TWI883611B - Switch mode power converter with synchronous rectifier implementing adaptive turn-off voltage and method thereof - Google Patents

Abstract

A power converter incorporating a synchronous rectifier implements adaptive turn-off voltage control for fast turn-off of the synchronous rectifier in the continuous conduction mode. In some embodiments, the synchronous rectifier turn off detection threshold is adaptively changed as a function of the detected operation mode of the power converter. In response to detecting the power converter being operated in the continuous conduction mode, the synchronous rectifier turn off detection threshold is set to a voltage value farther away from zero volt as compared to the nominal turn off detection threshold. In this manner, the synchronous rectifier can be turned off earlier while in the continuous conduction mode. When the synchronous rectifier can be turned off quickly, large reverse current or negative current as well as large drain voltage spike at the synchronous rectifier can be avoided. The reliability of the synchronous rectifier and the power converter is improved.

Description

Switching mode power converter with synchronous rectifier to achieve adaptive turn-off voltage and method thereof

The present invention relates to a switching regulator circuit and method, and more particularly to a switching regulator having a synchronous rectifier, wherein the synchronous rectifier realizes an adaptive turn-off voltage to quickly turn off in a continuous conduction mode.

Power converters are used in a wide range of electronic applications to convert AC voltage to DC voltage or to convert DC voltage from one voltage value to another. Commonly used power converters include switch-mode power supplies or switch-mode converters, also known as switching regulators or DC-DC converters. Switching regulators provide power functions through low-loss components such as capacitors, inductors, and transformers, and power switches that turn on and off to transfer energy from the input to the output in the form of discrete packets. Feedback control circuits are used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.

A flyback converter is a type of switch-mode power converter used in electronic devices such as televisions or computers, or mobile device chargers. Flyback converters can also be used to supply high voltage power to electronic devices such as televisions or monitors.

A flyback converter is an isolated power converter, typically used for AC to DC and DC to DC conversion, with galvanic isolation between the input and one or more outputs. More specifically, a flyback converter is a buck-boost converter that splits the inductor to form a transformer, thereby multiplying the voltage ratio and having the additional advantage of isolation. Synchronous rectification is often used to replace the diode rectifier to improve efficiency. Figure 1 is an example of a flyback converter using synchronous rectification. As shown in Figure 1, the typical structure of a flyback converter includes a primary switch (SW) coupled to the primary transformer winding of transformer Lm and a synchronous rectifier switch (SR) coupled to the secondary transformer winding of transformer Lm. An input voltage V _IN is provided between the primary winding and the primary switch. The primary switch is turned on and off by a control voltage V _GS to conduct a primary current Ipri. The primary switch and the synchronous rectifier are complementary in operation, one switch is turned on while the other is turned off. The conduction cycles of the primary switch SW and the synchronous rectifier SR do not overlap. The current flowing on the secondary side, referred to as the secondary current Isec, charges the output capacitor C3 to provide the output voltage V _O . In some cases, an active clamp may be implemented on the primary side to clamp the voltage at the drain terminal of the primary switch SW when the primary switch SW is turned off.

FIG. 2 shows exemplary signal waveforms for operating the flyback converter of FIG. 1 in constant frequency continuous conduction mode (CF CCM). FIG. 3 illustrates exemplary signal waveforms for operating the flyback converter of FIG. 1 in constant frequency discontinuous conduction mode (CF DCM). The flyback converter of FIG. 1 and the operating modes of FIGS. 2 and 3 are described in detail in the paper “Design Considerations and Performance Evaluation of Synchronous Rectification in Flyback Converters” by M. T. Zhang, M. M. Jovanovic, and F. C. Lee, Applied Power Electronics Conference and Expo, 1997, Proceedings of APEC '97, pp. 623-630, Vol. 2. In short, when operating in CCM operation mode, the secondary current Isec does not become a zero current value before the next switching cycle starts (primary switch SW is turned on), as shown in Figure 2. On the other hand, when operating in DCM operation mode, the secondary current Isec decreases to a zero current value before the next switching cycle starts, as shown in Figure 3.

In particular, when a power converter with a synchronous rectifier operates in continuous conduction mode, the secondary current does not decrease to zero before the next switching cycle, but when the main switch SW is turned on, the secondary current flowing through the synchronous rectifier will become zero. In practice, when the synchronous rectifier is instructed to turn off, the propagation delay and the gate driver discharge time cause the gate drive voltage _VGS to actually decrease to the voltage level that turns off the synchronous rectifier with a certain amount of delay. When the synchronous rectifier is turned off after the secondary current crosses zero current, a secondary reverse current is generated. In practice, if the synchronous rectifier is not turned off fast enough, a large reverse current may be generated, causing the drain-source voltage spike at both ends of the synchronous rectifier to be too high, thus affecting the reliability of the synchronous rectifier and the entire power converter.

The present invention discloses a power converter having adaptive shutdown control for a synchronous rectifier, substantially as shown and/or described below, for example in conjunction with at least one of the accompanying drawings, as more fully described in the scope of the invention application.

In some embodiments, a method of operating a power converter incorporating a synchronous rectifier and receiving an input voltage and providing an output voltage includes: detecting the start of a synchronous rectifier (SR) conduction cycle; detecting a gate voltage at a gate terminal of the synchronous rectifier near the end of the SR conduction cycle; in response to the detected gate voltage being less than a gate voltage target, selecting a first SR turn-off detection voltage as an SR turn-off detection threshold; in response to the detected gate voltage being greater than or equal to the gate voltage target, selecting a second SR turn-off detection voltage as an SR turn-off detection threshold; The R turn-off detection voltage is used as the SR turn-off detection threshold, the first and second SR turn-off detection voltages are negative voltage values, and the first SR turn-off detection voltage is closer to zero volts than the second SR turn-off detection voltage; in response to the synchronous rectifier being turned off in response to the drain voltage of the synchronous rectifier reaching the SR turn-off detection threshold, the SR conduction time of the current SR conduction cycle is stored; in response to the SR turn-off detection threshold being set to the second SR turn-off detection voltage, the SR turn-off detection threshold is reset to the first SR turn-off detection voltage.

In another embodiment, a power converter includes an input terminal for receiving an input voltage and an output terminal for providing an output voltage; a synchronous rectifier is coupled to the output terminal; and a controller is coupled to generate a gate control signal to drive a gate terminal of the synchronous rectifier during a plurality of synchronous rectifier (SR) conduction cycles. The controller detects a gate voltage at the gate terminal of the synchronous rectifier near the end of each SR conduction cycle. The controller sets the SR turn-off detection threshold to a first voltage in response to a detected gate voltage having a value less than a gate voltage target, and sets the SR turn-off detection threshold to a second voltage in response to a detected gate voltage value being greater than the gate voltage target value. The first and second voltages are both negative voltage values, and the first voltage is closer to zero volts than the second voltage. The controller uses the SR turn-off detection threshold to determine turn-off of the synchronous rectifier. In response to the controller signaling the synchronous rectifier to shut down in response to the SR turn-off detection threshold, the controller resets the SR turn-off detection threshold to the first voltage when the SR turn-off detection threshold has been set to the second voltage.

These and other advantages, aspects and novel features of the present invention, as well as the details of the illustrated embodiments thereof, will be more fully understood from the following description and accompanying drawings.

