Isolated power supply and control circuit and control method thereof

Abstract

The invention provides a control circuit of an isolated power supply, a primary side original signal generator generates a primary side original opening signal according to a voltage feedback signal of an output voltage of the isolated power supply, and a secondary side control signal generator generates a secondary side control signal according to a secondary side voltage signal and the primary side original opening signal. In the current switching period, if the turn-on time of the primary side main switching tube is earlier than the turn-on time of the secondary side synchronous rectifier tube, the secondary side control signal indicates the turn-on of the secondary side synchronous rectifier tube, so that the possibility of punch-through is avoided; the turn-off threshold value of the next switching period is adjusted according to the follow current time and the dead zone time of the current switching period, so that the follow current time approaches to the follow current reference value and the dead zone time approaches to the dead zone reference value, the secondary side synchronous rectifier tube is close to zero-current turn-off in a steady state, the switching loss is reduced, and the efficiency is improved. Correspondingly, the invention also provides an isolated power supply and a control method thereof.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic circuits, and in particular to an isolated power supply and a control circuit and a control method thereof. Background Art

[0002] Under the background of the increasing importance of environmental protection and energy saving, the requirements for power supply efficiency are getting higher and higher. For switching power supplies, the use of synchronous rectifiers to replace traditional freewheeling diodes is an effective way to improve efficiency. When the power supply is in the continuous conduction mode (CCM), in the process of controlling the synchronous rectifier, if the synchronous rectifier switch and the main switch are turned on at the same time, there may be a serious risk of explosion due to the commonality, so special attention needs to be paid during the control circuit design process. For non-isolated switching power supplies, the control of the synchronous rectifier can usually be synchronized with the control of the main switch, and the design difficulty is not great. However, for isolated switching power supplies, since the synchronous rectifier and its control circuit are located on the secondary side and the main switch and its control circuit are located on the primary side, the two are electrically isolated and the control difficulty is higher.

[0003] The primary and secondary sides of existing isolated power supplies are usually controlled independently, that is, the opening and closing of the secondary synchronous rectifier tube generally does not rely on the control logic of the primary main switch tube. Usually, the voltage signal on the secondary winding is detected to determine whether the primary main switch tube has been turned on, and the secondary synchronous rectifier tube is turned off after the primary main switch tube is turned on. This control method is highly portable and does not need to be coordinated with the primary controller. The main disadvantage is that the reliability is not high. In the discontinuous conduction mode (DCM), due to reasons such as transformer leakage inductance, the free resonance of the inductor may be judged as a signal that needs to be turned on, resulting in the problem of false opening, which affects the conversion efficiency; in addition, in the CCM mode, since the closing of the secondary synchronous rectifier tube always lags behind the opening of the primary main switch tube, a common phenomenon may still occur when the load and input voltage change. Often, the current in the common process is limited to avoid machine explosion, but this reduces efficiency.

[0004] Therefore, for isolated power supplies, there is an urgent need for a control solution that can reliably solve the common problems of the primary side main switch tube and the secondary side synchronous rectifier tube without losing efficiency. Summary of the invention

[0005] The object of the present invention is to provide an isolated power supply and a control circuit and a control method thereof, so as to solve the problem in the prior art that efficiency is lost when preventing the main switch tube and the synchronous rectifier tube from being in common.

[0006] In order to achieve the above object, the present invention provides a control circuit of an isolated power supply, comprising:

[0007] A primary side original signal generator, used for receiving a voltage feedback signal of the output voltage of the isolated power supply to generate a primary side original opening signal, wherein the primary side original opening signal includes information of a time when the primary side main switch tube of the isolated power supply should be turned on;

[0008] A secondary control signal generator is used to receive the secondary voltage signal on the secondary winding of the isolated power supply and the primary original opening signal, so as to obtain the information of the opening time of the secondary synchronous rectifier of the isolated power supply, the freewheeling time and dead time of the body diode of the secondary synchronous rectifier, and generate a secondary control signal;

[0009] Among them, in the current switching cycle, if the time when the primary main switch tube should be turned on is earlier than the time when the secondary synchronous rectifier tube should be turned on, the secondary control signal indicates to turn on the secondary synchronous rectifier tube; if the value of the secondary voltage signal is greater than the shutdown threshold of the current switching cycle, the secondary control signal indicates to turn off the secondary synchronous rectifier tube; and the secondary control signal generator also adjusts the shutdown threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle, so that the freewheeling time approaches the freewheeling reference value and the dead time approaches the dead reference value.

[0010] Optionally, the secondary side control signal generator further obtains a secondary side shutdown signal according to the secondary side control signal, the secondary side shutdown signal includes information about the actual shutdown time of the secondary side synchronous rectifier tube, and the control circuit of the isolated power supply further includes:

[0011] A logic controller is used to receive the primary side original turn-on signal and the secondary side turn-off signal to generate a primary side turn-on signal, wherein the primary side turn-on signal includes information about the actual turn-on time of the primary side main switch tube, and the primary side turn-on signal indicates that the actual turn-on time of the primary side main switch tube in the current switching cycle is not earlier than the actual turn-off time of the secondary side synchronous rectifier tube.

[0012] Optionally, in the current switching cycle, if the time when the primary main switch tube should be turned on is later than the actual time when the secondary synchronous rectifier tube is actually turned off, the primary turn-on signal indicates that the primary main switch tube is turned on at the time when the primary main switch tube should be turned on; if the time when the primary main switch tube should be turned on is earlier than the actual time when the secondary synchronous rectifier tube is actually turned off, the primary turn-on signal indicates that the primary main switch tube is turned on at or after the actual time when the secondary synchronous rectifier tube is actually turned off.

[0013] Optionally, the control circuit of the isolated power supply further includes:

[0014] A signal transmitter is used to receive the primary side original opening signal or the primary side opening signal, and modulate the primary side original opening signal or the primary side opening signal and send it from the secondary side to the primary side.

[0015] Optionally, the time when the secondary synchronous rectifier should be turned on is the time when the value of the secondary voltage signal drops to less than the turn-on threshold for the first time; or, the time when the secondary synchronous rectifier should be turned on is the time when the slope of the secondary voltage signal increases to greater than the turn-on slope threshold for the first time.

