Control device and control method

A control device applied to a flyback converter including an auxiliary switch includes: a current detector configured to detect an amplitude of a current of the flyback converter to obtain an amplitude of a negative magnetizing current of the flyback converter; and a comparator controller configured to compare the amplitude of the negative magnetizing current obtained by the current detector with a reference value, and turn off the auxiliary switch according to a comparison result. According to the present disclosure, it is able to achieve zero-voltage switching of a primary-side switch of the flyback converter with variable outputs.

CROSS REFERENCE

This application is based upon and claims priority to Chinese Patent Application No. 201710229571.6, filed on Apr. 10, 2017, and Chinese Patent Application No. 201710526927.2, filed on Jun. 30, 2017 the entire contents thereof are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the field of power electronic technologies, and more particularly, to a control device and control method applied to a flyback converter.

BACKGROUND

Flyback converter is widely adopted for low power off-line applications, such as notebook adapter, cell phone charger etc. Quasi-resonant (QR) flyback is particularly popular because of less switching loss. It operates in discontinuous current conduction mode, and achieves zero voltage switching (ZVS) at low line and partial hard switching at high line. Recently there is growing need for higher power density for adapters, and high switching frequency design accompanied by efficiency improvement is necessary to pass thermal requirement in a smaller volume. QR flyback typically operates below 150 kHz, and switching loss becomes dominant when switching frequency further increases, especially at high line condition. Soft switching flyback converter were introduced to run at higher switching frequency (300˜1 MHz) with optimized efficiency. With new control method, QR flyback with synchronous rectifier (SR) can achieve ZVS for both low line and high line conditions. Before turning on the primary main switch, the SR is controlled to get inverse secondary side current, and then the inverse current is transferred to primary side to discharge parasitic capacitance of the primary main switch for ZVS. Active clamp flyback (ACF) is a two-switch topology that achieves soft switching. Operating in continuous current mode (CCM), or discontinuous current mode (DCM), it can achieve full ZVS, and furthermore, ACF recycles leakage inductance energy, which benefits both efficiency and device stress.

So far, soft switching flyback converter mentioned above is developed for fixed output voltage application. For wide range output applications such as USB PD adapter, ZVS may be lost, or efficiency may be degraded due to excessive circulation current.

Therefore, it is an urgent need at present to develop a control device and a control method that can overcome the above technical problems.

It is to be noted that the above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.

SUMMARY

An objective of the present disclosure is to provide a control device and a control method, so as to overcome, at least to a certain extent, one or more problems caused by limitation and defects of related technologies.

According to an aspect of the present disclosure, there is provided a control device applied to a flyback converter including an auxiliary switch. The control device includes: a current detector configured to detect an amplitude of a current of the flyback converter to obtain an amplitude of a negative magnetizing current of the flyback converter; and a comparator controller configured to compare the amplitude of the negative magnetizing current obtained by the current detector with a reference value, and turn off the auxiliary switch according to a comparison result.

According to an aspect of the present disclosure, there is provided a switching power supply, which includes the control device according to any one of the above embodiments.

According to an aspect of the present disclosure, there is provided a control method applied to a flyback converter including an auxiliary switch. The control method includes: detecting an amplitude of a current of the flyback converter to obtain an amplitude of a negative magnetizing current of the flyback converter; comparing the amplitude of the negative magnetizing current with a reference value; and turning off the auxiliary switch according to a comparison result to achieve zero-voltage switching of a primary-side power switch of the flyback converter.

According to the control device and the control method provided by the exemplary embodiments of the present disclosure, an amplitude of a negative magnetizing current is obtained by detecting an amplitude of a current of the flyback converter, the obtained amplitude of the negative magnetizing current is compared with a reference value, and turn-off of the auxiliary switch is controlled according to a comparison result. On the one hand, the amplitude of the negative magnetizing current is obtained by detecting the amplitude of the current of the flyback converter, such that the amplitude of the negative magnetizing current with different output voltages may be detected. On the other hand, the obtained amplitude of the negative magnetizing current is compared with the reference value, and turn-off of the auxiliary switch is controlled according to a comparison result, such that zero-voltage switching of a primary-side switch with different output voltages may be achieved by reasonably setting the reference value.