D1: body diode

D2: body diode

Lm: Transformer

SW: Primary switch

SR: Synchronous Rectifier

V _IN : Input voltage

V _O : Output voltage

V _GS : Gate drive voltage

Ipri: primary current

Isec: Secondary current

V _SEC : Secondary voltage

TSR: SR conduction time

V _DS(SW) : Source voltage

V _DS : Drain voltage

Cin: Input decoupling capacitor

C _OUT : Output capacitor

V _OUT : Output voltage

M2(SR): synchronous rectifier switch

M1(SW): Primary switch

V _GS1 : Control voltage

V _GS2 : Gate voltage

V _{OUT_FB} : Feedback voltage

_LP : Transformer

10: Flyback converter

12: Input voltage node

14: Node

15: Node

16: Node

18: Node

20: Load

25: Passive clamping circuit

30: Primary side controller

40: Secondary side controller

V _THGON :SR conduction detection voltage

V _THGOFF :SR turn-off detection voltage

OP1: Operational amplifier

OP2: Operational amplifier

OP3: Operational amplifier

VD: Drain voltage

Tri-EN: enable signal

V _THREG : Regulation threshold voltage

S1: switch

42: Sensing circuit

44: Gate on/off control logic circuit

46: Three-state gate driver

48: Discharge current control circuit

V _GS(SR) : Gate voltage

V _DS(SR) : Drain voltage

52: Curve

54: Curve

56: Curve

V _{THREG_H} : High regulation threshold voltage

V _{THREG_L} : Low regulation threshold voltage

V _RESET : Reset threshold voltage

V _GDET : Gate voltage

62: Curve

64:Curve

66:Curve

80:Methods

V _{THGOFF_H} : High SR turn-off detection voltage

V _{THGOFF_L} : Low SR turn-off detection voltage

V _THGDET : Gate voltage target

Comp1: Comparator

Comp2: Comparator

V _{THGOFF_SEL} : Detection voltage selection signal

S2: switch

100: Secondary side controller

102: Sensing circuit

103: Node

104: Gate on/off control logic circuit

106: Three-state gate driver

108: Discharge current control circuit

110: Adaptive shutdown voltage control circuit

112: Register

114: Sample and hold circuit

116: Register

118:Multiplier

120: logical AND gate

122:SR trigger

I _DS : Drain-source current

Various embodiments of the present invention are disclosed in the following detailed description and accompanying drawings. Although the accompanying drawings depict various examples of the present invention, the present invention is not limited to the depicted examples. It should be understood that in the accompanying drawings, the same figure reference numerals represent the same structural elements. In addition, it should be understood that the depictions in the drawings are not necessarily drawn to scale.

Figure 1 is an example of a flyback converter using synchronous rectification.

FIG. 2 shows exemplary signal waveforms for operating the flyback converter of FIG. 1 in constant frequency, continuous conduction mode (CF CCM).

FIG. 3 shows exemplary signal waveforms for operating the flyback converter of FIG. 1 in constant frequency, discontinuous conduction mode (CF DCM).

Figure 4 is a schematic diagram of a flyback converter in an embodiment of the present invention.

FIG. 5 is a schematic diagram of a secondary-side controller in the flyback converter of FIG. 4 in an embodiment of the present invention.

FIG. 6 illustrates signal waveforms in a switching cycle of a synchronous rectifier in the flyback converter of FIG. 4 during the conduction period of the synchronous rectifier in some examples.

FIG. 7 illustrates signal waveforms in a switching cycle of a synchronous rectifier in the flyback converter of FIG. 4 during the conduction period of the synchronous rectifier in an alternative example.

Figure 8 shows the signal waveforms in the switching cycle of the synchronous rectifier in the flyback converter that implements the adaptive turn-off voltage control method in the embodiment of the present invention.

FIG. 9 is a flow chart illustrating an adaptive shutdown voltage control method that can be implemented in a power converter such as the flyback converter of FIG. 4 in an embodiment of the present invention.

Figure 10 shows the signal waveforms in the switching cycle of the synchronous rectifier in the flyback converter that implements the adaptive turn-off voltage control method in the embodiment of the present invention.

FIG. 11 is a schematic diagram of a secondary-side controller in the flyback converter of FIG. 4, which incorporates an adaptive turn-off voltage control circuit in an embodiment of the present invention.

A power converter including a synchronous rectifier implements adaptive shutdown voltage control to quickly shut down the synchronous rectifier in a continuous conduction mode. In some embodiments, the synchronous rectifier shutdown detection threshold is adaptively changed according to the detected operating mode of the power converter. In response to detecting that the power converter is operating in a continuous conduction mode, the synchronous rectifier shutdown detection threshold is set to a voltage value that is farther from zero volts than the nominal shutdown detection threshold. In this way, the synchronous rectifier can be turned off early in the continuous conduction mode. When the synchronous rectifier can be turned off quickly, a large reverse current or negative current and a large drain voltage spike can be avoided in the synchronous rectifier. The reliability of the synchronous rectifier and the power converter is improved.

In the present embodiment, the power converter is a flyback converter including a synchronous rectifier coupled to a secondary winding of the transmission. In other embodiments, the power converter may be any other type of switch-mode power supply in combination with the use of a synchronous rectifier. For example, the power converter may be a boost or buck-boost converter without a transformer, or any DC-DC converter, or an LLC SSR converter, or any power converter using synchronous rectifier voltage detection. In the following description, a flyback converter is used as an example to illustrate the implementation of the adaptive shutdown voltage control. The use of a flyback converter as the power converter is merely illustrative and not limiting.

FIG. 4 is a schematic diagram of a flyback converter in an embodiment of the present invention. Referring to FIG. 4, the flyback converter 10 includes a primary switch M1 (SW) coupled to the primary transformer winding of the transformer LP and a synchronous rectifier switch M2 (SR) coupled to the secondary transformer winding of the transformer LP. The input voltage V _IN is coupled across the primary winding and the primary switch, between the input voltage node 12 and the ground node 18. The input decoupling capacitor Cin can be coupled to the input voltage node 12. The primary switch is turned on and off by the controller control voltage V _GS1 to conduct the primary current Ipri to flow in the primary winding of the transformer. The synchronous rectifier switch is turned on and off by the gate voltage V _GS2 to conduct the secondary current Isec into the transformer secondary winding. In this specification, the term "primary current" refers to the current flowing through the transformer primary winding, and the terms "secondary current" and "synchronous rectification current" both refer to the current flowing through the transformer LP secondary winding, also known as the drain current of the synchronous rectifier. The output capacitor C _OUT is connected across the secondary winding and the synchronous rectifier, that is, between the output node 16 and the ground node 18. The output voltage V _OUT is generated at the output node 16 to drive the load 20. In some embodiments, a passive clamping circuit 25 may be provided on the primary side to clamp the voltage at the drain terminal (node 14) of the primary switch M1 when the primary switch M1 is turned off.

In an embodiment of the present invention, the primary switch M1 and the synchronous rectifier M2 are power switches, typically MOSFET devices. In the present embodiment, the primary switch M1 and the synchronous rectifier M2 are both constructed using NMOS transistors. The NMOS transistor of the primary switch M1 has a drain terminal coupled to the transformer LP (node 14), a source terminal coupled to the ground (node 18), and a gate driven by a control voltage V _GS1 . As an NMOS transistor, the primary switch M1 also has an associated parasitic body diode D1 across the transistor drain and source terminals. In this illustration, the body diode D1 is shown as being connected to both ends of the NMOS switch M1 with a dotted line to indicate that the body diode D1 is only a parasitic diode rather than an additional diode element. On the secondary side, the NMOS transistor of the synchronous rectifier switch M2 has a drain terminal coupled to the transformer LP (node 15), a source terminal coupled to ground (node 18), and a gate driven by a gate voltage V _GS2 . As an NMOS transistor, the synchronous rectifier switch M2 has an associated parasitic body diode D2 across the drain and source terminals of the transistor M2. Once again, the body diode D2 is shown as connected across the NMOS switch M2 with a dashed line to indicate that the diode D2 is a parasitic element formed as part of the NMOS transistor structure. In this specification, the secondary current is also referred to as the drain-source current I DS (or "leakage current I DS") of the MOSFET switch acting as a synchronous rectifier.

