Method for improving the primary-secondary disconnection problem of primary-side feedback converter

Abstract

The present invention discloses a method for improving the primary-secondary disconnection problem of a primary-side feedback converter. The double-clamped zero-voltage switching converter performs closed-loop control by sampling the clamping capacitor voltage according to the relationship between the primary-side clamping capacitor voltage and the secondary-side output voltage, thereby achieving the output isolation and voltage stabilization function. However, when switching between empty and full load, the clamping capacitor voltage cannot timely reflect the change in the output voltage, and there is a problem of primary-secondary disconnection. The present invention proposes two methods for improving the primary-secondary disconnection problem, including: by detecting the clamping capacitor voltage, when the converter switches between empty and full load, a dynamic load is added to the clamping capacitor to increase the discharge rate of the clamping capacitor; and the closed-loop point of the clamping capacitor voltage is increased when the converter is empty and full load, and the change in the discharge voltage of the clamping capacitor is reduced. The method proposed by the present invention is relatively simple, saves space, and can effectively avoid the problem of severe output voltage drop caused by the empty and full load switching of the converter, greatly improving the reliability of the subsequent electrical equipment.

Description

Technical Field

[0001] The invention belongs to the technical field of converters, and in particular to a method for improving the primary-secondary disconnection problem of a primary-side feedback converter. Background Art

[0002] For isolated DC / DC converters, in order to achieve control of the secondary output voltage, secondary feedback control or primary feedback control can be used. The converter using secondary feedback control has the advantages of high output voltage accuracy and good load regulation, but the sampling of the output voltage and the closed-loop regulation circuit are on the secondary side of the converter. The output signal of the closed-loop regulation circuit needs to be transmitted to the primary side of the converter through an optocoupler, and the introduction of the optocoupler and its peripheral circuits will reduce the power density of the converter. In addition, the current transfer ratio of the optocoupler will decay with time and temperature, affecting the reliability of the system. The converter using primary feedback control controls the signal that can reflect the secondary output voltage by sampling and feeding back the primary side of the converter, thereby indirectly achieving control of the secondary output voltage. The converter using primary feedback control can remove the optocoupler and peripheral circuits required for primary-secondary signal transmission, which can greatly improve the power density and reliability of the converter.

[0003] The double-clamped zero-voltage switching converter is a typical primary-side feedback converter. Figure 1 The circuit diagram shown is a circuit diagram including a first switch tube Q1, a second switch tube Q2, a third switch tube Q3, a fourth switch tube Q4, a secondary rectifier tube Q5, and a clamping capacitor C f , output capacitor C o , power transformer T and a main controller and synchronous rectification controller. One end of the primary side of the power transformer T is connected between the first switch tube Q1 and the second switch tube Q2, and the other end is connected between the third switch tube Q3 and the fourth switch tube Q4; one end of the secondary side of the power transformer T is connected to the output voltage v o The positive end of the double clamp zero voltage switching converter has four working stages in one switching cycle, such as Figure 2 As shown, it includes the input energy storage stage, the primary-secondary energy transfer stage, the resonance stage and the freewheeling stage.

[0004] In the primary-secondary energy transfer stage, the second switch tube Q2, the third switch tube Q3 and the secondary rectifier tube Q5 of the converter are turned on. Figure 3 As shown. At this time, the transformer primary voltage is clamped to v by the clamping capacitor voltage. c ; The secondary voltage is clamped to v by the output voltage o According to the transformer primary and secondary turns ratio, the clamping capacitor voltage v c and the output voltage vo Satisfies the following relationship: c =n·v o , where n is the transformer turns ratio. According to the clamping capacitor voltage v c and the output voltage v o The relationship between the sampling clamp capacitor voltage v c By performing closed-loop control, the output isolation and voltage regulation function can be realized.

[0005] However, when the load of the converter changes step by step, the clamping capacitor voltage v c and the output voltage v o During the transient process, the relationship described in the above formula may not be satisfied, resulting in the primary and secondary disconnection problem of the converter. If a load is applied during the primary and secondary disconnection period of the converter, the output voltage v of the converter may be o The drop is serious. In extreme cases, the output voltage v o The voltage drops to zero, seriously affecting the reliability of the subsequent electrical equipment. Accelerating the dynamic response speed of the loop can improve the primary and secondary disconnection problem to a certain extent, but the effect is very limited and it is easy to cause loop instability and output voltage oscillation. Summary of the invention

[0006] The object of the present invention is to provide a method for improving the primary-secondary disconnection problem of a primary-side feedback converter in view of the problems existing in the prior art.

