realizes reduction of capacitance value of the energy storage capacitor and uses the thin film capacitor instead of the electrolytic capacitor, so that the converter improves power density and prolongs the service life of the converter.