[0016] Optionally, the secondary side control signal generator includes:

[0017] A time detector, used to receive the secondary side voltage signal and the primary side original opening signal, and detect the freewheeling time and the dead time of the current switching cycle;

[0018] a threshold generator, configured to receive the freewheeling time and the dead time of the current switching cycle, and output the turn-off threshold of the next switching cycle according to a comparison result of the freewheeling time with the freewheeling reference value and a comparison result of the dead time with the dead time reference value; and,

[0019] A control signal generator is used to receive the primary side original turn-on signal, the secondary side voltage signal and the turn-off threshold of the current switching cycle to generate the secondary side control signal.

[0020] Optionally, if the freewheeling time of the current switching cycle is less than the freewheeling reference value or the dead time of the current switching cycle is less than the dead time reference value, the turn-off threshold of the next switching cycle generated by the threshold generator is less than the turn-off threshold of the current switching cycle; if the freewheeling time of the current switching cycle is greater than the freewheeling reference value and the dead time of the current switching cycle is greater than the dead time reference value, the turn-off threshold of the next switching cycle generated by the threshold generator is greater than the turn-off threshold of the current switching cycle.

[0021] The present invention also provides an isolated power supply, comprising:

[0022] An isolation converter having a primary side and a secondary side, wherein the primary side includes a primary main switch tube, and the secondary side includes a secondary synchronous rectifier tube;

[0023] The control circuit of the isolated power supply is used to control the on and off of the secondary-side synchronous rectifier tube.

[0024] Optionally, the isolation converter is a flyback isolation converter.

[0025] Optionally, the control circuit is located on the secondary side, and the isolated power supply also includes a primary side controller located on the primary side, and the primary side controller is used to receive a signal from the control circuit to control the on and off of the primary side main switch tube.

[0026] The present invention also provides a control method for an isolated power supply, comprising:

[0027] Receiving a voltage feedback signal of the output voltage of the isolated power supply to generate a primary original turn-on signal, wherein the primary original turn-on signal includes information of a time when a primary main switch tube of the isolated power supply should be turned on;

[0028] Receiving a secondary voltage signal on the secondary winding of the isolated power supply and the primary original opening signal to obtain information on the opening time of the secondary synchronous rectifier of the isolated power supply, the freewheeling time and dead time of the body diode of the secondary synchronous rectifier, and generating a secondary control signal;

[0029] Among them, in the current switching cycle, if the time when the primary main switch tube should be turned on is earlier than the time when the secondary synchronous rectifier tube should be turned on, the secondary control signal indicates to turn on the secondary synchronous rectifier tube; if the value of the secondary voltage signal is greater than the shutdown threshold of the current switching cycle, the secondary control signal indicates to turn off the synchronous rectifier tube; and the secondary control signal generator also adjusts the shutdown threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle, so that the freewheeling time approaches the freewheeling reference value and the dead time approaches the dead reference value.

[0030] Optionally, the step of adjusting the turn-off threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle includes:

[0031] Comparing the freewheeling time of the current switching cycle with the freewheeling reference value, and comparing the dead time of the current switching cycle with the dead reference value;

[0032] If the freewheeling time of the current switching cycle is less than the freewheeling reference value or the dead time of the current switching cycle is less than the dead time reference value, lower the shutdown threshold of the next switching cycle; if the freewheeling time of the current switching cycle is greater than the freewheeling reference value and the dead time of the current switching cycle is greater than the dead time reference value, increase the shutdown threshold of the next switching cycle.

[0033] The control circuit of the isolated power supply provided by the present invention includes a primary original signal generator and a secondary control signal generator. The primary original signal generator is used to receive a voltage feedback signal of the output voltage of the isolated power supply and generate a primary original turn-on signal containing information about the time when the primary main switch tube should be turned on. The secondary control signal generator is used to receive a secondary voltage signal on a secondary winding and a primary original turn-on signal to generate a secondary control signal. In the current switching cycle, if the primary side main switch tube should be turned on earlier than the secondary side synchronous rectifier tube should be turned on, the secondary side control signal will indicate to turn on the secondary side synchronous rectifier tube, avoiding the problem of the secondary side synchronous rectifier tube SR being turned on by mistake in the DCM mode, and eliminating the possibility of punch-through; at the same time, the secondary side control signal generator also adjusts the turn-off threshold of the next switching cycle according to the freewheeling time and dead time of the current switching cycle, so that the freewheeling time is close to the freewheeling reference value and the dead time is close to the dead reference value. If the value of the secondary side voltage signal is greater than the turn-off threshold of the current switching cycle, the secondary side control signal will indicate to turn off the secondary side synchronous rectifier tube, so that the secondary side synchronous rectifier tube is turned off close to zero current in the steady state, reducing the switching loss and improving the efficiency. Correspondingly, the present invention also provides an isolated power supply and a control method thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic diagram of the structure of an isolated power supply provided in Embodiment 1 of the present invention;

[0035] Figure 2 A specific structural block diagram of a secondary side control signal generator provided in Embodiment 1 of the present invention;

[0036] Figure 3 A possible specific structural block diagram of a threshold generator provided in Embodiment 1 of the present invention;

[0037] Figure 4A This is a working waveform diagram of the isolated power supply provided in the first embodiment of the present invention working in the CCM mode;

[0038] Figure 4B This is a working waveform diagram of the isolated power supply provided in the first embodiment of the present invention working in the DCM mode;

[0039] Figure 5 A schematic diagram of the structure of an isolated power supply provided in Embodiment 2 of the present invention;

[0040] Wherein, the accompanying drawings are marked as follows:

[0041] 100-isolated power supply; 101-control circuit; 102-secondary control signal generator; 112-time detector; 122-threshold generator; 1221-first comparator; 1222-second comparator; 1223-NOR gate; 1224-threshold adjuster 1224; 132-control signal generator; 142-shutdown signal generator; 103-primary original signal generator; 104-logic controller; 105-signal transmitter; 106-signal sensor; 107-load; 108-primary controller;

[0042] RB-rectifier bridge; Cin-input capacitor; CR-absorption circuit; T1-isolation transformer; P1-primary winding; Q1-primary main switch tube; AC-external AC power supply; S1-secondary winding; SR-secondary synchronous rectifier tube; Cout-output filter capacitor; VFB-voltage feedback signal; PSO-primary original opening signal; V_Forw-secondary voltage signal; K1-the time when the primary main switch tube should be turned on; SRoff-secondary shutdown signal; K1'-the actual turn-on time of the secondary synchronous rectifier tube; K2-the actual turn-off time of the secondary synchronous rectifier tube; Tbd-following time; Tdead-dead time; Von-turn-on threshold; Voff-turn-off threshold; K3-the time when the value of the secondary voltage signal rises to above 0V for the first time; Tbd_min-following reference value; Tdead_min-dead reference value; COMP1-

