Power conversion circuit and control method thereof

Abstract

A power conversion circuit and a control method thereof are provided. The power conversion circuit includes a transformer, a resonance capacitor, an upper bridge transistor, a lower bridge transistor, and a control circuit. The transformer includes a primary coil and a secondary coil. The primary coil is coupled between a switching node and a resonance node. The resonance capacitor is coupled between the resonance node and a ground terminal. The upper bridge transistor provides an input voltage to the switching node based on an upper bridge driving signal. The lower bridge transistor couples the switching node to the ground terminal based on a lower bridge driving signal. The control circuit operates in a normal mode to generate the upper bridge driving signal and the lower bridge driving signal, such that the secondary coil generates an output voltage. When the control circuit operates in a discharge mode, the control circuit discharges the resonance capacitor with the lower bridge transistor at a discharge current.

Description

Technical Field

[0001] This invention relates to a power conversion circuit and its control method, and more particularly to a resonant power conversion circuit and its control method for discharging a resonant capacitor. Background Technology

[0002] With the continuous development of portable electronic devices, the development trend of power conversion circuits, like most power products, is towards higher efficiency, higher power density, higher reliability, and lower cost. Since resonant power conversion circuits (including LLC resonant power conversion circuits and flyback power conversion circuits) are high-efficiency and high-power-density power conversion circuits, portable electronic devices are increasingly adopting resonant power conversion circuits.

[0003] However, current resonant power conversion circuits still have many shortcomings, so it is necessary to further optimize them. Summary of the Invention

[0004] This invention proposes a power conversion circuit and its control method. When the voltage across the resonant capacitor is unbalanced with the output voltage, the resonant capacitor can be discharged through the lower-bridge transistor. This helps reduce the maximum primary current of the primary coil and the maximum output current of the secondary coil, thereby reducing the conduction losses of the lower-bridge transistor and the rectifier transistor, and thus improving the conversion efficiency under low output voltage and light load conditions. Furthermore, it also avoids surges in the secondary coil, protecting circuit components from burnout.

[0005] In view of this, the present invention proposes a power conversion circuit including a transformer, a resonant capacitor, an upper-bridge transistor, a lower-bridge transistor, and a control circuit. The transformer includes a primary coil and a secondary coil, wherein the primary coil is coupled between a switching node and a resonant node. The resonant capacitor is coupled between the resonant node and a ground terminal. The upper-bridge transistor provides an input voltage to the switching node based on an upper-bridge drive signal. The lower-bridge transistor couples the switching node to the ground terminal based on a lower-bridge drive signal. The control circuit operates in a normal mode and generates the upper-bridge drive signal and the lower-bridge drive signal, causing the secondary coil to generate an output voltage. When the control circuit operates in a discharge mode, the control circuit discharges the resonant capacitor with a discharge current using the lower-bridge transistor.

[0006] According to one embodiment of the present invention, when the control circuit receives the input voltage, the control circuit executes a startup procedure. In the startup procedure, the control circuit operates in the discharge mode to discharge the resonant capacitor. After the startup procedure, the control circuit operates in the normal mode to generate the output voltage in the secondary coil.

[0007] According to another embodiment of the present invention, when the control circuit executes a protection mechanism and simultaneously turns off the upper bridge transistor and the lower bridge transistor, the control circuit operates in the discharge mode to discharge the resonant capacitor.

[0008] According to one embodiment of the present invention, the power conversion circuit further includes a current detection circuit. The current detection circuit detects a primary current flowing through the primary coil and the resonant capacitor, and generates a current detection signal. When the control circuit operates in the discharge mode, the control circuit controls the discharge current to be less than a predetermined current based on the current detection signal.

[0009] According to one embodiment of the present invention, the current detection circuit further includes a first detection resistor. The first detection resistor is coupled between the resonant capacitor and the ground terminal. The voltage across the first detection resistor generates the current detection signal.

[0010] According to another embodiment of the present invention, the current detection circuit further includes an isolation transformer and a second detection resistor. The isolation transformer includes an isolation primary coil and an isolation secondary coil. The second detection resistor is coupled to both ends of the isolation secondary coil. The isolation primary coil is coupled between the resonant capacitor and the ground terminal. The voltage across the second detection resistor generates the current detection signal.

[0011] According to one embodiment of the present invention, the control circuit further includes a current control circuit and a selection switch. The current control circuit generates a discharge signal based on the relationship between the current detection signal and a reference voltage. The selection switch provides one of the lower bridge drive signal and the discharge signal to the lower bridge transistor. When the control circuit operates in the discharge mode, the selection switch provides the discharge signal to the lower bridge transistor. When the control circuit operates in the normal mode, the selection switch provides the lower bridge drive signal to the lower bridge transistor.

[0012] According to one embodiment of the present invention, the current control circuit adjusts the discharge signal to control the discharge current to be less than the predetermined current. When the control circuit operates in the discharge mode, the lower bridge transistor operates in a linear region.

[0013] According to one embodiment of the present invention, the current control circuit further includes an error amplifier, a first control resistor, and a second control resistor. The error amplifier includes a positive input terminal, a negative input terminal, and an output terminal. The first control resistor is coupled between the current detection signal and the positive input terminal. The second control resistor is coupled between the positive input terminal and the reference voltage. The output terminal generates the discharge signal. The negative input terminal is coupled to the ground terminal.

[0014] According to one embodiment of the present invention, the control circuit further includes a voltage detection circuit. The voltage detection circuit is used to detect the voltage across the resonant capacitor to generate a voltage detection signal. The voltage detection signal includes a first voltage divider resistor and a second voltage divider resistor. The first voltage divider resistor is coupled to the resonant node. The second voltage divider resistor is coupled between the first voltage divider resistor and the ground terminal. The voltage across the second voltage divider resistor generates the voltage detection signal.