It should be understood that the above general description and the detailed description below are merely exemplary and explanatory, and do not limit the present disclosure.

DETAILED DESCRIPTION

The exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, the exemplary embodiments can be implemented in a variety of forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments are provided so that the present disclosure will be thorough and complete and will fully convey the concepts of exemplary embodiments to those skilled in the art. The features, structures, or characteristics described may be combined in one or more embodiments in any suitable manner. In the following description, numerous specific details are provided to give a full understanding of the embodiments of the present disclosure. Those skilled in the art will recognize, however, that the technical solution of the present disclosure may be practiced without one or more of the specific details described, or that other methods, components, materials, etc. may be employed. In other instances, well-known technical solutions are not shown or described in detail to avoid obscuring aspects of the present disclosure.

In addition, the accompanying drawings are merely exemplary illustration of the present disclosure, and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus repeated description thereof will be omitted. Some block diagrams shown in the figures are functional entities and not necessarily to be corresponding to a physically or logically individual entities. These functional entities may be implemented in software form, or implemented in one or more hardware modules or integrated circuits, or implemented in different networks and/or processor apparatuses and/or microcontroller apparatuses.

FIG. 1illustrates a circuit diagram of an active clamp flyback converter according to a typical structure, by which zero-voltage switching (ZVS) of a primary-side power switch S1may be achieved. An exemplary control method may be implemented by turning on a clamp switch S2for a set time (for example, t2-t3in the control waveform diagram as shown inFIG. 2) only before the primary-side power switch S1is turned on.

FIG. 3illustrates a schematic circuit diagram of an RCD clamp flyback converter according to a typical structure, by which zero-voltage switching (ZVS) of the primary-side power switch S1may be achieved by delaying in turning on a secondary-side synchronous rectifier SR. An exemplary control method for delaying in turning on the secondary-side synchronous rectifier SRmay be implemented by further maintaining, after a secondary-side current isof the synchronous rectifier SRdropping to zero, the synchronous rectifier turn-on for a set time, such as t1-t2in the control waveform diagram as shown inFIG. 4.

The above two methods for achieving the zero-voltage switching (ZVS) of the primary-side power switch S1are implemented by controlling the synchronous rectifier SRor the clamp switch S2turn-on for a set time, which is applicable to an application scenario where the output voltage is constant.

However, with the development of a power adapter, particularly the promotion of USB-PD Type-C, the application of variable outputs becomes more and more popular. For an application scenario of the variable outputs, the above control mode is not applicable any more. This is because no matter the RCD clamp flyback converter or the active clamp flyback converter has the following basic principle of achieving the zero-voltage switching (ZVS) of the primary-side power switch: before the primary-side power switch S1is turned on, a negative magnetizing current Im_n(t) is generated on an magnetizing inductor Lmof a transformer, the zero-voltage switching (ZVS) of the primary-side power switch S1is achieved by virtue of the negative magnetizing current Im_n(t), and the magnitude of the negative magnetizing current depends on the following formula:

where Lmis the magnetizing inductance value of the transformer, n is a turn ratio of the transformer, Vois an output voltage value of the converter, Im_n(t) is an amplitude of the negative magnetizing current, and t is on-time of the auxiliary switch, e.g. delayed on-time for the synchronous rectifier of the quasi-resonant flyback converter, or on-time for the clamp switch of the active clamp flyback converter.

As can be seen from the above formula, both the magnetizing inductance Lmand the turn ratio n are fixed for a design. When the output voltage Vois fixed, as can be seen from Formula (1), the on-time t is fixed and the fixed on-time t signifies a fixed amplitude of the negative magnetizing current. Therefore, turning on the synchronous rectifier SRor the clamp switch S2for the set time t is applicable to an application scenario where the output voltage is constant. When the output voltage is variable, the fixed on-time t signifies that the amplitude of the negative magnetizing current may vary with the change of the output voltage Vo. Taking the application of USB-PD Type-C as an example, the minimum output voltage is 5V, and the maximum output voltage is 20V. When the control method of the fixed on-time is adopted, either one of the following two results may occur.

