Sample-and-hold structure, controller, ac-dc switching power supply and sampling method

Abstract

The application provides a sample holding structure, a controller, an AC-DC switching power supply and a sampling method. The sample holding structure comprises a charging module, a discharging module, a control module and a sampling module. The discharging module comprises an energy storage element and a discharging unit. The charging module is used for charging the energy storage element. The discharging unit is used for discharging the energy storage element. The sampling module is used for collecting the voltage of the feedback end of the auxiliary winding of the transformer of the AC-DC switching power supply. The control module is used for sending a sampling signal to the sampling module at a set proportion of the transformer secondary side demagnetization time. The set proportion is directly proportional to the ratio of the charging current when the energy storage element is charged and the discharging current when the energy storage element is discharged. The application can determine the sampling time of the current cycle according to the demagnetization time of the current cycle, and is not affected by the change and size of the primary side peak current of an arbitrary cycle.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and more specifically, to a sample-and-hold structure, a controller, an AC-to-DC switching power supply, and a sampling method. Background Technology

[0002] Primary-side feedback (PSR) AC-DC control technology, compared with the traditional secondary-side feedback switching power supply structure, has the biggest advantage of eliminating isolation feedback devices. This saves space on the circuit board, reduces costs, and improves system reliability. Therefore, it is widely used in small and medium power chargers, adapters, and LED drivers.

[0003] Figure 1 This is a block diagram of a primary-side feedback AC-DC power supply in the prior art. (See attached diagram.) Figure 1 As shown, the primary-side feedback AC-DC drive power supply includes a rectifier bridge BR, an input capacitor Cin, a starting resistor R4, a power supply rectifier diode D1, a power supply capacitor C1, an upper voltage divider resistor R1 and a lower voltage divider resistor R2, a controller, a transformer 200, a power transistor N1, a current sensing resistor Rcs, a Schottky rectifier diode D2, and an output capacitor Cout. The transformer includes a primary winding Np, an auxiliary winding Na, and a secondary winding Ns. The upper and lower voltage divider resistors R1 and R2 form a sampling circuit. The FB pin is the voltage feedback input pin for the auxiliary winding Na of the transformer, and it obtains the signal from the sampling circuit formed by the upper and lower voltage divider resistors R1 and R2. Inside the controller, the FB pin is held and its operating frequency is controlled by a sampling control circuit. When the power transistor is turned on, the primary winding of the transformer is conducting, and the transformer stores energy. Since the polarity of the primary winding Np is opposite to that of the auxiliary winding Na and the secondary winding Ns, the FB pin has a negative voltage when the primary winding is on. When the power transistor is turned off and the system is in the demagnetizing stage, the secondary winding of the transformer releases energy. Since the polarity of the auxiliary winding Na and the secondary winding Ns is the same, the FB voltage is a positive voltage. At this time, the voltage of the secondary winding of the transformer is Vs = Vo + Vz, where Vo is the output voltage of the transformer, Vz is the voltage drop of the Schottky rectifier diode D2, and the voltage of the auxiliary winding Va = Vs × (NA / NS) = VFB × R2 / (R (1+R2), therefore Vo=VFB×R2×NS / [(R1+R2)*NA]-Vz, where NA is the number of turns of the auxiliary winding, NS is the number of turns of the secondary winding, and VFB is the feedback voltage of the auxiliary winding Na. That is, the output voltage is a function of the feedback voltage VFB. The controller samples the VFB voltage at this time and compares it with the reference voltage to control the switching frequency so that the output voltage Vo is stable at the set value. During the demagnetization stage, the secondary current decreases with time. When it decreases to 0, the secondary demagnetization ends. If the primary power transistor N1 does not turn on again at this time, FB enters resonance. The secondary demagnetization time is recorded as Tons.

[0004] In order to accurately detect the output voltage, the FB voltage sampling is very important. Because of the demagnetization stage of the secondary side, the voltage drop Vz is generated when the secondary power flows through the Schottky D2. The forward voltage drop Vz decreases with the decrease of the current, so the sampling is performed when the demagnetization is close to the end, so that the detected output voltage Vo is close to the real output voltage.

[0005] Figure 2 The existing sampling control circuit is shown in the figure, PFM is the primary side conduction signal, Tons is the secondary side demagnetization signal, the two signals generate three narrow pulse signals pulse1-3 through the pulse generator to control the switching tubes K1, K2 and K3. Specifically, pulse1 is generated at the falling edge of PFM, pulse2 is generated at the falling edge of Tons, and pulse3 is generated after a delay. The delay and the length of the narrow pulse are very short and can be ignored compared with the length of Tons.

[0006] Figure 3 The working waveform diagram of the above sampling control circuit is shown in the figure. At the falling edge of PFM in the Nth cycle, pulse1 comes, the switch K1 is briefly turned on, and the capacitor C2 is discharged to 0. The system enters the demagnetization stage of the Nth cycle, and Tons controls the switch K0 to be turned on. During the demagnetization stage Tons(N) of the entire Nth cycle, the current I charges the capacitor C2. Assuming that the charging voltage at the end of Tons(N) is V2(N)=I*Tons(N) / C2=I*Tons(N) / (2c); at the end of Tons(N), the narrow pulse pulse2 comes to control K2 to be briefly turned on, and the capacitor C3 is discharged to 0. Then after a very short delay, the narrow pulse pulse3 comes to control K3 to be briefly turned on, so that the capacitors C2 and C3 are connected together. Since C2=2c and C3=c, V2(N)=V3(N)=(2 / 3)*Va(N), and the voltage on C3 remains V3(N) thereafter.

[0007] When PFM of the (N+1)th cycle ends, Tons comes, the narrow pulse puse1 comes again, the switch K2 is briefly turned on, and the capacitor C2 is discharged to 0V. Then the constant current source I starts to charge C2, and the comparator enable end receives Tons to enable the comparator to start working. When the voltage on C2 rises to V3(N), the comparator flips, and cmp_out changes from low to high, and the pulse generator sends a sampling signal SH. The sampling time of the (N+1)th cycle is V3(N)*C2 / I=(2 / 3)*Va(N)*2c / I=(2 / 3)Tons(N), that is, the sampling time of the (N+1)th cycle is 2 / 3 of the demagnetization time of the Nth cycle.

