Method for controlling continuous current mode of flyback converter

Abstract

Disclosed in the present invention is a method for controlling a continuous current mode (CCM) of a flyback converter, comprising: when the flyback converter operates in a critical current mode, recording a current peak value ipk1 of a transformer and corresponding turn-off time Toff1, and recording same as a first period; if a current peak value ipk of a certain period is not higher than ipk1, turning on a flyback switch after the original turn-off logic ends, and recording same as a second period; if the current peak value ipk of a certain period is higher than ipk1 and the difference is Δi, controlling the switch to reduce the turn-off time of the period to Toff1-k·Δi, thus the current being unable to return to zero when the period ends, entering the CCM, and recording same as a third period; and then, by taking the current peak value ipk as a determination condition, the flyback converter switching between a discontinuous current mode (DCM) and the CCM. By means of the present invention, the duty ratio of the flyback converter entering the CCM can be stably controlled and adjusted, and the calculation of an output average current is also considered.

Description

A control method of continuous current mode of flyback converter TECHNICAL FIELD

[0001] The present application relates to the technical field of switching power supply chip control, and particularly relates to a control method of continuous current mode of flyback converter. BACKGROUND

[0002] Switching power supply is the energy source of most electrical and electronic products, and AC / DC conversion is the first step to obtain energy from the power grid. Generally, AC / DC is a rectifier bridge plus DC / DC conversion. As shown in FIG. 5, the positive and negative alternating sine AC voltage is first converted into a positive full-wave voltage through a rectifier bridge, and then a more stable voltage is formed through a filter capacitor to supply the DC / DC converter in the next stage. However, as the input voltage, the grid voltage has a very wide range of variation with the region: 80-280Vac, and the grid frequency also has a large fluctuation with the load: 47-63Hz. This limits the use of filter capacitors, which not only need to withstand the highest input voltage, but also need to be large enough to ensure the normal operation and performance of the DC / DC converter in the next stage. Using a filter capacitor with too small a capacitance value will make the voltage fluctuation on it larger and the minimum voltage lower.

[0003] As the main topology of small and medium power AC / DC switching power supply, the flyback converter is widely used in various consumer electronic products. Among them, the charger with the popularization of smart phones puts forward more stringent requirements on the power density of the whole machine power supply and the volume of each component. The filter capacitor after the rectifier bridge usually uses a 400V electrolytic capacitor, and the capacitance needs to reach 2μF / W or more of the full load power, and its volume accounts for 30-40% of the entire charger, which is the most difficult to reduce the volume of the component.

[0004] FIG. 6 shows a typical AC / DC flyback converter application circuit, in which the input is an AC voltage, BD is a rectifier bridge, C1 is a filter capacitor, Vbus is the voltage on C1, Q1 is the main switch in the primary side, Q2 is the rectifier switch in the secondary side, the transformer T has a primary winding with N p turns and a secondary winding with N s turns. C2 is the output filter capacitor, RL is the output load, and V o is the DC output voltage. When the flyback works in CCM, Q1 works in a pulse period with a certain duty cycle D, and Vo / Vbus satisfies N s / N p ·D / (1-D). When a smaller C1 capacitance value causes a large change in Vbus voltage, the flyback converter needs to respond to a larger duty cycle and a higher peak current in time to meet the output voltage without drop.

[0005] The existing peak current control technology can control the flyback converter in a closed loop, and a basic block diagram is shown in Fig. 7. The control circuit samples the current flowing through the switch Q1, compares the current peak value calculated by the loop through the comparator 1, and turns off Q1 when the current exceeds the value. After that, the oscillator generates a variable period pulse signal, or combines the auxiliary winding N a The detected resonance valley signal is used to turn on Q1. This technology has a simple implementation, can ensure that the current flowing through the transformer and Q1 is within a safe range, and can decouple the inductor current in the loop control, thereby improving the loop stability; but the disadvantage is that when the duty cycle D of CCM is greater than 50%, sub-harmonic oscillation will occur, and a fixed slope compensation needs to be added to the loop, which reduces the original peak current, and the compensation amount is sometimes insufficient or over-adjusted, on the other hand, the duty cycle will be limited to within 80% because the oscillator frequency is fixed, so that the output voltage cannot be maintained stable at a lower Vbus voltage.

