A flyback converter current limiting circuit and flyback converter

Abstract

The present invention relates to a flyback converter current limiting circuit and a flyback converter. The converter includes a power input terminal and an output terminal, a transformer, a rectifier circuit and a switch unit. The current limiting circuit includes: first and second current detection units, a first divider, a second divider, a delay unit, a comparison unit and a drive unit. The first input terminal of the first divider is connected to the second current detection unit, the second input terminal of the first divider is used to input a target current value of the converter, the output of the first divider is connected to the first input terminal of the second divider, the second input terminal of the second divider is connected to the output terminal of the delay unit, and the output terminal of the second divider is connected to the input terminal of the delay unit. The first input terminal of the comparison unit is connected to the output terminal of the delay unit, the second input terminal of the comparison unit is connected to the first current detection unit, the output terminal of the comparison unit is connected to the input terminal of the drive unit, and the output terminal of the drive unit is used to connect to the control terminal of the switch unit.

Description

Technical Field

[0001] The present invention relates to the technical field of switching power supplies, and more particularly to a flyback converter current limiting circuit and a flyback converter. Background Art

[0002] Flyback converters are widely used due to their simple structure and low cost. Figure 11 This is a typical flyback converter circuit diagram. The circuit primarily includes the primary main switch S1; a transformer TX1 with a primary winding Np and a secondary winding Ns; and an output rectifier D1. The control circuit output signal DRV is a pulse-width modulated signal. When DRV is high, the main switch S1 turns on, and the transformer TX1 stores energy from the input Vin. When the main switch S1 turns off, the transformer TX1 releases energy to the output through the rectifier D1.

[0003] During operation of a flyback converter, to protect the load and rectifier D1, the current flowing through the secondary winding Ns must be limited to within the load and rectifier D1's tolerances. Several traditional approaches are described below.

[0004] like Figure 12 In the method shown, the output current Io is detected to generate a signal, Io_sense. Io_sense is compared with the output current reference signal, Io_ref, to generate an error signal, err. The PI module performs loop compensation on the error signal (using proportional-integral compensation as an example) and generates a control signal to adjust the output current to the value set by the current reference signal by controlling the duty cycle of switch S1. This method can limit the converter's output current, but requires loop compensation. Due to system stability requirements, this method is generally slow and can only control the output current, not the secondary winding current. Therefore, it cannot prevent transient high currents. Furthermore, loop compensation components increase cost.

[0005] Another US patent is "US Patent no. 6,972,969 B1", patent name is "System and Method for Controlling Current Limit with Primary Side Sensing". This method uses the relationship between output current and power , where Po is the output power, Vin is the input voltage, Lm is the primary magnetizing inductance of the transformer, Ton is the on-time of S1, and Tp is the switching period. is the conversion efficiency. By knowing or measuring Vin, Lm, Tp, and Vo, as well as the target output current Io, the required on-time can be calculated to achieve current control. Compared to the above method, this method eliminates the need for loop compensation and can achieve rapid current limiting within the switching cycle. However, it requires more parameters to be known or measured, which can easily lead to large errors. Furthermore, this method only limits the output current and cannot provide real-time control of the secondary winding current.

[0006] There is also a US patent "US Patent no.7,443,700 B2", patent name "On-Time Control for Constant Current Mode in a Flyback Power Supply". In this method, the geometric relationship of the output current and the peak current control mode are used to detect the demagnetization time of the transformer. , using digital control to calculate the peak control voltage .here ,in is the transformer demagnetization time for the previous cycle; Vpp(n) is the peak control voltage required for the next cycle; Ts is the switching period; Rs is the sampling resistor for the primary current Ip; N is the transformer primary-to-secondary turns ratio; and IAS is the target output current. This method achieves rapid current limiting within the switching cycle without loop compensation and can limit the secondary current. However, if circuit parameters change, this method lacks real-time output current detection, making it impossible to derive current error to adjust the control results in real time. Furthermore, this method is based on the geometric relationship of the current in the transformer's discontinuous operating mode, which can result in significant errors when the transformer is in continuous operating mode. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a flyback converter current limiting circuit and a flyback converter to overcome the defects of the above-mentioned traditional control method.

