An enhanced quasi-resonant PWM control circuit

Abstract

The application discloses an enhanced quasi-resonant PWM control circuit, a transformer comprising a transformer primary, a transformer first secondary and a transformer second secondary, an input circuit connected to the transformer primary, the transformer primary connected to a first switch, the transformer first secondary connected to a second switch, the transformer second secondary connected to a third switch, the second switch connected to a first capacitor, and the third switch connected to a second capacitor; a first control circuit for controlling the on and off of the first switch; a second control circuit for controlling the on and off of the second switch and the on and off of the third switch; the input circuit transmits energy through the transformer; wherein the second control circuit controls the resonant energy of the resonant circuit by controlling the on and off time sequence and time of the second switch and the third switch. The application has the function of increasing the energy of the resonant circuit, further reducing the voltage across the first switch when it is turned on, reducing the energy loss when it is turned on, and improving the conversion efficiency of the converter.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of PWM control circuit, in particular to an enhanced quasi-resonant PWM control circuit. BACKGROUND

[0002] The existing quasi-resonant (QR) PWM flyback converter circuit is shown in Figure 1 , a control circuit controls the on and off of a first switch S1 to achieve energy output from an input circuit to a transformer primary, and the transformer primary generates a magnetic field to store energy in a transformer gap. After the first switch is turned off, the energy stored in the transformer gap is output through the transformer secondary, and the output is charged to a second capacitor through a diode and a feedback circuit to make the output a set voltage. After most of the energy stored in the transformer gap is output through the transformer secondary, the secondary output current and voltage begin to decrease, and when the voltage decreases to below the sum of the diode conduction voltage and the voltage of the second capacitor, the diode is turned off. At this time, the energy stored in the transformer gap cannot be transmitted to the transformer secondary output, and the energy flows into a resonant circuit composed of a leakage inductance L Lk , a magnetizing inductance L m , a transformer primary copper loss resistance R L , and a parasitic capacitance C S of the first switch S1. The energy flows in this resonant circuit and generates a resonant voltage fluctuation on the parasitic capacitance C S of the first switch S1, and the energy and voltage amplitude decrease over time. The voltage across the first switch S1 fluctuates due to the charging and discharging of the parasitic capacitance of the first switch S1 in the resonant circuit, and when the output power is full load, the control circuit detects that the voltage across the first switch S1 is at the lowest point and then turns on the first switch S1 to output energy from the input circuit to the transformer primary. When the load is low, the control circuit can choose to turn on the first switch S1 when the voltage is at the lowest point. Since the first switch S1 is turned on when the voltage across the two ends is at the lowest point, the energy loss caused by the short circuit of the parasitic capacitance when turned on is reduced, and the converter conversion efficiency is improved. The current and voltage waveforms of the existing quasi-resonant (QR) PWM flyback converter in operation are shown in Figure 2 .

[0003] The existing technology has the following disadvantages:

[0004] 1. When the first switch is turned on, the voltage across the first switch S1 is still high at the lowest point of the resonant voltage, causing a large energy loss when turned on. Moreover, when the input voltage is high, the voltage across the first switch S1 is pulled up at the lowest point of the resonant voltage, and the energy loss is more serious when the first switch is turned on, reducing the converter conversion efficiency.

[0005] 2. At lower load, the control circuit selects to turn on the first switch S1 at the second or later resonance minimum point. Because of the transformer primary copper loss resistance and other energy losses, the energy in the resonant circuit also decreases over time, and the resonant voltage amplitude on the parasitic capacitance of the first switch S1 also decreases at the same time. The voltage minimum point of the first switch S1 is continuously rising in each resonance period, and the voltage when the first switch is turned on is getting higher and higher, and the energy loss when the switch is turned on is increasing, which reduces the conversion efficiency of the converter. SUMMARY

[0006] The present application provides an enhanced quasi-resonant PWM control circuit to solve the problem of high voltage when the first switch is turned on, large energy loss when the switch is turned on, and reduced conversion efficiency of the converter in the prior art.

[0007] To solve the above technical problems, the present application provides an enhanced quasi-resonant PWM control circuit, which comprises an input circuit, a transformer, a first switch, a second switch, a third switch, a first capacitor, a second capacitor, a first control circuit and a second control circuit. The transformer comprises a transformer primary, a transformer first secondary and a transformer second secondary. The input circuit is connected to the transformer primary, the transformer primary is connected to the first switch, the transformer first secondary is connected to the second switch, the transformer second secondary is connected to the third switch, the second switch is connected to the first capacitor, and the third switch is connected to the second capacitor. The first control circuit is used to control the turn-on and turn-off of the first switch. The second control circuit is used to control the turn-on and turn-off of the second switch and the turn-on and turn-off of the third switch. The input circuit transmits energy through the transformer. The second control circuit controls the resonant energy of the resonant circuit by controlling the on-off time sequence and time of the second switch and the third switch.

