Single-stage PFC converter rear stage ripple elimination circuit

Abstract

The present invention designs a single-stage PFC converter post-stage ripple elimination circuit, which is composed of a single-stage PFC converter (1), a DC-DC converter (2), a filter capacitor and an output capacitor. The single-stage PFC converter (1) has three groups or two groups of outputs. When the single-stage PFC converter (1) has three groups of outputs, the voltage difference between the first group and the second group of outputs is used as the input of the DC-DC converter (2), and the output of the DC-DC converter (2) is superimposed on the third group of outputs. Control is performed so that the AC component of the output of the DC-DC converter (2) just offsets the second harmonic of the third group of outputs, thereby obtaining a ripple-free DC output. When the power of the whole machine is relatively small, the single-stage PFC converter (1) can remove the third group of outputs, while the operating voltage of the DC-DC converter (2) remains unchanged, and its output is a ripple-free DC. Advantages: The operating voltage and conversion power of the DC-DC converter (2) are significantly reduced, and low-voltage power devices can be used to improve efficiency and reduce costs. The present invention is a new technical solution for single-stage PFC+DC conversion, which can be widely used in high power factor switching power supplies.

Description

Technical Field

[0001] The invention relates to a single-stage PFC converter rear-stage ripple elimination circuit, which is a switching power supply technology and belongs to the field of power electronics technology. Background Art

[0002] At present, there are two types of technical solutions for high power factor isolated AC-DC converters, one is a single-stage conversion topology, and the other is a two-stage conversion topology.

[0003] The single-stage conversion topology mainly includes single-stage PFC (Power factor Correction) converters such as flyback and bridge, as well as combined single-stage converters consisting of PFC conversion and DC-DC conversion (sharing a set of switch tubes and control circuits). The single-stage PFC converter is characterized by a high power factor, but the DC output contains secondary harmonic ripple. The characteristic of the combined single-stage converter is that the ripple at the DC output and the power factor at the AC input can be optimized by compromise, which can reduce or eliminate the ripple at the DC output, but will reduce the power factor at the input.

[0004] The two-stage conversion topology solution is that the first stage is power factor correction AC-DC conversion, and the second stage is DC-DC conversion. There are two technical routes. One is that the first stage AC-DC is non-isolated, generally using the Boost topology; and the second stage DC-DC is isolated, mainly including topologies such as phase-shifted full-bridge, LLC conversion and single-ended conversion. The other is that the first stage AC-DC is isolated, that is, a single-stage PFC converter, mainly including topologies such as flyback and bridge; and the second stage DC-DC is non-isolated, mainly including topologies such as Buck, Boost, and Buck-Boost. The two-stage conversion topology can completely eliminate output ripple.

[0005] The main disadvantages of the two-stage conversion topology are: ① the circuit is complex. ② the overall efficiency is reduced. ③ the cost is high. These are all caused by the fact that all input electrical energy needs to go through two-stage power conversion before it can reach the load.

[0006] The above contents are only used to assist in understanding the technical solutions of the present invention, and do not constitute an admission that the above contents are all prior art. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and design a single-stage PFC converter rear-stage ripple elimination circuit. It is a new technical solution for single-stage PFC+non-isolated DC-DC conversion, which uses a simple circuit topology to reduce the operating voltage and conversion power of the rear-stage DC-DC converter, not only improving the overall efficiency, but also reducing the cost.

[0008] The technical solution of the present invention is as follows.

[0009] A single-stage PFC converter post-stage ripple elimination circuit is composed of a single-stage PFC converter (1), a DC-DC converter (2), filter capacitors (C1, C2, C3) and an output capacitor (Co). The single-stage PFC converter (1) has three groups of outputs or two groups of outputs, which are connected to a common ground or a common positive electrode. The DC-DC converter (2) is a non-isolated Buck topology or a Boost topology or a step-up-down topology, having an input positive electrode P, an input negative electrode N and an output electrode S; the input voltage of the DC-DC converter (2) is connected by the input positive electrode P and the input negative electrode N, and the output voltage of the DC-DC converter (2) is formed between the output electrode S and the input negative electrode N or the input positive electrode P. Among them,