[0008] The technical solution adopted by the present invention to solve its technical problem is: constructing a flyback converter current limiting circuit, the converter including a power input terminal, a power output terminal, a transformer, a rectifier circuit and a switch unit, wherein a first end of the primary input of the transformer is connected to the power input terminal, a second end of the primary input of the transformer is connected to a first end of the switch unit, a first end of the secondary output of the transformer is connected to an input terminal of the rectifier circuit, an output terminal of the rectifier circuit is connected to the power output terminal, and a second end of the secondary output of the transformer is grounded;

[0009] The current limiting circuit includes: a first current detection unit for obtaining the primary input current of the transformer and a second current detection unit for obtaining the secondary output current of the transformer; a first divider, a second divider, a delay unit, a comparison unit and a drive unit;

[0010] A first input terminal of the first divider is connected to the second current detection unit, a second input terminal of the first divider is used to input a target current value of the converter, an output terminal of the first divider is connected to the first input terminal of the second divider, a second input terminal of the second divider is connected to the output terminal of the delay unit, and an output terminal of the second divider is connected to the input terminal of the delay unit;

[0011] The first input end of the comparison unit is connected to the output end of the delay unit, the second input end of the comparison unit is connected to the first current detection unit, the output end of the comparison unit is connected to the input end of the drive unit, and the output end of the drive unit is used to connect to the control end of the switch unit.

[0012] Preferably, in the flyback converter current limiting circuit of the present invention, the first current detection unit includes a detection resistor, a first end of the detection resistor is connected to the second end of the switch unit and the inverting input end of the comparison unit, and a second end of the detection resistor is grounded.

[0013] Preferably, in the flyback converter current limiting circuit of the present invention, the second current detection unit includes a first sampling resistor, a first differential amplifier and a first averaging unit;

[0014] A first end of the first sampling resistor is connected to the output end of the rectifier circuit and the non-inverting input end of the first differential amplifier. A second end of the first sampling resistor is connected to the positive voltage output end of the power supply output end and the inverting input end of the first differential amplifier. The output end of the first differential amplifier is connected to the input end of the first averaging unit. The output end of the first averaging unit is connected to the first input end of the first divider.

[0015] Preferably, in the flyback converter current limiting circuit of the present invention, the second current detection unit further includes a first isolation unit;

[0016] An input end of the first isolation unit is connected to an output end of the first differential amplifier, and an output end of the first isolation unit is connected to an input end of the first averaging unit.

[0017] Preferably, in the flyback converter current limiting circuit of the present invention, the second current detection unit includes a second sampling resistor, a second differential amplifier, and a second averaging unit; a first end of the second sampling resistor is connected to the second end of the secondary output of the transformer and the inverting input end of the second differential amplifier, a second end of the second sampling resistor is connected to the negative voltage output end of the power supply output end and the non-inverting input end of the second differential amplifier, an output end of the second differential amplifier is connected to the input end of the second averaging unit, and an output end of the second averaging unit is connected to the first input end of the first divider; or

[0018] The second current detection unit includes a third differential amplifier, a third averaging unit and a second isolation unit. The non-inverting input end of the third differential amplifier is connected to the input end of the rectifier circuit, the inverting input end of the third differential amplifier is connected to the output end of the rectifier circuit, the output end of the third differential amplifier is connected to the input end of the second isolation unit, the output end of the second isolation unit is connected to the input end of the third averaging unit, and the output end of the third averaging unit is connected to the first input end of the first divider.

[0019] Preferably, in the flyback converter current limiting circuit of the present invention, the second current detection unit includes a first waveform analysis unit and a first controller;

[0020] The first controller is respectively connected to the output end of the delay unit, the output end of the comparison unit, the output end of the first waveform analysis unit and the first input end of the first divider;

[0021] The input end of the first waveform analysis unit is connected to the second end of the primary input of the transformer;

[0022] The first controller is used to obtain the demagnetization time of the transformer based on the peak current control amount output by the delay unit, the output period of the comparison unit and the input voltage of the first end of the switching unit to obtain the voltage detection value corresponding to the mean current output by the secondary side of the transformer.