[0015] According to one embodiment of the present invention, when the control circuit operates in the discharge mode, the control circuit determines whether to end the discharge of the resonant capacitor based on the relationship between the voltage detection signal and the output voltage. When the control circuit determines that the discharge of the resonant capacitor is complete, the selection switch provides the lower bridge drive signal to the lower bridge transistor.

[0016] According to one embodiment of the present invention, the control circuit determines the relationship between the voltage across the resonant capacitor and the output voltage based on the voltage detection signal. When the voltage across the resonant capacitor is less than the product of the output voltage and a turns ratio, the control circuit terminates the discharge mode. When the voltage across the resonant capacitor is not less than the product of the output voltage and the turns ratio, the control circuit continues to operate in the discharge mode. The turns ratio is equal to the number of turns of the primary coil divided by the number of turns of the secondary coil.

[0017] According to one embodiment of the present invention, when the control circuit operates in the discharge mode for a predetermined time, the control circuit ends the discharge mode and stops discharging the resonant capacitor. When the control circuit operates in the discharge mode, the control circuit intermittently turns on the lower bridge transistor to control the temperature of the lower bridge transistor.

[0018] The present invention further proposes a control method for controlling a power conversion circuit. The power conversion circuit includes a resonant capacitor coupled between a resonant node and a ground terminal, a transformer including a primary coil and a secondary coil, an upper-bridge transistor providing an input voltage to a switching node, and a lower-bridge transistor coupling the switching node to the ground terminal, wherein the primary coil is coupled between the switching node and the resonant node. The control method includes: determining whether the operation is in a normal mode or a discharge mode; when operating in the normal mode, driving the upper-bridge transistor and the lower-bridge transistor to generate an output voltage in the secondary coil; and when operating in the discharge mode, discharging the resonant capacitor with a discharge current using the lower-bridge transistor.

[0019] According to an embodiment of the present invention, the step of determining whether to operate in the normal mode or the discharge mode further includes: operating in the discharge mode before operating in the normal mode.

[0020] According to an embodiment of the present invention, the step of determining whether to operate in the normal mode or the discharge mode further includes: when the input voltage is received, executing a startup procedure; in the startup procedure, operating in the discharge mode; and when the startup procedure ends, operating in the normal mode.

[0021] According to another embodiment of the present invention, the step of determining whether to operate in the normal mode or the discharge mode further includes: operating in the discharge mode when a protection mechanism is executed and the upper bridge transistor and the lower bridge transistor are turned off simultaneously; and operating in the normal mode when the protection mechanism ends.

[0022] According to an embodiment of the present invention, the step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: detecting the discharge current and generating a current detection signal; and controlling the lower bridge transistor based on the current detection signal so that the discharge current is less than a predetermined current.

[0023] According to an embodiment of the present invention, the step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: generating a discharge signal using an error amplifier based on the relationship between the current detection signal and a reference voltage; and controlling the lower bridge transistor using the discharge signal so that the discharge current is less than the predetermined current. When operating in the discharge mode, the lower bridge transistor operates in a linear region.

[0024] According to another embodiment of the present invention, the step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: detecting the voltage across the resonant capacitor and generating a voltage detection signal; determining whether to continue discharging the resonant capacitor based on the relationship between the voltage across the resonant capacitor and the output voltage; stopping the discharge of the resonant capacitor when the voltage across the resonant capacitor is less than the output voltage multiplied by a turns ratio; and continuing to discharge the resonant capacitor when the voltage across the resonant capacitor is not less than the output voltage multiplied by the turns ratio. The turns ratio is equal to the number of turns of the primary coil divided by the number of turns of the secondary coil. Attached Figure Description

[0025] Figure 1 This is a circuit diagram showing a power conversion circuit according to an embodiment of the present invention;

[0026] Figure 2 This is a waveform diagram showing the power conversion circuit according to an embodiment of the present invention;

[0027] Figure 3 This is a circuit diagram showing a power conversion circuit according to another embodiment of the present invention;

[0028] Figure 4 This is a circuit diagram showing a power conversion circuit according to yet another embodiment of the present invention;

[0029] Figure 5 This is a circuit diagram showing a power conversion circuit according to yet another embodiment of the present invention;

[0030] Figure 6 This is a circuit diagram showing a power conversion circuit according to yet another embodiment of the present invention; and

[0031] Figure 7 This is a flowchart illustrating a control method according to an embodiment of the present invention.

[0032] [Symbol Explanation]