[0006] Secondly, the existing peak current control technology also takes into account the calculation of the output average current to ensure the consistency of the overcurrent point in a wide range of input and output conditions, i.e. primary constant current, without optocoupler feedback. As shown in Fig. 8, the output average current can be obtained by periodically averaging the area of the transformer current in the falling stage. The calculation in DCM needs to use the auxiliary winding in Fig. 7 to detect the demagnetization time of the transformer current, and then use the area of a triangle formula; the calculation in CCM needs to analyze the complete transformer current signal, such as detecting the starting value or midpoint value of the current trapezoid. The disadvantage of this calculation method is that the complete transformer current signal is easily affected by the turn-on noise and sampling delay. Without relevant compensation measures, the calculation error of the output average current in CCM is more than 10%.

[0007] Based on the above considerations, a control method is needed that can further reduce the C1 filter capacitor while taking into account the calculation of the output average current in DCM and CCM. SUMMARY

[0008] The purpose of the present application is to provide a control method for flyback converter continuous current mode, which can stably control and adjust the degree and speed of the duty cycle when the flyback converter enters CCM, and take into account the calculation of the output average current, realize the application of smaller input filter capacitor in AC / DC rectifier bridge of flyback converter, reduce the output voltage ripple, and finally realize higher power density.

[0009] The purpose of the present application can be achieved by the following technical solution, a control method for flyback converter continuous current mode, comprising the following steps:

[0010] Step one: based on the existing peak current control technology, when the flyback converter works in critical current mode, record the transformer current peak value as i pk1 and the corresponding off time T off1 , recorded as the first cycle.

[0011] Step two: if the current peak value i pk of a certain cycle is not higher than i pk1 , then end the original off logic and turn on the flyback switch, recorded as the second cycle.

[0012] Step three: if the current peak value i pk of a certain cycle is higher than i pk1 , the difference is Δi, control the switch to reduce the off time of this cycle to T off1 -k·Δi, then the current cannot return to zero at the end of this cycle, thus entering CCM, recorded as the third cycle, where k is a preset proportion coefficient.

[0013] After that, take the current peak value i pk as the judgment condition, the flyback converter converts between DCM and CCM.

[0014] As a further scheme of the present application: in step one, the working state can be changed in a certain cycle, that is, by giving the current peak value of this cycle as i pk1 and turning on in critical current mode to obtain the off time T off1 .

[0015] As a further scheme of the present application: in step one, the critical current mode is not limited to turning on the switch immediately after the transformer current returns to zero, but also includes turning on at the first valley of resonance, with a difference of half a system resonance period between the two time points.

[0016] As a further scheme of the present application: in step three, when the minimum input voltage of the flyback converter is lower, the required CCM duty cycle is larger, and the value of k needs to be increased, so that the off time of the third cycle is reduced more.

[0017] As a further scheme of the present application, the output average current calculation method is:

[0018] Where i pk is the current peak value of each cycle, Δi is i pk -i pk1 (a positive number or zero), N ps is the turns ratio of the transformer primary to secondary, V o is the flyback output voltage value, L m is the transformer excitation inductance value, T off is the off time or transformer current demagnetization time, T sis the period time.

[0019] As a further aspect of the present application: when in the first and second periods, the current peak i pk is less than i pk1 , Δi needs to be substituted as zero in the output average current calculation method, which is simplified as:

[0020] As a further aspect of the present application: when calculating the output average current, when in the first and second periods, T off is the transformer current demagnetization time; when in the third period, since the transformer current cannot return to zero at the end of the period, T off is T off1 -k·Δi.