[0008] Optionally, the resonant circuit comprises a transformer primary leakage inductance, a transformer primary excitation inductance, a transformer primary copper loss resistance and a parasitic capacitance of the first switch.

[0009] Optionally, it comprises a first working mode and a second working mode. The first working mode is used when the output power is full load or close to full load. The second working mode is used when the output power is lower than full load.

[0010] Optionally, in the first working mode, the second switch and the third switch are turned off, and the first switch is turned on; the transformer primary is grounded through the first switch, the input circuit outputs energy to the transformer primary, the transformer primary generates a magnetic field and stores energy in the transformer gap; the first switch is turned off, and the second switch and the third switch are turned on; the energy stored in the transformer gap is output through the transformer first secondary and the transformer second secondary, and the energy is charged to the first capacitor through the second switch and to the second capacitor through the third switch, and the voltage of the second capacitor rises to provide energy for the load; when the first control circuit or the second control circuit detects that the energy of the transformer gap decreases to a preset point, the second switch and the third switch are turned off at the same time.

[0011] Optionally, the simultaneous turning off of the second switch and the third switch makes the energy of the transformer gap unable to be transmitted to the transformer secondary circuit, and when the energy of the transformer gap is unable to be transmitted to the transformer secondary circuit, the energy is injected into a resonance circuit composed of the leakage inductance of the transformer primary, the excitation inductance of the transformer primary, the copper loss resistance of the transformer primary, and the parasitic capacitance of the first switch.

[0012] Optionally, since the transmission of the energy of the transformer gap to the transformer secondary circuit is terminated early due to the turning off of the second switch and the third switch, the energy provided to the resonance circuit increases, the voltage fluctuation on the parasitic capacitance of the first switch increases due to resonance, the lowest point of the voltage across the first switch becomes lower, and the first switch is turned on again when the control circuit detects that the voltage across the first switch is at the lowest point, the input circuit outputs energy to the transformer primary, and the next working period is entered.

[0013] Optionally, in the second working mode, the second switch and the third switch are turned off, and the first switch is turned on; the transformer primary is grounded through the first switch, the input circuit outputs energy to the transformer primary, the transformer primary generates a magnetic field and stores energy in the transformer gap; the first switch is turned off, and the second switch and the third switch are turned on; the energy stored in the transformer gap is output through the transformer first secondary and the transformer second secondary, and the energy is charged to the first capacitor through the second switch and to the second capacitor through the third switch, and the voltage of the second capacitor rises to provide energy for the load; when the first control circuit or the second control circuit detects that the energy of the transformer gap decreases to a preset point, the second switch and the third switch are turned off at the same time.

[0014] Optionally, the simultaneous turning off of the second switch and the third switch makes the energy of the transformer gap unable to be transmitted to the transformer secondary circuit, and when the energy of the transformer gap is unable to be transmitted to the transformer secondary circuit, the energy is injected into a resonance circuit composed of the leakage inductance of the transformer primary, the excitation inductance of the transformer primary, the copper loss resistance of the transformer primary, and the parasitic capacitance of the first switch.

[0015] Optionally, when the load is low, the control circuit selects to turn on the third switch at the moment when the voltage across the second or the first switch is the lowest, to inject energy into the resonant circuit, and at the same time, the voltage across the parasitic capacitance of the first switch is increased due to the voltage fluctuation caused by the resonance, so that the lowest point of the voltage across the first switch becomes lower; the third switch is turned off, and then the first switch is turned on, so that the input circuit outputs energy to the transformer primary, and enters the next working period.

[0016] The application provides an enhanced quasi-resonant PWM control circuit, which comprises an input circuit, a transformer, a first switch, a second switch, a third switch, a first capacitor, a second capacitor, a first control circuit and a second control circuit; the transformer comprises a transformer primary, a transformer first secondary and a transformer second secondary; the input circuit is connected with the transformer primary; the transformer primary is connected with the first switch; the transformer first secondary is connected with the second switch; the transformer second secondary is connected with the third switch; the second switch is connected with the first capacitor; the third switch is connected with the second capacitor; the first control circuit is used for controlling the turn-on and turn-off of the first switch; the second control circuit is used for controlling the turn-on and turn-off of the second switch and the turn-on and turn-off of the third switch; the input circuit transmits energy through the transformer; wherein the second control circuit controls the resonant energy of the resonant circuit by controlling the on-off time sequence and time of the second switch and the third switch. The application has the function of increasing the resonant circuit energy, further reduces the voltage across the first switch when the first switch is turned on, reduces the energy loss when the first switch is turned on, and improves the conversion efficiency of the converter. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the application, the following will briefly introduce the drawings needed to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can also be obtained by those skilled in the art without any creative effort.