[0008] If the Tons of each cycle is the same when the primary feedback AC-DC power supply works, that is, Tons(N) = Tons(N+1), it is equivalent to sampling time at 2 / 3 of the current cycle Tons, so that the sampling method can accurately sample the output voltage every cycle; To keep Tons the same every cycle, under the condition that the system element parameters are fixed, it is necessary to ensure that the CS (Current Sense, primary current) peak value of each cycle is the same, and the output voltage is also the same. Therefore, in the traditional primary feedback scheme, the CS of adjacent cycles cannot change significantly, otherwise, assuming that the Tons of the Nth cycle is 9us, the sampling time of the (N+1)th cycle is 6us, but if the CS of the (N+1)th cycle decreases, Tons(N+1) will also decrease by the same proportion, at this time, if the sampling is still at 6us, the sampled voltage will change, and in the extreme case, if the CS of the (N+1)th cycle decreases by more than 1 / 3, Tons(N+1)<6us, at this time, if the sampling is at 6us, the demagnetization has already ended, FB enters resonance, and the sampled voltage cannot reflect the size of the output voltage, that is, the sampling is wrong. That is, the traditional sampling scheme can only be applied to the case where the CS of adjacent cycles cannot change significantly.

[0009] With the increasing demand for power supply systems, the performance of the primary feedback system also needs to be further improved, especially when the load has a dynamic response, the PSR system needs to quickly respond to the changing needs of the load, at this time, not only does the PSR system require a fast changing working frequency, but also the CS peak value needs to change quickly, that is, when the load suddenly becomes lighter, the CS peak value of the next cycle needs to be quickly reduced, and vice versa, when the load becomes heavier, the CS peak value of the next cycle needs to be quickly increased, such a quick response can make the PSR system have good dynamic response performance; Since the CS of adjacent two cycles changes quickly, the demagnetization time also changes proportionally, and the existing sampling method cannot meet the requirement of quick response when the CS of adjacent cycles changes quickly. SUMMARY

[0010] In view of one or more of the problems in the prior art, the present application provides a sample and hold structure, comprising a charging module, a discharging module, a control module and a sampling module, the discharging module comprises an energy storage element and a discharging unit, the charging module is used for charging the energy storage element of the discharging module, the discharging unit is used for discharging the energy storage element, the sampling module is used for collecting the voltage of the feedback end of the auxiliary winding of the transformer of the AC to DC switching power supply, and the control module is used for sending a sampling signal to the sampling module at a set proportion of the demagnetization time of the transformer secondary side, the set proportion is proportional to the ratio of the charging current of the charging module to the discharging current of the discharging unit when the energy storage element is charged.

[0011] According to one aspect of the present application, the charging module comprises a first current mirror, a first constant voltage source and a first switch, the first constant voltage source is used to make the voltage of the feedback end of the AC-DC switching power supply constant, the output current of the auxiliary winding of the transformer of the AC-DC switching power supply is collected, the first current mirror amplifies the output current, and the first switch controls the charging of the energy storage element of the discharging module by the amplified current of the first current mirror.

[0012] According to one aspect of the present application, the first current mirror comprises two MOS transistors, preferably, the MOS transistors are PMOS transistors, a first PMOS transistor 12 is connected with a second PMOS transistor 13 at the gate, the drain and the gate of the first PMOS transistor 12 are electrically connected with the first constant voltage source, and the drain of the second PMOS transistor 13 is electrically connected with the first switch 11.

[0013] According to one aspect of the present application, the first constant voltage source comprises a first transistor 14, a second transistor 15 and a constant current source 16, the base of the first transistor 14 and the second transistor 15 are connected, the collector of the first transistor is electrically connected with the first current mirror, and the constant current source 16 is electrically connected with the collector and the base of the second transistor, preferably, the first transistor 14 and the second transistor 15 are NPN transistors.

[0014] According to one aspect of the present application, the discharging unit comprises a second current mirror and a second constant voltage source, the energy storage element is electrically connected with the second constant voltage source, the second constant voltage source is electrically connected with the second current mirror, and the second constant voltage source provides the voltage for the input end and the output end of the second current mirror, and the voltage provided by the second constant voltage source for the output end of the second current mirror is a multiple of the voltage provided for the input end.

[0015] Preferably, the second current mirror comprises two MOS transistors, and further preferably, the MOS transistors are NMOS transistors, a first NMOS transistor 22 is connected with a second NMOS transistor 23 at the gate.

[0016] Preferably, the second constant voltage source comprises a second switch 24, a first operational amplifier 25, a third switch 26, a second capacitor 27, a third NMOS transistor 28 and a fourth switch 29, the positive input end of the first operational amplifier is electrically connected with the feedback end of the AC-DC switching power supply through the second switch, the positive input end of the first operational amplifier is also electrically connected with the input end of the second current mirror, the negative input end of the first operational amplifier is electrically connected with the output end of the second current mirror, the output end of the first operational amplifier is respectively electrically connected with the gate of the second capacitor and the third NMOS transistor through the third switch, the source of the third NMOS transistor is electrically connected with the output end of the second current mirror, the drain of the third NMOS transistor is electrically connected with the energy storage element through the fourth switch, and preferentially, the energy storage element is a capacitor.

[0017] According to one aspect of the present application, the control module comprises a comparator 31 and a time sequence generator 32, one input of the comparator is electrically connected with the energy storage element, the voltage of the other input of the comparator is a first set value, the output of the comparator is electrically connected with the time sequence generator, the output of the comparator is reversed when the voltage of the energy storage element is less than the first set value, and the time sequence generator controls the sampling of the sampling module.

[0018] According to one aspect of the present application, the sampling module comprises a fifth switch 42 and a third capacitor 41, and the third capacitor is electrically connected with the feedback end of the AC-DC switching power supply through the fifth switch.

[0019] According to one aspect of the present application, the charging module comprises a first switch 11, a first current mirror comprising a first PMOS 12 and a second PMOS 13, and a first constant voltage source comprising a first transistor 14, a second transistor 15 and a constant current source 16; the discharging module comprises a first capacitor, a second current mirror comprising a first NMOS 22 and a second NMOS 23, and a second constant voltage source comprising a second switch 24, a first operational amplifier 25, a third switch 26, a second capacitor 27, a third NMOS 28 and a fourth switch 29; the control module comprises a comparator 31 and a time sequence generator 32; the sampling module comprises a fifth switch 42 and a third capacitor 41; the feedback end of the AC-DC switching power supply is electrically connected with the emitter of the first transistor, electrically connected with the non-inverting input end of the first operational amplifier through the second switch, and electrically connected with the third capacitor through the fifth switch; the first PMOS is electrically connected with the gate of the second PMOS, the drain of the first PMOS is electrically connected with the collector of the first transistor, the drain of the second PMOS is electrically connected with the input end of the first switch, the bases of the first transistor and the second transistor are connected, the constant current source is electrically connected with the base and the collector of the second transistor; the output end of the first switch and the input end of the fourth switch are electrically connected with the first capacitor respectively, the output end of the fourth switch is electrically connected with the drain of the third NMOS, the gate of the third NMOS and the output end of the third switch are electrically connected with the second capacitor respectively, the source of the third NMOS is electrically connected with the drain of the second NMOS, the inverting input end of the first operational amplifier is electrically connected with the drain of the second NMOS, the output end of the first operational amplifier is electrically connected with the input end of the third switch, the gates of the first NMOS and the second NMOS are connected, the non-inverting input end of the first operational amplifier is electrically connected with the drain of the first NMOS, one input end of the comparator is electrically connected with the first capacitor, the other input end of the comparator is a first set value, the output end of the comparator is electrically connected with the time sequence generator, the time sequence generator collects the output signal of the comparator, the output signal of the feedback end and the PFM signal of the first switch, and the time sequence generator sends the control signals of the second switch, the third switch, the fourth switch and the fifth switch and the secondary side demagnetization time signal of the AC-DC switching power supply, and preferably, the set ratio is the ratio of the amplification multiples of the first current mirror and the second current mirror.

