Maximum current sampling circuit, multi-tube isolating circuit and switching power supply

Abstract

The invention provides a maximum current sampling circuit, a multi-tube isolation circuit and a switching power supply. The maximum current sampling circuit comprises a voltage output end and a plurality of branch sampling modules. The branch sampling module comprises a current transformer, an absorption unit, a first resistor and a first diode; a primary winding of the current transformer is connected in series with a corresponding primary winding in the multi-tube isolation circuit; the dotted terminal of the secondary winding of the current transformer is connected with the first end of the absorption unit; the second end of the absorption unit is connected with the first end of the first resistor and the anode of the first diode; the synonym end of the secondary winding of the current transformer, the third end of the absorption unit and the second end of the first resistor are connected and grounded; the cathode of each first diode is connected with a voltage output end, and the voltage output end is used for outputting a sampling voltage corresponding to the maximum current collected by the plurality of branch sampling modules. According to the invention, the occurrence of circuit overcurrent damage can be reduced.

Description

Technical Field

[0001] The present application relates to the technical field of electronic circuits, and in particular to a maximum current sampling circuit, a multi-transistor isolation circuit, and a switching power supply. Background Art

[0002] Multi-transistor isolation circuits are generally controlled by sampling the sum of the primary currents. When the sampled sum of the primary currents exceeds the set current limit value, the current limiting protection function of the pulse width modulation (PWM) control chip will be triggered to prevent overcurrent damage to the multi-transistor isolation circuit.

[0003] When multiple primary currents are highly consistent, using the sum of the sampled primary currents for control can achieve better current limiting. However, due to the influence of transformer parasitic parameters, multiple primary currents often exhibit uneven current distribution. When using the sum of the primary currents for current limiting control, the sum of the primary currents may not trigger current limiting protection, but may cause circuit damage. Summary of the Invention

[0004] The embodiments of the present application provide a maximum current sampling circuit, a multi-transistor isolation circuit, and a switching power supply to sample the primary current of the multi-transistor isolation circuit and reduce the occurrence of overcurrent damage in the circuit.

[0005] In a first aspect, an embodiment of the present application provides a maximum current sampling circuit, comprising a voltage output terminal and a plurality of branch sampling modules; wherein the number of the branch sampling modules is the same as the number of primary windings in a multi-transistor isolation circuit, and the branch sampling modules are used to collect currents of their corresponding primary windings in the multi-transistor isolation circuit;

[0006] The branch sampling module includes a current transformer, an absorption unit, a first resistor and a first diode;

[0007] The primary winding of the current transformer is connected in series with the corresponding primary winding in the multi-transistor isolation circuit;

[0008] The same-name end of the secondary winding of the current transformer is connected to the first end of the absorption unit; the second end of the absorption unit is connected to the first end of the first resistor and the anode of the first diode; the opposite-name end of the secondary winding of the current transformer, the third end of the absorption unit and the second end of the first resistor are connected and grounded;

[0009] The cathode of each first diode is connected to the voltage output end, and the voltage output end is used to output a sampling voltage corresponding to the maximum current collected by the multiple branch sampling modules.

[0010] In a possible implementation, the absorption unit includes a second resistor, a second diode, and a third diode;

[0011] The first end of the second resistor is connected to the anode of the third diode to serve as the first end of the absorption unit;

[0012] The cathode of the third diode serves as the second end of the absorption unit;

[0013] The second end of the second resistor is connected to the cathode of the second diode; the anode of the second diode serves as the third end of the absorption unit.

[0014] In a possible implementation, the maximum current sampling circuit further includes a total current limiting module and multiple branch current limiting modules; wherein the number of the branch current limiting modules is the same as the number of the branch sampling modules, and one branch current limiting module is provided in one branch sampling module;

[0015] The second end of the absorption unit in each branch sampling module is connected to the first end of the first resistor and the anode of the first diode through the branch current limiting module provided therein;

[0016] The third end of each branch current limiting circuit is connected to the first end of the total current limiting module, the second end of the total current limiting module is connected to the auxiliary power supply, and the third end of the total current limiting module is connected to the voltage output end.

[0017] In a possible implementation, the branch current limiting module includes a third resistor, a fourth resistor, a fifth resistor, and a first switch tube;

[0018] The first end of the third resistor is connected to the first end of the fourth resistor and the second end of the corresponding absorption unit; the second end of the third resistor is connected to the first end of the fifth resistor, the emitter of the first switching tube, the first end of the corresponding first resistor and the anode of the corresponding first diode;

[0019] The second end of the fourth resistor is connected to the second end of the fifth resistor and the base of the first switch tube; the collector of the first switch tube is connected to the first end of the total current limiting module.

