US-12627239-B2 - Synchronous rectification control method for resonant power supply by optimizing maximum condition width for turn-off time

US12627239B2US 12627239 B2US12627239 B2US 12627239B2US-12627239-B2

Abstract

A synchronous rectification control method for a resonant power supply includes steps as follows. First, setting an initial conduction width of a synchronous rectification switch, which is less than a maximum conduction width. Afterward, detecting a voltage waveform across two ends of a body diode of the synchronous rectification switch. Afterward, calculating a resonant frequency according to the voltage waveform. Finally, determining the maximum conduction width of the synchronous rectification switch.

Inventors

Lin-Ya TSAI
Chien-An Lai

Assignees

DELTA ELECTRONICS, INC.

Dates

Publication Date: 20260512
Application Date: 20230626
Priority Date: 20230324

Claims (9)

1 . A synchronous rectification control method for a resonant power supply, comprising steps of: setting an initial condition width of a synchronous rectification switch, wherein the initial condition width is less than a maximum condition width, detecting a voltage waveform across two ends of a body diode of the synchronous rectification switch, calculating a resonant frequency according to the voltage waveform, and determining the maximum condition width of the synchronous rectification switch, wherein turning on the synchronous rectification switch at a first time, turning off the synchronous rectification switch at a second time, and turning on the synchronous rectification switch at a third time; calculating a first time difference between the second time and the first time, and calculating a second time difference between the third time and the second time; selecting the smaller of the first time difference and the second time difference as the maximum condition width.
2 . The synchronous rectification control method as claimed in claim 1 , wherein generating a first pulse voltage at the first time, generating a second pulse voltage at the second time, and generating a third pulse voltage at the third time; calculating the first time difference according to the first pulse voltage and the second pulse voltage, and calculating the second time difference according to the second pulse voltage and the third pulse voltage.
3 . The synchronous rectification control method as claimed in claim 2 , wherein the first time difference is less than the second time difference.
4 . The synchronous rectification control method as claimed in claim 1 , wherein the synchronous rectification switch is disposed on a secondary side of the resonant power supply, and the voltage waveform across two ends of the body diode is a voltage waveform between a drain and a source of the synchronous rectification switch.
5 . The synchronous rectification control method as claimed in claim 4 , wherein the resonant power supply comprises a dual comparator, and the dual comparator is configured to receive a voltage at the drain and a voltage at the source.
6 . The synchronous rectification control method as claimed in claim 1 , wherein acquiring a plurality of time differences, acquiring an average value of the plurality of time differences, and calculating the resonant frequency.
7 . The synchronous rectification control method as claimed in claim 1 , wherein delaying to turn off the synchronous rectification switch when the synchronous rectification switch operates at a zero-current condition.
8 . The synchronous rectification control method as claimed in claim 1 , wherein periodically updating the maximum condition width.
9 . The synchronous rectification control method as claimed in claim 1 , wherein controlling an input DC voltage of the resonant power supply to decrease so that the resonant power supply operates below a first resonant frequency point, and maintaining an output of the resonant power supply at a half-loading condition.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority of application Ser. No. 20/231,0297636.6 filed in China on Mar. 24, 2023 under 35 U.S.C. § 119, the entire contents of all of which are hereby incorporated by reference. BACKGROUND Technical Field The present disclosure relates to a control method for a resonant power supply, and more particularly to a synchronous rectification control method for a resonant power supply. Description of Related Art The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art. The LLC resonant converter, which has lower switching losses, has now become a widely adopted power topology. The LLC resonant converter has the advantage of higher efficiency and switching frequency compared to the hard-switching topology, as it can achieve zero-voltage turned-on of the power diode at the primary side and zero-current turned-off of the rectification switch at the secondary side within a wide load range. For the LLC resonant converter, the turned-on point of the synchronous rectification switch is basically the same as that of the power diode on the primary side, but the timing of turning off the synchronous rectification switch will affect the overall system's operation efficiency and characteristics. When the synchronous rectification switch is turned off earlier than the optimal turned-off point, the current will flow through the body diode of the synchronous rectification switch, thereby increasing its losses. When the synchronous rectification switch is turned off later than the optimal turned-off point, the secondary-side current will affect the operation state of the primary-side resonant tank of the LLC resonant converter, thereby causing distortion of the resonant current. In the existing technologies, there is a lack of synchronous rectification control method for resonant power supplies that can effectively solve the above-mentioned problems. Accordingly, the present disclosure provides a synchronous rectification control method to solve the existing problems: when the time of controlling the conduction of synchronous rectification switch is greater than the maximum conduction width, it will cause distortion of the resonant current, resulting in a risk of unsafe operation. When the time of controlling the conduction of synchronous rectification switch is too small compared to the maximum conduction width, it will increase the losses of the synchronous rectification switch and be detrimental to the design of converter efficiency. SUMMARY An objective of the present disclosure is to provide a synchronous rectification control method for a resonant power supply to solve the problems of existing technology. In order to achieve the above-mentioned objective, the synchronous rectification control method includes steps of: setting an initial condition width of a synchronous rectification switch, wherein the initial condition width is less than a maximum condition width, detecting a voltage waveform across two ends of a body diode of the synchronous rectification switch, calculating a resonant frequency according to the voltage waveform, and determining the maximum condition width of the synchronous rectification switch. In one embodiment, turning on the synchronous rectification switch at a first time, turning off the synchronous rectification switch at a second time, and turning on the synchronous rectification switch at a third time; calculating a first time difference between the second time and the first time, and calculating a second time difference between the third time and the second time; selecting the smaller of the first time difference and the second time difference as the maximum condition width. In one embodiment, generating a first pulse voltage at the first time, generating a second pulse voltage at the second time, and generating a third pulse voltage at the third time; calculating the first time difference according to the first pulse voltage and the second pulse voltage, and calculating the second time difference according to the second pulse voltage and the third pulse voltage. In one embodiment, the first time difference is less than the second time difference. In one embodiment, the synchronous rectification switch is disposed on a primary side of the resonant power supply, and the voltage waveform across two ends of the body diode is a voltage waveform between a drain and a source of the synchronous rectification switch. In one embodiment, the resonant power supply includes a dual comparator, and the dual comparator receives a voltage at the drain and a voltage at the source. In one embodiment, acquiring a plurality of time differences, acquiring an average value of the plurality of time differences, and calculating the resonant frequency. In one embodiment, delaying to turn off the synchronous rectification switch when the synchronous rectification swit