Demagnetization error time calibration circuit, control circuit and flyback switching converter

Abstract

The invention relates to a demagnetization error time calibration circuit, a control circuit and a flyback switching converter. The demagnetization error time calibration circuit comprises an error time detection circuit and a calibration circuit, a first delay duration of an error time delay pulse relative to a resonance initial signal is adjusted, and the first delay duration is calibrated to be the delay duration of a first zero-crossing signal relative to the resonance initial signal in a normal working state; the obtained calibrated first delay time length can be used for detecting whether the secondary side power switch is delayed to be turned off or not, and is used for communication between a secondary side circuit and a primary side circuit and the like; the demagnetization error time can be used for eliminating an error of a switching period caused by taking a first zero crossing point moment of a primary side feedback signal as a demagnetization end point, and the performance of the flyback switching converter is improved. The control circuit and the flyback switching converter comprise the demagnetization error time calibration circuit.

Description

Technical Field

[0001] The invention relates to the field of switching power supplies, and in particular to a demagnetization error time calibration circuit and a control circuit for a flyback switching converter and a flyback switching converter. Background Art

[0002] The output end of the flyback topology switching converter obtains energy when the primary winding is disconnected from the power supply. During the period when the main switch tube coupled to the primary winding is turned on, the circuit only stores but does not transfer energy. During the period when the main switch tube is turned off, energy is transferred to the load through the transformer.

[0003] In the discontinuous conduction mode (DCM), the time when the main switch is turned off includes not only the demagnetization time but also the dead time. During the dead time, due to the resonance of the parasitic capacitance at the drain end of the main switch and the primary inductance of the transformer, the secondary current and the primary feedback signal of the voltage obtained by connecting to the auxiliary winding of the transformer (hereinafter referred to as the primary feedback signal FB) will resonate, so that the primary feedback signal FB of each switching cycle does not drop to 0 at the end of the demagnetization of the secondary circuit, but is delayed for a resonance time before dropping to 0 for the first time (i.e., the first zero crossing). Therefore, there is an error in the demagnetization time obtained by the first zero crossing of the primary feedback signal FB.

[0004] In the control of the flyback switching converter, it is necessary to obtain the demagnetization error time of the primary feedback signal FB. For example, in the constant current output state, the demagnetization time after the main switch is turned off is proportional to the switching cycle, but the demagnetization time with the first zero crossing of the primary feedback signal FB as the demagnetization end point is longer than the actual demagnetization time, which will cause the calculated switching cycle to be too large and the output current to be too small. Therefore, the demagnetization error time needs to be calibrated to eliminate the deviation of the switching cycle. For another example, changing the turn-off time of the secondary power switch will affect the resonant slope of the primary feedback signal FB, and the demagnetization error time can be used to determine whether the turn-off time of the secondary power switch has changed. Summary of the invention

[0005] The present invention provides a demagnetization error time calibration circuit for a flyback switching converter, which can calibrate and lock the delay time of the first zero-crossing signal of the primary feedback signal in the flyback switching converter after demagnetization relative to the resonant initial signal under normal working conditions, and the calibrated delay time is used to determine the demagnetization error time. The present invention also provides a control circuit for a flyback switching converter and a flyback switching converter.

[0006] In one aspect, the present invention provides a demagnetization error time calibration circuit for a flyback switching converter, wherein the flyback switching converter includes a transformer, a primary circuit, and a secondary circuit, and has a primary feedback signal, and the demagnetization error time calibration circuit includes:

[0007] an error time detection circuit, used for collecting a resonant initial signal determined by a resonant reference value and a first zero-crossing signal of the primary feedback signal after the secondary circuit is demagnetized; and

[0008] A calibration circuit is used to generate a periodic error time delay pulse, wherein the error time delay pulse has an initial first delay time relative to the resonant initial signal, and adjust the first delay time of the next switching cycle according to the comparison result between the first zero-crossing signal detected in the current cycle and the error time delay pulse, and calibrate the first delay time to the delay time of the first zero-crossing signal relative to the resonant initial signal under normal working conditions through pulse adjustment of multiple switching cycles, so as to determine the demagnetization error time through the calibrated first delay time.

[0009] By using the demagnetization error time calibration circuit provided by the present invention, by adjusting the first delay time of the error time delay pulse relative to the resonant initial signal, the first delay time is calibrated to the delay time of the first zero-crossing signal relative to the resonant initial signal under normal working conditions to determine the demagnetization error time. The resonant initial signal is determined by a resonant reference value. The calibrated first delay time can be used to detect whether the secondary power switch is delayed and is used for communication between the secondary circuit and the primary circuit. The demagnetization error time can be used to eliminate the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal as the demagnetization end point, which helps to improve the performance of the flyback switching converter.

[0010] On the other hand, the present invention provides a control circuit for a flyback switching converter, the control circuit comprising the above-mentioned demagnetization error time calibration circuit, a communication time threshold circuit and a communication signal detection circuit, the communication time threshold circuit being used to generate a periodic communication time threshold pulse, the communication time threshold pulse having a second delay time relative to the resonant initial signal, the second delay time being smaller than the first delay time, the communication signal detection circuit being used to detect whether the first zero-crossing signal of each cycle is ahead of or behind the communication time threshold pulse, and when the first zero-crossing signal is ahead of the communication time threshold pulse, it is determined that the secondary circuit has sent a communication signal.

[0011] By using the control circuit provided on the other hand, it is detected whether the first zero-crossing signal in each cycle leads or lags behind the communication time threshold pulse, so as to obtain the change in the resonance time of the primary feedback signal from the start of resonance to the first zero crossing (i.e., the change in the resonance slope), and to determine whether the secondary circuit sends a communication signal. Compared with optocoupler communication, magnetic coupling communication, and protocol chip, no additional components or chips are required, the cost is low, and it can be used to detect and transmit communication signals of different contents.

[0012] On the other hand, the present invention provides a control circuit for a flyback switching converter, the control circuit comprising the above-mentioned demagnetization error time calibration circuit and the demagnetization time correction circuit, the demagnetization error time calibration circuit being used to generate a calibrated first delay time, the demagnetization time correction circuit being used to detect the zero-crossing time of the primary feedback signal from an initial value to a first zero-crossing point, and the zero-crossing time is corrected according to the demagnetization error time determined by the calibrated first delay time to obtain the demagnetization time.

