TWI869103B - Asymmetric half-bridge flyback switching power supply and its control circuit - Google Patents

Abstract

An asymmetric half-bridge flyback switching power supply and a control circuit thereof are provided, wherein the asymmetric half-bridge flyback switching power supply includes a transformer, a resonant capacitor, and a first transistor and a second transistor constituting a half-bridge circuit, the resonant circuit consisting of the leakage inductance of the transformer and the resonant capacitor is coupled to two ends of the second transistor, and the control circuit is configured to: detect whether the negative current flowing through the leakage inductance of the transformer reaches a first current threshold during the demagnetization process of the transformer; and when the negative current flowing through the leakage inductance of the transformer reaches the first current threshold, control the first transistor to continue to be in an off state and control the second transistor to change from an on state to an off state.

Description

Asymmetric half-bridge flyback switching power supply and its control circuit

The present invention relates to the field of circuits, and more specifically to an asymmetric half-bridge flyback switching power supply and its control circuit.

As the application scope of consumer electronic devices becomes wider and wider, the demand for portable charging devices or adapters is increasing, and the power density requirements for switching power supplies are getting higher and higher. Traditional switching power supplies mostly use a flyback architecture. Although the structure is simple, its power conversion efficiency is low and the size is large. As the output power increases, its shortcomings become more and more obvious. Half-bridge resonant (LLC) switching power supplies have higher power conversion efficiency and higher power density when the output power is large, but their cost is relatively high and the effect is poor in applications with a wide output voltage range. In power supply applications with high output power, asymmetric half-bridge flyback switching power supplies can greatly improve power conversion efficiency, reduce size, increase power density and have lower cost compared to traditional flyback switching power supplies, so they are becoming more and more popular.

According to the control circuit for an asymmetric half-bridge flyback switching power supply of an embodiment of the present invention, the asymmetric half-bridge flyback switching power supply includes a transformer, a resonant capacitor, and a first transistor and a second transistor constituting a half-bridge circuit, the resonant circuit consisting of the leakage inductance of the transformer and the resonant capacitor is coupled to the two ends of the second transistor, and the control circuit is configured to: detect whether the negative current flowing through the leakage inductance of the transformer reaches the first current threshold during the demagnetization of the transformer; and when the negative current flowing through the leakage inductance of the transformer reaches the first current threshold, control the first transistor to continue to be in the off state and control the second transistor to change from the on state to the off state.

According to the embodiment of the present invention, the asymmetric half-bridge flyback switching power supply includes the above-mentioned control circuit.

100: Asymmetric half-bridge flyback switching power supply

102: Controller

104:SR controller

106: Feedback circuit

400A, 400B, 400C: Control method for asymmetric half-bridge flyback switching power supply 100

Cout: output capacitance

Cr: resonant capacitor

GH: The upper tube control signal used to control the on and off of the upper power transistor S1

GL: The lower tube control signal used to control the on and off of the lower transistor S2

i _Lm : magnetizing current of transformer T1

i _Lr : Current flowing through the leakage inductance Lr of transformer T1

Ip: Peak current flowing through the leakage inductance Lr of transformer T1

i _sec : Current flowing through the secondary winding of transformer T1

Lr: leakage inductance of transformer T1

N: turns ratio between primary winding and secondary winding of transformer T1

Rcs: Current sampling resistor

S1: Power-on transistor of half-bridge circuit

S2: Lower transistor of half-bridge circuit

S3: Synchronous Rectifier (SR) switch

S402, S404, S406A, S406B, S406C, S408, S408C: Steps

T1: Transformer

TL431: Voltage regulator

V _Cr , Vcr0: Voltage on resonant capacitor Cr

V _HB : Midpoint voltage of half-bridge circuit

Vout: system output voltage

The present invention can be better understood from the following description of the specific implementation of the present invention in conjunction with the drawings, wherein:

FIG1 shows a schematic diagram of an example circuit structure of an asymmetric half-bridge flyback switching power supply according to an embodiment of the present invention.

FIG2 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 is in a stable operating state.

