LLC resonant converter for automotive DC-DC converters

Abstract

The present invention discloses an LLC resonant converter applied to a vehicle-mounted DC-DC converter, which improves the hardware circuit, wherein the inverter module adopts a full-bridge inverter circuit with three bridge arms, the resonant cavity adopts two resonant cavities connected in parallel and symmetrically to each other, there are two transformers, and the secondary rectifier module adopts a full-bridge rectifier circuit with three bridge arms, in conjunction with an asymmetric PWM control method, the voltage gain of the LLC resonant converter is controlled by the size of the phase shift angle to switch between the full-bridge and half-bridge LLC resonant converters, thereby controlling the gain of the entire circuit by adjusting the phase shift angle. The present invention will not increase the loss while widening the voltage gain range, and has good stability when switching the working mode.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic voltage conversion, and in particular relates to an LLC resonant converter applied to a vehicle-mounted DC-DC converter. Background Art

[0002] In traditional DC-DC converters, the internal resistance of the power supply will increase energy consumption under wide voltage input conditions, resulting in reduced efficiency. In addition, in order to achieve stable voltage conversion requirements, the circuit design becomes relatively more complicated. As a type of DC-DC converter, LLC resonant converter can achieve soft switching characteristics within the full load range and is widely used in high-frequency situations. For this reason, LLC resonant converters are widely used in wide voltage input conditions.

[0003] Although LLC resonant converters can be applied to wide voltage input conditions, when the input voltage is too wide, in order to achieve high gain, its quality factor Q value and voltage ratio need to be very small, which leads to slight frequency changes. If the voltage regulation is too wide, it will cause system instability. In recent years, in order to realize the application of LLC resonant converters under wide voltage input conditions, scholars at home and abroad have conducted in-depth research on how to widen its voltage gain while ensuring its high efficiency and high performance.

[0004] Common methods for broadening the voltage gain range of traditional LLC resonant converters include hybrid control strategy, changing the resonant cavity voltage, mode switching, adding circuits or bridge arms, changing resonant cavity parameters, etc. LLC topology hybrid control strategies usually adopt frequency modulation control (PFM) and pulse width modulation (PWM) hybrid control, frequency modulation control (PFM) and phase shift control hybrid control, etc. Changing the resonant cavity voltage: While keeping the switching frequency unchanged, the input voltage level is changed by pulse width modulation (PWM), thereby changing the output voltage. This method simplifies the circuit design because the switching frequency remains unchanged, but the ZVS of the primary switch tube is easily lost. Mode switching: The converter works in multiple modes, and different modes have different output voltage ranges. A wider output voltage range is obtained by switching modes, but this method is more complex to control. Adding circuits or bridge arms: Adding a circuit or a series auxiliary bridge arm on the primary input side of the transformer to adjust the bus voltage, thereby broadening the output voltage range, but its cost is relatively high.

[0005] In order to further broaden its voltage gain range, reduce its frequency adjustment range, and retain the LLC soft switching advantage, Chinese invention patent application CN115085556A adds a new asymmetric PWM control method on the basis of the traditional LLC frequency modulation control strategy, and adds a bidirectional switch tube to the secondary side of the converter for voltage doubling rectification, which greatly broadens the voltage gain of the converter, thereby broadening the output voltage range of the converter. In terms of structure, its bidirectional switch tube is composed of a fifth diode D5, a sixth diode D6, a seventh diode D7, an eighth diode D8 and a switch tube Q5. By controlling the on and off of the secondary side switch tube Q5, a double voltage gain is achieved. Compared with the traditional voltage doubling rectification method, the additional bidirectional switch tube is composed of four diodes and a field effect tube, which will cause its loss to be larger. In terms of control, it adopts a hybrid control strategy to achieve low-voltage output by reducing the duty cycle. However, it can be seen from simulation experiments that when the duty cycle changes in a wide range, the converter will lose its soft switching characteristics, so long-term use in hybrid control will affect the stability of the converter. Summary of the invention

[0006] The object of the present invention is to provide an LLC resonant converter for vehicle-mounted DC-DC converters which can broaden its voltage gain range without increasing losses and has good stability when switching working modes.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] LLC resonant converter applied to vehicle-mounted DC-DC converter, including inverter module, resonant cavity, transformer and secondary side rectifier module;

[0009] The inverter module is a full-bridge inverter circuit having a first bridge arm, a second bridge arm and a third bridge arm, wherein the first bridge arm, the second bridge arm and the third bridge arm each have two switch tubes, and through frequency modulation control, the duty ratio of the switch signals of the first bridge arm, the second bridge arm and the third bridge arm is 50%;

[0010] The resonant cavity comprises a first resonant cavity and a second resonant cavity which are connected in parallel and symmetrically to each other, the first resonant cavity and the second resonant cavity are both composed of a resonant capacitor, an excitation inductor and a resonant inductor which are connected in series to each other, one end of the first resonant cavity is connected to the midpoint of the first bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module, one end of the second resonant cavity is connected to the midpoint of the second bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module;