[0010] When the three output groups or two output groups of the single-stage PFC converter (1) are connected to a common ground, the negative electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a ground terminal GND, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N, and the output electrode S of the DC-DC converter (2) serves as the positive terminal Vo; the positive electrode Vo1 of the first output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode Vo2 of the second output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the positive electrode Vo3 of the third output group is connected to the output electrode S of the DC-DC converter (2). The positive electrode of the filter capacitor (C1) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C1) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C2) is connected to the input negative electrode N of the DC-DC converter (2), and the negative electrode of the filter capacitor (C2) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed. The positive electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the negative electrode of the output capacitor (Co) is connected to the ground terminal GND or connected to the input negative electrode N of the DC-DC converter (2). The load is connected via the positive terminal Vo and the ground terminal GND.

[0011] When the three output groups or two output groups of the single-stage PFC converter (1) are connected with a common positive electrode, the positive electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a positive terminal Vo, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input positive electrode P, and the output electrode S of the DC-DC converter (2) serves as the ground terminal GND; the ground GND1 of the first output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2), and the ground GND2 of the second output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the ground GND3 of the third output group is connected to the output electrode S of the DC-DC converter (2). The negative electrode of the filter capacitor (C1) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C1) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C2) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode of the filter capacitor (C2) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed. The negative electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the positive electrode of the output capacitor (Co) is connected to the positive terminal Vo or connected to the input positive electrode P of the DC-DC converter (2). The load is connected via the positive terminal Vo and the ground terminal GND.

[0012] Compared with the prior art, the present invention has the following advantages.

[0013] 1) In the invention, the input voltage of the subsequent DC-DC converter (2) is equal to the difference between the voltage of the first group of outputs and the voltage of the second group of outputs of the single-stage PFC converter (1); that is, the operating voltage is significantly reduced, and low-voltage power devices can be used.

[0014] 2) In the invention, the conversion power of the subsequent DC-DC converter (2) is much smaller than the total power of the two output groups of the single-stage PFC converter (2), and even smaller than the total power of the three output groups, so its power loss is further reduced.

[0015] 3) The invention has a simple and ingenious circuit topology, which reduces the operating voltage and conversion power of the subsequent DC-DC converter (2), improves the efficiency of the whole machine, reduces the circuit cost, and has universal practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is the common ground connection schematic diagram of the post-stage ripple elimination circuit of a single-stage PFC converter.

[0017] Figure 2It is the common positive connection principle diagram of the rear-stage ripple elimination circuit of a single-stage PFC converter.

[0018] Among them, 1 is a single-stage PFC converter, 2 is a DC-DC converter, C1, C2, C3 are filter capacitors, and Co is an output capacitor. a ——AC input power, Vo——positive terminal, GND——ground terminal.

[0019] Figure 3 It is a synchronous Buck topology diagram of the DC-DC converter of the invention.

[0020] Figure 4 It is a non-synchronous Buck common negative electrode output topology diagram of the DC-DC converter of the invention.

[0021] Figure 5 It is a non-synchronous Buck common positive output topology diagram of the DC-DC converter of the invention.

[0022] Among them, Q1, Q2 are switching tubes, D1 is a diode, and Lo is a filter inductor. DETAILED DESCRIPTION

[0023] The present invention will be described and analyzed in detail below with reference to the accompanying drawings and preferred embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention and not all of them.

[0024] It should be further noted that the descriptions such as “first”, “second” and “third” involved in the present invention are only used for indicative purposes and should not be understood as indicating their relative importance or implicitly indicating the number of technical features.

[0025] 1. Preferred embodiments of the present invention

[0026] like Figure 1 , Figure 2 As shown, a single-stage PFC converter rear-stage ripple elimination circuit is composed of a single-stage PFC converter (1), a DC-DC converter (2), filter capacitors (C1, C2, C3) and an output capacitor (Co). The single-stage PFC converter (1) has three groups of outputs or two groups of outputs, which are connected to a common ground or a common positive electrode. The DC-DC converter (2) is a non-isolated Buck topology or a Boost topology or a step-up-down topology, and has an input positive electrode P, an input negative electrode N and an output electrode S; the input voltage of the DC-DC converter (2) is connected by the input positive electrode P and the input negative electrode N, and the output voltage of the DC-DC converter (2) is formed between the output electrode S and the input negative electrode N or the input positive electrode P.