[0013] The corrected demagnetization time obtained by the control circuit provided in the further aspect can be used to calculate the switching cycle and further perform constant current compensation, thereby eliminating the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal FB as the demagnetization end point, and improving the accuracy of constant current compensation. In the control circuit, the demagnetization time correction circuit corrects the zero-crossing time of the primary feedback signal from the initial value to the first zero-crossing point according to the demagnetization error time determined by the calibrated first delay time, and does not need to collect a complete resonant cycle. Even if the resonance of the primary feedback signal is less than one resonant cycle after the secondary circuit demagnetization is completed, constant current compensation can still be achieved without being limited by the input line voltage.

[0014] On the other hand, the present invention provides a flyback switching converter, the flyback switching converter includes the above-mentioned demagnetization error time calibration circuit for the flyback switching converter or includes the above-mentioned control circuit for the flyback switching converter. The demagnetization error time calibration circuit adjusts the first delay time of the error time delay pulse relative to the resonant initial signal, and calibrates the first delay time to the delay time of the first zero-crossing signal relative to the resonant initial signal under normal working conditions to determine the demagnetization error time, the resonant initial signal is determined by a resonant reference value, and the demagnetization error time can be used to eliminate the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal FB as the demagnetization end point and detect whether the power switch in the secondary circuit is delayed in shutting down, etc., which helps to improve the performance of the flyback switching converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is the waveform of the secondary output current in the flyback switching converter.

[0016] Figure 2 Schematic diagram of the resonant waveform of the primary feedback signal FB when the secondary power switch is normally turned off and delayed turned off.

[0017] Figure 3 Schematic diagram of a demagnetization error time calibration circuit for a flyback switching converter according to an embodiment of the present invention.

[0018] Figure 4 It is a schematic diagram of realizing communication between a secondary circuit and a primary circuit by using a control circuit according to an embodiment of the present invention.

[0019] Figure 5 1 is a circuit diagram of a control circuit for a flyback switching converter according to an embodiment of the present invention.

[0020] Figure 6 It is a timing diagram of multiple signals used for demagnetization error time calibration and primary-secondary side communication detection in one embodiment of the present invention.

[0021] Figure 7 Schematic diagram of a demagnetization time correction circuit in one embodiment of the present invention.

[0022] Figure 8 Is adopted Figure 7 The demagnetization time correction circuit shown is a timing diagram of multiple signals when correcting the demagnetization time. DETAILED DESCRIPTION

[0023] The demagnetization error time calibration circuit, control circuit and flyback switching converter of the present invention are further described in detail below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer according to the following description. It should be noted that the accompanying drawings are all in a very simplified form and are not in precise proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0024] Embodiments of the present invention relate to a flyback switching converter, and also to a demagnetization error time calibration circuit and a control circuit for a flyback switching converter. The flyback switching converter includes a transformer for energy conversion and a primary circuit and a secondary circuit. For example, in an AC-DC flyback switching converter, the primary circuit rectifies and filters the AC power supply and is coupled to the primary winding of the transformer through a main switch tube, and the secondary circuit is coupled to the secondary winding of the transformer, and the secondary circuit may include a freewheeling diode, a secondary power switch, and an output capacitor. In addition, the flyback switching converter may also have a primary feedback signal FB, and the primary feedback signal FB is obtained, for example, by a voltage-dividing resistor string coupled to the auxiliary winding of the transformer, and the primary feedback signal FB may be collected by a control circuit (or control chip) that controls the primary circuit.

[0025] The demagnetization error time calibration circuit and control circuit for a flyback switching converter and the flyback switching converter according to the embodiments of the present invention are described below.

[0026] <Demagnetization error time calibration circuit for flyback switching converter>

[0027] Figure 1This is the waveform of the secondary output current in the flyback switching converter. Figure 1 , secondary output current I out It can be calculated using the following expression (1):

[0028]

[0029] Among them, T DEM Indicates the demagnetization time, I pk represents the peak current, Np / Ns represents the turns ratio of the primary and secondary sides of the transformer, T s Represents the switching cycle.

[0030] In the above expression (1), the switching period T s is the demagnetization time T DEM The product of the coefficient k, that is, T s =k*T DEM When the demagnetization time T DEM The calculation error T error , then the corresponding switching period T s The error is generated, and the switching period T s2 It can be calculated using expression (2):

[0031] T s2 = k*(T DEM +T error ) (2)

[0032] It can be seen that when the demagnetization time T DEM The calculation of the switching cycle produces an error, and the secondary output current I obtained by using the above expression (1) is out There are errors accordingly, such as the demagnetization time T DEM If it is larger than the actual value, the calculated switching period is larger than the actual value. The secondary output current I calculated by expression (1) is out Smaller than the actual output current.

[0033] In order to obtain the demagnetization time T DEM In some schemes, the moment when the voltage of the primary feedback signal FB drops to 0 (i.e., the first zero crossing) is collected as the demagnetization end moment. However, due to the resonance of the parasitic capacitance at the drain end of the main switch tube and the primary inductance of the transformer, the primary feedback signal FB will be delayed for a resonance period before the first zero crossing after the demagnetization of the secondary circuit is completed. That is, the moment when the primary feedback signal FB first crosses zero is delayed compared to the actual demagnetization end calculation moment, and the delayed time is the demagnetization error time.

[0034] The moment when the primary feedback signal FB first crosses the zero point (or the resonant slope of the primary feedback signal FB) is affected by the change of the secondary circuit. Figure 2It is a schematic diagram of the resonant waveform of the primary feedback signal FB when the secondary power switch is normally turned off and delayed turned off. Figure 2 During the switching cycle of the flyback switching converter, after the secondary circuit is demagnetized, the primary feedback signal FB resonates. When the secondary power switch is turned off normally, the waveform of the primary feedback signal FB after the demagnetization is free resonant is as follows: Figure 2 As shown in L1 in the figure, the resonance time (T dn2 ) is 1 / 4 of the resonant period (i.e. T res2 is the resonant period of the primary feedback signal FB), the resonant period T res2 It can be calculated by expression (3):

[0035]

[0036] Wherein, LC represents the product of the initial current L and the initial voltage C.