Result A: If the on-time may exactly satisfy the condition of the zero-voltage switching (ZVS) of the primary-side power switch when the output voltage is 5V, the amplitude of the negative magnetizing current generated in an application scenario where the output voltage is 20V will be four times as much as that in an application scenario where the output voltage is 5V. Extra loss may be introduced by excessively large negative magnetizing current, and thus a negative effect may be applied on the efficiency of the converter.

Result B: If the on-time may exactly satisfy the condition of the zero-voltage switching (ZVS) of the primary-side power switch when the output voltage is 20V, the amplitude of the negative magnetizing current generated in an application scenario where the output voltage is 5V will be only ¼ of that in an application scenario where the output voltage is 20V. The primary-side power switch may be unable to achieve the zero-voltage switching due to excessively small negative magnetizing current.

Based on the above contents, in an exemplary embodiment, there is provided a control device600, referring toFIG. 6, which is used for controlling a flyback converter610including an auxiliary switch. As shown inFIG. 6, the control device600may include: a current detector620and a comparator controller630.

The current detector620is configured to detect an amplitude of a current of the flyback converter610to obtain an amplitude of a negative magnetizing current of the flyback converter610.

The comparator controller630is configured to compare the amplitude of the negative magnetizing current obtained by the current detector620with a reference value, and turn off the auxiliary switch according to a comparison result.

According to the control device600in this exemplary embodiment, following advantages may be achieved. On the one hand, an amplitude of a negative magnetizing current is obtained by detecting an amplitude of a current of the flyback converter610, such that the amplitude of the negative magnetizing current with different outputs may be obtained. On the other hand, the obtained amplitude of the negative magnetizing current is compared with a reference value, and auxiliary switch is turned off according to a comparison result, such that zero-voltage switching of a primary-side switch with different outputs may be achieved by reasonably setting the reference value.

In this exemplary embodiment, the flyback converter610further includes a primary-side switching circuit, a secondary-side rectifier, a transformer, and an output capacitor (not shown). The primary-side switching circuit includes a primary-side power switch, the secondary-side rectifier includes a first end and a second end, where the first end and the second end are electrically connected to the transformer and the output capacitor respectively. Before the primary-side power switch is turned on, the flyback converter610is caused to generate a negative magnetizing current by turning on and turning off the auxiliary switch. The reference value is set, and amplitude information of the negative magnetizing current is obtained by detecting the current of the flyback converter. When the amplitude of the negative magnetizing current is greater than or equal to the reference value, the comparator controller630outputs a control signal to turn off the auxiliary switch. Afterward, the negative magnetizing current serves as an initial value, and the zero-voltage switching (ZVS) of the primary-side power switch is achieved via resonance of the magnetizing inductor Lmand a parasitic capacitor CEQof a primary-side circuit. By reasonably setting the reference value of the negative magnetizing current, the zero-voltage switching (ZVS) of the primary-side power switch may be achieved with different outputs within a whole load range and a whole input voltage range. In this embodiment, the parasitic capacitor CEQconsists of a parasitic capacitor of the primary-side power switch S1and a parasitic capacitor of a primary-side coil of the transformer T.

It is to be noted that in this exemplary embodiment, the output voltage of the flyback converter610is variable. For example, the output voltage of the flyback converter610may be 5V, 9V, 15V or 20V and so on, which is not specially limited in the present disclosure.

Further, as shown inFIG. 6, the control device600may include a reference calculator640, which is configured to generate the reference value Im_Naccording to information on the flyback converter610.

In addition, in some exemplary embodiments, the flyback converter610may be the active clamp flyback converter as shown inFIG. 1or the RCD clamp flyback converter as shown inFIG. 3orFIG. 5, but the flyback converter in the exemplary embodiments of the present disclosure is not limited thereto.