[0027] like Figure 1As shown, when the three output groups or two output groups of the single-stage PFC converter (1) are connected to a common ground, the negative electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a ground terminal GND, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N, and the output electrode S of the DC-DC converter (2) serves as the positive terminal Vo; the positive electrode Vo1 of the first output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode Vo2 of the second output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the positive electrode Vo3 of the third output group is connected to the output electrode S of the DC-DC converter (2). The positive electrode of the filter capacitor (C1) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C1) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C2) is connected to the input negative electrode N of the DC-DC converter (2), and the negative electrode of the filter capacitor (C2) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed. The positive electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the negative electrode of the output capacitor (Co) is connected to the ground terminal GND or connected to the input negative electrode N of the DC-DC converter (2). The load is connected via the positive terminal Vo and the ground terminal GND.

[0028] like Figure 2As shown, when the three output groups or two output groups of the single-stage PFC converter (1) are connected with a common positive electrode, the positive electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a positive terminal Vo, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input positive electrode P, and the output electrode S of the DC-DC converter (2) serves as the ground terminal GND; the ground GND1 of the first output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2), and the ground GND2 of the second output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the ground GND3 of the third output group is connected to the output electrode S of the DC-DC converter (2). The negative electrode of the filter capacitor (C1) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C1) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C2) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode of the filter capacitor (C2) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed. The negative electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the positive electrode of the output capacitor (Co) is connected to the positive terminal Vo or connected to the input positive electrode P of the DC-DC converter (2). The load is connected via the positive terminal Vo and the ground terminal GND.

[0029] The DC-DC converter (2) may adopt a Buck topology, a Boost topology, or a step-up-down topology. When the conversion power is large, synchronous conversion is adopted to improve efficiency; when the conversion power is small, asynchronous conversion is adopted to reduce cost.

[0030] like Figure 3 As shown, the DC-DC converter (2) adopts a synchronous Buck topology, which is composed of two switching tubes (Q1, Q2) and a filter inductor (Lo), and has an input positive electrode P, an input negative electrode N and an output electrode S. The drain of the switching tube (Q1) is connected to the input positive electrode P, and the source of the switching tube (Q2) is connected to the input negative electrode N; one end of the filter inductor (Lo) is connected to the source of the switching tube (Q1) and the drain of the switching tube (Q2), and the other end of the filter inductor (Lo) is connected to the output electrode S. The output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N or the input positive electrode P.

[0031] like Figure 4 As shown, if the DC-DC converter (2) adopts the non-synchronous Buck common cathode output topology, then Figure 3The switch tube (Q2) in the synchronous Buck topology shown is replaced by a diode (D1), that is, it is composed of a switch tube (Q1), a diode (D1) and a filter inductor (Lo). The drain of the switch tube (Q1) is connected to the input positive electrode P, and the anode of the diode (D1) is connected to the input negative electrode N; one end of the filter inductor (Lo) is connected to the source of the switch tube (Q1) and the cathode of the diode (D1), and the other end of the filter inductor (Lo) is connected to the output electrode S. The output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N. This topology is only applicable to the solution where three or two groups of outputs of the single-stage PFC converter (1) are connected to a common ground.

[0032] like Figure 5 As shown, if the DC-DC converter (2) adopts the non-synchronous Buck common positive output topology, then Figure 3 The switch tube (Q1) in the synchronous Buck topology shown is replaced by a diode (D1), that is, it is composed of a switch tube (Q2), a diode (D1) and a filter inductor (Lo). The source of the switch tube (Q2) is connected to the input negative electrode N, and the cathode of the diode (D1) is connected to the input positive electrode P; one end of the filter inductor (Lo) is connected to the drain of the switch tube (Q2) and the anode of the diode (D1), and the other end of the filter inductor (Lo) is connected to the output electrode S. The output voltage of the DC-DC converter (2) is formed by the output electrode S and the input positive electrode P. This topology is only applicable to the solution in which three or two groups of outputs of the single-stage PFC converter (1) are connected with a common positive electrode.