[0021] The present application has the beneficial effects: the present application controls the off time of the flyback converter, to stably control and adjust the degree and speed of the duty cycle when entering CCM, and takes into account the calculation of the output average current, so as to realize the application of smaller input filter capacitance in the flyback converter after the AC / DC rectifier bridge, reduce the output voltage ripple, and finally realize higher power density. BRIEF DESCRIPTION OF DRAWINGS

[0022] The present application will be further described below with reference to the accompanying drawings.

[0023] Fig. 1 is a transformer current waveform of the flyback converter in DCM and CCM according to the method of the present application;

[0024] Fig. 2 is a flowchart of the control according to the method of the present application;

[0025] Fig. 3 is an example of the control according to the method of the first embodiment of the present application;

[0026] Fig. 4 is an example of the control according to the method of the second embodiment of the present application;

[0027] Fig. 5 is a filter voltage fluctuation curve under different filter capacitance values;

[0028] Fig. 6 is a typical AC / DC flyback converter circuit;

[0029] Fig. 7 is an example of peak current control of a flyback converter;

[0030] Fig. 8 is a transformer current waveform of a flyback converter in CCM and DCM. DETAILED DESCRIPTION

[0031] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0032] In order to make the technical personnel better understand the present application, the following will be combined with the drawings in the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by the ordinary skilled in the art without creative labor should belong to the scope of protection of the present application.

[0033] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the application described herein can be implemented. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not necessarily limit to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0034] In the present embodiment, a control block diagram of a flyback converter continuous current mode is provided, as shown in FIG. 2 and FIG. 3, the transformer current sampling uses R1 to convert the current signal into a voltage signal. On the basis of the existing peak current control technology, the sample-and-hold 1, the comparator 2, the subtractors 1 and 2, the proportional coefficient k and N are added in the dashed box ps ·V o / L m ; the output average current calculation unit introduces ΔVi and the proportional coefficient N ps ·Vo / L m ; the slope compensation unit is omitted, and the complete transformer current signal is not needed to be introduced into the output average current calculation unit. When the demagnetization / valley bottom detection unit detects the critical current mode, the sample-and-hold 1 is enabled to latch T off at T off1 when the comparator 2 flips, realizing the first period; when in DCM, the sample-and-hold 1 does not work, the subtractor 1 outputs ΔVi as 0, realizing the second period; when it is needed to enter CCM, ΔVi>0, the oscillator turns on Q1 according to the output T off1 -k·ΔVi time of the subtractor 2, realizing the third period. Based on this structure, the flow steps of FIG. 2 can be realized, and the output average current calculation unit does not need to distinguish DCM or CCM.

[0035] The calculation method for the average output current can be derived from the area 3 in Figure 1. Since the slope of the transformer current's descent is only affected by the output voltage and hardly changes, a parallelogram can be constructed to simplify the calculation of area 3. Part of the upper base of the trapezoid in area 3 is Δi, and the other part is proportional to the reduced turn-off time k·Δi. This relationship represents the slope N of the transformer current's descent. ps ·V o / L m The lower base of the trapezoid is i in the current period. pk The high is the reduced T off Therefore, the average output current for that period can be calculated.

[0036] In one specific embodiment, setting the scaling factor k to 0 means that the third period is at any i pk >i pk1 Below, the shutdown time will remain at T. off1 The control structure remains unchanged and can still enter CCM, but the duty cycle rises at a slower rate. The control structure of this embodiment can be further simplified; subtractor 2 and the two proportional coefficients can be omitted, and the output averaging unit is more streamlined.

[0037] In another embodiment, a control block diagram for the continuous current mode of a flyback converter is provided, as shown in Figure 4. The proportional coefficient k can vary according to the operating conditions; the proportional coefficient N... ps ·V o / L m It is then broken down into the actual physical fixed quantity N. ps / L m and change V o Two coefficients, k and V. o Both can be controlled by the demagnetization / valley detection unit. This unit can be controlled by the auxiliary winding N. a Indirectly detect the input and output voltage information of the flyback converter, thereby reasonably changing k and V. o The coefficient allows for more precise control of the duty cycle entering the CCM, adapting to different application scenarios.