FIG3 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 is in an abnormal operating state.

FIG4A shows a flow chart of a control method for the asymmetric half-bridge flyback switching power supply shown in FIG1.

FIG4B shows a flow chart of another control method for the asymmetric half-bridge flyback switching power supply shown in FIG1.

FIG4C shows a flow chart of another control method for the asymmetric half-bridge flyback switching power supply shown in FIG1.

FIG5 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 adopts the control method shown in FIG4A or FIG4B.

FIG6 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 adopts the control method shown in FIG4C.

The features and exemplary embodiments of various aspects of the present invention are described in detail below. In the detailed description below, many specific details are set forth in order to provide a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention can be implemented without some of these specific details. The following description of the embodiments is intended only to provide a better understanding of the present invention by illustrating examples of the present invention. The present invention is in no way limited to any specific configuration and algorithm set forth below, but rather covers any modification, substitution, and improvement of elements, components, and algorithms without departing from the spirit of the present invention. In the drawings and the following description, well-known structures and techniques are not shown in order to avoid unnecessary ambiguity of the present invention. In addition, it should be noted that the term "A and B are connected" used here can mean "A and B are directly connected" or "A and B are indirectly connected via one or more other components."

FIG1 shows a schematic diagram of an example circuit structure of an asymmetric half-bridge flyback switching power supply according to an embodiment of the present invention. In the asymmetric half-bridge flyback switching power supply 100 shown in FIG1 , S1 and S2 are the upper transistor and the lower transistor of the half-bridge circuit, respectively, and can be implemented by switching devices such as metal-oxide-semiconductor field-effect transistor (MOSFET), bipolar junction transistor, gallium nitride (GaN), etc., and are turned on or off under the control of the controller 102; T1 is a transformer, Lr is the leakage inductance of the transformer T1, and the auxiliary winding of the transformer T1 feeds back the working state of the transformer T1 to the controller 102; Cr is a resonant capacitor, which forms a resonant circuit with the leakage inductance Lr of the transformer T1; Rcs is a current sampling resistor; S3 is a synchronous rectifier (Synchronous Rectifier (SR) switch, which is turned on or off under the control of SR controller 104; Cout is the output capacitor; the feedback circuit 106 feeds back the output feedback signal related to the system output voltage Vout to the controller 102 through the voltage regulator TL431 and the optocoupler; the controller 102 controls the on and off of the upper transistor S1 and the lower transistor S2 based on the output feedback signal.

As shown in FIG. 1 , taking the critical conduction mode (CrM) as an example, ignoring the dead time between the upper transistor S1 and the lower transistor S2, the working process of the asymmetric half-bridge flyback switching power supply 100 in a stable working state mainly includes the following two stages: a magnetizing stage, in which the upper transistor S1 is in the on state, the lower transistor S2 is in the off state, the transformer T1 and the resonant capacitor Cr store energy, and the current i flowing through the leakage inductance Lr of the transformer T1 As _Lr increases, the voltage on the resonant capacitor Cr increases; in the demagnetization stage, the lower transistor S2 is in the on state, the upper transistor S1 is in the off state, the transformer T1 is demagnetized, the resonant capacitor Cr resonates with the leakage inductance Lr of the transformer T1 and transfers energy to the secondary side of the transformer T1.

It should be understood by those skilled in the art that the circuit structure of the asymmetric half-bridge flyback switching power supply 100 according to the embodiment of the present invention is not limited to the connection method shown in FIG1 . For example, the resonant circuit can also be connected to both ends of the upper transistor S1. Similarly, the circuit structure of the feedback circuit 106 is not limited to the connection method shown in FIG1 .

FIG2 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 is in a stable operating state, wherein GH represents the upper tube control signal for controlling the on and off of the upper transistor S1, GL represents the lower tube control signal for controlling the on and off of the lower transistor S2, V _HB represents the midpoint voltage of the half-bridge circuit, V _Cr represents the voltage on the resonant capacitor Cr, i _Lr represents the current flowing through the leakage inductance Lr of the transformer T1, i _Lm represents the magnetizing current of the transformer T1 (in the magnetizing stage, the magnetizing current of the transformer T1 is the current flowing through the leakage inductance Lr, and in the demagnetizing stage, the magnetizing current of the transformer T1 is shown in dotted lines), i _sec represents the current flowing through the secondary winding of transformer T1.

