Llc resonant converter and control method thereof

Abstract

This invention provides an LLC converter comprising a square wave generator, an LLC resonant circuit, a first synchronous rectifier switch, an operating power supply capacitor, and a synchronous rectifier controller. The square wave generator includes at least two power switches connected between an input power line and an input ground line to provide a square wave input at an input terminal. The LLC resonant circuit is connected to the input terminal and includes a primary winding of a transformer connected in series and a resonant capacitor. The transformer includes a first secondary winding. The first synchronous rectifier switch has a first detection terminal. The first synchronous rectifier switch can be connected to the first secondary winding and an output ground line. The first secondary winding is connected between the first detection terminal and an output power line. The operating power supply capacitor provides an operating power supply. The synchronous rectifier controller controls the first synchronous rectifier switch based on a first detected voltage at the first detection terminal. The synchronous rectifier controller includes a power controller configured to draw current from the first detection terminal to establish the operating power supply and power the synchronous rectifier controller.

Description

[Technical Field]

[0001] This invention relates generally to synchronous rectification of LLC converters, and more particularly to the establishment of the operating power supply for the synchronous rectification controller in an LLC converter. [Previous Technology]

[0001] LLC converters are a type of resonant converter, which typically offers a stable output waveform, high conversion efficiency, and high output power. Generally, a resonant converter converts a DC input power supply into a sinusoidal signal. This conversion is achieved through a switch network, which provides a square-wave voltage to a resonant tank. After filtering by the resonant tank, the fundamental frequency of the square-wave voltage is roughly retained, thus approximating a sinusoidal input current. Due to the inductance effect, an alternating current is generated on the secondary side of the LLC converter, which, after rectification, can be used to establish the output power supply.

[0002] LLC converters are generally used for high current output. To improve conversion efficiency, synchronous rectification can be used on the secondary side of the LLC converter, that is, a synchronous rectifier switch and a synchronous rectifier controller can be used instead of the traditional single rectifier diode. In this way, the high power consumption caused by the approximately fixed forward bias voltage of the rectifier diode when conducting high current can be reduced or eliminated.

[0003] Although synchronous rectification may increase conversion efficiency, it may also introduce other problems that require special handling. [Summary of the Invention]

[0004] This embodiment of the invention provides an LLC converter, which includes a square wave generator, an LLC resonant circuit, a first synchronous rectifier switch, an operating power supply capacitor, and a synchronous rectifier controller. The square wave generator includes at least two power switches connected between an input power line and an input ground line to provide a square wave input at an input terminal. The LLC resonant circuit is connected to the input terminal and includes a primary winding of a transformer connected in series and a resonant capacitor. The transformer includes a first secondary winding. The first synchronous rectifier switch has a first detection terminal. The first synchronous rectifier switch can be connected to the first secondary winding and an output ground line. The first secondary winding is connected between the first detection terminal and an output power line. The operating power supply capacitor is used to provide an operating power supply. The synchronous rectifier controller controls the first synchronous rectifier switch according to a first detection voltage on the first detection terminal. The synchronous rectifier controller has a power controller architecture configured to draw current from the first detection terminal to establish the operating power supply and supply power to the synchronous rectifier controller.

[0005] This embodiment of the invention provides a control method for an LLC converter, comprising: providing a square wave voltage to an input terminal of a resonant circuit, wherein the resonant circuit is connected between the input terminal and a power supply line, and includes a primary winding of a transformer connected in series and a resonant capacitor, the transformer including a first secondary winding; providing a first synchronous rectifier switch and a synchronous rectifier controller, wherein the first synchronous rectifier switch is connected between a first detection terminal and an output ground line and is controlled by the synchronous rectifier controller, the first secondary winding is connected between the first detection terminal and an output power supply line; and drawing current from the first detection terminal, charging an operating power supply capacitor through the synchronous rectifier controller to establish an operating power supply and supply power to the synchronous rectifier controller. [Simplified Explanation of the Diagram]

[0006] Figure 1 shows an LLC converter.

[0006] Figure 2 shows the signal waveforms of the square wave signal VSW, control signal SG1, detection voltage VDT1, inductor current ILS1, control signal SG2, detection voltage VDT2, and inductor current ILS2 at the endpoint SW in Figure 1.

[0006] Figure 3 shows an example of a synchronous rectifier switch.

[0006] Figure 4 shows an LLC converter implemented according to the present invention.

[0006] Figure 5 shows a synchronous rectifier controller according to the present invention.