[0038] This invention does not limit the implementation of addition, subtraction, multiplication, and division in the method for calculating the average output current, nor does it limit any use of the calculation result, such as for loop control, power calculation, or triggering protection.

[0039] One of the core aspects of this invention is that by using filter capacitors C1 with different capacitance values, the degree and speed of duty cycle when the flyback converter enters CCM can be stably controlled and adjusted when Vbus changes drastically, by changing the proportional coefficient k. If a larger value of k is used, the switching frequency will increase rapidly with the increase of peak current, without being limited by the preset frequency of the oscillator, and the maximum duty cycle of CCM can exceed 85%.

[0040] One of the core points of the present application: because the turn-off time is controlled, there is no sub-harmonic oscillation of large and small turn-off time when entering CCM, no slope compensation unit is needed, the maximum peak current is not lost, the utilization rate of the transformer is improved, and the adaptability of flyback is enhanced.

[0041] One of the core points of the present application: when calculating the output average current, there is no need to distinguish between DCM and CCM working conditions, and a complete transformer current signal is not needed, only peak current information is needed, the influence of Q1 opening noise and sampling delay is avoided, and the calculation accuracy can be improved.

[0042] It can be understood that the above embodiments are only exemplary embodiments adopted for illustrating the principles of the present application, and the present application is not limited thereto. Various modifications and improvements can be made by those of ordinary skill in the art without departing from the spirit and essence of the present application, and these modifications and improvements are also considered to be within the protection scope of the present application.

Claims

1. A control method of a flyback converter continuous current mode, characterized in that, The method comprises the following steps: Step one: on the basis of peak current control technology, when the flyback converter works in critical current mode, record the transformer current peak value as i pk1 and the corresponding turn-off time T off1 , denoted as the first period; Step two: if the current peak value i pk No higher than i pk1 Then, according to the original off logic, the flyback switch is turned on, which is recorded as the second cycle. Step three: if the current peak value i pk High i pk1 , the difference is Δi, the control switch makes the off time of the period reduced to T off1 -k·Δi, then the current can not return to zero at the end of the period, so as to enter CCM, recorded as the third period, wherein k is a preset proportion coefficient; After that, the current peak i pk is used as a judgment condition for the flyback converter to switch between DCM and CCM.

2. The control method of a flyback converter in continuous current mode according to claim 1, characterized in that, In step one, the working state can be changed in a certain period, i.e. by giving the current peak value i pk1 and open in critical current mode to obtain the off time T off1 .

3. The control method of a flyback converter continuous current mode according to claim 1, characterized in that, In step one, the critical current mode is not limited to turning on the switch immediately after the transformer current returns to zero, but also includes turning on at the first valley of the resonance, and the two time points differ by half a system resonance period.

4. The control method of a flyback converter continuous current mode according to claim 1, characterized in that, In step three, the lower the minimum input voltage of the flyback converter, the greater the required CCM duty cycle, and the k value needs to be adjusted to reduce the third period of turn-off time more.

5. The control method of a flyback converter continuous current mode according to claim 1, characterized in that, The output average current calculation method is: Wherein, i pk is the current peak value of each cycle, Δi is i pk -i pk1 , N ps is the turns ratio of the transformer primary to secondary, V o is the flyback output voltage value, L m is the transformer excitation inductance value, T off is the off time or transformer current demagnetization time, T s is the cycle time.

6. The control method of a flyback converter continuous current mode according to claim 5, characterized in that, When in the first cycle, the second cycle, the current peak i pk Less than i pk1 , Δi as zero value into the output average current calculation method, simplified as:

7. The control method of a flyback converter continuous current mode according to claim 4, characterized in that, In calculating the output average current, when in the first period, the second period, T off is the transformer current demagnetization time; when in the third period, since the transformer current cannot return to zero at the end of the period, T off is T off1 -k Δi.

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