[0023] Preferably, in the flyback converter current limiting circuit of the present invention, the first controller outputs the control parameter according to the following formula: :

[0024]

[0025] in, is the voltage value corresponding to the primary peak current control amount of the transformer, is the turns ratio between the primary input and secondary output of the transformer, is the demagnetization time of the transformer, is the switching period of the switching unit, wherein the demagnetization time of the transformer The method is obtained according to the input voltage of the first terminal of the switch unit.

[0026] Preferably, in the flyback converter current limiting circuit of the present invention, the converter further includes an auxiliary coil, and the second current detection unit includes a first voltage-dividing resistor, a second voltage-dividing resistor, a second waveform analysis unit, and a second controller;

[0027] The second controller is respectively connected to the output end of the delay unit, the output end of the comparison unit, the output end of the second waveform analysis unit and the first input end of the first divider;

[0028] An input end of the second waveform analysis unit is connected to a first end of the first voltage-dividing resistor and a first end of the second voltage-dividing resistor, a second end of the first voltage-dividing resistor is connected to a first end of the auxiliary coil, wherein a second end of the auxiliary coil and the second voltage-dividing resistor are grounded;

[0029] The second controller is used to obtain a voltage detection value corresponding to the mean current output by the secondary side of the transformer based on the peak current control amount output by the delay unit, the output cycle of the comparison unit and the demagnetization time of the transformer detected by the auxiliary coil.

[0030] Preferably, in the flyback converter current limiting circuit of the present invention, the second controller outputs the control parameter according to the following formula: :

[0031]

[0032] in, is the voltage value corresponding to the primary peak current control amount of the transformer, is the turns ratio between the primary input and secondary output of the transformer, is the demagnetization time of the transformer, is the switching period of the switching unit, wherein the demagnetization time of the transformer The voltage of the auxiliary coil is obtained.

[0033] The present invention also constructs a flyback converter, comprising: a power input terminal, a power output terminal, a transformer, a rectifier circuit, a switch unit, and a current limiting circuit as described in any one of the above;

[0034] The first end of the primary input of the transformer is connected to the power input end, the second end of the primary input of the transformer is connected to the first end of the switch unit, the first end of the secondary output of the transformer is connected to the input end of the rectifier circuit, the output end of the rectifier circuit is connected to the power output end, and the second end of the secondary output of the transformer is grounded.

[0035] The current limiting circuit is connected to the control end of the switch unit.

[0036] A flyback converter current limiting circuit and a flyback converter implementing the present invention have the following beneficial effects: they can effectively utilize the secondary current detection value and the primary peak current control value in the current cycle to correct the control value of the next cycle, thereby achieving simple and accurate secondary current control, and reducing the control error in the transformer continuous mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0038] Figure 1 This is a circuit diagram of an embodiment of a flyback converter current limiting circuit of the present invention;

[0039] Figure 2 1 is a schematic diagram of operating waveforms of an embodiment of a current limiting circuit for a flyback converter according to the present invention;

[0040] Figure 3 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0041] Figure 4 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0042] Figure 5 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0043] Figure 6 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0044] Figure 7 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0045] Figure 8 1 is a circuit diagram of another embodiment of a flyback converter current limiting circuit of the present invention;

[0046] Figure 9 1 is a schematic diagram of operating waveforms of another embodiment of a current limiting circuit for a flyback converter according to the present invention;

[0047] Figure 10 1 is a schematic diagram of operating waveforms of another embodiment of a current limiting circuit for a flyback converter according to the present invention;

[0048] Figure 11 This is a schematic diagram of a current flyback converter circuit;

[0049] Figure 12 This is a schematic diagram of the current limiting circuit of a flyback converter. DETAILED DESCRIPTION

