Power converting circuit and control method thereof

Abstract

A power conversion circuit includes 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, with the primary coil coupled between a switching node and a resonant node. The resonant capacitor is coupled between the resonant node and ground. 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 ground based on a lower-bridge drive signal. The control circuit generates upper-bridge drive signals and lower-bridge drive signals. When the power conversion circuit starts, the control circuit generates a pre-charge signal to pre-charge the resonant capacitor.

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 that can achieve zero-voltage switching of the upper bridge transistor during startup. Prior 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, power conversion circuits in portable electronic devices are gradually moving towards 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 presents a power conversion circuit and its control method. When the power conversion circuit starts up, pre-charging the resonant capacitor during the startup procedure reduces voltage surges at the switching nodes. After the startup procedure ends, turning on the lower bridge transistor first and then the upper bridge transistor helps the upper bridge transistor achieve zero-voltage switching, which further stabilizes the voltage at the switching nodes, thereby reducing noise, improving the reliability of circuit components, and increasing the conversion efficiency of the power conversion circuit.

[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 generates the upper-bridge drive signal and the lower-bridge drive signal. When the power conversion circuit is started, the control circuit generates a pre-charge signal to pre-charge the resonant capacitor.

[0006] According to some embodiments of the present invention, the control circuit further includes a charging diode. The pre-charge signal pre-charges the resonant capacitor via the charging diode.

[0007] According to some embodiments of the present invention, the power conversion circuit described above is used to convert the input voltage into an output voltage.

[0008] According to some embodiments of the present invention, the power conversion circuit further includes a rectifier circuit. The rectifier circuit is used to convert the energy of the secondary coil into the output voltage.

[0009] According to some embodiments of the present invention, the control circuit generates a charging current from the input voltage. The control circuit uses the charging current to generate a supply voltage that powers the control circuit. The control circuit uses the charging current to generate a pre-charge signal for pre-charging the resonant capacitor.

[0010] According to some embodiments of the present invention, when the supply voltage exceeds a threshold voltage, the control circuit stops generating the pre-charge signal. When the pre-charge signal stops being generated, the control circuit generates the upper bridge drive signal and the lower bridge drive signal.

[0011] According to some embodiments of the present invention, when the pre-charge signal stops being generated, the control circuit first turns on the lower bridge transistor and then turns on the upper bridge transistor, so that the upper bridge transistor can achieve zero voltage conversion.

[0012] According to some embodiments of the present invention, the power conversion circuit further includes a charging resistor. The charging resistor is coupled to the input voltage and generates a charging current. The control circuit includes a startup circuit. The startup circuit is used to generate the pre-charge signal.

[0013] According to some embodiments of the present invention, the startup circuit includes a normally open transistor, a startup transistor, a startup resistor, and a startup diode. The normally open transistor receives the charging current. The startup transistor includes a gate terminal, a drain terminal, and a source terminal, wherein the drain terminal is coupled to the normally open transistor, and the source terminal generates the pre-charge signal. The startup resistor is coupled between the gate terminal and the drain terminal. The startup diode includes an anode terminal and a cathode terminal, wherein the anode terminal is coupled to the source terminal, and the cathode terminal generates a supply voltage. The control circuit is powered by the supply voltage.

[0014] According to some embodiments of the present invention, the startup circuit further includes a comparator. The comparator compares the supply voltage with a threshold voltage to generate a comparison signal. The comparison signal is provided to the gate terminal. When the supply voltage exceeds the threshold voltage, the comparator does not turn on the startup transistor to stop generating the charging current and the pre-charge signal.

[0015] According to some embodiments of the present invention, the transformer further includes an auxiliary coil, a supply capacitor, and a supply diode. The auxiliary coil generates an auxiliary coil voltage. The supply capacitor is used to maintain the supply voltage. The supply diode is used to unidirectionally charge the supply capacitor using the auxiliary coil voltage to generate the supply voltage, and to prevent the supply voltage from affecting the operation of the transformer. When the start-up transistor is not conducting, the auxiliary coil generates the supply voltage to power the control circuit.