[0033] 2. Working Principle of the Invention

[0034] The working principle of the post-stage ripple elimination circuit of the single-stage PFC converter is analyzed in detail from the following four aspects. These four aspects can be summarized as: the output voltage of the single-stage PFC converter; the input voltage of the DC-DC converter (2); the conversion power distribution and the overall efficiency analysis; and the capacity selection of the filter capacitor.

[0035] 2.1 Output voltage of single-stage PFC converter

[0036] The so-called single-stage PFC (Power Factor Correction) converter refers to a conversion circuit that outputs a DC voltage and implements power factor correction through a single-stage power conversion. Its output is a DC voltage V D and DC current I D , the input quantity is AC voltage u a and the alternating current i a The control strategy of the converter is to stabilize the required DC output while achieving power factor correction at the input.

[0037] The so-called power factor correction (PFC) is the AC current i aTracking AC voltage u a , so that their waveforms are consistent and their phases are the same, thus achieving a high power factor. Theoretically, the power factor PF≤1. When PF=1, we have:

[0038]

[0039] In formula (E-1), V a is the AC voltage u a The effective value of I a is the alternating current i a The effective value of ω is the angular frequency of the sinusoidal alternating current. Assuming the efficiency of the single-stage PFC converter is η, the AC input power P of the single-stage PFC converter is a And DC output power P D They are:

[0040]

[0041] The output power P D Decomposition into DC components Plus AC In the form of:

[0042]

[0043] Similarly, the DC output voltage V D , which can be decomposed into the DC component Plus AC of the form, namely:

[0044]

[0045] Taking the resistive load as an example, the following analyzes the AC component in the DC output voltage Because the load containing a reactance component (inductive reactance or capacitive reactance) is connected in parallel with the filter capacitor, it can be equivalent to a parallel model of a capacitor and a resistor.

[0046] According to the law of conservation of energy, linear superposition theorem and circuit theory, the following differential equation is obtained:

[0047]

[0048] Where C is the filter capacitor, R o is the load resistance. Considering the actual Formula (E-5) is simplified to:

[0049]

[0050] Solving the differential equation of (E-6), we can obtain the AC component The expression is as follows:

[0051]

[0052] It can be seen from formula (E-7) that the AC component of the output voltage of the single-stage PFC converter is The angular frequency of the second harmonic is twice the angular frequency of the input AC voltage, so it is called the second harmonic. Increasing the filter capacitor C can reduce the second harmonic, but it cannot be completely eliminated. If you want to completely eliminate the second harmonic, you need to use other technical means.

[0053] 2.2 Input voltage of DC-DC converter (2)

[0054] There are six basic topologies of non-isolated DC-DC converters, namely Buck, Boost, Buck-Boost, Sepic, Cuk, and Zeta. Buck is a step-down conversion, Boost is a step-up conversion, Buck-Boost is a step-up-down conversion, Sepic, Cuk, and Zeta are also step-up-down conversions, and use more devices than Buck-Boost, but the input-output characteristics are improved. When a non-isolated DC-DC converter operates in the current continuous mode, its voltage transfer function is as follows:

[0055]

[0056] In formula (E-8), D is the on-duty cycle of the switch tube, that is, the duty cycle of PWM control; V in With V out They are the input voltage and output voltage of the non-isolated DC-DC converter respectively.

[0057] The DC-DC converter (2) in the invention can adopt one of the six basic topologies mentioned above. Figure 1 and Figure 3 , taking the output common ground connection of a single-stage PFC converter (1) and the DC-DC converter (2) using a synchronous Buck topology as an example, a detailed analysis is conducted.