[0037] When the power switch in the secondary circuit is turned off with delay (the opening time remains unchanged, and the conduction time is extended), there is a negative current on the secondary side, which will continue to flow and demagnetize through the primary inductance of the transformer. Although the primary follow-through current will not cause the resonant period of the parasitic capacitance at the drain end of the main switch tube and the primary inductance of the transformer to change, it will change the initial energy of the resonance, so that the free resonant waveform of the primary feedback signal FB after the secondary demagnetization is superimposed with an oscillating waveform caused by the initial energy stored in the primary inductance. Therefore, when the secondary power switch is turned off with delay, the primary feedback signal FB will cross zero for the first time after a period of resonance delay after the demagnetization, but the first zero crossing time is earlier than the free resonance. At this time, the resonant waveform is as follows: Figure 2 As shown in L2 in the figure, the resonance time (T dn1 ) is less than 1 / 4 of the resonant period (i.e. T res1 is the resonant period of the primary feedback signal FB after the secondary power switch is delayed shut down), but the total duration of the resonant period of the primary feedback signal FB remains unchanged, T res1 It can be expressed by expression (4):

[0038]

[0039] From the above description, it can be known that in the control of the flyback switching converter, calibrating and locking the demagnetization error time of the primary feedback signal FB is beneficial to the operation of the flyback switching converter. For example, the calibrated demagnetization error time can be used to correct the error of the switching cycle to obtain an accurate output current I outIn addition, since changing the turn-off time of the secondary power switch will cause a change in the resonant slope of the primary feedback signal FB, the demagnetization error time can be used to determine whether the turn-off time of the secondary power switch has changed, and can be used to detect whether the secondary circuit has sent a communication signal to the primary circuit by changing the turn-off time of the secondary power switch.

[0040] The demagnetization error time calibration circuit for a flyback switching converter according to an embodiment of the present invention is further described below.

[0041] Figure 3 1 is a schematic diagram of a demagnetization error time calibration circuit for a flyback switching converter in an embodiment of the present invention. In the embodiment of the present invention, the demagnetization error time calibration circuit includes an error time detection circuit 10, and the error time detection circuit 10 is used to collect a resonant initial signal Vfb-up determined by a resonant reference value and a first zero-crossing signal Vfb-dn of the primary feedback signal FB after the secondary circuit demagnetization is completed.

[0042] As an example, the demagnetization error time calibration circuit may start to work when the flyback switching converter is turned on and operates in DCM. For example, the flyback switching converter is in a constant voltage or constant current state. Figure 6 1 is a timing diagram of multiple signals used for demagnetization error time calibration and primary-secondary communication detection in one embodiment of the present invention. Figure 6 In each switching cycle, the primary feedback signal FB goes through 1 / 4 of the resonance cycle from the end of demagnetization (corresponding to the resonance maximum value Vref) to the first zero crossing.

[0043] In this embodiment, the resonance initial signal Vfb-up collected by the error time detection circuit 10 is determined by a resonance reference value, and the resonance reference value is smaller than the resonance maximum value Vref of the primary feedback signal FB. Figure 2 and Figure 6 Exemplarily, the error time detection circuit 10 includes a first comparator I1 and a second comparator I2, wherein the first comparator I1 is used to compare the primary feedback signal FB with the resonance reference value to generate a resonance initial signal Vfb-up, and the second comparator I2 is used to compare the primary feedback signal FB with the ground point potential (gnd) to generate a first zero-crossing signal Vfb-dn.

[0044] Since the resonance maximum value Vref is difficult to detect accurately, in this embodiment, 90% of the resonance maximum value Vref is used as the resonance reference value. By comparing the primary feedback signal FB with the resonance reference value, the resonance initial signal Vfb-up can be obtained. At this time, the resonance time length from the resonance initial signal Vfb-up obtained from the resonance reference value (90% of the resonance maximum value Vref) to the first zero-crossing signal Vfb-dn is 1 / 6 of the resonance period of the primary feedback signal FB (1 / 6T res ). The setting of the resonance initial signal Vfb-up is not limited to this. For example, the resonance initial signal Vfb-up can be obtained by comparing the voltage of the primary feedback signal FB with the resonance reference value when the resonance initial voltage Vref drops to 99% to 70.7%. The proportion of the corresponding demagnetization error time in the resonance period changes accordingly, and the designer can set it according to different circuit designs and requirements. Figure 6 As shown, in the first two switching cycles, the resonance initial signal Vfb-up is at a high level when the primary feedback signal FB is 100% to 90% of the resonance maximum value Vref, and is at a low level after the primary feedback signal FB drops to 90% of the resonance maximum value Vref.

[0045] The resonant reference value can be set based on the calibration capability of the demagnetization error time calibration circuit. Specifically, the closer the resonant reference value is to the resonant maximum value Vref, the longer the calibration time is, and the larger the detection range of the calibration circuit needs to be; for example, when the resonant reference value takes the resonant maximum value Vref, the first delay time after calibration is 1 / 4 of the resonant cycle, and the corresponding calibration time is about 300ns; when the resonant reference value takes 0.9Vref, the first delay time after calibration is 1 / 6 of the resonant cycle, and the corresponding calibration time is only 200ns. In this embodiment, the calibration capability of the demagnetization error time calibration circuit is reflected in the hardware mainly as the different number of current sources required in the delay subcircuit described below.

[0046] The first zero-crossing signal Vfb-up indicates the moment when the voltage of the primary feedback signal FB drops to 0 for the first time after the primary feedback signal FB starts to resonate in each switching cycle. Figure 6 As shown, in the first two switching cycles, the first zero-crossing signal Vfb-dn is at a high level before the primary feedback signal FB crosses zero for the first time, and is at a low level after the first zero-crossing. In this embodiment, 90% of the resonance maximum value Vref is used as the resonance reference value, and the falling edges of the resonance initial signal Vfb-up and the first zero-crossing signal Vfb-dn are separated by 1 / 6 of the resonance cycle.

[0047] Reference Figure 3 and Figure 6The demagnetization error time calibration circuit in the embodiment of the present invention further includes a calibration circuit 20, the calibration circuit is used to generate a periodic error time delay pulse Timer_cali, the error time delay pulse Timer_cali has an initial first delay time length T1 relative to the above-mentioned resonant initial signal Vfb-up, the calibration circuit adjusts the first delay time length T1 of the next switching cycle according to the comparison result between the first zero-crossing signal Vfb-dn detected in this cycle and the error time delay pulse Timer_cali, and adjusts the first delay time length T1 of the next switching cycle through multi-cycle pulse adjustment. The first delay time T1 is calibrated to the delay time of the first zero-crossing signal Vfb-dn relative to the resonant initial signal Vfb-up under normal working conditions (for example, 1 / 6 of the resonant cycle in this embodiment), so as to determine the demagnetization error time by using the first delay time T1 calibrated by pulse adjustment. For example, when determining the demagnetization error time, the calibrated first delay time T1 can be used to transform the corresponding 1 / 4 of the resonant cycle (for example, by performing open-loop compensation to obtain the product of the first delay time T1 and a proportional coefficient), thereby obtaining the demagnetization error time, and realizing the calibration lock of the demagnetization error time. The above-mentioned "normal working state" refers to the working state of the flyback switching converter when the secondary power switch is normally turned off without early or delayed shutdown.