[0011] The transformer comprises a first transformer and a second transformer, the primary coil of the first transformer is connected in parallel with the excitation inductance of the first resonant cavity, and the primary coil of the second transformer is connected in parallel with the excitation inductance of the second resonant cavity;

[0012] The parameters of the components of the first resonant cavity and the second resonant cavity are the same, and the parameters of the first transformer and the second transformer are the same;

[0013] The secondary rectifier module is a full-bridge rectifier circuit having a first bridge arm, a second bridge arm and a third bridge arm, the two ends of the secondary coil of the first transformer are respectively connected to the midpoint of the first bridge arm of the secondary rectifier module and the midpoint of the second bridge arm of the secondary rectifier module, and the two ends of the secondary coil of the second transformer are respectively connected to the midpoint of the second bridge arm of the secondary rectifier module and the midpoint of the third bridge arm of the secondary rectifier module;

[0014] The method for switching the working mode of the LLC resonant converter is as follows:

[0015] When the LLC resonant converter is working, detecting the input voltage of the LLC resonant converter;

[0016] When the input voltage of the LLC resonant converter is lower than the first set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the HGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the working mode of the LLC resonant converter in the previous cycle is not the HGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to a threshold close to the HGM working mode switching threshold. Through the buffer adjustment of these ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to a threshold close to the HGM working mode switching threshold, the mode is switched, so that the LLC resonant converter smoothly enters the HGM working mode. At this time, the LLC resonant converter is equivalent to two full-bridge LLC resonant converters in parallel.

[0017] When the input voltage of the LLC resonant converter is greater than or equal to the first set voltage value but less than or equal to the second set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the TGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the LLC resonant converter in the previous cycle is not the TGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to a threshold close to the TGM working mode switching threshold. Through the buffer adjustment of these ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to a threshold close to the TGM working mode switching threshold, the mode is switched, so that the LLC resonant converter smoothly enters the TGM working mode. At this time, the LLC resonant converter is equivalent to two half-bridge LLC resonant converters in parallel.

[0018] When the input voltage of the LLC resonant converter is greater than the second set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the LGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the working mode of the LLC resonant converter in the previous cycle is not the LGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to a threshold close to the LGM working mode switching threshold. Through the buffer adjustment of ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to a threshold close to the LGM working mode switching threshold, the mode is switched, so that the LLC resonant converter smoothly enters the LGM working mode. At this time, the LLC resonant converter is equivalent to a half-bridge LLC resonant converter.

[0019] The first set voltage value is 160V, and the second set voltage value is 320V.

[0020] The operating frequency threshold of the LLC resonant converter when the HGM operating mode is switched to the TGM operating mode is 130 khz, and the operating frequency threshold when the TGM operating mode is switched to the HGM operating mode is 70 khz.

[0021] The operating frequency threshold of the LLC resonant converter when the TGM operating mode is switched to the LGM operating mode is 130 khz, and the operating frequency threshold when the LGM operating mode is switched to the TGM operating mode is 70 khz.

[0022] The operating frequency threshold of the LLC resonant converter when the HGM operating mode is switched to the LGM operating mode is 130 khz, and the operating frequency threshold when the LGM operating mode is switched to the HGM operating mode is 70 khz.

[0023] When the LLC resonant converter is in the HGM operating mode, the upper arm switch tube of the first bridge arm, the upper arm switch tube of the second bridge arm and the lower arm switch tube of the third bridge arm are alternately and complementaryly turned on with the lower arm switch tube of the first bridge arm, the lower arm switch tube of the second bridge arm and the upper arm switch tube of the third bridge arm.

[0024] When the LLC resonant converter is in the TGM operating mode, the upper arm switch tube of the third bridge arm is turned off, the lower arm switch tube of the third bridge arm is turned on, and the upper arm switch tube of the first bridge arm and the upper arm switch tube of the second bridge arm are alternately and complementaryly turned on with the lower arm switch tube of the first bridge arm and the lower arm switch tube of the second bridge arm.

[0025] When the LLC resonant converter is in the LGM operating mode, the upper arm switch tube of the second bridge arm and the upper arm switch tube of the third bridge arm remain turned off, the lower arm switch tube of the second bridge arm and the lower arm switch tube of the third bridge arm remain turned on, and the upper arm switch tube of the first bridge arm and the lower arm switch tube of the first bridge arm are alternately and complementary turned on.

[0026] After adopting the above scheme, the LLC resonant converter applied to the vehicle-mounted DC-DC converter of the present invention improves the hardware circuit, the inverter module adopts a full-bridge inverter circuit with three bridge arms, the harmonic resonant cavity adopts two resonant cavities connected in parallel and symmetrically to each other, there are two transformers, and the secondary rectifier module adopts a full-bridge rectifier circuit with three bridge arms. In conjunction with the asymmetric PWM control method, the voltage gain of the LLC resonant converter is controlled by the size of the phase shift angle to switch between the full-bridge and half-bridge LLC resonant converters, thereby controlling the gain of the entire circuit by adjusting the phase shift angle.