[0058] When the DC-DC converter (2) completely eliminates the ripple (i.e., the second harmonic) at the Vo terminal, the output voltage has only a DC component. According to equation (E-4), let the three output voltages (V o1 、V o2 、V o3 )for:

[0059]

[0060] In this example, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N. Then the input voltage (i.e., the working voltage) V of the DC-DC converter (2) isin , output voltage V out They are:

[0061]

[0062] According to formula (E-8), if the DC-DC converter (2) operates in the current continuous mode, its voltage transfer function is:

[0063]

[0064] In formula (E-11), D t is the on-duty ratio of switch Q1, which changes with time. Ignoring the dead time, the on-duty ratio of switch Q2 is (1-D t ). Then the output voltage With V o1 、V o2 The relationship is:

[0065]

[0066] It can be seen that if the second harmonic in Vo is to be completely eliminated, only feedback control D t Just satisfy formula (E-12).

[0067] Because 0≤D t ≤1, so a necessary condition for the normal operation of the DC-DC converter (2) is:

[0068]

[0069] Without loss of generality, we can set:

[0070]

[0071] n is called the margin coefficient. Considering a margin of 10 to 50%, n can generally be taken as 1.1 to 1.5.

[0072] According to formula (E-10) and formula (E-14), it can be deduced that:

[0073]

[0074] From formula (E-15), it can be seen that the working voltage V of the DC-DC converter (2) is in Only with Associated, given and You can determine V in The input voltage of the DC-DC converter (2) is the two output voltages (V o1 、V o2), that is, the operating voltage is significantly reduced. Therefore, the DC-DC converter (2) can use low-voltage power devices with low on-resistance, thereby improving efficiency and reducing costs.

[0075] 2.3 Conversion power distribution and overall efficiency analysis

[0076] If the single-stage PFC converter (1) has a third output V o3 According to formula (E-2), the third group output power P of the single-stage PFC converter (1) is o3 for:

[0077] P o3 = k·P o ·(1-cos(2ωt))=k·P o -k·P o ·cos(2ωt) (E-16)

[0078] In formula (E-16), P o is the total output power of the invention, k is P o3 Total output power P o The proportional coefficient is 0≤k≤1. If the single-stage PFC converter (1) does not have a third output group V o3 , then k=0.

[0079] From the perspective of energy transmission, if the second harmonic is to be completely eliminated at the Vo terminal, the output current I of the DC-DC converter (2) must be DC Satisfies the following formula:

[0080]

[0081] Combining equations (E-10) and (E-17), we can obtain the conversion power P of the DC-DC converter (2) DC for:

[0082]

[0083] According to formula (E-18), the three special cases are: k = 0, k = 0.5 and k = 1. They correspond to the third group of output power P of the single-stage PFC converter (1) respectively. o3 Total output power P o The ratio is 0%, 50%, 100%. That is:

[0084]

[0085] One point to note: when k=1, the DC-DC converter (2) is equivalent to an active filter on the DC side.

[0086] The invention, the single-stage PFC converter (1) directly outputs to Vo The power at the end is P o -P DC Therefore, the input power P of the single-stage PFC converter (1) is obtained a for:

[0087]

[0088] Wherein, η2 is the efficiency of the DC-DC converter (2), and η is the efficiency of the single-stage PFC converter (1). According to formula (E-20), the overall efficiency η of the invention is derived: e for:

[0089]

[0090] In the traditional two-stage conversion topology, the output power of the single-stage PFC conversion is all output after the subsequent DC-DC conversion, and its overall efficiency is (η·η2). It can be seen from formula (E-21) that the overall efficiency η of the invention is e Higher than the overall efficiency (η·η2) of the traditional two-stage conversion topology, P DC The smaller the size, the higher the overall efficiency.

[0091] 2.4 Selection of filter capacitor capacity

[0092] Depending on the topology of the DC-DC converter (2) and the selection of filter capacitors (C1, C2, C3), the capacity of the filter capacitors may vary. Figure 1 and Figure 3 , taking the output voltage of a single-stage PFC converter (1) with a common ground and a DC-DC converter (2) using a synchronous Buck topology as an example, a detailed analysis is conducted.

[0093] When the DC-DC converter (2) adopts the Buck topology, there are two situations for selecting the filter capacitors (C1, C2, C3). One is to only retain the filter capacitors (C2, C3), and the other is to only retain the filter capacitors (C1, C2).

[0094] 2.4.1 Only retain filter capacitors C2 and C3 and remove C1.