[0033] 100, 300, 400, 500, 600: Power conversion circuit

[0034] 111: Upper-bridge transistor

[0035] 111D: Parasitic diode on the upper bridge

[0036] 112: Lower-bridge transistor

[0037] 112D: Lower-bridge parasitic diode

[0038] 120: Rectifier circuit

[0039] 130: Secondary control circuit

[0040] 140, 310, 610: Control circuit

[0041] 150: Level shifting circuit

[0042] CR: Resonant capacitor

[0043] TM: Transformer

[0044] PD: Optical Coupler

[0045] HSD: Upper Bridge Driver Circuit

[0046] LSD: Lower bridge driver circuit

[0047] HSG: Upper Bridge Gate Drive Signal

[0048] LSG: Lower Bridge Gate Drive Signal

[0049] VIN: Input voltage

[0050] SW: Switch Node

[0051] NR: Resonant Node

[0052] VCR: Resonant Voltage

[0053] PS: Primary coil

[0054] SS: Secondary coil

[0055] IOUT: Output current

[0056] IP: Primary Current

[0057] VOUT: Output voltage

[0058] TR: Rectifier Transistor

[0059] COUT: Output capacitor

[0060] VD: Drain voltage

[0061] DR: Rectifier Parasitic Diode

[0062] IFB: Feedback Current

[0063] SG: Gate signal

[0064] VFB: Feedback Voltage

[0065] SH: Overpass drive signal

[0066] SL: Lower bridge drive signal

[0067] 200: Waveform Diagram

[0068] TW: Bridge connection time

[0069] IM: Magnetizing current

[0070] TDS: Demagnetization time

[0071] TSL: Downbridge conduction time

[0072] TRL: First Dead Zone Time

[0073] TRH: Second Dead Zone Time

[0074] T1: First Time Point

[0075] T2: Second Time Point

[0076] T3: Third Time Point

[0077] T4: Fourth Time Point

[0078] T5: Fifth Time Point

[0079] T6: Sixth Time Point

[0080] T7: Seventh Time Point

[0081] IOM: Maximum Output Current

[0082] IPM: Maximum primary current

[0083] 311: Current control circuit

[0084] 312: Selector Switch

[0085] 320: Current detection circuit

[0086] CS: Current detection signal

[0087] DCG: Discharge signal

[0088] SEL: Selection signal

[0089] IDCG: Discharge Current

[0090] EA: Error Amplifier

[0091] RC1: First control resistor

[0092] RC2: Second control resistor

[0093] INP: Positive Input Terminal

[0094] INN: Negative input terminal

[0095] O: Output terminal

[0096] VREF: Reference Voltage

[0097] RD1: First sensing resistor

[0098] ITM: Isolation Transformer

[0099] RD2: Second sensing resistor

[0100] IPS: Isolation Primary Coil

[0101] ISS: Isolation Secondary Coil

[0102] 611: Voltage Detection Circuit

[0103] VS: Voltage detection signal

[0104] RV1: First voltage divider resistor

[0105] RV2: Second voltage divider resistor Detailed Implementation

[0106] The following description is an embodiment of this application. Its purpose is to illustrate the general principles of this application and should not be regarded as a limitation of this application. The scope of this application shall be defined by the claims.

[0107] It is worth noting that the following disclosure provides multiple embodiments or examples for practicing different features of this application. The specific element examples and arrangements described below are merely for briefly illustrating the spirit of this application and are not intended to limit its scope. Furthermore, the same element symbols or words may be repeated in multiple examples in the following description. However, the purpose of repetition is solely to provide a simplified and clear explanation and is not intended to limit the relationship between the various embodiments and / or configurations discussed below.

[0108] Furthermore, the descriptions in the following specification of a feature being connected to, coupled to, and / or formed on another feature may actually include multiple different embodiments, including features that are in direct contact, or additional features that are formed between features, such that the features are not in direct contact.

[0109] Furthermore, relative terms such as "lower" or "bottom" and "higher" or "top" may be used in the embodiments to describe the relative relationship of one element of the diagram to another element. It is understood that if the arrangement of the diagram is flipped so that it is upside down, the element depicted on the "lower" side will become the element on the "higher" side.

[0110] It is understood that although terms such as "first," "second," and "third" may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms, and these terms are only used to distinguish different elements, components, regions, layers, and / or portions. Therefore, a first element, component, region, layer, and / or portion discussed below may be referred to as a second element, component, region, layer, and / or portion without departing from the teachings of some embodiments of this application.

[0111] Some embodiments of this application can be understood in conjunction with the accompanying drawings, which are also considered part of the description of the embodiments of this application. It should be understood that the drawings of the embodiments of this application are not drawn to scale with actual devices and components. The shape and thickness of the embodiments may be exaggerated in the drawings to clearly show the features of the embodiments of this application. Furthermore, the structures and devices in the drawings are drawn schematically to clearly show the features of the embodiments of this application.

[0112] Here, the terms "about," "approximately," and "roughly" generally indicate within 20% of a given value or range, preferably within 10%, and even more preferably within 5%, or within 3%, or within 2%, or within 1%, or within 0.5%. The quantities given here are approximate quantities, meaning that the meaning of "about," "approximately," and "roughly" may be implied even without specific mention of them.

[0113] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It is understood that these terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with the relevant art and the background or context of this application, and should not be interpreted in an idealized or overly formal manner, unless specifically defined in the embodiments of this application.

[0114] In some embodiments of this application, terms such as "connection" and "interconnection" are used to refer to two structures being in direct contact, unless otherwise defined. This can also refer to two structures not being in direct contact, with other structures disposed between them. Furthermore, these terms can also include cases where both structures are movable or both structures are fixed.

[0115] In a diagram, similar elements and / or features may have the same element symbol. Elements of the same type can be distinguished by adding letters or numbers after the element symbol to differentiate similar elements and / or features.

[0116] Figure 1This is a circuit diagram showing a power conversion circuit according to an embodiment of the present invention. Figure 1 As shown, the power conversion circuit 100 includes an upper bridge transistor 111, a lower bridge transistor 112, a resonant capacitor CR, a transformer TM, a rectifier circuit 120, a secondary control circuit 130, an optocoupler PD, a control circuit 140, a level shifting circuit 150, an upper bridge drive circuit HSD, and a lower bridge drive circuit LSD.

[0117] The upper-bridge transistor 111 provides the input voltage VIN to the switching node SW based on the upper-bridge gate drive signal HSG. According to one embodiment of the invention, the upper-bridge transistor 111 includes an upper-bridge parasitic diode 111D, wherein the upper-bridge parasitic diode 111D is coupled between the switching node SW and the input voltage VIN. The lower-bridge transistor 112 couples the switching node SW to ground based on the lower-bridge gate drive signal LSG. According to one embodiment of the invention, the lower-bridge transistor 112 includes a lower-bridge parasitic diode 112D, wherein the lower-bridge parasitic diode 112D is coupled between the switching node SW and ground.

[0118] The resonant capacitor CR is coupled between the resonant node NR and the ground terminal, and a resonant voltage VCR is generated across the capacitor CR. The transformer TM includes a primary coil PS and a secondary coil SS. The primary coil PS is coupled between the switching node SW and the resonant node NR. The output current IOUT generated by the secondary coil SS is rectified by the rectifier circuit 120 to generate the output voltage VOUT.

[0119] According to some embodiments of the present invention, the primary coil PS and the resonant capacitor CR are connected in series between the switching node SW and the ground terminal. In other words, the resonant capacitor CR can also be coupled between the switching node SW and the resonant node NR, and the primary coil PS can be coupled between the resonant node NR and the ground terminal.

