Overvoltage detection circuit and flyback converter

Abstract

The utility model provides an overvoltage detection circuit and a flyback converter. The overvoltage detection circuit comprises a first capacitor; the setting unit sets the voltage of the first capacitor at the initial moment of the second time period according to the first time period and the input voltage; the discharging unit is used for discharging the first capacitor with a second current in a second time period; and the judgment unit outputs an overvoltage judgment signal according to the voltage of the first capacitor at the end moment of the second time period and a preset threshold voltage. Based on the principle that volt-second balance and output overvoltage judgment of an inductor in a flyback converter are combined, the utility model provides the overvoltage detection circuit. The voltage of the first capacitor is set according to the input voltage in the inductor charging time period in each switching period, and the voltage of the first capacitor is set according to the second current which is in a first proportion to the preset protection voltage in the inductor discharging time period; therefore, whether the output of the flyback converter is overvoltage or not can be judged directly through the voltage change of the first capacitor after the discharge of the inductor is finished.

Description

Technical Field

[0001] The utility model belongs to the technical field of power electronics, and in particular relates to an overvoltage detection circuit and a flyback converter. Background Art

[0002] Output overvoltage protection (OVP) is a commonly used protection method in switching power supply protection circuits. It can effectively identify abnormal conditions when the switching power supply is in output overvoltage, and control the switching state of the transistor in the switching power supply to prevent damage to the switching power supply.

[0003] At present, the more common output overvoltage protection scheme in the prior art is: setting a voltage divider resistor at the output end, collecting the voltage of the voltage divider resistor, and then comparing the sampled voltage with the preset voltage threshold to determine whether the output is overvoltage. However, this scheme will increase power consumption due to the existence of the voltage divider resistor, which is not conducive to improving the working efficiency of the switching power supply.

[0004] Therefore, the output overvoltage protection method in the prior art needs to be improved to obtain a simpler and more efficient output overvoltage protection method. Utility Model Content

[0005] In order to solve the technical problem of high power consumption of the output overvoltage protection scheme in the prior art, the utility model proposes an overvoltage detection circuit and a flyback converter, wherein the overvoltage detection circuit for detecting whether the output of the flyback converter is overvoltage includes:

[0006] a first capacitor;

[0007] a setting unit, configured to set the voltage of the first capacitor at an initial moment of a second time period according to a first time period and an input voltage of the flyback converter, wherein the first time period represents a time period during which the inductor current in the flyback converter continues to rise, and the second time period represents a time period during which the inductor current in the flyback converter continues to fall;

[0008] a discharge unit, discharging the first capacitor with a second current in the second time period, wherein the second current is in a first proportion to a preset protection voltage, and the preset protection voltage represents a maximum output voltage of the flyback converter during normal operation;

[0009] The judgment unit outputs an overvoltage judgment signal according to the voltage of the first capacitor at the end of the second time period and a preset threshold voltage.

[0010] Further, the setting unit includes a first comparator, a first switch and a voltage setting branch, and the judging unit includes the first comparator, wherein:

[0011] The positive input terminal of the first comparator receives the preset threshold voltage in the second time period, the negative input terminal of the first comparator is connected to the first capacitor, and the first switch is connected between the output terminal and the negative input terminal of the first comparator.

[0012] The voltage setting branch sets the voltage of the first capacitor at the initial moment of the second time period, and the first switch is closed so that the voltage of the first capacitor follows the voltage of the positive input terminal.

[0013] Preferably, the output end of the voltage setting branch is connected to the positive input end during the first time period, and the first switch is closed during the first time period.

[0014] Preferably, the output end of the voltage setting branch is connected to the first capacitor, the positive input end receives the preset threshold voltage during the first time period, and the first switch is closed at the end of the second time period.

[0015] Preferably, the voltage setting branch includes a multiplier and a second switch, the input end of the multiplier receives the input voltage and the first time period, the output end of the multiplier is connected to the positive input end through the second switch, the third switch is connected between the ground end and the positive input end, the ground end voltage is the preset threshold voltage, the second switch is closed in the first time period, and the third switch is closed in the second time period.

[0016] Preferably, the voltage setting branch comprises a first power supply, and the first power supply charges the first capacitor with a first current in the first time period, and the first current is in a first proportion to the input voltage.

[0017] Preferably, the judgment unit further includes a D trigger, the input end of the D trigger is connected to the output end of the first comparator, and the clock end receives a clock signal, and the clock signal triggers the D trigger to generate the overvoltage judgment signal.

