US-12620906-B2 - Synchronous rectification controller and control method thereof

US12620906B2US 12620906 B2US12620906 B2US 12620906B2US-12620906-B2

Abstract

A control method is disclosed to generate a control signal in a power supply with a synchronous rectifier. The control signal turns ON the synchronous rectifier to provide a channel approximately short-circuited if the control signal is greater than a threshold voltage, and turns OFF the synchronous rectifier to turn the channel into an open circuit if the control signal is between the threshold voltage and a ground reference voltage. A detection signal is generated based on one end of the channel, and compared with a predetermined level higher than the ground reference voltage. An excitation time is a period when the detection signal exceeds the predetermined level. When the excitation time exceeds a preset time, the control signal is precharged to a sub-threshold voltage between the threshold voltage and the ground reference voltage.

Inventors

Tsung-Chien Wu
Jun-Hao Huang
Chung-Wei Lin
Ming-Chang Tsou

Assignees

LEADTREND TECHNOLOGY CORPORATION

Dates

Publication Date: 20260505
Application Date: 20240416
Priority Date: 20230719

Claims (20)

1 . A control method for synchronous rectification in use of a power supply having primary and secondary sides isolated from each other, wherein the power supply includes on the secondary side a synchronous rectifier with a control terminal, at which a control signal turns ON the synchronous rectifier to provide a channel approximately short-circuited if the control signal is greater than a threshold voltage, and turns OFF the synchronous rectifier to turn the channel into an open circuit if the control signal is between the threshold voltage and a ground reference voltage, the control method comprising: receiving a detection signal generated based on one end of the channel; comparing the detection signal with a predetermined level, wherein the predetermined level is higher than the ground reference voltage, and an excitation time is a period when the detection signal exceeds the predetermined level; checking if the excitation time lasts a preset time to generate a satisfaction signal; and precharging the control signal to a sub-threshold voltage between the threshold voltage and the ground reference voltage in response to the satisfaction signal.
2 . The control method of claim 1 , further comprising: providing an operational amplifier to adjust the control signal in response to the detection signal and regulate the detection signal at a default reference level less than the ground reference voltage.
3 . The control method of claim 2 , further comprising: disabling the operational amplifier in response to the satisfaction signal.
4 . The control method of claim 3 , further comprising: precharging the control signal to the sub-threshold voltage and disabling the operational amplifier after the excitation time lasts the preset time and before the excitation time ends.
5 . The control method of claim 2 , comprising: pulling up the control signal to a constant ON voltage higher than the threshold voltage when the detection signal is less than an ON-reference level after the excitation time ends.
6 . The control method of claim 5 , further comprising: providing a pull-up switch for pulling up the control signal; turning OFF the pull-up switch after turning ON the pull-up switch for a predetermined time; and enabling the operational amplifier to regulate the detection signal at the default reference level after turning OFF the pull-up switch.
7 . The control method of claim 1 , comprising: comparing the detection signal and the ground reference voltage; and turning ON a pull-down switch to clamp the control signal to the ground reference voltage if the detection signal exceeds the ground reference voltage.
8 . The control method of claim 1 , comprising: comparing the detection signal with an ON-reference level to generate a forward-bias signal, wherein the ON-reference level is below the ground reference voltage; and precharging the control signal to the sub-threshold voltage in response to the forward-bias signal.
9 . A synchronous rectifier controller in use of a power supply with primary and second sides isolated from each other, wherein on the secondary side the power supply comprises a synchronous rectifier with a control terminal, a control signal at the control terminal turns ON the synchronous rectifier to provide a channel approximately short-circuited if the control signal exceeds a threshold voltage, the control signal substantially turns OFF the synchronous rectifier to turn the channel into an open circuit if the control signal is between the threshold voltage and a ground reference voltage, the synchronous rectifier controller comprising: a primary-side ON detector, comparing a detection signal at one end of the channel with a predetermined level, wherein the predetermined level is higher than the ground reference voltage, an excitation time is a period when the detection signal exceeds the predetermined level, and the primary-side ON detector provides a satisfaction signal if the excitation time lasts a preset time; and a precharge circuit precharging the control signal to a sub-threshold voltage between the threshold voltage and the ground reference voltage in response to the satisfaction signal.
10 . The synchronous rectifier controller of claim 9 , wherein the primary-side ON detector comprises: a comparator, comparing the detection signal with the predetermined level to detect the excitation time; and a debounce circuit connected to the comparator, for generating a confirmation signal if the excitation time lasts the preset time; and a register for recording the conformation signal to generate the satisfaction signal.
11 . The synchronous rectifier controller of claim 9 , wherein the precharge circuit comprises: a current source charging the control signal; and a comparator comparing the control signal with the sub-threshold voltage to control the current source.
12 . The synchronous rectifier controller of claim 9 , comprising: a driver for pulling up the control signal, comprising: a pull-up switch for pulling up the control signal to a constant ON voltage higher than the threshold voltage, so as to turn ON the synchronous rectifier; and a control circuit for controlling the pull-up switch in response to the satisfaction signal.
13 . The synchronous rectifier controller of claim 12 , wherein the control circuit is a pulse generator for turning ON the pull-up switch for a predetermined time.
14 . The synchronous rectifier controller of claim 9 , further comprising an operational amplifier for driving the control signal to regulate the detection signal at a default reference level less than the ground reference voltage.
15 . The synchronous rectifier controller of claim 14 , wherein after the excitation time has lasted the preset time and before the excitation time ends, the precharge circuit precharges the control signal to the sub-threshold voltage and the operational amplifier is disabled.
16 . The synchronous rectifier controller of claim 9 , comprising: a pull-down switch connected to the control terminal for fixing the control signal at the ground reference voltage; and a pull-down controller to turn ON the pull-down switch when the detection signal exceeds the ground reference voltage.
17 . The synchronous rectifier controller of claim 9 , comprising: a forward-bias voltage detector, for comparing the detection signal with an ON-reference level to generate a forward-bias signal, wherein the ON-reference level is below the ground reference voltage; wherein, in response to the forward-bias signal, the precharge circuit precharges the control signal to the sub-threshold voltage if the detection signal is less than the ON-reference level.
18 . A control method for controlling a synchronous rectifier with a control terminal, at which a control signal turns ON the synchronous rectifier to provide a channel approximately short-circuited if the control signal is greater than a threshold voltage, and turns OFF the synchronous rectifier to turn the channel into an open circuit if the control signal is between the threshold voltage and a ground reference voltage, wherein one end of the channel provides a detection signal, and the other end of the channel provides the ground reference voltage, the control method comprising: comparing the detection signal with a predetermined level to determine an excitation time, wherein the predetermined level is higher than the ground reference voltage, and the excitation time is a period when the detection signal exceeds the predetermined level; checking if the excitation time lasts a preset time to generate a satisfaction signal; and precharging the control signal to a sub-threshold voltage between the threshold voltage and the ground reference voltage in response to the satisfaction signal.
19 . The control method of claim 18 , further comprising: providing an operational amplifier to adjust the control signal in response to the detection signal and regulate the detection signal at a default reference level less than the ground reference voltage.
20 . The control method of claim 19 , further comprising: disabling the operational amplifier in response to the satisfaction signal and precharging the control signal to the sub-threshold voltage and disabling the operational amplifier after the excitation time lasts the preset time and before the excitation time ends.

