TWI751725B - Llc resonant converter and method for control the same - Google Patents

Abstract

An LLC resonant converter converts input voltage to output voltage and the LLC resonant converter includes a transformer and a primary side circuit coupled to the transformer. The primary side circuit includes a first bridge arm, a second bridge arm and a control unit. The first bridge arm includes a first switch and a second switch connected in series, and the second bridge arm includes a third switch and a fourth switch connected in series. The control unit provides a first control signal to control the first switch and provides a fourth control signal to control the fourth switch. The control unit adjusts a switching frequency of the first control signal and the fourth control signal according to the output voltage. When the switching frequency increases to a frequency threshold, the switching frequency is controlled to be fixed at the frequency threshold, and the first control signal and the fourth control signal are controlled to have a variable phase shift amount.

Description

LLC resonant converter and control method thereof

The present invention relates to an LLC resonant converter and a control method thereof, in particular to a control method of the LLC resonant converter when the load is light or no-load.

The LLC resonant converter is a DC-DC power converter, which has a zero-voltage switch on the primary side (turn on) and a zero-current switch on the secondary side (turn off), so it has higher output power than other converters. , The advantages of high conversion efficiency. It is easier to achieve high efficiency and high power density performance by using a synchronous rectifier switch on the secondary side.

However, the current control method of the LLC resonant converter, with reference to FIG. 1 , is that the first switch Q1 and the fourth switch Q4 on the primary side use the same phase PWM control signal, and the second switch Q2 and the third switch Q3 use the same phase PWM control signal. PWM control signal. In order to keep the output voltage stable within the specification range when the load is light or no-load, the current practice usually adopts several methods: 1. Operate the switching frequency at a higher frequency, but this method may It will cause the output voltage to be unstable under no-load, and need to add dummy load or limit the minimum load current. 2. The switching frequency of the switch is operated at a higher frequency and is matched with a burst control mode (Burst mode) or reduces the responsibility of the primary side switch This method may cause the switching element on the primary side to bear a large voltage stress, and a large voltage ripple will be generated at the output end.

Therefore, how to design an LLC resonant converter and its control method, so that the LLC resonant converter can reduce the output voltage ripple and reduce the voltage stress on the switching element under the condition of light load or no load when the LLC resonant converter is working normally, It is a major subject that the author of this case intends to study.

In order to solve the above problems, the present invention provides an LLC resonant converter to overcome the problems of the prior art. Therefore, the LLC resonant converter of the present invention converts the input voltage into an output voltage, and the LLC resonant converter includes: a transformer. The primary side circuit is coupled to the primary side winding of the transformer. The primary side circuit includes: a first bridge arm that receives an input voltage, and includes a first switch and a second switch connected in series. and a second bridge arm, the first bridge arm is connected in parallel, and includes a third switch and a fourth switch connected in series. and a control unit, which provides a first control signal to control the first switch, and provides a fourth control signal to control the fourth switch. The control unit adjusts the switching frequency of the first control signal and the fourth control signal according to the output voltage, when the switching frequency increases to the frequency threshold, the control switching frequency is fixed at the frequency threshold, and the first control signal and the fourth control signal have The amount of phase shift that changes.

In order to solve the above problems, the present invention provides a control method of an LLC resonant converter to overcome the problems of the prior art. Therefore, the LLC resonant converter of the present invention includes a primary side circuit having a first bridge arm and a second bridge arm coupled in parallel, and the first bridge arm includes the first switch and the second switch connected in series, and the second bridge arm includes a series connection The control method includes the following steps: providing a first control signal to control the first switch, and providing a fourth control signal to control the fourth switch to control the LLC A resonant converter converts an input voltage to an output voltage. The switching frequency and phase shift amount of the first control signal and the fourth control signal are adjusted according to the output voltage. When the switching frequency increases to the frequency threshold, the switching frequency is controlled to be fixed at the frequency threshold, and the phase shift amount of the first control signal and the fourth control signal is adjusted.