[0095] At this time, when the switch tube Q1 of the DC-DC converter (2) is turned on, both filter capacitors C2 and C3 are discharged; when the switch tube Q2 is turned on, filter capacitor C2 is discharged but filter capacitor C3 is not discharged. The voltages of filter capacitors C2 and C3 are respectively (V C2 、V C3 ), for the convenience of expression, set (V C2 、V C3 The proportional relationship between the DC component and the AC component of ) is as follows:

[0096]

[0097] Generally choose: δ,σ∈[0.1,0.3]. Because (V C2 =V o2 , V C3 =V o1 -V o2 ),and and The phases are basically consistent, so from equations (E-15), (E-14), (E-12) and (E-22) we can get:

[0098]

[0099]

[0100] According to formula (E-16), V of the single-stage PFC converter (1) o3 The output power at the end is P o3 , so V o1 Terminal and V o2 Output to V o The power at the end is (P o -P o3 ). (P o -P o3 ) can be decomposed into two parts, namely the output power P of the filter capacitor C2 C2O And the output power P of filter capacitor C3 C3O , according to formula (E-24) and formula (E-17):

[0101]

[0102] According to the energy conservation and formula (E-2), the instantaneous reactive power (i.e., AC component) superimposed on the filter capacitors C2 and C3 through the single-stage PFC converter (1) and the DC-DC converter (2) is They are:

[0103]

[0104] It can be seen from formula (E-26) that the total reactive power With P o3 Total output power P o The proportionality coefficient k has nothing to do with k, which only affects the distribution of reactive power on the filter capacitors (C2, C3). This is in line with the law of conservation of energy, because only the filter capacitors (C2, C3) are responsible for filtering out the second harmonic. and It contains not only the second harmonic but also the fourth harmonic, but its amplitude is small and can be ignored. The simulation analysis verifies the correctness of this conclusion.

[0105] Only consider and The second harmonic in, according to formula (E-7), assuming The expression is:

[0106]

[0107] Substituting equation (E-27) and proportional equation (E-22) and equation (E-23) into equation (E-26), simplifying according to the sum and difference angle formula of trigonometric functions, we obtain:

[0108]

[0109] According to formula (E-7), filter capacitors C2, C3 and their ripple voltage The relationships between them are:

[0110]

[0111]

[0112] From formula (E-30), the relationship between filter capacitors C2 and C3 can be obtained as follows:

[0113]

[0114] To further simplify the design, set:

[0115]

[0116] Substituting equation (E-23) and equation (E-31) into equation (E-32), we obtain:

[0117]

[0118] 2.4.2 Only retain filter capacitors C1 and C2 and remove C3.

[0119] At this time, when the switch tube Q1 of the DC-DC converter (2) is turned on, the filter capacitor C1 discharges but C2 does not discharge; when the switch tube Q2 is turned on, the filter capacitor C2 discharges but C1 does not discharge. The voltages of the filter capacitors C1 and C2 are respectively (V C1 、V C2 ), for the convenience of expression, set (V C1 、V C2 The proportional relationship between the DC component and the AC component of ) is as follows:

[0120]

[0121] Generally, δ=0.1~0.3. Because (VC1 =V o1 , V C2 =V o2 ), so from formula (E-15), formula (E-14), formula (E-12) and formula (E-34), we can get:

[0122]

[0123]

[0124]

[0125] According to formula (E-16), V of the single-stage PFC converter (1) o3 The output power at the end is P o3 , so V o1 Terminal and V o2 Output to V o The power at the end is (P o -P o3 ). (P o -P o3 ) can be decomposed into two parts, namely the output power P of the filter capacitor C1 C1O And the output power P of filter capacitor C2 C2O According to formula (E-36), formula (E-37) and formula (E-17), we get:

[0126]

[0127] According to the law of energy conservation and formula (E-2), the instantaneous reactive power (i.e., AC component) superimposed on the filter capacitors C1 and C2 through the single-stage PFC converter (1) and the DC-DC converter (2) is They are:

[0128]

[0129] From formula (E-39), we can see that the total reactive power With P o3 Total output power P o The proportionality coefficient k has nothing to do with k, which only affects the distribution of reactive power on the filter capacitors (C1, C2). This is in line with the law of conservation of energy, because only the filter capacitors (C1, C2) are responsible for filtering out the second harmonic. and The wave contains not only the second harmonic, but also the fourth and higher harmonics, but their amplitudes are small and can be ignored. The simulation analysis verifies the correctness of this conclusion.

