CN-121984331-A - Synchronous rectification simulation backflow solving method

CN121984331ACN 121984331 ACN121984331 ACN 121984331ACN-121984331-A

Abstract

The application relates to a synchronous rectification simulation backward flow solving method, which relates to the technical field of synchronous rectification and comprises the following steps of firstly, pre-judging backward flow risk, synchronously detecting the instantaneous values of drain-source voltage Vds and flowing current Ids of a synchronous rectification switching tube at the turn-off decision time of the synchronous rectification switching tube, judging that a high backward flow risk scene exists if the Vds is lower than a preset low-voltage threshold value and the Ids is higher than a preset current threshold value, starting pre-turn-off impedance modulation immediately and adaptively enhancing the modulation depth, and otherwise, executing conventional turn-off logic. According to the application, through the linkage of the risk prejudgment and the pre-turn-off impedance modulation, the switching tube is placed in the critical conduction region in the early turn-off stage, so that the on resistance of the switching tube is naturally increased, the current change rate is gently limited, the severe voltage recoil and ringing caused by current mutation are avoided from the source, and the switching tube is effectively protected.

Inventors

NIE XIAOZHI
LIU WANLE

Assignees

苏州美思迪赛半导体技术有限公司

Dates

Publication Date: 20260505
Application Date: 20251211

Claims (8)

1. The synchronous rectification simulation backward flow solving method is characterized by comprising the following steps of: Step one, the risk of backward flow is prejudged, at the turn-off decision time of the synchronous rectification switch tube, the drain-source voltage Vds and the instantaneous value of the flowing current Ids are synchronously detected, if the condition that Vds is lower than a preset low-voltage threshold value and Ids is higher than a preset current threshold value is met, a high backward flow risk scene is judged to exist, then the pre-turn-off impedance modulation is started and the modulation depth is adaptively enhanced, otherwise, the conventional turn-off logic is executed.
2. The synchronous rectification emulated backward solution according to claim 1, wherein when the backward risk pre-judging step determines that there is a high backward risk scene, the following steps are sequentially performed: Step two, pre-turn-off impedance modulation, after the synchronous rectification switching tube is judged to be turned off and before the synchronous rectification switching tube is completely turned off, controlling the grid driving voltage of the synchronous rectification switching tube to drop from a first level to a preset second level, wherein the second level enables the switching tube to work in a critical conduction area, and the conduction resistance is increased, so that the amplitude and the change rate of potential backward current are naturally restrained within the turn-off delay time; Step three, the dynamic time window is shielded, after the synchronous rectification switch tube is completely closed, a dynamic shielding signal with preset duration is generated and activated immediately, and any possible synchronous rectification start signal is unconditionally shielded within the signal validity period, so that the false start caused by immune turn-off ringing is realized; And step four, system-level time sequence interlocking, namely setting the minimum turn-off time of the driving signal of the primary side main switching tube according to the time length of the dynamic shielding signal, and ensuring that the time interval between any two continuous turn-on moments of the driving signal is not smaller than the time length of the dynamic shielding signal, so that the forced interlocking of the primary switching tube and the secondary switching tube on the time sequence is realized, and the common phenomenon is stopped.
3. A synchronous rectification emulated reverse flow solution according to claim 2, characterized in that the second level is set between 2V and 3V, the decision being based on the secondary winding voltage back-pressure detection.
4. A synchronous rectification emulated backward solution according to claim 2, characterized in that the duration of the dynamic masking signal is 3 μs to 4 μs, the value of which is set according to the statistical duration of the ringing after switching off.
5. A synchronous rectification emulated reverse flow solution as claimed in claim 2, wherein the system-level timing interlock is implemented indirectly by the primary side controller by detecting the transformer auxiliary winding voltage waveform.
6. The synchronous rectification emulated backward solution according to claim 1, wherein the modulation depth is represented by a rate at which the gate driving voltage drops from the first level to the second level and a specific voltage value of the second level, and when the determined backward risk level is high, the control voltage drops to a lower second level value at a faster rate, so that the synchronous rectification switching tube enters the high impedance state more quickly.
7. The method of claim 2, wherein the pre-turn-off impedance modulation in the second step is not a fixed value, but a real-time current value flowing through the switching tube before turn-off is adaptively adjusted, specifically, a negative correlation mapping relation between the real-time current value and a target second level voltage value is established, and when the real-time current value is larger before turn-off, the voltage value of the second level is lower, so that the switching tube can exit from the deep linear region more quickly when receiving heavy current turn-off, and the on resistance is increased more significantly.
8. The synchronous rectification simulated backflow solving method according to claim 2, wherein the dynamic time window in the third step is shielded, the preset duration is not a fixed value, but is adaptively set according to the ringing characteristic frequency triggered by the current change rate di/dt collected in the pre-turn-off impedance modulation stage in the second step, specifically, the oscillation period of the ringing waveform is detected and analyzed, the duration of the dynamic shielding signal is set to cover the ringing for a plurality of periods, and the duration is inversely proportional to the main frequency of the ringing.

