A synchronous rectification control method for LLC resonant converters

Abstract

This invention discloses a synchronous rectification control method for an LLC resonant converter, applicable to an LLC resonant converter composed of an inverter circuit, an LLC resonant cavity, a transformer, and a secondary-side rectifier network. This synchronous rectification control method achieves reliable switching of the synchronous rectifier diodes across the entire voltage range based on the feedback voltage signals of the LLC resonant converter under different operating modes. Especially during mode switching, by first uniformly turning off the drive signals of each synchronous rectifier diode and then gradually soft-starting them with a smaller pulse width, the method avoids the energy backflow problem caused by mis-turning of the secondary-side synchronous rectifier diodes during mode switching, achieving smooth and reliable switching between modes. The synchronous rectification control method proposed in this invention has higher practicality and reliability, requires no additional hardware circuitry, can be directly implemented through controller software programming, and offers greater flexibility, extending to synchronous rectification control schemes under a wider voltage range and more modes.

Description

Technical Field

[0001] This invention relates to the field of LLC resonant converter technology, and more specifically to a synchronous rectification control method. Background Technology

[0002] With the rapid development of power electronics, switching converters are being used more and more widely, and high power density and high efficiency have become industry trends. LLC resonant converters, with their advantages of low noise, low stress, and low switching losses, occupy an important position in the power supply industry. Although LLC converters can achieve zero-current turn-off of the secondary rectifier diodes, the conduction losses of the rectifier diodes are still relatively large in high-current applications. Therefore, synchronous rectification technology has been proposed in the industry. The main synchronous rectification schemes for LLC resonant converters are as follows:

[0003] 1. Detect the drain-source voltage of the secondary-side synchronous rectifier diode and compare this voltage with a preset turn-on / turn-off voltage threshold to obtain the drive signal for the synchronous MOSFET; currently, many dedicated synchronous rectifier chips are designed based on this principle, such as... Figure 1 As shown. This method is easily affected by PCB traces and parasitic parameters. Since the secondary-side synchronous rectifier tube is often selected as a switching tube with low on-resistance, the detection voltage is easily interfered with, and the overly sensitive detection threshold cannot guarantee the reliable switching of the synchronous rectifier tube.

[0004] 2. Detecting system current to obtain the drive signal typically takes two forms: one detects the secondary current, requiring two current detection devices; the other detects the primary resonant current, but the primary current generally contains both excitation current and load current, requiring specialized equipment for processing and calculation, making it more complex. Both methods require additional current detection circuitry, increasing cost and size, and pose a risk of false triggering if detection accuracy is insufficient.

[0005] 3. The digital control method typically divides the LLC resonant converter into two operating regions. The drive signal of the primary-side switching transistor is used to obtain the drive signal of the secondary-side synchronous rectifier transistor, and the entire judgment and calculation process is implemented by the controller. However, this method does not consider the handling method when the synchronous rectifier drive signal switches between the two operating regions in actual use, and the operating characteristics of conventional LLC resonant converters are not ideal when the switching frequency is higher than the resonant frequency.

[0006] To broaden the application scenarios of LLC resonant converters, maximize their advantages, and resolve the contradiction between wide gain and high efficiency in traditional LLC resonant converters, invention application No. 201910840309.4, entitled "A Wide Gain Range LLC Resonant Converter and Its Control Method," discloses an LLC resonant converter topology for achieving fixed-frequency control, such as... Figure 2 As shown, this LLC resonant converter adds a clamping branch between the primary-side resonant capacitor and resonant inductor, and achieves output voltage regulation by controlling the duty cycle of the primary-side switching transistor. When the input voltage is in the low-voltage range, the circuit operates in FBLLC mode, and when the input voltage is in the high-voltage range, the circuit operates in HBLLC mode, thus allowing for a wider gain range. However, it does not consider the switching between FBLLC and HBLLC modes in practical applications, nor the synchronous rectification control issue during switching between different modes. Summary of the Invention

