Flyback converter and power supply system

Abstract

The application discloses a flyback converter and a power supply system, the flyback converter comprising: a transformer; a first switch tube and a second switch tube; a first inductor and a first capacitor; a control circuit, the control circuit comprising: an under-voltage protection module configured to set an under-voltage protection threshold proportional to an output voltage of the flyback converter, and trigger under-voltage protection of the flyback converter when an input voltage of the flyback converter is less than the under-voltage protection threshold. By setting an adaptive under-voltage protection threshold, the phenomenon of system restart after input power failure or shutdown of AHB can be avoided, so that in fast charging applications containing AHB topology structure, the PFC circuit can be turned off to optimize standby power consumption and light load efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of switching power supply, in particular to a flyback converter and a power supply system. BACKGROUND

[0002] Flyback converter belongs to a kind of switching power supply, and is widely used in the design of low-power switching power supply, such as mobile phone charger, LED drive etc. Traditional switching power supply system realizes current output based on single-stage or multi-stage driver. Among them, single-stage system is usually composed of dc / dc converter;And multi-stage system, for example, is composed of power factor correction (PFC) circuit and dc / dc converter. Among them, the BOOST topology structure is generally used in PFC circuit and used as a front stage to provide a stable output voltage as the input of the rear stage, and the dc / dc converter is used as the rear stage to convert the output voltage provided by the PFC circuit into the required DC voltage. Taking mobile phone charger as an example, the input power needs to be set with PFC circuit in PD fast charging system with a certain value, such as 75W or more.

[0003] The rear circuit of PFC circuit usually adopts single-tube flyback (flyback converter) to realize wide range input and wide range output, and it is just because of the advantages of wide range input and wide range output of single-tube flyback converter that it can turn off PFC circuit when 5V output standby or light load, optimizing standby power consumption or light load efficiency. Asymmetric half bridge converter (AHB) has the advantage of higher efficiency than single-tube flyback converter, but the output / input gain range of asymmetric half bridge converter is lower than that of single-tube flyback converter. In the optimal design of the system, asymmetric half bridge converter can only meet the wide range output, and cannot meet the wide range input at the same time.

[0004] In order to optimize the standby power consumption or light load efficiency of asymmetric half bridge converter, it is necessary to turn off PFC circuit when 5V output standby or light load. But in the case of PFC circuit being turned off, the input voltage of asymmetric half bridge converter is relatively low under low voltage input (such as 90V AC voltage), so it is necessary to set the Brown in (operating voltage threshold, that is, when the input voltage reaches the threshold, it starts to work) and Brown out (under voltage protection threshold) of asymmetric half bridge converter relatively low, for example, set its operating voltage threshold below 90V, such as 80V, and set its under voltage protection threshold to, for example, 70V, so that asymmetric half bridge converter can still work normally under the condition of PFC circuit being turned off and low voltage input.

[0005] If the working voltage threshold and the under-voltage protection threshold of the asymmetric half-bridge flyback converter are set to be relatively low, the following problems will occur: when the flyback converter is in a high-voltage output condition and the input is turned off, because the gain range of the asymmetric half-bridge flyback converter is smaller than that of the single-tube flyback converter, the output voltage of the asymmetric half-bridge flyback converter starts to drop before the input voltage of the asymmetric half-bridge flyback converter drops to the under-voltage protection threshold, at this time, the output under-voltage protection of the protocol chip or the overload protection (OLP) of the control circuit of the asymmetric half-bridge flyback converter is triggered, and after a certain period of protection triggering, the control circuit or the protocol chip of the asymmetric half-bridge flyback converter will try to restart the system, because the input voltage of the asymmetric half-bridge flyback converter has not dropped to the under-voltage protection threshold at this time, the output of the converter is re-established, which does not meet the requirements of the system control.

[0006] Therefore, it is necessary to provide an improved technical solution to overcome the above technical problems in the prior art. SUMMARY

[0007] In order to solve the above technical problems, the present application provides a flyback converter and a power supply system, by setting a dynamic under-voltage protection threshold, the phenomenon of system restart after the input power failure or shutdown of the asymmetric half-bridge flyback converter can be avoided, so that in the fast charging application containing the asymmetric half-bridge flyback topology structure, the standby power consumption and light load efficiency can be optimized by turning off the PFC circuit, and the structure is simple.

[0008] According to a first aspect of the present disclosure, a flyback converter is provided, comprising: a transformer having a primary winding, a secondary winding and an auxiliary winding;

[0009] A first switch tube and a second switch tube are connected in series between an input end and a reference ground;

[0010] A first inductor and a first capacitor form a resonance circuit with the primary winding and the second switch tube;

[0011] A control circuit is configured to control the switching state of the first switch tube and the second switch tube,

[0012] The control circuit includes an under-voltage protection module,

[0013] The under-voltage protection module sets an under-voltage protection threshold proportional to the output voltage of the flyback converter, and triggers the under-voltage protection of the flyback converter when the input voltage of the flyback converter is less than the under-voltage protection threshold.

[0014] Optionally, the under-voltage protection threshold is directly proportional to the output voltage of the flyback converter.

[0015] Optionally, the undervoltage protection module includes setting a duty cycle threshold based on the undervoltage protection threshold, and triggering undervoltage protection by comparing the operating duty cycle of the flyback converter with the duty cycle threshold.

[0016] Optionally, when the operating duty cycle of the flyback converter is greater than the duty cycle threshold, it indicates that the input voltage is less than the undervoltage protection threshold.

[0017] Optionally, the duty cycle threshold is the ratio of the output voltage of the flyback converter to the undervoltage protection threshold.

[0018] Optionally, the duty cycle threshold is less than the maximum operating duty cycle of the flyback converter.