[0043]

[0044] Signal; PSC-primary side start signal; PSG-primary side control signal; Ip-primary side current; Is-secondary side current. DETAILED DESCRIPTION

[0045] The specific implementation of the present invention will be described in more detail below in conjunction with the schematic diagram. The advantages and features of the present invention will become clearer based on the following description. It should be noted that the drawings are all in a very simplified form and are not in exact proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0046] Embodiment 1

[0047] Figure 1 FIG. 1 is a schematic diagram of the structure of the isolated power supply 100 provided in this embodiment. Figure 1As shown, this embodiment provides an isolated power supply 100 and a control circuit 101 thereof, wherein the isolated power supply adopts a flyback power supply topology, including a primary side and a secondary side. The primary side includes a rectifier bridge RB, an input capacitor Cin, an absorption circuit CR, a primary winding P1 of an isolation transformer T1, and a primary main switch tube Q1, and the input end of the rectifier bridge RB is coupled to an external AC power supply AC; the secondary side includes a secondary winding S1 of the isolation transformer T1, a secondary synchronous rectifier tube SR, a control circuit 101, an output filter capacitor Cout, and a load 107. Since the basic topology of the flyback isolated power supply is well known to those skilled in the art, it will not be described in detail here.

[0048] Below Figure 1 Taking the flyback topology shown in the figure as an example, the control circuit 101 of the isolated power supply is described in detail. It should be noted that although Figure 1 The isolated power supply shown is a flyback topology, but the isolated power supply using the control circuit 101 is not limited to the flyback topology. A person of ordinary skill in the art can understand that any isolated power supply topology that has common risks between the primary main switch tube and the secondary synchronous rectifier tube, such as single-ended forward, dual-tube forward, active clamped forward, resonant half-bridge LLC, resonant full-bridge LLC, phase-shifted full-bridge, etc., can be adapted to the control circuit 101 shown in this embodiment.

[0049] like Figure 1 As shown, the control circuit 101 includes a primary side original signal generator 103 and a secondary side control signal generator 102 .

[0050] The primary side original signal generator 103 is used to receive the voltage feedback signal VFB of the output voltage VOUT of the isolated power supply and generate the primary side original opening signal PSO, the primary side original opening signal PSO at least includes the information of the time K1 at which the primary side main switch tube Q1 should be turned on, and is used to indicate turning on the primary side main switch tube Q1. In some embodiments, the primary side original signal generator 103 may include comparing the voltage feedback signal VFB with a voltage reference value, and when the voltage feedback signal VFB drops to the voltage reference value, the primary side original opening signal PSO is generated, and the time when the voltage feedback signal VFB drops to the voltage reference value is used as the time K1 at which the primary side main switch tube Q1 should be turned on.

[0051] The secondary control signal generator 102 is used to receive the secondary voltage signal V_Forw and the primary original turn-on signal PSO on the secondary winding S1, and generate a secondary control signal SRG according to the secondary voltage signal V_Forw and the primary original turn-on signal PSO. The secondary control signal SRG includes information about the actual turn-on time K1' and the actual turn-off time K2 of the secondary synchronous rectifier SR, which are used to indicate the turn-on and turn-off of the secondary synchronous rectifier SR, respectively.

[0052] Specifically, after the secondary side control signal generator 102 receives the secondary side voltage signal V_Forw and the primary side original turn-on signal PSO, it first obtains the information of the turn-on time of the secondary side synchronous rectifier SR, the freewheeling time Tbd and the dead time Tdead of the body diode of the secondary side synchronous rectifier SR according to the secondary side voltage signal V_Forw and the primary side original turn-on signal PSO.

[0053] In this embodiment, the time when the secondary synchronous rectifier SR should be turned on is the time when the value of the secondary voltage signal V_Forw drops to less than an on-threshold value Von for the first time, and the on-threshold value Von can be, for example, a negative value such as -0.8 V. In some embodiments, the time K4 when the secondary synchronous rectifier SR should be turned on can also be the time when the slope of the secondary voltage signal V_Forw increases to greater than an on-slope threshold value for the first time.

[0054] The freewheeling time Tbd of the body diode of the secondary synchronous rectifier SR is the time between the actual turn-off moment K2 of the secondary synchronous rectifier SR and the moment K3 when the value of the secondary voltage signal V_Forw first rises to a value higher than 0V; the dead time Tdead is the time between the actual turn-off moment K2 of the secondary synchronous rectifier SR and the moment K1 when the primary main switch Q1 should be turned on.

[0055] It should be understood that, in this article, "the time K1 at which the primary main switch Q1 should be turned on" means the time at which the primary main switch Q1 should be turned on, obtained only based on the design principle of the feedback control loop of the isolated power supply, using the voltage feedback signal VFB and other possible feedback parameters (such as the current feedback signal of the output current of the isolated power supply), as well as the internal clock signal and / or feedback loop compensation. The "time K1 at which the primary main switch Q1 should be turned on" may be obtained by directly including the information of the time K1 at which it should be turned on in a certain signal generated, or by obtaining the time information contained in a certain signal through a specific operation, such as adding or subtracting a specific time offset, to obtain the value of the time K1 at which it should be turned on. The "voltage on the secondary winding S1" is the voltage at the non-fixed potential end of the secondary winding S1, that is, the voltage at the common connection end between the secondary synchronous rectifier SR and the secondary winding S1. The term "secondary voltage signal V_Forw on the secondary winding S1" is defined as a signal that can represent the voltage on the secondary winding S1, for example, a signal formed by the voltage on the secondary winding S1 passing through a voltage divider, or a signal directly sampled from a non-fixed potential terminal on the secondary winding S1. Figure 1In the embodiment shown, the secondary synchronous rectifier SR is connected between the secondary winding S1 and the secondary ground SGND. At this time, the same-name end of the secondary winding S1 directly provides the output voltage VOUT. The output voltage VOUT does not change in the steady state, so it is a fixed potential end; the opposite-name end of the secondary winding S1 is connected to the secondary synchronous rectifier SR, and the voltage on the opposite-name end is the voltage on the secondary winding S1; in other embodiments, the secondary synchronous rectifier SR can also be connected to Figure 1 Between the secondary winding S1 and the output terminal of the isolated power supply shown in FIG. 1 , the voltage on the same-name terminal is the voltage on the secondary winding S1 .