[0007] Therefore, this invention proposes a synchronous rectification control method for a wide-gain LLC resonant converter. Without adding hardware circuitry, the synchronous rectification control timing sequence under different operating modes is obtained based on the feedback voltage signal of the LLC resonant converter, achieving reliable switching of the synchronous rectifier diodes across the entire voltage range. Furthermore, by employing a method of uniformly turning off the synchronous rectification pulse width before soft-starting during mode switching, the energy backflow problem caused by mis-turning of the secondary synchronous rectifier diodes during mode switching is avoided, thus achieving smooth and reliable switching between modes.

[0008] The technical problem of this invention is solved by the following solution:

[0009] A synchronous rectification control method for an LLC resonant converter is disclosed, applied to an LLC resonant converter including an inverter circuit, an LLC resonant cavity, a transformer, and a secondary-side rectifier network. A clamping branch including switching transistors S5 and S6 is added to the LLC resonant cavity. By controlling the conduction time of switching transistors S5 and S6, fixed-frequency PWM control is added to broaden the output voltage gain. The synchronous rectification control method of this invention determines the driving signal pulse width of the secondary-side synchronous rectifier transistors according to the operating state of the LLC resonant converter under different modes.

[0010] Specifically, when the converter operates in variable frequency PFM mode, the converter's operating frequency is adjusted according to the output voltage gain. At this time, the drive signals of each synchronous rectifier diode are also synchronously frequency-adjusted, and their pulse width is a constant pulse width signal related to the drive signal of the corresponding inverter circuit switch transistor. This constant pulse width signal is half of the resonant period. When the converter operates in fixed frequency PWM mode, the converter's operating frequency is fixed at the resonant frequency. The duty cycle of the switch transistor (S1) in the inverter circuit is adjusted according to the output voltage gain. At this time, the pulse width of the drive signal of each synchronous rectifier diode is consistent with the pulse width of the drive signal of the corresponding inverter circuit switch transistor.

[0011] Specifically, when the converter switches from the frequency conversion PFM mode to the frequency conversion PWM mode, the drive signals of each synchronous rectifier are first turned off uniformly, and then gradually soft-started from a smaller pulse width to the synchronous rectification pulse width corresponding to the frequency conversion PWM mode; when the converter switches from the frequency conversion PWM mode to the frequency conversion PFM mode, the drive signals of each synchronous rectifier are first turned off uniformly, and then gradually soft-started from a smaller pulse width to the synchronous rectification pulse width corresponding to the frequency conversion PFM mode.

[0012] Specifically, the above control method is implemented by the controller through software programming. By adjusting the synchronous rectification pulse width of the LLC resonant converter in real time under different operating states, the synchronous rectification control of the converter is ensured to be efficient and reliable.

[0013] The inverter circuit of the LLC resonant converter can be a full-bridge topology consisting of four switching transistors or a half-bridge topology consisting of two switching transistors.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. The synchronous rectification control method proposed in this invention does not require auxiliary circuits such as current transformers and synchronous rectification chips, which improves efficiency while effectively saving system cost and size;

[0016] 2. The synchronous rectification control method proposed in this invention has higher practicality and reliability. It can be directly implemented through controller software programming and can achieve reliable switching of synchronous rectifier tubes across the entire voltage range. In particular, during mode switching, the synchronous rectification drive signal avoids the energy backflow problem caused by the mis-conduction of synchronous rectifier tubes on the secondary side during mode switching by first turning off and then soft-starting the synchronous rectification drive signal, thus achieving smooth and reliable switching between modes.