As can be seen from Figure 2, when the asymmetric half-bridge flyback switching power supply 100 is in a stable working state, the voltage on the resonant capacitor Cr fluctuates around N*Vout, where N is the turns ratio between the primary winding and the secondary winding of the transformer T1, and Vout is the system output voltage. Assuming that at the moment when the on-state transistor S1 changes from the on state to the off state, the peak current flowing through the leakage inductance Lr of the transformer T1 is Ip, and the voltage on the resonant capacitor Cr is Vcr0, then the resonant current flowing through the leakage inductance Lr of the transformer T1 in the demagnetization stage is:

in:

Combining equations (1) to (3), it can be seen that the maximum resonant current flowing through the leakage inductance Lr of transformer T1 during the demagnetization stage can be achieved:

When the leakage inductance Lr and the resonant capacitor Cr of the transformer T1 remain unchanged, the resonant parameters of the resonant circuit composed of them remain unchanged. Therefore, when the upper transistor S1 changes from the on state to the off state, the greater the peak current Ip flowing through the leakage inductance Lr of the transformer T1, and the greater the difference between the voltage Vcr0 on the resonant capacitor Cr and the reflected voltage N.Vout of the system output voltage _Vout , the greater the resonant current flowing through the leakage inductance Lr of the transformer T1 in the demagnetization stage.

When the asymmetric half-bridge flyback switching power supply 100 has an abnormality, including but not limited to, the peak current Ip flowing through the leakage inductance Lr of the transformer T1 increases, the voltage Vcr0 on the resonant capacitor Cr increases, the system output voltage Vout decreases, etc., the resonant current flowing through the leakage inductance Lr of the transformer T1 is large. If it is not limited, it will cause the current stress of the lower transistor S2 and the secondary winding of the transformer T1 to increase, which may cause damage to the device in severe cases.

Since the resonant current is a negative current relative to the demagnetization current and the magnetization current, under normal circumstances, a negative current limit protection (for example, the limit is Ineg_limit) can be added to the resonant current. That is, when the resonant current is detected to be too large, the lower transistor S2 is immediately turned off, and the resonant current will no longer continue to increase. However, the asymmetric half-bridge flyback switching power supply is different from the traditional flyback switching power supply. After turning off the lower transistor S2, the following problems may occur: Under normal working conditions, the charging current and the discharging current of the resonant capacitor Cr are balanced, and the voltage V _Cr on the resonant capacitor Cr fluctuates around N*Vout. If the lower transistor S2 is turned off when a large negative current is detected, the discharge circuit of the resonant capacitor Cr will be turned off, the charge and discharge balance of the resonant capacitor Cr will be broken, and the voltage V _Cr on the resonant capacitor Cr will increase. Combined with the previous analysis, it can be seen that the increase in the voltage V _Cr on the resonant capacitor Cr will cause the resonant current to continue to increase, resulting in a positive feedback result, which further worsens the situation. Figure 3 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in Figure 1 is in an abnormal working state. The signals represented by GH, GL, V _HB , V _Cr , i _Lr , and i _Lm are the same as those in Figure 2, and will not be repeated here.

In view of the above situation, a control circuit for an asymmetric half-bridge flyback switching power supply according to an embodiment of the present invention is proposed, which can limit the maximum resonant current of the asymmetric half-bridge flyback switching power supply during operation and improve the reliability of the operation of the asymmetric half-bridge flyback switching power supply. At the same time, when an abnormality occurs in the asymmetric half-bridge flyback switching power supply, it can be restored to a stable working state.