[0056] It should be understood that the turn-on time K1 of the primary main switch tube Q1 and the turn-on time, actual turn-on time K1' and actual turn-off time K2 of the secondary synchronous rectifier tube SR are for each switching cycle, and multiple turn-on times K1 of the primary main switch tube Q1 and multiple turn-on times, actual turn-on times K1' and actual turn-off times K2 of the secondary synchronous rectifier tube SR will be generated in multiple switching cycles. Correspondingly, the freewheeling time Tbd and the dead time Tdead are also for each switching cycle, and multiple freewheeling times Tbd and dead times Tdead will be generated in multiple switching cycles.

[0057] In the current switching cycle, the secondary synchronous rectifier SR will be turned on only when the turn-on time K1 of the primary main switch Q1 is earlier than the turn-on time of the secondary synchronous rectifier SR (the secondary synchronous rectifier SR will be turned on only after the primary main switch Q1 is turned on). This avoids the problem of the secondary synchronous rectifier SR being turned on incorrectly due to the voltage resonance waveform on the secondary winding S1 in the DCM mode reaching the pre-set condition for turning on the secondary synchronous rectifier SR, and eliminates the possibility of punch-through.

[0058] Furthermore, in the current switching cycle, if the value of the secondary voltage signal V_Forw is greater than the turn-off threshold Voff_n of the current switching cycle, the secondary control signal SRG indicates to turn off the synchronous rectifier SR. In this way, the turn-off of the secondary synchronous rectifier SR does not depend on the conduction detection of the primary main switch Q1. When working in a steady state, the secondary control signal generator 102 can basically ensure that the secondary synchronous rectifier SR and the primary main switch Q1 are prevented from being connected in common without additional anti-punch-through measures or anti-explosion measures. At the same time, the secondary control signal generator 102 also adjusts the turn-off threshold Voff_n+1 of the next switching cycle according to the freewheeling time Tbd_n and the dead time Tdead_n of the current switching cycle, so that the freewheeling time is close to the freewheeling reference value Tbd_min and the dead time is close to the dead reference value Tdead_min, so that the secondary synchronous rectifier SR is turned off close to zero current in the steady state, preventing oscillation after zero crossing due to the continued opening of the secondary synchronous rectifier SR, reducing switching losses and improving efficiency.

[0059] Figure 2 Detailed structural block diagram of the secondary side control signal generator 102 provided in this embodiment. Figure 2 As shown, the secondary-side control signal generator 102 includes a time detector 112 , a threshold generator 122 , a control signal generator 132 and a shutdown signal generator 142 .

[0060] The time detector 112 is used to receive the secondary voltage signal V_Forw and the primary original opening signal PSO, and detect the freewheeling time Tbd_n and the dead time Tdead_n of the current switching cycle. Regarding how to detect the freewheeling time and the dead time, there are corresponding design schemes in the prior art, such as by detecting the change of the control signal, the driving signal or the winding voltage of each switch tube, etc. The present invention will not be repeated here. Those skilled in the art can understand that any method that can detect the freewheeling time and the dead time can be applied to the present embodiment to obtain the corresponding effect.

[0061] The threshold generator 122 is used to receive the freewheeling time Tbd_n and the dead time Tdead_n of the current switching cycle, and output the turn-off threshold Voff_n+1 of the next switching cycle according to the comparison result Tdead_n between the freewheeling time Tbd_n and the freewheeling reference value Tbd_min and the comparison result between the dead time Tdead_n and the dead reference value Tdead_min.

[0062] The freewheeling reference value Tbd_min and the dead zone reference value Tdead_min may be, for example, 100 ns, and the initial value of the shutdown threshold may be, for example, set to -0.2 V, but should not be limited thereto.

[0063] Specifically, if the freewheeling time Tbd_n of the current switching cycle is less than the freewheeling reference value Tbd_min, it indicates that the secondary winding is about to end demagnetization and the freewheeling time is already very small. If the secondary synchronous rectifier SR is turned off later, current reflux may occur; if the dead time Tdead_n of the current switching cycle is less than the dead time reference value Tdead_min, it indicates that the closing time of the secondary synchronous rectifier SR affects the control of the primary main switch Q1 by the original turn-on signal PSO of the primary side, and the secondary synchronous rectifier SR needs to be turned off in advance; in both cases, the secondary synchronous rectifier SR needs to be turned off in advance. Therefore, if the freewheeling time Tbd_n of the current switching cycle is less than the freewheeling reference value Tbd_min or the dead time Tdead_n of the current switching cycle is less than the dead time reference value Tdead_min, the turn-off threshold Voff_n+1 of the next switching cycle is less than the turn-off threshold Voff_n of the current switching cycle, which is equivalent to lowering the turn-off threshold Voff_n+1 of the next switching cycle to turn off the secondary synchronous rectifier SR in advance.

[0064] Correspondingly, if the freewheeling time Tbd_n of the current switching cycle is greater than the freewheeling reference value Tbd_min and the dead time Tdead_n of the current switching cycle is greater than the dead time reference value Tdead_min, it indicates that the closing time of the secondary synchronous rectifier tube SR will not affect the control of the primary original turn-on signal PSO on the primary main switch tube Q1, and the freewheeling time is large enough, and the turn-off threshold Voff_n+1 of the next switching cycle is greater than the turn-off threshold Voff_n of the current switching cycle, which is equivalent to increasing the turn-off threshold Voff_n+1 of the next switching cycle to delay the turn-off of the secondary synchronous rectifier tube SR, thereby improving efficiency.

[0065] Figure 3 A possible specific structural block diagram of the threshold generator 122 provided in this embodiment. Figure 3 As shown, the threshold generator 122 includes a first comparator 1221 , a second comparator 1222 , a NOR gate 1223 and a threshold adjuster 1224 .

[0066] Specifically, the negative input terminal of the first comparator 1221 is used to receive the freewheeling time Tbd_n of the current switching cycle, the positive input terminal of the first comparator 1221 is used to receive the freewheeling reference value Tbd_min, the first comparator 1221 compares the freewheeling time Tbd_n of the current switching cycle with the freewheeling reference value Tbd_min, and generates a first comparison signal COMP1 reflecting the size information of the freewheeling time Tbd_n of the current switching cycle and the freewheeling reference value Tbd_min. The negative input terminal of the second comparator 1222 is used to receive the dead time Tdead_n of the current switching cycle, the positive input terminal of the second comparator 1222 is used to receive the dead time reference value Tdead_min, the second comparator 1222 compares the dead time Tdead_n of the current switching cycle with the dead time reference value Tdead_min, and generates a second comparison signal COMP2 reflecting the size information of the dead time Tdead_n of the current switching cycle and the dead time reference value Tdead_min.