Implementation Method

[0007] In this specification, some identical symbols represent components with the same or similar structure, function, or principle, which can be deduced by those with ordinary knowledge in the industry based on the teachings of this specification. For the sake of brevity, components with the same symbols will not be repeated.

[0008] Figure 1 shows an LLC converter 100 used to convert the input power supply VIN on the primary side into the output power supply VOUT on the secondary side. The LLC converter 100 uses synchronous rectification with two synchronous rectifier switches SR1 and SR2 to rectify the individual inductor currents ILS1 and ILS output by the two secondary windings LS1 and LS2.

[0009] On the primary side, the upper arm switch HS and the lower arm switch LS are connected in series between the input power line IN and the input ground line GNDI through the input terminal SW, forming a half-bridge structure, which can be regarded as a square wave generator. The input capacitor CI acts as a filter capacitor, connected between the input power line IN and the input ground line GNDI, which can stabilize the voltage of the input power supply VIN. The upper arm switch HS and the lower arm switch LS are controlled by the upper arm control signal HI and the lower arm control signal LO, respectively.

[0010] The resonant inductor LR, the primary winding LP of the transformer TF, and the resonant capacitor CR are connected in series between the input terminal SW and the input ground line GNDI to form an LLC resonant tank TNK. In one embodiment, the resonant inductor LR may not be an independent component, but rather a leakage inductance in the primary winding LP that is not inductively coupled to the secondary windings LS1 and LS2.

[0011] The upper arm switch HS and the lower arm switch LS are alternately turned on, providing a square wave voltage to the input terminal SW of the resonant circuit, causing the LLC resonant circuit TNK to resonate. An AC current IR is generated on the resonant inductor LR. Through the inductive coupling of the transformer TF, the secondary windings LS1 and LS2 also generate corresponding inductor currents ILS1 and ILS2. The synchronous rectifier switches SR1 and SR2 on the secondary side provide full-wave rectification. The synchronous rectifier switch SR1 is connected between the output ground line GNDO and the detection terminal DT1, while the synchronous rectifier switch SR2 is connected between the output ground line GNDO and the detection terminal DT2. The output capacitor CO provides low-pass filtering, generating an output power supply VOUT across the output power line OUT and the output ground line GNDO, which is used to power the load 102.

[0012] In Figure 1, the output power supply VOUT also serves as the operating power supply VCC for the synchronous rectifier controller 104, supplying the electrical energy required for its operation. The synchronous rectifier controller 104 generates control signals SG1 and SG2 at the control terminals G1 and G2 based on the detected voltages VDT1 and VDT2 at the detection terminals DT1 and DT2, respectively, to control the synchronous rectifier switches SR1 and SR2. For example, the bias detector BVD1 controls the drive circuit DR1 based on the detected voltage VDT1, which can generate a control signal SG1 with an appropriate voltage to control the synchronous rectifier switch SR1. Figure 2 shows the signal waveforms at the terminal SW in Figure 1: square wave signal VSW, control signal SG1, detected voltage VDT1, inductor current ILS1, control signal SG2, detected voltage VDT2, and inductor current ILS2. In Figure 2, time point t11 ​​to time point t12 can be regarded as half a switching cycle, while time point t12 to time point t13 can be regarded as the other half switching cycle.

[0013] Taking half a switching cycle from time point t11 ​​to time point t12 as an example, near time point t11, the detected voltage VDT2 decreases due to the decrease in the cross voltage of the primary winding LP. Once the detected voltage VDT2 turns negative, the synchronous rectifier controller 104 sets the control signal SG2 to a logical "1", turns on the synchronous rectifier switch SR2, clamps the detected voltage VDT2 to approximately 0V, and the inductor current ILS2 also oscillates and rises with the resonance of the LLC resonant circuit TNK, and then oscillates and decreases approximately before time point t12. Near time point t11, also due to inductor induction, the detected voltage VDT1 will start to rise from a negative value very close to 0V. Once the detected voltage VDT1 turns positive, the synchronous rectifier controller 104 sets the control signal SG1 to a logical "0", turns off the synchronous rectifier switch SR1, keeps the inductor current ILS1 at 0A, and the detected voltage VDT1 will continue to rise, and then stabilize at about twice the output power supply VOUT, as shown in Figure 2.

[0014] In Figure 2, the signal waveforms in the other half of the switching cycle from time point t12 to time point t13 can be found in the previous explanation of the first half of the switching cycle, and will not be repeated here.