[0017] 3. The synchronous rectification control method proposed in this invention is more flexible and can be extended to synchronous rectification control schemes under a wider voltage range and more modes. Attached Figure Description

[0018] Figure 1 This is a circuit schematic diagram of the synchronous rectification chip method used in the prior art;

[0019] Figure 2 This is a circuit schematic diagram of a wide-gain LLC resonant converter according to a preferred embodiment of the present invention;

[0020] Figure 3 The main timing diagram of the wide-gain LLC resonant converter operating in the frequency conversion PFM mode, which is a preferred embodiment of the present invention;

[0021] Figure 4The main timing diagram of the wide-gain LLC resonant converter operating in fixed-frequency PWM mode, which is a preferred embodiment of the present invention;

[0022] Figure 5 This is a timing diagram of the wide-gain LLC resonant converter of the present invention when switching from frequency conversion PFM mode to fixed frequency PWM mode in a preferred embodiment of the present invention.

[0023] Figure 6 The following is a timing diagram of the wide-gain LLC resonant converter of the present invention when it switches from fixed-frequency PWM mode to variable-frequency PFM mode. Detailed Implementation

[0024] like Figure 2 As shown, a wide-gain LLC resonant converter applicable to this invention includes an inverter circuit 10, an LLC resonant cavity 20, a transformer T, and a rectifier network 30 connected sequentially from input to output. Vin is the input power supply of the wide-gain LLC resonant converter, and R0 is the output load of the converter.

[0025] The inverter circuit 10 consists of switching transistors S1, S2, S3, and S4. The LLC resonant cavity 20 includes a resonant inductor Lr, a magnetizing inductor Lm, and a resonant capacitor Cr, and also includes a clamping branch composed of switching transistors S5 and S6. The rectifier network 30 includes synchronous rectifiers SR1 and SR2 and a filter capacitor C0. The bridge arm switching transistors S1 or S4 of the primary-side inverter circuit 10 correspond to the secondary-side synchronous rectifier SR1, and the bridge arm switching transistors S3 or S2 of the primary-side inverter circuit 10 correspond to the secondary-side synchronous rectifier SR2.

[0026] The drain of switching transistor S1 is connected to the drain of switching transistor S2 and the positive terminal of the input power supply Vin. The source of switching transistor S1 is connected to the drain of switching transistor S3 and one end of the resonant capacitor Cr. The other end of the resonant capacitor Cr is connected to one end of the resonant inductor Lr and the drain of switching transistor S5. The other end of the resonant inductor Lr is connected to one end of the magnetizing inductor Lm and the same-name terminal of the primary winding Np of transformer T. The opposite-name terminal of the primary winding Np of transformer T is connected to the other end of the magnetizing inductor Lm, the source of switching transistor S2, the drain of switching transistor S4, and the drain of switching transistor S6. The source of switching transistor S4 is connected to the source of switching transistor S3 and... The negative terminal of the input power supply Vin is connected to the source of the switching transistor S6, which is connected to the source of the switching transistor S5. The same-name terminal of the secondary winding Ns1 of the transformer T is connected to the drain of the secondary synchronous rectifier SR2. The source of the synchronous rectifier SR2 is connected to the source of the synchronous rectifier SR1, one end of the filter capacitor Co, and the negative terminal of the output load Ro. The opposite-name terminal of the secondary winding Ns2 of the transformer T is connected to the drain of the secondary synchronous rectifier SR1. The opposite-name terminal of the secondary winding Ns1 of the transformer T is connected to the same-name terminal of the secondary winding Ns2 of the transformer T, and together they are connected to the other end of the filter capacitor Co and the positive terminal of the output load Ro.

[0027] The wide-gain LLC resonant converter adds a clamping branch to the resonant cavity of the traditional LLC resonant circuit and changes the single-frequency conversion PFM control to multi-mode switching control, specifically:

[0028] In the low-voltage input range, it operates in variable-frequency PFM mode, where switches S5-S6 are continuously off. The output voltage V0 is controlled by adjusting the switching frequencies of switches S1-S4. In the high-voltage input range, it operates in fixed-frequency PWM mode, where the switching frequencies of switches S1-S6 are equal. Switches S1 and S5 are complementary in conduction, and switches S2 and S6 are complementary in conduction. The output voltage V0 is controlled by adjusting the duty cycle of switch S1.