[0067] The first input terminal of the NOR gate 1223 is used to receive the first comparison signal COMP1 , and the second input terminal is used to receive the second comparison signal COMP2 , and performs a NOR logic operation on the first comparison signal COMP1 and the second comparison signal COMP2 to generate a threshold control signal Voff_ctrl.

[0068] The input end of the threshold adjuster 1224 is used to receive the threshold control signal Voff_ctrl, and adjust the turn-off threshold Voff_n of the current switching cycle according to the threshold control signal Voff_ctrl to generate the turn-off threshold Voff_n+1 of the next switching cycle. Specifically, the turn-off threshold Voff_n of the current switching cycle is built into the threshold adjuster 1224. If the freewheeling time Tbd_n of the current switching cycle is less than the freewheeling reference value Tdead_min or the dead time Tdead_n of the current switching cycle is less than the dead time reference value Tdead_min, the threshold control signal Voff_ctrl indicates that the turn-off threshold Voff_n needs to be lowered, and the threshold adjuster 1224 lowers the turn-off threshold Voff_n according to the indication of the threshold control signal Voff_ctrl, and uses the lowered turn-off threshold Voff_n as the next switch. The turn-off threshold Voff_n+1 of the cycle is output; if the freewheeling time Tbd_n of the current switching cycle is greater than the freewheeling reference value Tdead_min and the dead time Tdead_n of the current switching cycle is greater than the dead time reference value Tdead_min, the threshold control signal Voff_ctrl indicates that the turn-off threshold Voff_n needs to be increased, and the threshold adjuster 1224 increases the turn-off threshold Voff_n according to the indication of the threshold control signal Voff_ctrl, and outputs the increased turn-off threshold Voff_n as the turn-off threshold Voff_n+1 of the next switching cycle.

[0069] Furthermore, the control signal generator 132 is used to receive the primary side original turn-on signal PSO, the secondary side voltage signal V_Forw and the turn-off threshold Voff_n of the current switching cycle to generate a secondary side control signal SRG to control the on and off of the secondary side synchronous rectifier SR.

[0070] Next, the shutdown signal generator 142 receives the secondary control signal SRG to generate a secondary shutdown signal SRoff, which includes information about the actual shutdown time K2 of the secondary synchronous rectifier SR. For example, the shutdown signal generator 142 can obtain the secondary shutdown signal SRoff by collecting the falling edge of the secondary control signal SRG.

[0071] The secondary-side shutdown signal SRoff may be generated after the secondary-side control signal SRG. The actual turn-on time K1' and the actual turn-off time K2 of the secondary-side synchronous rectifier SR indicated by the secondary-side control signal SRG are theoretical actual turn-on time and actual turn-off time, which may be slightly earlier than the actual turn-on time and actual turn-off time of the secondary-side synchronous rectifier SR. However, no matter what the relationship between the secondary-side shutdown signal SRoff and the secondary-side control signal SRG is, as long as the secondary-side control signal SRG contains the information of the actual turn-on time K1' and the actual turn-off time K2 of the secondary-side synchronous rectifier SR, the purpose of the present invention can be achieved.

[0072] In some embodiments, the secondary-side control signal SRG may not directly or indirectly generate the secondary-side shutdown signal SRoff, and the secondary-side shutdown signal SRoff may have a common source signal with the secondary-side control signal SRG, which source signal includes information about the actual turn-on time K1' and the actual turn-off time K2 of the secondary-side synchronous rectifier SR or includes all parameters that can calculate the actual turn-on time K1' and the actual turn-off time K2 of the secondary-side synchronous rectifier SR.

[0073] In some other embodiments, the secondary-side control signal SRG may be directly used as a driving signal of the secondary-side synchronous rectifier SR to act on the gate of the secondary-side synchronous rectifier SR to turn it on or off.

[0074] Please continue reading Figure 1 , the control circuit 101 further includes a logic controller 104, which is used to receive the secondary side shutdown signal SRoff and the primary side original turn-on signal PSO to generate a primary side turn-on signal PSC, the primary side turn-on signal PSC at least including the actual turn-on time of the primary side main switch tube Q1, used to indicate the turn-on of the primary side main switch tube Q1. Specifically, the logic controller 104 can determine the actual turn-off time K2 of the secondary side synchronous rectifier tube SR based on the secondary side shutdown signal SRoff, and determine the turn-on time K1 of the primary side main switch tube Q1 based on the primary side original turn-on signal PSO. The logic controller 104 further determines the actual turn-on time of the primary side main switch tube Q1 based on the turn-on time K1 of the primary side main switch tube Q1 and the actual turn-off time K2 of the secondary side synchronous rectifier tube SR to generate the primary side turn-on signal PSC, the primary side turn-on signal PSC indicating that the actual turn-on time of the primary side main switch tube Q1 in the current switching cycle is not earlier than the actual turn-off time K2 of the secondary side synchronous rectifier tube SR.

[0075] Further, in the current switching cycle, if the time K1 at which the primary main switch tube Q1 should be turned on is later than the actual time K2 at which the secondary synchronous rectifier tube SR is actually turned off, the primary turn-on signal PSC indicates that the primary main switch tube Q1 is turned on at the time K1 at which the primary main switch tube Q1 should be turned on, that is, the actual turn-on time of the primary main switch tube Q1 corresponds to the time K1 at which the primary main switch tube Q1 should be turned on; if the time K1 at which the primary main switch tube Q1 should be turned on is earlier than the actual time K2 at which the secondary synchronous rectifier tube SR is actually turned off, the primary turn-on signal PSC indicates that the primary main switch tube Q1 is turned on at or after the actual time K2 at which the secondary synchronous rectifier tube SR is actually turned off, that is, the actual turn-on time of the primary main switch tube Q1 is delayed to be no earlier than the actual time K2 at which the secondary synchronous rectifier tube SR is actually turned off.

[0076] In some embodiments, the primary-side turn-on signal PSC may be a level signal, for example, a rising edge may prompt the turn-on of the primary-side main switch Q1 .