The main purpose and effect of the present invention is to control the control method of the switching elements in the first bridge arm and the second bridge arm by providing a first control signal and a fourth control signal with a fixed frequency plus phase shift control, so that the LLC resonates The converter can be under the condition of light load or no load without increasing the switching frequency or reducing the duty ratio of the first control signal and the fourth control signal, and still can reduce the ripple of the output voltage and reduce the first bridge The effect of voltage stress and current stress on the arm and the second bridge arm.

In order to further understand the technology, means and effect adopted by the present invention to achieve the predetermined purpose, please refer to the following detailed description and accompanying drawings of the present invention. For specific understanding, however, the accompanying drawings are only provided for reference and description, and are not intended to limit the present invention.

10, 10': LLC resonant converter

122: The first bridge arm

Q1: The first switch

Q2: Second switch

124: Second bridge arm

Q3: The third switch

Q4: Fourth switch

Q1, Q2, Q3, Q4: switch

126: Resonant unit

T: Transformer

12: Primary side circuit

14, 14': Secondary side circuit

142, 142': rectifier circuit

QR1: The first rectifier switch

QR2: Second rectifier switch

QR3: The third rectifier switch

QR4: Fourth rectifier switch

Co: output capacitance

16: Control unit

162: Compare Unit

164: Voltage Control Unit

166: Frequency adjustment unit

168: Phase adjustment unit

170: Drive control unit

171: PWM unit

172: Drive circuit

20: load

Vin: input voltage

Vo: output voltage

Uv: voltage upper limit value

Lv: Voltage lower limit value

Vref: reference voltage

Io: output current

Sc1, Sc2, Sc3, Sc4, Sr1, Sr2, Sr3, Sr4: Control signal

Sc1: The first control signal

Sc2: The second control signal

Sc3: The third control signal

Sc4: Fourth control signal

Sr1, Sr2, Sr3, Sr4: rectification control signal

Ser: error signal

Scm: Frequency command

Sp: Phase adjustment signal

Sa: enable signal

Sb: Disable signal

f: frequency

fsw: switching frequency

fmax: frequency threshold

Ps: phase shift amount

Pmax: maximum phase shift amount

Pt: phase shift threshold

Pp: Threshold value

I, II, III: Regions

BM: Burst Control Mode

(S100)~(S360): Steps

Fig. 1 is a circuit structure diagram of the LLC resonant converter of the first embodiment of the present invention; Fig. 2 is a circuit structure diagram of the LLC resonant converter of the second embodiment of the present invention; Fig. 3 is a circuit block diagram of the control unit of the present invention; 4 is a control curve diagram of the LLC resonant converter of the present invention; FIG. 5 is a schematic waveform diagram of the LLC resonant converter of the present invention in burst control mode; and FIG. 6 is a control flow chart of the LLC resonant converter of the present invention.

Hereby, the technical content and detailed description of the present invention are described as follows in conjunction with the drawings:

Please refer to FIG. 1 , which is a circuit structure diagram of the LLC resonant converter according to the first embodiment of the present invention. The LLC resonant converter 10 includes a transformer T, a primary side circuit 12 , a secondary side circuit 14 and a control unit 16 . The transformer T is coupled to the primary side circuit 12 and the secondary side circuit 14 . The control unit 16 controls the primary side circuit 12 and the secondary side circuit 14 through the control signals Sc1 , Sc2 , Sc3 , Sc4 , Sr1 , and Sr2 to convert the input voltage Vin into the output voltage Vo and provide power to the load 20 . As shown in FIG. 1 , the secondary side circuit 14 includes a rectifier circuit 142 and an output capacitor Co. The rectifier circuit 142 can be a first rectifier switch QR1 and a second rectifier switch QR2, and is combined with a center-tapped structure of the secondary winding of the transformer T to form a half-bridge structure. The output capacitor Co is coupled to the rectifier circuit 142 and the load 20 . Wherein, the first rectification switch QR1 and the second rectification switch QR2 are not limited to synchronous rectification switches, but may also be diodes. The primary side circuit 12 includes a first bridge arm 122 , a second bridge arm 124 , and a resonance unit 126 coupled in parallel. The first bridge arm 122 includes a first switch Q1 and a second switch Q2 coupled in series, and the second bridge arm 124 includes a third switch Q3 and a fourth switch Q4 coupled in series. One end of the resonance unit 126 is coupled to the common contact of the first switch Q1 and the second switch Q2, and the other end of the resonance unit 126 is coupled to the common contact of the third switch Q3 and the fourth switch Q4 through the primary side winding of the transformer T .