[0020] According to the second aspect of the present application, a controller is provided, comprising the above-mentioned sample-and-hold structure.

[0021] According to the third aspect of the present application, an AC-DC switching power supply is provided, comprising a transformer and the above-mentioned controller.

[0022] According to the fourth aspect of the present application, a method for sampling an AC-DC switching power supply using the sampling and holding structure is provided, comprising:

[0023] The voltage of the feedback end of the auxiliary winding of the transformer is collected at a set proportion of the off-time of the secondary side of the transformer in the same cycle, which is directly proportional to the ratio of the charging current when the energy storage element is charging and the discharging current when the energy storage element is discharging.

[0024] According to the fourth aspect of the present application, the step of collecting the voltage of the feedback end of the auxiliary winding of the transformer at a set proportion of the off-time of the secondary side of the transformer in the same cycle comprises:

[0025] When the primary side of the transformer of the AC-DC switching power supply is conducting, the feedback end is used to detect the negative voltage of the auxiliary winding and convert it into a current to charge the energy storage element from a first set value until the primary side is turned off.

[0026] In the first set time period at the beginning of the demagnetization stage, the feedback end is used to convert the positive voltage of the auxiliary winding into a current to discharge the energy storage element.

[0027] After the first set time period, the discharging current is maintained by the sampling and holding method to continue discharging the energy storage element until the voltage of the energy storage element is less than the first set value, and at the same time, the feedback end is used to detect the output voltage of the transformer in the demagnetization stage after the first time period and is sampled and held as a feedback voltage when the voltage of the energy storage element is less than the first set value, so that the output voltage of the transformer is maintained at a second set value.

[0028] The sampling and holding structure of the present application controls the charging and discharging of the energy storage element, so that the sampling unit samples at a set proportion of the off-time of the secondary side of the transformer, which can determine the sampling time according to the off-time of the current cycle and is not affected by the change and size of the peak current of the primary side in any cycle.

[0029] The application includes the above-mentioned controller with the holding structure, the AC-DC switching power supply and the sampling method. The feedback signal FB is divided by time multiplexing auxiliary winding voltage division feedback signal. When the primary side is turned on, the feedback end is used to detect the negative voltage Va of the auxiliary winding, and the voltage is converted into current through the upper voltage dividing resistor R1. The first capacitor is charged from the initial voltage of the first set value. The whole charging continues until the primary side is turned off. At the beginning of the demagnetization stage, the feedback end is used to convert the positive voltage Va of the auxiliary winding into current through R1. The first capacitor is discharged. After the first set time period, the discharge current is maintained by sampling and holding, and the first capacitor continues to be discharged until the first capacitor voltage is less than the first set value. At the same time, after the first set time period, the demagnetization stage, the FB signal restores the normal voltage division of Va by R1 and R2, which is used to detect the output voltage Vo. When the first capacitor voltage is less than the first set value, it is sampled and held, which is used for the negative feedback loop adjustment of the AC-DC switching power supply, so that the output voltage is maintained near the second set value. The sampling can be realized at the set proportion of the demagnetization time Tons in the current cycle, which is not affected by the change and size of the primary side peak current in any cycle, and meets the requirement of fast response when the adjacent cycle CS changes rapidly. BRIEF DESCRIPTION OF DRAWINGS

[0030] Other objects and advantages of the present application can be more fully understood and appreciated by reference to the following detailed description taken in conjunction with the accompanying drawings of which:

[0031] Figure 1 is a structure block diagram of the prior art primary side feedback AC-DC power supply;

[0032] Figure 2 is a schematic diagram of the sampling control circuit of the prior art;

[0033] Figure 3 is a schematic diagram of each signal of the prior art primary side feedback AC-DC power supply;

[0034] Figure 4 is a schematic diagram of the sampling and holding structure described in the application;

[0035] Figure 5 is a schematic diagram of the AC-DC switching power supply described in the application;

[0036] Figure 6 is a schematic diagram of each signal of the AC-DC switching power supply described in the application. DETAILED DESCRIPTION

[0037] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It can be evident, however, that such embodiment(s) can be practiced without these specific details.

[0038] In the following, various embodiments according to the present application will be described in detail with reference to the accompanying drawings.

[0039] Embodiment 1: Sample and hold structure 1

[0040] Figure 4 is a schematic diagram of the sample and hold structure according to the present application, as Figure 4 shown, the sample and hold structure 1 samples the voltage of the feedback end of the transformer at a set proportion of the transformer secondary side demagnetization time, so that the output voltage of the transformer which is a function of the voltage of the feedback end is sampled and held. The sample and hold structure 1 according to the present application comprises a charging module 10, a discharging module 20, a control module 30 and a sampling module 40, the discharging module comprises an energy storage element and a discharging unit, the charging module is used for charging the energy storage element of the discharging module, the discharging unit is used for discharging the energy storage element, the sampling module is used for collecting the voltage of the feedback end of the auxiliary winding of the transformer of the AC to DC switching power supply, and the control module is used for sending a sampling signal to the sampling module at a set proportion of the transformer secondary side demagnetization time, the set proportion is directly proportional to the ratio of the charging current when the charging module charges the energy storage element and the discharging current when the discharging unit discharges the energy storage element.