[0048] Specifically, refer to Figure 3 The calibration circuit 20 includes a delay subcircuit 21, a sampling subcircuit 22 and a counting subcircuit 23. The specific description is as follows.

[0049] The delay subcircuit 21 is used to generate the periodic error time delay voltage pulse Timer_cali, and the error time delay pulse Timer_cali has an initial first delay time length T1 relative to the resonant initial signal Vfb-up. Figure 3 As shown, in one embodiment, the delay subcircuit 21 includes a current source group CSG, a charging capacitor C1, a pulse switch S1 and a third comparator I3, the current source group CSG includes a plurality of current sources arranged in parallel, the current source group responds to the calibration code Sw<1:N> (feedback by the counting subcircuit 23) to control the number of current sources turned on, the first end of the charging capacitor C1 is coupled to the output end of the current source group CSG, the second end of the charging capacitor C1 is grounded, the pulse switch S1 responds to the resonance initial signal Vfb-up to ground the first end of the charging capacitor C1 during the conduction period, the non-inverting input end of the third comparator I3 is coupled to the first end of the charging capacitor C1, the inverting input end inputs the resonance maximum value Vref, and the third comparator I3 outputs a signal of the error time delay pulse Timer_cali. The structure of the delay subcircuit 21 is not limited to Figure 3In other embodiments, other circuit structures may be used to implement the output and adjustment of the error time delay pulse Timer_cali.

[0050] The sampling subcircuit 22 is used to sample the first zero-crossing signal Vfb-dn of the primary feedback signal FB using the error time delay pulse Timer_cali, and output the sampling result. The error time delay pulse Timer_cali is generated once in each switching cycle, and in each switching cycle, the starting point (or rising edge) of the error time delay pulse Timer_cali has a first delay time length T1 relative to the falling edge of the resonant initial signal Vfb-up.

[0051] Reference Figure 6 In this embodiment, the rising edge of the error time delay pulse Timer_cali is determined by the falling edge of the resonance initial signal Vfb-up and the first delay time T1. According to the sampling result of the error time delay pulse Timer_cali on the falling edge of the first zero-crossing signal Vfb-dn, the comparison result of the first zero-crossing signal Vfb-dn and the error time delay pulse Timer_cali can be obtained.

[0052] like Figure 3 As shown, exemplarily, the sampling subcircuit 22 adopts a D flip-flop, for example. The clock signal input terminal of the D flip-flop is coupled to the output terminal of the third comparator I3, and the other input terminal inputs the first zero-crossing signal Vfb-dn. The D flip-flop samples the falling edge of the first zero-crossing signal Vfb-dn using the rising edge of the error time delay pulse Timer_cali, and outputs the sampling result.

[0053] The counting subcircuit 23 is used to change the calibration code Sw<1:N> according to the sampling result output by the sampling subcircuit 22 and feed the calibration code Sw<1:N> back to the delay subcircuit 21. The delay subcircuit 21 responds to the calibration code Sw<1:N> and adjusts the first delay time T1, thus forming a feedback loop. Figure 3 As shown, the counting subcircuit 23 is, for example, a counter, whose input end is coupled to the output end of the D flip-flop in the sampling subcircuit 22. According to the sampling result output by the D flip-flop, the counter increases or decreases the least significant bit of the calibration code Sw<1:N> and outputs it. After receiving the feedback calibration code Sw<1:N>, the delay subcircuit 21 compares it with the previous calibration code Sw<1:N>, and adjusts the number of current sources turned on in the current source group according to the change, so as to achieve the purpose of adjusting the first delay time length T1.

[0054] Specifically, when the error time delay pulse Timer_cali does not sample the first zero-crossing signal Vfb-dn (for example, the sampling result is "0"), the corresponding sampling result indicates that the starting point of the error time delay pulse Timer_cali lags behind the falling edge of the first zero-crossing signal Vfb-dn, and the least significant bit of the calibration code Sw<1:N> is reduced by "1". The delay sub-circuit 21 shortens the first delay time T1 of the next switching cycle in response to the corresponding calibration code Sw<1:N> (for example, the number of turned-on current sources is increased), so that the starting point of the error time delay pulse Timer_cali is closer to the first zero-crossing signal Vfb-dn. The falling edge is close; when the error time delay pulse Timer_cali samples the first zero-crossing signal Vfb-dn, the corresponding sampling result indicates that the starting point of the error time delay pulse Timer_cali is ahead of the falling edge of the first zero-crossing signal (for example, the sampling result is "1"), and the lowest bit of the calibration code Sw<1:N> is added with "1", and the delay sub-circuit 21 increases the first delay time T1 of the next switching cycle in response to the corresponding calibration code Sw<1:N> (for example, reducing the number of turned-on current sources), so that the starting point of the error time delay pulse Timer_cali is close to the falling edge of the first zero-crossing signal Vfb-dn.

[0055] After the delay subcircuit 21 receives the updated calibration code Sw<1:N> each time, the change amplitude of the number of turned-on current sources can be set as needed, and a value greater than or equal to 1 can be selected. The smaller the number of current sources adjusted each time, the higher the calibration accuracy. Using the above feedback loop, after adjustment of multiple switching cycles, the sampling result can be made to alternate between 1 and 0, and the lowest bit of the calibration code Sw<1:N> is cyclically added "1" and subtracted "1", representing a steady state. At this time, the first delay time T1 of the starting point of the error time delay pulse Timer_cali relative to the resonant initial signal Vfb-up is equal to the delay time of the first zero-crossing signal Vfb-dn relative to the resonant initial signal Vfb-up, which is, for example, 1 / 6 of the resonant period of the primary feedback signal FB in this embodiment.

[0056] By using the demagnetization error time calibration circuit described in the above embodiment, by adjusting the first delay time T1 of the error time delay pulse Timer_cali relative to the resonant initial signal Vfb-up, the first delay time T1 is calibrated to the delay time of the first zero-crossing signal Vfb-dn relative to the resonant initial signal Vfb-up under normal working conditions to determine the demagnetization error time. The calibrated first delay time T1 can be used to detect whether the secondary power switch is delayed and turned off and is used for communication between the secondary circuit and the primary circuit. The demagnetization error time can be used to eliminate the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal as the demagnetization end point, which helps to improve the performance of the flyback switching converter.