[0019] Optionally, if the duty cycle of the flyback converter is greater than the duty cycle threshold for a period of time or if the duty cycle is greater than the duty cycle threshold in N consecutive comparisons, then undervoltage protection is triggered, where N is an integer greater than 1.

[0020] Optionally, the undervoltage protection module is configured to sample predetermined parameters of the flyback converter to obtain the input voltage and undervoltage protection threshold of the flyback converter, and to trigger undervoltage protection of the flyback converter when the input voltage is less than the undervoltage protection threshold.

[0021] Optionally, the undervoltage protection module includes:

[0022] The sample-and-hold unit is configured to sample and hold predetermined parameters of the flyback converter during the conduction of the first switch, and output a first sampled signal based on the sampling result;

[0023] The first sampling unit is configured to sample predetermined parameters of the flyback converter during the conduction period of the second switch and output a second sampling signal based on the sampling result.

[0024] The processing unit is configured to receive the first sampling signal and the second sampling signal, and to perform calculations on the first sampling signal and the second sampling signal to obtain the input voltage and the undervoltage protection threshold. The processing unit triggers the undervoltage protection of the flyback converter when the input voltage is less than the undervoltage protection threshold.

[0025] Optionally, the processing unit is configured to perform an addition operation on the absolute value of the first sampled signal and the absolute value of the second sampled signal to obtain a first voltage signal characterizing the input voltage;

[0026] The absolute value of the second sampled signal is multiplied by a first preset coefficient to obtain a second voltage signal characterizing the undervoltage protection threshold.

[0027] Optionally, the processing unit is configured to perform a subtraction operation on the absolute value of the first sampling signal and the absolute value of the product of the second sampling signal and a second preset coefficient to obtain a third voltage signal including information characterizing the input voltage and the undervoltage protection threshold.

[0028] The processing unit is further configured to compare the third voltage signal with a zero voltage signal, and trigger undervoltage protection of the flyback converter when the third voltage signal is less than the zero voltage signal.

[0029] Optionally, the processing unit is configured to compare the absolute value of the first sampling signal and the absolute value of the product of the second sampling signal and a third preset coefficient, and trigger undervoltage protection of the flyback converter when the absolute value of the first sampling signal is less than the absolute value of the product of the second sampling signal and the third preset coefficient.

[0030] Optionally, the transformer includes a primary winding, a secondary winding, and an auxiliary winding, and the predetermined parameter is the voltage across any winding in the transformer; or

[0031] The predetermined parameter is the current flowing through the voltage detection pin of the control circuit.

[0032] Optionally, the flyback converter further includes:

[0033] A startup module for starting the control circuit when it is detected that the input voltage is higher than a preset operating voltage threshold.

[0034] Optionally, the startup module includes:

[0035] A third resistor connected in parallel across the two ends of the first capacitor;

[0036] A comparison unit configured to compare the input voltage with a preset operating voltage threshold, and start the control circuit when the input voltage is greater than the preset operating voltage threshold.​​​​​​​​​​

[0040] a fourth resistor connected between a voltage detection pin of the control circuit and a first end of an auxiliary winding of the transformer, a second end of the auxiliary winding being connected with a reference ground;

[0041] a third switch connected between the voltage detection pin of the control circuit and the reference ground;

[0042] a control unit connected with a control end of the third switch, for controlling the third switch to be turned on during the first switch and the second switch are turned on.

[0043] According to a second aspect of the present disclosure, a power supply system is provided, comprising: a rectifier circuit for rectifying an input alternating current power supply;

[0044] a power factor correction circuit connected with the rectifier circuit, for performing power factor correction on an output signal of the rectifier circuit and outputting a second voltage signal; and

[0045] a flyback converter as described above, for outputting a predetermined direct current voltage signal according to the output signal of the power factor correction circuit.

[0046] The present disclosure has at least the following advantages:

[0047] The under-voltage protection module in the flyback converter of the embodiment of the present disclosure can effectively avoid the phenomenon of output restart of the flyback converter (for example, asymmetric half-bridge flyback converter) after input power failure or shutdown, so that in fast charging applications containing asymmetric half-bridge flyback topology structure, standby power consumption and light load efficiency can be optimized by shutting down the PFC circuit, and the structure is simple.

[0048] It should be noted that the foregoing general description and the following detailed description are only exemplary and explanatory, and cannot limit the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1a FIG. 1 shows a circuit structure schematic diagram of an existing asymmetric half-bridge flyback converter;

[0050] Figure 1b FIG. 2 shows a circuit structure schematic diagram of another existing asymmetric half-bridge flyback converter;

[0051] Figure 2 FIG. 3 shows a timing waveform diagram of the asymmetric half-bridge flyback converter shown in FIG. 1; Figure 1b

[0052] Figure 3 FIG. 4 shows a circuit structure schematic diagram of a power supply system provided according to a first embodiment of the present disclosure;​

[0053] Figure 4 A schematic diagram of the circuit structure of a power supply system provided according to a second embodiment of the present invention is shown;

[0054] Figure 5 Show Figure 3 and Figure 4 Internal structure diagram of the undervoltage protection module;

[0055] Figure 6 A schematic diagram of the circuit structure of a power supply system provided according to a fourth embodiment of the present invention is shown. Detailed Implementation

[0056] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0057] Figure 1a and Figure 1b The circuit structures of two existing asymmetric half-bridge flyback converters are shown respectively, such as Figure 1a and 1b As shown, where Figure 1a The upper switch Q2 is the second switch, and the lower switch Q1 is the first switch; Figure 1b The upper switch Q1 is the first switch, and the lower switch Q2 is the second switch. The two circuits operate on essentially the same principle, differing only in the winding positions. Figure 1b For example, its operating waveform in critical mode (BCM mode) is as follows: Figure 2 As shown, Vgs1 and Vgs2 are the drive signal waveforms of the first switch Q1 and the second switch Q2, respectively; i Lm Vds_Q1 is the excitation current waveform on the primary winding Np; Vaux is the voltage across the auxiliary winding Na; Vds_Q1 is the drain-source voltage of the first switching transistor.