Description

Synchronous rectification simulation backflow solving method Technical Field The application relates to the technical field of synchronous rectification, in particular to a synchronous rectification simulation backward flow solving method. Background In modern switching power supplies, especially high-frequency and efficient AC-DC or DC-DC converters, synchronous rectification technology has become a key means for improving efficiency, and it uses a power MOSFET with extremely low on resistance to replace a traditional schottky diode as a rectifying device, so as to significantly reduce the conduction loss on the secondary side and improve the efficiency of the whole machine, however, synchronous rectification brings efficiency advantages, and simultaneously introduces technical challenges not found in traditional diode rectification, and one of the most prominent problems is current backflow. The current flowing backward generally means that under a specific working state, the current reversely flows into the synchronous rectification switch tube, so that energy is abnormally recharged from the output end to the secondary winding of the transformer, and the phenomenon not only can cause extra loss and reduce the system efficiency, but also can cause serious voltage ringing and electromagnetic interference, even cause overstress damage of devices, and seriously threaten the reliability and stability of a power supply. The reverse flow risk is particularly prominent in two typical situations, namely, firstly, when a control logic decides to turn off a synchronous rectification switch tube under a Continuous Conduction Mode (CCM) or heavy load condition, if the synchronous rectification switch tube is in a deep linear region with large current conduction and extremely low tube voltage drop, the fast turn-off can lead a channel providing a follow current path for inductive current to suddenly disappear, the rapidly-changed current can excite high-frequency oscillation on parasitic parameters of a circuit, and form huge current and voltage stress on a body diode or parasitic capacitance of the switch tube, namely, serious voltage spike and ringing are formed, secondly, the violent fluctuation of the drain-source voltage of the switch tube easily crosses the turn-on threshold value of a synchronous rectification controller in the turn-off process and the subsequent ringing stage, so that the switch tube is turned on by mistake to form unexpected conduction pulses, and the phenomenon of 'common-on' of simultaneous conduction of a primary side main switch tube and a secondary side synchronous rectification switch tube can be possibly caused, and direct energy short circuit is caused, and devices are burnt. The traditional solutions have limitations, such as that the turn-off time sequence is simply optimized or the fixed dead time is set to be difficult to adapt to wide-range load change, while the turn-off speed is reduced to slow down the current change rate, but turn-off loss is increased to offset the efficiency advantage of synchronous rectification, and the fixed ringing shielding window is adopted to prevent insufficient shielding or excessive shielding caused by the fact that the actual ringing duration cannot be matched, so that the effective turn-on time is sacrificed to influence the efficiency. Disclosure of Invention In order to solve the problems, the application provides a synchronous rectification simulation backward flow solving method. The application provides a synchronous rectification simulation backward flow solving method, which adopts the following technical scheme: a synchronous rectification simulation backward flow solving method comprises the following steps: Step one, the risk of backward flow is prejudged, at the turn-off decision time of the synchronous rectification switch tube, the drain-source voltage Vds and the instantaneous value of the flowing current Ids are synchronously detected, if the condition that Vds is lower than a preset low-voltage threshold value and Ids is higher than a preset current threshold value is met, a high backward flow risk scene is judged to exist, then the pre-turn-off impedance modulation is started and the modulation depth is adaptively enhanced, otherwise, the conventional turn-off logic is executed. As a preferable technical scheme of the present application, when the backflow risk pre-judging step judges that a high backflow risk scene exists, the following steps are sequentially executed: Step two, pre-turn-off impedance modulation, after the synchronous rectification switching tube is judged to be turned off and before the synchronous rectification switching tube is completely turned off, controlling the grid driving voltage of the synchronous rectification switching tube to drop from a first level to a preset second level, wherein the second level enables the switching tube to work in a critical conduction area, and the conduction resistance is