[0057] <Control circuit for flyback switching converter 1>

[0058] like Figure 2 As shown, changing the turn-off moment of the secondary power switch will cause a change in the resonant slope of the primary feedback signal FB. The first delay time calibrated by the demagnetization error time calibration circuit described in the above embodiment can be used to determine whether the turn-off moment of the secondary power switch has changed, so that a communication signal can be sent to the primary circuit by changing the turn-off moment of the secondary power switch, and the communication information between the secondary circuit and the primary circuit can be obtained by detecting the change in the resonant slope of the primary feedback signal FB.

[0059] Figure 4 Schematic diagram of using a control circuit of an embodiment of the present invention to realize communication between a secondary circuit and a primary circuit. Figure 2 , Figure 4 and Figure 6 According to one embodiment of the present invention, the flyback switching converter includes a power stage composed of a primary circuit and a transformer, and the power stage forms a secondary output voltage and an output current under the control of a primary side feedback (PSR) controller. In order to obtain the information of the communication between the secondary circuit and the primary circuit, the secondary power switch can be delayed to be turned off through an external instruction to send a communication signal to the primary circuit. When the secondary power switch is delayed to be turned off, the negative current formed on the secondary side is demagnetized by the primary inductance of the transformer, so that the primary inductance stores the initial energy (such as Figure 6 The secondary side injects energy and starts the primary-secondary side communication) and changes the resonant slope of the primary side feedback signal FB through the auxiliary winding. By detecting the slope of the primary side feedback signal FB (such as Figure 4 The “FB slope detection” shown in the figure) can control the primary circuit accordingly to form a response, thereby achieving communication between the secondary circuit and the primary circuit.

[0060] Some embodiments of the present invention relate to a control circuit for a flyback switching converter. Figure 4The process shown realizes the communication between the secondary circuit and the primary circuit. The control circuit includes the demagnetization error time calibration circuit described in the above embodiment, so as to generate the calibrated first delay time T1 by using the demagnetization error time calibration circuit, and then the circuit detects the slope change of the primary feedback signal FB. The specific description is as follows.

[0061] Figure 5 1 is a circuit diagram of a control circuit for a flyback switching converter according to an embodiment of the present invention. Figure 5 In one embodiment of the present invention, a control circuit for a flyback switching converter includes the above-mentioned demagnetization error time calibration circuit, a communication time threshold circuit 30 and a communication signal detection circuit 40.

[0062] Reference Figure 5 and Figure 6 The communication time threshold circuit 30 is used to generate a periodic communication time threshold pulse Timer, and the communication time threshold pulse Timer has a second delay time T2 relative to the above-mentioned resonant initial signal Vfb-up, and the second delay time T2 is smaller than the first delay time T1.

[0063] Specifically, the communication time threshold circuit 30 includes, for example, a fourth comparator I4, which is used to compare a periodic output voltage with a communication reference voltage to form the communication time threshold pulse Timer, and the communication reference voltage is less than the resonance maximum value Vref of the primary feedback signal FB. In this embodiment, the non-inverting input terminal of the fourth comparator I4 is coupled to the output terminal of the charging capacitor C1 in the demagnetization error time calibration circuit, and the communication reference voltage is input to the inverting input terminal of the fourth comparator I4. As an example, the communication reference voltage is 1 / 2 of the resonance maximum value Vref, that is, 0.5Vref, the communication time threshold pulse Timer is a periodic pulse, and the second delay time length T2 of the communication time threshold pulse Timer relative to the resonance initial signal Vfb-up is less than the first delay time length T1 of the error time delay pulse Timer_cali relative to the resonance initial signal Vfb-up. The second delay time length T2 provides a communication time threshold, so as to detect the slope change of the primary feedback signal FB and then determine whether the secondary power switch is delayed to be turned off.

[0064] The communication reference voltage can be set according to the minimum energy injected by the secondary side into the primary side required for the primary side to trigger communication. The greater the minimum energy required, the greater the difference between the second delay time T2 and the demagnetization error time when the primary side is not communicating. In this embodiment, the demagnetization error time that has been calibrated and locked is, for example, 1 / 6 of the resonant period, and a time less than 1 / 6 of the resonant period (such as 1 / 2, 2 / 3, 1 / 3 or 1 / 4 of 1 / 6 of the resonant period) can be selected as the second delay time T2, i.e., the communication time threshold. As an example, the communication reference voltage is 0.5Vref, and the second delay time T2 is 1 / 2 of the first delay time T1 (T2=0.5T1).

[0065] Reference Figure 5 The control circuit also includes a communication signal detection circuit 40, which detects whether the first zero-crossing signal Vfb-dn of the same switching cycle is ahead of or behind the communication time threshold pulse Timer. When the first zero-crossing signal Vfb-dn is ahead of the communication time threshold pulse Timer, it is determined that the secondary circuit has sent a communication signal.

[0066] like Figure 5 As shown, the communication code generation subcircuit 41 may include an OR-N operation logic and an RS trigger. The OR-N operation logic is coupled to the error time detection circuit and the communication time threshold circuit 30 to perform an OR-N operation on the resonance initial signal Vfb-up, the first zero-crossing signal Vfb-dn and the communication time threshold pulse Timer. The set end of the RS trigger is connected to the output end of the OR-N operation logic and outputs the corresponding communication code Code.

[0067] Reference Figure 6 , using the communication code generation sub-circuit 41, when the falling edge of the first zero-crossing signal Vfb-dn in the same switching cycle lags behind the rising edge of the communication time threshold pulse Timer (such as Figure 6 The first two switching cycles shown in the figure indicate that the time required for the first zero crossing of the primary feedback signal FB relative to the resonance initial signal Vfb-up is greater than the currently set communication time threshold, and the communication code Code output by the communication code generation subcircuit 41 is "0", indicating that the secondary circuit does not send a communication signal; when the falling edge of the first zero crossing signal Vfb-dn of the same switching cycle is ahead of the rising edge of the communication time threshold pulse Timer (such as Figure 6 The third switching cycle shown in the figure indicates that the time required for the first zero crossing of the primary feedback signal FB relative to the resonance initial signal Vfb-up is less than the currently set communication time threshold, and the communication code Code output by the communication code generation subcircuit 41 is "1", and the communication code signal is high level (such as Figure 6The pulse of “1” is detected as shown).