[0058] refer to Figure 2 , Figure 1b The working principle of the asymmetric half-bridge flyback converter shown is as follows:

[0059] During the time interval t0-t1, the first control signal Vgs1 is high, and the first switch Q1 is turned on. During this time interval, the energy from the input voltage Vin is supplied to the transformer TR through the circuit consisting of the first switch Q1, the magnetizing inductor Lm, the primary winding Np, and the first capacitor Cr, resulting in a magnetizing current i. LmFirstly, it linearly increases from negative linear decrease to zero. In this process, the first inductor L1, the transformer TR and the first capacitor Cr store energy, and the auxiliary winding Na is-(Na / Np)*(Vin-Vc). Wherein, Na / Np is the turns ratio of the auxiliary winding Na and the primary winding Np in the transformer TR, and Vc is the voltage across the first capacitor Cr.

[0060] At t1, the first control signal Vgs1 becomes low, and the first switch Q1 is turned off.

[0061] In the period of t1-t2, the first switch Q1 is in the off state, and the second switch Q2 has not yet turned on. In this dead time, because the first inductor L1 and the primary winding Np continue to flow, the junction capacitance C1 of the first switch Q1, the junction capacitance C2 of the second switch Q2, the first capacitor Cr, the first inductor L1 and the primary winding Np resonate, the energy of the junction capacitance C2 of the second switch Q2 is extracted, the drain-source voltage Vds_Q2 of the second switch Q2 drops, and the junction capacitance C1 of the first switch Q1 is charged, the drain-source voltage Vds_Q1 of the first switch Q1 rises. At the same time, the voltage Vaux on the auxiliary winding Na starts to rise from negative voltage.

[0062] At t2, the junction capacitance voltage of the first switch Q1 reaches the highest, and the junction capacitance voltage of the second switch Q2 is extracted to zero voltage. At this time, the second control signal Vgs2 becomes high, and the second switch Q2 is turned on, so that the zero voltage turn-on of the second switch Q2 can be realized. At the same time, the voltage Vaux across the auxiliary winding Na also reaches the highest, which is (Na / Np)*Vc.

[0063] In the period of t2-t3, in the secondary side part of the asymmetric half-bridge flyback converter, the rectifier diode D1 is forward biased, and the energy stored in the primary side of the transformer TR begins to release to the secondary side, and the magnetizing current i Lm Linearly decreases.

[0064] At t3, the time is the initial turn-off time of the asymmetric half-bridge flyback converter, and at t3, the magnetizing current i Lm Linearly decreases to near zero current.

[0065] In the period of t3-t4, the period is the delay turn-on time of the second switch Q2, and the first inductor L1 and the first capacitor Cr resonate in this period, and the energy stored in the first capacitor Cr is also released to the secondary side through the forward process, and the magnetizing current i Lm Enters negative direction.

[0066] At the moment t4, the second driving signal Vgs2 received by the gate of the second switch Q2 becomes low, and the second switch Q2 is turned off.

[0067] During the period t4-t5, both the first switch Q1 and the second switch Q2 are in the off state, because the magnetizing current i Lm The freewheeling current so the first inductor L1 and the primary winding Np freewheel, so the junction capacitance C1 of the first switch Q1, the junction capacitance C2 of the second switch Q2, the first capacitor Cr, the first inductor L1 and the primary winding Np resonate, the energy of the junction capacitance C1 of the first switch Q1 is extracted, the drain-source voltage Vds_Q1 of the first switch Q1 drops, at the same time, the junction capacitance C2 of the second switch Q2 is charged, and the drain-source voltage Vds_Q1 of the second switch Q2 rises. At the same time, the voltage Vaux across the auxiliary winding Na linearly drops.

[0068] At the moment t5, the voltage Vaux across the auxiliary winding Na drops to the lowest, -(Na / Np)*(Vin-Vc), at the same time, the junction capacitance voltage of the second switch Q2 reaches the highest, the junction capacitance voltage of the first switch Q1 is extracted to zero voltage, and the first control signal Vgs1 becomes high, realizing the zero voltage turn-on of the first switch Q1. Thus, one cycle is completed, and then the same working process is repeated.

[0069] The asymmetric half-bridge flyback converter in the BCM mode additionally opens the second switch Q2 for a period of time (such as t ZVS ), generates a negative magnetizing current i Lm , and further realizes the zero voltage turn-on of the first switch Q1.

[0070] As shown in Figure 3 and Figure 4 , the power supply system in the embodiment of the application comprises: an alternating current power supply 1, a rectifier circuit 2, a power factor correction circuit 3, and a flyback converter 5. Exemplarily, the flyback converter 5 comprises, but is not limited to, the asymmetric half-bridge flyback converter shown in Figure 1a or Figure 1b .

[0071] The rectifier circuit 2 is used for rectifying the alternating current power supply input by the alternating current power supply 1. Optionally, the rectifier circuit 2 can be a full-bridge rectifier circuit or a half-bridge rectifier circuit.

[0072] The power factor correction circuit 3 is connected with the rectifier circuit 2, and is used for performing power factor correction on the voltage signal output by the rectifier circuit 2. The power factor correction circuit 3 generally adopts a BOOST topology structure and serves as a front stage to provide a stable corrected voltage signal (such as 400V) based on the rectified voltage signal as the input of a rear stage circuit such as the flyback converter 5.

[0073] The flyback converter 5 is used to output a predetermined DC voltage signal according to the output signal of the power factor correction circuit 3 to drive a subsequent load.

[0074] In the embodiment of the present application, the flyback converter 5 is, for example, an asymmetric half-bridge flyback converter, which comprises a transformer 51 including a primary winding Np, a secondary winding Ns and an auxiliary winding Na, a first switch Q1 and a second switch Q2 connected in series between an input terminal and a reference ground and constituting a half-bridge, a first inductor Lk, a first capacitor Cr and a control circuit 52.