[0050] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0051] like Figure 1 As shown, in a first embodiment of a flyback converter current limiting circuit of the present invention, the converter includes a power input terminal 100, a power output terminal 200, a transformer, a rectifier circuit 400 and a switch unit 500, a first end of the primary input of the transformer is connected to the power input terminal 100, a second end of the primary input of the transformer is connected to the first end of the switch unit 500, a first end of the secondary output of the transformer is connected to the input end of the rectifier circuit 400, an output end of the rectifier circuit 400 is connected to the power output terminal 200, and a second end of the secondary output of the transformer is grounded; the current limiting circuit includes: a first current detection unit 310 for obtaining the primary input current of the transformer and a second current detection unit 320 for obtaining the secondary output current of the transformer; and a first divider 331, a second divider 332, a delay circuit 333, and a time delay circuit 334. Unit 333, comparison unit 334 and drive unit 335; the first input end of the first divider 331 is connected to the second current detection unit 320, the second input end of the first divider 331 is used to input the target current value of the converter, the output of the first divider 331 is connected to the first input end of the second divider 332, the second input end of the second divider 332 is connected to the output end of the delay unit 333, and the output end of the second divider 332 is connected to the input end of the delay unit 333; the first input end of the comparison unit 334 is connected to the output end of the delay unit 333, the second input end of the comparison unit 334 is connected to the first current detection unit 310, the output end of the comparison unit 334 is connected to the input end of the drive unit 335, and the output end of the drive unit 335 is used to connect to the control end of the switch unit 500. Specifically, the high and low levels output by the driving unit 335 drive the switch unit 500 to be turned on or off, and the energy storage of the primary coil of the transformer Tx1 (corresponding to the primary input of the transformer Tx1) is controlled by the on or off time of the switch unit 500, so as to ultimately control the current of the secondary coil of the transformer Tx1 (corresponding to the secondary output of the transformer Tx1). Figure 2As shown, when the output level DRV of the driving unit 335 is high, the switch unit 500 (corresponding to the mark S1) is turned on, the primary current Ip in the primary winding Np of the transformer Tx1 increases, and the transformer stores energy. When the output level DRV of the driving unit 335 is low, the switch unit S1 is turned off, and the current of the primary input 100 of the converter stops. At this time, the input current of the power input terminal 100 of the transformer reaches the peak current. , the transformer energy is released to the load to form the output current of the secondary side of the transformer , the current is output to the power supply output 200 through the secondary coil of the transformer. The process from the beginning to the end is called transformer demagnetization (or magnetic recovery). , that is, the average current in the switching cycle of the secondary output of the transformer can be controlled . The output current The following relationship is satisfied:

[0052] (1)

[0053] here, is the average current on the secondary side of the transformer during the switching cycle (the current flowing through the rectifier D1); is the peak current output from the secondary side of the transformer; is the demagnetization time of the transformer; is the switching period of the switching unit S1; according to the characteristics of the transformer,

[0054] (2)

[0055] here, is the primary input current of the transformer The peak value (corresponding to the input current of 100 at the power input terminal), is the turns ratio of the transformer's primary and secondary windings, Therefore, the average value of the current output by the secondary side of the transformer during the switching cycle can be expressed as

[0056] (3)

[0057] Assume that the secondary current limit target of the transformer is , according to (3), if we keep The ratio of (when the converter works at a stable point, and The peak value of the primary input current of the transformer needs to be equal to the target value in formula (4) , so that the current output by the secondary side of the transformer is equal to the target limit:

[0058] (4)

[0059] From (3) and (4), we can conclude that

[0060] (5)

[0061] Formula (5) shows that as long as the average current at the secondary output of the transformer in the current cycle can be obtained and the peak current of the primary input of the transformer , according to the target current , we can calculate the peak current value required in the next cycle , so that the current output by the secondary side of the transformer in the next cycle reaches the target limit. According to the peak current control principle, the new The opening time of S1 in the next cycle can be determined. The ratio of the average current output by the secondary side of the transformer to the target current in the current switching cycle is:

[0062] (6)

[0063] Then there is

[0064] (7)

[0065] In the current limiting circuit, the second current detection unit 320 is used to obtain the average value of the current output by the secondary side of the transformer within the cycle. , and obtain the average value within the current cycle through the first divider 331 With target current The ratio of the corresponding current ratio ; and obtain the peak current of the primary input of the transformer through the second divider 332 Compared with the above current The ratio of the peak current control value in the next cycle is obtained . The control amount By delaying to the next switching cycle, the current value output by the secondary side of the transformer in the next cycle can be accurately limited by controlling the control terminal of the switch unit 335.