[0027] Compared with the prior art, the LLC resonant converter of the present invention has the following beneficial effects:

[0028] 1. It has three working modes. Different working modes have different output voltage ranges. By switching the working modes, a wider output voltage range can be obtained, thereby widening its voltage gain range;

[0029] 2. Since the current of the primary circuit is small, although there are additional devices on the primary side in the present invention, the increased loss can be ignored. Although the secondary rectifier module has an additional bridge arm, each working mode still only uses two of the bridge arms, which will not increase the loss.

[0030] 3. The present invention adopts an asymmetric PWM control method. When the working mode is to be switched, the phase shift angle is controlled to become larger within ten cycles so that the working frequency required by the LLC resonant converter is reduced to a threshold close to the next working mode switching. Through ten cycles of buffer adjustment, the sudden changes in the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process are avoided. The mode switching is performed when the working frequency required by the LLC resonant converter drops to a threshold close to the next working mode switching, so that the LLC resonant converter smoothly enters the next working mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the control principle block diagram of LLC resonant converter;

[0032] Figure 2 Schematic diagram of the circuit structure of the LLC resonant converter in the present invention;

[0033] Figure 3 Schematic diagrams of circuit principles of the LLC resonant converter in the present invention when it is in (a) HGM working mode, (b) TGM working mode, and (c) LGM working mode;

[0034] Figure 4 is the f of the LLC resonant converter in the HGM working mode in the present invention s <f r Waveform diagram of

[0035] Figure 5 Schematic diagram of the circuit principle of the LLC resonant converter in the present invention when it works in the HGM working mode in three working stages (a) [t0, t1] working state, (b) [t1, t2] working state and (c) [t2, t3] working state;

[0036] Figure 6 is the overall voltage gain waveform diagram of the LLC resonant converter in the present invention;

[0037] Figure 7 It is a schematic diagram of the control flow of switching the working mode of the LLC resonant converter in the present invention;

[0038] Figure 8 It is a schematic diagram of the changes in the output voltage, output current and resonant cavity current waveform during the TGM-HGM working mode switching process in the present invention;

[0039] Fig. 9 The driving waveform, output voltage, output current and switching waveform of the resonant cavity current of the switch tube S6 using the asymmetric PWM control strategy in the present invention are shown. DETAILED DESCRIPTION

[0040] The control principle of LLC resonant converter, such as Figure 1 As shown, when working, the voltage is input to the LLC resonant converter, and the LLC resonant converter performs DC-DC conversion and then outputs it to the load. The controller samples the input voltage, output voltage and output current of the LLC resonant converter respectively, and outputs a control signal to the drive circuit, which outputs a drive signal to each switch tube of the LLC resonant converter to control each switch tube of the LLC resonant converter to turn on or off. In the present invention, the controller uses the TMS280049C of the C2000 series 32-bit MCU of TI Company, which has a main frequency of up to 100Mhz, has multi-channel PWM output, and has three independent 12-bit ADCs.

[0041] The LLC resonant converter of the present invention applied to a vehicle-mounted DC-DC converter, such as Figure 2 As shown, it includes an inverter module, a resonant cavity, a transformer and a secondary rectifier module;

[0042] The inverter module is a full-bridge inverter circuit having a first bridge arm, a second bridge arm and a third bridge arm. The first bridge arm of the inverter module is composed of switch tubes S1 and S2, the second bridge arm of the inverter module is composed of switch tubes S3 and S4, and the third bridge arm of the inverter module is composed of switch tubes S5 and S6. The drains of the switch tubes S1, S3 and S5 are all connected to the input voltage V in The positive electrode of the switch tubes S2, S4 and S6 are all connected to the input voltage V in The gates of the switch tubes S1-S6 are connected to the output end of the drive circuit; the source of the switch tube S1 and the drain of the switch tube S2 are connected together to form the midpoint of the first bridge arm of the inverter module ( Figure 2 , Figure 3 and Figure 5 The source of the switch tube S3 and the drain of the switch tube S4 are connected together to form the midpoint of the second bridge arm of the inverter module ( Figure 2 , Figure 3 and Figure 5 The source of the switch tube S5 and the drain of the switch tube S6 are connected together to form the midpoint of the third bridge arm of the inverter module ( Figure 2 , Figure 3 and Figure 5 The LLC resonant converter is frequency modulated by the controller, and the duty ratios of the switching signals of the first bridge arm of the inverter module, the second bridge arm of the inverter module, and the third bridge arm of the inverter module are all 50%.