[0075] The drain of the first switch Q1 is connected to the input terminal to receive a second voltage signal (which is the input voltage of the flyback converter 5, denoted as Vin in this text), and the gate of the first switch Q1 is connected to the control circuit 52; the drain of the second switch Q2 is connected to the source of the first switch Q1, the source of the second switch Q2 is connected to the reference ground, and the gate of the second switch Q2 is connected to the control circuit 52; and the capacitor C11 and the capacitor C12 are the junction capacitances of the first switch Q1 and the second switch Q2 respectively. In the same switching period, the first switch Q1 and the second switch Q2 are turned on at different times to transfer the input voltage Vin from the primary part to the secondary part of the transformer 51. In a possible embodiment, the first switch Q1 and the second switch Q2 are both NMOS field effect transistors.

[0076] The first inductor Lk, the primary winding Np, the first capacitor Cr and the second switch Q2 form a resonant circuit. In this embodiment, the first inductor Lk is connected between the drain of the second switch Q2 and the first end of the primary winding Np, and the first capacitor Cr is connected between the source of the second switch Q2 and the second end of the primary winding Np. In a possible embodiment, the first inductor Lk is the leakage inductance of the primary winding Np, and the first capacitor Cr is the resonant capacitor.

[0077] The secondary side of the flyback converter 5 includes a rectifier diode D1 and an output capacitor Co. In the present embodiment, the anode of the rectifier diode D1 is connected to the non-common terminal of the secondary winding Ns, and the cathode of the rectifier diode D1 is connected to the output terminal of the flyback converter 5. The positive terminal of the output capacitor Co is connected to the output terminal of the flyback converter 5, and the negative terminal of the output capacitor Co is connected to the reference ground, and the common terminal of the secondary winding Ns is also connected to the reference ground. Further, the output terminal of the flyback converter 5 is connected to a load, which receives the electrical energy (e.g. voltage and current) converted by the flyback converter 5. In some examples, the electrical energy converted by the flyback converter 5 is further filtered before reaching the load. In some examples, the filter is a sub-component of the flyback converter 5, an external component of the flyback converter 5, and / or a sub-component of the load. In any case, the load can use the filtered or unfiltered electrical energy from the flyback converter 5 to perform a function. Optionally, the load can include, but is not limited to, a computing device and related components, such as a microprocessor, an electrical component, a circuit, a laptop computer, a desktop computer, a tablet computer, a mobile phone, a battery, a speaker, a lighting unit such as an LED, a component related to an automobile / ship / aeroplane / train, a motor, a transformer, or any other type of electrical device and / or circuit that receives voltage or current from a flyback converter.

[0078] The control circuit 52 is configured to generate a first control signal Vgs1 and a second control signal Vgs2 to control the switching state of the first switch Q1 and the second switch Q2, respectively.

[0079] Further, the control circuit 52 includes an under-voltage protection module 521. The under-voltage protection module 521 is configured to sample a predetermined parameter of the flyback converter to obtain an input voltage of the flyback converter and an under-voltage protection threshold value proportional to the output voltage Vo of the flyback converter, and trigger an under-voltage protection of the flyback converter when the input voltage is less than the under-voltage protection threshold value. Preferably, the under-voltage protection threshold value is directly proportional to the output voltage Vo of the flyback converter.

[0080] Reference Figure 5The under-voltage protection module 521 further comprises: a sample-and-hold unit 5211, a first sampling unit 5212, and a processing unit 5213. The sample-and-hold unit 5211 is configured to sample and hold a predetermined parameter of the flyback converter during the conduction of the first switch tube Q1, and output a first sampling signal according to the sampling result. The first sampling unit 5212 is configured to sample the predetermined parameter of the flyback converter during the conduction of the second switch tube Q2, and output a second sampling signal according to the sampling result. The processing unit 5213 is connected with the sample-and-hold unit 5211 and the first sampling unit 5212 respectively, and is configured to receive the first sampling signal and the second sampling signal, and perform operation processing on the first sampling signal and the second sampling signal to obtain the input voltage of the flyback converter and an under-voltage protection threshold. The processing unit 5213 is further configured to trigger the under-voltage protection of the flyback converter when the input voltage of the flyback converter is less than the under-voltage protection threshold.

[0081] It can be understood that, since the under-voltage protection threshold obtained in the embodiment of the present application varies in proportion to the output voltage Vo of the flyback converter 5 (the higher the output voltage, the higher the under-voltage protection threshold), even if the output voltage of the asymmetric half-bridge flyback converter triggers the output UVP under-voltage protection of the protocol chip or the overload protection of the control circuit 52 in the case of high-voltage output of the asymmetric half-bridge flyback converter 5, the input voltage of the flyback converter can timely trigger the input under-voltage protection to meet the system requirements, effectively avoiding the phenomenon of system restart after the input power failure or shutdown of the asymmetric half-bridge flyback converter 5, so that the standby power consumption and light load efficiency of the system can be optimized by shutting down the PFC circuit 3 in the power supply system containing the asymmetric half-bridge flyback topology, for example, the power supply system shown in FIG. 1. Figure 3 or Figure 4 It can be understood that, since the under-voltage protection threshold obtained in the embodiment of the present application varies in proportion to the output voltage Vo of the flyback converter 5 (the higher the output voltage, the higher the under-voltage protection threshold), even if the output voltage of the asymmetric half-bridge flyback converter triggers the output UVP under-voltage protection of the protocol chip or the overload protection of the control circuit 52 in the case of high-voltage output of the asymmetric half-bridge flyback converter 5, the input voltage of the flyback converter can timely trigger the input under-voltage protection to meet the system requirements, effectively avoiding the phenomenon of system restart after the input power failure or shutdown of the asymmetric half-bridge flyback converter 5, so that the standby power consumption and light load efficiency of the system can be optimized by shutting down the PFC circuit 3 in the power supply system containing the asymmetric half-bridge flyback topology, for example, the power supply system shown in FIG. 1.