[0041] In one embodiment, the charging module is used for inputting a pulse frequency modulation signal to control the charging of the energy storage element, the control module is used for collecting a feedback signal of the feedback end of the AC to DC switching power supply, and the control module controls the time-sharing conduction of the discharging unit and the sampling module according to the feedback signal and the pulse frequency modulation signal, specifically: the feedback signal and the pulse frequency modulation signal indicate the secondary side demagnetization time of the transformer of the AC to DC switching power supply, the control module controls the discharging of the energy storage element of the discharging module, and when the voltage of the energy storage element is less than a first set value, the control module controls the sampling and holding of the voltage of the feedback end of the AC to DC switching power supply by the sampling module, so as to sample and hold the output voltage which is a function of the voltage of the feedback end; wherein the first set value is the voltage of the energy storage element when the discharging time of the energy storage element reaches the set proportion of the secondary side demagnetization time.

[0042] The specific constitution of the sample and hold structure of the present embodiment will be described in detail below.

[0043] Charging module 10

[0044] As Figure 4As shown, the charging module 10 comprises a first current mirror, a first constant voltage source and a first switch 11, the first constant voltage source is used to make the voltage of the feedback end of the AC-DC switching power supply constant, the output current of the auxiliary winding of the transformer of the AC-DC switching power supply is collected, the first current mirror amplifies the output current, and the first switch controls the charging of the energy storage element of the discharging module by the amplified current of the first current mirror.

[0045] In one embodiment, the first current mirror comprises two MOS transistors, preferably, the MOS transistors are PMOS transistors, as shown in Figure 4 As shown, a first PMOS transistor 12 and a second PMOS transistor 13 are connected at the gate, the drain and the gate of the first PMOS transistor 12 are electrically connected to the first constant voltage source, and the drain of the second PMOS transistor 13 is electrically connected to the first switch 11.

[0046] In one embodiment, as shown in Figure 4 The first constant voltage source comprises a first transistor 14, a second transistor 15 and a constant current source 16, the base of the first transistor 14 and the base of the second transistor 15 are connected, the collector of the first transistor is electrically connected to the first current mirror, the constant current source 16 is electrically connected to the collector and the base of the second transistor, and preferably, the first transistor 14 and the second transistor 15 are NPN transistors.

[0047] Discharging module 20

[0048] The discharging module comprises a discharging unit and an energy storage element, the discharging unit comprises a second current mirror and a second constant voltage source, the energy storage element is electrically connected to the second constant voltage source, the second constant voltage source is electrically connected to the second current mirror, and the second constant voltage source provides voltage for the input end and the output end of the second current mirror, and the voltage provided by the second constant voltage source for the output end of the second current mirror is a multiple of the voltage provided for the input end.

[0049] As shown in Figure 4 The second current mirror comprises two MOS transistors, preferably, the MOS transistors are NMOS transistors, a first NMOS transistor 22 and a second NMOS transistor 23, as shown in Figure 4 The gate of the first NMOS transistor 22 and the gate of the second NMOS transistor 23 are connected.

[0050] As shown in Figure 4As shown, the second constant voltage source includes a second switch 24, a first operational amplifier 25, a third switch 26, a second capacitor 27, a third NMOS transistor 28, and a fourth switch 29. The positive input terminal of the first operational amplifier is electrically connected to the feedback terminal of the AC-to-DC switching power supply through the second switch 24. The positive input terminal of the first operational amplifier is also electrically connected to the input terminal of the second current mirror. The inverting input terminal of the first operational amplifier is electrically connected to the output terminal of the second current mirror. The output terminal of the first operational amplifier is electrically connected to the gate of the second capacitor and the third NMOS transistor through the third switch. The source of the third NMOS transistor is electrically connected to the output terminal of the second current mirror. The drain of the third NMOS transistor is electrically connected to the energy storage element through the fourth switch.

[0051] like Figure 4 As shown, the energy storage element is a capacitor, namely the first capacitor 21.

[0052] Control module 30

[0053] like Figure 4 As shown, the control module 30 includes a comparator 31 and a timing generator 32. One input terminal of the comparator is electrically connected to the energy storage element, and the voltage value at the other end of the comparator is a first set value. The output terminal of the comparator is electrically connected to the timing generator. When the voltage of the energy storage element is less than the first set value, the output terminal of the comparator flips, and the timing generator controls the sampling module to sample.

[0054] Sampling module 40

[0055] like Figure 4 As shown, the sampling module 40 includes a fifth switch 42 and a third capacitor 41. The third capacitor is electrically connected to the feedback terminal of the AC-to-DC switching power supply through the fifth switch.

[0056] In a preferred embodiment, such as Figure 4As shown, the charging module 10 comprises a first switch 11, a first current mirror comprising a first PMOS tube 12 and a second PMOS tube 13, and a first constant voltage source comprising a first transistor 14, a second transistor 15 and a constant current source 16; the discharging module 20 comprises a first capacitor 21, a second current mirror comprising a first NMOS tube 22 and a second NMOS tube 23, and a second constant voltage source comprising a second switch 24, a first operational amplifier 25, a third switch 26, a second capacitor 27, a third NMOS tube 28 and a fourth switch 29; the control module 30 comprises a comparator 31 and a time sequence generator 32; the sampling module 40 comprises a fifth switch 42 and a third capacitor 41; the feedback end FB of the AC-DC switching power supply is electrically connected with the emitter of the first transistor, electrically connected with the non-inverting input end of the first operational amplifier through the second switch, and electrically connected with the third capacitor through the fifth switch; the first PMOS tube is electrically connected with the gate of the second PMOS tube, the drain of the first PMOS tube is electrically connected with the collector of the first transistor, the drain of the second PMOS tube is electrically connected with the input end of the first switch, the bases of the first transistor and the second transistor are connected, the constant current source is electrically connected with the base and the collector of the second transistor; the output end of the first switch and the input end of the fourth switch are electrically connected with the first capacitor respectively, the output end of the fourth switch is electrically connected with the drain of the third NMOS tube, the gate of the third NMOS tube and the output end of the third switch are electrically connected with the second capacitor respectively, the source of the third NMOS tube is electrically connected with the drain of the second NMOS tube, the inverting input end of the first operational amplifier is electrically connected with the drain of the second NMOS tube, the output end of the first operational amplifier is electrically connected with the input end of the third switch, the gates of the first NMOS tube and the second NMOS tube are connected, the non-inverting input end of the first operational amplifier is electrically connected with the drain of the first NMOS tube, one input end of the comparator is electrically connected with the first capacitor, the other input end of the comparator is a first set value, the output end of the comparator is electrically connected with the time sequence generator, the time sequence generator collects the output signal (disc) of the comparator, the output signal (FB) of the feedback end and the PFM signal of the first switch, and the time sequence generator sends the control signal (Tons_delay2) of the second switch, the control signal (Tons_delay1) of the third switch, the control signal (Tons_disc) of the fourth switch, the control signal (SH, which is also the sampling signal sent by the control module to the sampling module) of the fifth switch and the secondary demagnetization time signal (Tons) of the AC-DC switching power supply.