[0066] Optional, such as Figures 3 to 7 In the flyback converter current limiting circuit of the present application, the first current detection unit 310 includes a detection resistor, the first end of the detection resistor is connected to the switch unit 500 (corresponding to Figure 3The second end of the detection resistor is connected to the reverse input end of the comparison unit 334. The second end of the detection resistor is grounded. Specifically, the current input to the primary side of the transformer can be obtained through the detection resistor, wherein the detection resistor is connected to the second end of the switch unit S1 and the ground. When the switch unit 500 is turned on, the primary coil of the transformer Tx1 is turned on. The detection resistor generates a corresponding voltage due to the current flowing through it. The corresponding current detection result can be obtained through the voltage detection result. Figures 3 to 8 Medium resistor Rip.

[0067] like Figure 3 As shown, in one embodiment, the second current detection unit 320 includes a first sampling resistor 3211, a first differential amplifier 3221, and a first averaging unit 3231. The first end of the first sampling resistor 3211 is connected to the output of the rectifier circuit 400 and the non-inverting input of the first differential amplifier 3221. The second end of the first sampling resistor 3211 is connected to the positive voltage output of the power supply output terminal 200 and the inverting input of the first differential amplifier 3221. The output of the first differential amplifier 3221 is connected to the input of the first averaging unit 3231, and the output of the first averaging unit 3231 is connected to the first input of the first divider 331. Specifically, the output of the rectifier circuit 400 can be detected by the first sampling resistor 3211, that is, the current at the positive output of the power supply output terminal 200 is detected to obtain the detection current output by the secondary side of the transformer. This detection current forms a voltage difference across the first sampling resistor 3211, and the first differential amplifier 3221 amplifies this voltage difference to obtain a corresponding voltage detection result. The voltage detection result corresponds to the secondary output current detection result of the transformer, and the detection result is averaged by the first averaging unit 3231 to obtain the corresponding average current detection result of the secondary output of the transformer. Figure 3 In the specific embodiment shown, the current of the primary input of the transformer is detected by the sampling resistor Rip and converted into a voltage signal. The rectified current output from the secondary side of the transformer is detected by the resistor Ris and converted into a voltage signal. Through Rip and Ris, the current input from the primary side of the converter and the current output from the secondary side of the transformer can be converted into voltage signals, which are convenient for the control system to process. Corresponding current , The corresponding peak control voltage is the peak current of the primary side. is the voltage value corresponding to the target current limit, in formula (8), To calculate the new peak current control voltage value required for the next cycle:

[0068] (8)

[0069] like Figure 4 As shown, in one embodiment, the second current detection unit 320 further includes a first isolation unit 3241; the input end of the first isolation unit 3241 is connected to the output end of the first differential amplifier 3221, and the output end of the first isolation unit 3241 is connected to the input end of the first averaging unit 3231. Specifically, when the primary input of the transformer and the secondary output of the transformer do not share a common ground, it is also necessary to set isolation between the primary input of the transformer and the secondary output of the transformer. The first isolation unit 3241 can be set in the second current detection unit 320, so that the current detection corresponding to the secondary output of the transformer is transmitted to the primary input of the transformer through the first isolation unit 3241. The first isolation unit 3241 can be implemented by optical isolation or magnetic coupling according to the application. Typical methods include linear optocouplers and amplitude modulation transformers, or digital optocouplers for transmission after analog-to-digital conversion.

[0070] like Figure 5 As shown, in one embodiment, the second current detection unit 320 includes a second sampling resistor 3212, a second differential amplifier 3222, and a second averaging unit 3232. The first end of the second sampling resistor 3212 is connected to the second end of the secondary output of the transformer and the inverting input of the second differential amplifier 3222. The second end of the second sampling resistor 3212 is connected to the negative voltage output of the power supply output and the non-inverting input of the second differential amplifier 3222. The output of the second differential amplifier 3222 is connected to the input of the second averaging unit 3232, and the output of the second averaging unit 3232 is connected to the first input of the first divider 331. Specifically, the second sampling resistor 3212 can detect the negative output of the secondary winding of the transformer, that is, perform current detection at the negative output of the power supply output. This output current forms a voltage difference across the second sampling resistor 3212, which is amplified by the second differential amplifier 3222 to ultimately obtain a corresponding voltage detection result. The voltage detection result corresponds to the current detection result of the secondary output of the transformer, and the corresponding average current detection result can be obtained by averaging the detection result through the second averaging unit 3232. In this embodiment, the current detection of the secondary output of the transformer is indirectly obtained by output load current detection. This process can control the average value of the rectifier circuit 400 by limiting the long-term average load current. However, since it cannot limit the current of the secondary output of the transformer in real time, it can be applied to the need to control the output current, but the rectifier circuit 400 (corresponding to Figure 5 The design of the rectifier tube D1) does not require high real-time current control.