[0082] Based on the working principle of the transformer 51, it can be known that the voltage across the primary winding Np, the voltage across the secondary winding Ns, and the voltage Vaux across the auxiliary winding Na in the transformer 51 have a certain proportional relationship with each other. Further, if the voltage detection pin Vs of the control circuit 52 in the flyback converter 5 can withstand negative voltage, in some embodiments, the predetermined parameter of the flyback converter 5 is the voltage across any winding in the transformer 51, at this time, the first sampling signal is the voltage signal obtained by sampling during the conduction of the first switch tube Q1, and the second sampling signal is the voltage signal obtained by sampling during the conduction of the second switch tube Q2. Preferably, the voltage Vaux across the auxiliary winding Na. For example, taking the voltage Vaux across the auxiliary winding Na as the predetermined parameter, at this time, the voltage detection pin Vs of the control circuit 52 can be directly connected with one end of the auxiliary winding Na, and the other end of the auxiliary winding Na is connected with the reference ground.

[0083] In some other embodiments, the predetermined parameter of the symmetrical half-bridge flyback converter 5 is the voltage division of the voltage across any winding of the transformer TR. For example, the predetermined parameter is the voltage division of the voltage Vaux across the auxiliary winding Na, and thus Figure 3 The flyback converter 5 further comprises a first resistor R1 and a second resistor R2. The first resistor R1 and the second resistor R2 are connected in series between the opposite end and the same end of the auxiliary winding Na, and the connection node of the first resistor R1 and the second resistor R2 is connected to the voltage detection pin Vs of the control circuit 52.

[0084] Optionally, in the embodiment, the voltage Vaux across the auxiliary winding Na can be sampled by the voltage detection pin Vs of the control circuit 52 after being divided by the first resistor R1 and the second resistor R2. Alternatively, the control circuit 52 can directly sample the voltage across the auxiliary winding Na.

[0085] Referring to Figure 2 , it can be seen that during the conduction of the first switch Q1, the voltage across the auxiliary winding Na is

[0086] Vaux1 = -(Na / Np)*(Vin-Vc) (1),

[0087] wherein Vaux1 is the first sampling signal, Na / Np is the turns ratio of the auxiliary winding to the primary winding of the transformer 51, Vin is the voltage value of the input voltage, and Vc is the voltage value across the first capacitor Cr.

[0088] During the conduction of the second switch Q2, the voltage across the auxiliary winding Na is

[0089] Vaux2 = (Na / Np)*Vc (2),

[0090] wherein Vaux2 is the second sampling signal.

[0091] Based on the working principle of the flyback converter, it can be seen that

[0092] Vc = (Np / Ns)*Vo (3),

[0093] wherein Np / Ns is the turns ratio of the primary winding to the secondary winding of the transformer 51, and Vo is the voltage value of the output voltage.

[0094] The process of obtaining the input voltage and the under-voltage protection threshold based on the operation and processing of the first sampling signal and the second sampling signal by the processing unit 5213 will be described in detail below with reference to the above formulas (1), (2) and (3).

[0095] Exemplarily, in the first embodiment of the present application, the processing unit 5213 is configured to perform an addition operation on the absolute value of the first sampling signal and the absolute value of the second sampling signal to obtain a first voltage signal (denoted as V1) representing the input voltage of the flyback converter 5. Specifically, after adding the formula (1) and the formula (2), it can be obtained that:

[0096] V1 = |Vaux1| + |Vaux2| = (Na / Np) * Vin (4).

[0097] As can be seen from the formula (4), the first voltage signal V1 is in a positive proportional relationship with the input voltage Vin of the flyback converter 5, and the first voltage signal V1 can be used to represent the input voltage Vin of the flyback converter 5.

[0098] In the present embodiment, the processing unit 5213 is configured to perform a multiplication operation on the absolute value of the second sampling signal and a first preset coefficient (denoted as k1) to obtain a second voltage signal (denoted as V2) representing the under-voltage protection threshold of the flyback converter 5. Specifically, from the formula (2) and the formula (3), it can be obtained that:

[0099] V2 = k1 * |Vaux2| = k1 * (Np / Ns) * Vo (5).

[0100] Wherein, k1 is greater than 0. And as can be seen from the formula (5), the second voltage signal V2 is in a positive proportional relationship with the output voltage Vo of the flyback converter 5, and the second voltage signal V2 can be used to represent the under-voltage protection threshold of the flyback converter 5 which is proportional to the output voltage Vo.

[0101] In the second embodiment of the present application, the processing unit 5213 is configured to perform a subtraction operation on the absolute value of the first sampling signal and the absolute value of the product of the second sampling signal and a second preset coefficient (denoted as k2) to obtain a third voltage signal (denoted as V3) containing the input voltage Vin and the under-voltage protection threshold of the flyback converter 5. Specifically,

[0102] V3 = |Vaux1| - |k2 * Vaux2| = (Na / Np) * Vin - (1 + k2) * (Na / Np) * Vc (6),

[0103] Wherein, k2 is greater than 0.

[0104] Further, in the present embodiment, the processing unit 5213 is further configured to compare the third voltage signal V3 with a zero voltage signal, and trigger the under-voltage protection of the flyback converter 5 when the third voltage signal V3 is less than the zero voltage signal, i.e.