[0043] The resonant cavity comprises a first resonant cavity and a second resonant cavity which are connected in parallel and symmetrically to each other. The first resonant cavity is composed of a resonant capacitor C connected in series with each other. r1, Excitation inductance L m1 and resonant inductor L r1 The second resonant cavity is composed of resonant capacitors C connected in series. r2 , Excitation inductance L m2 and resonant inductor L r2 One end of the first resonant cavity is connected to the midpoint of the first bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module. One end of the second resonant cavity is connected to the midpoint of the second bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module.

[0044] The transformer comprises a first transformer T r1 and the second transformer T r2 , the first transformer T r1 The primary coil and the excitation inductance L of the first resonant cavity m1 Phase parallel, the second transformer T r2 The primary coil and the excitation inductance L of the second resonant cavity m2 Phase parallel connection;

[0045] The parameters of the components of the first resonant cavity and the second resonant cavity are the same. r1 and the second transformer T r2 The parameters are the same;

[0046] The secondary rectifier module is a full-bridge rectifier circuit having a first bridge arm, a second bridge arm and a third bridge arm. The first bridge arm of the secondary rectifier module is composed of diodes D1 and D2, the second bridge arm of the secondary rectifier module is composed of diodes D3 and D4, and the third bridge arm of the secondary rectifier module is composed of diodes D5 and D6. The cathodes of the diodes D1, D3 and D5 are connected to one end of the load R0, and the anodes of the diodes D2, D4 and D6 are connected to the load R0. o The anode of diode D1 and the cathode of diode D2 are connected together to form the midpoint of the first bridge arm of the secondary rectifier module, the anode of diode D3 and the cathode of diode D4 are connected together to form the midpoint of the second bridge arm of the secondary rectifier module, and the anode of diode D5 and the cathode of diode D6 are connected together to form the midpoint of the third bridge arm of the secondary rectifier module. The first transformer T r1 The two ends of the secondary coil are respectively connected to the midpoint of the first bridge arm of the secondary rectifier module and the midpoint of the second bridge arm of the secondary rectifier module. r2 The two ends of the secondary coil are respectively connected to the midpoint of the second bridge arm of the secondary rectifier module and the midpoint of the third bridge arm of the secondary rectifier module.

[0047] The LLC resonant converter of the present invention has three working modes, namely HGM working mode (high gain mode), TGM working mode (medium gain mode) and LGM working mode (low gain mode). in When the LLC resonant converter input voltage V in When the LLC resonant converter input voltage V in When the voltage is high, it works in LGM working mode.

[0048] As a preferred embodiment, in the present invention, when the input voltage range is 80V-160V, it is low voltage; when the input voltage range is 160V-320V, it is medium-low voltage; when the input voltage range is 320V-500V, it is high voltage.

[0049] In the present invention, when the LLC resonant converter operates in the HGM operating mode, Figure 3 As shown in (a), the LLC resonant converter is equivalent to two full-bridge LLC resonant converters in parallel; when the LLC resonant converter works in the TGM working mode, as shown in Figure 3 As shown in (b), the LLC resonant converter is equivalent to two half-bridge LLC resonant converters in parallel; when working in the LGM working mode, as shown in Figure 3 As shown in (c), the LLC resonant converter is equivalent to the half-bridge LLC resonant converter. Since the time domain analysis of the three working modes is similar, the steady-state waveform of the HGM working mode is mainly analyzed below.

[0050] When the LLC resonant converter operates in the HGM mode, Figure 3 As shown in (a), in the primary inverter module, the switch tubes S1, S3, S6 and the switch tubes S2, S4, S5 are alternately turned on, and the duty cycle is a fixed value of 50%. At this time, the circuit works in the dual full-bridge mode. Under steady-state conditions, f s <f r When (f s is the operating frequency of the LLC resonant converter, f r is the series resonant frequency), the switching waveforms of the switch tubes S1-S6, and the midpoint voltage u of the two bridge arms ab and u bc The waveform of the resonant current i r1 、i r2 , the current i of the excitation inductor m1 、i m2 , and the output current i of the secondary diodes D1 and D2 d1 、i d2 like Figure 4 shown.

[0051] (1) Working stage I [t0, t1]: Before t0, switches S2, S4, and S5 are turned on, the diode of the secondary rectifier module is open and no energy is transmitted, and the load R o By the filter capacitor C o When the power supply is on, the switch tubes S2, S4, and S5 are turned off and the working phase I begins. Figure 5 As shown in (a), from time t0, the circuit enters the dead zone, all switches enter the cut-off state, and the resonant current i r1 、i r2 The junction capacitance of the switch tubes S1, S3, and S6 is discharged, and the junction capacitance of the switch tubes S2, S4, and S5 is charged. When the drain-source voltage of the switch tubes S1, S3, and S6 drops to zero, the current flows through the body diode, creating conditions for achieving ZVS (zero voltage switching circuit). At this time, the charging and discharging process of the junction capacitance ends.