[0077] In this way, the control circuit 101 not only independently determines the actual turn-off time K2 of the secondary-side synchronous rectifier tube SR without relying on the detection of the conduction of the primary-side main switch tube Q1, but also uses the actual turn-off time K2 of the secondary-side synchronous rectifier tube SR as the basis for determining the actual turn-on time of the primary-side main switch tube Q1, thereby achieving the purpose of preventing penetration without delaying the primary-side turn-on signal PSC. In an environment without a risk of punch-through, for example, when the isolated power supply is in the DCM mode or in the CCM mode, the primary main switch tube Q1 can select the turn-on time K1 to determine the actual turn-on time of the primary main switch tube Q1 under the feedback control operation of the control feedback loop of the primary main switch tube Q1 in the control circuit 101 without adding any additional delay processing, so that the feedback response and circuit performance are not affected by the anti-punch-through design. At the same time, when the isolated power supply operates under the condition of a risk of punch-through, for example, when a load jump suddenly occurs in the CCM mode, the primary main switch tube Q1 can only be turned on after the secondary synchronous rectifier tube SR is turned off when there is a certain risk of punch-through, which not only eliminates the possibility of punch-through, but also because the control circuit 101 intervenes and corrects the actual turn-on time of the primary main switch tube Q1 in a more accurate manner only when it is determined that a high risk of punch-through occurs, the impact on the feedback response performance is also controlled to a smaller extent.

[0078] Furthermore, the isolated power system 100 further includes a primary side controller 108 and a signal sensor 106 .

[0079] The primary controller 108 is located on the primary side, receives the primary turn-on signal PSC, and generates the primary control signal PSG according to the information of the actual turn-on time of the primary main switch tube Q1 contained in the primary turn-on signal PSC. At this time, the primary controller 108 can be combined with other suitable feedback signals, such as a signal for detecting the current of the primary main switch tube Q1 or a signal for detecting the demagnetization of the drain of the primary main switch tube Q1 to determine the actual turn-off time of the primary main switch tube Q1. For ordinary technicians in this field, it can be designed according to specific needs and feedback characteristics to use suitable signals to determine the actual turn-off time of the primary main switch tube Q1, and the present invention is not limited to this.

[0080] The signal sensor 106 is used to sense the output voltage VOUT of the isolated power supply and generate a voltage feedback signal VFB. In this embodiment, the signal sensor 106 is located outside the control circuit 101 and is composed of discrete components. In other embodiments, the signal sensor 106 can also be integrated with the control circuit 101 in a die or chip. Common signal sensors 106 may include a resistor divider, etc., which are well known to those skilled in the art and will not be described in detail.

[0081] Furthermore, in this embodiment, the control circuit 101 is integrally connected to the secondary side of the isolated power supply 100. Figure 1 As shown, the control circuit 101 further includes a signal transmitter 105, which is used to receive the primary side opening signal PSC, and modulate the primary side opening signal PSC and send it from the secondary side to the primary side. Specifically, the signal transmitter 105 can use any common primary-secondary side isolation communication method, such as optical coupling, magnetic coupling or capacitive coupling transmission, or use on-off keying (OOK) technology to modulate and generate pulses to send the primary side opening signal PSC.

[0082] Figure 4A and Figure 4B for Figure 1 The working waveform diagram of the control circuit 101, Figure 4A : is a working waveform diagram of the isolated power supply 100 working in the CCM mode. Figure 4B The isolated power supply 100 is a working waveform diagram when working in the DCM mode. Figure 1 , 4A and Figure 4B As an example, the working principle of the control circuit 101 is explained. For the convenience of description, the segmented primary current Ip and secondary current Is are combined into the same waveform here and below, and are collectively referred to as "inductor current". Those skilled in the art can understand that the meaning of the term "inductor current" in the illustrated embodiment is not a real current, but a combination of the waveforms of the primary current Ip and the secondary current Is. At the same time, when explaining the waveform diagram here and below, for the convenience of explanation, it is assumed that the conduction time of the secondary synchronous rectifier SR can be adjusted with the change of the load 107.

[0083] like Figure 4A and Figure 4BAs shown, at time K1, the primary side main switch tube Q1 is turned on, the secondary side voltage signal V_Frow rises to a positive voltage, and the inductor current starts to rise until the primary side main switch tube Q1 is turned off, and the secondary side voltage signal V_Frow drops from a positive voltage to a negative voltage. In the freewheeling stage of the secondary side, since the secondary side control signal generator 102 has received the rising edge of the primary side original turn-on signal PSO (indicating that the turn-on time K1 of the primary side main switch tube Q1 has arrived), as long as the value of the secondary side voltage signal V_Frow is less than the turn-on threshold Von for the first time (indicating that the turn-on time of the secondary side synchronous rectifier tube SR has arrived), the secondary side control signal SRG jumps to a high level at time K1', indicating that the secondary side synchronous rectifier tube SR is turned on. In terms of timing, it can be ensured that the secondary side synchronous rectifier tube SR is turned on only after the primary side main switch tube Q1 is turned on. After that, the inductor current starts to decrease. From then on, until the next K1 moment, regardless of whether the waveform of the secondary voltage signal V_Forw reaches the pre-set condition for turning on the secondary synchronous rectifier SR (such as the value of the secondary voltage signal V_Frow being less than the turn-on threshold Von), the secondary synchronous rectifier SR will not be turned on. When the inductor current decreases to the point where the value of the secondary voltage signal V_Frow reaches a value greater than the turn-off threshold Voff_n of the current switching cycle (indicating that the demagnetization stage is about to end), the secondary control signal SRG jumps to a low level at K2 moment, indicating that the secondary synchronous rectifier SR is turned off.

[0084] like Figure 1 and Figure 4A As shown, in DCM mode, assuming that after the end of switching cycle n-1, the freewheeling time Tbd_n-1 of switching cycle n-1 is greater than the freewheeling reference value Tbd_min, the turn-off threshold Voff_n of switching cycle n is increased, so that the secondary synchronous rectifier tube SR is delayed to turn off in switching cycle n; assuming that after the end of switching cycle n, the freewheeling time Tbd_n in switching cycle n is less than the freewheeling reference value Tbd_min, the turn-off threshold Voff_n+1 of switching cycle n+1 is reduced, so that the secondary synchronous rectifier tube SR is turned off in advance in switching cycle n+1, and the freewheeling time Tbd_n+1 of switching cycle n+1 is increased. By properly selecting the adjustment range of the turn-off threshold Voff, the freewheeling time Tbd in steady state can be close to the freewheeling reference value Tbd_min.