[0016] According to some embodiments of the present invention, when the auxiliary coil generates the supply voltage, the starting diode is used to isolate the supply voltage and the source terminal to prevent the supply voltage from affecting the pre-charge signal.

[0017] According to some embodiments of the present invention, the power conversion circuit described above is a resonant flyback power conversion circuit.

[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: pre-charging the resonant capacitor when the input voltage is provided to the power conversion circuit; determining whether the voltage across the resonant capacitor exceeds a target voltage; and driving the upper-bridge transistor and the lower-bridge transistor when the voltage across the resonant capacitor exceeds the target voltage.

[0019] According to some embodiments of the present invention, the step of pre-charging the resonant capacitor when the power conversion circuit is started further includes: generating a charging current from the input voltage; pre-charging the resonant capacitor using the charging current; and generating a supply voltage using the charging current.

[0020] According to some embodiments of the present invention, the power conversion circuit further includes a control circuit for executing the control method. The control circuit is powered by the supply voltage.

[0021] According to some embodiments of the present invention, the step of determining whether the voltage across the resonant capacitor reaches the target voltage further includes: determining whether the supply voltage exceeds a threshold voltage; and stopping the generation of the charging current when the supply voltage exceeds the threshold voltage. The supply voltage is positively correlated with the voltage across the resonant capacitor.

[0022] According to some embodiments of the present invention, when the pre-charging of the resonant capacitor is stopped, the supply voltage is generated by an auxiliary coil of one of the transformers.

[0023] According to some embodiments of the present invention, the step of driving the upper bridge transistor and the lower bridge transistor when the voltage across the resonant capacitor exceeds the target voltage further includes: turning on the lower bridge transistor when the voltage across the resonant capacitor exceeds the target voltage; and turning on the upper bridge transistor after the lower bridge transistor is not turned on.

[0024] According to some embodiments of the present invention, when the lower bridge transistor is turned on first and then the upper bridge transistor is turned on, the current in the primary coil helps the upper bridge transistor achieve zero-voltage switching, thereby increasing the conversion efficiency of the power conversion circuit and increasing the voltage stability of the switching node. Simple Explanation of the Diagram

[0025] Figure 1 shows a block diagram of a power conversion circuit according to one embodiment of the present invention; Figure 2 shows waveforms of a power conversion circuit according to one embodiment of the present invention; Figure 3 shows a block diagram of a power conversion circuit according to another embodiment of the present invention; Figure 4 shows a block diagram of a startup circuit according to one embodiment of the present invention; and Figure 5 shows a flowchart of a control method according to one embodiment of the present invention. Implementation

[0026] The following description is an embodiment of this disclosure. Its purpose is to illustrate the general principles of this disclosure and should not be regarded as a limitation thereof. The scope of this disclosure shall be defined by the claims.

[0027] It is worth noting that the following disclosure provides multiple embodiments or examples for practicing different features of this disclosure. The specific examples and arrangements of components described below are only used to briefly illustrate the spirit of this disclosure and are not intended to limit the scope of this disclosure. Furthermore, the same component symbols or words may be used repeatedly in multiple examples in the following description. However, the purpose of repetition is only to provide a simplified and clear explanation and is not intended to limit the relationship between the multiple embodiments and / or configurations discussed below.

[0028] Furthermore, the description 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 features are not in direct contact.

[0029] 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.

[0030] 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 disclosed herein.

[0031] This disclosure includes several embodiments that can be understood in conjunction with the accompanying drawings, which are considered part of the description of these embodiments. It should be understood that the drawings 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 disclosed embodiments. Furthermore, the structures and devices in the drawings are shown schematically to clearly demonstrate the features of the disclosed embodiments.

[0032] 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 these terms.

[0033] 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 disclosure 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 disclosure, and should not be interpreted in an idealized or overly formal manner, unless specifically defined in the embodiments of this disclosure.