[0018] Further, when the voltage of the first capacitor at the end of the second time period is greater than the preset threshold voltage, the judgment unit outputs an invalid overvoltage judgment signal, and the invalid overvoltage judgment signal represents an output overvoltage of the flyback converter.

[0019] Preferably, when the flyback converter operates in DCM mode or BCM mode, the clock end is connected to the output end of the zero-crossing detection circuit, the zero-crossing detection circuit detects the zero value of the inductor current, and the zero-crossing detection signal output by the zero-crossing detection circuit is the clock signal.

[0020] A flyback converter comprises a rectifier tube and the overvoltage detection circuit described above. When the overvoltage detection circuit detects that the output of the flyback converter is overvoltage, the rectifier tube is turned off.

[0021] Based on the principle of combining the volt-second balance of the inductor in the flyback converter and the judgment of the output overvoltage, the above-mentioned overvoltage detection circuit is proposed in the utility model. In each switching cycle, the voltage of the first capacitor is set according to the input voltage during the time period of inductor charging, and the voltage of the first capacitor is set according to the second current that is in a first proportion to the preset protection voltage during the time period of inductor discharge. Therefore, it is possible to directly judge whether the output of the flyback converter is overvoltage by the voltage change of the first capacitor after the inductor discharge ends. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the structural block diagram of the overvoltage detection circuit;

[0023] Figure 2 is a further detailed structural block diagram of the overvoltage detection circuit in the first embodiment;

[0024] Figure 3 The specific circuit structure of the overvoltage detection circuit in the first embodiment;

[0025] Figure 4 is a further detailed structural block diagram of the overvoltage detection circuit in the second embodiment;

[0026] Figure 5 The specific circuit structure of the overvoltage detection circuit in the second embodiment;

[0027] Figure 6 Specific circuit structure of the overvoltage detection circuit in the first embodiment in BCM mode and CCM mode. DETAILED DESCRIPTION

[0028] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0029] In order to enable the public to have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, and those skilled in the art can fully understand the present invention without these detailed descriptions.

[0030] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and use non-precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0031] The existing output overvoltage protection scheme of the flyback converter is generally to collect a sample voltage representing the output voltage by setting a voltage divider resistor (the voltage divider resistor can be directly connected to the output end or set through an auxiliary winding), and then judge whether the output is overvoltage by comparing the sample voltage with the maximum output voltage during normal operation of the flyback converter. This scheme will inevitably cause certain losses, thereby affecting the efficiency of the flyback converter.

[0032] In view of this situation, the utility model proposes an overvoltage detection circuit, which is applied to a flyback converter to determine whether the output of the flyback converter is overvoltage. Figure 1 As shown, the overvoltage detection circuit includes:

[0033] a first capacitor;

[0034] a setting unit, wherein the setting unit sets the voltage of the first capacitor at the initial moment of the second time period according to the first time period and the input voltage of the flyback converter, the first time period being a time period in which the inductor current in the flyback converter continues to rise, and the second time period being a time period in which the inductor current continues to fall;

[0035] a discharge unit, discharging the first capacitor with a second current in a second time period, the second current being in a first proportion to a preset protection voltage, the preset protection voltage representing a maximum output voltage of the flyback converter during normal operation;

[0036] The judgment unit outputs an overvoltage judgment signal according to the magnitude relationship between the voltage of the first capacitor at the end of the second time period and the preset threshold voltage, and the output overvoltage judgment signal indicates whether the output of the flyback converter is overvoltage.

[0037] In the first embodiment, if Figure 2 and Figure 3As shown, the overvoltage detection circuit includes: a first capacitor C1, a setting unit, a discharge unit and a judgment unit. Among them, the setting unit includes a voltage setting branch, a first comparator CMP1 and a first switch S1, and the judgment unit includes a first comparator CMP1 and a D trigger. The positive input end of the first comparator CMP1 receives a preset threshold voltage VDC (the specific value of the preset threshold voltage VDC is not limited and can be set according to requirements), the reverse input end of the first comparator CMP1 is connected to the first capacitor C1, the input end of the D trigger is connected to the output end of the first comparator CMP1, the clock end of the D trigger receives a clock signal, the first switch S1 is connected between the output end and the reverse input end of the first comparator CMP1, the output end of the voltage setting branch is connected to the first capacitor C1, and the voltage setting branch includes a first current source, and the first current source charges the first capacitor C1 with a first current I1 in a first time period, and the first current I1 is in a first proportion to the input voltage Vin. In the first time period, the voltage setting branch charges the first capacitor C1 with the first current I1, and in the second time period, the discharge unit discharges the first capacitor C1 with the second current I2. The first comparator CMP1 compares the voltage of the first capacitor C1 at the end of the second time period with the preset threshold voltage VDC to output a first comparison signal R1. The D flip-flop outputs an overvoltage judgment signal OVP according to the first comparison signal R1 under the triggering of the clock signal to indicate whether the output of the flyback converter is overvoltage. In this embodiment, when the voltage of the first capacitor C1 at the end of the second time period is greater than the preset threshold voltage VDC, the first comparator CMP1 outputs the first comparison signal R1 in an invalid state, and the D flip-flop follows to output the overvoltage judgment signal OVP in an invalid state, and the overvoltage judgment signal OVP in an invalid state indicates that the output of the flyback converter is overvoltage.