[0085] like Figure 1 and Figure 4BAs shown in the figure, in CCM mode, assuming that after the switching cycle n-1 ends, the dead time Tdead_n-1 is greater than the dead time reference value Tdead_min, the turn-off threshold Voff_n of the switching cycle n is increased, so that the secondary synchronous rectifier tube SR is delayed to turn off in the switching cycle n; if after the switching cycle n ends, the dead time Tdead in the switching cycle n is less than the dead time reference value Tdead_min, the turn-off threshold Voff_n+1 of the switching cycle n+1 is reduced, so that the secondary synchronous rectifier tube SR is turned off in advance in the switching cycle n+1, and the dead time Tdead_n+1 of the switching cycle n+1 is increased. By properly selecting the adjustment range of the turn-off threshold Voff, the dead time Tdead in the steady state can be close to the dead time reference value Tdead_min.

[0086] Based on this, this embodiment further provides a control method for an isolated power supply, and the control method for an isolated power supply includes:

[0087] Step S100: receiving a voltage feedback signal VFB of the output voltage VOUT of the isolated power supply 100 to generate a primary original turn-on signal PSO, wherein the primary original turn-on signal PSO includes information of a turn-on time K1 of the primary main switch Q1 of the isolated power supply 100;

[0088] Step S200: receiving the secondary voltage signal V_Forw and the primary original opening signal PSO on the secondary winding S1 of the isolated power supply 100, so as to obtain the information of the opening time of the secondary synchronous rectifier SR of the isolated power supply 100, the freewheeling time Tbd and the dead time Tdead of the body diode of the secondary synchronous rectifier SR, and generate the secondary control signal SRG;

[0089] Among them, in the current switching cycle, if the turn-on time K1 of the primary side main switch tube Q1 is earlier than the turn-on time of the secondary side synchronous rectifier tube SR, the secondary side control signal SRG indicates to turn on the secondary side synchronous rectifier tube SR, and if the value of the secondary side voltage signal V_Forw is greater than the turn-off threshold Voff_n of the current switching cycle, the secondary side control signal SRG indicates to turn off the secondary side synchronous rectifier tube SR; and the secondary side control signal generator 102 also adjusts the turn-off threshold Voff_n+1 of the next switching cycle according to the freewheeling time Tbd_n and the dead time Tdead_n of the current switching cycle, so that the freewheeling time approaches the freewheeling reference value Tbd_min and the dead time approaches the dead reference value Tdead_min.

[0090] Furthermore, the step of adjusting the turn-off threshold Voff_n+1 of the next switching cycle according to the freewheeling time Tbd_n and the dead time Tdead_n of the current switching cycle includes:

[0091] Step S201: comparing the freewheeling time Tbd_n of the current switching cycle with the freewheeling reference value Tbd_min, and comparing the dead time Tdead_n of the current switching cycle with the dead reference value Tdead_min;

[0092] Step S202: If the freewheeling time Tbd_n of the current switching cycle is less than the freewheeling reference value Tbd_min or the dead time Tdead_n of the current switching cycle is less than the dead time reference value Tdead_min, reduce the shutdown threshold Voff_n+1 of the next switching cycle; if the freewheeling time Tbd_n of the current switching cycle is greater than the freewheeling reference value Tbd_min and the dead time Tdead_n of the current cycle is greater than the dead time reference value Tdead_min, increase the shutdown threshold Tdead_min of the next switching cycle.

[0093] Embodiment 2

[0094] Figure 5 FIG. 1 is a schematic diagram of an isolated power supply 100 provided in this embodiment. Figure 5 As shown, the difference from the first embodiment is that in this embodiment, the logic controller is omitted, and the signal transmitter 105 directly modulates the primary side original opening signal PSO and sends it from the secondary side to the primary side. In this way, in the current switching cycle, the actual opening time of the primary side main switch tube Q1 will correspond to the opening time K1.

[0095] It should be noted that the primary controller 108 can be combined with other appropriate feedback signals, such as a signal for detecting the current of the primary main switch tube Q1 or a signal for detecting the demagnetization of the drain of the primary main switch tube Q1 to determine the actual turn-off time of the primary main switch tube Q1 to generate the primary control signal PSG. For those skilled in the art, the actual turn-off time of the primary main switch tube Q1 can be determined by using an appropriate signal according to specific needs and feedback characteristics, and the present invention is not limited to this.

[0096] In summary, the control circuit of the isolated power supply provided in the embodiment of the present invention includes a primary original signal generator and a secondary control signal generator. The primary original signal generator is used to receive the voltage feedback signal of the output voltage of the isolated power supply, and generate a primary original turn-on signal containing information about the time when the primary main switch tube should be turned on. The secondary control signal generator is used to receive the secondary voltage signal and the primary original turn-on signal on the secondary winding to generate a secondary control signal. In the current switching cycle, if the primary side main switch tube should be turned on earlier than the secondary side synchronous rectifier tube should be turned on, the secondary side control signal will indicate to turn on the secondary side synchronous rectifier tube, avoiding the problem of the secondary side synchronous rectifier tube SR being turned on by mistake in the DCM mode, and eliminating the possibility of punch-through; at the same time, the secondary side control signal generator also adjusts the turn-off threshold of the next switching cycle according to the freewheeling time and dead time of the current switching cycle, so that the freewheeling time is close to the freewheeling reference value and the dead time is close to the dead reference value. If the value of the secondary side voltage signal is greater than the turn-off threshold of the current switching cycle, the secondary side control signal will indicate to turn off the secondary side synchronous rectifier tube, so that the secondary side synchronous rectifier tube is turned off close to zero current in the steady state, reducing the switching loss and improving the efficiency. Correspondingly, the present invention also provides an isolated power supply and a control method thereof.

[0097] It should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same or similar parts between the various embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part description.

[0098] It should also be noted that, although the present invention has been disclosed as a preferred embodiment, the above embodiment is not intended to limit the present invention. For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments of equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

[0099] It should also be understood that, unless otherwise specified or indicated, the terms "first", "second", "third", etc. in the specification are merely used to distinguish between the various components, elements, steps, etc. in the specification, and are not used to indicate the logical relationship or sequential relationship between the various components, elements, steps, etc.

[0100] It should also be recognized that the terms described herein are only used to describe specific embodiments and are not intended to limit the scope of the invention. It should be noted that the singular forms "a" and "an" used herein and in the appended claims include plural references unless the context clearly indicates otherwise. For example, a reference to "a step" or "a device" means a reference to one or more steps or devices, and may include secondary steps and secondary devices. All conjunctions used should be understood in the broadest sense. And, the word "or" should be understood to have the definition of a logical "or", rather than a logical "exclusive or", unless the context clearly indicates otherwise. In addition, the implementation of the method and / or device in the embodiments of the present invention may include performing the selected task manually, automatically, or in combination.