The control unit 16 adjusts the first control signal Sc1 for controlling the first switch Q1, the second control signal Sc2 for controlling the second switch Q2, and the first control signal Sc2 for controlling the third switch Q3 by detecting the actual output voltage Vo of the LLC resonant converter 10. Three control signals Sc3, and a fourth control signal Sc4 for controlling the fourth switch Q4. The waveforms of the first control signal Sc1 and the second control signal Sc2 are substantially complementary, and the waveforms of the third control signal Sc3 and the fourth control signal Sc4 are substantially complementary. The control unit 16 also provides the rectification control signals Sr1 and Sr2 to control the first rectification switch QR1 and the second rectification switch QR1 of the rectification circuit 142 respectively. The rectification switch QR2 enables the rectification circuit 142 to perform synchronous rectification. It is worth mentioning that the resonant unit 126 in the LLC resonant converter is not limited to the connection method shown in the figure, and the structure that can generate two resonant frequencies by LC should be included in the scope of this embodiment.

Please refer to FIG. 2 , which is a circuit structure diagram of the LLC resonant converter according to the second embodiment of the present invention, and also refer to FIG. 1 . The difference between the LLC resonant converter 10' of the embodiment of Fig. 2 and the LLC resonant converter 10 of Fig. 1 is that the rectifier circuit 142' of the secondary side circuit 14' of the LLC resonant converter 10' is a full-bridge structure. It includes a first rectification switch QR1, a second rectification switch QR2, a third rectification switch QR3 and a fourth rectification switch QR4, and the control unit 16 provides rectification control signals Sr1, Sr2, Sr3, and Sr4 to control the rectification switches QR1, QR2, QR3, QR4, so that the rectification circuit 142' can perform synchronous rectification. The circuit structure and control method not mentioned in this embodiment are the same as those in FIG. 1 , and will not be repeated here.

Please refer to FIG. 3 , which is a circuit block diagram of the control unit of the present invention, and refer to FIGS. 1 to 2 in combination. The control unit 16 includes a comparison unit 162 , a voltage control unit 164 , a frequency adjustment unit 166 and a phase adjustment unit 168 . The comparison unit 162 receives the feedback of the actual output voltage Vo and the reference voltage Vref as the target value of the output voltage Vo. The voltage control unit 164 is coupled to the comparison unit 162 and the frequency adjustment unit 166 , and the phase adjustment unit 168 is coupled to the frequency adjustment unit 166 . The frequency adjusting unit 166 and the phase adjusting unit 168 are coupled to the driving circuit 172 to provide control signals Sc1 , Sc2 , Sc3 and Sc4 to control the on or off of the switches Q1 , Q2 , Q3 and Q4 of the primary side circuit 12 .

The comparison unit 162 calculates the difference between the output voltage Vo and the reference voltage Vref to generate an error signal Ser. The voltage control unit 164 generates a frequency command Scm for determining the switching frequency fsw of the control signals Sc1 , Sc2 , Sc3 , and Sc4 according to the error signal Ser. Among them, since the frequency command Scm is generated according to the error between the output voltage Vo and the reference voltage Vref, the output voltage Vo will vary with the load 20 Therefore, the frequency command Scm reflects the lightness of the load. When the load is heavier, the frequency command Scm value will correspondingly decrease, and when the load is lighter, the frequency command Scm value will be correspondingly increased.