[0038] Further, the first switch S1 is closed at the end of the second time period so that the potential of the first capacitor C1 follows the voltage of the positive input terminal of the first comparator CMP1, that is, at the initial moment of the next switching cycle, the potential of the first capacitor C1 is restored to the preset threshold voltage VDC, so the overvoltage detection circuit can judge whether the output of the flyback converter is overvoltage in the next switching cycle according to the voltage of the first capacitor at the end of the second time period and the magnitude of the preset threshold voltage. In this embodiment, after the first switch S1 is closed, it forms a voltage follower with the first comparator CMP1. At this time, the voltage of the first capacitor C1 can follow the voltage of the positive input terminal of the first comparator only by closing the first switch S1, so there is no need to set a discharge branch for the first capacitor C1 to discharge and restore the potential. In addition, the voltage follower formed by the first comparator CMP1 and the first switch S1 to restore the voltage of the first capacitor can also eliminate the error caused by the comparator input offset to the output overvoltage detection process, thereby improving the detection accuracy of the overvoltage detection circuit.

[0039] Furthermore, in the first embodiment, when the flyback converter operates in the DCM mode, the first switch S1 is controlled to be turned on when the main power tube and the rectifier tube are turned off at the same time, so that the potential of the first capacitor C1 is restored to the preset threshold voltage VDC. However, when the flyback converter operates in the CCM mode and the BCM mode, the first capacitor C1 does not have enough time to restore the voltage to the preset threshold voltage VDC at the end of the second time period. Therefore, two overvoltage detection circuits can be set up in the CCM mode and the BCM mode to detect whether the output is overvoltage. The two overvoltage detection circuits work alternately and periodically in any two adjacent switching cycles. Among them, the specific structures of the two overvoltage detection circuits are the same, which will not be repeated here. For details, please refer to Figure 6 . In any switching cycle, the first overvoltage detection circuit detects whether the output of the flyback converter is overvoltage, and at this time, the potential of the first capacitor in the second overvoltage detection circuit is restored; in the next switching cycle, the second overvoltage detection circuit is replaced to detect whether the output of the flyback converter is overvoltage, and the potential of the first capacitor in the first overvoltage detection circuit is restored. Specifically, the switch controller can realize the periodic alternating operation of the first overvoltage detection circuit and the second overvoltage detection circuit by controlling the on-off timing of switch K1, switch K2, switch S1, switch K3, switch K4 and switch S1'.

[0040] In the second embodiment, if Figure 4 and Figure 5As shown, the overvoltage detection circuit includes: a first capacitor C1, a setting unit, a discharge unit and a judgment unit. Among them, the setting unit includes a voltage setting branch, a first comparator CMP1 and a first switch S1, and the judgment unit includes a first comparator CMP1 and a D trigger. The positive input end of the first comparator CMP1 is connected to the output end of the voltage setting branch in the first time period and receives a preset threshold voltage VDC in the second time period. The reverse input end of the first comparator CMP1 is connected to the first capacitor C1, the first switch S1 is connected between the output end and the reverse input end of the first comparator CMP1, the output end of the first comparator CMP1 is connected to the input end of the D trigger, and the clock end of the D trigger receives a clock signal. Among them, the voltage setting branch includes a multiplier and a second switch S2, the input end of the multiplier receives the first time period and the input voltage to output the product of the two, the output end of the multiplier is connected to the positive input end of the first comparator CMP1 through the second switch S2, and the third switch S3 is connected between the positive input end of the first comparator and the ground end, and the ground end voltage is the preset threshold voltage VDC. Therefore, in the first time period, the first switch S1 and the second switch S2 are closed, and the positive input terminal of the first comparator CMP1 receives the product of the first time period and the input voltage, so the potential of the first capacitor C1 at the initial moment of the second time period follows the product of the first time period and the input voltage; in the second time period, the first switch S1 and the second switch S2 are turned off, the third switch S3 is closed, and the discharge unit discharges the first capacitor C1 with the second current I2. At the end of the second time period, the first comparator CMP1 compares the voltage of the first capacitor C1 with the preset threshold voltage VDC and outputs the first comparison signal R1, and the D flip-flop follows the first comparison signal R1 to output the overvoltage judgment signal OVP. Specifically, in this embodiment, when the voltage of the first capacitor C1 at the end of the second time period is greater than the preset threshold voltage, the first comparator outputs the first comparison signal R1 in an invalid state, and the D flip-flop follows to output the overvoltage judgment signal OVP in an invalid state, and the overvoltage judgment signal OVP in an invalid state indicates the output overvoltage of the flyback converter.