Further, in some exemplary embodiments, the auxiliary switch of the flyback converter610may be the clamp switch S2as shown inFIG. 1or the synchronous rectifier SRas shown inFIG. 3, but the auxiliary switch in the exemplary embodiments of the present disclosure is not limited thereto. For example, the secondary side as shown inFIG. 5is the RCD clamp flyback converter with diode rectifier, and the auxiliary switch thereof may be a switch Sauxconnected in parallel with a diode D1, or the auxiliary switch thereof may be a switch Saux_VCCconnected in series with an auxiliary winding Waux.

It is to be noted that in some exemplary embodiments, an operating mode of the flyback converter may be a discontinuous current mode (DCM) or a discontinuous current mode boundary (DCMB), which is not specially limited by the present disclosure.

Further, in this exemplary embodiment, when the flyback converter is the active clamp flyback converter as shown inFIG. 1, the auxiliary switch is the clamp switch S2. The negative magnetizing current may be generated by the non-complementary control mode as shown inFIG. 2, or the negative magnetizing current may be generated by the complementary control mode as shown inFIG. 7.

Further, in this exemplary embodiment, when the flyback converter is the RCD clamp flyback converter as shown inFIG. 3, the auxiliary switch is the synchronous rectifier SR. The flyback converter may generate the negative magnetizing current by the delay conduction control mode of synchronous rectifier as shown inFIG. 4. In one embodiment, the negative magnetizing current may be generated by the second conduction control mode of synchronous rectifier as shown inFIG. 8, in which the second conduction refers to the conduction from t2to t3.

FIG. 9illustrates a specific embodiment of a control device. As shown inFIG. 9, the control device900is configured to control the flyback converter910. The control device900includes: a current detector920, a comparator controller930, and a reference calculator940.

The flyback converter910is an RCD clamp flyback converter, including a primary-side switching circuit, a secondary-side rectifier, a transformer T, and an output capacitor Co. The primary-side switching circuit includes a primary-side power switch S1, the secondary-side rectifier includes a synchronous rectifier SR, and the secondary-side rectifier is electrically connected to the transformer T and the output capacitor Corespectively.

In this embodiment, the current detector920receives the amplitude ISof the current flowing through the synchronous rectifier SR. The current detector920obtains the amplitude Im_nof the negative magnetizing current of a primary-side coil according to the current amplitude ISof the synchronous rectifier SR, and delivers the amplitude Im_nof the negative magnetizing current to the comparator controller930. The comparator controller930compares the amplitude Im_nof the negative magnetizing current with a reference value Im_Nof the reference calculator940. When the amplitude Im_nof the negative magnetizing current is greater than or equal to the reference value Im_N, the comparator controller930outputs a control signal to turn off the synchronous rectifier SR.

In an exemplary embodiment, reference is made toFIG. 4,FIG. 8andFIG. 9for principles of the method for detecting the amplitude of the negative magnetizing current of the RCD clamp flyback converter. When the synchronous rectifier is delayed in turn-on (such as t1˜t2as shown inFIG. 4) or when the synchronous rectifier is turned on twice (namely, the synchronous rectifier SRis turned on twice within one switching cycle, for example, the first turn-on of t0˜t1and the second turn-on of t2˜t3as shown inFIG. 8), during t2˜t3as shown inFIG. 8, a negative current Ismay be generated on a secondary-side coil of the transformer T. When the synchronous rectifier SRis turned off, the current is transferred to a primary-side coil, and the negative magnetizing current Im_nof the primary-side coil is generated. According to a circuit principle, an amplitude Im_nof a negative magnetizing current of a primary-side coil may be obtained by detecting an amplitude Isof a current flowing through the secondary-side synchronous rectifier SR, and a specific calculation formula is as below:

where Im_ndenotes the amplitude of the negative magnetizing current, Isdenotes the amplitude of the current flowing through the synchronous rectifier, and n denotes the turn ratio of the transformer.