[0007] The technical solution to achieve the purpose of the present invention is: a method for improving the primary-secondary disconnection problem of a primary-side feedback converter, the method comprising: reducing the primary-secondary disconnection time ΔT.

[0008] Furthermore, the implementation method of reducing the primary-secondary disconnection time ΔT includes: increasing the clamping capacitor C f Discharge rate k.

[0009] Furthermore, the implementation method of increasing the discharge rate k of the clamping capacitor includes: f The discharge resistor R connected in parallel f And the switch tube Q f , use the clamping capacitor C f Voltage c Control switch tube Q f , and introduces a hysteresis loop.

[0010] Furthermore, the switch tube Q f The control signal S f It is expressed as:

[0011]

[0012] In the formula, K1 is the proportionality coefficient, vc is the clamping capacitor C f Voltage, V set1 and V set2 are the lower and upper limits of the hysteresis control, respectively.

[0013] Furthermore, the implementation method of reducing the primary-secondary disconnection time ΔT includes: reducing the clamping capacitor C f Discharge voltage change ΔV.

[0014] Furthermore, the reduction of the clamping capacitance C f The discharge voltage change ΔV is achieved by increasing the clamping capacitor C when no-load f Voltage closed loop point, at the same time, in order to ensure the converter output voltage v at rated load o unchanged, it is necessary to make the converter output voltage v o As the load current i o decreases with the increase of .

[0015] Furthermore, for the double-clamped zero-voltage switching converter, the converter output voltage v o As the load current i o The implementation methods of increasing and decreasing include: ref -K3·v err As the closed-loop reference value of the converter, V ref is the reference value, K3 is the proportional coefficient, v err It is the output signal of the error amplifier in the double-clamped zero-voltage switching converter.

[0016] Furthermore, the reduction of the clamping capacitance C f The discharge voltage change ΔV can be achieved by increasing the output capacitance C o .

[0017] Compared with the prior art, the present invention has the following significant advantages:

[0018] The present invention proposes a method of detecting the clamping capacitor voltage, adding a dynamic load to the clamping capacitor when the converter switches between no-load and full-load, increasing the discharge rate of the clamping capacitor, and improving the primary-secondary side disconnection problem; and a method of increasing the clamping capacitor voltage closed-loop point when no-load, reducing the change in the clamping capacitor discharge voltage, and improving the primary-secondary side disconnection problem. The method proposed by the present invention is relatively simple, saves space, and can effectively avoid the problem of severe output voltage drop caused by no-load and full-load switching of the converter, greatly improving the reliability of the subsequent electrical equipment.

[0019] The present invention is further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1This is the structural diagram of the double-clamped zero-voltage switching converter.

[0021] Figure 2 This is the time domain waveform of the double-clamped zero-voltage switching converter.

[0022] Figure 3 This is the equivalent circuit diagram of the primary-secondary energy transfer of the double-clamped zero-voltage switching converter.

[0023] Figure 4 This is the relationship diagram between the clamping capacitor voltage and the output voltage of the double-clamped zero-voltage switching converter when it is fully loaded and no-loaded.

[0024] Figure 5 This is the equivalent circuit diagram of the natural discharge of the clamping capacitor and the output capacitor.

[0025] Figure 6 This is the equivalent circuit diagram of the converter's primary side working in Buck-Boost mode and the natural discharge of the secondary side output capacitor.

[0026] Figure 7 This is a simulation waveform diagram when the loading time is between t1 and t2.

[0027] Figure 8 Schematic diagram of the method to improve the original secondary side disconnection problem.

[0028] Fig. 9 Schematic diagram for increasing the discharge rate of the clamping capacitor.

[0029] Fig.10 Add dynamic load equivalent circuit diagram for clamping capacitor.

[0030] Fig.11 Schematic diagram of the dynamic load control method of the clamping capacitor, where Figure (a) is the hysteresis control circuit and Figure (b) is the hysteresis output schematic diagram.

[0031] Fig.12 Simulation waveforms showing the addition of a dynamic load to the clamp capacitor.

[0032] Fig.13 Schematic diagram of reducing voltage change in order to increase the closed-loop point of the clamping capacitor voltage.

[0033] Fig.14 This is the output voltage droop characteristic curve.

[0034] Fig.15 Schematic diagram of how to implement the output voltage droop characteristic.

[0035] Fig.16 To improve the closed-loop point simulation waveform of the clamping capacitor voltage.