[0020] In a possible implementation, the total current limiting module includes a sixth resistor, a seventh resistor, an eighth resistor, and a second switch tube;

[0021] A junction point where the first end of the sixth resistor and the first end of the seventh resistor are connected serves as the first end of the total current limiting module;

[0022] A junction where the second end of the sixth resistor and the emitter of the second switch tube are connected serves as the second end of the total current limiting module;

[0023] The second end of the seventh resistor is connected to the base of the second switching tube;

[0024] The collector of the second switching tube is connected to the voltage output end through the eighth resistor.

[0025] In a possible implementation, the maximum current sampling circuit further includes a first voltage-dividing resistor, a second voltage-dividing resistor, and a first capacitor;

[0026] The cathode of the first diode in each branch sampling module is connected to the first end of the second voltage-dividing resistor, the first end of the first capacitor and the voltage output end through the first voltage-dividing resistor;

[0027] The second end of the second voltage-dividing resistor and the second end of the first capacitor are grounded.

[0028] In a possible implementation, a ratio of the sum of the resistances of the first voltage-dividing resistor and the second voltage-dividing resistor to the resistance of one first resistor is a preset multiple.

[0029] In a possible implementation, the resistance of the first resistor is determined according to a turns ratio of the current transformer, a voltage drop of a diode, and a voltage of the auxiliary power supply.

[0030] In a second aspect, an embodiment of the present application provides a multi-transistor isolation circuit, comprising the maximum current sampling circuit as described in the first aspect or any possible implementation of the first aspect.

[0031] In a third aspect, an embodiment of the present application provides a switching power supply, comprising the multi-transistor isolation circuit as described in the second aspect above.

[0032] Compared with the prior art, the embodiments of the present application have the following beneficial effects:

[0033] The embodiment of the present application sets up multiple branch sampling modules, and each branch sampling module can be used to separately collect the current of the primary winding in the multi-tube isolation circuit; through the absorption unit, the first resistor and the first diode set on the secondary side of the current transformer in the branch sampling module, the current collected by the current transformer can be first converted into a voltage using the first resistor; since the cathodes of all the first diodes are connected to the same contact, the maximum voltage can be screened out through each first diode and output using the voltage output end, so that the sampling voltage corresponding to the maximum current of each primary winding in the multi-tube isolation circuit can be output, so that the subsequent control chip can accurately perform current limiting control on the multi-tube isolation circuit according to the sampling voltage corresponding to the maximum current, thereby avoiding the situation where the current of one primary winding is too large, causing overcurrent damage to the circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0035] Figure 1 Schematic diagram of the module structure of the maximum current sampling circuit provided in an embodiment of the present application;

[0036] Figure 2 This is a schematic structural diagram of a maximum current sampling circuit provided in one embodiment of the present application;

[0037] Figure 3 is a structural diagram of a maximum current sampling circuit provided by another embodiment of the present application;

[0038] Figure 4 is a structural diagram of a maximum current sampling circuit provided in yet another embodiment of the present application;

[0039] Figure 5 1 is a waveform diagram of a collected voltage provided by an embodiment of the present application;

[0040] Figure 6 This is a waveform diagram of a collected voltage provided in another embodiment of the present application. DETAILED DESCRIPTION

[0041] To help those skilled in the art better understand this solution, the following will clearly describe the technical solutions in the embodiments of this solution in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of this solution, not all of it. Based on the embodiments of this solution, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of this solution.

[0042] Throughout the specification, claims, and accompanying figures of this solution, the term "including" and any variations thereof mean "including, but not limited to," and are intended to cover non-exclusive inclusions and are not limited to the examples listed herein. Furthermore, the terms "first" and "second," etc., are used to distinguish between different objects, not to describe a specific order.

[0043] The inventors have discovered that when controlling a multi-transistor isolation circuit by sampling the sum of the primary currents, if the multiple primary currents have good consistency, the current limiting control effect of sampling the sum of the primary currents will also be better. However, due to the influence of transformer parasitic parameters, the multiple primary currents often have uneven current distribution. For example, the current of one primary winding may be too high, while the current of another primary winding may be too low. In this case, if the current of one primary winding is too high, but the total current of the sampled primary windings is insufficient to trigger the current limiting protection, the circuit may be damaged.

[0044] In order to reduce the possibility of overcurrent damage to the circuit, in the embodiment of the present application, a branch sampling module is set for each primary winding, and the first resistor therein is used to convert the collected current of each primary winding into a voltage. Then, the first diode is connected to the common cathode between each branch sampling module, and the largest voltage is screened out as the sampling voltage output, so that the current limiting protection of the multi-tube isolation circuit is controlled by the largest voltage. That is, by collecting the largest current in each primary winding for current limiting protection, the circuit damage caused by excessive current in one primary winding can be avoided, and the current limiting protection of the multi-tube isolation circuit can be triggered in a timely and accurate manner.

[0045] In order to make the purpose, technical solutions and advantages of this application clearer, specific embodiments will be described below with reference to the accompanying drawings.