In other embodiments, detection of the amplitude of the current flowing through the synchronous rectifier SRmay be achieved via a current transformer, an on-state resistance of the synchronous rectifier, or a sampling resistor.

FIG. 10is another specific embodiment of a control device. As shown inFIG. 10, the control device1000is configured to control the flyback converter1010. The control device1000includes: a current detector1020, a comparator controller1030, and a reference calculator1040.

The flyback converter1010is an active clamp flyback converter, including a primary-side switching circuit, a secondary-side rectifier, a transformer T and an output capacitor Co. The primary-side switching circuit includes a primary-side power switch S1and a clamp switch S2, the secondary-side rectifier includes a synchronous rectifier SR, and the secondary-side rectifier is electrically connected to the transformer T and the output capacitor Corespectively.

In this embodiment, the current detector1020receives the detected amplitude IS2of the current flowing through the clamp switch S2, and receives the detected amplitude ISof the current flowing through a secondary rectifier. The current detector1020obtains an amplitude Im_nof the negative magnetizing current according to the current amplitudes IS2and the IS, and delivers the amplitude Im_nto the comparator controller1030. The comparator controller1030compares the amplitude Im_nof the negative magnetizing current with a reference value Im_Nof the reference calculator1040. When the amplitude Im_nof the negative magnetizing current is greater than or equal to the reference value Im_N, the comparator controller1030outputs a control signal to turn off the clamp switch S2.

In an embodiment, the active clamp flyback converter may adopt a non-complementary control mode (such as t3˜t4as shown inFIG. 2) and a complementary control mode (such as t1˜t2as shown inFIG. 7). No matter which control mode is adopted by the active clamp flyback converter, principles of the method for detecting the amplitude of the negative magnetizing current are as below: referring toFIG. 2,FIG. 7andFIG. 11, when the clamp switch is turned on, and the amplitude Im_nof the negative magnetizing current may be indirectly obtained by detecting the amplitude IS2of the current flowing through the clamp switch and the amplitude ISof the current flowing through the secondary-side rectifier, and a specific calculation formula is as below:

where Im_ndenotes the amplitude of the negative magnetizing current, Is2denotes the amplitude of the current flowing through the clamp switch, Isdenotes the amplitude of the current flowing through the secondary-side rectifier, and the n denotes the turn ratio of the transformer.

It is to be noted that in other exemplary embodiments of the present disclosure, detection of the amplitude of the current flowing through the clamp switch S2and the amplitude of the current flowing through the secondary-side rectifier may be achieved via a current transformer, a sampling resistor, or an on-state resistance of a power device, which is not specially limited in the present disclosure. For example, the amplitude IS2of the current flowing through the clamp switch may be detected via the current transformer, the sampling resistor, or the on-state resistance of the clamp switch; and the amplitude ISof the current flowing through the secondary-side rectifier of the flyback converter may be detected via the current transformer, the sampling resistor, or the on-state resistance of the secondary-side rectifier.

In addition, in some exemplary embodiments of the present disclosure, the control device further includes a reference calculator940or a reference calculator1040, which is configured to set the reference value Im_N. For setting the reference value, in case of low voltage input (Vbus<nVo), the zero-voltage switching (ZVS) of a primary-side power switch may be achieved without the aid of the negative magnetizing current; and in case of high voltage input (Vbus>nVo), to achieve the zero-voltage switching (ZVS) of the primary-side power switch, the minimum amplitude of the negative magnetizing current may satisfy the formula (4):

where Im_Ndenotes the reference value, Vbusdenotes an input voltage, and CEQdenotes a parasitic capacitance value.

According to the above Formula (4), for a concrete circuit design, the turn ratio n, the magnetizing inductance Lmand the parasitic capacitance CEQare fixed. To achieve the zero-voltage switching (ZVS) of the primary-side power switch, the reference value Im_Nis dependent on the input voltage Vbusand the output voltage VO. Thus, the reference calculator may adjust the reference value Im_Ndependent on the input voltage Vbusand the output voltage VOof the flyback converter.