[0052] (2) Working phase II [t1, t2]: When the time t1 starts, Figure 5 As shown in (b), the switches S1, S3, and S6 complete ZVS, and the diodes D1 and D6 of the secondary rectifier module are turned on. At this time, the midpoint voltage u between nodes a and b is ab The midpoint voltage u between nodes b and c bc The input voltage is V in , the resonant inductance L in the two resonant cavities r1 , L r2 , resonant capacitor C r1 , C r2 The current starts to rise, and the excitation inductance L m1 , L m2 is forward clamped, the magnetizing inductance L m1 , L m2 The current starts to rise linearly, and the circuit continuously supplies the resonant capacitor C r1 , C r2 Charging, when the resonant inductor L r1 and the magnetizing inductance L m1 When the current flowing into the primary side of the ideal transformer is zero, no energy is transferred to the secondary side. Figure 5 As shown in (c), the current flowing through the diode of the secondary rectifier module passes through zero, achieving secondary side ZCS.

[0053] (3) Working phase III [t2, t3], such as Figure 5 As shown in (c), since the primary side no longer transmits energy to the secondary side, the diodes D1 and D6 of the secondary side rectifier module are turned off with zero current, and the excitation inductor L m1 , L m2No longer clamped by the output voltage, at time t3, the switch tubes S1, S3, and S6 begin to turn off.

[0054] Since the working state of the negative half cycle is the same as that of the first half cycle, it will not be described in detail here. The overall voltage gain of the LLC resonant converter of the present invention is as follows: Figure 6 As shown, here the quality factor Q is set to 0.25 and the inductance ratio K is set to 3.

[0055] The LLC resonant converter of the present invention has an HGM working mode, a TGM working mode and an LGM working mode, and the switching of each working mode adopts an asymmetric PWM control method.

[0056] In the present invention, the control process of switching the working mode of the LLC resonant converter is as follows: Figure 7 shown.

[0057] First, initialize the controller chip and peripheral modules such as EPWM (pulse width modulator) and ADC (analog-to-digital converter), then clear all interrupts, initialize the interrupt vector table, set the GPIO clock to enable, trigger ADC sampling through EPWM, and wait for interrupt after sampling.

[0058] After entering the ADC interrupt, the cycle starts, and the output identifiers Flag1=0 and Flag2=0. The controller monitors the input voltage value of the LLC resonant converter and executes the corresponding working mode (HGM working mode, TGM working mode and LGM working mode) according to the three voltage ranges of low voltage, medium-low voltage and high voltage mentioned above.

[0059] In the present invention, the identifiers of the TGM working mode are set to Flag1=1, Flag2=1, the identifier of the HGM working mode is set to Flag1=0, and the identifier of the LGM working mode is set to Flag2=0.

[0060] When the voltage is less than 160V, if the identifier Flag1=0, it means that the HGM working mode was executed in the previous cycle, and the HGM working mode will continue to be executed, otherwise the working mode needs to be switched, and then the cycle ends and the output identifier Flag1=0.

[0061] When the voltage is greater than 320V, if the identifier Flag2=0, it means that the previous cycle executed the LGM working mode, then the LGM working mode will continue to be executed, otherwise the working mode needs to be switched, and then the cycle ends, and the output identifier Flag2=0.

[0062] When the voltage is between 160V and 320V, the corresponding mode switching is performed according to the values ​​of identifier Flag1 and identifier Flag2. If identifier Flag1=0, it means that the HGM working mode was executed in the previous cycle, and the mode switching is required. At the end of the cycle, Flag1=1 and Flag2=0 are output; if identifier Flag1≠0, when identifier Flag2=0, the working mode switching is required, and at the end of the cycle, Flag1=1 and Flag2=0 are output; if identifier Flag1≠0 and identifier Flag2≠0, it means that the TGM working mode was executed in the previous cycle, then it continues to work in the TGM working mode, and at the end of the cycle, Flag1=1 and Flag2=1 are output.

[0063] After the ADC interrupt ends, the sampled ADC signal is used to calculate the control quantity of the next cycle through closed-loop PI control, and then the frequency and control information are updated to the main circuit. Finally, the interrupt is reset and the cycle is waited for or the interrupt is entered again.

[0064] Since the LLC resonant converter will produce sudden changes in output voltage, output current and resonant cavity current during the working mode switching process, such as Figure 8 As shown, it is the output voltage V during the TGM-HGM working mode switching process o , output current i o and the resonant cavity current i Cr Waveform diagram, the part circled by the ellipse in the figure is the sudden change waveform of voltage and current during the mode switching process. Figure 8 It can be seen that when the TGM working mode switches to the HGM working mode, the load will undergo a sharp mutation in a transient process and last for a long time. Similarly, the resonant current i Cr A spike current will appear. This sudden mode jump will seriously affect the stability of the system, and excessive spike current will cause damage to the device.