[0036] Fig.17 Schematic diagram of reducing voltage change by increasing output capacitance. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0038] In one embodiment, a double-clamped zero-voltage switching converter is used as an example to illustrate a method for improving the primary-secondary disconnection problem of a primary-side feedback converter. It is worth noting that the method is not limited to the double-clamped zero-voltage switching converter.

[0039] Figure 4 The clamping capacitor voltage v of the double-clamped zero-voltage switching converter after full load switching is given. c and the output voltage v o At time t0, the load is suddenly unloaded, and the clamping capacitor voltage v c and the output voltage v o At time t1, the output of the converter error amplifier is adjusted to zero, the first switch tube Q1, the third switch tube Q3 and the secondary rectifier tube Q5 are turned off, and the clamping capacitor and the output capacitor begin to discharge naturally.

[0040] In the actual design of the converter, the output capacitor C o The capacitance is usually much larger than the clamping capacitor C f capacity, so the clamp capacitor discharges much faster than the output capacitor. Due to the fast discharge rate of the clamp capacitor, after t1, v c <n·v o , the primary and secondary sides of the converter are disconnected. When the clamp capacitor discharges naturally to time t2, the clamp capacitor voltage is equal to the clamp capacitor voltage closed loop point, but still less than n·v o At this time, the first switch tube Q1, the second switch tube Q2, the third switch tube Q3 and the fourth switch tube Q4 are working, and the secondary rectifier tube Q5 is turned off; the primary side works in Buck-Boost mode, the clamping capacitor voltage is supported, the output capacitor is in a natural discharge state, and the primary and secondary sides are in a disconnected state.

[0041] According to the above analysis, there are two situations in which the original and secondary sides are disconnected after the converter is fully loaded and switched to no-load:

[0042] Case 1: The primary and secondary sides are disconnected at t1-t2, and the primary clamping capacitor and the output capacitor are in a natural discharge state. The equivalent circuit is as follows: Figure 5 shown.

[0043] Case 2: The primary and secondary sides are disconnected from each other at t2 to t3, the primary side works in Buck-Boost mode, and the secondary side output capacitor is in a natural discharge state. The equivalent circuit is as follows: Figure 6 shown.

[0044] When the converter is loaded between t1 and t2, due to the clamping capacitor voltage v c Higher than the clamp capacitor voltage closed-loop point, the clamp capacitor is in a natural discharge state and cannot reflect the output voltage v o Due to the rapid drop caused by loading, the sudden increase of load will cause the output voltage v o When the load is applied between t2 and t3, the primary side works in Buck-Boost mode, and the clamping capacitor voltage is supported; when the output voltage v o Fall to When the secondary rectifier tube Q5 is turned on, the energy is transferred from the primary side to the secondary side through the transformer, and the output voltage v o Get support.

[0045] Figure 7 The simulation waveform loaded between t1 and t2 is given. Figure 7 It can be seen that the clamping capacitor voltage v c If the capacitor is loaded before it is discharged to the closed-loop voltage point of the clamp capacitor, the sudden increase of the load will cause the output voltage v o Rapid drop, when the converter output capacitor C o When the capacity is small, the output voltage v o may drop to zero. The output voltage v o The rapid drop of voltage seriously affects the reliability of subsequent electrical equipment.

[0046] according to Figure 4 , reducing the primary-secondary disconnection time ΔT shown in Case 1 can improve the primary-secondary disconnection problem, so that the clamping capacitor voltage is supported when the converter is loaded, and the problem of the output voltage dropping rapidly to zero due to a sudden increase in load can be avoided.

[0047] Define the discharge rate of the clamp capacitor as k, and the voltage difference between the clamp capacitor overshooting to the highest point and discharging to the support point as ΔV. Then the primary-secondary side disconnection time ΔT can be expressed as:

[0048]

[0049] It can be seen from formula (2) that the disconnection time ΔT is inversely proportional to the discharge rate k, and the disconnection time is proportional to the voltage change ΔV. Therefore, increasing the discharge rate k and reducing the voltage change ΔV can shorten the disconnection time ΔT and improve the primary and secondary disconnection problem.

[0050] The present invention aims at the primary and secondary side disconnection problem existing in the no-load and full-load switching of the double-clamped zero voltage switching converter, and proposes a method to reduce the disconnection time ΔT and improve the primary and secondary side disconnection problem. Figure 8Specifically: 1. Increase the discharge rate k of the clamping capacitor; 2. Reduce the voltage change ΔV.