[0015] The signal waveform in Figure 2 shows the state when the output power supply VOUT is ready, meaning that the output power supply VOUT has stabilized at the preset target voltage VTAR, for example, 5V. However, during the power-on process, that is, before the output power supply VOUT rises from 0V to the target voltage VTAR, the synchronous rectifier switches SR1 and SR2 may, due to capacitive coupling, conduct slightly when they should be off, causing leakage. In the worst case, this may prevent the output power supply VOUT voltage from rising to the target voltage VTAR at all.

[0016] Figure 3 illustrates the synchronous rectifier switch SR1. Figure 3 also shows two components parasitic on the synchronous rectifier switch SR1: a drain-to-gate capacitor CGD1 connected between the control terminal G1 and the detection terminal DT1, and a body diode DB1 connected between the output ground line GNDO and the detection terminal DT1. When the output power supply VOUT voltage is not high enough, the synchronous rectifier controller 104 in Figure 1 lacks the operating power supply VCC and cannot control the synchronous rectifier switch SR1 to open or close. In this case, theoretically, the synchronous rectifier switch SR1 should remain in the closed state, with rectification provided by the body diode DB1. In other words, when the output power supply VOUT voltage is not high enough, the control signals SG1 and SG2 should remain logically "0". However, near time point t11 ​​in Figure 2, the detection voltage VDT1 has a rising edge. This rising edge may slightly raise the voltage at the control terminal G1 through capacitive coupling of the drain capacitor CGD1, causing the synchronous rectifier switch SR1, which should be turned off, to conduct slightly and leak current. Therefore, in the LLC converter 100 of Figure 1, the voltage of the output power supply VOUT may take a long time to rise to the target voltage VTAR. In the worst case, the voltage of the output power supply VOUT may not rise to the target voltage VTAR at all.

[0017] Figure 4 shows an LLC converter 200 according to the present invention, used to convert an input power supply VIN on the primary side into an output power supply VOUT on the secondary side. The LLC converter 200 is similar to the LLC converter 100; any similarities or identical features can be found in the previous teachings regarding the LLC converter 100, and will not be repeated here. Compared to the LLC converter 100 in Figure 1, the LLC converter 200 of Figure 4 additionally has a rectifier diode DP and an operating power supply capacitor CVCC. The operating power supply capacitor CVCC is used to provide the operating power supply VCC. Compared to the synchronous rectifier controller 104 in Figure 1, the synchronous rectifier controller 204 of Figure 4 additionally has a power controller 205, which has a rectifier diode D1 and a linear regulator 206.

[0018] In Figure 4, the operating power supply VCC on the operating power supply capacitor CVCC supplies power to the synchronous rectifier controller 204. The operating power supply VCC can be established by the output power supply VOUT through the rectifier diode DP, or by the power controller 205 drawing current from the detection terminal DT1.

[0019] In one embodiment, as long as the operating power supply VCC is higher than 4.5V, the synchronous rectification controller 204 can normally control the synchronous rectification switches SR1 and SR2; and the linear regulator 206 is configured to draw current from the detection terminal DT1 to charge the operating power supply capacitor CVCC, thereby pulling the operating power supply VCC up to 4.5V. When the voltage of the operating power supply VCC exceeds 4.5V, the linear regulator 206 stops drawing current from the detection terminal DT1. Referring to Figure 2, during the power-on process, as long as the output power supply VOUT is higher than 2.25V, the detection voltage VDT1 on the detection terminal DT1 is approximately 4.5V during the half-switching cycle from time point t11 ​​to t12. This voltage is likely high enough to provide current through diode D1 and the linear regulator 206 to charge the operating power supply capacitor CVCC, pulling the operating power supply VCC up to 4.5V, allowing the synchronous rectification controller 204 to normally control the synchronous rectification switches SR1 and SR2.

[0020] Assume the target voltage VTAR of the output power supply VOUT is 5V. When the LLC converter 200 in Figure 4 regulates the output power supply VOUT to 5V, the output power supply VOUT will maintain the operating power supply VCC at approximately 5V through the rectifier diode DP, and the linear regulator 206 will stop drawing current from the sensing terminal DT1. In other words, when the output power supply VOUT is already regulated at the target voltage VTAR, the linear regulator 206 will hardly generate any energy loss.