[0018] Figure 1 is a prior quasi-resonant (QR) PWM flyback converter circuit;

[0019] Figure 2 is the current and voltage waveform of the prior quasi-resonant (QR) PWM flyback converter when it is in operation;

[0020] Figure 3 is a circuit schematic diagram of an embodiment of the enhanced quasi-resonant PWM control circuit of the application;

[0021] Figure 4 is Figure 3 is a circuit schematic diagram of the first switch of the control circuit of

[0022] Figure 5 is Figure 3a circuit schematic diagram of the first switch in the control circuit turning off the third switch and turning on the second switch;

[0023] Figure 6 is Figure 3 a circuit schematic diagram of the first switch and the second switch in the control circuit turning off the third switch and turning on the second switch;

[0024] Figure 7 is each current and voltage waveform when the enhanced quasi-resonant PWM control circuit in the application operates in the first working mode;

[0025] Figure 8 is each current and voltage waveform when the enhanced quasi-resonant PWM control circuit in the application operates in the second working mode. DETAILED DESCRIPTION

[0026] In order for those skilled in the art to better understand the technical solutions of the present application, the enhanced quasi-resonant PWM control circuit provided by the present application is further described in detail below in combination with the drawings and specific embodiments.

[0027] The present application provides an enhanced quasi-resonant PWM control circuit, please refer to Figure 3 , Figure 3 is a schematic diagram of an embodiment of the enhanced quasi-resonant PWM control circuit in the application. In this embodiment, the enhanced quasi-resonant PWM control circuit can include an input circuit, a transformer, a first switch S1, a second switch S2, a third switch S3, a first capacitor C1, a second capacitor C2, a first control circuit and a second control circuit.

[0028] The transformer includes a transformer primary, a transformer first secondary and a transformer second secondary. The input circuit is connected to the transformer primary. The transformer primary is connected to the first switch S1. The transformer first secondary is connected to the second switch S2. The transformer second secondary is connected to the third switch S3. The second switch S2 is connected to the first capacitor C1. The third switch S3 is connected to the second capacitor C2.

[0029] It should be noted that the transformer secondary can include one or more than one group. In this embodiment, the transformer secondary can include the transformer first secondary and the transformer second secondary.

[0030] Among them, the first control circuit is used to control the turn-on and turn-off of the first switch S1; the second control circuit is used to control the turn-on and turn-off of the second switch S2 and the turn-on and turn-off of the third switch S3; the input circuit can transmit energy through the transformer.

[0031] In this embodiment, the second control circuit can control the resonance energy of the resonance circuit by controlling the on-off time sequence and time of the second switch S2 and the third switch S3. The resonance circuit includes the transformer primary leakage inductance L Lk, transformer primary excitation inductance L m , transformer primary copper loss resistance R L , and the parasitic capacitance C of the first switch S1 S .

[0032] Further, the enhanced quasi-resonant PWM control circuit can include a first working mode and a second working mode; the first working mode is used when the output power is full load or close to full load; the second working mode is used when the output power is lower than full load.

[0033] It should be noted that, in theory, the first working mode is used when the output power is full load, but in actual operation, when the output power is close to full load, it can also be treated as full load and the first working mode is adopted.

[0034] 1) In the first working mode (when the output power is close to full load or full load):

[0035] The second switch S2 and the third switch S3 are turned off, and the first switch S1 is turned on; the transformer primary is grounded through the first switch S1, the input circuit outputs energy to the transformer primary, the transformer primary generates a magnetic field and stores energy in the transformer gap (such as Figure 4 and Figure 7 , t0 to t1).

[0036] The first switch S1 is turned off, and the second switch S2 and the third switch S3 are turned on, the energy stored in the transformer gap is output through the transformer secondary, charged to the first capacitor C1 through the second switch S2, and charged to the second capacitor C2 through the third switch S3, and the voltage of the second capacitor C2 rises (such as Figure 7 , t1 to t2); when the first control circuit or the second control circuit detects that the energy of the transformer gap decreases to a preset point, the second switch S2 and the third switch S3 are turned off at the same time (such as Figure 5 and Figure 7 , t2).