[0051] 1. Increase the clamping capacitor discharge rate k

[0052] Connect an additional discharge resistor R in parallel with the clamp capacitor f Can increase the clamp capacitor discharge rate k, and the resistor R f The smaller it is, the greater the discharge rate k is, and the shorter the disconnection time ΔT is. Fig. 9 As shown, increase the discharge resistor R f The disconnection time ΔT is significantly shortened from t1 to t2 to t1 to t 2' .

[0053] However, when the converter is working in steady state, there is no problem of primary and secondary disconnection, and the discharge resistor R f The existence of increases the extra loss, making the converter efficiency lower. At the same time, the discharge resistor R f The existence of increases the voltage pulsation of the clamping capacitor. For the double-clamped zero-voltage switching converter, it is necessary to sample the clamping capacitor voltage to achieve output isolation and voltage regulation. The increase in the voltage pulsation of the clamping capacitor reduces the stability of the converter at light load.

[0054] The present invention proposes to increase the discharge rate k of the clamping capacitor by adding a dynamic load at the clamping capacitor, thereby shortening the primary-secondary disconnection time ΔT. Specifically, the discharge resistor R f The switch tube Q is introduced into the branch f , so that the discharge resistor R f Only when the clamping capacitor voltage v c When overshoot occurs, the discharge of the clamp capacitor is accelerated. The circuit is as follows: Fig.10 As shown. Using the clamping capacitor voltage v c Control switch tube Q f , and introduce a certain hysteresis loop, such as Fig.11 As shown, K1 is the proportionality coefficient. c Higher than V set2 When the switch tube Q f The control signal S f Set to 1, switch tube Q f Open, resistance R f Discharge the clamping capacitor; K1·v c Lower than V set1 When the switch tube Q f The control signal S f Set to 0, switch Q f Off, resistance R f Disconnect; V set1 and V set2 are the lower and upper limits of the hysteresis control, respectively. Fig.11 (b) As shown above, the switch tube Qf The control signal S f It can be expressed as:

[0055]

[0056] V set1 and V set2 The settings can be adjusted flexibly according to the actual situation. For example, V set1 105% of the converter closed-loop control reference value, V set2 is 110% of the reference value; so when the load is suddenly unloaded, the clamp capacitor voltage v c When the closed loop point reaches 110%, the switch tube Q f Closed, discharge resistor R f Start to accelerate the discharge of the clamping capacitor; the clamping capacitor voltage v c When the discharge reaches 105% of the closed loop point, the switch tube Q f Disconnect and the clamp capacitor discharges naturally.

[0057] Fig.12 The simulation waveform of the clamping capacitor adding dynamic load is given. It can be seen from the figure that when the clamping capacitor surges due to the sudden load unloading, the switch tube Q f Open, discharge resistor R f Discharge the clamp capacitor and accelerate the discharge of the clamp capacitor to the closed loop point, so that the clamp capacitor voltage v c Obtain support; shorten the primary and secondary side disconnection time, and improve the primary and secondary side disconnection problem of the converter. Figure 7 , loaded at the same time, the output voltage v o The drop is significantly reduced.

[0058] 2. Reduce the voltage change ΔV

[0059] The present invention proposes to reduce the voltage variation ΔV by increasing the closed-loop point of the clamping capacitor voltage when no-load, such as Fig.13 It can be seen that after the closed-loop point of the no-load clamp capacitor voltage is increased, the primary-secondary disconnection time shown in Case 1 is shortened from t1~t2 to t1~t2", and the total primary-secondary disconnection time is shortened from t1~t3 to t1~t 3' .

[0060] In order to ensure the output voltage v at rated load o After increasing the closed-loop point of the clamping capacitor voltage at no-load, the output voltage v o It shows a drooping characteristic, that is, the output voltage v o As the load current i o decreases with the increase of Fig.14 As shown. o is the output voltage corresponding to the rated load, V o'is the output voltage corresponding to no load.

[0061] For the double-clamped zero-voltage switching converter, the output signal of the error amplifier is v err and load current i o The relationship is as follows:

[0062]

[0063] Where τ is a constant, L is the converter excitation inductance, V in and v out Corresponding to the input and output voltages respectively.

[0064] From formula (4), we can see that v err As the load increases, v err Small, heavy load v err Larger. V ref -K3·v err As the closed-loop reference value of the converter, the output voltage decreases as the load increases, such as Fig.15 shown.