[0021] During the power-on process, the LLC converter 200 in Figure 4 can control the synchronous rectifier switches SR1 and SR2 relatively early. As shown in Figure 4 and the related description, the synchronous rectifier controller 204 in Figure 4 can start normal operation as long as the output power supply VOUT is higher than 2.25V, because the operating power supply VCC will be approximately 4.5V at this time. In contrast, as shown in Figure 1 and the related description, if the synchronous rectifier controller 104 in Figure 1 also requires an operating power supply VCC of 4.5V to operate normally, then the output power supply VOUT in Figure 1 must be higher than 4.5V for the synchronous rectifier controller 104 to operate normally. Therefore, compared to Figure 1, the LLC converter 200 in Figure 4 controls the synchronous rectifier switches SR1 and SR2 relatively early during the power-on process, resulting in a more stable power-on process.

[0022] Figure 5 shows a synchronous rectifier controller 304, which in one embodiment can replace the synchronous rectifier controller 204 in Figure 4. Compared to the power controller 205, the power controller 305 additionally has a rectifier diode D2 connected between the linear regulator 206 and the sensing terminal DT2, as shown in Figure 5.

[0023] The synchronous rectifier controller 304 can not only draw current from the detection terminal DT1 to establish the operating power supply VCC during the first half of the switching cycle (time point t11 ​​to t12) in Figure 2, but also draw current from the detection terminal DT2 to establish the operating power supply VCC during the second half of the switching cycle (time point t12 to t13) in Figure 2.

[0024] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made in accordance with the scope of the patent application of the present invention shall be covered by the present invention.

Claims

1. An LLC converter comprising: a square wave generator including at least two power switches connected between an input power line and an input ground line to provide a square wave input at an input terminal; and an LLC resonant circuit connected to the input terminal, including a primary winding of a transformer connected in series and a resonant capacitor, wherein... The transformer includes a primary and a secondary winding; The system includes a first synchronous rectifier switch having a first detection terminal for connecting the first secondary winding to an output ground line, wherein the first secondary winding is connected between the first detection terminal and an output power line; an operating power capacitor for providing an operating power supply; and a synchronous rectifier controller for controlling the first synchronous rectifier switch based on a first detection voltage on the first detection terminal. The synchronous rectifier controller includes a power controller configured to draw current from the first detection terminal to establish the operating power supply and supply power to the synchronous rectifier controller.

2. As in the LLC converter of claim 1, wherein, The transformer further includes a second secondary winding, and the LLC converter further includes: a second synchronous rectifier switch having a second detection terminal for connecting the second secondary winding to the output ground line, wherein the second secondary winding is connected between the second detection terminal and the output power line; and a synchronous rectifier controller for controlling the second synchronous rectifier switch based on a second detection voltage on the second detection terminal.

3. Such as the LLC converter in claim 2, wherein, The power controller architecture draws current from the second sensing terminal to establish the operating power supply.

4. The LLC converter of claim 1 further includes a rectifier diode connected between the output power line and the operating power capacitor.

5. The LLC converter as described in claim 1, wherein, The power controller architecture is based on the principle that when the operating power supply exceeds a first preset voltage, it stops drawing current from the first detection terminal.

6. An LLC converter as described in claim 5, wherein, The LLC converter regulates one of the output power supplies on the output power line to a second preset voltage, and the first preset voltage is lower than the second preset voltage.

7. An LLC converter as described in claim 1, wherein, The power controller is a low dropout regulator.

8. An LLC converter as claimed in claim 1, wherein, This square wave generator has a half-bridge structure.

9. A control method for an LLC converter, comprising: providing a square wave voltage to an input terminal of a resonant circuit, wherein, The resonant circuit is connected between the input terminal and a power line, and includes a primary winding of a transformer connected in series and a resonant capacitor. The transformer includes a first secondary winding. A first synchronous rectifier switch and a synchronous rectifier controller are provided. The first synchronous rectifier switch is connected between a first detection terminal and an output ground line and is controlled by the synchronous rectifier controller. The first secondary winding is connected between the first detection terminal and an output power line. Current is drawn from the first detection terminal and charged to an operating power capacitor through the synchronous rectifier controller to establish an operating power supply and supply power to the synchronous rectifier controller.

10. The control method as described in claim 9, wherein, The transformer includes a second secondary winding, and the control method includes: providing a second synchronous rectifier switch connected between a second detection terminal and the output ground line, controlled by the synchronous rectifier controller; the second secondary winding being connected between the second detection terminal and the output power line; and drawing current from the second detection terminal and charging the operating power capacitor through the synchronous rectifier controller.

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