[0071] In the above embodiment, the resistor Ris is used to detect the current on the secondary side of the transformer. In practical applications, other methods can be used to detect the current on the secondary side of the transformer. Figure 6In one embodiment, the second current detection unit 320 includes a third differential amplifier 3223, a third averaging unit 3233 and a second isolation unit 3242, that is, the same-direction input end of the third differential amplifier 3223 is connected to the input end of the rectifier circuit 400, the reverse input end of the third differential amplifier 3223 is connected to the output end of the rectifier circuit 400, the output end of the third differential amplifier 3223 is connected to the input end of the second isolation unit 3242, the output end of the second isolation unit 3242 is connected to the input end of the third averaging unit 3233, and the output end of the third averaging unit 3233 is connected to the first input end of the first divider. Specifically, when the rectifier circuit 400 is a synchronous rectifier tube S2, it can directly implement current detection using the on-resistance of the synchronous rectifier tube S2. That is, without adding a special detection resistor, the efficiency of the converter is improved. The detection implementation method can refer to Figure 4 Example of .

[0072] like Figure 7 As shown, in one embodiment, the second current detection unit 320 includes a first waveform analysis unit 3251 and a first controller 3261; the first controller 3261 is respectively connected to the output end of the delay unit 333, the output end of the comparison unit 334, the output end of the first waveform analysis unit 3251 and the first input end of the first divider 331; the input end of the first waveform analysis unit 3251 is connected to the second end of the primary coil of the transformer; the first controller 3261 is used to obtain the demagnetization time of the transformer according to the primary peak current control amount output by the delay unit 333, the output cycle of the comparison unit 334 and the input voltage of the first end of the switch unit to obtain the voltage detection value corresponding to the average current output by the secondary side of the transformer. Specifically, in the flyback converter, the drain voltage of the switch unit S1 can be used. Analyze and obtain the transformer demagnetization time The first controller obtains the voltage detection value corresponding to the current output by the secondary side of the transformer according to the acquired parameters and the following formula:

[0073] (9).

[0074] like Figure 8As shown, in one embodiment, the converter further includes an auxiliary winding Na, and the second current detection unit 320 includes a first voltage-dividing resistor 327, a second voltage-dividing resistor 328, a second waveform analysis unit 3252, and a second controller 3262. The second controller 3262 is respectively connected to the output end of the delay unit 333, the output end of the comparison unit 334, the output end of the second waveform analysis unit 3252, and the first input end of the first divider 331. The input end of the second waveform analysis unit 3252 is connected to the first end of the first voltage-dividing resistor 327 and the first end of the second voltage-dividing resistor 328. The second end of the first voltage-dividing resistor 327 is connected to the first end of the auxiliary winding Na, wherein the second end of the auxiliary winding Na and the second voltage-dividing resistor 328 are grounded. The second controller 362 is configured to calculate a voltage detection value corresponding to the current outputted by the secondary side of the transformer based on the controlled voltage value of the primary side peak current outputted by the delay unit 333, the output period of the comparison unit 334, and the demagnetization time of the transformer detected by the auxiliary winding. Specifically, in the isolated flyback converter, the voltage corresponding to the voltage detection result of the auxiliary winding Na can be used. Get transformer demagnetization time The second controller obtains the voltage detection value corresponding to the current output by the secondary side of the transformer according to the acquired parameters and the following formula:

[0075] (9)

[0076] I understand. Figure 7 and Figure 8 In a corresponding embodiment, the secondary current can be indirectly calculated by detecting the demagnetization time of the transformer.

[0077] like Figure 9 As shown, this is the waveform of the transformer working in discontinuous mode. In discontinuous mode, is the demagnetization time of the transformer, which is determined by the waveform analysis unit according to or From the time when the driving voltage DRV is turned off (from high to low) to the time when the output detection voltage corresponding to the second current detection unit is or until the slope changes abruptly. is the switching period of the converter operation, which can be obtained from the cycle time of the drive voltage DRV (from the first rising edge to the second rising edge).