So configured, the primary switch M1 and the synchronous rectifier M2 are each driven by their respective controller circuits to control the on and off operations of the switches. Specifically, the primary side controller 30 is coupled to drive the gate terminal of the primary switch M1, and the secondary side controller 40 is coupled to drive the gate terminal of the synchronous rectifier M2. The primary side controller 30 and the secondary side controller 40 can be constructed in various ways based on the control scheme selected for the flyback converter 10. In other words, the flyback converter 10 is a power stage and different control schemes can be used to control the flyback converter power stage. In operation, the switching of the primary switch is synchronized with the switching of the synchronous rectifier. In most embodiments, either the primary side controller is the master controller and the secondary side controller is the slave controller, or the secondary side controller is the master controller and the primary side controller is the slave controller. The master controller is typically implemented as a PWM controller. Examples of control schemes that can be used in the flyback converter 10 include voltage mode control, peak current mode control, and input voltage feedforward control. Each control scheme uses a different feedback signal to control and maintain a constant output voltage and provide load regulation. The specific implementation of the control scheme in the flyback converter 10 is not critical to the practice of the present invention. It can be understood by those with ordinary knowledge in the art to which the present invention belongs that adaptive turn-off voltage control can be applied in any control scheme to achieve fast turn-off of the synchronous rectifier in continuous conduction mode. In this illustration, a primary side controller and a secondary side controller are provided. In other embodiments, the primary side controller and the secondary side controller may be configured as a single controller or control circuit that generates control signals for the primary switch and the synchronous rectifier switch.

In one example, a flyback converter power stage implements a control scheme in which the secondary side is the primary controller. In this case, the secondary side controller is a PWM controller configured to regulate the output voltage V _OUT . Alternatively, the flyback converter power stage can be implemented with a primary side controller as the primary controller. In this case, the primary side controller includes a PWM controller configured to regulate the output voltage V _OUT , for example, through a feedback voltage V _{OUT_FB} . The secondary side controller includes logic circuitry to control the synchronous rectifier in response to a drain voltage V _DS detected at the drain terminal of the synchronous rectifier MOSFET.

FIG. 5 is a schematic diagram of a secondary-side controller in the flyback converter of FIG. 4 in an embodiment of the present invention. Referring to FIG. 5, a secondary-side controller 40 for generating a gate voltage V _GS2 to control a synchronous rectifier MOSFET M2 includes a drain voltage V _DS sensing circuit 42 for sensing a drain voltage V _DS of the synchronous rectifier M2. The sensed or detected drain voltage is coupled to a pair of operational amplifiers OP1 and OP2 to be compared with respective detection threshold voltages to generate a gate on/off control signal for the synchronous rectifier M2. Specifically, the operational amplifier OP1 compares the detected drain voltage (at the negative input terminal) with the SR turn-on detection voltage V _THGON (at the positive input terminal) to determine when the synchronous rectifier M2 should be turned on, and the operational amplifier OP2 compares the detected drain voltage (at the positive input terminal) with the SR turn-off detection voltage V _THGOFF (at the negative input terminal) to determine when the synchronous rectifier M2 should be turned off. In this operating state, the sensed drain voltage VD has a negative voltage value, and the SR turn-on detection voltage V _THGON and the SR turn-off detection voltage V _THGOFF are both negative voltage values. The secondary side controller 40 includes a gate on/off control logic circuit 44 that receives the output signals from the operational amplifiers OP1 and OP2 and generates an on/off control signal. The on/off control signal is coupled to a tri-state gate driver 46 that provides a gate voltage V _GS2 to drive the gate terminal of the synchronous rectifier M2 when enabled by the enable signal Tri-EN. In this example, the secondary side controller 40 also includes an operational amplifier OP3 for comparing the sensed drain voltage (at the positive input) with the regulation threshold voltage V _THREG (at the negative input). When the sensed drain voltage reaches the regulation threshold voltage V _THREG , operational amplifier OP3 closes switch S1 to allow the discharge current control circuit 48 to discharge the gate voltage V _GS2 in a controlled manner, as will be explained in more detail below.

The flyback converter 10 can be operated in a discontinuous conduction mode or a continuous conduction mode. When operating in the continuous conduction mode of operation, the secondary current Isec does not reach a zero current value before the next switching cycle starts (primary switch M1 is turned on). On the other hand, when operating in the discontinuous conduction mode of operation, the secondary current Isec decreases to a zero current value before the next switching cycle starts. In an embodiment of the present invention, the secondary side controller 40 includes an adaptive turn-off voltage control circuit to modify the turn-off voltage of the synchronous rectifier when operating in the continuous conduction mode, as will be explained in more detail below.

The general operation of the flyback converter 10 will now be described. Referring to FIGS. 4 and 5 , various control schemes may be used to control the flyback converter 10. Regardless of the control scheme used, the primary switch SW (M1) and the synchronous rectifier SR (M2) operate complementarily with one switch turned on and the other turned off. The conduction periods of the primary switch SW and the synchronous rectifier SR do not overlap. When the primary switch SW is turned on, the primary winding of the transformer _LP is connected to the input voltage V _IN , and the primary current Ipri increases linearly with the increase in magnetic flux in the transformer. Energy is stored in the transformer _LP . At this time, the voltage _VSEC induced in the secondary winding is opposite to the polarity of the primary winding, reverse biasing the body diode D2 of the synchronous rectifier SR. No secondary current Isec flows and the charge stored on the output capacitor _COUT supplies power to the load 20. With the primary switch SW turned on, the drain-to-source voltage _VDS(SW) of the primary switch SW at node 14 is at or near zero volts. At the same time, the secondary voltage _VSEC of the synchronous rectifier SR (node 15), also known as the drain-source voltage _VDS(SR) or _VDS of the synchronous rectifier, is driven to a positive voltage that is proportional to the input voltage _VIN .

After the on-time period of the primary switch expires, the primary switch is turned off and the synchronous rectifier is turned on after a non-overlapping period. When the primary switch is turned off, the primary current Ipri decreases and the magnetic flux drops. The voltage across the secondary winding reverses, making the secondary voltage positive at the dotted line end or negative at the drain of the synchronous rectifier (node 15), thereby causing the body diode D2 of the synchronous rectifier SR to become forward biased. As a result, current flows through the secondary winding as the secondary current Isec. The secondary current Isec is also the drain-source current or drain current of the synchronous rectifier. The secondary current Isec increases to a peak current value. The synchronous rectifier SR is turned on after a non-overlapping cycle to conduct the secondary current Isec and help transfer the stored energy from the transformer core to the output capacitor _COUT . The output capacitor _COUT is recharged and supplies power to the load 20. The output voltage _VOUT (node 16) is maintained by the charge on the output capacitor _COUT . When the primary switch SW is turned off, the drain-source voltage _VDS(SW) of the primary switch SW (node 14) swings to a high voltage value. In some examples, a voltage clamping circuit such as a passive clamping circuit 25 is used to clamp the drain voltage at the primary switch to the maximum allowable voltage value to protect the primary switch.