The frequency adjustment unit 166 accordingly provides the frequency adjustment signal Sfa according to the frequency command Scm to adjust the switching frequencies fsw of the control signals Sc1 , Sc2 , Sc3 and Sc4 . When the frequency adjustment unit 166 determines according to the frequency command Scm that the switching frequencies fsw of the control signals Sc1, Sc2, Sc3 and Sc4 will be greater than the frequency threshold fmax preset by the LLC resonant converter 10, the frequency adjustment unit 166 adjusts the control signals Sc1, Sc1, The switching frequency fsw of Sc2, Sc3, Sc4 is set to a fixed frequency equal to the frequency threshold value fmax. After the frequency adjustment unit 166 sets the switching frequency fsw to a fixed frequency, the frequency adjustment unit 166 provides the phase adjustment signal Sp to the phase adjustment unit 168 to notify the phase adjustment unit 168 to provide the phase shift signal Sps to start adjusting the first control signal Sc1 and the first control signal Sps. The phase shift amount Ps of the four control signals Sc4 gradually increases the waveform phase difference between the two from the threshold value Pp (for example, but not limited to, usually 0°). Since the waveforms of the first control signal Sc1 and the second control signal Sc2 are substantially complementary, and the waveforms of the third control signal Sc3 and the fourth control signal Sc4 are substantially complementary, this step is equivalent to adjusting the second control signal Sc2 and the third control signal Sc2. The phase shift amount Ps between the three control signals Sc3.

Further, the control unit 16 further includes a pulse width modulation unit 171 and a driving circuit 172 . The pulse width modulation unit 171 receives the frequency adjustment signal Sfa and the phase shift signal Sps and modulates the control signals Sc1 , Sc2 , Sc3 and Sc4 accordingly. The frequency adjustment signal Sfa and the phase shift signal Sps can be output in the form of triangular waves, and after comparing with the reference voltage Vref, the corresponding control signals Sc1 , Sc2 , Sc3 and Sc4 are output. The driving circuit 172 receives the control signals Sc1, Sc2, Sc3, and Sc4, and drives the first bridge arm 122 and the second bridge arm 124 according to the control signals Sc1, Sc2, Sc3, and Sc4. It is worth mentioning that the pulse width modulation unit 171 may include various implementations, and the circuit diagram in FIG. 3 is only one of the various implementations, which is not limited herein. In addition, the driving circuit 172 is a driving device for successfully driving the high-power switch by using the weak current signal. When the control signals Sc1 , Sc2 , Sc3 , and Sc4 can drive the first bridge arm 122 and the second bridge arm 124 smoothly without the drive circuit 172 , the drive circuit 172 does not need to be installed.

Referring back to FIG. 3 , the control unit 16 further includes a drive control unit 170 , and the control unit 16 further detects the output current Io (not shown). The drive control unit 170 is coupled to the drive circuit 172 and determines whether to provide the enable signal Sa or the disable signal Sb to the drive circuit 172 according to the magnitude of the output voltage Vo and the output current Io, so as to further control the enable or disable of the drive circuit 172 , the way of judging enable or disable will be further explained later. In an embodiment of the present invention, in the absence of the driving circuit 172, the driving control unit 170 can also be coupled to the PWM unit 171 through a simple logic circuit (such as but not limited to AND gate, OR gate), The output of the pulse width modulation unit 171 is controlled by using the enable signal Sa and the disable signal Sb. It is worth mentioning that, in an embodiment of the present invention, the control signals Sc1 and Sc2 can be reversed with those of Sc3 and Sc4. That is, the output of the upper comparator of the PWM unit 171 in FIG. 3 controls the third switch Q3 and the fourth switch Q4, and the output of the lower comparator controls the first switch Q1 and the second switch Q2. In addition, in an embodiment of the present invention, it is not limited that the components in the control unit 16 must be implemented according to this structure, such as any component, circuit or software that can achieve the same function (for example, the comparison function is not limited to only using a comparator). The program (and the use of writing a control software program to enable the controller to control the LLC resonant converter according to the program) should be included in the scope of this embodiment.