[0034] In some embodiments disclosed herein, terms such as "connection" and "interconnection" used to refer to joining or linking, unless otherwise defined, may refer to two structures being in direct contact, or to two structures not being in direct contact, with other structures disposed between them. Furthermore, these terms regarding joining or linking may also include situations where both structures are movable or both structures are fixed.

[0035] In a diagram, similar elements and / or features may have the same element symbol. Various elements of the same type can be distinguished by adding letters or numbers after the element symbol to differentiate similar elements and / or similar features.

[0036] Figure 1 shows a block diagram of a power conversion circuit according to one embodiment of the present invention. As shown in Figure 1, the power conversion circuit 100 includes an upper bridge transistor 111, a lower bridge transistor 112, a resonant capacitor CR, a transformer TM, a voltage generation circuit 120, a rectifier circuit 130, a secondary control circuit 140, an optocoupler PD, a control circuit 150, a level shifting circuit 160, an upper bridge drive circuit HSD, and a lower bridge drive circuit LSD.

[0037] 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 present 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 present 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 the ground.

[0038] 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, a secondary coil SS, and an auxiliary coil AS. The primary coil PS is coupled between the switching node SW and the resonant node NR. The auxiliary coil AS is coupled between the auxiliary node NA and the ground terminal, and an auxiliary coil voltage VNA is generated at the auxiliary node NA. The output current IOUT generated by the secondary coil SS is rectified by the rectifier circuit 130 to generate the output voltage VOUT.

[0039] 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.

[0040] The voltage generating circuit 120 is used to generate a supply voltage VDD using the auxiliary coil voltage VNA. The voltage generating circuit 120 includes a supply diode DSP and a supply capacitor CSP. The supply diode DSP is used to unidirectionally charge the supply capacitor CSP using the auxiliary coil voltage VNA to generate the supply voltage VDD, thereby preventing the supply voltage VDD from affecting the operation of the transformer TM. According to one embodiment of the present invention, when the auxiliary coil voltage VNA generated by the auxiliary coil AS is less than the supply voltage VDD, the supply capacitor CSP is used to maintain the supply voltage VDD.

[0041] The rectifier circuit 130 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 and generate the output voltage VOUT. When the rectifier transistor TR is not turned on, the voltage across the drain terminal to the source terminal of the rectifier transistor TR is the drain voltage VD.

[0042] The secondary control circuit 140 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 140 further generates a gate signal SG to convert the output current IOUT generated by the secondary coil SS into the output voltage VOUT.

[0043] The control circuit 150 is powered by the supply voltage VDD and generates an upper bridge drive signal SH and a lower bridge drive signal SL based on the feedback voltage VFB. The level shifting circuit 160 shifts the voltage level of the upper bridge drive signal SH to the input voltage VIN. The upper bridge drive circuit HSD generates an upper bridge gate drive signal HSG based on the shifted signal to drive the upper bridge transistor 111. The lower bridge drive circuit LSD generates a lower bridge gate drive signal LSG based on the lower bridge drive signal SL to drive the lower bridge transistor 112.

[0044] According to some embodiments of the present invention, the control circuit 150 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.

[0045] Figure 2 shows a waveform diagram of a power conversion circuit according to one embodiment of the present invention. The following description of waveform 200 will be provided in conjunction with the power conversion circuit 100 in Figure 1 for detailed explanation. 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), wherein 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.

[0046] When the upper bridge transistor 111 is not conducting (i.e., the upper bridge drive signal SH is at a low logic level), the transformer TM demagnetizes. During the demagnetization time TDS, the transformer TM generates an output current IOUT, and the conduction 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 conduction time TSL under 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: (Formula 1)

[0047] In this context, NP represents the number of turns of the primary coil PS, NS represents 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.

[0048] The demagnetization time TDS system is shown in Formula 2: (Formula 2)

[0049] When the upper bridge transistor 111 is turned on, This refers to the voltage used to magnetize the transformer TM.

[0050] At the second time point T2, the upper bridge drive signal SH transitions to a low logic level and the upper bridge transistor 111 is not turned on. At the third time point T3, the lower bridge drive signal SL transitions to a high logic level and the lower bridge transistor 112 is turned on. 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 when the upper bridge transistor 111 is not turned on to when the lower bridge transistor 112 is turned on.