The control scheme implemented in a flyback converter includes a feedback control loop to monitor the output voltage V _OUT . The applied control scheme controls the on-time of the synchronous rectifiers or the off-time of the primary switch to maintain the output voltage at the desired voltage value under various load conditions. At the specified time, the primary-side or secondary-side controller of the flyback converter starts the next switching cycle by turning off the synchronous rectifiers and turning on the primary switch. The above operation is repeated.

In the case where the flyback converter 10 operates in continuous conduction mode, at a specified time, such as when the drain voltage has dropped to a predetermined gate turn-off detection threshold V _THGOFF , the secondary side controller signals the synchronous rectifier to turn off. However, due to propagation delays and gate driver discharge time, the gate voltage V _GS2 of the synchronous rectifier M2 is often delayed in turning off the synchronous rectifier switch. As a result, the secondary current Isec experiences a negative current or reverse current excursion, as further described in detail with reference to FIG. 6 .

FIG6 shows signal waveforms in the switching cycle of the synchronous rectifier in the flyback converter of FIG4 during the conduction period of the synchronous rectifier in some examples. Referring to FIG6, at time T0 of the switching cycle, the primary switch has been turned off and the voltage across the secondary winding has reversed, the secondary current Isec (curve 56), which is also the drain-to-source current or the current of the drain synchronous rectifier, is conducted through the forward biased body diode of the synchronous rectifier M2, and the drain voltage V _DS(SR) (curve 54) of the drain terminal of the synchronous rectifier M2 drops to a negative voltage value. When the drain voltage V _DS(SR) drops to a negative voltage value greater than the SR conduction detection voltage V _THGON , the synchronous rectifier M2 receives a conduction signal, and the gate voltage V _GS(SR) (curve 52) driving the synchronous rectifier gradually increases. The gate voltage V _GS(SR) refers to the gate voltage V _GS2 in Figures 4 and 5. Therefore, the synchronous rectifier turns on and conducts the secondary current Isec to the output capacitor. In fact, the drain voltage V _DS of the synchronous rectifier is a function of the secondary current Isec and the on-resistance RDSon of the synchronous rectifier switch. In other words, the drain voltage V _DS follows the secondary current Isec.

During the conduction period of the synchronous rectifier, the secondary current Isec conducts current to transfer the energy stored in the secondary winding of the transformer LP to the output capacitor _COUT . As the energy is transferred, the secondary current Isec (or drain current) decreases and the drain voltage _VDS(SR) decreases accordingly. In some embodiments, the drain voltage is measured to serve as a representative of the drain current of the synchronous rectifier. In this example, when the voltage V _DS(SR) drops to the regulation threshold V _THREG (time T1), the gate voltage V _GS(SR) is regulated to maintain the drain current as the drain current continues to decrease. As long as the gate voltage can be reduced to meet the drain current demand, the drain voltage V _DS(SR) will be regulated near the regulation voltage level. At time T2, the drain current has dropped to the zero current level and the drain voltage V _DS(SR) drops to the SR turn-off detection voltage V _THGOFF , which indicates that the synchronous rectifier M2 will be turned off. However, due to the propagation delays inherent in the secondary-side controller (Figure 5) and the time required for the gate driver to discharge, there is a delay when the synchronous rectifier M2 is actually turned off. In addition, during this time, the slope of the secondary current can be very large. This causes the secondary current to cross zero current and become negative (or reverse current), as shown in Figure 6. When the primary switch (M1) is turned on, the negative secondary current must be consumed, which causes a large voltage to swing on the drain voltage V _DS(SR) of the synchronous rectifier (M2). The drain voltage V A large voltage swing on _DS(SR) is undesirable as it may affect the reliability of the synchronous rectifier switches.

FIG. 7 illustrates signal waveforms during the switching cycle of the synchronous rectifier in the flyback converter of FIG. 4 during the conduction period of the synchronous rectifier in an alternative example. In particular, FIG. 7 illustrates the use of hysteresis regulation in the regulation threshold voltage. In some applications, a hysteresis regulation threshold including a high regulation threshold voltage V _{THREG_H} and a low regulation threshold voltage V _{THREG_L} is used to reduce the gate voltage while maintaining a reduced secondary current Isec during regulation. The synchronous rectifier drain voltage V _DS is allowed to swing between a high regulation threshold voltage and a low regulation threshold voltage, while the synchronous rectifier gate voltage V _GS(SR) is gradually reduced to regulate the reduction of the secondary current.

In the control schemes of Figures 6 and 7, the SR turn-off detection voltage V _THGOFF is selected to be very close to 0V. Typically, the SR turn-off detection voltage V _THGOFF is about -3mV. The SR turn-off detection voltage V _THGOFF is set close to 0V to shorten the dead time between switching cycles. However, since the turn-off detection threshold voltage is so close to 0V, and the falling slope of the secondary current is large at this time, the secondary current Isec will generate a large negative current, resulting in a large drain voltage swing. Using a hysteresis adjustment threshold does not solve the negative secondary current problem.

In an embodiment of the present invention, a power converter, such as a flyback converter, implements an adaptive turn-off voltage control method, in which the SR turn-off detection voltage of the synchronous rectifier is adaptively changed according to the operating conditions of the power converter. When the power converter operates in a discontinuous conduction mode, the SR turn-off detection voltage remains unchanged and close to zero volts to ensure that the dead time between switching cycles is minimized. However, when the power converter operates in a continuous conduction mode, the SR turn-off detection voltage is adjusted to be further away from zero voltage, thereby triggering the synchronous rectifier to turn off earlier in the switching cycle, thereby preventing large negative secondary currents.

It should be noted that in discontinuous conduction mode, the secondary current decreases slowly, that is, the slope of the secondary current is small. In this case, even if the SR turn-off detection voltage is close to zero volts, there will be only a small amount of negative current when the synchronous rectifier sends a shutdown signal. However, in continuous conduction mode, the secondary current decreases very quickly, that is, the slope of the secondary current is large. In this case, when the SR turn-off detection voltage is close to zero volts and the synchronous rectifier sends a shutdown signal, the secondary current will swing to a larger negative current value.

FIG. 8 shows the signal waveforms in the switching cycle of the synchronous rectifier in the flyback converter implementing the adaptive turn-off voltage control method in an embodiment of the present invention. In some embodiments, the flyback converter is implemented using the topology of the flyback converter 10 of FIG. 4. Referring to FIG. 8, at time T0 of the switching cycle, the drain voltage V _DS(SR) (curve 64) of the synchronous rectifier M2 drops to a negative voltage level exceeding the SR conduction detection voltage V _THGON , which indicates the conduction of the synchronous rectifier. The gate voltage V _GS(SR) (curve 62) driving the synchronous rectifier ramps up to turn on the synchronous rectifier. The gate voltage V _GS(SR) in FIG. 8 refers to the gate voltage V _GS2 in FIG. 4 and FIG. 5 . The synchronous rectifier M2 is turned on and conducts the secondary current Isec (curve 66) to the output capacitor. As described above, the drain voltage V _DS(SR) of the synchronous rectifier is a function of the drain current of the synchronous rectifier (i.e., the secondary current Isec) and the on-resistance RDSon of the synchronous rectifier switch. In other words, the drain voltage V _DS(SR) follows the drain current Isec.

During the conduction period of the synchronous rectifier, the secondary current Isec conducts current to transfer the energy stored in the secondary winding of the transformer LP to the output capacitor C _OUT . As the energy is transferred, the secondary current Isec decreases and the drain voltage V _DS(SR) decreases accordingly. In this example, when the voltage V _DS drops to the regulation threshold V _THREG (time T1), the drain-source voltage V _DS is regulated near the regulation voltage level, and the gate voltage V _GS(SR) is reduced to regulate the secondary current as the secondary current continues to decrease.