However, to adjust the reference value Im_N, two variables need to be monitored when the above method is used: the input voltage Vbusand the output voltage VO, which makes the above control so complex. The effect of the output voltage on the reference value may be neglectable when the flyback converter operates in case of high voltage input (Vbus>nVo). That is, the reference value is merely related to the input voltage, and thus setting the reference value may be greatly simplified.

Thus, the reference calculator may set the reference value Im_Nbased on the input voltage Vbusof the flyback converter.

In some embodiments, there may be two methods for setting the reference value as below.

In a method for setting a fixed reference value, to achieve the zero-voltage switching (ZVS) of the primary-side power switch within a whole input voltage range, the reference value is set according to the maximum input voltage, namely:

where Vbus_maxdenotes the maximum value of the input voltage.

As to the method for setting a fixed reference value, when the bus voltage is the maximum value, the zero-voltage switching (ZVS) of the primary-side power switch may be exactly satisfied. However, when the input voltage is a low voltage, the amplitude of the negative magnetizing current generated by this control method is greater than the amplitude of the negative magnetizing current required for achieving the zero-voltage switching (ZVS) of the primary-side power switch, by which extra loss may be caused and thus the efficiency optimization may be impacted. The method for setting a fixed reference value may be used in an application scenario where efficiency requirements are not so critical.

For an application scenario where the efficiency requirements are critical, a method for setting a reference value varying with the input voltage may be employed to optimize the efficiency of the converter. Therefore, the reference value may be set as:

For a certain circuit design, the magnetizing inductance Lmand the parasitic capacitance CEQare fixed. As can be known from the above Formula (7), the reference value Im_Nis proportional to the input voltage Vbus, and the reference calculator may directly calculate the reference value Im_Naccording to the input voltage value Vbusdetected by an input voltage detector. As shown inFIG. 12andFIG. 13, an input voltage Vbussignal is detected by an input voltage detector. Specifically, the input voltage detector may be a resistance voltage dividing network; and the input voltage Vbussignal is scaled to a required reference value by a proportion calculator.

FIG. 12illustrates still another specific embodiment of a control device. The structure inFIG. 12is similar to that inFIG. 9, but the structure inFIG. 12further includes a specific example of the reference calculator. As shown inFIG. 12, the control device1200further includes an input voltage detector1250. In this embodiment, the input voltage detector1250includes a first resistor R1and a second resistor R2. The input voltage information Vbusis detected by dividing voltage of the first resistor R1and the second resistor R2. The input voltage detector1250transmits the input voltage information Vbusto a reference calculator1240to output a reference value Im_N, which is delivered to a comparator controller1230. The current detector1220receives a detected amplitude ISof the current flowing through the synchronous rectifier, obtains the amplitude Im_nof the negative magnetizing current according to the current amplitude IS, and delivers the amplitude Im_nto a comparator controller1230. The comparator controller1230compares the amplitude Im_nof the negative magnetizing current obtained by the current detector1220with a reference value Im_Nof the reference calculator1240. When the Im_nis greater than or equal to the reference value Im_N, the comparator controller1230outputs a control signal to turn off the synchronous rectifier SR.

FIG. 13illustrates still another specific embodiment of a control device. The structure inFIG. 13is similar to that inFIG. 10, but the structure inFIG. 13further includes another example of the reference calculator. As shown inFIG. 13, the control device1300further includes an input voltage detector1350. In this embodiment, the input voltage detector1350includes a first resistor R1and a second resistor R2. The input voltage information Vbusis detected by way of resistance voltage division. The input voltage detector1350transmits the input voltage information Vbusto a reference calculator1340to output a reference value Im_N. The current detector1320receives a detected amplitude IS2of the current flowing through the clamp switch S2, and receives the amplitude ISof the current flowing through the secondary rectifier. The current detector1320obtains the amplitude Im_nof the negative magnetizing current according to the current amplitudes IS2and the IS, and delivers the amplitude Im_nof the negative magnetizing current to a comparator controller1330. The comparator controller1330compares the amplitude Im_nof the negative magnetizing current with a reference value Im_Nof the reference calculator1340. When the Im_nis greater than or equal to the reference value Im_N, the comparator controller1330outputs a control signal to turn off the clamp switch S2.