[0065] Fig.16 The simulation waveforms of the converter no-load and full-load switching are given after the strategy of increasing the closed-loop point of the clamping capacitor voltage when the converter is no-load is adopted. Fig.16 It can be seen that by increasing the closed-loop voltage point of the clamp capacitor when the converter is unloaded, the clamp capacitor can be supported by naturally discharging to a higher closed-loop voltage point after the load is suddenly unloaded, reducing the voltage change ΔV, shortening the time of primary-secondary disconnection, and improving the primary-secondary disconnection of the converter. Figure 7 , loaded at the same time, the output voltage v o The drop is significantly reduced.

[0066] In addition, increasing the output capacitance can effectively limit the voltage overshoot caused by converter unloading and can also reduce the voltage change ΔV, such as Fig.17 As shown. When the voltage overshoot value after unloading decreases, the original secondary side disconnection time shown in Case 1 is shortened from t1~t2 to t1~t2” ' At the same time, the total primary-secondary disconnection time is shortened from t1~t3 to t1~t3".

[0067] The technical solution of the present invention is not limited to the above-mentioned embodiments, and any technical solution obtained by modification or equivalent replacement of the technical solution of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving the primary-secondary disconnection problem of a primary-side feedback converter, the method is implemented based on a double-clamped zero-voltage switching converter, the double-clamped zero-voltage switching converter includes a first switch tube Q 1. Second switch tube Q 2. The third switch tube Q 3. The fourth switch tube Q 4. Secondary rectifier tube Q 5. Clamping capacitor C f , output capacitor C o , Power Transformer T , a main controller and a synchronous rectification controller; the power transformer T One end of the primary side is connected to the first switch tube Q 1's S pole and the second switch tube Q 2, the first switch tube Q 1's D pole and the second switch tube Q The S pole of 2 is the input voltage v in , and the second switch tube Q 2 has its S pole grounded; the power transformer T The other end of the primary side is connected to the third switch tube Q 3's S pole and the fourth switch tube Q 4, the third switch tube Q 3's D pole and the fourth switch tube Q 4. Connect the clamp capacitor between the S poles of C f , power transformer T The secondary side is connected to the output voltage v o The positive end of the fifth switch tube Q 5's D pole, the fifth switch tube Q 5 S pole connected to output voltage v o The negative terminal of the output voltage v o Connect the output capacitor between the positive and negative terminals of C o The main controller is used to generate control signals for the four switch tubes. S 1. S 2. S 3 and S 4. Synchronous rectifier controller generates secondary rectifier tube Q 5 control signal S 5; characterized in that, The method comprises: reducing the primary and secondary side disconnection time ; The reduction of the original secondary side disengagement time Ways to achieve this include: increasing the clamping capacitance C f Discharge rate k , or reduce the clamping capacitance C f Discharge voltage change , or increase the output capacitance C o .

2. The method for improving the primary-secondary disconnection problem of a primary-side feedback converter according to claim 1, characterized in that: The increased clamping capacitance C f Discharge rate k The implementation methods include: C f Discharge resistors connected in parallel R f And switch tube Q f , use clamping capacitors C f Voltage v c Control switch tube Q f The opening of the clamp capacitor can increase the discharge rate of the clamp capacitor. k , and introduces a hysteresis loop.

3. The method for improving the primary-secondary disconnection problem of a primary-side feedback converter according to claim 2, characterized in that: The switch tube Q f The control signal S f It is expressed as: （1） In the formula, K 1 is the proportionality factor, v c Clamping capacitor C f The voltage, V set1 and V set2 are the lower and upper limits of the hysteresis control respectively; when Greater than the hysteresis control upper limit V set2 , switch tube Q f The control signal S f =1, the switch tube Q f Open, discharge resistor R f Discharge the clamp capacitor. C f Discharge rate k Increase; when the clamp capacitor voltage discharges to Less than the hysteresis control limit V set1 , switch tube Q f The control signal S f is 0, the switch tube Q f Shut down.

4. The method for improving the primary-secondary disconnection problem of a primary-side feedback converter according to claim 3, characterized in that: The reduced clamp capacitance C f Discharge voltage change The implementation methods include: increasing the clamping capacitance when no load C f voltage closed loop point, and at the same time make the converter output voltage v o As the load current i o decreases with the increase of .

5. The method for improving the primary-secondary disconnection problem of a primary-side feedback converter according to claim 4, characterized in that: For the double-clamped zero-voltage switching converter, the converter output voltage v o As the load current i o The implementation methods of reducing the increase of include: V ref - K 3∙ v err As the closed-loop reference value of the converter, V ref is the base value, K 3 is the proportionality coefficient, v err It is the output signal of the error amplifier in the double-clamped zero-voltage switching converter.

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