[0051] 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, thereby pulling down the voltage of the switching node SW, so that the lower bridge transistor 112 achieves zero-voltage switching. At the third time point T3, the trans-voltage system of the primary coil PS is the resonant voltage VCR of the resonant capacitor CR.

[0052] Between the third time point T3 and the fourth time point T4, the upper bridge transistor 111 is not conducting, while the lower bridge transistor 112 conducts under zero-voltage switching. The rectifier transistor TR is conducting, 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. Furthermore, the primary current IP remains positive and flows into the resonant capacitor CR.

[0053] 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.

[0054] During the period from time point T4 to time point T5, the upper-bridge transistor 111 remains non-conducting while the lower-bridge transistor 112 remains on. The energy of the transformer TM is continuously transferred to the secondary coil SS, and at this time, the energy is provided by the resonant capacitor CR. Furthermore, since the lower-bridge transistor 112 remains on, the energy of the resonant capacitor CR is used to bring the magnetizing current IM to a negative value.

[0055] At the fifth time point T5, the rectifier transistor TR does not conduct based on the gate signal SG, thus ending the demagnetization time TDS. From the fifth time point T5 to the sixth 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 de-conducted.

[0056] During the period from the sixth time point T6 to the seventh time point T7, both the upper bridge transistor 111 and the lower bridge transistor 112 are not conducting. Furthermore, the primary current IP, induced as a negative current during the period from the fifth time point T5 to the sixth time point 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 the period from the sixth time point T6 to the seventh time point T7 is the dead time from when the lower bridge transistor 112 is not conducting to when the upper bridge transistor 111 is conducting.

[0057] 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.

[0058] According to some embodiments of the present invention, when the power conversion circuit 100 is powered on, since both the output voltage VOUT and the resonant voltage VCR are zero, no negative primary current IP is generated (as shown at time points T5 to T7 in Figure 2), thus failing to assist the upper bridge transistor 111 in achieving zero-voltage switching. Furthermore, as shown in Equation 2, the demagnetization time TDS will be very long, easily causing hard switching of the upper bridge transistor 111, resulting in a voltage spike at the switching node SW. This voltage spike generates noise and reduces component reliability. According to some embodiments of the present invention, the power conversion circuit 100 startup can also be considered as the input voltage VIN being supplied to the power conversion circuit 100.

[0059] For example, when the power conversion circuit 100 starts and the upper bridge transistor 111 is turned on, a voltage surge will occur at the switching node SW. Since the resonant voltage VCR of the resonant capacitor CR is zero, the voltage at the switching node SW is transmitted to the secondary coil SS via the transformer TM. Furthermore, since the rectifier transistor TR is not turned on when the upper bridge transistor 111 is turned on, the energy in the secondary coil SS has nowhere to dissipate, causing the drain voltage VD to rise, and may even burn out the rectifier transistor TR.

[0060] In order to avoid noise caused by surges generated during switching node SW, and in order to protect the rectifier transistor TR and improve the conversion efficiency of power conversion circuit 100, it is necessary to optimize power conversion circuit 100.

[0061] Figure 3 shows a block diagram of a power conversion circuit according to another embodiment of the present invention. Compared with the power conversion circuit 100 in Figure 1, the power conversion circuit 300 in Figure 3 further includes a charging resistor RH, and the control circuit 310 of the power conversion circuit 300 further includes a startup circuit 311. When the power conversion circuit 300 is started, the control circuit 310 uses the charging resistor RH to generate a charging current ICHG from the input voltage VIN, and the startup circuit 311 uses the charging current ICHG to precharge the resonant capacitor CR and generate a supply voltage VDD.