[0057] Preferably, the set proportion in the sampling at the set proportion of the transformer secondary demagnetization time is the ratio of the amplification multiples of the first current mirror and the second current mirror.

[0058] Existing sampling control circuits delay sampling based on the secondary demagnetization of the previous cycle. They must ensure that the demagnetization time of two adjacent cycles is basically the same to ensure that the sampling of the current cycle does not fail. They cannot react immediately to sudden changes (such as load changes or input voltage changes). However, the holding structure described in this invention samples in real time based on the secondary demagnetization of the current cycle. Regardless of how the demagnetization time of the cycle changes, it can ensure effective sampling of the current cycle and can react quickly to sudden changes.

[0059] Example 2: Controller 100

[0060] like Figure 5 As shown, the controller of the present invention is used to control the PFM frequency and sample and hold the feedback terminal FB of the transformer auxiliary winding. The controller 100 includes the sample and hold structure 1 of the above embodiments.

[0061] like Figure 5 As shown, the controller 100 also includes a second operational amplifier 2, a frequency control module 3, an RS flip-flop 4, a drive unit 5, and a cycle-by-cycle current limiting unit 6. The frequency control module sends an initial pulse frequency modulation signal (PFM signal) to the sample-and-hold structure, which is used to acquire the feedback signal from the feedback terminal of the AC-to-DC switching power supply. The second operational amplifier is used to amplify the difference between the sampled signal (FB_sh) and the reference signal (FB_ref) and output a continuous analog signal. The frequency control module is used to control the continuous change of the frequency of the pulse frequency modulation signal according to the continuous analog signal output by the second operational amplifier. The RS flip-flop is used to send the pulse frequency modulation signal to the drive unit. The drive unit is used to amplify the pulse frequency modulation signal output by the RS flip-flop, thereby controlling the conduction and shutdown of the primary side of the transformer. The cycle-by-cycle current limiting unit is used to compare the current value (CS) of the primary side of the transformer with the current limiting reference (CS_ref) in each cycle. When the current value of the primary side of the transformer reaches the current limiting reference, a periodic shutdown is performed.

[0062] The second operational amplifier mentioned above determines the operating frequency of the controller. The higher the operating frequency, the greater the energy transmitted. When the output voltage is lower than the reference, the operating frequency is increased to increase the transmitted energy and increase the output voltage. When the output voltage is higher than the reference, the operating frequency is decreased to reduce the transmitted energy and decrease the output voltage. The controller controls the output power of the transformer by adjusting the operating frequency.

[0063] Preferably, the controller further includes a built-in power supply 7 to provide internal power for the use of a holding circuit.

[0064] The controller can accurately sample at a fixed proportion of the current cycle off-time Tons, regardless of the changes in any cycle CS, output voltage, input voltage, and the proportion of the sampling time will not change with these parameters.

[0065] Embodiment 3: AC-DC switching power supply

[0066] As shown in Figure 5 , the AC-DC switching power supply includes a transformer 200 and the controller 100 of the above embodiments.

[0067] As shown in Figure 5 , the transformer 200 includes a primary winding Np, an auxiliary winding Na and a secondary winding Ns, the polarity of the primary winding is opposite to the same end of the auxiliary winding and the secondary winding, and the auxiliary winding is electrically connected to the sampling and holding structure as a feedback end.

[0068] As shown in Figure 5 , the AC-DC switching power supply further includes a rectifier bridge BR, a rectifier diode D2, a smoothing capacitor Cout, a power tube N1 and a current detection resistor Rcs, the rectifier bridge inputs AC power, the output of the rectifier bridge is electrically connected to one end of the primary winding of the transformer, the other end of the primary winding is electrically connected to the input end of the power tube, the control end of the power tube is electrically connected to the controller, the output end of the power tube is electrically connected to the current detection resistor, and the rectifier diode and the smoothing capacitor are connected in series and then connected in parallel across the secondary winding of the transformer.

[0069] Preferably, the transformer 200 further includes a series-connected upper voltage dividing resistor R1 and a lower voltage dividing resistor R2, the upper voltage dividing resistor and the lower voltage dividing resistor are connected in series across the auxiliary winding of the transformer, and the feedback end FB is led out between the upper voltage dividing resistor R1 and the lower voltage dividing resistor R2.

[0070] Embodiment 4: Sampling method

[0071] In one embodiment, the method for sampling the AC-DC switching power supply by using the sampling and holding structure of the above embodiments includes:

[0072] The voltage of the feedback end of the auxiliary winding of the transformer is collected at a set proportion of the off-time of the transformer in the same cycle, and the set proportion is directly proportional to the ratio of the charging current when the energy storage element is charged and the discharging current when the energy storage element is discharged.

[0073] The sampling time of the present application is completely fixed at the set proportion of the off-time of the current cycle, which can immediately respond to the sudden changes in the current cycle, and will not fail to sample in the face of any sudden changes.

[0074] Preferably, the step of collecting the voltage of the feedback end of the auxiliary winding of the transformer at the set proportion of the demagnetization time of the secondary winding of the transformer in the same cycle comprises:

[0075] When the primary winding of the transformer of the AC-DC switching power supply is turned on, the feedback end is used to detect the negative voltage of the auxiliary winding, which is converted into a current to charge the energy storage element from a first set value until the primary winding is turned off.

[0076] In the first set time period at the beginning of the demagnetization stage, the feedback end is used to convert the positive voltage of the auxiliary winding into a current to discharge the energy storage element.

[0077] After the first time period, the discharge current is maintained by the sample-and-hold method to continue discharging the energy storage element until the voltage of the energy storage element is less than the first set value. At the same time, in the demagnetization stage after the first time period, the feedback end is used to detect the output voltage of the transformer and is sampled and held as a feedback voltage when the voltage of the energy storage element is less than the first set value, so that the output voltage is maintained within a second set value set error range.

[0078] Preferably, the first set time period is not greater than 1 / 6Tons-1 / 2Tons. If the first set time period is too short, it will be affected by the resonance at the beginning of demagnetization. If the first set time period is too long, it will affect the sampling.

[0079] The primary winding feedback of the AC-DC switching power supply is divided into two parts in time sequence. The front part is to sample and hold the feedback voltage of the auxiliary winding of the transformer. The back part is to process the sampled feedback voltage, such as the control of the constant voltage loop. The present application controls the sampling time of the front part at a set proportion of the current cycle to dynamically respond to the demagnetization time of the secondary winding of the transformer in the current cycle, so as to realize effective and rapid sampling and holding of the feedback voltage.