[0078] like Figure 10 As shown in Figure 2, the transformer operates in continuous mode. The initial current is not zero. The secondary current of the transformer The final current is also not zero. In this working mode, the average value of the current output by the secondary side of the converter during the switching cycle can be expressed as

[0079] (10)

[0080] The current and peak current are not in proportional relationship, but the average value of the secondary current can be obtained by detection. , and the known target current , we can still use the methods (6)-(8) to gradually introduce new control quantities based on the control quantities and current feedback values ​​of the previous cycle to converge the secondary current of the transformer to the target value.

[0081] In addition, the present application provides a flyback converter comprising a primary input for inputting a voltage and a secondary output for providing a voltage output. The first end of the transformer primary input is connected to the power supply input, the second end of the transformer primary input is connected to the first end of the switch unit, the first end of the transformer secondary output is connected to the input of the rectifier circuit, the output of the rectifier circuit is connected to the power supply output, and the second end of the transformer secondary output is grounded. The current limiting circuit is connected to the control end of the switch unit. Specifically, during operation, the converter controls the current of the converter primary input through the current limiting circuit. This circuit can limit the secondary current and the average output current without requiring loop compensation, thereby enabling direct response within the switching cycle and rapid secondary current limiting. The control process is simple, requires few control parameters, and can modify the control value for the next cycle based on the secondary current feedback value and the control value of the previous cycle, resulting in high accuracy. This circuit can also achieve relatively accurate current control when the converter is operating in continuous mode.

[0082] It is understandable that the above embodiments only express the preferred implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the patent scope of the present invention. It should be pointed out that for ordinary technicians in this field, without departing from the concept of the present invention, the above technical features can be freely combined, and several modifications and improvements can be made, including the use of digital circuits, which all fall within the scope of protection of the present invention. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should fall within the scope of coverage of the claims of the present invention.

Claims

1. A flyback converter current limiting circuit, characterized in that: The converter includes a power input terminal, a power output terminal, a transformer, a rectifier circuit, and a switch unit, wherein a first terminal of the primary input of the transformer is connected to the power input terminal, a second terminal of the primary input of the transformer is connected to a first terminal of the switch unit, a first terminal of the secondary output of the transformer is connected to an input terminal of the rectifier circuit, an output terminal of the rectifier circuit is connected to the power output terminal, and a second terminal of the secondary output of the transformer is grounded; The current limiting circuit includes: a first current detection unit for obtaining the primary input current of the transformer and a second current detection unit for obtaining the secondary output current of the transformer; a first divider, a second divider, a delay unit, a comparison unit and a drive unit; A first input terminal of the first divider is connected to the second current detection unit, a second input terminal of the first divider is used to input a target current value of the converter, an output terminal of the first divider is connected to the first input terminal of the second divider, a second input terminal of the second divider is connected to the output terminal of the delay unit, and an output terminal of the second divider is connected to the input terminal of the delay unit; The first input end of the comparison unit is connected to the output end of the delay unit, the second input end of the comparison unit is connected to the first current detection unit, the output end of the comparison unit is connected to the input end of the driving unit, and the output end of the driving unit is used to connect to the control end of the switch unit; In which, the second current detection unit is used to obtain the average value of the current output by the secondary side of the transformer within the cycle, and obtain the ratio of the average value within the current cycle to the target current value through the first divider to obtain the current ratio, and obtain the ratio of the current peak value of the primary side input of the transformer to the current ratio through the second divider to obtain the peak current control amount in the next cycle.

2. The flyback converter current limiting circuit according to claim 1, wherein: The first current detection unit includes a detection resistor, a first end of the detection resistor is connected to the second end of the switch unit and the inverting input end of the comparison unit, and a second end of the detection resistor is grounded.

3. The flyback converter current limiting circuit according to claim 1, wherein: The second current detection unit includes a first sampling resistor, a first differential amplifier and a first averaging unit; A first end of the first sampling resistor is connected to the output end of the rectifier circuit and the non-inverting input end of the first differential amplifier. A second end of the first sampling resistor is connected to the positive voltage output end of the power supply output end and the inverting input end of the first differential amplifier. The output end of the first differential amplifier is connected to the input end of the first averaging unit. The output end of the first averaging unit is connected to the first input end of the first divider.