[0046] Figure 1 The structural diagram of the maximum current sampling circuit provided in the embodiment of the present application is detailed as follows:

[0047] The maximum current sampling circuit provided in the present application includes a voltage output terminal Vcs and multiple branch sampling modules 1; wherein, the number of branch sampling modules 1 is the same as the number of primary windings in the multi-tube isolation circuit, and the branch sampling module 1 is used to collect the current of the corresponding primary winding in the multi-tube isolation circuit.

[0048] Here, the maximum current sampling circuit provided by the present application can be applied to a multi-tube isolation circuit, which can be a multi-tube flyback circuit or a multi-tube forward circuit. For example, when applied to a multi-tube flyback circuit, the maximum current sampling circuit of this embodiment can be used to collect the maximum current of the primary winding in the multi-tube flyback circuit. In this case, the number of branch sampling modules 1 is the same as the number of primary windings in the multi-tube flyback circuit, and the branch sampling modules 1 are used to collect the current of their corresponding primary windings in the multi-tube flyback circuit.

[0049] In this embodiment, there may be n branch sampling modules 1, each of which includes a current transformer Ti, an absorption unit 1i, a first resistor R1i, and a first diode D1i. The value of i may be 1, 2, 3, ..., n. Figure 1As shown, T1, T2, T3 ... Tn represent current transformers, 11, 12, 13 ... 1n represent absorption units, R11, R12, R13 ... R1n represent first resistors, and D11, D12, D13 ... D1n represent first diodes.

[0050] The primary winding of the current transformer Ti is connected in series with the corresponding primary winding in the multi-transistor isolation circuit, and is used to collect the primary current of the corresponding primary winding in the multi-transistor isolation circuit. Figure 1 In FIG. 1 , Ip1, Ip2, Ip3, ..., and Ipn represent the collected primary currents, and Ipi is used for illustration below.

[0051] The same-name end of the secondary winding of the current transformer Ti is connected to the first end of the absorption unit 1i; the second end of the absorption unit 1i is connected to the first end of the first resistor R1i and the anode of the first diode D1i; the opposite-name end of the secondary winding of the current transformer Ti, the third end of the absorption unit 1i and the second end of the first resistor R1i are connected and grounded.

[0052] In this embodiment, the sampled primary current can be converted into a voltage through the first resistor R1i, so that the maximum voltage can be subsequently screened out through each first diode D1i.

[0053] The leakage inductance of the current transformer Ti can cause magnetic flux leakage outside the core, interfering with the normal operation of nearby components. Furthermore, this leakage inductance can cause current reversal, generating negative current. Therefore, to accurately sample the primary current of each primary winding in a multi-transistor isolation circuit, an absorption unit is required to demagnetize and remove negative current, allowing only positive current to flow into subsequent components.

[0054] The cathode of each first diode D1i is connected to the voltage output terminal Vcs, and the voltage output terminal Vcs is used to output the sampling voltage corresponding to the maximum current collected by the multiple branch sampling modules 1.

[0055] In this embodiment, since the cathodes of all first diodes D1i are connected to the same point, i.e., the first diodes D1i serve as common-cathode diodes. The branch sampling module 1 with the highest voltage will conduct, and since the cathode voltages of the common-cathode diodes are the same, the anode voltages of the first diodes D1i in other branch sampling modules 1 with lower voltages are lower, failing to meet the conduction condition. Therefore, the common-cathode diodes can be used to filter out the maximum voltage of each branch sampling module 1 for output.

[0056] Optionally, the resistance of the first resistor R1i can be as large as possible to make the voltage signal across the first resistor R1i as large as possible, so that the voltage drop of the first diode D1i can be ignored, thereby ensuring the accuracy of the sampled voltage.

[0057] The embodiment of the present application sets up multiple branch sampling modules, and each branch sampling module can be used to separately collect the current of the primary winding in the multi-tube isolation circuit; through the absorption unit, the first resistor and the first diode set on the secondary side of the current transformer in the branch sampling module, the current collected by the current transformer can be first converted into a voltage using the first resistor; since the cathodes of all the first diodes are connected to the same contact, the maximum voltage can be screened out through each first diode and output using the voltage output end, so that the sampling voltage corresponding to the maximum current of each primary winding in the multi-tube isolation circuit can be output, so that the subsequent control chip can accurately perform current limiting control on the multi-tube isolation circuit according to the sampling voltage corresponding to the maximum current, thereby avoiding the situation where the current of one primary winding is too large, causing overcurrent damage to the circuit.

[0058] In some embodiments, as Figure 2 As shown, the absorption unit 1i includes a second resistor R2i, a second diode D2i and a third diode D3i, and the value of i can be 1, 2, 3...n. Figure 2 , R21, R22, R23 ... R2n represent second resistors, D21, D22, D23 ... D2n represent second diodes, and D31, D32, D33 ... D3n represent third diodes.