[0080] In one specific embodiment, as Figures 4-6As shown, the charging module 10 of the sample and hold structure 1 is composed of a first PMOS tube 12, a second PMOS tube 13, a first transistor (NPN) 14, a second transistor 15, a constant current source 16 and a first switch 11, wherein the source levels of the first PMOS tube 12 and the second PMOS tube 13 are both connected to the internal power supply VDD of the controller 100, the gate of the first PMOS tube 12 and the gate of the second PMOS tube 13 are connected together and connected to the drain of the first PMOS tube 12 and the collector of the first transistor 14, the drain of the second PMOS tube 13 is connected to one end of the first switch 11, the other end of the first switch 11 is connected to the upper plate of the first capacitor 21 of the discharging module 20, one end of the fourth switch 29 and the positive input end of the comparator 31 of the control module, the emitter of the first transistor 14 is connected to the FB pin, the base of the first transistor 14 and the base of the second transistor 15 are connected together and connected to the collector of the second transistor 15 and one end of the constant current source 16, the emitter of the second transistor 15 is grounded, and the other end of the constant current source 16 is connected to the internal power supply VDD. When the primary side of the transformer 200 is turned on, the voltage Va of the auxiliary winding is a negative voltage, because the emitter of the second transistor 15 is grounded, the base voltage Vb of the second transistor is equal to Vbe26, Vbe26 is the base-emitter bias voltage of the second transistor 15, the FB voltage VFB is equal to Vb-Vbe25, Vbe25 is the base-emitter bias voltage of the first transistor 14, because the Vbe of the transistor and the collector current Ic are in an exponential relationship, therefore, under the condition that the collector current deviation of the first transistor 14 and the second transistor 15 is not very large, it can be approximately considered that Vbe25=Vbe26, then VFB is equal to 0V, so the two ends of the lower voltage dividing resistor R2 are both 0V, so R2 has no current, and the current flowing through the upper voltage dividing resistor R1 is equal to I1,

[0081]

[0082] wherein Vin is the input voltage of the primary winding of the transformer, NP is the number of turns of the primary winding of the transformer, and R1 is the resistance value of the upper voltage dividing resistor;

[0083] The first current mirror composed of the first PMOS tube 12 and the second PMOS tube 13 is an N1 times relationship,

[0084]

[0085] wherein I2 is the output current of the first current mirror, and N1 is the current amplification multiple of the first current mirror;

[0086] In addition, the primary side conduction time tonp of the transformer is:

[0087]

[0088] Wherein, Ipp is the peak current of the primary winding of the transformer, Lp is the inductance of the primary winding of the transformer.

[0089] The first switch 11 controlled by the PFM signal charges the first capacitor 21 in the primary conduction tonp time, and the fourth switch 29 is off at this time, so the first current mirror output current I2 charges the first capacitor 21 in the tonp time The amount of charge is:

[0090]

[0091] After the primary conduction time tonp ends, the secondary side demagnetization stage is entered, the FB voltage VFB>0, the current I1=0, and the PFM becomes low after the PFM is off, and the first switch 11 is off, and the charging is completed.

[0092] The discharge module 20 is composed of the first capacitor 21, the first NMOS tube 22, the second NMOS tube 23, the second switch 24, the first operational amplifier 25, the third switch 26, the second capacitor 27, the third NMOS tube 28 and the fourth switch 29, wherein one end of the second switch 24 is connected with FB, the other end is connected with the drain end of the first NMOS tube 22 and the positive input end of the first operational amplifier 25, the control end of the second switch 24 is connected with the signal Tons_delay2 of the time sequence generator 32, the gates of the first NMOS tube 22 and the second NMOS tube 23 are connected with the internal power supply VDD, and the source levels are grounded, the first NMOS tube 22 and the second NMOS tube 23 work in the linear region, which is equivalent to two switch tubes, and the Vds is very small, about tens of millivolts, the drain end of the second NMOS tube 23 is connected with the source end of the third NMOS tube 28 and the reverse input end of the first operational amplifier 25, the output end of the first operational amplifier 25 is connected with one end of the third switch 26, the other end of the third switch 26 is connected with the upper plate of the second capacitor 27 and the gate of the third NMOS tube 28, the lower plate of the second capacitor 27 is grounded, the drain end of the third NMOS tube 28 is connected with one end of the fourth switch 29, the other end of the fourth switch 29 is connected with the first capacitor 21, and the control end of the fourth switch 29 is connected with the signal Tons_disc of the time sequence generator 32. When the primary conduction ends and the secondary side demagnetization starts, the second switch 24 and the third switch 26 are turned on within the first set time period (1.1us) of demagnetization (Tons), the gates of the first NMOS tube 22 and the second NMOS tube 23 are connected with the internal power supply VDD, and are in the linear region, the on-resistance Rds_on is small, the current I3 flows through the first NMOS tube 22 from the auxiliary winding voltage Va through the upper voltage dividing resistor R1 and the second switch, and the drain-source voltage Vds of the first NMOS tube 22 is Figure 4Vb2 is about tens of millivolt, which can be ignored relative to Va voltage, and R1 and R2 resistance values are of the same order of magnitude, so there is only tens of millivolt voltage on R2, and the current relative to R1 can also be ignored, thus obtaining current I3

[0093]

[0094] wherein, Va is the auxiliary winding voltage, Vs is the secondary winding voltage, Vo is the output voltage, and Vz is the voltage drop of the output rectifier diode D2.

[0095] The demagnetization time Tons of the secondary side can be obtained by the following formula:

[0096]

[0097] wherein, Ips is the peak current of the transformer secondary side, and Ls is the inductance of the transformer secondary winding.

[0098] At this time, since the third switch 26 is turned on, the output Vg1 of the first operational amplifier 25 = Vg2, and the first operational amplifier 25 adjusts the output voltage Vg1 through negative feedback so that Vb2 = Vb3, that is, the Vds of the first NMOS tube 22 and the second NMOS tube 23 are equal, and since their Vgs are also equal (equal to VDD), the drain current of the second NMOS tube 23 is N2 times the drain current of the first NMOS tube 22, that is, I4 = N2 * I3. At the second set time period after the demagnetization starts (the second set time period is less than the first set time period, for example, 1us), Tons_delay1 flips from high level to low level, the third switch 26 is turned off, and the Vg2 voltage is sampled and held on the second capacitor 27, so I4 will maintain the previous current. From the moment the secondary side demagnetization starts, Tons_disc flips to high level, the fourth switch 29 is turned on, and I4 discharges the first capacitor 21 all the time until the Vsaw voltage is lower than the first set value (such as 0.5V), and the comparator 31 output flips. The charge amount of I4 discharging the first capacitor 21 over time is as follows:

[0099]

[0100] wherein, t is the discharge time of the first capacitor.