[0062] According to some embodiments of the present invention, when the power conversion circuit 300 is started, the start-up circuit 311 first pre-charges the resonant capacitor CR, and then the control circuit 310 drives the upper bridge transistor 111 and the lower bridge transistor 112. According to some embodiments of the present invention, after the start-up circuit 311 pre-charges the resonant capacitor CR, the control circuit 310 first turns on the lower bridge transistor 112 and then turns on the upper bridge transistor 111, so that the upper bridge transistor 111 can switch at zero voltage and stabilize the voltage of the switching node SW, thereby protecting the rectifier transistor TR from burnout.

[0063] Figure 4 shows a block diagram of a startup circuit according to one embodiment of the present invention. As shown in Figure 4, the startup circuit 400 includes a normally open transistor TNO, a startup transistor TST, a startup resistor RST, a startup diode DST, a charging diode DCHG, and a comparator CMP.

[0064] The gate of the normally open transistor TNO is coupled to ground and receives the charging current ICHG generated by the charging resistor RH in Figure 3. According to one embodiment of the present invention, the charging resistor RH can step down the input voltage VIN and then provide it to the startup circuit 400. The startup transistor TST includes a gate terminal G, a drain terminal D, and a source terminal S, wherein the startup resistor RST is coupled to the gate terminal G and the drain terminal D, and the source terminal S generates a pre-charge signal PCHG. The startup diode DST is coupled between the pre-charge signal PCHG and the supply voltage VDD in Figure 3, and the charging diode DCHG is coupled between the pre-charge signal PCHG and the resonant voltage VCR.

[0065] According to some embodiments of the present invention, the start diode DST is used to unidirectionally provide the precharge signal PCHG to the supply voltage VDD, and the charging diode DCHG is used to unidirectionally provide the precharge signal PCHG to the resonant voltage VCR, while the supply voltage VDD and the resonant voltage VCR cannot be provided to the precharge signal PCHG in the reverse direction. The comparator CMP is used to compare the supply voltage VDD and the threshold voltage VTH, and provides the comparison result RCM to the gate terminal G. According to some embodiments of the present invention, when the supply voltage VDD exceeds the threshold voltage VTH, the comparator CMP does not turn on the start transistor TST, and stops generating the precharge signal PCHG.

[0066] According to some embodiments of the present invention, when the comparator CMP does not turn on the startup transistor TST, the output terminal of the comparator CMP couples the gate terminal G of the startup transistor TST to the ground terminal, wherein the resistance values ​​of the charging resistor RH and the startup resistor RST are used to determine the magnitude of the charging current ICHG flowing to the ground terminal. According to some embodiments of the present invention, when the input voltage VIN is provided to the power conversion circuit 300, that is, when the power conversion circuit 300 is started, the comparator CMP floats the gate terminal G of the startup transistor TST, and the input voltage VIN is provided to the gate terminal G through the charging resistor RH, the normally open transistor TNO, and the startup resistor RST, thereby turning on the startup transistor TST.

[0067] According to some embodiments of the present invention, when the input voltage VIN is provided to the power conversion circuit 300 of Figure 3 to start the power conversion circuit 300, the start-up circuit 311 uses the charging current ICHG through the charging diode DCHG to precharge the resonant capacitor CR, thereby generating the resonant voltage VCR.

[0068] Furthermore, the charging current ICHG simultaneously charges the supply capacitor CSP in Figure 3 via the startup diode DST, generating a supply voltage VDD, which powers the control circuit 310. In other words, when the power conversion circuit 300 is started, the startup circuit 400 generates the supply voltage VDD to power the control circuit 310. According to some embodiments of the present invention, the supply voltage VDD can power the comparator CMP.

[0069] According to other embodiments of the present invention, when the supply voltage VDD exceeds the threshold voltage VTH, the comparison result RCM generated by the comparator CMP is used to prevent the start-up transistor TST from conducting, indicating that the power conversion circuit 300 has completed startup. The control circuit 310 further drives the upper bridge transistor 111 and the lower bridge transistor 112 based on the comparison result RCM. When the start-up transistor TST is not conducting, the startup circuit 400 stops generating the pre-charge signal PCHG and simultaneously stops generating the supply voltage VDD.