[0130] Only consider and Substitute equations (E-34) and (E-35) into equation (E-39) and simplify according to the sum and difference angle formula of trigonometric functions to obtain:

[0131]

[0132]

[0133] According to formula (E-7), the filter capacitors C1 and C2 are related to the ripple voltage The relationships between them are:

[0134]

[0135]

[0136] So far, the present invention has been analyzed in detail by taking the case where the output of the single-stage PFC converter (1) is connected to a common ground and the DC-DC converter (2) adopts a synchronous Buck topology as an example. When the output of the single-stage PFC converter (1) is connected to a common positive electrode and the DC-DC converter (2) adopts other non-isolated topologies, the analysis process can be carried out according to similar principles and will not be described in detail here.

[0137] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly or indirectly applied in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A single-stage PFC converter rear-stage ripple elimination circuit, comprising a single-stage PFC converter (1), a DC-DC converter (2), filter capacitors (C1, C2, C3) and an output capacitor (Co); the single-stage PFC converter (1) has three groups of outputs or two groups of outputs, which are connected to a common ground or a common positive electrode; the DC-DC converter (2) is a non-isolated Buck topology or a Boost topology or a step-up-down topology, having an input positive electrode P, an input negative electrode N and an output electrode S; the input voltage of the DC-DC converter (2) is connected by the input positive electrode P and the input negative electrode N, and the output voltage of the DC-DC converter (2) is formed between the output electrode S and the input negative electrode N or the input positive electrode P; the characteristics are: When the three output groups or two output groups of the single-stage PFC converter (1) are connected to a common ground, the negative electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a ground terminal GND, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input negative electrode N, and the output electrode S of the DC-DC converter (2) serves as the positive terminal Vo; the positive electrode Vo1 of the first output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode Vo2 of the second output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the positive electrode Vo3 of the third output group is connected to the output electrode S of the DC-DC converter (2); the filter The positive electrode of the wave capacitor (C1) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C1) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C2) is connected to the input negative electrode N of the DC-DC converter (2), and the negative electrode of the filter capacitor (C2) is connected to the ground terminal GND; the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2), and the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed; the positive electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the negative electrode of the output capacitor (Co) is connected to the ground terminal GND or connected to the input negative electrode N of the DC-DC converter (2); When the three output groups or two output groups of the single-stage PFC converter (1) are connected with a common positive electrode, the positive electrodes of the three output groups or two output groups of the single-stage PFC converter (1) are connected together to form a positive terminal Vo, the output voltage of the DC-DC converter (2) is formed by the output electrode S and the input positive electrode P, and the output electrode S of the DC-DC converter (2) serves as the ground terminal GND; the ground GND1 of the first output group of the single-stage PFC converter (1) is connected to the input negative electrode N of the DC-DC converter (2), and the ground GND2 of the second output group of the single-stage PFC converter (1) is connected to the input positive electrode P of the DC-DC converter (2); if the single-stage PFC converter (1) has three output groups, the ground GND3 of the third output group is connected to the output electrode S of the DC-DC converter (2); The negative electrode of the filter capacitor (C1) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C1) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C2) is connected to the input positive electrode P of the DC-DC converter (2), and the positive electrode of the filter capacitor (C2) is connected to the positive terminal Vo; the negative electrode of the filter capacitor (C3) is connected to the input negative electrode N of the DC-DC converter (2), and the positive electrode of the filter capacitor (C3) is connected to the input positive electrode P of the DC-DC converter (2); or any one of the three filter capacitors (C1, C2, C3) is removed; the negative electrode of the output capacitor (Co) is connected to the output electrode S of the DC-DC converter (2), and the positive electrode of the output capacitor (Co) is connected to the positive terminal Vo or connected to the input positive electrode P of the DC-DC converter (2).

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