FIG. 4A is a flow chart showing a control method for the asymmetric half-bridge flyback switching power supply shown in FIG. 1 . As shown in FIG. 1 and FIG. 4A , a control method 400A for an asymmetric half-bridge flyback switching power supply 100 can be implemented by a controller 102 and includes the following processing: step S402, in the magnetization stage, controlling the upper transistor S1 to be in the on state and controlling the lower transistor S2 to be in the off state; and step S404, in the demagnetization stage, controlling the upper transistor S1 to be in the off state and controlling the lower transistor S2 to be in the on state, and when it is detected that the negative current flowing through the leakage inductance Lr of the transformer T1 reaches a first current threshold value (e.g., Ineg_limit), controlling the upper transistor S1 to continue to be in the off state and controlling the lower transistor S2 to change from the on state to the off state.

As shown in FIG. 4A , in some embodiments, after controlling the upper transistor S1 to continue to be in the off state and controlling the lower transistor S2 to change from the on state to the off state, the control method 400A for the asymmetric half-bridge flyback switching power supply 100 may further include: step S406A, when it is detected that the negative current flowing through the leakage inductance Lr of the transformer T1 reaches the second current threshold, controlling the upper transistor S1 to continue to be in the off state and controlling the lower transistor S2 to change from the off state to the on state, wherein the second current threshold is less than the first current threshold.

As shown in FIG. 4A , in some embodiments, the control method 400A for the asymmetric half-bridge flyback switching power supply 100 may further include: step S408, after the demagnetization of the transformer T1 is completed, controlling the upper transistor S1 to change from the off state to the on state and controlling the lower transistor S2 to change from the on state to the off state, so that the transformer T1 is magnetized.

FIG4B shows a flow chart of another control method for the asymmetric half-bridge flyback switching power supply shown in FIG1. As shown in FIG1, FIG4A, and FIG4B, the control method 400B for the asymmetric half-bridge flyback switching power supply 100 can also be implemented by the controller 102, and the difference from the control method 400A shown in FIG4A is only in step S406B, that is, after the first predetermined time has passed after the upper transistor S1 is controlled to continue to be in the off state and the lower transistor S2 is controlled to change from the on state to the off state, the upper transistor S1 is controlled to continue to be in the off state and the lower transistor S2 is controlled to change from the off state to the on state.

The main idea of the control methods 400A and 400B shown in FIG4A and FIG4B is that, during the demagnetization process of the transformer T1, when it is detected that the negative current flowing through the leakage inductance Lr of the transformer T1 reaches the first current threshold, the lower transistor S2 is controlled to change from the on state to the off state, and then when the negative current flowing through the leakage inductance Lr of the transformer T1 decreases to a certain extent, the lower transistor S2 is controlled to change from the off state to the on state, until the negative current flowing through the leakage inductance Lr of the transformer T1 reaches the first current threshold again or the demagnetization of the transformer T1 is completed. In this way, the maximum resonant current flowing through the lower transistor S2 can be limited, the current stress of the lower transistor S2 can be reduced, and the reliability of the system can be improved. In addition, the resonant capacitor Cr can be continuously discharged during the demagnetization process of the transformer T1, so that the voltage V _Cr on the resonant capacitor Cr fluctuates around N*Vout. FIG5 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 adopts the control method shown in FIG4A or FIG4B, wherein the signals represented by GH, GL, V _HB , V _Cr , i _Lr , i _Lm , and i _sec are the same as those in FIG2 and are not described in detail here.

FIG4C shows a flow chart of another control method for the asymmetric half-bridge flyback switching power supply shown in FIG1. As shown in FIG1, FIG4A, and FIG4C, the control method 400C for the asymmetric half-bridge flyback switching power supply 100 can also be implemented by the controller 102, and the difference from the control method 400A shown in FIG4A is that: Step S406C, after the demagnetization of the transformer T1 is completed, the upper transistor S1 is controlled to continue to be in the off state and the lower transistor S2 is controlled to change from the off state to the on state or in the off state. The upper transistor S1 is controlled to continue to be in the off state and the lower transistor S2 is controlled to change from the off state to the on state after a second predetermined time has passed, and the upper transistor S1 is controlled to change from the off state to the on state and the lower transistor S2 is controlled to change from the on state to the off state, so that the transformer T1 is magnetized.