[0029] The timing sequence for controlling the synchronous rectification drive signal on the secondary side of the wide-gain LLC resonant converter, in response to strain frequency PFM mode, fixed frequency PWM mode, and the switching between the two, is as follows:

[0030] When the wide-gain LLC resonant converter operates in the frequency conversion PFM mode, the converter's operating frequency is adjusted according to the output voltage gain. At this time, the drive signals of each synchronous rectifier (SR1 or SR2) are also synchronously frequency-converted. The pulse width of each synchronous rectifier is a constant pulse width signal, which is related to the drive signal of the switching transistor in the inverter circuit 10 that corresponds to the synchronous rectifier. This constant pulse width signal is half of the resonant period.

[0031] When the wide-gain LLC resonant converter operates in fixed-frequency PWM mode, the converter's operating frequency is the resonant frequency. The switching transistor S1 adjusts its duty cycle according to the change in output voltage gain. At this time, the pulse width of the drive signal of each synchronous rectifier transistor and the drive signal of the corresponding switching transistor in the inverter circuit 10 are consistent.

[0032] When the wide-gain LLC resonant converter is switching from the frequency conversion PFM mode to the fixed frequency PWM mode, the drive signals of each synchronous rectifier are first turned off uniformly, and then gradually soft-started from a smaller pulse width to the corresponding synchronous rectifier pulse width under the fixed frequency PWM mode.

[0033] When the wide-gain LLC resonant converter is switching from fixed-frequency PWM mode to variable-frequency PFM mode, the drive signals of each synchronous rectifier are first turned off uniformly, and then gradually soft-started from a smaller pulse width to the corresponding synchronous rectifier pulse width under the variable-frequency PFM mode.

[0034] In practical implementation, a reasonable dead time must be set between the two complementary primary-side switching signals to achieve zero-voltage turn-on of the corresponding switching transistors; the drive signal of the secondary-side synchronous rectifier transistor needs to be turned off earlier than the drive signal of its corresponding primary-side switching transistor, and a certain safety margin should be reserved to prevent energy backflow.

[0035] The following is in conjunction with the appendix Figures 3 to 6 This section provides a detailed explanation of the synchronous rectification control process of a wide-gain LLC resonant converter under different operating modes.

[0036] In this embodiment, the input voltage range is selected as 40~160VDC, and the resonant frequency f is... r If the frequency is set to 500kHz, the resonant cavity parameters are calculated as follows: Lr=0.892uH, Lm=2.676uH, Cr=113.6nF, and the primary-to-secondary turns ratio of the transformer is 3:1.

[0037] When the input voltage of the wide-gain LLC resonant converter is between 40V and 80V, the converter operates in the frequency conversion PFM mode, at which time switches S5 and S6 remain off. Figure 3 This is the main timing diagram for a wide-gain LLC resonant converter using frequency conversion control. Vgs1 / 4 are the drive signals for switches S1 and S4, Vgs2 / 3 are the drive signals for switches S2 and S3, and Vg... SR1 Vg is the drive signal for the synchronous rectifier diode SR1. SR2 The drive signal for synchronous rectifier SR2, i Lr i Lm i SR1 i SR2These represent the resonant current, the magnetizing current, and the current flowing through the secondary synchronous rectifier diodes SR1 and SR2, respectively. The energy transferred from the wide-gain LLC resonant converter to the next stage is actually the resonant current i. Lr With excitation current i Lm The energy difference is transferred to the load through the secondary synchronous rectifier diode, therefore the current flowing through the secondary synchronous rectifier diode is the resonant current i. Lr With excitation current i Lm The difference. The converter operates in frequency conversion PFM mode, and the operating frequency f of the primary-side switching transistor. s Less than or equal to the resonant frequency f r By changing the switching frequency f of the primary-side switching transistor s To change the output voltage gain, the drive signals of each synchronous rectifier diode also change frequency, which is also the switching frequency f. s The on-time t of the synchronous rectifier diode SR_on This is a constant pulse width, corresponding to the respective bridge arm switches in the inverter circuit, and this constant pulse width signal is half of the resonant period, i.e. To allow for a certain safety margin, the conduction time of the synchronous rectifier diode pulse in this mode is taken as... .