[0068] The initial value of the first delay time T1 can be set to be relatively small. Since the second delay time T2 is in a certain proportion to the first delay time T1, the initial value of the second delay time T2 is also relatively small. In this way, in the process of the delay subcircuit 21 adjusting the first delay time T1, the communication code Code obtained by the communication code generation subcircuit 41 is "0", which does not affect the detection result of the communication signal detection circuit 40.

[0069] In order to improve the anti-interference characteristics of the system, optionally, the communication signal detection circuit 40 has a communication code generation subcircuit 41, an encoding subcircuit 42 and a decoding subcircuit 43, the communication code generation subcircuit 41 is used to detect whether the first zero-crossing signal Vfb-dn of each cycle is ahead or behind the communication time threshold pulse Timer, and output a corresponding one-bit communication code Code, the encoding subcircuit 42 is used to obtain the communication code output by the communication code generation subcircuit 41, and generate a communication code sequence, the decoding subcircuit 43 is used to obtain the communication code sequence and decode it, when the communication code of the specified bit in the communication code sequence is the same as the target communication code, it is determined that the secondary circuit has sent a communication signal. The communication code generation subcircuit 41, the encoding subcircuit 42 and the decoding subcircuit 43 can adopt known circuits that can realize the above functions.

[0070] The encoding subcircuit 42, for example, includes an N-bit shift register. Considering that the first zero crossing of the primary feedback signal FB when the communication code Code is "1" is advanced relative to when the primary circuit and the secondary circuit are not communicating, for example, when the communication code Code is "1", the delay time of the first zero crossing signal Vfb-dn relative to the resonant initial signal is no longer 1 / 6 of the resonant period under the constant voltage or constant current output state, and therefore, there is no need to detect the first zero crossing signal Vfb-dn to calibrate the demagnetization error time, and the pulse adjustment of the first delay time T1 by the demagnetization error time calibration circuit can be suspended. Figure 5 As shown, in this embodiment, when the first zero-crossing signal Vfb-dn is ahead of the communication time threshold pulse Timer, the communication signal detection circuit 40 sends a shielding signal to the calibration circuit 20 to keep the first delay time length T1 obtained by the previous switch cycle calibration unchanged. After detecting that the first zero-crossing signal Vfb-dn lags behind the communication time threshold pulse Timer, the shielding signal can be released to continue to calibrate the first delay time length T1. Figure 5As shown, the communication code Code generated by the communication code generation subcircuit 41 is coupled to the encoding subcircuit 42, and can also be coupled to the counting subcircuit 23 in the above-mentioned demagnetization error time calibration circuit, and perform an OR operation with the clock (gd) of the counting subcircuit. After the OR operation, when the communication code Code is "1", the clock of the counting subcircuit 23 is shielded, and when the communication code is "0", it indicates that the primary circuit is not injected with energy by the secondary circuit, and the clock constraint will be released. When the clock of the counting subcircuit 23 is shielded, it no longer updates the calibration code Sw<1:N>, and the delay subcircuit 21 uses the calibration code Sw<1:N> fed back before the clock of the counting subcircuit 23 is shielded to set the number of current sources turned on in the current source group, so that the first delay time length T1 uses the value of the previous switching cycle, so that the first delay time length T1 generated by the demagnetization error time calibration circuit can be ensured to be unchanged after the secondary power switch is delayed and turned off, so as to facilitate accurate primary and secondary communication detection.

[0071] After determining that the power switch of the secondary circuit is delayed in shutting down and a communication signal is sent, a corresponding instruction can be sent to the primary circuit of the flyback switching converter by a primary feedback controller (i.e., a PSR controller) included in or associated with the control circuit, and the primary circuit responds according to the corresponding instruction, that is, the communication between the secondary circuit and the primary circuit is completed. As an example, according to the communication signal sent by the secondary circuit, the primary circuit changes the reference voltage of the constant voltage output loop, thereby changing the output voltage. In some embodiments, the communication signal may include output voltage gear adjustment, peak current increase or decrease, and improved dynamic response, etc.

[0072] By utilizing the control circuit for a flyback switching converter, it is determined whether the secondary circuit sends a communication signal by obtaining the change in the resonance time of the primary feedback signal from the start of resonance to the first zero crossing (i.e., the change in the resonance slope). Compared with optocoupler communication, magnetic coupling communication, and protocol chip methods, no additional components or chips are required, the cost is low, and it can be used to detect and transmit communication signals of different contents.

[0073] <Control circuit for flyback switching converter 2>

[0074] like Figure 1 As shown, the demagnetization time T after the main switch is turned off DEM With the switching period T s In the existing solutions on the market, the demagnetization time T is usually taken as the time from the start of demagnetization to the first zero crossing of the primary feedback signal FB. DEM , the demagnetization time T DEMCalculating 1 / 4 resonant cycle more than the actual demagnetization time will cause the calculated switching cycle to be too large and the output current to be too small. Therefore, when calculating the switching cycle and the output current, the time from the end of demagnetization to the first zero crossing of the primary feedback signal FB can be subtracted from 1 / 4 resonant cycle, and then used as the demagnetization time to calculate the switching cycle, thereby eliminating the error of the switching cycle. In order to obtain the 1 / 4 resonant cycle, some existing solutions detect multiple valleys of the resonant signal of the primary feedback signal FB to obtain a complete resonant cycle, and then divide it by 4 to obtain 1 / 4 resonant cycle. However, when the resonance of the primary feedback signal FB after the demagnetization of the secondary circuit is less than one resonant cycle (such as Figure 8 As shown, only one valley bottom is experienced), and the complete resonant cycle cannot be collected at this time, making it difficult to eliminate the error of the switching cycle using this method.

[0075] Figure 7 Schematic diagram of a demagnetization time correction circuit in one embodiment of the present invention. Figure 8 Is adopted Figure 7 The demagnetization time correction circuit shown in the figure obtains the timing diagram of multiple signals during the demagnetization time. Figure 7 and Figure 8 This embodiment relates to a control circuit for a flyback switching converter, the control circuit comprising a demagnetization error time calibration circuit and a demagnetization time correction circuit as described in the above embodiment. In the control circuit, the demagnetization error time calibration circuit is used to generate a calibrated first delay time T1, the demagnetization time correction circuit is used to detect the zero-crossing time of the primary feedback signal FB from the initial value to the first zero-crossing point, and the zero-crossing time is corrected according to the demagnetization error time determined by the calibrated first delay time T1 generated by the demagnetization time correction circuit to obtain the demagnetization time T DEM .