[0065] In order to solve the problem of voltage and current mutations during mode switching, the LLC resonant converter of the present invention proposes a transition switching mode based on flexible switching. The proposed transition switching mode is based on asymmetric phase shift of a single bridge arm. By controlling the leading bridge arm to be complementary turned on with a duty cycle of 50%, but in the lagging bridge arm, the duty cycle of the switch tube S5 begins to shrink with the end of a cycle as the end point, and its contraction duty cycle is dS5. The duty cycle of the switch tube S6 simultaneously expands with a duty cycle complementary to dS5, and its expansion duty cycle is dS6. The size of the expansion duty cycle is equal to dS6-T / 2, and its contraction duty cycle is T / 2-dS5. According to theoretical analysis, it can be seen that the expansion duty cycle can reach up to 180°, and the reduction duty cycle can also reach 180°. At this time, the switch tube S5 remains closed and the switch tube S6 remains open. Similarly, when the LLC resonant converter switches from the TGM working mode to the LGM working mode, the switch tube S5 remains closed, the switch tube S6 remains off, the duty cycle of the switch tube S3 begins to shrink, and the duty cycle of the switch tube S4 begins to expand. Finally, the switch tube S3 remains off and the switch tube S4 remains on, completing the mode switching.

[0066] (1)

[0067] In formula (1), V ab is the equivalent input voltage of LLC resonant converter; T is the period; V in is the input voltage, and θ is the phase shift angle.

[0068] From formula (1), we can know that the equivalent input voltage V in the negative half cycle can be controlled by adjusting the phase shift angle θ. ab By deducing the voltage gain of the full-bridge LLC resonant converter, we know that:

[0069] (2)

[0070] (3)

[0071] Where H is the transfer function; V O is the output voltage; n is the unknown coefficient, which refers to the output voltage V O Multiples of; Q is the quality factor; C r is the resonant capacitor; L r is the resonant inductor; f n is the normalized switching frequency; L m is the magnetizing inductance; M is the current gain; K is Lm : L r Inductance ratio; R ac is the primary equivalent resistance; ω s is the angular frequency; j is an imaginary unit;

[0072] From the derivation of formulas (2) and (3), it can be found that when the value of the phase shift angle θ approaches 0, the equivalent voltage gain of the negative half cycle is equivalent to that of the half-bridge LLC resonant converter. When the value of the phase shift angle θ approaches 180°, its equivalent voltage gain is equivalent to 0. Therefore, the total voltage gain range of the LLC resonant converter of this control mode is between the half-bridge and full-bridge LLC resonant converters. The gain of the entire circuit is mainly controlled by adjusting the phase shift angle θ.

[0073] Through the analysis of the above control methods, it can be known that when the asymmetric PWM control method is adopted, the voltage gain of the LLC resonant converter can be controlled by the size of the phase shift angle θ to switch between the full-bridge and half-bridge LLC resonant converters. in When the phase shift angle θ is fixed, the larger the phase shift angle θ is, the smaller the voltage gain is. When the gain is reduced, the operating frequency of the system needs to be reduced to stabilize the output voltage. Therefore, in the present invention, an asymmetric PWM control method can be used to switch the working mode. The main control method is to control the phase shift angle θ to become larger within ten cycles, so that the operating frequency required by the system is reduced to a threshold close to the next mode switching to complete the switching.

[0074] First, many current studies regard asymmetric PWM control as a mode switching working mode, so that it can be used as a transition while expanding the voltage gain. However, it can be found from its steady-state waveform that when the phase shift angle θ is larger, it is more difficult to achieve soft switching. The longer this mode exists, the greater the loss of components in the circuit, and the efficiency will also be reduced. Therefore, the present invention regards this asymmetric control as a fast response stage, which is divided into two parts by the value of the phase shift angle θ. From the previous analysis, it can be seen that when the LLC resonant converter works in the HGM working mode and switches to the TGM working mode, the working The operating frequency is 130khz, and the operating frequency when switching from TGM working mode to HGM working mode is 70khz, so a phase shift angle θ is designed to reduce the operating frequency to close to the operating frequency (such as to 110khz), and then, the second phase shift angle θ is used to reduce the operating frequency to close to the operating frequency (such as to 90khz). There is no need to use asymmetric phase shifting to make the value of the phase shift angle θ reach 180°, because when the phase shift angle θ is too large, the duty cycle of the contraction tube of the lagging bridge arm will be too low. When the duty cycle is close to 0, it is not conducive to the zero voltage opening of the switch tube.

[0075] Through simulation and theoretical analysis, it can be seen that when the operating frequency is 110khz, the phase shift angle is 36°;

[0076] When the operating frequency is 90khz, the phase shift angle is 72°.

[0077] When the HGM working mode is to be switched to the TGM working mode, the phase shift angle is first controlled to 72° and then to 36° within ten cycles, so that the required working frequency of the LLC resonant converter is reduced to 110 khz (close to the threshold for switching from the HGM working mode to the TGM working mode). Through ten cycles of buffer adjustment, the sudden changes in the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process are avoided. When the required working frequency of the LLC resonant converter drops to 110 khz, the mode is switched, so that the LLC resonant converter smoothly enters the TGM working mode, and then the operation of each switch tube is controlled according to the TGM working mode.