[0070] Furthermore, when the startup circuit 400 stops generating the pre-charge signal PCHG, the control circuit 310 drives the upper bridge transistor 111 and the lower bridge transistor 112 based on the comparison result RCM, causing the auxiliary coil AS to generate a supply voltage VDD to continuously power the control circuit 310. According to one embodiment of the present invention, when the power conversion circuit 300 starts, since the supply voltage VDD is positively correlated with the resonant voltage VCR, determining whether the supply voltage VDD exceeds the threshold voltage VTH is equivalent to determining whether the resonant voltage VCR exceeds the target voltage. In other words, when the supply voltage VDD exceeds the threshold voltage VTH, it means that the resonant voltage VCR exceeds the target voltage. Therefore, the control circuit 310 can end the startup procedure based on the comparison result RCM and begin driving the upper bridge transistor 111 and the lower bridge transistor 112.

[0071] According to some embodiments of the present invention, after the power conversion circuit 300 finishes the startup program, the lower bridge transistor 112 is turned on first and then the upper bridge transistor 111 is turned on, which can generate the negative current of the primary current IP shown at the fifth time point T5 to the seventh time point T7 in Figure 2, which helps the upper bridge transistor 111 to achieve zero voltage conversion.

[0072] Figure 5 shows a flowchart of a control method according to one embodiment of the present invention. The following description of the control method 500 will be provided in conjunction with the power conversion circuit 300 in Figure 3 and the startup circuit 400 in Figure 4 for detailed explanation.

[0073] First, when the input voltage VIN is supplied to the power conversion circuit 300 (i.e., the power conversion circuit 300 is started), the resonant capacitor CR is pre-charged using the start-up circuit 311 in Figure 3 and the start-up circuit 400 in Figure 4 (step S510). Next, it is determined whether the voltage across the resonant capacitor CR (i.e., the resonant voltage VCR) exceeds the target voltage (step S520).

[0074] According to some embodiments of the present invention, as shown in Figure 4, since the pre-charge signal PCHG generates the resonant voltage VCR and the supply voltage VDD via the charging diode DCHG and the activation diode DST respectively, the supply voltage VDD is positively correlated with the resonant voltage VCR. In other words, the comparator CMP determining whether the supply voltage VDD exceeds the threshold voltage VTH is equivalent to the comparator CMP determining whether the resonant voltage VCR exceeds the target voltage.

[0075] When step S520 is determined to be yes, it means that the power conversion circuit 300 has completed the startup procedure. The control circuit 310 first turns on the lower bridge transistor 112 (step S530), and then turns on the upper bridge transistor 111 (step S540). This helps the upper bridge transistor 111 achieve zero voltage conversion and stabilize the voltage of the switching node SW, thereby reducing noise and improving the reliability of the rectifier transistor TR.

[0076] According to some embodiments of the present invention, in steps S510 and S520, the supply voltage VDD supplying power to the control circuit 310 is generated by the startup circuit 400 in Figure 4. In steps S530 and S540, the supply voltage VDD supplying power to the control circuit 310 is generated by the auxiliary coil AS in Figure 3 charging the supply capacitor CSP via the supply diode DSP.

[0077] This invention presents a power conversion circuit and its control method. When the power conversion circuit starts up, pre-charging the resonant capacitor during the startup procedure reduces voltage surges at the switching nodes. After the startup procedure ends, turning on the lower bridge transistor first and then the upper bridge transistor helps the upper bridge transistor achieve zero-voltage switching, which further stabilizes the voltage at the switching nodes, thereby reducing noise, improving the reliability of circuit components, and increasing the conversion efficiency of the power conversion circuit.

[0078] While the embodiments and advantages of this disclosure have been disclosed above, it should be understood that anyone skilled in the art can make modifications, substitutions, and refinements without departing from the spirit and scope of this disclosure. Furthermore, the scope of protection of this disclosure is not limited to the processes, machines, manufacturing, material composition, apparatus, methods, and steps described in the specific embodiments of this specification. Anyone skilled in the art can understand from the disclosure of some embodiments of this disclosure the current or future development of processes, machines, manufacturing, material composition, apparatus, 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 disclosure. Therefore, the scope of protection of this disclosure includes the aforementioned processes, machines, manufacturing, material composition, apparatus, methods, and steps. In addition, each claim constitutes an individual embodiment, and the scope of protection of this disclosure also includes combinations of various claim claims and embodiments.