[0038] When the input voltage of the wide-gain LLC resonant converter is between 80V and 160V, the converter operates in fixed-frequency PWM mode. Figure 4 This is the main timing diagram for a wide-gain LLC resonant converter using fixed-frequency PWM control. Vgs5 and Vgs6 are the drive signals for switches S5 and S6, respectively. The meanings of the remaining signals are the same as... Figure 3 Consistent. The converter operates in fixed-frequency PWM mode, with the circuit operating frequency fixed at the resonant frequency f. r The output voltage gain is changed by altering the duty cycle D of the switching transistor S1 in the inverter circuit 10. At this time, the conduction time of the drive signals for each synchronous rectifier diode is consistent with the conduction time of the drive signals for the corresponding switching transistors in the inverter circuit 10. To allow for a certain safety margin, the conduction time of the synchronous rectifier diode pulse in this mode is taken as... ,in D <0.5, .

[0039] When the input voltage of the wide-gain LLC resonant converter is adjusted from 70V to 80V, the converter will switch from variable-frequency PFM mode to fixed-frequency PWM mode. Because the switching frequency f is near the mode switching point... s Approximately equal to the resonant frequency f rAt this point, the conduction time of the synchronous rectifier diodes in the variable frequency PFM mode is maintained at half a resonant cycle. If this is not adjusted, a sudden switch to the fixed frequency PWM mode will result in secondary-side energy backflow, causing a sharp decline in system performance and even system failure. Therefore, at the instant of mode switching, all drive signals of the secondary-side synchronous rectifier diodes should be turned off. After the mode switching is completed, a small pulse width (60ns~100ns) should be immediately assigned to each synchronous rectifier diode, followed by a slow soft-start to the corresponding conduction time in the current fixed frequency PWM mode. (Appendix) Figure 5 This is a timing diagram of the resonant converter switching from frequency-converting PFM mode to frequency-fixed PWM mode.

[0040] When the input voltage of the wide-gain LLC resonant converter is adjusted from 80V to 70V, the converter will switch from fixed-frequency PWM mode to variable-frequency PFM mode. Since the conduction time of the synchronous rectifier diodes is set differently in different modes, in fixed-frequency PWM mode, the conduction time of the synchronous rectifier diodes is consistent with the conduction time of the switching transistor drive signal in inverter circuit 10. If the converter directly switches to PFM mode, the pulse width of the synchronous rectifier diodes will suddenly change to half of the resonant period, causing a sudden change in the system at the moment of mode switching and a sharp decline in system performance. Similarly, at the moment of mode switching, all drive signals of the secondary synchronous rectifier diodes are turned off. After the mode switching is completed, a small value (60ns~100ns) is immediately assigned to the pulse width of each synchronous rectifier diode, and then a slow soft-start is performed to the corresponding conduction time in the current variable-frequency PFM mode. (Appendix) Figure 6 This is a timing diagram of the resonant converter switching from fixed-frequency PWM mode to variable-frequency PFM mode.

[0041] As can be seen from the above description of the converter's operation process, all switching devices in the converter can achieve zero-voltage turn-on, and the secondary-side rectifier devices can also achieve zero-current turn-off. All switching devices can achieve soft switching; and the reliable conduction of the secondary-side synchronous rectifier tube is guaranteed across the entire voltage range, effectively improving system efficiency.