In addition, in an exemplary embodiment, there is further provided a control method, which may be applied to the flyback converter as shown inFIG. 6-FIG. 13. The flyback converter includes an auxiliary switch. Referring toFIG. 14, the control method may include following steps.

In step S1410, an amplitude of a current of the flyback converter is detected to obtain an amplitude of a negative magnetizing current of the flyback converter.

In step S1420, the obtained amplitude of the negative magnetizing current is compared with a reference value.

In step S1430, the auxiliary switch is turned off according to a comparison result to achieve zero-voltage switching of a primary-side power switch of the flyback converter.

On the one hand, the amplitude of the negative magnetizing current is obtained by detecting the amplitude of the current of the flyback converter, such that the amplitude of the negative magnetizing current with different outputs may be acquired. On the other hand, the acquired amplitude of the negative magnetizing current is compared with a reference value, and turn-off of the auxiliary switch is controlled according to a comparison result, such that zero-voltage switching of the primary-side switch with different outputs within a whole input voltage range (such as 90˜264 Vac) may be achieved by reasonably setting the reference value.

Further, in some exemplary embodiments, the control method may further include: turning off the auxiliary switch when the amplitude of the negative magnetizing current is greater than or equal to the reference value.

Further, in some exemplary embodiments, the control method may further include: setting the reference value based on an input voltage of the flyback converter.

Further, in some exemplary embodiments, setting the reference value based on an input voltage of the flyback converter may include: setting the reference value based on the maximum value of the input voltage of the flyback converter.

Furthermore, in some exemplary embodiments, the control method may further include: setting the reference value based on the input voltage of the flyback converter and an output voltage of the flyback converter.

Further, in some exemplary embodiments, when the auxiliary switch is a synchronous rectifier, the control method further includes: generating the negative magnetizing current in the flyback converter by means of a delay conduction control mode or a second conduction control mode.

Further, in some exemplary embodiments, when the auxiliary switch is a synchronous rectifier, the detecting the amplitude of the current of the flyback converter may include: detecting the amplitude of the current flowing through the synchronous rectifier by means of a current transformer, a sampling resistor, or an on-state resistance of the synchronous rectifier.

Further, in some exemplary embodiments, when the auxiliary switch is a clamp switch, the control method further includes: generating the negative magnetizing current in the flyback converter by means of a complementary control mode or a non-complementary control mode.

Further, in some exemplary embodiments, when the auxiliary switch is a clamp switch, the detecting an amplitude of a current of the flyback converter may include:

detecting the amplitude of the current flowing through the clamp switch by means of a current transformer, a sampling resistor, or an on-state resistance of the clamp switch; and detecting the amplitude of the current flowing through a secondary-side rectifier by means of another current transformer, another sampling resistor, or an on-state resistance of the secondary-side rectifier.

Further, in some exemplary embodiments, to achieve zero-voltage switching of the primary-side power switch of the flyback converter may include: achieving the zero-voltage switching of the primary-side power switch of the flyback converter via resonance of a magnetizing inductor and a parasitic capacitor in the flyback converter.

Each step in the control method of this exemplary embodiment corresponds, one to one, to functions of each unit or module in the control device, and thus is not unnecessarily elaborated herein.

Furthermore, another exemplary embodiment of the present disclosure provides a switching power supply, which may include the control device according to any one of the preceding embodiments. The switching power supply of this exemplary embodiment adopts the control device, and thus at least has all the corresponding advantages of the control device.

Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure disclosed here. This application is intended to cover any variations, uses, or adaptations of the present disclosure following the general principles thereof and including such departures from the present disclosure as come within prior art. It is intended that the specification and embodiments be considered as exemplary only, with a scope of the present disclosure being indicated by the following claims.