[0120] The rectifier circuit 120 converts the output current IOUT generated by the secondary coil SS into an output voltage VOUT, and includes a rectifier transistor TR and an output capacitor COUT. According to some embodiments of the present invention, the rectifier transistor TR further includes a rectifier parasitic diode DR. The rectifier transistor TR is turned on based on a gate signal SG, causing the output current IOUT from the secondary coil SS to charge the output capacitor COUT to generate the output voltage VOUT. When the rectifier transistor TR is turned off, the voltage across the drain terminal to the source terminal of the rectifier transistor TR is the drain voltage VD.

[0121] The secondary control circuit 130 generates a feedback current IFB based on the output voltage VOUT, wherein the feedback current IFB generates a feedback voltage VFB via the optocoupler PD. The secondary control circuit 130 further uses the gate signal SG to turn on the rectifier transistor TR, so that the output current IOUT generated by the secondary coil SS charges the output capacitor COUT, thereby generating the output voltage VOUT.

[0122] Control circuit 140 generates an upper bridge drive signal SH and a lower bridge drive signal SL based on the feedback voltage VFB. Level shift circuit 150 shifts the voltage level of upper bridge drive signal SH to the input voltage VIN. Upper bridge drive circuit HSD generates upper bridge gate drive signal HSG based on the shifted signal to drive upper bridge transistor 111. Lower bridge drive circuit LSD generates lower bridge gate drive signal LSG based on lower bridge drive signal SL to drive lower bridge transistor 112.

[0123] According to some embodiments of the present invention, the control circuit 140 further generates an upper bridge drive signal SH and a lower bridge drive signal SL based on the voltage of the switching node SW, so that both the upper bridge transistor 111 and the lower bridge transistor 112 achieve zero voltage switching (ZVS), thereby improving the conversion efficiency of the power conversion circuit 100. According to some embodiments of the present invention, the power conversion circuit 100 may be a resonant power conversion circuit. According to some embodiments of the present invention, the power conversion circuit 100 may be a resonant flyback power conversion circuit. According to some embodiments of the present invention, the power conversion circuit 100 may be an asymmetrical half-bridge flyback power converter.

[0124] Figure 2 This is a waveform diagram showing a power conversion circuit according to an embodiment of the present invention. The following description of waveform diagram 200 will be accompanied by… Figure 1 The power conversion circuit 100 is described in detail below. Between a first time point T1 and a second time point T2, the upper-bridge transistor 111 is turned on based on the upper-bridge drive signal SH (i.e., high logic level), where the upper-bridge turn-on time TW is the turn-on time of the upper-bridge transistor 111. During the upper-bridge turn-on time TW, the transformer TM is magnetized, generating a magnetizing current IM. As the turn-on time TW increases, the magnetizing current IM of the transformer TM, the primary current IP flowing through the primary coil PS, and the resonant voltage VCR all continuously increase. In other words, the upper-bridge turn-on time TW is the magnetization time of the transformer TM.

[0125] When the upper-bridge transistor 111 is turned off (i.e., the upper-bridge drive signal SH is at a low logic level), the transformer 10 demagnetizes. During the demagnetization time TDS, the transformer 10 generates an output current IOUT, and the on-time of the lower-bridge transistor 112 (i.e., the lower-bridge drive signal SL is at a high logic level) corresponds to the demagnetization time TDS. According to some embodiments of the present invention, the lower-bridge on-time TSL of the lower-bridge drive signal SL is equal to or greater than the demagnetization time TDS. During the demagnetization time TDS, the voltage across the primary coil PS is equal to the resonant voltage VCR, and the output voltage VOUT is as shown in Equation 1:

[0126]

[0127] (Formula 1)

[0128] Where NP is the number of turns of the primary coil PS, NS is the number of turns of the secondary coil SS, and the turns ratio n is the number of turns of the primary coil PS divided by the number of turns of the secondary coil SS.

[0129] Demagnetization time TDS is shown in Formula 2:

[0130] (Formula 2)

[0131] When the upper bridge transistor 111 is turned on, The voltage used to magnetize the transformer TM.

[0132] At the second time point T2, the upper bridge drive signal SH transitions to a low logic level, turning off the upper bridge transistor 111. At the third time point T3, the lower bridge drive signal SL transitions to a high logic level, turning on the lower bridge transistor 112. According to some embodiments of the present invention, the first dead time TRL from the second time point T2 to the third time point T3 is the dead time from the turn-off of the upper bridge transistor 111 to the turn-on of the lower bridge transistor 112. According to some embodiments of the present invention, between the second time point T2 and the third time point T3, the primary current IP reaches the maximum primary current IPM.

[0133] According to some embodiments of the present invention, during the first dead time TRL, the circulating current generated by the primary coil PS turns on the lower bridge parasitic diode 112D, and pulls down the voltage of the switching node SW, causing the lower bridge transistor 112 to achieve zero-voltage switching. At the third time point T3, the voltage across the primary coil PS is the resonant voltage VCR of the resonant capacitor CR.

[0134] Between the third time point T3 and the fourth time point T4, the upper bridge transistor 111 is off, and the lower bridge transistor 112 is on under zero-voltage switching. The rectifier transistor TR is on, causing the output current IOUT to flow through the rectifier transistor TR and generate the output voltage VOUT, where the output voltage VOUT is equal to the resonant voltage VCR divided by the number of turns n, as shown in Equation 1. In addition, the primary current IP remains positive and flows into the resonant capacitor CR.

[0135] According to some embodiments of the present invention, the leakage inductance of the primary coil PS and the resonant capacitor CR form a resonant tank. The output current IOUT is of sine wave type, and its frequency is determined by the resonant frequency of the resonant circuit. The primary current IP is the magnetizing current IM plus the reflection of the output current IOUT.