[0079] 100, 300: Power conversion circuit 111: Upper Bridge Transistor 111D: Upperbridge parasitic diode 112: Lower bridge transistor 112D: Lower bridge parasitic diode 120: Voltage generation circuit 130: Rectifier circuit 140: Secondary control circuit 150, 310: Control circuit 160: Level shifting circuit 311,400: Startup circuit CR: Resonant capacitor TM: Transformer PD: Optical Coupler HSD: Upper Bridge Driver Circuit LSD: Lower bridge driver circuit HSG: Upper bridge gate drive signal LSG: Lower bridge gate drive signal VIN: Input voltage SW: Switch Node NR: Resonant Node VCR: Resonant Voltage PS: Primary coil SS: Secondary coil AS: Auxiliary coil NA: Auxiliary Node VNA: Auxiliary coil voltage IOUT: Output current VOUT: Output voltage VDD: Supply voltage DSP: Supply diode CSP: Supply capacitors TR: Rectifier Transistor COUT: Output capacitor VD: Drain Voltage DR: Rectifier Parasitic Diode IFB: Feedback Current SG: Gate signal VFB: Feedback Voltage SH: Overpass drive signal SL: Lower bridge drive signal HSG: Upper bridge gate drive signal LSG: Lower bridge gate drive signal 200: Waveform Diagram TW: Bridge connection time IM: Magnetizing current TDS: Demagnetization time TSL: Downbridge conduction time TRL: First Dead Zone Time TRH: Second Dead Zone Time T1: First Time Point T2: Second Time Point T3: Third Time Point T4: Fourth Time Point T5: Fifth Time Point T6: Sixth Time Point T7: Seventh Time Point RH: Charging resistance ICHG: Charging Current TNO: Normally Open Transistor TST: Start Transistor RST: Start-up resistor DST: Start the diode DCHG: Charged Diode CMP: Comparator G: Gate extreme D: Extreme S: Source Extreme PCHG: Precharge signal VTH: Threshold voltage RCM: Comparison Results 500: Control Method S510~S540: Procedure Flow

Claims

1. A power conversion circuit, comprising: 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 upper-bridge transistor provides an input voltage to the switching node based on an upper-bridge drive signal; a lower-bridge transistor couples the switching node to the ground terminal based on a lower-bridge drive signal; and a control circuit that generates the upper-bridge drive signal and the lower-bridge drive signal; wherein when the power conversion circuit is activated, the control circuit generates a pre-charge signal to pre-charge the resonant capacitor; wherein the control circuit further includes a charging diode; wherein the pre-charge signal pre-charges the resonant capacitor via the charging diode.

2. The power conversion circuit of claim 1 further includes: A rectifier circuit for converting the energy of the secondary coil into an output voltage, and includes: an output capacitor; and a rectifier transistor for charging the output capacitor with an output current output from the secondary coil to generate an output voltage.

3. As in claim 2, when the power conversion circuit is started, the control circuit first pre-charges the resonant capacitor, then turns on the lower bridge transistor first and then turns on the upper bridge transistor.

4. The power conversion circuit of claim 1, wherein the control circuit generates a charging current from the input voltage; wherein the control circuit uses the charging current to generate a supply voltage for powering the control circuit; wherein the control circuit uses the charging current to generate a pre-charge signal for pre-charging the resonant capacitor.

5. The power conversion circuit of claim 4, wherein when the supply voltage exceeds a threshold voltage, the control circuit stops generating the pre-charge signal; wherein when the pre-charge signal stops generating, the control circuit generates the upper bridge drive signal and the lower bridge drive signal.