[0101] If the charge and discharge charge amounts of the first capacitor are equal in each cycle, that is, Qc = Qdisc, then

[0102]

[0103] The control module is composed of a comparator 31 and a time sequence generator 32. The positive input end of the comparator 31 is connected to the upper plate of the first capacitor 21, i.e. the Vsaw signal, the reverse input end is connected to the first set value, the output end disc is used as the input end of the time sequence generator 32, the other two input ends of the time sequence generator 32 are connected to the PFM signal and the FB pin respectively, and the five output signals of the time sequence generator 32 are: the secondary side demagnetization time signal Tons, the first delay signal Tons_delay1 (for example, about 1us) and the second delay signal Tons_delay2 (for example, about 1.1us) of the secondary side demagnetization signal, the signal Tons_disc from the start of Tons to the end of disc flip, and the sampling signal SH. When the demagnetization stage is opened, the first capacitor 21 is discharged, and the Vsaw decreases. When it decreases to slightly less than the first set value, the output disc of the comparator 31 flips from high level to low level, and the Tons_disc signal also flips from high level to low level, the fourth switch 29 is turned off, the discharging is ended, and the Vsaw is maintained at the first set value. The first set value of the comparator 31 of the control module is the set proportion of the secondary side demagnetization time Tons, and the set proportion is the ratio of the amplification multiple of the first current mirror of the charging module and the amplification multiple of the second current mirror of the discharging module. In the next period when the primary side is turned on, the charging starts, and the Vsaw also starts to rise from the first set value, and the process is repeated. Figure 6 The working waveforms of the signals of the time-sharing sampling AC-DC switching power supply are shown.

[0104] The sampling module 40 is composed of a fifth switch 42 and a third capacitor 41. One end of the fifth switch 42 is connected to the FB pin, the other end is connected to the upper plate of the third capacitor 41, the control end of the fifth switch 42 is the sampling signal SH, and the lower plate of the third capacitor 41 is connected to the ground. During the secondary side demagnetization time Tons, The time sequence generator sends a sampling narrow pulse signal SH, which is used for temporarily turning on the fifth switch 42, sampling the FB voltage signal at this moment and maintaining it on the third capacitor 41 as an output voltage feedback sampling signal, and sending the output voltage feedback sampling signal to the input end of the chip operational amplifier to participate in the loop control.

[0105] The sampling and holding structure of the application is a time-sharing sampling control method for the PSR AC-DC switching power supply. The method can accurately sample at a fixed proportion of the demagnetization time in the current period by using time-sharing multiplexing of the auxiliary winding voltage division feedback signal.

[0106] In one embodiment, The discharge time of the first capacitor 21 is t = 0.8 * Tons. The first capacitor is discharged to 0.5V. At this time, comparator 31 flips, Tons_disc flips to low, the fourth switch 29 turns off, and the voltage Vsaw on the first capacitor 21 remains at 0.5V. The next charging cycle also starts from 0.5V. Therefore, through the above time-division multiplexing, the value... Under these conditions, 80% of the demagnetization time in this cycle can be accurately detected, and then sampled at 80%. Regardless of the primary current or the length of the Tons time in this cycle, the 80% ratio will not change due to variations in these parameters. Tons_delay2 also toggles to low about 100ns after Tons_delay1. This ensures no current flows through the FB pin, and FB returns to the voltage division of Va, serving as the feedback sampling signal for the output voltage. This signal is sampled in the latter part of the demagnetization phase and participates in loop regulation.

[0107] The sampling method of existing primary-side feedback AC-DC converters first detects the demagnetization time Tons(n-1) of the previous cycle, and then takes a fixed proportion (e.g., 2 / 3) of the previous cycle's demagnetization time as the sampling time during the current cycle's demagnetization. This sampling method has two drawbacks: first, there is a certain error in the detection of Tons between adjacent cycles; second, if there is a significant change in the primary-side peak current between adjacent cycles (e.g., dynamic load changes, extreme cases such as switching from no-load to full-load or vice versa), sampling will fail. In other words, the sampling time of existing technology depends on the demagnetization time of the previous cycle. Effective sampling requires that the demagnetization times of adjacent cycles be the same or only slightly different; otherwise, the sampled signal will jump and be inaccurate. If the demagnetization time of the current cycle changes significantly, the sampled signal of the current cycle is still based on the demagnetization time of the previous cycle and is completely unrelated to the current cycle's demagnetization time. This results in flaws in the sampling time and may even lead to sampling failure.

[0108] The present invention addresses the aforementioned problems of the prior art by using the demagnetization time of the current cycle as a reference and fixing it at a set proportion of the demagnetization time of the current cycle, regardless of the previous cycle. No matter how the demagnetization time of the current cycle changes, the sampling is always performed at the aforementioned set point, which can accurately sample the output voltage.

[0109] Although exemplary embodiments of the invention have been shown in the foregoing disclosure, it should be noted that various changes and modifications can be made without departing from the scope defined by the claims. Furthermore, while elements of the invention may be described or claimed individually, it is also contemplated that multiple elements may be included, unless explicitly limited to a single element.

Claims

1. A sample-and-hold structure, characterized in that, The system includes a charging module, a discharging module, a control module, and a sampling module. The discharging module includes an energy storage element and a discharging unit. The charging module is used to charge the energy storage element of the discharging module, and the discharging unit is used to discharge the energy storage element. The sampling module is used to collect the voltage at the feedback terminal of the auxiliary winding of the transformer of the AC-to-DC switching power supply. The control module is used to send a sampling signal to the sampling module at a set ratio of the demagnetization time of the transformer secondary side. The set ratio is proportional to the ratio of the charging current when the charging module charges the energy storage element and the discharging current when the discharging unit discharges the energy storage element. The discharge unit includes a second current mirror and a second constant voltage source. The energy storage element is electrically connected to the second constant voltage source, which is also electrically connected to the second current mirror. The second constant voltage source provides voltage to the input and output terminals of the second current mirror. The voltage provided by the second constant voltage source to the output terminal of the second current mirror is a multiple of the voltage provided to the input terminal. The second current mirror includes two MOS transistors, which are NMOS transistors. The gates of the first NMOS transistor (22) and the second NMOS transistor (23) are connected. The second constant voltage source includes a second switch (24), a first operational amplifier (25), a third switch (26), and a second capacitor (27). The first operational amplifier has a third NMOS transistor (28) and a fourth switch (29). The positive input terminal of the first operational amplifier is electrically connected to the feedback terminal of the AC-to-DC switching power supply through the second switch. The positive input terminal of the first operational amplifier is also electrically connected to the input terminal of the second current mirror. The inverting input terminal of the first operational amplifier is electrically connected to the output terminal of the second current mirror. The output terminal of the first operational amplifier is electrically connected to the gate of the second capacitor and the third NMOS transistor through the third switch. The source of the third NMOS transistor is electrically connected to the output terminal of the second current mirror. The drain of the third NMOS transistor is electrically connected to the energy storage element through the fourth switch.