[0041] In summary, compared with the design of the first embodiment, in the second embodiment, the overvoltage detection circuit uses the voltage follower formed by the first comparator and the first switch to set the voltage of the first capacitor in the first time period, so there is sufficient time in each switching cycle to adjust the voltage of the first capacitor according to the input voltage in the first time period, so there is no need to set multiple overvoltage detection circuits to perform overvoltage detection in BCM mode or CCM mode, and the structural design and control logic are simplified. In addition, using the voltage follower formed by the first comparator and the first switch to set the voltage of the first capacitor can also eliminate the error caused by the comparator input offset to the output overvoltage detection process, thereby improving the detection accuracy of the overvoltage detection circuit.

[0042] Furthermore, in the COT control mode, the error amplifier in the feedback loop of the flyback converter can be used to characterize the input voltage based on the output voltage and the compensation voltage output by the reference voltage, so that the multiplier can set the voltage of the first capacitor based on the compensation voltage and the first time period.

[0043] Furthermore, in the first embodiment and the second embodiment, the clock end of the D flip-flop can be connected to a zero-crossing detection circuit in the DCM mode and the BCM mode. The zero-crossing detection circuit is used to detect the zero value of the inductor current. The zero-crossing detection signal output by the zero-crossing detection circuit is the clock signal. Whenever the zero-crossing detection signal represents the arrival of the zero value of the inductor current, the D flip-flop is triggered to follow the input to generate an output. Using the zero-crossing detection signal instead of the clock signal can be combined with the specific working mode of the flyback converter to achieve precise control of the overvoltage detection circuit.

[0044] Further, the working principle of the above overvoltage detection circuit is analyzed:

[0045] For the flyback converter, according to the volt-second balance principle:

[0046] Where Vin represents the input voltage of the flyback converter, k represents the primary-to-secondary turns ratio of the transformer, L represents the primary inductance of the flyback converter, Ton represents the time period during which the inductor current continues to rise, and Toff represents the time period during which the inductor current continues to fall;

[0047] If the output of the flyback converter is overvoltage, then: V out *k>V ref , after replacing the equation, we can get: V in *T on >V ref *T off Among them, Vref represents the maximum output voltage of the flyback converter during normal operation. It should be noted that "Vref represents the maximum output voltage of the flyback converter during normal operation" does not mean "the voltage value of Vref is the maximum output voltage of the flyback converter during normal operation". The value of Vref after the original-secondary side conversion is the maximum output voltage of the flyback converter during normal operation;

[0048] In the first embodiment, the first capacitor is analyzed to know that the charging amount of the first capacitor in the first time period is K1*V in *T on , the discharge amount of the first capacitor in the second time period is K1*V ref *T off , K1 represents the first ratio. Therefore, it is only necessary to determine whether the voltage of the first capacitor is greater than the preset threshold voltage (i.e., determine whether V in *T onIs it greater than V ref *T off ) can tell whether the output of the flyback converter is over-voltage.

[0049] In the second embodiment, the analysis of the first capacitor shows that the voltage of the first capacitor at the initial moment of the second time period (the end moment of the first time period) is V in *T on / RC1, the voltage of the first capacitor reduced in the second time period is V ref *T off Therefore, it is only necessary to determine whether the voltage of the first capacitor is greater than the preset threshold voltage (i.e., determine whether V in *T on Is it greater than V ref *T off ) can tell whether the output of the flyback converter is over-voltage.

[0050] In summary, based on the principle of combining the volt-second balance of the inductor in the flyback converter and the judgment of the output overvoltage, the above-mentioned overvoltage detection circuit is proposed in the utility model. In each switching cycle, the voltage of the first capacitor is set according to the input voltage during the time period of inductor charging, and the voltage of the first capacitor is set according to the second current that is in a first proportion to the preset protection voltage during the time period of inductor discharge. In this way, it is possible to directly judge whether the output of the flyback converter is overvoltage by the voltage change of the first capacitor after the inductor discharge is completed.