[0136] During the period from the fourth time point T4 to the fifth time point T5, the upper-bridge transistor 111 remains off and the lower-bridge transistor 112 remains on. Energy from the transformer TM is continuously transferred to the secondary coil SS, and this energy is provided by the resonant capacitor CR. Furthermore, since the lower-bridge transistor 112 remains on, the energy from the resonant capacitor CR is used to bring the magnetizing current IM to a negative value. According to some embodiments of the present invention, between the fourth time point T4 and the fifth time point T5, the output current IOUT reaches the maximum output current IOM.

[0137] At time point T5, the rectifier transistor TR is turned off based on the gate signal SG, thus ending the demagnetization time TDS. From time point T5 to time point T6, the resonant capacitor CR continuously reverse-magnetizes the primary coil PS, causing the primary current IP to remain negative until the lower bridge transistor 112 is turned off.

[0138] During time points T6 to T7, both the upper-bridge transistor 111 and the lower-bridge transistor 112 are off. Furthermore, the primary current IP, induced as a negative current during time points T5 to T6, turns on the upper-bridge parasitic diode 111D, causing the voltage at the switching node SW to rise to the input voltage VIN. According to some embodiments of the present invention, the second dead time TRH during time points T6 to T7 is the dead time from the turn-off of the lower-bridge transistor 112 to the turn-on of the upper-bridge transistor 111.

[0139] At time point T7, the upper bridge drive signal SH is at a high logic level. As the voltage of the switching node SW rises to the input voltage VIN, the upper bridge transistor 111 can be turned on in a zero-voltage switching state.

[0140] Because the resonant capacitor CR is connected in parallel with the primary coil PS during the demagnetization of the transformer TM, the resonant voltage VCR is the output voltage VOUT multiplied by the number of turns of the transformer TM. When the difference between the resonant voltage VCR and the output voltage VOUT multiplied by the number of turns is too large, the drain voltage VD will generate a very high voltage spike, which will reduce the reliability of the rectifier transistor TR and even damage the rectifier transistor TR.

[0141] Furthermore, when the difference between the resonant voltage VCR and the output voltage VOUT multiplied by the turns ratio is too large, it will significantly increase the maximum output current IOM and the maximum primary current IP during transformer TM demagnetization, thereby reducing conversion efficiency. To protect circuit components and improve conversion efficiency, it is necessary to optimize the power conversion circuit 100.

[0142] Figure 3 This is a circuit diagram showing a power conversion circuit according to another embodiment of the present invention. The power conversion circuit 300 is connected to... Figure 1 Compared to the power conversion circuit 100, the power conversion circuit 300 further includes a current detection circuit 320, and the control circuit 140 is replaced by a control circuit 310.

[0143] like Figure 3 As shown, the current detection circuit 320 detects the primary current IP and generates a current detection signal CS. The control circuit 310 includes a current control circuit 311 and a selection switch 312. The current control circuit 311 generates a discharge signal DCG based on the current detection signal CS generated by the current detection circuit 320. The selection switch 312 provides one of the lower bridge drive signal SL and the discharge signal DCG to the lower bridge drive circuit LSD based on the selection signal SEL, thereby driving the lower bridge transistor 112.

[0144] According to one embodiment of the present invention, the current detection circuit 320 detects the discharge current IDCG and generates a current detection signal CS. The current control circuit 311 dynamically adjusts the discharge signal DCG based on the current detection signal CS, so that the discharge current IDCG of the resonant capacitor CR discharged through the lower bridge transistor 112 is less than a predetermined current. According to some embodiments of the present invention, the predetermined current can be set according to a predetermined discharge time. According to another embodiment of the present invention, the discharge current IDCG can also be controlled to a fixed value. According to some embodiments of the present invention, when the resonant capacitor CR discharges through the lower bridge transistor 112, the lower bridge transistor 112 operates in the linear region.

[0145] According to an embodiment of the present invention, when the control circuit 310 operates in normal mode, the control circuit 310 provides the lower bridge drive signal SL to the lower bridge drive circuit LSD using the selection signal SEL, and drives the upper bridge transistor 111 and the lower bridge transistor 112 using the upper bridge drive signal SH and the lower bridge drive signal SL respectively, thereby generating an output voltage VOUT in the secondary coil SS.

[0146] According to another embodiment of the present invention, when the control circuit 310 operates in the discharge mode, the control circuit 310 provides the discharge signal DCG to the lower bridge drive circuit LSD using the selection signal SEL, so that the resonant capacitor CR is discharged via the lower bridge transistor 112 with the discharge current IDCG.

[0147] According to some embodiments of the present invention, when the control circuit 310 operates in normal mode, the current detection circuit 320 detects the primary current IP. When the control circuit 310 operates in discharge mode, the current detection circuit 320 detects the discharge current IDCG.

[0148] According to an embodiment of the present invention, when the control circuit 310 receives the input voltage VIN ( Figure 3 When (not shown), the control circuit 310 executes the startup procedure and operates in discharge mode during the startup procedure, discharging the resonant capacitor CR with the discharge current IDCG. In other words, when the input voltage VIN is provided to the power conversion circuit 300, the control circuit 310 executes the startup procedure.

[0149] According to another embodiment of the present invention, when the control circuit 310 executes the protection mechanism and simultaneously turns off the upper bridge transistor 111 and the lower bridge transistor 112, the control circuit 310 operates in discharge mode and discharges the resonant capacitor CR with a discharge current IDCG. According to some embodiments of the present invention, the protection mechanism includes overcurrent protection, overvoltage protection, and other protection mechanisms to prevent the power conversion circuit 300 from burning out.

[0150] like Figure 3 As shown, the current control circuit 311 includes an error amplifier EA, a first control resistor RC1, and a second control resistor RC2. The error amplifier EA includes a positive input terminal INP, a negative input terminal INN, and an output terminal O. The first control resistor RC1 is coupled between the current detection signal SC and the positive input terminal INP, and the second control resistor RC2 is coupled between the positive input terminal INP and the reference voltage VREF. The negative input terminal INN is coupled to ground, and the output terminal O generates a discharge signal DCG.