[0037] Turning off the second switch S2 and the third switch S3 makes the energy of the transformer gap unable to be transmitted to the transformer secondary circuit, when the energy of the transformer gap cannot be transmitted to the transformer secondary circuit, the energy will be injected into the resonant circuit composed of the transformer primary leakage inductance L Lk , transformer primary excitation inductance L m , transformer primary copper loss resistance R L , and the parasitic capacitance C of the first switch S1 S .

[0038] Because turning off the second switch S2 and the third switch S3 makes the energy of the transformer gap transmitted to the transformer secondary circuit terminate early, the energy provided to the resonant circuit increases, and the parasitic capacitance C of the first switch S1 SThe voltage fluctuation caused by resonance increases the lowest voltage between the first switch S1, and the control circuit detects the lowest voltage between the first switch S1 and then turns on the first switch S1 (as shown in Figure 4 and Figure 7 , t0-t1), the input circuit outputs energy to the transformer primary, and enters the next working period.

[0039] 2) In the second working mode (when the output power is lower than full load):

[0040] The second switch S2 and the third switch S3 are turned off, the first switch S1 is turned on, the transformer primary is grounded through the first switch S1, the input circuit outputs energy to the transformer primary, the transformer primary generates a magnetic field and stores energy in the transformer gap (as shown in Figure 4 and Figure 8 , t0-t1).

[0041] The first switch S1 is turned off, the second switch S2 and the third switch S3 are turned on, the energy stored in the transformer gap is output through the transformer secondary, and the second capacitor C2 is charged through the second switch S2 and the third switch S3, and the voltage of the second capacitor C2 rises (as shown in Figure 4 and Figure 8 , t1-t2); when the first control circuit or the second control circuit detects that the energy of the transformer gap decreases to a preset point, the second switch S2 and the third switch S3 are turned off at the same time (as shown in Figure 5 and Figure 8 , t2).

[0042] Turning off the second switch S2 and the third switch S3 makes the energy of the transformer gap unable to be transmitted to the transformer secondary circuit, and the energy of the transformer gap cannot be transmitted to the transformer secondary circuit, and the energy is injected into the resonance circuit composed of the transformer primary leakage inductance L Lk , the transformer primary excitation inductance L m , the transformer primary copper loss resistance R L and the parasitic capacitance C S of the first switch S1.

[0043] When the load is low, the control circuit selects to turn on the third switch S3 at the second or subsequent lowest voltage point to inject energy into the resonance circuit (as shown in Figure 6 and Figure 8 , t3-t4), and the parasitic capacitance C S of the first switch S1 increases due to the voltage fluctuation caused by resonance, making the lowest voltage between the first switch S1 lower; the third switch S3 is turned off, and the first switch S1 is turned on again (as shown in Figure 4 , t0-t1), the input circuit outputs energy to the transformer primary, and enters the next working period.

[0044] In summary, the application provides an enhanced quasi-resonant PWM control circuit, comprising: an input circuit, a transformer, a first switch, a second switch, a third switch, a first capacitor, a second capacitor, a first control circuit and a second control circuit; the transformer comprises a transformer primary, a transformer first secondary and a transformer second secondary, the input circuit is connected to the transformer primary, the transformer primary is connected to the first switch, the transformer first secondary is connected to the second switch, the transformer second secondary is connected to the third switch, the second switch is connected to the first capacitor, and the third switch is connected to the second capacitor; the first control circuit is used to control the turn-on and turn-off of the first switch; the second control circuit is used to control the turn-on and turn-off of the second switch and the turn-on and turn-off of the third switch; the input circuit transmits energy through the transformer; wherein the second control circuit controls the resonant energy of the resonant circuit by controlling the on-off time sequence and time of the second switch and the third switch. The application has the function of increasing the energy of the resonant circuit, further reduces the voltage across the first switch when it is turned on, reduces the energy loss when it is turned on, and improves the conversion efficiency of the converter; compared with the prior art, the circuit structure is simple to change, the loss of the first switch is reduced, the reliability is improved, and the conversion efficiency is improved.

[0045] It can be understood that the specific embodiments described herein are only used to explain the application, but not to limit the application. In addition, for the convenience of description, only part of the structure related to the application is shown in the drawings, not all the structures. The step numbers used in the description are only for the convenience of description, and are not limited to the execution sequence of the steps. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the application.

[0046] The terms "first", "second", and the like in the application are used to distinguish different objects, not to describe a specific order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units not listed, or optionally includes other steps or units inherent to the process, method, product or device.