[0076] like Figure 7 and Figure 8 As shown, the demagnetization time correction circuit may include an edge detection subcircuit, an edge delay subcircuit and a correction subcircuit, which are specifically described as follows.

[0077] The edge detection subcircuit generates a feedback signal rising edge pulse T based on the primary feedback signal FB DEM-up And the feedback signal falling edge pulse T DEM-dn , the feedback signal rising edge pulse T DEM-up Corresponding to the demagnetization start time, that is, the initial value of the primary feedback signal FB, the feedback signal falling edge pulse T DEM-dn Corresponding to the first zero-crossing moment of the primary feedback signal FB. Based on the rising edge pulse T of the feedback signal DEM-up And the feedback signal falling edge pulse T DEM-dnThe zero-crossing time T from the initial value of the primary feedback signal FB to the first zero-crossing point can be obtained. DEM-error .

[0078] The edge delay subcircuit converts the calibrated first delay time T1 generated by the demagnetization time correction circuit into a demagnetization error time, and uses the demagnetization error time to delay the rising edge pulse T of the feedback signal DEM-up And get the rising edge delay pulse T DEM-updl .

[0079] Specifically, when the calibrated first delay time T1 is used to convert the demagnetization error time, since the demagnetization error time is proportional to the calibrated first delay time T1, the demagnetization error time is 1 / 4 of the resonance period of the primary feedback signal FB. For example, a suitable compensation circuit can be selected for conversion so that the calibrated first delay time T1 is multiplied by a proportional coefficient to obtain a signal of the demagnetization error time. When the demagnetization error time is used to delay the rising edge pulse T of the feedback signal DEM-up And get the rising edge delay pulse T DEM-updl When a delay circuit Td is used, the feedback signal rising edge pulse T DEM-up Delay 1 / 4 resonant period ( T delay is the delay time, T res is the resonance period), and the rising edge delay pulse T DEM-updl .

[0080] The correction subcircuit utilizes the rising edge delay pulse T DEM-updl And the feedback signal falling edge pulse T DEM-dn Get the demagnetization time pulse T DEM-cali Specifically, the rising edge delay pulse T DEM-updl And the feedback signal falling edge pulse T DEM-dn The signal is input into the set terminal S and reset terminal R of an RS flip-flop respectively, and the demagnetization time pulse T can be output. DEM-cali , the demagnetization time pulse T DEM-cali The pulse width is equal to the demagnetization time obtained by correction, that is, the zero-crossing time T is realized. DEM-error Correction. Figure 8 As shown, the demagnetization time pulse T DEM-cali The actual demagnetization signal (T DEM ) are equal in pulse width, that is, T DEM-cali =T DEM .

[0081] The corrected demagnetization time obtained by using the control circuit for the flyback switching converter can be used to calculate the switching cycle of the flyback switching converter and further perform constant current compensation, thereby eliminating the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal FB as the demagnetization end point, and improving the accuracy of constant current compensation. In the control circuit, the demagnetization time correction circuit corrects the zero-crossing time of the primary feedback signal from the initial value to the first zero-crossing point based on the demagnetization error time determined by the calibrated first delay time length T1, and does not need to collect a complete resonant cycle. Even if the resonance of the primary feedback signal FB is less than one resonant cycle (for example, after the secondary circuit demagnetization is completed), the demagnetization error time of the primary feedback signal FB is corrected. Figure 8 When the number of valleys shown is 1), constant current compensation can still be achieved without being limited by the input line voltage.

[0082] <Flyback Switching Converter>

[0083] An embodiment of the present invention also relates to a flyback switching converter, the flyback switching converter includes the demagnetization error time calibration circuit for the flyback switching converter described in the above embodiment, or the flyback switching converter includes the control circuit for the flyback switching converter described in the above embodiment. The flyback switching converter includes a transformer for energy conversion and a primary circuit and a secondary circuit. The flyback switching converter also has a primary feedback signal FB, and the primary feedback signal FB is formed by, for example, connecting to an auxiliary winding of the transformer. The flyback switching converter can be an AC-DC flyback switching converter or a DC-DC converter. In the flyback switching converter, the demagnetization error time calibration circuit adjusts the first delay time T1 of the error time delay pulse Timer_cali relative to the resonant initial signal Vfb-up, and calibrates the first delay time to the delay time of the first zero-crossing signal Vfb-dn relative to the resonant initial signal Vfb-up under normal working conditions, so as to determine the demagnetization error time. The demagnetization error time can be used to eliminate the error of the switching cycle caused by taking the first zero-crossing point of the primary feedback signal FB as the demagnetization end point and to detect whether the power switch in the secondary circuit is delayed in shutting down, etc., which helps to improve the performance of the flyback switching converter.

[0084] It should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0085] The above description is only a description of the preferred embodiment of the present invention, and is not any limitation on the scope of rights of the present invention. Any technical personnel in this field can make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A demagnetization error time calibration circuit for a flyback switching converter, characterized in that: The flyback switching converter includes a transformer, a primary circuit and a secondary circuit, and has a primary feedback signal. The demagnetization error time calibration circuit includes: an error time detection circuit, used for collecting a resonant initial signal determined by a resonant reference value and a first zero-crossing signal of the primary feedback signal after the secondary circuit is demagnetized; and A calibration circuit is used to generate a periodic error time delay pulse, wherein the error time delay pulse has an initial first delay time relative to the resonant initial signal, and adjust the first delay time of the next switching cycle according to the comparison result between the first zero-crossing signal detected in the current cycle and the error time delay pulse, and calibrate the first delay time to the delay time of the first zero-crossing signal relative to the resonant initial signal under normal working conditions through pulse adjustment of multiple switching cycles, so as to determine the demagnetization error time through the calibrated first delay time.

2. The demagnetization error time calibration circuit according to claim 1, characterized in that: The rising edge of the error time delay pulse is determined by the falling edge of the resonance initial signal and the first delay time, and the comparison result is obtained according to the sampling result of the error time delay pulse on the falling edge of the first zero-crossing signal.

3. The demagnetization error time calibration circuit according to claim 1, wherein: When the error time delay pulse detected in this cycle samples the falling edge of the first zero-crossing signal, the first delay time of the next switching cycle is increased; when the error time delay pulse detected in this cycle does not sample the falling edge of the first zero-crossing signal, the first delay time of the next switching cycle is shortened.