[0078] Similarly, when the TGM working mode is to switch to the HGM working mode, the phase shift angle is first controlled to 36° within ten cycles, and then the phase shift angle is controlled to 72°, so that the required working frequency of the LLC resonant converter is reduced to 90 khz (close to the threshold for switching from the TGM working mode to the HGM working mode). Through ten cycles of buffering adjustment, the sudden changes in the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process are avoided. When the required working frequency of the LLC resonant converter drops to 90 khz, the mode is switched, so that the LLC resonant converter smoothly enters the HGM working mode, and then the operation of each switch tube is controlled according to the HGM working mode.

[0079] Similarly, when the TGM working mode is switched to the LGM working mode, the working frequency is 130 khz, and when the LGM working mode is switched to the TGM working mode, the working frequency is 70 khz. When the TGM working mode is to switch to the LGM working mode, the switch tube S3 is controlled to remain on and the switch tube S4 is controlled to remain off. As above, 110khz and 90khz are selected as the working points for ten cycles. When the working frequency is 110khz, the phase shift angle is 36°, and when the working frequency is 90khz, the phase shift angle is 72°. Therefore, in ten cycles, the phase shift angle is first controlled to 72°, and then to 36°, so that the required working frequency of the LLC resonant converter is reduced to 110khz (close to the threshold for switching from the TGM working mode to the LGM working mode). Through the buffer adjustment of ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process is avoided. When the required working frequency of the LLC resonant converter drops to 110khz, the mode is switched, so that the LLC resonant converter smoothly enters the LGM working mode, and then the operation of each switch tube is controlled according to the LGM working mode.

[0080] Similarly, when the LGM working mode is to switch to the TGM working mode, the phase shift angle is first controlled to 36° within ten cycles, and then to 72°, so that the required working frequency of the LLC resonant converter drops to 90 khz (close to the threshold for switching from the LGM working mode to the TGM working mode). Through ten cycles of buffering adjustment, the sudden changes in the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process are avoided. When the required working frequency of the LLC resonant converter drops to 90 khz, the mode is switched, so that the LLC resonant converter smoothly enters the TGM working mode, and then the operation of each switch tube is controlled according to the TGM working mode.

[0081] The switching between the HGM working mode and the LGM working mode is based on the same principle and will not be described in detail here.

[0082] In the present invention, the driving waveforms of the switch tubes S4 and S6 and the output voltage V o , output current i o and the resonant cavity current i Cr The switching waveform is as follows Fig. 9 As shown. Fig. 9 It can be seen that when the working mode is switched from the TGM working mode to the HGM working mode, by controlling the duty cycle of the lagging bridge arm, the peak value and duration of the load mutation are reduced and the resonant cavity almost does not generate a peak current.

[0083] Compared with the prior art, the LLC resonant converter of the present invention has the following beneficial effects:

[0084] 1. It has three working modes. Different working modes have different output voltage ranges. By switching the working modes, a wider output voltage range can be obtained, thereby widening its voltage gain range;

[0085] 2. Since the current of the primary circuit is small, although there are additional devices on the primary side in the present invention, the increased loss can be ignored. Although the secondary rectifier module has an additional bridge arm, each working mode still only uses two of the bridge arms, which will not increase the loss.

[0086] 3. The present invention adopts an asymmetric PWM control method. When the working mode is to be switched, the phase shift angle is controlled to become larger within ten cycles so that the working frequency required by the LLC resonant converter is reduced to a threshold close to the next working mode switching. Through ten cycles of buffer adjustment, the sudden changes in the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process are avoided. The mode switching is performed when the working frequency required by the LLC resonant converter drops to a threshold close to the next working mode switching, so that the LLC resonant converter smoothly enters the next working mode.