[0042] The synchronous rectification control method proposed in this invention does not require auxiliary circuits such as current transformers and synchronous rectification chips, thus improving efficiency while effectively saving system cost and size. Furthermore, the synchronous rectification control method proposed in this invention has higher practicality and reliability, and can be directly implemented through controller software programming, enabling reliable switching of synchronous rectifier tubes across the entire voltage range. Especially during mode switching, the synchronous rectification drive signal avoids the energy backflow problem caused by mis-conduction of secondary synchronous rectifier tubes during mode switching by first turning off and then soft-starting, achieving smooth and reliable switching between modes.

[0043] The above description of the embodiments is only for the purpose of helping to understand the inventive concept of this application and is not intended to limit the present invention. For those skilled in the art, any modifications, equivalent substitutions, improvements, etc., made without departing from the principle of the present invention should be included within the protection scope of the present invention.

Claims

1. A synchronous rectification control method for an LLC resonant converter, the LLC resonant converter comprising an inverter circuit, an LLC resonant cavity, a transformer, and a rectifier network, wherein the LLC resonant cavity includes a clamping branch, and the LLC resonant converter can realize multi-mode switching control, characterized in that: When the LLC resonant converter operates in the frequency conversion PFM mode, the pulse width of the synchronous rectifier in the rectifier network is a constant pulse width signal, which is half of the resonant period of the LLC resonant converter. When the LLC resonant converter operates in fixed-frequency PWM mode, the pulse width of the drive signal of the synchronous rectifier in the rectifier network and the drive signal of the corresponding switching transistor in the inverter circuit are the same. When the LLC resonant converter operates in mode switching mode, the drive signal of the synchronous rectifier in the rectifier network is first turned off uniformly, and then gradually soft-started from a smaller pulse width to the synchronous rectifier pulse width of the corresponding mode after mode switching.

2. The method of claim 1, wherein: The inverter circuit includes switching transistors S1, S2, S3, and S4. The rectifier network includes synchronous rectifier transistors SR1 and SR2 and a filter capacitor Co. The drain of switching transistor S1 is connected to the drain of switching transistor S2 and the positive terminal of the input power supply Vin. The source of switching transistor S1 is connected to the drain of switching transistor S3 and one end of the LLC resonant cavity. The source of switching transistor S2 is connected to the drain of switching transistor S4 and the other end of the LLC resonant cavity. The source of switching transistor S3 is connected to the source of switching transistor S4 and the negative terminal of the input power supply Vin. The secondary winding N of the transformer... The same-name terminal of S1 is connected to the drain of synchronous rectifier SR2. The source of synchronous rectifier SR2 is connected to the source of synchronous rectifier SR1 and one end of filter capacitor Co. The opposite-name terminal of transformer secondary winding Ns2 is connected to the drain of synchronous rectifier SR1. The opposite-name terminal of transformer secondary winding Ns1 is connected to the same-name terminal of transformer T secondary winding Ns2, and together they are connected to the other end of filter capacitor Co. Switch S1 or switch S4 has a corresponding relationship with synchronous rectifier SR1, and switch S3 or switch S2 has a corresponding relationship with synchronous rectifier SR2.

3. The method of claim 2, wherein: In the variable frequency PFM mode, the operating frequency of the LLC resonant converter is adjusted according to the change of its output voltage gain, and the drive signals of synchronous rectifier tubes SR1 and SR2 are also synchronously frequency-adjusted.

4. The synchronous rectification control method for an LLC resonant converter according to claim 2, characterized in that: In the fixed-frequency PWM mode, the operating frequency of the LLC resonant converter is the resonant frequency. The duty cycle of the switch S1 is adjusted according to the change of the output voltage gain of the LLC resonant converter. The pulse width of the drive signals of the synchronous rectifiers SR1 and SR2 is consistent with the pulse width of the drive signals of the corresponding switches in the inverter circuit.

5. The synchronous rectification control method for an LLC resonant converter according to any one of claims 1 to 4, characterized in that, The synchronous rectification control method is implemented by the controller through software programming. By adjusting the synchronous rectification pulse width of the LLC resonant converter in real time under different operating states, the synchronous rectification control of the converter is ensured.

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