In an embodiment of the present invention, the adaptive turn-off voltage control method implements two SR turn-off detection voltage values, a first SR turn-off detection voltage V _{THGOFF_H} having a voltage value close to zero volts and a second SR turn-off detection voltage V _{THGOFF_L} having a voltage value far from zero volts. In one example, the voltage V _{THGOFF_H} is -3mV and the voltage V _{THGOFF_L} is -30mV. The adaptive turn-off voltage control method detects the operating mode of the flyback converter. In response to the flyback converter operating in the discontinuous conduction mode, the method selects the first SR turn-off detection voltage V _{THGOFF_H} , which sets the gate turn-off threshold to close to zero volts. Alternatively, in response to the flyback converter operating in the continuous conduction mode, the method selects a second SR turn-off detection voltage V _{THGOFF_L} , which sets the turn-off threshold far from zero volts. In the present embodiment, the first SR turn-off detection voltage V _{THGOFF_H} is a nominal gate turn-off detection voltage, and the method switches to the second SR turn-off detection voltage V _{THGOFF_L} when the continuous conduction mode is detected, and returns to the first SR turn-off detection voltage V _{THGOFF_H} when the drain voltage reaches a predetermined reset voltage value V _RESET to prepare the flyback converter for the next switching cycle. In some embodiments, the reset voltage value is a positive voltage, such as 3-4V in some examples.

The adaptive turn-off voltage control method detects the operating mode of the flyback converter at time T2, which is close to the end of the synchronous rectifier conduction period. In the example shown in Figure 8, the method detects the operating mode of the flyback converter at 90% of the expected "on" cycle (or conduction time) of the synchronous rectifier and determines that the flyback converter operates in continuous conduction mode. Accordingly, the gate turn-off threshold is switched from the nominal first SR turn-off detection voltage V _{THGOFF_H} to the second SR turn-off detection voltage V _{THGOFF_L} . At the same time, the secondary current continues to decrease. At time T3, the secondary current decreases to a near zero current level and the drain voltage V _DS reaches the second SR turn-off detection V _{THGOFF_L} , which indicates that the synchronous rectifier M2 will be turned off. Due to the inherent propagation delay of the secondary-side controller (Figure 5) and the time required for the gate driver to discharge, the actual turn-off of the synchronous rectifier is delayed. At time T4, the synchronous rectifier M2 is actually turned off. At some time (before time T4 in this example), the drain voltage V _DS has reached the reset threshold voltage V _RESET , and the gate turn-off voltage is reset to the nominal first gate turn-off threshold voltage V _{THGOFF — H} for the next switching cycle.

By using the second SR turn-off detection voltage V _{THGOFF_L} , the synchronous rectifier M2 is turned off earlier than when the nominal gate turn-off detection threshold (V _{THGOFF_H} ) is used. Therefore, the negative current swing of the secondary current is reduced, and the corresponding voltage swing on the drain voltage V _DS is also reduced. In this way, the synchronous rectifier switch can be protected from the stress of undesirable or excessive voltage swings and its reliability is improved.

The operation of the adaptive shutdown voltage control method of the present invention will be explained in more detail below with reference to FIG. 9. FIG. 9 is a flow chart for illustrating a power converter that can be used in an embodiment of the adaptive shutdown voltage control method of the present invention, such as the flyback converter of FIG. 4. Referring to FIG. 9, method 80 has recorded or stored the synchronous rectifier conduction time from the previous switching cycle (82). Method 80 begins by detecting the start of a new SR conduction cycle (84). Method 80 checks the synchronous rectifier gate voltage V _GS(SR) when the current SR conduction cycle is nearing the end. In an embodiment of the present invention, the method 80 uses the recorded SR conduction time of the previous switching cycle as a reference value and takes X% of this SR conduction time to determine whether the flyback converter operation is approaching the end of the current SR conduction cycle. That is, the method 80 uses X% of the recorded SR conduction time of the previous switching cycle as the gate detection time in the current conduction cycle. In some examples, X% is 90% or between 85% and 95%. In practice, the operating frequency of the flyback converter is relatively constant, so the SR conduction time of each cycle will be relatively constant. Therefore, in one embodiment, the method 80 detects the synchronous rectifier gate voltage V _GS(SR) (86) when the time in the current SR conduction cycle is 90% of the conduction time of the previous SR conduction cycle. Referring to FIG. 8, in the current SR conduction cycle N, the time T2 is associated with 90% of the SR conduction time of the previous SR conduction cycle N-1, represented as 90% of T _SR(N-1) .

The method 80 compares the detected gate voltage (denoted as V _GDET in FIG. 8 ) with the gate voltage target V _THGDET and determines whether the detected gate voltage V _GDET is greater than or equal to the gate voltage target V _THGDET ( 88 ). In the case that the detected gate voltage V _GDET is less than the gate voltage target V _THGDET , the method determines that the flyback converter operates in a discontinuous conduction mode, and the method maintains the SR turn-off detection voltage at a high detection level V _{THGOFF_H} , closer to zero volts ( 90 ). In the case that the detected gate voltage V _GDET is greater than or equal to the gate voltage target V _THGDET , the method determines that the flyback converter operates in continuous conduction mode and the method changes the SR turn-off detection voltage to a low detection level V _{THGOFF — L} , far from zero volts ( 92 ).

In particular, in an embodiment of the present invention, the adaptive turn-off voltage control method 80 uses the gate voltage of the synchronous rectifier as a representative of the operating state of the flyback converter. When the flyback converter operates in a discontinuous conduction mode (DCM), the gate voltage of the synchronous rectifier will be very small at the end of the conduction cycle. That is, in the discontinuous conduction mode, the gate voltage V _GS(SR) of the synchronous rectifier will be less than the gate voltage target V _THGDET . On the other hand, when the flyback converter operates in a continuous conduction mode (CCM), the gate voltage of the synchronous rectifier is still large at the end of the conduction cycle. That is, in continuous conduction mode, the gate voltage V _GS(SR) of the synchronous rectifier will be greater than or at least equal to the gate voltage target V _THGDET . In some examples, for a flyback converter, the gate voltage target V _THGDET is 3.5-4V, but can have different voltage values depending on the power converter topology and other operating conditions.

Therefore, by detecting the gate voltage V _GS(SR) of the synchronous rectifier near the end of the SR conduction period (and converting the detected gate voltage V By comparing _GDET with the gate voltage target, method 80 can determine the operating mode of the synchronous rectifier and can set the SR turn-off detection voltage accordingly to achieve fast shutdown when the synchronous rectifier operates in continuous conduction mode. After selecting the appropriate SR turn-off detection voltage (90 or 92), when the drain voltage reaches the selected SR turn-off detection voltage, the flyback converter turns off the synchronous rectifier. At the end of the SR conduction cycle, method 80 records or stores the SR conduction time of the current SR conduction cycle after the synchronous rectifier is turned off (94). At the same time, method 80 further detects the drain voltage V _DS(SR) of the synchronous rectifier to determine the drain voltage V The method 80 determines whether the drain voltage _{V DS(SR)} has reached the reset threshold voltage V _RESET (96). When the drain voltage V _DS(SR) has reached the reset threshold voltage V _RESET , the method 80 resets the SR turn-off detection voltage to a high detection level V _{THGOFF_H} , closer to zero volts if applicable (98). It is understood that in the case where the SR turn-off detection voltage does not change to the low detection level V _{THGOFF_L} , there is no need to reset the gate turn-off threshold voltage because it is already at the high detection level V _{THGOFF_H} , i.e., the nominal detection level.