Please refer to FIG. 4 , which is a control curve diagram of the LLC resonant converter of the present invention, and refer to FIGS. 1 to 3 in combination. In FIG. 4 , the vertical axis shows the magnitude of the frequency f and the phase shift P at the same time, and the higher the frequency f is, the smaller the phase shift P is. The horizontal axis represents the amount of error between the output voltage Vo and the reference voltage Vref (ie, the corresponding load). It should be noted that, according to the circuit specification of the LLC resonant converter 10 , there will be a preset switching frequency curve (represented by a solid line). At the corresponding frequency command Scm, the control unit 16 will The determined switching frequency curve provides the switching frequency fsw corresponding to the control signals Sc1, Sc2, Sc3, and Sc4. The curve formed by the other dotted line is the phase shift curve.

In region I, the LLC resonant converter 10 is the operating condition when the load 20 is normally pumped. At this time, the control unit 16 enters the frequency conversion control mode, so that the phase shift amount Ps of the first control signal Sc1 and the fourth control signal Sc4 is equal to the threshold value Pp, and has a switching frequency fsw that varies with the load. Since the second control signal Sc2 and the first control signal Sc1 are complementary, and the third control signal Sc3 and the fourth control signal Sc4 are complementary, the phase shift amount Ps between the second control signal Sc2 and the third control signal Sc3 is also equal to the Limit Pp, and has a switching frequency fsw that varies with the size of the load. When the load 20 becomes heavier, the switching frequency fsw becomes smaller, and when the load 20 becomes lighter, the switching frequency fsw becomes larger. When the switching frequency fsw continues to increase to the frequency threshold fmax due to the lightening of the load, the control curve enters the region II.

In region II, the control unit 16 controls the switching frequency fsw of the first control signal Sc1 and the fourth control signal Sc4 to be a fixed frequency equal to the frequency threshold fmax, and starts to change the phase shift amount Ps between them to enter the phase shift control model. At this time, the control unit 16 further determines whether the output current Io is less than the current threshold value, and when the output current Io is not less than the current threshold value (for example, but not limited to 6A), it means that the LLC resonant converter 10 is in a light load condition, and the control unit 16 continues to operate at In the phase shift control mode, the switching frequency of the first control signal Sc1 and the fourth control signal Sc4 is kept constant, and the phase shift amount Ps increases as the output current Io decreases. When the phase shift amount Ps continues to increase to the phase shift threshold value Pt, the control curve enters the region III.

In the region III, when the load is no-load or near no-load, the control unit 16 maintains the switching frequency fsw of the first control signal Sc1 and the fourth control signal Sc4 equal to the frequency threshold fmax. The phase adjustment unit 168 has also adjusted the phase shift amount Ps to the upper phase shift threshold Pt (for example, but not limited to, usually 165° to 135°, which can be customized according to the circuit specification of the LLC resonant converter 10 ) to maintain the phase. The shift amount Ps is equal to the phase shift the threshold Pt. At this time, the control unit 16 continuously monitors the output current Io. When the output current Io is smaller than the current threshold, the LLC resonant converter 10 enters the burst control mode. In this mode, the control unit 16 further controls the drive control unit according to the magnitude of the output voltage Vo. 170 enable or disable. It is worth mentioning that when the control curve is still in the region II, that is, when the phase shift amount Ps has not reached the phase shift threshold value Pt, if the control unit 16 detects that the output current Io is less than the current threshold value, the LLC resonant converter 10 directly enters the phase shift threshold. When the control mode is sent, the phase shift amount Ps of the control signals Sc1, Sc4 (or Sc2, Sc3) at this time will be equal to the phase shift amount Ps (between the threshold Pp and the phase shift threshold Pt) when the control mode changes.