[0087] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An LLC resonant converter applied to a vehicle-mounted DC-DC converter, comprising an inverter module, a resonant cavity, a transformer and a secondary rectifier module; characterized in that: The inverter module is a full-bridge inverter circuit having a first bridge arm, a second bridge arm and a third bridge arm, wherein the first bridge arm, the second bridge arm and the third bridge arm each have two switch tubes, and through frequency modulation control, the duty ratio of the switch signals of the first bridge arm, the second bridge arm and the third bridge arm is 50%; The resonant cavity comprises a first resonant cavity and a second resonant cavity which are connected in parallel and symmetrically to each other, the first resonant cavity and the second resonant cavity are both composed of a resonant capacitor, an excitation inductor and a resonant inductor which are connected in series to each other, one end of the first resonant cavity is connected to the midpoint of the first bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module, one end of the second resonant cavity is connected to the midpoint of the second bridge arm of the inverter module, and the other end is connected to the midpoint of the third bridge arm of the inverter module; The transformer comprises a first transformer and a second transformer, the primary coil of the first transformer is connected in parallel with the excitation inductance of the first resonant cavity, and the primary coil of the second transformer is connected in parallel with the excitation inductance of the second resonant cavity; The parameters of the components of the first resonant cavity and the second resonant cavity are the same, and the parameters of the first transformer and the second transformer are the same; The secondary rectifier module is a full-bridge rectifier circuit having a first bridge arm, a second bridge arm and a third bridge arm, the two ends of the secondary coil of the first transformer are respectively connected to the midpoint of the first bridge arm of the secondary rectifier module and the midpoint of the second bridge arm of the secondary rectifier module, and the two ends of the secondary coil of the second transformer are respectively connected to the midpoint of the second bridge arm of the secondary rectifier module and the midpoint of the third bridge arm of the secondary rectifier module; The method for switching the working mode of the LLC resonant converter is as follows: When the LLC resonant converter is working, detecting the input voltage of the LLC resonant converter; When the input voltage of the LLC resonant converter is lower than the first set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the HGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the working mode of the LLC resonant converter in the previous cycle is not the HGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to the threshold value of the HGM working mode switching. Through the buffer adjustment of these ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump in the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to the threshold value of the HGM working mode switching, the mode switching is performed, so that the LLC resonant converter smoothly enters the HGM working mode. At this time, the LLC resonant converter is equivalent to two full-bridge LLC resonant converters in parallel. When the input voltage of the LLC resonant converter is greater than or equal to the first set voltage value but less than or equal to the second set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the TGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the working mode of the LLC resonant converter in the previous cycle is not the TGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to the threshold value of the TGM working mode switching. Through the buffer adjustment of these ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump in the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to the threshold value of the TGM working mode switching, the mode switching is performed, so that the LLC resonant converter smoothly enters the TGM working mode. At this time, the LLC resonant converter is equivalent to two half-bridge LLC resonant converters in parallel. When the input voltage of the LLC resonant converter is greater than the second set voltage value, the working mode of the LLC resonant converter in the previous cycle is detected. If the working mode of the LLC resonant converter in the previous cycle is the LGM working mode, the LLC resonant converter continues to work according to the existing working mode. If the working mode of the LLC resonant converter in the previous cycle is not the LGM working mode, an asymmetric PWM control method is used to control the phase shift angle to become larger within ten cycles, so that the working frequency required by the LLC resonant converter is reduced to the threshold value of the LGM working mode switching. Through the buffer adjustment of ten cycles, the sudden change of the voltage and current of the resonant cavity caused by the large frequency jump during the mode switching process is avoided. When the working frequency required by the LLC resonant converter drops to the threshold value of the LGM working mode switching, the mode is switched, so that the LLC resonant converter smoothly enters the LGM working mode. At this time, the LLC resonant converter is equivalent to a half-bridge LLC resonant converter.

2. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: The first set voltage value is 160V, and the second set voltage value is 320V.

3. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: The operating frequency threshold of the LLC resonant converter when the HGM operating mode is switched to the TGM operating mode is 130 khz, and the operating frequency threshold when the TGM operating mode is switched to the HGM operating mode is 70 khz.

4. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: The operating frequency threshold of the LLC resonant converter when the TGM operating mode is switched to the LGM operating mode is 130 khz, and the operating frequency threshold when the LGM operating mode is switched to the TGM operating mode is 70 khz.

5. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: The operating frequency threshold of the LLC resonant converter when the HGM operating mode is switched to the LGM operating mode is 130 khz, and the operating frequency threshold when the LGM operating mode is switched to the HGM operating mode is 70 khz.

6. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: When the LLC resonant converter is in the HGM operating mode, the upper arm switch tube of the first bridge arm, the upper arm switch tube of the second bridge arm and the lower arm switch tube of the third bridge arm are alternately and complementaryly turned on with the lower arm switch tube of the first bridge arm, the lower arm switch tube of the second bridge arm and the upper arm switch tube of the third bridge arm.

7. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: When the LLC resonant converter is in the TGM operating mode, the upper arm switch tube of the third bridge arm is turned off, the lower arm switch tube of the third bridge arm is turned on, and the upper arm switch tube of the first bridge arm and the upper arm switch tube of the second bridge arm are alternately and complementaryly turned on with the lower arm switch tube of the first bridge arm and the lower arm switch tube of the second bridge arm.

8. The LLC resonant converter for vehicle-mounted DC-DC converter according to claim 1, characterized in that: When the LLC resonant converter is in the LGM operating mode, the upper arm switch tube of the second bridge arm and the upper arm switch tube of the third bridge arm remain turned off, the lower arm switch tube of the second bridge arm and the lower arm switch tube of the third bridge arm remain turned on, and the upper arm switch tube of the first bridge arm and the lower arm switch tube of the first bridge arm are alternately and complementary turned on.

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