The method 80 then returns to detecting the start of the next synchronous rectifier conduction cycle (84). The process continues again to detect the gate voltage of the synchronous rectifier near the end of the conduction cycle to determine the operating mode and adaptively adjust the SR turn-off detection voltage based on the detected operating mode. When it is detected that the flyback converter is operating in continuous conduction mode, by changing the SR turn-off detection voltage to a lower voltage value (e.g., -30mV vs. -3mV), the synchronous rectifier will be turned off earlier in the conduction time, which has the effect of reducing the negative secondary current offset and the drain voltage swing on the synchronous rectifier. Importantly, method 80 only changes the SR turn-off detection voltage for continuous conduction mode and keeps the SR turn-off detection voltage for discontinuous conduction mode at a nominal level. In this way, the SR turn-off detection voltage can be kept close to zero volts to avoid excessive dead time between switching cycles. At the same time, fast synchronous rectifier turn-off is achieved by moving the SR turn-off detection voltage away from zero voltage.

FIG. 10 shows signal waveforms in the switching cycle of a synchronous rectifier in a flyback converter implementing an adaptive turn-off voltage control method in an embodiment of the present invention. In particular, FIG. 10 illustrates the use of hysteresis in the regulation threshold voltage and the adaptive turn-off voltage control method. The hysteresis regulation threshold, including a high regulation threshold voltage V _{THREG_H} and a low regulation threshold voltage V _{THREG_L} , is used to better regulate the secondary current Isec during regulation. The drain voltage V _DS(SR) is allowed to swing between the high and low regulation threshold voltages, while the gate voltage V _GS(SR) of the synchronous rectifier is reduced in a stepwise manner to regulate the reduction of the secondary current. The use of a hysteresis regulating threshold does not change the operation of the adaptive turn-off voltage control method of the present invention. In particular, the method detects the gate voltage V _GS(SR) at time T2, which is 90% of the SR conduction time recorded from the previous switching cycle. When the method determines that the gate voltage V _GS(SR) is greater than the gate voltage target V _THGDET , the SR turn-off detection voltage is changed to a lower voltage value V _{THGOFF_L} to make the synchronous rectifier turn-off detection occur faster, as explained above. When the drain voltage V _DS(SR) reaches the reset threshold voltage V _RESET , the gate turn-off threshold is reset to the nominal turn-off voltage V _{THGOFF_H} .

FIG. 11 is a schematic diagram of a secondary-side controller in the flyback converter of FIG. 4, which incorporates an adaptive shutdown voltage control circuit in an embodiment of the present invention. In some embodiments, the adaptive shutdown voltage control circuit implements the adaptive shutdown voltage control method of FIG. 9. Referring to FIG. 11, a secondary-side controller 100 for generating a gate voltage _VGS2 to control a synchronous rectifier MOSFET M2 includes a drain voltage _VDS sensing circuit 102 for sensing the drain voltage _VDS of the synchronous rectifier M2. The sensed or detected drain voltage VD (node 103) is coupled to a pair of operational amplifiers OP1 and OP2 to be compared with respective detection threshold voltages to generate a gate on/off control signal for the synchronous rectifier M2. Specifically, the operational amplifier OP1 compares the sensed drain voltage VD (node 103) with the SR turn-on detection voltage V _THGON to determine when the synchronous rectifier M2 should be turned on, and the operational amplifier OP2 compares the sensed drain voltage VD (node 103) with the SR turn-off detection voltage V _THGOFF to determine when the synchronous rectifier M2 should be turned off. The secondary side controller 100 includes a gate on/off control logic circuit 104 that receives output signals from operational amplifiers OP1 and OP2 and generates an on/off control signal. The on/off control signal is coupled to a tri-state gate driver 106 that provides a gate voltage V _GS2 to drive the synchronous rectifier M2 when enabled by an enable signal Tri-EN.

In this example, the secondary side controller 100 further includes an operational amplifier OP3 for comparing the sensed drain voltage VD (node 103) with the regulation threshold voltage V _THREG . When the sensed drain voltage VD (node 103) reaches the regulation threshold voltage V _THREG , the operational amplifier OP3 closes the switch S1 to allow the discharge current control circuit 108 to discharge the gate voltage V _GS2 in a controlled manner. In particular, when the sensed drain voltage VD (node 103) decreases to the regulation threshold V _THREG , the drain-source voltage V _DS of the synchronous rectifier is regulated near the regulation voltage level, and the gate voltage V _GS2 is discharged by the discharge current control circuit 108. In other words, the gate voltage V _GS2 is reduced to regulate the secondary current flowing in the secondary winding as the secondary current continues to decrease during the conduction cycle of the synchronous rectifier, as shown in FIG8. Additional operational amplifiers can be used to provide additional regulation thresholds, such as when hysteresis regulation is required.

The secondary side controller 100 also includes an adaptive turn-off voltage control circuit 110 to select a desired voltage value for the SR turn-off detection voltage. The control circuit 110 includes a register 112 for storing the SR conduction time TSR of each synchronous rectifier conduction cycle. The SR conduction time TSR(N-1) of the previous conduction cycle is sampled or captured by a sampling and holding circuit 114. The sampled SR conduction time TSR(N-1) is multiplied by a multiplication factor close to but less than 1 at a multiplier 118 to produce a factorized SR conduction time. In this example, the multiplier 118 multiplies the sampled SR conduction time TSR(N-1) by 0.9 to obtain 90% of the SR conduction time of the previous cycle. In other embodiments, other multiplication factors may be used. For example, the multiplication factor of the multiplier 118 can be between 0.85 and 0.95. At the same time, the register 116 stores the SR conduction time TSR(N) in the current conduction cycle. The SR conduction time TSR(N) is compared with the indexed SR conduction time from the multiplier 118 through the comparator Comp1. Once the current SR conduction time reaches the indexed SR conduction time, the comparator Comp1 outputs a high effective level. That is, if the current SR conduction cycle reaches 90% of the conduction time of the previous SR conduction cycle, the comparator Comp1 outputs a high effective level.

The output of the comparator Comp1 is provided as an input to the logical AND gate 120. The logical AND gate 120 operates on three inputs and outputs a high active level when the conditions associated with the three inputs are met. The first input of the logical AND gate 120 is the output of the comparator Comp1, indicating whether the SR conduction time of the current SR conduction cycle is close to the end of the conduction cycle, that is, whether the SR conduction time has reached 90% of the conduction time of the previous SR conduction cycle. The second input of the logical AND gate 120 is the enable signal Tri-EN, which is used to start the tri-state gate driver 106. The third input of the logical AND gate 120 is the output of the comparator Comp2, which compares the gate voltage V _GS2 of the synchronous rectifier with the gate voltage target V _THGDET . The logical AND gate 120 outputs a high active level when three conditions are met: (1) the SR conduction time has reached 90% of the conduction time of the previous SR conduction cycle; (2) the enable signal Tri-En of the tri-state gate driver is enabled; (3) the gate voltage V _GS2 of the synchronous rectifier is equal to or greater than the gate voltage target V _THGDET . When the three conditions are met, the adaptive turn-off voltage control circuit 110 determines that the flyback converter operates in the continuous conduction mode. When any one of the three conditions is not met, the adaptive turn-off voltage control circuit 110 determines that the flyback converter operates in the discontinuous conduction mode.