[0047] In this document, the term "embodiment" means that the specific features, structures or characteristics described in connection with the embodiment can be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily mean the same embodiment, nor is it independent or alternative to other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0048] The above merely describes the embodiments of the present application, and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation, or direct or indirect application in other related technical fields, which is made according to the content of the present application specification and drawings, is also included in the patent protection scope of the present application.

Claims

1. An enhanced quasi-resonant PWM control circuit, characterized in that: include: An input circuit, a transformer, a first switch, a second switch, a third switch, a first capacitor, a second capacitor, a first control circuit, and a second control circuit; The transformer includes a transformer primary, a transformer first secondary, and a transformer second secondary, the input circuit is connected to the transformer primary, the transformer primary is connected to the first switch, the transformer first secondary is connected to the second switch, the transformer second secondary is connected to the third switch, the second switch is connected to the first capacitor, and the third switch is connected to the second capacitor; The first control circuit is used to control the switching on and off of the first switch; The second control circuit is used to control the on and off of the second switch and the on and off of the third switch; the input circuit transmits energy through the transformer; Wherein, the second control circuit controls the resonant energy of the resonant circuit by controlling the on-off timing and time of the second switch and the third switch; The enhanced quasi-resonant PWM control circuit includes a first operating mode and a second operating mode; The first operating mode is used when the output power is full load; the second operating mode is used when the output power is lower than full load; In the first operating mode, the second switch and the third switch are disconnected, and the first switch is connected; the transformer primary is grounded via the first switch, the input circuit outputs energy to the transformer primary, the transformer primary generates a magnetic field and stores the energy in the transformer magnetic gap; the first switch is disconnected, the second switch and the third switch are connected, the energy stored in the transformer magnetic gap is output via the transformer first secondary and the transformer second secondary, the first capacitor is charged via the second switch, and the second capacitor is charged via the third switch, the voltage of the second capacitor increases, and energy is provided to the load; when the first control circuit or the second control circuit detects that the energy in the transformer magnetic gap is reduced to a preset point, the second switch and the third switch are simultaneously turned off; when the first control circuit detects that the voltage across the first switch is at the lowest point, the first switch is turned on again, and the input circuit outputs energy to the transformer primary, and the next operating cycle begins; In the second operating mode, the second and third switches are disconnected, and the first switch is connected. The transformer primary is grounded through the first switch, and the input circuit outputs energy to the transformer primary, generating a magnetic field and storing the energy in the transformer magnetic gap. The first switch is disconnected, and the second and third switches are connected. The energy stored in the transformer magnetic gap is output through the first and second secondary transformers, charging the first capacitor through the second switch and the second capacitor through the third switch. The voltage of the second capacitor increases, providing energy to the load. When the first control circuit or the second control circuit detects that the energy in the transformer magnetic gap has decreased to a predetermined value, the second and third switches are simultaneously turned off. When the load is low, the second control circuit selects to turn on the third switch at the lowest point of the voltage across the second or subsequent first switches, injecting energy into the resonant circuit. Simultaneously, the voltage across the parasitic capacitance of the first switch increases due to the voltage fluctuation generated by the resonance, causing the lowest point of the voltage across the first switch to become lower. The third switch is disconnected, and the first switch is then connected again, causing the input circuit to output energy to the transformer primary, and the next operating cycle begins.

2. The enhanced quasi-resonant PWM control circuit according to claim 1, characterized in that: The resonant circuit includes the leakage inductance of the transformer, the magnetizing inductance of the primary of the transformer, the copper loss resistance of the primary of the transformer, and the parasitic capacitance of the first switch.

3. The enhanced quasi-resonant PWM control circuit according to claim 1, characterized in that: In the first operating mode, the second switch and the third switch are simultaneously turned off, so that the energy of the transformer magnetic gap cannot be transferred to the transformer secondary circuit. When the energy of the transformer magnetic gap cannot be transferred to the transformer secondary circuit, the energy will be injected into the resonant circuit composed of the leakage inductance of the transformer primary, the magnetizing inductance of the transformer primary, the copper loss resistance of the transformer primary, and the parasitic capacitance of the first switch.

4. The enhanced quasi-resonant PWM control circuit according to claim 1, characterized in that: In the second operating mode, the second switch and the third switch are simultaneously turned off, so that the energy of the transformer magnetic gap cannot be transferred to the transformer secondary circuit. When the energy of the transformer magnetic gap cannot be transferred to the transformer secondary circuit, the energy will be injected into the resonant circuit composed of the leakage inductance of the transformer primary, the magnetizing inductance of the transformer primary, the copper loss resistance of the transformer primary, and the parasitic capacitance of the first switch.

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