Claims

1. A control circuit of an isolated power supply, characterized in that: include: A primary side original signal generator, used for receiving a voltage feedback signal of the output voltage of the isolated power supply to generate a primary side original opening signal, wherein the primary side original opening signal includes information of a time when the primary side main switch tube of the isolated power supply should be turned on; A secondary control signal generator is used to receive the secondary voltage signal on the secondary winding of the isolated power supply and the primary original opening signal, so as to obtain the information of the opening time of the secondary synchronous rectifier of the isolated power supply, the freewheeling time and dead time of the body diode of the secondary synchronous rectifier, and generate a secondary control signal; Among them, in the current switching cycle, if the time when the primary main switch tube should be turned on is earlier than the time when the secondary synchronous rectifier tube should be turned on, the secondary control signal indicates to turn on the secondary synchronous rectifier tube; if the value of the secondary voltage signal is greater than the shutdown threshold of the current switching cycle, the secondary control signal indicates to turn off the secondary synchronous rectifier tube; and the secondary control signal generator also adjusts the shutdown threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle, so that the freewheeling time approaches the freewheeling reference value and the dead time approaches the dead reference value.

2. The control circuit of the isolated power supply according to claim 1, characterized in that: The secondary side control signal generator further obtains a secondary side shutdown signal according to the secondary side control signal, wherein the secondary side shutdown signal includes information about the actual shutdown time of the secondary side synchronous rectifier tube. The control circuit of the isolated power supply further includes: A logic controller is used to receive the primary side original turn-on signal and the secondary side turn-off signal to generate a primary side turn-on signal, wherein the primary side turn-on signal includes information about the actual turn-on time of the primary side main switch tube, and the primary side turn-on signal indicates that the actual turn-on time of the primary side main switch tube in the current switching cycle is not earlier than the actual turn-off time of the secondary side synchronous rectifier tube.

3. The control circuit of the isolated power supply according to claim 2, characterized in that: In the current switching cycle, if the time when the primary main switch should be turned on is later than the actual time when the secondary synchronous rectifier is turned off, the primary turn-on signal indicates that the primary main switch is turned on at the time when the primary main switch should be turned on; If the time when the primary main switch should be turned on is earlier than the actual time when the secondary synchronous rectifier is turned off, the primary turn-on signal indicates that the primary main switch is turned on at or after the actual time when the secondary synchronous rectifier is turned off.

4. The control circuit of the isolated power supply according to claim 2 or 3, characterized in that: The control circuit of the isolated power supply further includes: A signal transmitter is used to receive the primary side original opening signal or the primary side opening signal, and modulate the primary side original opening signal or the primary side opening signal and send it from the secondary side to the primary side.

5. The control circuit of the isolated power supply according to claim 1, characterized in that: The secondary synchronous rectifier should be turned on when the value of the secondary voltage signal drops to less than the turn-on threshold for the first time; or, the secondary synchronous rectifier should be turned on when the slope of the secondary voltage signal increases to greater than the turn-on slope threshold for the first time.

6. The control circuit of the isolated power supply according to claim 1, characterized in that: The secondary side control signal generator comprises: A time detector, used to receive the secondary side voltage signal and the primary side original opening signal, and detect the freewheeling time and the dead time of the current switching cycle; a threshold generator, configured to receive the freewheeling time and the dead time of the current switching cycle, and output the turn-off threshold of the next switching cycle according to a comparison result of the freewheeling time with the freewheeling reference value and a comparison result of the dead time with the dead time reference value; and, A control signal generator is used to receive the primary side original turn-on signal, the secondary side voltage signal and the turn-off threshold of the current switching cycle to generate the secondary side control signal.

7. The control circuit of the isolated power supply according to claim 6, characterized in that: If the freewheeling time of the current switching cycle is less than the freewheeling reference value or the dead time of the current switching cycle is less than the dead time reference value, the turn-off threshold of the next switching cycle generated by the threshold generator is less than the turn-off threshold of the current switching cycle; if the freewheeling time of the current switching cycle is greater than the freewheeling reference value and the dead time of the current switching cycle is greater than the dead time reference value, the turn-off threshold of the next switching cycle generated by the threshold generator is greater than the turn-off threshold of the current switching cycle.

8. An isolated power supply, characterized in that: include: An isolation converter having a primary side and a secondary side, wherein the primary side includes a primary main switch tube, and the secondary side includes a secondary synchronous rectifier tube; The control circuit of the isolated power supply according to any one of claims 1 to 7 is used to control the on and off of the secondary synchronous rectifier.

9. The isolated power supply according to claim 8, characterized in that: The isolation converter is a flyback isolation converter.

10. The isolated power supply according to claim 8, characterized in that: The control circuit is located on the secondary side, and the isolated power supply also includes a primary side controller located on the primary side. The primary side controller is used to receive a signal from the control circuit to control the on and off of the primary side main switch tube.

11. A control method for an isolated power supply, characterized in that: include: Receiving a voltage feedback signal of the output voltage of the isolated power supply to generate a primary original turn-on signal, wherein the primary original turn-on signal includes information of a time when a primary main switch tube of the isolated power supply should be turned on; Receiving a secondary voltage signal on the secondary winding of the isolated power supply and the primary original opening signal to obtain information on the opening time of the secondary synchronous rectifier of the isolated power supply, the freewheeling time and dead time of the body diode of the secondary synchronous rectifier, and generating a secondary control signal; Among them, in the current switching cycle, if the time when the primary main switch tube should be turned on is earlier than the time when the secondary synchronous rectifier tube should be turned on, the secondary control signal indicates to turn on the secondary synchronous rectifier tube; if the value of the secondary voltage signal is greater than the shutdown threshold of the current switching cycle, the secondary control signal indicates to turn off the synchronous rectifier tube; and the secondary control signal generator also adjusts the shutdown threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle, so that the freewheeling time approaches the freewheeling reference value and the dead time approaches the dead reference value.

12. The control method of the isolated power supply according to claim 11, characterized in that: The step of adjusting the turn-off threshold of the next switching cycle according to the freewheeling time and the dead time of the current switching cycle includes: Comparing the freewheeling time of the current switching cycle with the freewheeling reference value, and comparing the dead time of the current switching cycle with the dead reference value; If the freewheeling time of the current switching cycle is less than the freewheeling reference value or the dead time of the current switching cycle is less than the dead time reference value, lower the shutdown threshold of the next switching cycle; if the freewheeling time of the current switching cycle is greater than the freewheeling reference value and the dead time of the current switching cycle is greater than the dead time reference value, increase the shutdown threshold of the next switching cycle.

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