2. The sampling and holding structure according to claim 1, characterized in that, The charging module includes a first current mirror, a first constant voltage source, and a first switch. The first constant voltage source is used to keep the voltage at the feedback terminal of the AC-to-DC switching power supply constant and to collect the output current of the auxiliary winding of the transformer of the AC-to-DC switching power supply. The first current mirror amplifies the output current, and the first switch controls the current amplified by the first current mirror to charge the energy storage element of the discharge module.

3. The sampling and holding structure according to claim 2, characterized in that, The first current mirror includes two MOS transistors, which are PMOS transistors. The gates of the first PMOS transistor (12) and the second PMOS transistor (13) are connected. The drain and gate of the first PMOS transistor (12) are electrically connected to the first constant voltage source. The drain of the second PMOS transistor (13) is electrically connected to the first switch (11). The first constant voltage source includes a first transistor (14), a second transistor (15), and a constant current source (16). The bases of the first transistor (14) and the second transistor (15) are connected. The collector of the first transistor is electrically connected to a first current mirror. The constant current source (16) is electrically connected to the collector and base of the second transistor.

4. The sampling and holding structure according to claim 3, characterized in that, The first transistor (14) and the second transistor (15) are NPN transistors.

5. The sampling and holding structure according to claim 1, characterized in that, The energy storage element is a capacitor.

6. The sampling and holding structure according to claim 1, characterized in that, The control module includes a comparator (31) and a timing generator (32). One input terminal of the comparator is electrically connected to the energy storage element, and the voltage value at the other end of the comparator is a first set value. The output terminal of the comparator is electrically connected to the timing generator. When the voltage of the energy storage element is less than the first set value, the output terminal of the comparator flips, and the timing generator controls the sampling module to sample.

7. The sampling and holding structure according to claim 1, characterized in that, The sampling module includes a fifth switch (42) and a third capacitor (41), the third capacitor being electrically connected to the feedback terminal of the AC-to-DC switching power supply through the fifth switch.

8. The sampling and holding structure according to claim 1, characterized in that, The charging module includes a first switch (11), a first current mirror, and a first constant voltage source. The first current mirror includes a first PMOS transistor (12) and a second PMOS transistor (13). The first constant voltage source includes a first transistor (14), a second transistor (15), and a constant current source (16). The discharging module includes a first capacitor, a second current mirror, and a second constant voltage source. The second current mirror includes a first NMOS transistor (22) and a second NMOS transistor (23). The second constant voltage source includes a second switch (24), a first operational amplifier (25), a third switch (26), and a second capacitor (27). The control module includes a comparator (31) and a timing generator (32); the sampling module includes a fifth switch (42) and a third capacitor (41); the feedback terminal of the AC-to-DC switching power supply is electrically connected to the emitter of the first transistor, electrically connected to the non-inverting input terminal of the first operational amplifier through the second switch, and electrically connected to the third capacitor through the fifth switch; the gates of the first PMOS transistor and the second PMOS transistor are electrically connected, the drain and gate of the first PMOS transistor are electrically connected to the collector of the first transistor, and the drain of the second PMOS transistor is electrically connected to the collector of the first transistor. The first switch is electrically connected to the input terminal of the first switch, and the bases of the first and second transistors are connected to each other. A constant current source is electrically connected to the base and collector of the second transistor. The output terminal of the first switch and the input terminal of the fourth switch are respectively connected to the first capacitor. The output terminal of the fourth switch is electrically connected to the drain of the third NMOS transistor. The gate of the third NMOS transistor and the output terminal of the third switch are respectively connected to the second capacitor. The source of the third NMOS transistor is electrically connected to the drain of the second NMOS transistor. The inverting input terminal of the first operational amplifier is electrically connected to the drain of the second NMOS transistor. The output terminal of the first operational amplifier is connected to the third switch. The input terminals are electrically connected, the gates of the first NMOS transistor and the second NMOS transistor are connected, the non-inverting input terminal of the first operational amplifier is electrically connected to the drain of the first NMOS transistor, one input terminal of the comparator is electrically connected to the first capacitor, the other input terminal of the comparator is a first set value, the output terminal of the comparator is electrically connected to a timing generator, the timing generator collects the comparator output signal, the feedback terminal output signal and the PFM signal of the first switch, the timing generator issues control signals for the second switch, the third switch, the fourth switch and the fifth switch and the secondary side demagnetization time signal of the AC to DC switching power supply.

9. The sampling and holding structure according to claim 8, characterized in that, The set ratio is the ratio of the magnification of the first current mirror and the second current mirror.

10. A controller, characterized in that, Includes the sample-and-hold structure as described in any one of claims 1-9.

11. An AC-to-DC switching power supply, characterized in that, Includes a transformer and the controller as described in claim 10.

12. A method for sampling an AC-to-DC switching power supply using the sample-and-hold structure described in claim 1, characterized in that, include: The voltage at the feedback terminal of the transformer auxiliary winding is collected at a set ratio of the demagnetization time of the transformer secondary side in the same period. The set ratio is proportional to the ratio of the charging current when the energy storage element is charging and the discharging current when the energy storage element is discharging.

13. The method according to claim 12, characterized in that, The step of acquiring the voltage at the feedback terminal of the transformer auxiliary winding at a set proportion of the transformer secondary demagnetization time within the same period includes: When the primary side of the transformer in the AC-to-DC switching power supply is turned on, the feedback terminal is used to detect the negative voltage of the auxiliary winding, convert it into current, and charge the energy storage element from the first set value until the primary side is turned off. During the first set time period at the start of the demagnetization phase, the feedback terminal is used to convert the positive voltage of the auxiliary winding into current to discharge the aforementioned energy storage element. After the first set time period, the discharge current is maintained at a constant magnitude by sampling and holding, and continues to discharge the energy storage element until the voltage of the energy storage element is less than the first set value. At the same time, during the demagnetization phase after the first time period, the feedback terminal is used to detect the output voltage of the transformer, and when the voltage of the energy storage element is less than the first set value, it is sampled and held as the feedback voltage so that the output voltage of the transformer is maintained at the second set value.

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