Please refer to FIG. 5 , which is a schematic diagram of waveforms of the LLC resonant converter of the present invention in the burst control mode. Please refer to FIGS. 1 to 4 for further details. In the burst control mode of FIG. 5 , the control unit 16 controls the phase shift amount Ps between the first control signal Sc1 and the fourth control signal Sc4 to be a constant value, so that the switching between the first switch Q1 and the fourth switch Q4 has Fixed phase shift amount Ps. Furthermore, the control unit 16 controls the switching frequency fsw of the first control signal Sc1 and the fourth control signal Sc4 to be fixed to a frequency threshold fmax. The waveforms of the second control signal Sc2 and the first control signal Sc1 are complementary, and the waveforms of the third control signal Sc3 and the fourth control signal Sc4 are complementary, so the switching of the second switch Q3 and the third switch Q3 also has The same fixed phase shift amount Ps and fixed switching frequency fsw. The control unit 16 further detects the magnitude of the output voltage Vo, and when the output voltage Vo reaches the upper voltage limit Uv, the drive control unit 170 sends the disable signal Sb to the drive circuit 172 to make it in the disable mode without sending the control signal Sc1, Sc2, Sc3, Sc4, at this time, the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4 are all turned off, so that the output voltage Vo drops. When the output voltage Vo drops to the lower voltage limit Lv, the drive control unit 170 sends an enable signal Sa to the drive circuit 172 to make it work normally, and the switches Q1, Q2, Q3, and Q4 are controlled according to the control signals Sc1, Sc2, Sc3, and Sc4. On or off, the output voltage Vo starts to rise. Through this control method, the control unit 16 can control the output voltage Vo to keep the voltage upper limit value Uv and A predetermined range between the voltage lower limit value Lv (for example, but not limited to, the ripple of the output voltage Vo is maintained within a variation range of ±3%).

Please refer to FIG. 6 for a flowchart of the control method of the LLC resonant converter of the present invention, and refer to FIGS. 1 to 5 in combination. The control method of the LLC resonant converter firstly includes comparing the output voltage with a reference voltage to generate a frequency command ( S100 ). The comparison unit 162 in the control unit 16 compares the output voltage Vo with the reference voltage Vref, and provides an error signal Ser according to the comparison result. The voltage control unit 164 then generates a frequency command Scm according to the error signal Ser, and the frequency command Scm is the target value of the switching frequency fsw of the control signals Sc1 , Sc2 , Sc3 and Sc4 . Then, it is judged whether the frequency command is larger than the frequency threshold (S120). The frequency adjustment unit 166 receives the frequency command Scm, and determines whether the frequency command Scm is greater than the frequency threshold. When the result of the step (S120) is "No", the control unit controls the LLC resonant converter to enter the frequency conversion control mode (220), and enables the driving circuit (S360). At this time, it represents that the LLC resonant converter 10 is operating under the normal load extraction of the load 20 , the control unit 16 controls the phase shift amount Ps between the first control signal Sc1 and the second control signal Sc2 to maintain the threshold value Pp, and the control unit 16 The switching frequency fsw of the first control signal Sc1 and the second control signal Sc2 is controlled to be a variable value. Then, the drive control unit 170 provides the enable signal Sa to the drive circuit 172 to make the drive circuit work normally, so the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4 are based on the control signals Sc1, Sc2, Sc3 , Sc4 is turned on or off. When the result of the step (S120) is "Yes", that is, when the switching frequency fsw is greater than the frequency threshold, the switching frequency fsw is set to a fixed frequency, and phase shift control is performed (S140). When the switching frequency fsw is greater than the frequency threshold fmax, it means that the load 20 is currently in a light load state, and at this time, the switching frequency fsw is limited to the frequency threshold fmax. The frequency adjustment unit 166 further provides the phase adjustment signal Sp to the phase adjustment unit 168 to change the phase shift amount Ps of the first control signal Sc1 and the fourth control signal Sc4.

Then, it is judged whether the phase shift amount is smaller than the phase shift threshold value (S160). When the result of the step (S160) is "No", the control phase shift amount is maintained at the phase shift threshold value (S180). When the phase shift amount Ps is not less than the phase shift threshold value Pt, it means that the load 20 is under a lighter load, and the control unit 16 controls the phase shift amount Ps between the first control signal Sc1 and the fourth control signal Sc4 to maintain the phase shift threshold value Pt. Then, it is further judged whether the output current is less than the current threshold (S200).