[0151] Figure 4 This is a circuit diagram showing a power conversion circuit according to another embodiment of the present invention. The power conversion circuit 400 is connected to... Figure 3Compared to the power conversion circuit 300, the current detection circuit 320 of the power conversion circuit 400 includes a first detection resistor RD1, wherein the first detection resistor RD1 is coupled between the resonant capacitor CR and the ground terminal.

[0152] According to some embodiments of the present invention, when the primary current IP in normal mode and the discharge current IDCG in discharge mode flow through the first detection resistor RD1, the voltage across the first detection resistor RD1 is the current detection signal CS. According to some embodiments of the present invention, since the resistance value of the first detection resistor RD1 is very small, the resonant voltage VCR can be regarded as the voltage across the resonant capacitor CR.

[0153] Figure 5 This is a circuit diagram showing a power conversion circuit according to another embodiment of the present invention. The power conversion circuit 500 and... Figure 3 Compared to the power conversion circuit 300, the current detection circuit 320 of the power conversion circuit 500 includes an isolation transformer ITM and a second detection resistor RD2.

[0154] The isolation transformer ITM includes an isolation primary coil IPS and an isolation secondary coil ISS. The isolation primary coil IPS is coupled between the resonant capacitor CR and ground. A second sensing resistor RD2 is coupled to both ends of the isolation secondary coil ISS. Furthermore, the voltage across the second sensing resistor RD2 generates a current detection signal CS. According to some embodiments of the present invention, the isolation transformer ITM maps the primary current IP or discharge current IDCG flowing through the isolation primary coil IPS to the secondary coil SS, and the current flows through the second sensing resistor RD2 to generate the current detection signal CS. According to some embodiments of the present invention, since the voltage across the isolation primary coil IPS is very small, the resonant voltage VCR can be considered as the voltage across the resonant capacitor CR.

[0155] Figure 6 This is a circuit diagram showing a power conversion circuit according to another embodiment of the present invention. The power conversion circuit 600 is connected to... Figure 3 Compared to the power conversion circuit 300, the control circuit 610 of the power conversion circuit 600 further includes a voltage detection circuit 611.

[0156] like Figure 6 As shown, the voltage detection circuit 611 is used to detect the resonant voltage VCR and generate a voltage detection signal VS. In other words, the voltage detection circuit 611 is used to detect the voltage across the resonant capacitor CR and generate a voltage detection signal VS. Figure 6In one embodiment, the voltage detection circuit 611 includes a first voltage divider resistor RV1 and a second voltage divider resistor RV2, wherein the first voltage divider resistor RV1 and the second voltage divider resistor RV2 are used to divide the resonant voltage VCR to generate a voltage detection signal VS. According to some embodiments of the present invention, the voltage across the second voltage divider resistor RV2 generates the voltage detection signal VS.

[0157] According to some embodiments of the present invention, when the control circuit 610 operates in discharge mode, the control circuit 610 determines whether to terminate the discharge of the resonant capacitor CR based on the relationship between the voltage detection signal VS and the output voltage VOUT. According to some embodiments of the present invention, when the voltage division ratio of the first voltage divider resistor RV1 and the second voltage divider resistor RV2 is equal to the turns ratio of the transformer TM, the control circuit 610 determines whether the voltage detection signal VS is equal to the output voltage VOUT, and decides whether to terminate the discharge of the resonant capacitor CR, wherein the turns ratio is equal to the number of turns of the primary coil PS divided by the number of turns of the secondary coil SS.

[0158] In other words, when the voltage detection signal VS is less than the output voltage VOUT, the control circuit 610 ends the discharge mode and stops discharging the resonant capacitor CR. When the voltage detection signal VS is not less than the output voltage VOUT, the control circuit 610 continues to operate in the discharge mode and continues to discharge the resonant capacitor CR. According to some embodiments of the present invention, when the discharge mode ends, the control circuit 610 can return to the normal mode to generate the output voltage VOUT.

[0159] In detail, when the voltage across the resonant capacitor CR is less than the product of the output voltage VOUT and the turns ratio of the transformer TM, the control circuit 610 stops discharging the resonant capacitor CR. When the voltage across the resonant capacitor CR is not less than the product of the output voltage VOUT and the turns ratio of the transformer TM, the control circuit 610 continues to discharge the resonant capacitor CR.

[0160] According to other embodiments of the present invention, the control circuit 610 may also continuously discharge the resonant capacitor CR for a predetermined time and then immediately end the discharge mode. According to some embodiments of the present invention, the predetermined time is not less than the time required for the resonant capacitor CR to discharge across the voltage to zero. According to some embodiments of the present invention, since the temperature of the lower bridge transistor 112 will rise when the resonant capacitor CR discharges, the control circuit 610 may intermittently turn on the lower bridge transistor 112 to control the temperature of the lower bridge transistor 112, thereby protecting the lower bridge transistor 112 from burnout.

[0161] Figure 7 This is a flowchart illustrating a control method according to an embodiment of the present invention. The following description of the control method 700 will be accompanied by… Figure 3 The power conversion circuit 300 is described in detail below.

[0162] First, the control circuit 310 determines whether the operation is in normal mode or discharge mode (step S710). When the operation is determined to be in normal mode, the upper bridge transistor 111 and the lower bridge transistor 112 are driven to generate an output voltage VOUT in the secondary coil SS (step S720). When the operation is determined to be in discharge mode, the lower bridge transistor 112 discharges the resonant capacitor CR with a discharge current IDCG (step S730).

[0163] According to one embodiment of the present invention, operation is in discharge mode before operation in normal mode. According to another embodiment of the present invention, when the control circuit 310 receives the input voltage VIN ( Figure 3 (Not shown) When the startup procedure is executed, the control circuit 310 operates in discharge mode. After the startup procedure ends, the control circuit operates in normal mode.