The main idea of the control method 400C shown in FIG4C is that, during the demagnetization process of the transformer T1, when it is detected that the negative current flowing through the leakage inductance Lr of the transformer T1 reaches the first current threshold, the lower transistor S2 is controlled to change from the on state to the off state, and after the demagnetization of the transformer T1 is completed, the lower transistor S2 is controlled to change from the off state to the on state to discharge the resonant capacitor Cr, and then the upper transistor S1 is controlled to change from the off state to the on state and the lower transistor S2 is controlled to change from the on state to the off state, so that the transformer T1 is magnetized. At the same time, the peak current Ip can be reduced in the next switching cycle, reducing the charging current of the resonant capacitor Cr, so that the resonant capacitor Cr can reach the charge-discharge balance faster. FIG6 shows the operating waveforms of multiple signals when the asymmetric half-bridge flyback switching power supply shown in FIG1 adopts the control method shown in FIG4C, wherein the signals represented by GH, GL, V _HB , V _Cr , i _Lr , and i _Lm are the same as those in FIG2 and will not be repeated here.

The above description takes the critical conduction mode (CrM) as an example. The above control scheme can also be applied to discontinuous conduction mode (DCM) and high load mode (Burst), that is, inserting a rest phase between the demagnetization phase and the magnetization phase.

The present invention may be implemented in other specific forms without departing from its spirit and essential features. For example, the algorithm described in a specific embodiment may be modified, and the system architecture does not deviate from the basic spirit of the present invention. Therefore, the present embodiments are considered to be illustrative rather than restrictive in all aspects, the scope of the present invention is defined by the attached claims rather than the above description, and all changes that fall within the meaning and scope of equivalents of the claims are therefore included in the scope of the present invention.

400A: Control method for asymmetric half-bridge flyback switching power supply 100

S402, S404, S406A, S408: Steps

Claims (6)

A control circuit for an asymmetric half-bridge flyback switching power supply, characterized in that the asymmetric half-bridge flyback switching power supply comprises a transformer, a resonant capacitor, and a first transistor and a second transistor constituting a half-bridge circuit, a resonant circuit consisting of a leakage inductance of the transformer and the resonant capacitor is coupled to two ends of the second transistor, and the control circuit is configured to: detect whether a negative current flowing through the leakage inductance of the transformer reaches a first current threshold during demagnetization of the transformer; and detect whether a negative current flowing through the leakage inductance of the transformer reaches a first current threshold when the leakage inductance of the transformer reaches a first current threshold. When the negative current of the transformer reaches the first current threshold, the first transistor is controlled to continue to be in the off state and the second transistor is controlled to change from the on state to the off state; whether the negative current flowing through the leakage inductance of the transformer reaches the second current threshold, wherein the second current threshold is less than the first current threshold; and when the negative current flowing through the leakage inductance of the transformer reaches the second current threshold, the first transistor is controlled to continue to be in the off state and the second transistor is controlled to change from the off state to the on state. The control circuit as described in claim 1 is further configured to control the first transistor to continue to be in the off state and control the second transistor to change from the off state to the on state after a first predetermined time has passed after the first transistor is controlled to continue to be in the off state and the second transistor is controlled to change from the on state to the off state. The control circuit as described in claim 1 or 2 is further configured to control the first transistor to change from an off state to an on state and control the second transistor to change from an on state to an off state after the demagnetization of the transformer is completed. The control circuit as described in claim 1 is further configured to control the first transistor to continue to be in the off state and control the second transistor to change from the off state to the on state or switch between the off state and the on state multiple times after the demagnetization of the transformer is completed, so as to discharge the resonant capacitor. The control circuit as described in claim 4 is further configured to control the first transistor to change from the off state to the on state and control the second transistor to change from the on state to the off state after a second predetermined time has passed after the first transistor is controlled to continue to be in the off state and the second transistor is controlled to change from the off state to the on state. An asymmetric half-bridge flyback switching power supply, comprising a control circuit as described in any one of claims 1 to 5.

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