[0105] V3<0....................(7)。

[0106] Wherein, formula (6) and (3) are brought into formula (7), formula (7) can be converted to:

[0107] Vin<(1+k2)*Vo*(Np / Ns)....................(8),

[0108] Wherein, (1+k2)*Vo*(Np / Ns) is the input voltage undershoot protection threshold of the flyback converter 5. According to formula (8), when the input voltage Vin of the flyback converter 5 is less than the undershoot protection threshold, the undershoot protection of the flyback converter 5 will be triggered, which is equivalent to setting a dynamic undershoot protection threshold in the embodiment of the application.

[0109] In the third embodiment of the application, the processing unit 5213 is configured to compare the absolute value of the first sampling signal and the absolute value of the product of the second sampling signal and a third preset coefficient (denoted as k3), and trigger the undershoot protection of the flyback converter when the absolute value of the first sampling signal is less than the absolute value of the product of the second sampling signal and the third preset coefficient k3.

[0110] Further, when comparing the first sampling signal Vaux1 and the second sampling signal Vaux2, if the first sampling signal Vaux1 and the second sampling signal Vaux2 satisfy the following formula (9), the undershoot protection of the flyback converter 5 is triggered:

[0111] |Vaux1|<|k3*Vaux2|...................(9)。

[0112] Wherein, formula (1), (2) and (3) are brought into formula (9), formula (9) can be converted to:

[0113] Vin<(1+k3)*Vo*(Np / Ns)..................(10),

[0114] Wherein, (1+k3)*Vo*(Np / Ns) is the under-voltage protection threshold of the flyback converter 5. According to the formula (10), when the input voltage Vin of the flyback converter 5 is less than the under-voltage protection threshold, the under-voltage protection of the flyback converter 5 is triggered, which is equivalent to setting a dynamic under-voltage protection threshold in the embodiment of the present application. In the embodiment, after the first sampling unit 5211 and the second sampling unit 5212 obtain the first sampling signal and the second sampling signal, the under-voltage protection of the flyback converter can be realized by directly comparing the first sampling signal and the second sampling signal. The required signal processing process is simple, the under-voltage protection can be more accurate, and the cost is lower.

[0115] In the above embodiments of the present application, since the under-voltage protection threshold of the flyback converter 5 varies in proportion to the output voltage Vo (the higher the output voltage, the higher the under-voltage protection threshold), even if the output voltage of the asymmetric half-bridge flyback converter triggers the output UVP under-voltage protection of the protocol chip or the overload protection of the control circuit 52 under the high-voltage output condition of the flyback converter 5, the input voltage of the asymmetric half-bridge flyback converter can trigger the input under-voltage protection in time to meet the system requirements, effectively avoiding the phenomenon that the output of the asymmetric half-bridge flyback converter 5 restarts after the input power failure or shutdown, so that in the power supply system shown in the fast charging application such as Figure 3 or Figure 4 the standby power consumption and the light load efficiency of the system can be optimized by turning off the PFC circuit 3.

[0116] According to the above formula (10), the proportional coefficient between the under-voltage protection threshold of the flyback converter 5 and the output voltage Vo is (1+k3)*(Np / Ns), wherein the third preset coefficient k3 satisfies:

[0117]

[0118] Wherein, Dmax is the preset maximum working duty ratio of the flyback converter. In a preferred embodiment of the present application, k3=0.5±0.1.

[0119] In the present application, the values of the above-mentioned preset coefficients (k1, k2, k3) can be set to be all the same or only partially the same, or all different, which can be reasonably selected according to the actual situation, and the present application does not limit this.

[0120] In the fourth embodiment of the present application, the under-voltage protection module 521 is further configured to set a duty ratio threshold (denoted as D UVP ) according to the under-voltage protection threshold, and trigger the under-voltage protection of the flyback converter 5 according to the working duty ratio (denoted as D) of the flyback converter 5 and the duty ratio threshold D UVPthe working duty cycle D of the flyback converter 5 is greater than a duty cycle threshold D UVP , which indicates that the input voltage Vin is less than the under-voltage protection threshold.

[0121] Specifically, as shown in Figure 6 , the under-voltage protection module 521 comprises a second sampling unit 5214 and a processing unit 5213, the second sampling unit 5214 is configured to sample the control signal Vgs1 of the main switch Q1 of the flyback converter 5, i.e. the switch not connected to the resonant loop (for example, the first switch Q1 in Figure 1a 、 Figure 1b 、 Figure 3 and Figure 4 ) to obtain the duty cycle of the control signal Vgs1 of the main switch Q1, i.e. the working duty cycle D of the flyback converter 5. For example, the second sampling unit 5214 can count the number of rising edges of the control signal Vgs1 within a preset time to obtain the period of the control signal Vgs1, and then time the length of the high level within one period of the control signal Vgs1 to obtain the current working duty cycle of the flyback converter 5. It should be noted that the second sampling unit 5214 can also use other conventional duty cycle sampling methods to sample the duty cycle of the control signal Vgs1 of the main switch Q1 to obtain the working duty cycle D of the flyback converter 5, which is not limited in the present application.

[0122] In the embodiment, the duty cycle threshold D UVP is configured as the ratio of the output voltage Vo of the flyback converter 5 to the under-voltage protection threshold. Since the under-voltage protection threshold is proportional to the output voltage Vo in the present application, the ratio of the output voltage Vo to the under-voltage protection threshold should be a fixed constant. Furthermore, in the embodiment, a suitable duty cycle threshold D UVP can be directly set in advance, and then only the working duty cycle D of the flyback converter 5 needs to be sampled and compared with the pre-set duty cycle threshold D UVP to realize the under-voltage protection of the flyback converter 5, which requires less sampling of signals and simple processing method.

[0123] For example, after converting the above formula (10), we can obtain:

[0124]

[0125] wherein, let

[0126] According to formula (11) and formula (13), the pre-set duty cycle threshold D UVPDmax. And for asymmetric half-bridge flyback converter topology, we have:

[0127]

[0128] Further, by substituting equation (13) and equation (14) into equation (12), we have:

[0129] D > D UVP .....................(15).