In an alternative embodiment, the logical AND gate 120 may omit the enable signal Tri-En as an input and only evaluate the remaining two conditions: 90% of the on-time and the gate voltage at the gate voltage target. In the case where the tri-state gate driver 106 is not enabled (the enable signal Tri-En is not enabled), whether the synchronous rectifier M2 is tri-state driven or not and the operation of the adaptive turn-off voltage control circuit 110 is irrelevant. The use of the enable signal Tri-En at the logical AND gate 120 in FIG. 11 is illustrative only.

The adaptive shutdown voltage control circuit 110 includes a set-reset (SR) trigger 122. The set input of the SR trigger 122 receives the output of the logical AND gate 120. Therefore, when the condition evaluated by the logical AND gate is met, the set input of the SR trigger 122 is enabled and the output (Q) of the trigger is enabled (e.g., logical "1"). The reset input of the SR trigger 122 receives a signal representing a comparison of the drain voltage VD (node 103) of the synchronous rectifier with the reset threshold voltage V _RESET . When the sensed drain voltage VD (node 103) reaches the reset threshold voltage V _RESET , the reset input of the SR trigger 122 is enabled. When the reset input of the SR trigger 122 is enabled, the output (Q) of the trigger 122 is de-enabled (e.g., logical "0"). The output (Q) of the SR trigger 122 is the detection voltage selection signal V _{THGOFF_SEL} and is coupled to control the switch S2 to select one of the two detection voltages.

Specifically, when the SR trigger 122 is set and the detection voltage selection signal V _{THGOFF_SEL} is enabled (e.g., logic "1"), the switch S2 selects the low SR turn-off detection voltage V _{THGOFF_L} . On the other hand, when the SR trigger 122 is reset and the detection voltage selection signal V _{THGOFF_SEL} is set low (e.g., logic "0"), the switch S2 selects the high SR turn-off detection voltage V _{THGOFF_H} . The switch S2 provides the selected detection voltage as the SR turn-off detection voltage V _THGOFF provided to the operational amplifier OP2. Thus, the adaptive turn-off voltage control circuit 110 detects that the flyback converter operates in continuous conduction mode and selects a low SR turn-off detection voltage V _{THGOFF_L} to turn off the synchronous rectifier faster. Alternatively, the adaptive turn-off voltage control circuit 110 detects that the flyback converter operates in discontinuous conduction mode and selects or maintains a high SR turn-off detection voltage V _{THGOFF_H} to ensure a short dead time between switching cycles. At the end of the current SR conduction cycle, the adaptive turn-off voltage control circuit 110 stores the SR conduction time in the register 112 for use in the next switching cycle.

As configured in this way, the adaptive turn-off voltage control circuit 110 operates to adaptively select the SR turn-off detection voltage according to the operation mode of the flyback converter operation. The flyback converter can achieve fast turn-off of the synchronous rectifier, thereby having the effect of reducing the negative current offset of the secondary current, thereby achieving the effect of reducing the large voltage swing of the synchronous rectifier drain voltage.

In the above description, a flyback converter including a transformer is described. It is understood that the adaptive shutdown voltage control circuit and method can be applied to other types of power converters or switching regulators, with or without transformer isolation. As used herein, the terms "primary current" and "secondary current" refer to the current flowing through the primary switch and the current flowing through the synchronous rectifier, respectively. The use of transformer-isolated power converters in this specification is illustrative only and not restrictive.

In this detailed description, a process step described for one embodiment may be used in a different embodiment even if the process step is not explicitly described in the different embodiment. When a method comprising two or more defined steps is mentioned herein, the defined steps may be performed in any order or simultaneously unless the context indicates otherwise or specific instructions are provided herein. In addition, unless the context dictates otherwise or explicit instructions are provided, the method may also include one or more other steps performed before any defined step, between two defined steps, or after all defined steps.

In this detailed description, various embodiments or examples of the present invention can be implemented in many ways, including as a process; an apparatus; a system; and a composition of matter. A detailed description of one or more embodiments of the present invention and accompanying drawings illustrating the principles of the present invention are provided above. The present invention is described in conjunction with these embodiments, but the present invention is not limited to any embodiment. Many modifications and variations within the scope of the present invention are possible. The scope of the present invention is limited only by the scope of the invention application, and the present invention includes many alternatives, modifications and equivalents. In order to provide a thorough understanding of the present invention, many specific details are set forth in the description. These details are provided for illustrative purposes, and the present invention may be practiced in accordance with the claims without some or all of these specific details. For the sake of clarity, technical materials known in the art related to the present invention are not described in detail so as not to unnecessarily obscure the present invention. The present invention is defined by the appended claims.

D1: body diode

D2: body diode

_LP : Transformer

V _IN : Input voltage

Ipri: primary current

Isec: Secondary current

V _SEC : Secondary voltage

V _DS(SW) : Source voltage

V _DS : Drain voltage

Cin: Input decoupling capacitor

C _OUT : Output capacitor

V _OUT : Output voltage

M2(SR): synchronous rectifier switch

M1(SW): Primary switch

V _GS1 : Control voltage

V _GS2 : Gate voltage

V _{OUT_FB} : Feedback voltage

10: Flyback converter

12: Input voltage node

14: Node

15: Node

16: Node

18: Node

20: Load

25: Passive clamping circuit

30: Primary test controller

40: Secondary side controller

Claims (20)