4. The demagnetization error time calibration circuit as claimed in claim 1, characterized in that: The error time detection circuit comprises: a first comparator, configured to compare the primary feedback signal with the resonance reference value to generate the resonance initial signal; and The second comparator is used to compare the primary feedback signal with the ground point potential to generate the first zero-crossing signal.

5. The demagnetization error time calibration circuit according to claim 1, wherein: The resonance reference value is smaller than the resonance maximum value of the primary feedback signal.

6. The demagnetization error time calibration circuit as claimed in claim 5, characterized in that: When the resonance reference value is 90% of the resonance maximum value, the demagnetization error time is 1 / 6 of the resonance period of the primary feedback signal.

7. The demagnetization error time calibration circuit according to claim 1, wherein: The calibration circuit comprises: A delay subcircuit, used for generating a periodic error time delay pulse, wherein the error time delay pulse has an initial first delay time length relative to the resonant initial signal; a sampling subcircuit, sampling the first zero-crossing signal using the error time-delayed pulse, and outputting a sampling result; and The counting subcircuit is used to change the calibration code according to the sampling result and feed the calibration code back to the delay subcircuit, and the delay subcircuit responds to the calibration code and adjusts the first delay time.

8. The demagnetization error time calibration circuit as claimed in claim 7, characterized in that: When the sampling result of this cycle indicates that the starting point of the error time delay pulse is ahead of the first zero-crossing signal, the delay sub-circuit increases the first delay time of the next switching cycle in response to the corresponding calibration code; when the sampling result of this cycle indicates that the starting point of the error time delay pulse lags behind the first zero-crossing signal, the delay sub-circuit shortens the first delay time of the next switching cycle in response to the corresponding calibration code.

9. The demagnetization error time calibration circuit as claimed in claim 7, characterized in that: The delay subcircuit comprises: A current source group, comprising a plurality of current sources arranged in parallel, wherein the current source group controls the number of current sources turned on in response to the calibration code fed back by the counting subcircuit; A charging capacitor, a first end of which is coupled to the output end of the current source group, and a second end of which is grounded; a pulse switch, responsive to the resonance initial signal to ground the first end of the charging capacitor during a conduction period; and The third comparator is used to compare the voltage value on the charging capacitor with the resonance maximum value to periodically form the error time delay pulse.

10. The demagnetization error time calibration circuit according to claim 7, wherein: The sampling subcircuit comprises: A D flip-flop has one input terminal coupled to the output terminal of the delay sub-circuit and the other input terminal inputting the first zero-crossing signal. The D flip-flop utilizes the rising edge of the error time delay pulse to sample the falling edge of the first zero-crossing signal and outputs the sampling result.

11. A control circuit for a flyback switching converter, characterized in that: The control circuit comprises: The demagnetization error time calibration circuit according to any one of claims 1 to 10, wherein the demagnetization error time calibration circuit is used to generate a first delay time length for calibration; A communication time threshold circuit, used for generating a periodic communication time threshold pulse, wherein the communication time threshold pulse has a second delay time length relative to the resonant initial signal, and the second delay time length is less than the first delay time length; and The communication signal detection circuit is used to detect whether the first zero-crossing signal in the same switching cycle is ahead of or behind the communication time threshold pulse. When the first zero-crossing signal is ahead of the communication time threshold pulse, it is determined that the secondary circuit has sent a communication signal.

12. The control circuit according to claim 11, characterized in that: The second delay duration is half of the first delay duration.

13. The control circuit according to claim 11, characterized in that: When the first zero-crossing signal precedes the communication time threshold pulse, the communication signal detection circuit sends a shielding signal to the calibration circuit to keep the first delay time obtained in the previous switching cycle unchanged, and after detecting that the first zero-crossing signal lags behind the communication time threshold pulse, the shielding signal is released to continue calibrating the first delay time.

14. The control circuit according to claim 11, characterized in that: The communication time threshold circuit comprises: The fourth comparator is used for comparing a periodic output voltage with a communication reference voltage to form the communication time threshold pulse, wherein the communication reference voltage is smaller than the resonant maximum value of the primary feedback signal.

15. The control circuit according to claim 11, characterized in that: The communication signal detection circuit comprises: A communication code generating subcircuit is used to detect whether the first zero-crossing signal of each cycle is ahead of or behind the communication time threshold pulse, and output a corresponding one-bit communication code; an encoding subcircuit, used for obtaining the communication code output by the communication code generating subcircuit and generating a communication code sequence; and The decoding subcircuit is used to obtain and decode the communication code sequence, and when the communication code of the designated bit in the communication code sequence is the same as the target communication code, it is determined that the secondary circuit has sent a communication signal.

16. The control circuit according to claim 15, characterized in that: The communication code generation subcircuit comprises: an OR-NOT operation logic, coupled to the error time detection circuit and the communication time threshold circuit, to perform an OR-NOT operation on the resonance initial signal, the first zero-crossing signal and the communication time threshold pulse signal; and The RS flip-flop has a set end coupled to the output end of the NOR operation logic, and the RS flip-flop outputs the corresponding communication code.

17. A control circuit for a flyback switching converter, characterized in that: The control circuit comprises: The demagnetization error time calibration circuit according to any one of claims 1 to 10, wherein the demagnetization error time calibration circuit is used to generate a first delay time length for calibration; and The demagnetization time correction circuit is used to detect the zero-crossing time of the primary feedback signal from the initial value to the first zero-crossing point, and to correct the zero-crossing time according to the demagnetization error time determined by the calibrated first delay time to obtain the demagnetization time.

18. The control circuit for a flyback switching converter according to claim 17, characterized in that: The demagnetization time correction circuit comprises: An edge detection subcircuit generates a feedback signal rising edge pulse and a feedback signal falling edge pulse based on the primary feedback signal; an edge delay subcircuit, converting the calibrated first delay time into a demagnetization error time, and utilizing the demagnetization error time to delay the rising edge pulse of the feedback signal to obtain a rising edge delayed pulse; and The correction subcircuit obtains a demagnetization time pulse by using the rising edge delay pulse and the falling edge pulse of the feedback signal, and the pulse width of the demagnetization time pulse is the demagnetization time obtained by correction.

19. A flyback switching converter, characterized in that: It comprises the demagnetization error time calibration circuit as described in any one of claims 1 to 10; or, it comprises the control circuit as described in any one of claims 11 to 16; or, it comprises the control circuit as described in claim 17 or 18.

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