[0051] Furthermore, in order to prevent the overvoltage protection from being falsely triggered in a single cycle, when using the above-mentioned overvoltage detection circuit, it can be set that the output of the flyback converter is only determined to be overvoltage if N consecutive cycles are all judged to be output overvoltage.

[0052] In addition, the utility model also proposes a flyback converter, which includes the overvoltage detection circuit proposed above to determine whether the output is overvoltage. When the overvoltage detection circuit determines that the output is overvoltage, the secondary rectifier tube in the flyback converter is turned off.

[0053] It should be noted that the specific implementation and corresponding illustrations given are merely a way of describing the implementation method of the utility model, and do not limit the specific structure of the implementation scheme of the utility model. Various changes or modifications can be made to these implementation schemes without departing from the principle and essence of the utility model, but these changes and modifications are all within the scope of protection of the utility model.

[0054] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0055] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. An overvoltage detection circuit for detecting whether the output of a flyback converter is overvoltage, characterized in that: The overvoltage detection circuit comprises: a first capacitor; a setting unit, configured to set the voltage of the first capacitor at an initial moment of a second time period according to a first time period and an input voltage of the flyback converter, wherein the first time period represents a time period during which the inductor current in the flyback converter continues to rise, and the second time period represents a time period during which the inductor current in the flyback converter continues to fall; a discharge unit, discharging the first capacitor with a second current in the second time period, wherein the second current is in a first proportion to a preset protection voltage, and the preset protection voltage represents a maximum output voltage of the flyback converter during normal operation; The judgment unit outputs an overvoltage judgment signal according to the voltage of the first capacitor at the end of the second time period and a preset threshold voltage.

2. The overvoltage detection circuit according to claim 1, characterized in that: The setting unit includes a first comparator, a first switch and a voltage setting branch, and the judging unit includes the first comparator, wherein: The positive input terminal of the first comparator receives the preset threshold voltage in the second time period, the negative input terminal of the first comparator is connected to the first capacitor, and the first switch is connected between the output terminal and the negative input terminal of the first comparator. The voltage setting branch sets the voltage of the first capacitor at the initial moment of the second time period, and the first switch is closed so that the voltage of the first capacitor follows the voltage of the positive input terminal.

3. The overvoltage detection circuit according to claim 2, characterized in that: The output end of the voltage setting branch is connected to the positive input end in the first time period, and the first switch is closed in the first time period.

4. The overvoltage detection circuit according to claim 2, characterized in that: The output end of the voltage setting branch is connected to the first capacitor, the positive input end receives the preset threshold voltage in the first time period, and the first switch is closed at the end of the second time period.

5. The overvoltage detection circuit according to claim 3, characterized in that: The voltage setting branch includes a multiplier and a second switch, the input end of the multiplier receives the input voltage and the first time period, the output end of the multiplier is connected to the positive input end through the second switch, the third switch is connected between the ground end and the positive input end, the ground end voltage is the preset threshold voltage, the second switch is closed in the first time period, and the third switch is closed in the second time period.

6. The overvoltage detection circuit according to claim 4, characterized in that: The voltage setting branch includes a first power supply, and the first power supply charges the first capacitor with a first current in the first time period, and the first current is in a first proportion to the input voltage.

7. The overvoltage detection circuit according to claim 5 or 6, characterized in that: The judgment unit further includes a D trigger, wherein an input end of the D trigger is connected to an output end of the first comparator, and a clock end of the D trigger receives a clock signal, and the clock signal triggers the D trigger to generate the overvoltage judgment signal.

8. The overvoltage detection circuit according to claim 7, characterized in that: When the voltage of the first capacitor at the end of the second time period is greater than the preset threshold voltage, the judgment unit outputs an invalid overvoltage judgment signal, and the invalid overvoltage judgment signal indicates that the output of the flyback converter is overvoltage.

9. The overvoltage detection circuit according to claim 7, characterized in that: When the flyback converter operates in DCM mode or BCM mode, the clock end is connected to the output end of the zero-crossing detection circuit, the zero-crossing detection circuit detects the zero value of the inductor current, and the zero-crossing detection signal output by the zero-crossing detection circuit is the clock signal.

10. A flyback converter, comprising a rectifier tube, characterized in that: It also includes the overvoltage detection circuit according to any one of claims 1 to 9, and when the overvoltage detection circuit detects that the output of the flyback converter is overvoltage, the rectifier tube is turned off.

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