When the result of the step (S200) is "No", then the step (S360) is entered to enable the driving circuit, and the driving control unit 170 provides the enable signal Sa to the driving circuit 172 to make the driving circuit work normally, so the first switch Q1, the first switch Q1, the The second switch Q2, the third switch Q3 and the fourth switch Q4 are turned on or off according to the control signals Sc1, Sc2, Sc3 and Sc4. When the result of the step (S200) is “Yes”, that is, the output current Io is less than the current threshold, which means that the load 20 is no-load or close to no-load. At this time, the control unit 16 controls the LLC resonant converter 10 to enter the burst control Mode BM. Wherein, when the result of step ( S160 ) is “Yes”, the control unit 16 also controls the LLC resonant converter 10 to proceed to step ( S200 ) with the current operating frequency fsw and phase shift amount Ps. In the burst control mode BM, the control unit 16 determines whether or not the output voltage value is greater than the voltage upper limit value (S300). The voltage upper limit value Uv can be determined according to the upper voltage limit that the load 20 of the LLC resonant converter 10 can accept. When the result of the step (S300) is "Yes", the driving circuit is disabled by the control signal (S320). That is, the drive control unit 170 sends the disable signal Sb to disable the drive circuit 172, so that the switches Q1, Q2, Q3, and Q4 are all turned off without being controlled by the control signals Sc1, Sc2, Sc3, and Sc4. When the result of step (S300) is "No", it is determined whether the output voltage value is smaller than the voltage lower limit value (S340). The voltage lower limit value Lv can be determined according to the voltage lower limit that the load 20 of the LLC resonant converter 10 can accept. Therefore, when the result of the step (S340) is "Yes", the driving circuit is enabled by the control signal (S360). That is, the drive control unit 170 sends the enable signal Sa to make the drive circuit 172 work normally, so that the switches Q1 , Q2 , Q3 , Q4 are turned on or off according to the control signals Sc1 , Sc2 , Sc3 , Sc4 . The result in step (S340) is "No" , return to step (S200). The judgment of the output voltage and the voltage upper/lower limit values in steps ( S300 ) and ( S340 ) can be assisted by the comparison unit 162 calculating the error between the output voltage Vo and the reference voltage Vref.

The above descriptions are only detailed descriptions and drawings of the preferred embodiments of the present invention, but the features of the present invention are not limited thereto, and are not intended to limit the present invention. The entire scope of the present invention should be defined as the following claims All the embodiments that conform to the spirit of the scope of the patent application of the present invention and its similar variations shall be included in the scope of the present invention. Modifications can be covered by the following patent scope of this case.

10: LLC Resonant Converter

122: The first bridge arm

Q1: The first switch

Q2: Second switch

124: Second bridge arm

Q3: The third switch

Q4: Fourth switch

126: Resonant unit

T: Transformer

12: Primary side circuit

14: Secondary side circuit

142: Rectifier circuit

QR1: The first rectifier switch

QR2: Second rectifier switch

Co: output capacitance

16: Control unit

20: load

Vin: input voltage

Vo: output voltage

Io: output current

Sc1: The first control signal

Sc2: The second control signal

Sc3: The third control signal

Sc4: Fourth control signal

Sr1, Sr2: rectification control signal

Claims (16)