4. The flyback converter current limiting circuit according to claim 3, wherein: The second current detection unit further includes a first isolation unit; An input end of the first isolation unit is connected to an output end of the first differential amplifier, and an output end of the first isolation unit is connected to an input end of the first averaging unit.

5. The flyback converter current limiting circuit according to claim 1, wherein: The second current detection unit includes a second sampling resistor, a second differential amplifier and a second averaging unit; a first end of the second sampling resistor is connected to the second end of the secondary output of the transformer and the inverting input end of the second differential amplifier, a second end of the second sampling resistor is connected to the negative voltage output end of the power supply output end and the non-inverting input end of the second differential amplifier, an output end of the second differential amplifier is connected to the input end of the second averaging unit, and an output end of the second averaging unit is connected to the first input end of the first divider; or The second current detection unit includes a third differential amplifier, a third averaging unit and a second isolation unit. The non-inverting input end of the third differential amplifier is connected to the input end of the rectifier circuit, the inverting input end of the third differential amplifier is connected to the output end of the rectifier circuit, the output end of the third differential amplifier is connected to the input end of the second isolation unit, the output end of the second isolation unit is connected to the input end of the third averaging unit, and the output end of the third averaging unit is connected to the first input end of the first divider.

6. The flyback converter current limiting circuit according to claim 1, wherein: The second current detection unit includes a first waveform analysis unit and a first controller; The first controller is respectively connected to the output end of the delay unit, the output end of the comparison unit, the output end of the first waveform analysis unit and the first input end of the first divider; The input end of the first waveform analysis unit is connected to the second end of the primary input of the transformer; The first controller is used to obtain the demagnetization time of the transformer based on the peak current control amount output by the delay unit, the output period of the comparison unit and the input voltage of the first end of the switching unit to obtain the voltage detection value corresponding to the mean current output by the secondary side of the transformer.

7. The flyback converter current limiting circuit according to claim 6, wherein: The first controller outputs the control parameter according to the following formula : in, is the voltage value corresponding to the primary peak current control amount of the transformer, is the turns ratio between the primary input and secondary output of the transformer, is the demagnetization time of the transformer, is the switching period of the switching unit, wherein the demagnetization time of the transformer The method is obtained according to the input voltage of the first terminal of the switch unit.

8. The flyback converter current limiting circuit according to claim 1, wherein: The converter further includes an auxiliary coil, and the second current detection unit includes a first voltage-dividing resistor, a second voltage-dividing resistor, a second waveform analysis unit and a second controller; The second controller is respectively connected to the output end of the delay unit, the output end of the comparison unit, the output end of the second waveform analysis unit and the first input end of the first divider; An input end of the second waveform analysis unit is connected to a first end of the first voltage-dividing resistor and a first end of the second voltage-dividing resistor, a second end of the first voltage-dividing resistor is connected to a first end of the auxiliary coil, wherein a second end of the auxiliary coil and the second voltage-dividing resistor are grounded; The second controller is used to obtain a voltage detection value corresponding to the mean current output by the secondary side of the transformer based on the peak current control amount output by the delay unit, the output cycle of the comparison unit and the demagnetization time of the transformer detected by the auxiliary coil.

9. The flyback converter current limiting circuit according to claim 8, wherein: The second controller outputs the control parameter according to the following formula : in, is the voltage value corresponding to the primary peak current control amount of the transformer, is the turns ratio between the primary input and secondary output of the transformer, is the demagnetization time of the transformer, is the switching period of the switching unit, wherein the demagnetization time of the transformer The voltage of the auxiliary coil is obtained.

10. A flyback converter, characterized in that: include: A power input terminal, a power output terminal, a transformer, a rectifier circuit, a switch unit, and a current limiting circuit according to any one of claims 1 to 9; The first end of the primary input of the transformer is connected to the power input end, the second end of the primary input of the transformer is connected to the first end of the switch unit, the first end of the secondary output of the transformer is connected to the input end of the rectifier circuit, the output end of the rectifier circuit is connected to the power output end, and the second end of the secondary output of the transformer is grounded. The current limiting circuit is connected to the control end of the switch unit.

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