[0164] According to some embodiments of the present invention, after steps S720 and S730, the process returns to step S710. That is, while operating in either the normal mode or the discharge mode, it is continuously determined whether to continue operating in either the normal mode or the discharge mode, or to enter the other of the normal mode or the discharge mode.

[0165] According to another embodiment of the present invention, when the control circuit 310 executes a protection mechanism and simultaneously turns off the upper bridge transistor 111 and the lower bridge transistor 112, it operates in discharge mode. When the protection mechanism and discharge mode end, the control circuit 310 operates in normal mode. According to some embodiments of the present invention, after step S730, the control circuit 310 returns to step S710 and adjusts the discharge signal DCG based on the discharge current IDCG, thereby controlling the on-resistance of the lower bridge transistor 112 to control the discharge current IDCG to be less than a predetermined current.

[0166] According to another embodiment of the present invention, the control circuit 310 can also control the on-resistance of the lower bridge transistor 112, so that the discharge current IDCG is a fixed value. According to one embodiment of the present invention, when the resonant capacitor CR discharges through the lower bridge transistor 112, the lower bridge transistor 112 operates in the linear region.

[0167] According to some embodiments of the present invention, when the control circuit 610 operates in the discharge mode, the control circuit 610 further detects the voltage across the resonant capacitor CR, and decides whether to continue to operate in the discharge mode and discharge the resonant capacitor CR based on the relationship between the voltage across the resonant capacitor CR and the output voltage VOUT.

[0168] According to an embodiment of the present invention, when the control circuit 610 determines that the voltage across the resonant capacitor CR is less than the product of the output voltage VOUT and the number of turns ratio, the control circuit 610 ends the discharge mode and stops discharging the resonant capacitor CR, wherein the number of turns ratio is the number of turns of the primary coil PS divided by the number of turns of the secondary coil SS.

[0169] According to another embodiment of the present invention, when the control circuit 610 determines that the voltage across the resonant capacitor CR is not less than the product of the output voltage VOUT and the number of turns ratio, the control circuit 610 continues to operate in the discharge mode and continuously discharges the resonant capacitor CR.

[0170] like Figure 6 As shown in the embodiment, the voltage division ratio of the adjustable voltage detection circuit 611 is equal to the number of turns ratio. When the voltage detection signal VS is less than the output voltage VOUT, it means that the voltage across the resonant capacitor CR is less than the product of the output voltage VOUT and the number of turns ratio, and the discharge mode ends. When the voltage detection signal VS is not less than the output voltage VOUT, it means that the voltage across the resonant capacitor CR is not less than the product of the output voltage VOUT and the number of turns ratio, and the operation continues in the discharge mode.

[0171] According to other embodiments of the present invention, when the control circuit 310 operates in discharge mode, an operation time is counted. When the operation time reaches a predetermined time, the control circuit 310 ends the discharge mode and enters the normal mode, wherein the predetermined time is not less than the time required for the resonant capacitor CR to discharge to zero. According to some embodiments of the present invention, when operating in discharge mode, the lower bridge transistor 112 may be intermittently turned on to control the temperature of the lower bridge transistor 112.

[0172] This invention proposes a power conversion circuit and its control method. When the voltage across the resonant capacitor is unbalanced with the output voltage, the resonant capacitor can be discharged through the lower-bridge transistor. This helps reduce the maximum primary current of the primary coil and the maximum output current of the secondary coil, thereby reducing the conduction losses of the lower-bridge transistor and the rectifier transistor, and thus improving the conversion efficiency under low output voltage and light load conditions. Furthermore, it also avoids surges in the secondary coil, protecting circuit components from burnout.

[0173] While the embodiments and advantages of this application have been disclosed above, it should be understood that any person skilled in the art can make modifications, substitutions, and refinements without departing from the spirit and scope of this application. Furthermore, the scope of protection of this application is not limited to the processes, machines, manufacturing methods, material compositions, apparatuses, methods, and steps described in the specific embodiments herein. Any person skilled in the art can understand, from the disclosure of some embodiments of this application, current or future developed processes, machines, manufacturing methods, material compositions, apparatuses, methods, and steps, as long as they can perform substantially the same function or obtain substantially the same results in the embodiments described herein, and can be used according to some embodiments of this application. Therefore, the scope of protection of this application includes the aforementioned processes, machines, manufacturing methods, material compositions, apparatuses, methods, and steps. In addition, each claim constitutes an individual embodiment, and the scope of protection of this application also includes combinations of various claims and embodiments.

Claims

1. A power conversion circuit, characterized in that, include: A transformer includes a primary coil and a secondary coil, wherein the primary coil is coupled between a switching node and a resonant node; A resonant capacitor is coupled between the resonant node and a ground terminal; An on-bridge transistor provides an input voltage to the switching node based on an on-bridge drive signal; A lower bridge transistor, based on a lower bridge drive signal, couples the switching node to the ground terminal; and A control circuit, operating in a normal mode, generates the upper bridge drive signal and the lower bridge drive signal, causing the secondary coil to generate an output voltage. When the control circuit operates in a discharge mode, the control circuit uses the lower bridge transistor to discharge the resonant capacitor with a discharge current.

2. The power conversion circuit as described in claim 1, characterized in that, When the control circuit receives the input voltage, the control circuit executes a startup procedure; In the startup procedure, the control circuit operates in the discharge mode to discharge the resonant capacitor; After the startup procedure is initiated, the control circuit operates in the normal mode and generates the output voltage in the secondary coil.

3. The power conversion circuit as described in claim 1, characterized in that, When the control circuit executes a protection mechanism to simultaneously turn off the upper bridge transistor and the lower bridge transistor, the control circuit operates in the discharge mode to discharge the resonant capacitor.

4. The power conversion circuit as described in claim 1, characterized in that, Including: A current detection circuit is used to detect a primary current flowing through the primary coil and the resonant capacitor, and generate a current detection signal. When the control circuit operates in the discharge mode, it controls the discharge current to be less than a predetermined current based on the current detection signal.