A switching mode power converter having a synchronous rectifier that implements an adaptive turn-off voltage, wherein a method of operating the power converter incorporating the synchronous rectifier and receiving an input voltage and providing an output voltage comprises: Detecting the start of a synchronous rectifier (SR) conduction cycle; Detecting a gate voltage at a gate terminal of the synchronous rectifier (SR) near the end of the synchronous rectifier (SR) conduction cycle; In response to the detected gate voltage being less than a gate voltage target, selecting a first SR turn-off detection voltage as a turn-off detection threshold of the synchronous rectifier (SR); In response to the detected gate voltage being greater than or equal to the gate voltage target, a second SR turn-off detection voltage is selected as the turn-off detection threshold of the synchronous rectifier (SR), the first SR turn-off detection voltage and the second SR turn-off detection voltage are negative voltage values, and the first SR turn-off detection voltage is closer to zero volts than the second SR turn-off detection voltage; In response to the synchronous rectifier being turned off after the drain voltage of the synchronous rectifier reaches the SR turn-off detection threshold, the SR conduction time (T) of the current conduction cycle of the synchronous rectifier (SR) is stored; and In response to the synchronous rectifier (SR) turn-off detection threshold being set to the second SR turn-off detection voltage, the synchronous rectifier (SR) turn-off detection threshold is reset to the first SR turn-off detection voltage. The method as described in claim 1, wherein resetting the synchronous rectifier (SR) turn-off detection threshold comprises: In response to the synchronous rectifier (SR) turn-off detection threshold being set to the second SR turn-off detection voltage, detecting the drain voltage of the synchronous rectifier; and In response to the detected drain voltage having reached the reset threshold voltage, setting the synchronous rectifier (SR) turn-off detection threshold to the first SR turn-off detection voltage. The method of claim 2, wherein the reset threshold voltage comprises a positive voltage value. The method as described in claim 1, wherein the gate voltage is detected when the gate terminal of the synchronous rectifier approaches the end of the synchronous rectifier (SR) conduction cycle, including: Sampling the stored SR conduction time (T) of the previous synchronous rectifier (SR) conduction cycle; Determining the gate detection time as X% of the SR conduction time (T) of the previous synchronous rectifier (SR) conduction cycle; and Detecting the gate voltage of the synchronous rectifier gate terminal during gate detection. The method of claim 4, wherein X% comprises values between 85% and 95%. The method as described in claim 4 further includes: Repeatedly detecting the start of the synchronous rectifier (SR) conduction cycle to reset the synchronous rectifier (SR) turn-off detection threshold of the next synchronous rectifier (SR) conduction cycle, and the stored synchronous rectifier (SR) conduction time becomes the SR conduction time (T) of the previous stored synchronous rectifier (SR) conduction cycle. A method as described in claim 1, wherein the power converter includes a flyback converter, the flyback converter includes a transformer, the transformer having a primary winding receiving the input voltage and a secondary winding providing the output voltage, a primary switch coupled to the primary winding and the synchronous rectifier coupled to the secondary winding. The method of claim 1, wherein the first SR turn-off detection voltage comprises -3 mV, and the second SR turn-off detection voltage comprises -30 mV. A method as described in claim 1, wherein the power converter operates in a discontinuous conduction mode in response to the detected gate voltage being less than the gate voltage target, and the power converter operates in a continuous conduction mode in response to the detected gate voltage being greater than or equal to the gate voltage target. A switching mode power converter having a synchronous rectifier that realizes an adaptive shutdown voltage, wherein the power converter comprises: an input terminal receiving an input voltage, and an output terminal providing an output voltage; a synchronous rectifier (SR) coupled to the output terminal; and a controller coupled to generate a gate control signal to drive the gate terminal of the synchronous rectifier (SR) during a plurality of synchronous rectifier (SR) conduction cycles; The controller detects a gate voltage at a gate terminal of the synchronous rectifier (SR) when each conduction cycle of the synchronous rectifier (SR) is near the end, and in response to the detected gate voltage being less than a gate voltage target, the controller sets a turn-off detection threshold of the synchronous rectifier (SR) to a first voltage, and in response to the detected gate voltage having a value greater than or equal to the gate voltage target, sets the turn-off detection threshold of the synchronous rectifier (SR) to a second voltage, wherein the first voltage and the second voltage are The first voltage is a negative voltage value, and the first voltage is closer to zero volts than the second voltage. The controller uses the synchronous rectifier (SR) shutdown detection threshold to determine the shutdown of the synchronous rectifier (SR), and in response to the controller sending a signal to shut down the synchronous rectifier in response to the synchronous rectifier (SR) shutdown detection threshold, when the synchronous rectifier (SR) shutdown detection threshold has been set to the second voltage, the controller resets the synchronous rectifier (SR) shutdown detection threshold to the first voltage. A power converter as described in claim 10, wherein in response to setting the synchronous rectifier (SR) turn-off detection threshold to the second voltage, the controller detects the drain voltage at the drain terminal of the synchronous rectifier and in response to the detected drain voltage having reached a reset threshold voltage, the controller sets the synchronous rectifier (SR) turn-off detection threshold to the first SR turn-off detection voltage. A power converter as described in claim 11, wherein the reset threshold voltage includes a positive voltage value. A power converter as described in claim 10, wherein the controller detects the gate voltage at the gate terminal of the synchronous rectifier near the end of each synchronous rectifier (SR) conduction cycle by sampling the stored SR conduction time (T) of the previous SR conduction cycle and determining the gate detection time to be X% of the SR conduction time (T) of the previous SR conduction cycle, and the controller detects the gate voltage at the gate terminal of the synchronous rectifier (SR) during the gate detection time. A power converter as described in claim 13, wherein X% of the SR conduction time (T) includes values between 85% and 95%. A power converter as described in claim 13, wherein the controller stores the SR on-time (T) of the current conduction cycle of the synchronous rectifier (SR) after the synchronous rectifier (SR) is turned off in response to the drain voltage of the synchronous rectifier (SR) reaching the synchronous rectifier (SR) turn-off detection threshold, and the controller repeatedly detects the gate voltage to reset the synchronous rectifier (SR) turn-off detection threshold for the next conduction cycle of the synchronous rectifier (SR), and the stored SR on-time (T) becomes the stored SR on-time (T) of the previous conduction cycle of the synchronous rectifier (SR). A power converter as described in claim 10, wherein the power converter includes a flyback converter, the flyback converter including: A transformer having a primary winding for receiving an input voltage and a secondary winding for providing an output voltage; A primary switch coupled to the primary winding; The synchronous rectifier (SR) coupled to the secondary winding; An output capacitor coupled to the secondary winding; and A primary-side controller coupled to generate a control signal to drive the primary switch. A power converter as described in claim 10, wherein the first voltage includes -3mV and the second voltage includes -30mV. A power converter as described in claim 10, wherein the power converter operates in a discontinuous conduction mode in response to the detected gate voltage being less than the gate voltage target, and the power converter operates in a continuous conduction mode in response to the detected gate voltage being greater than or equal to the gate voltage target. A power converter as described in claim 11, wherein the controller includes: A drain voltage detection circuit for detecting the drain voltage of the synchronous rectifier (SR) drain terminal; A first operational amplifier having a first input terminal coupled to receive the detected drain voltage and a second input terminal coupled to receive the synchronous rectifier (SR) conduction threshold, the first operational amplifier having an output terminal and outputting an enable signal in response to the detected drain voltage being more negative than the synchronous rectifier (SR) conduction threshold; A second operational amplifier having a first input terminal coupled to receive the synchronous rectifier (SR) turn-off threshold and a second input terminal coupled to receive the detected drain voltage, the second operational amplifier having an output terminal and outputting an enable signal in response to the detected drain voltage being greater than the synchronous rectifier (SR) turn-off threshold; and a gate control circuit receiving output signals of the first operational amplifier and the second operational amplifier, and driving a gate driver in response to the output signal indicating that the synchronous rectifier (SR) is to be turned on or off, the gate driver generating a gate control signal to drive the gate terminal of the synchronous rectifier (SR). A power converter as described in claim 19, wherein the controller further comprises: a first register for storing the SR conduction time (T) of the previous conduction cycle of the synchronous rectifier (SR); a sample-hold circuit for sampling the stored SR conduction time (T); a multiplier configured to multiply the stored SR conduction time (T) by X% to determine the gate detection time; a second register for storing the time value of the conduction time in progress in the current conduction cycle; The first comparator compares the gate detection time with the time value stored in the second register. The first comparator triggers the first comparator output enable signal in response to the time value stored in the second register being equal to the gate detection time, otherwise the first comparator output enable signal is released; The second comparator compares the detected gate voltage with the gate voltage target. The second comparator triggers the second comparator output enable signal in response to the gate voltage being greater than the gate voltage target, otherwise the second comparator output enable signal is released; Logical AND Gate The logical AND gate is configured to receive the comparator output signals of the first comparator and the second comparator, and the logical AND gate triggers the output signal in response to the output signals of the first comparator and the second comparator, otherwise the output signal of the logical AND gate is released; The set-reset trigger has the function of receiving the logical AND gate. The invention relates to a set input terminal of an output signal of a synchronous rectifier (SR), a reset terminal receiving a reset signal, the reset signal being triggered in response to the detected drain voltage having reached a reset threshold voltage, the set-reset trigger generating a selection signal at the output signal, the selection signal being set in response to the output signal of the logical AND gate and being reset in response to the reset signal being triggered; and a switch being controlled by the selection signal to select a first voltage or a second voltage as the synchronous rectifier (SR) turn-off detection threshold, the switch being reset in response to the selection signal to select the first voltage and being set in response to the selection signal to select the second voltage.