An LLC resonant converter converts an input voltage into an output voltage, the LLC resonant converter comprises: a transformer; a primary side circuit coupled to a primary side winding of the transformer, the primary side circuit comprising: a first bridge an arm that receives the input voltage and includes a first switch and a second switch connected in series; and a second bridge arm that is connected in parallel with the first bridge arm and includes a third switch and a fourth switch connected in series; and a control unit that provides a first control signal to control the first switch, and provides a fourth control signal to control the fourth switch; wherein the control unit adjusts the first control signal and the fourth control signal according to the output voltage a switching frequency, when the switching frequency increases to a frequency threshold, the switching frequency is controlled to be fixed at the frequency threshold, and the first control signal and the fourth control signal are controlled to have a varying phase shift; and wherein, When the switching frequency is equal to the frequency threshold, the control unit controls the phase shift amount not to exceed a phase shift threshold. The LLC resonant converter of claim 1, wherein when the control unit detects that an output current of the LLC resonant converter is less than a current threshold, the control unit enters a burst control mode. The LLC resonant converter as described in claim 2, wherein in the burst control mode, the control unit controls the switching frequency to be the frequency threshold, and controls the phase shift amount to be a certain value. The LLC resonant converter of claim 2, wherein in the burst control mode, the control unit disables the first control signal when the output voltage is greater than a voltage upper limit value signal and the fourth control signal, and the control unit enables the first control signal and the fourth control signal when the output voltage is less than a voltage lower limit value. The LLC resonant converter as described in claim 1, wherein when the switching frequency is less than the frequency threshold, the control unit fixes the phase shift amount as a threshold value, and controls the first control signal and the first control signal Four control signals have the switching frequency that varies. The LLC resonant converter of claim 1, wherein the control unit comprises: a comparison unit for comparing the output voltage with a reference voltage to provide an error signal; a voltage control unit for generating according to the error signal a frequency command; a frequency adjustment unit for determining the switching frequency according to the frequency command and providing a phase adjustment signal; a phase adjustment unit for adjusting the phase shift amount according to the phase adjustment signal; and a pulse width modulation unit for The outputs of the frequency adjusting unit and the phase adjusting unit adjust the first control signal and the fourth control signal. The LLC resonant converter of claim 1, wherein the control unit comprises: a driving circuit, which drives the first switch according to the first control signal, and drives the fourth switch according to the fourth control signal. The LLC resonant converter as described in claim 7, wherein the control unit further comprises: a drive control unit for providing a disable signal or an enable signal to the drive circuit. The LLC resonant converter as described in claim 1, wherein the control unit comprises: A second control signal is provided to control the second switch, and a third control signal is provided to control the third switch; the second control signal and the first control signal are complementary signals, and the third control signal and the fourth control signal are complementary The control signal is a complementary signal. A control method of an LLC resonant converter, the LLC resonant converter includes a primary side circuit having a first bridge arm and a second bridge arm coupled in parallel, and the first bridge arm includes a first switch connected in series and a second switch, the second bridge arm includes a third switch and a fourth switch connected in series, the control method includes the following steps: providing a first control signal to control the first switch, and providing a fourth control signal controlling the fourth switch to control the LLC resonant converter to convert an input voltage into an output voltage; adjusting a switching frequency and a phase shift of the first control signal and the fourth control signal according to the output voltage; When the switching frequency increases to a frequency threshold, control the switching frequency to be fixed at the frequency threshold and adjust the phase shift amount of the first control signal and the fourth control signal; and when the switching frequency is fixed at the frequency threshold, control The phase shift amount starts to increase from a threshold value and does not exceed a phase shift threshold value. The control method as described in claim 10, wherein the control method further comprises the following steps: detecting an output current, and when the output current is less than a current threshold, fixing the phase shift amount to enter a burst control model. The control method described in item 11 of the scope of the application, wherein the control method further comprises the following steps: In the burst control mode, when the output voltage is greater than a voltage upper limit value, the first control signal and the fourth control signal are disabled; and in the burst control mode, when the output voltage is less than a voltage When the lower limit value is set, the first control signal and the fourth control signal are enabled. The control method according to claim 11, wherein the control method further comprises the following steps: when the output current is greater than the current threshold, enabling the first control signal and the fourth control signal. The control method as described in claim 10, wherein the control method further comprises the following steps: when the switching frequency is less than the frequency threshold, controlling the switching of the first control signal and the fourth control signal to have a change frequency, and the phase shift amount is fixed as a threshold value. The control method according to claim 10, wherein the control method further comprises the following steps: comparing the output voltage with a reference voltage to provide an error signal; generating a frequency command according to the error signal; according to the frequency command A frequency adjustment signal is provided to adjust the switching frequency, and when the switching frequency is a fixed frequency, a phase adjustment signal is provided; and a phase shift signal is provided according to the phase adjustment signal to adjust the phase shift amount. The control method of claim 10, wherein the control method further comprises the following steps: providing a second control signal complementary to the first control signal to control the second switch, and providing a second control signal complementary to the fourth control signal A complementary third control signal controls the third switch.

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