5. The power conversion circuit as described in claim 4, characterized in that, The current detection circuit further includes: A first detection resistor is coupled between the resonant capacitor and the ground terminal; The current detection signal is generated by the voltage across the first detection resistor.

6. The power conversion circuit as described in claim 4, characterized in that, The current detection circuit further includes: An isolation transformer, comprising an isolation primary coil and an isolation secondary coil; and A second detection resistor is coupled to both ends of the isolation secondary coil; The isolation primary coil is coupled between the resonant capacitor and the ground terminal; The current detection signal is generated by the voltage across the second detection resistor.

7. The power conversion circuit as described in claim 4, characterized in that, The control circuit further includes: A current control circuit generates a discharge signal based on the relationship between the current detection signal and a reference voltage; and A selection switch provides one of the lower bridge drive signal and the discharge signal to the lower bridge transistor; When the control circuit operates in the discharge mode, the selection switch provides the discharge signal to the lower bridge transistor. When the control circuit operates in the normal mode, the selection switch provides the lower bridge drive signal to the lower bridge transistor.

8. The power conversion circuit as described in claim 7, characterized in that, The current control circuit adjusts the discharge signal, thereby controlling the discharge current to be less than the predetermined current. When the control circuit operates in the discharge mode, the lower bridge transistor operates in a linear region.

9. The power conversion circuit as described in claim 7, characterized in that, The current control circuit further includes: An error amplifier includes a positive input terminal, a negative input terminal, and an output terminal; A first control resistor is coupled between the current detection signal and the positive input terminal; and A second control resistor is coupled between the positive input terminal and the reference voltage; The output terminal generates the discharge signal; The negative input terminal is coupled to the ground terminal.

10. The power conversion circuit as described in claim 7, characterized in that, The control circuit further includes: A voltage detection circuit is used to detect the voltage across the resonant capacitor and generate a voltage detection signal; The voltage detection signal includes: A first voltage divider resistor is coupled to the resonant node; and A second voltage divider resistor is coupled between the first voltage divider resistor and the ground terminal; The voltage detection signal is generated by the voltage across the second voltage divider resistor.

11. The power conversion circuit as described in claim 10, characterized in that, When the control circuit operates in the discharge mode, the control circuit determines whether to stop discharging the resonant capacitor based on the relationship between the voltage detection signal and the output voltage. When the control circuit determines that the discharge of the resonant capacitor is complete, the selection switch provides the lower bridge drive signal to the lower bridge transistor.

12. The power conversion circuit as described in claim 10, characterized in that, The control circuit determines the relationship between the voltage across the resonant capacitor and the output voltage based on the voltage detection signal. When the voltage across the resonant capacitor is less than the product of the output voltage and the number of turns, the control circuit terminates the discharge mode. Where the voltage across the resonant capacitor is not less than the product of the output voltage and the number of turns ratio, the control circuit continues to operate in the discharge mode; The turns ratio is equal to the number of turns of the primary coil divided by the number of turns of the secondary coil.

13. The power conversion circuit as described in claim 1, characterized in that, When the control circuit operates in the discharge mode for a predetermined time, the control circuit ends the discharge mode and stops discharging the resonant capacitor. When the control circuit operates in the discharge mode, the control circuit intermittently turns on the lower bridge transistor to control the temperature of the lower bridge transistor.

14. A control method for controlling a power conversion circuit, characterized in that, The power conversion circuit includes a resonant capacitor coupled between a resonant node and a ground terminal, a transformer including a primary coil and a secondary coil, an upper-bridge transistor that provides an input voltage to a switching node, and a lower-bridge transistor that couples the switching node to the ground terminal, wherein the primary coil is coupled between the switching node and the resonant node, and the control method includes: Determine whether the operation is in a normal mode or a discharge mode; When operating in the normal mode, the upper bridge transistor and the lower bridge transistor are driven to generate an output voltage in the secondary coil; and When operating in the discharge mode, the resonant capacitor is discharged with a discharge current using the lower bridge transistor.

15. The control method as described in claim 14, characterized in that, The step of determining whether the operation is in the normal mode or the discharge mode further includes: Before operating in the normal mode, the system operates in the discharge mode.

16. The control method as described in claim 14, characterized in that, The step of determining whether the operation is in the normal mode or the discharge mode further includes: Upon receiving the input voltage, a startup procedure is executed; In the startup procedure, operation is performed in the discharge mode; and When the startup process ends, it operates in the normal mode.

17. The control method as described in claim 14, characterized in that, The step of determining whether the operation is in the normal mode or the discharge mode further includes: When a protection mechanism is executed to simultaneously shut down both the upper-bridge transistor and the lower-bridge transistor, the system operates in the discharge mode; and When the protection mechanism ends, it operates in the normal mode.

18. The control method as described in claim 14, characterized in that, The step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: The discharge current is detected, and a current detection signal is generated; and The lower bridge transistor is controlled based on the current detection signal, such that the discharge current is less than a predetermined current.

19. The control method as described in claim 18, characterized in that, The step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: A discharge signal is generated using an error amplifier based on the relationship between the current detection signal and a reference voltage; and The discharge signal is used to control the lower bridge transistor, so that the discharge current is less than the predetermined current; When operating in the discharge mode, the lower bridge transistor operates in a linear region.

20. The control method as described in claim 18, characterized in that, The step of discharging the resonant capacitor with the discharge current using the lower bridge transistor when operating in the discharge mode further includes: A voltage detection signal is generated by detecting the voltage across the resonant capacitor. Based on the relationship between the voltage across the resonant capacitor and the output voltage, a decision is made as to whether to continue discharging the resonant capacitor. When the voltage across the resonant capacitor is less than the output voltage multiplied by the turns ratio, discharging of the resonant capacitor stops; and When the voltage across the resonant capacitor is not less than the output voltage multiplied by the number of turns, the resonant capacitor continues to discharge. The turns ratio is equal to the number of turns of the primary coil divided by the number of turns of the secondary coil.

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