[0130] Further, the second sampling unit 5214 sends the sampled working duty ratio D of the flyback converter 5 to the processing unit 5213 or other units or modules with corresponding data comparison function, and the processing unit 5213 compares the working duty ratio D with the duty ratio threshold D UVP previously set according to the under-voltage protection threshold, and triggers the under-voltage protection of the flyback converter 5 when the working duty ratio D is greater than the internal set duty ratio threshold D UVP .

[0131] Further, the processing unit 5213 or other units or modules with corresponding data comparison function can also be configured to trigger the under-voltage protection of the flyback converter 5 when the working duty ratio D is greater than the duty ratio threshold D UVP for a period of time or when the working duty ratio D is greater than the duty ratio threshold D UVP for N consecutive times, where N is an integer greater than 1. By multiple comparisons, the trigger of the under-voltage protection can be more accurate, and the system stability is better.

[0132] In yet some embodiments, the predetermined parameter of the flyback converter 5 is the current Is flowing through the voltage detection pin Vs of the control circuit 52, which can be applied to some controller chips that do not support negative voltage. In this case, the first sampling signal is the corresponding first current signal sampled during the conduction period of the first switch Q1, and the second sampling signal is the corresponding second current signal sampled during the conduction period of the second switch Q2. In this case, with reference to Figure 4 , the asymmetric half-bridge flyback converter 5 further comprises a fourth resistor RFB1, a third switch Q3 and a control unit 522. The fourth resistor RFB1 is connected between the voltage detection pin Vs of the control circuit 52 and the non-identical terminal of the auxiliary winding Na, and the identical terminal of the auxiliary winding Na is connected to the reference ground. The third switch Q3 is connected between the voltage detection pin Vs of the control circuit 52 and the reference ground. The control unit 522 is connected to the control terminal of the third switch Q3, and is used to control the conduction of the third switch Q3 during the conduction period of the first switch Q1 and the second switch Q2.

[0133] Optionally, in the embodiment, the third switch Q3 can be integrated in the control circuit 52 or can be arranged outside the control circuit 52, which is not limited in the application. In one possible embodiment, the third switch Q3 is an NMOS field effect transistor.

[0134] Reference Figure 4 In the third embodiment, during the conduction of the first switch Q1, the first sampling signal (denoted as Is1) obtained after sampling and holding is:

[0135]

[0136] During the conduction of the second switch Q2, the second sampling signal (denoted as Is2) obtained after sampling is:

[0137]

[0138] Wherein, Na is the number of turns of the auxiliary winding, Np is the number of turns of the primary winding, Vin is the input voltage, Vc is the voltage across the first capacitor Cr, and RFB1 is the resistance value of the third resistor.

[0139] Further, when comparing the first sampling signal Is1 and the second sampling signal Is2, if the first sampling signal Is1 and the second sampling signal Is2 satisfy the following formula (18), the under-voltage protection of the flyback converter 5 is triggered:

[0140] |Is1|<|k4*Is2|................(18).

[0141] Wherein, by substituting the formula (16), (17) and (3) into the formula (18), the same result as the aforementioned formula (10) can be obtained.

[0142] It should be noted that after obtaining the first sampling signal Is1 and the second sampling signal Is2, the same or similar principles as in the first embodiment to the fourth embodiment can be used to process the first sampling signal Is1 and the second sampling signal Is2 to achieve the under-voltage protection of the flyback converter 5, which will not be described here.

[0143] Further, the flyback converter 5 further comprises a starting module 53. The starting module 53 is configured to start the control circuit 52 when it is detected that the input voltage Vin is higher than a preset working voltage threshold. In the embodiment, the preset working voltage threshold is a fixed voltage value, and the voltage value is set to be relatively small, so that the flyback converter 5 can be started when the input voltage Vin is relatively low. Figure 3 or Figure 4 As shown in the power supply system, the flyback converter 5 can work normally even if the power factor correction circuit 3 is not started.

[0144] The starting module 53 further comprises a third resistor R3 and a comparison unit 531. The third resistor R3 is connected in parallel to the first capacitor Cr. The comparison unit 531 is configured to compare the input voltage Vin with a preset working voltage threshold, and start the control circuit 52 when the input voltage Vin is greater than the preset working voltage threshold. In this embodiment, by connecting the third resistor R3 in parallel to the first capacitor Cr, the voltage of the first capacitor Cr can be reduced to 0V before the flyback converter 5 starts, so that the starting module 53 can detect the input voltage Vin by directly sampling the predetermined parameters of the flyback converter 5 during the conduction of the first switch Q1.

[0145] Exemplarily, in Figure 3 or Figure 4 In one practical application of the power supply system shown in the figure, when the power supply system is working, the power factor correction circuit 3 is not started, and the preset working voltage threshold of the flyback converter 5 is set to be relatively low, for example, 80V. When the input voltage Vin of the flyback converter 5 is greater than the preset working voltage threshold, the flyback converter 5 can enter the working state without starting the power factor correction circuit 3, and output a low output signal, for example, 5V. Then, the power factor correction circuit 3 starts to work in the boost mode, the under-voltage protection threshold of the flyback converter 5 can be set to be higher than the working voltage threshold, and is dynamically set in proportion to the output voltage of the flyback converter 5. When the system is powered off, the output voltage of the flyback converter 5 starts to decrease and triggers the output UVLO under-voltage protection or overload protection, thereby discharging the power supply voltage of the system to the under-voltage protection point of the UVLO, so as to ensure that the system will not restart when it is powered off.