6. The power conversion circuit of claim 5, wherein when the pre-charge signal stops being generated, the control circuit first turns on the lower bridge transistor and then turns on the upper bridge transistor, so that the upper bridge transistor can achieve zero voltage conversion.

7. The power conversion circuit of claim 1 further includes: A charging resistor is coupled to the above input voltage and generates a charging current. The aforementioned control circuit includes: a startup circuit for generating the aforementioned pre-charge signal.

8. The power conversion circuit as claimed in claim 7, wherein the aforementioned startup circuit includes: A normally open transistor receives the charging current; a startup transistor includes a gate, a drain, and a source, wherein the drain is coupled to the normally open transistor, and the source generates the pre-charge signal; a startup resistor is coupled between the gate and the drain; and a startup diode includes an anode and a cathode, wherein the anode is coupled to the source, and the cathode generates a supply voltage; wherein the control circuit is powered by the supply voltage.

9. The power conversion circuit of claim 8, wherein the aforementioned startup circuit further includes: A comparator compares the supply voltage with a threshold voltage to generate a comparison signal; wherein the comparison signal is provided to the gate terminal; wherein when the supply voltage exceeds the threshold voltage, the comparator does not turn on the start-up transistor to stop generating the charging current and the pre-charge signal.

10. The power conversion circuit of claim 9, wherein the transformer further comprises: An auxiliary coil generates an auxiliary coil voltage; a supply capacitor maintains the supply voltage; and a supply diode is used to unidirectionally charge the supply capacitor using the auxiliary coil voltage to generate the supply voltage, while preventing the supply voltage from affecting the operation of the transformer; wherein when the start-up transistor is not conducting, the auxiliary coil generates the supply voltage to power the control circuit.

11. The power conversion circuit of claim 10, wherein when the auxiliary coil generates the supply voltage, the start-up diode is used to isolate the supply voltage and the source terminal to prevent the supply voltage from affecting the pre-charge signal.

12. The power conversion circuit of claim 8, wherein the charging resistor is used to step down the input voltage before supplying it to the startup circuit.

13. A control method for controlling a power conversion circuit, wherein 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 for providing an input voltage to a switching node, and a lower-bridge transistor for coupling the switching node to the ground terminal, wherein the primary coil is coupled between the switching node and the resonant node, wherein the control method includes: When the input voltage is supplied to the power conversion circuit, the resonant capacitor is pre-charged; it is determined whether the voltage across the resonant capacitor exceeds a target voltage; and when the voltage across the resonant capacitor exceeds the target voltage, the upper bridge transistor and the lower bridge transistor are driven.

14. The control method of claim 13, wherein the step of pre-charging the resonant capacitor when the power conversion circuit is started further includes: A charging current is generated from the above input voltage; the above charging current is used to precharge the above resonant capacitor; And a supply voltage is generated using the aforementioned charging current.

15. The control method of claim 14, wherein the power conversion circuit further includes a control circuit for executing the control method; wherein the control circuit is powered by the supply voltage.

16. The control method of claim 14, wherein the step of determining whether the voltage across the resonant capacitor reaches the target voltage further includes: Determine whether the supply voltage exceeds a threshold voltage; and when the supply voltage exceeds the threshold voltage, stop generating the charging current; wherein the supply voltage is positively correlated with the voltage across the resonant capacitor.

17. The control method of claim 16, wherein when the pre-charging of the resonant capacitor is stopped, the supply voltage is generated by an auxiliary coil of one of the transformers.

18. The control method of claim 13, wherein the step of driving the upper bridge transistor and the lower bridge transistor when the voltage across the resonant capacitor exceeds the target voltage further includes: When the voltage across the resonant capacitor exceeds the target voltage, the lower bridge transistor is turned on; and when the lower bridge transistor is turned off, the upper bridge transistor is turned on.

19. The control method of claim 18, wherein when the lower bridge transistor is turned on first and then the upper bridge transistor is turned on, the current of the primary coil helps the upper bridge transistor achieve zero-voltage switching, thereby increasing the conversion efficiency of the power conversion circuit and increasing the voltage stability of the switching node.

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