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to and the benefit of Taiwan Application Series Number 112126895 filed on Jul. 19, 2023, which is incorporated by reference in its entirety. BACKGROUND The present disclosure relates generally to synchronous rectification, and more particularly to control methods and apparatuses that precharge a control signal at a control terminal of a synchronous rectifier. Synchronous rectification is a technology that uses actively controlled electronic switches, such as power MOSFETs or bipolar transistors, to replace diodes for rectification, in order to improve power supply efficiency. In conventional semiconductor diodes, the voltage drop is substantially at around 0.5 to 1V, and within the operating current range, this voltage drop does not vary significantly with the current. In contrast, the voltage drop across an actively controlled synchronous rectifier behaves more like a resistor, resulting in a tiny voltage drop at low currents. The timing of switching ON and OFF a synchronous rectifier needs careful control. If the turning-on of a synchronous rectifier occurs too early, it may lead to the occurrence of reverse current, significantly reducing power supply efficiency. Conversely, if the turn-on happens too late, the majority of the current may flow through a body diode of the synchronous rectifier for example, losing the primary purpose of synchronous rectification. BRIEF DESCRIPTION OF THE DRAWINGS Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted. The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: FIG. 1 illustrates a flyback power supply implemented according to embodiments of the present invention; FIG. 2 illustrates the SR controller shown in FIG. 1; FIG. 3 illustrates a control method used for the SR controller in FIG. 2; FIG. 4 shows some signal waveforms in FIGS. 1 and 2 from moment t10 to moment t1E within switching cycle TCYC1; and FIG. 5 illustrates switching cycle TCYC2 from moment t20 to moment t30, as well as switching cycle TCYC3 from moment t30 to moment t3E. DETAILED DESCRIPTION In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale. FIG. 1 illustrates flyback power supply 10 implemented according to embodiments of the present invention, serving as an isolated AC to DC power converter for converting AC power source VAC on primary side PRM into DC output power source VOUT on secondary side SEC. Flyback power supply 10 is just one embodiment of the present invention and is not intended to limit the invention. For example, embodiments of the present invention can also include LLC converters, active-clamp flyback power converters (ACF), or asymmetric half-bridge (AHB) power supplies. In FIG. 1, transformer TF divides flyback power supply 10 into primary side PRM and secondary side SEC. On primary side PRM, bridge rectifier 12 rectifies AC power source VAC, providing input power source VIN on the input power line IN and input ground reference voltage on input ground line 26. Power controller 13 generates pulse-width modulation (