[0146] In summary, in this application, the predetermined parameters of the flyback converter are sampled during the conduction of the first switch and the second switch, respectively, to obtain the first sampling signal and the second sampling signal which are proportional to the input voltage of the flyback converter and the turns ratio of the converter, respectively. Then, in the process of comparing the first sampling signal and the second sampling signal to realize the under-voltage comparison of the flyback converter, the under-voltage protection threshold which is proportional to the output voltage of the flyback converter can be obtained, which can effectively avoid the phenomenon that the output of the flyback converter (for example, the asymmetric half-bridge flyback converter) restarts after the input power is cut off or the system is powered off, so that in the fast charging application containing the asymmetric half-bridge flyback topology, the standby power consumption and the light load efficiency can be optimized by closing the PFC circuit, and the structure is simple.

[0147] It should be noted that the above-mentioned embodiments are merely used to clearly illustrate the technical solutions of the present application, and should not be construed as limitations to the present application. Based on the above-mentioned embodiments, those skilled in the art can make other variations or modifications without departing from the spirit of the present application. The present application is not required to enumerate all of the embodiments, and the variations or modifications made without departing from the spirit of the present application should fall within the scope of the present application.

Claims

1. A flyback converter, wherein, The application relates to a transformer, a first switch tube and a second switch tube, a first inductor and a first capacitor, and a control circuit. The transformer has a primary winding, a secondary winding and an auxiliary winding. The first switch tube and the second switch tube are connected in series between an input end and a reference ground. The first inductor and the first capacitor form a resonance circuit with the primary winding and the second switch tube. The control circuit is used for controlling the switching state of the first switch tube and the second switch tube. The control circuit comprises an under-voltage protection module. The under-voltage protection module sets an under-voltage protection threshold value proportional to the output voltage of the flyback converter, and triggers the under-voltage protection of the flyback converter when the input voltage of the flyback converter is smaller than the under-voltage protection threshold value.

2. The flyback converter of claim 1, wherein, The under-voltage protection threshold value is directly proportional to the output voltage of the flyback converter. The under-voltage protection module sets a duty cycle threshold value according to the under-voltage protection threshold value.

3. The flyback converter of claim 2, wherein, The under-voltage protection is triggered according to the comparison between the working duty cycle of the flyback converter and the duty cycle threshold value.

4. The flyback converter of claim 2, wherein, When the working duty cycle of the flyback converter is greater than the duty cycle threshold value, it indicates that the input voltage is smaller than the under-voltage protection threshold value.

5. The flyback converter of claim 2, wherein, The duty cycle threshold value is the ratio of the output voltage of the flyback converter to the under-voltage protection threshold value.

6. The flyback converter of claim 2, wherein, The duty cycle threshold value is smaller than the maximum working duty cycle of the flyback converter.

7. The flyback converter of claim 1, wherein, When the working duty cycle of the flyback converter is greater than the duty cycle threshold value for a period of time or for N consecutive times, the under-voltage protection is triggered, wherein N is an integer greater than 1.

8. The flyback converter of claim 7, wherein, The under-voltage protection module is configured to sample predetermined parameters of the flyback converter to obtain the input voltage and the under-voltage protection threshold value of the flyback converter, and trigger the under-voltage protection of the flyback converter when the input voltage is smaller than the under-voltage protection threshold value. The under-voltage protection module comprises: a sample-and-hold unit configured to sample and hold the predetermined parameters of the flyback converter during the conduction of the first switch tube, and output a first sampling signal according to the sampling result; a first sampling unit configured to sample the predetermined parameters of the flyback converter during the conduction of the second switch tube, and output a second sampling signal according to the sampling result; 9. The flyback converter of claim 8, wherein, a processing unit configured to receive the first sampling signal and the second sampling signal, and perform operation processing on the first sampling signal and the second sampling signal to obtain the input voltage and the under-voltage protection threshold value, and trigger the under-voltage protection of the flyback converter when the input voltage is smaller than the under-voltage protection threshold value. The processing unit is configured to perform addition operation on the absolute value of the first sampling signal and the absolute value of the second sampling signal to obtain a first voltage signal representing the input voltage.

10. The flyback converter of claim 8, wherein, The processing unit is configured to perform multiplication operation on the absolute value of the second sampling signal and a first preset coefficient to obtain a second voltage signal representing the under-voltage protection threshold value. The processing unit is configured to perform subtraction operation on the absolute value of the first sampling signal and the absolute value of the product of the second sampling signal and a second preset coefficient to obtain a third voltage signal containing information representing the input voltage and the under-voltage protection threshold value. The processing unit is further configured to compare the third voltage signal with a zero voltage signal, and trigger an under-voltage protection of the flyback converter when the third voltage signal is less than the zero voltage signal.

11. The flyback converter of claim 8, wherein, The processing unit is configured to compare an absolute value of the first sampling signal with an absolute value of a product of the second sampling signal and a third preset coefficient, and trigger an under-voltage protection of the flyback converter when the absolute value of the first sampling signal is less than the absolute value of the product of the second sampling signal and the third preset coefficient.

12. The flyback converter of any of claims 7-11, wherein, The transformer comprises a primary winding, a secondary winding and an auxiliary winding, and the predetermined parameter is a voltage across any winding of the transformer; or The predetermined parameter is a current flowing through a voltage detection pin of the control circuit.

13. The flyback converter of claim 1, wherein, The flyback converter further comprises: a starting module configured to start the control circuit when the input voltage is detected to be higher than a preset operating voltage threshold.

14. The flyback converter of claim 13, wherein, The starting module comprises: a third resistor connected in parallel across the first capacitor; a comparison unit configured to compare the input voltage with the preset operating voltage threshold, and start the control circuit when the input voltage is greater than the preset operating voltage threshold.

15. A power supply system, wherein, The flyback converter comprises: a rectification circuit configured to rectify an input alternating current power supply; a power factor correction circuit connected with the rectification circuit and configured to perform power factor correction on an output signal of the rectification circuit; and a flyback converter according to any one of claims 1-14, configured to output a predetermined direct current voltage signal according to the output signal of the power factor correction circuit. ​

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