[0059] A first end of the second resistor R2i is connected to an anode of the third diode D3i, serving as a first end of the absorption unit 1i, and is used to be connected to a same-name end of the secondary winding of the current transformer Ti.

[0060] The cathode of the third diode D3i serves as the second end of the absorption unit 1i, and is used to connect the first end of the first resistor R1i and the anode of the first diode D1i.

[0061] The second end of the second resistor R2i is connected to the cathode of the second diode D2i; the anode of the second diode D2i serves as the third end of the absorption unit 1i, which is used to connect to the opposite-name end of the secondary winding of the current transformer Ti.

[0062] In this embodiment, by providing the second resistor R2i and the second diode D2i, demagnetization can be achieved, thereby preventing the leakage magnetic field of the current transformer Ti from interfering with nearby components.

[0063] By providing the third diode D3i, negative current can be removed and the device is only turned on when there is positive current. At the same time, the second resistor R2i and the second diode D2i can also consume negative current to ensure the accuracy of subsequent maximum current sampling.

[0064] In some embodiments, the maximum current sampling circuit may further include a first voltage-dividing resistor Rf1 , a second voltage-dividing resistor Rf2 , and a first capacitor C1 .

[0065] The cathode of the first diode D1i in each branch sampling module 1 is connected to the first end of the second voltage dividing resistor Rf2, the first end of the first capacitor C1 and the voltage output terminal Vcs through the first voltage dividing resistor Rf1; the second end of the second voltage dividing resistor Rf2 and the second end of the first capacitor C1 are grounded.

[0066] Since the maximum current sampling circuit provided in this embodiment can be used to control the multi-tube isolation circuit and trigger the current limiting protection function of its PWM control chip, that is, the voltage output terminal Vcs of the maximum current sampling circuit needs to be connected to the PWM control chip, and the sampling voltage output by the voltage output terminal Vcs is input into the PWM chip. Therefore, the output sampling voltage of the voltage output terminal Vcs needs to meet the requirements of the PWM control chip.

[0067] Based on this, a first voltage-dividing resistor Rf1 and a second voltage-dividing resistor Rf2 may be set to divide the voltage, and input a sampling voltage that meets the voltage range requirement of the PWM control chip.

[0068] In addition, a first capacitor C1 may be provided for filtering to remove high-frequency noise and interference, making the sampled voltage smoother and more stable, thereby ensuring the control effect of the PWM control chip on the multi-tube isolation circuit.

[0069] Optionally, the ratio of the sum of the resistances of the first voltage-dividing resistor Rf1 and the second voltage-dividing resistor Rf2 to the resistance of the first resistor R1i is a preset multiple.

[0070] In this embodiment, the sum of the resistances of the first and second voltage-dividing resistors Rf1 and Rf2 can be one order of magnitude higher than the resistance of the first resistor R1i to ensure sufficient sampling accuracy. For example, the ratio of the sum of the resistances of the first and second voltage-dividing resistors Rf1 and Rf2 to the resistance of the first resistor R1i can be in the range of 8 to 50.

[0071] In a specific embodiment, the resistance of the first voltage-dividing resistor Rf1 can be 50Ω, the resistance of the second voltage-dividing resistor Rf2 can be 2400Ω, and the resistance of a first resistor R1i can be 100Ω, that is, the ratio of the sum of the resistances of the first voltage-dividing resistor Rf1 and the second voltage-dividing resistor Rf2 to the resistance of a first resistor R1i is 29.

[0072] The above mainly introduces the maximum current sampling circuit in detail. The inventors have considered that the current limiting protection of the multi-tube isolation circuit requires a certain action time. In order to perform current limiting protection in a timely and effective manner, the PWM control chip can start the current limiting protection in advance. This is described in detail below.

[0073] In some embodiments, as Figure 3As shown, the maximum current sampling circuit may further include a total current limiting module 3 and multiple branch current limiting modules 4; wherein, the number of branch current limiting modules 4 is the same as the number of branch sampling modules 1, and one branch current limiting module 4 is provided in one branch sampling module 1.

[0074] The second end of the absorption unit 1i in each branch sampling module 1 is connected to the first end of the first resistor R1i and the anode of the first diode D1i through the branch current limiting module 4 provided therein; the third end of each branch current limiting circuit 4 is connected to the first end of the total current limiting module 3, the second end of the total current limiting module 3 is connected to the auxiliary power supply Vaux, and the third end of the total current limiting module 3 is connected to the voltage output end Vcs.

[0075] In this embodiment, a branch current limiting module 4 is further provided in a branch sampling module 1 between the absorption unit 1i, the first resistor R1i, and the first diode D1i. All branch current limiting modules 4 are also connected to the total current limiting module 3. When the primary current Ipi reaches a preset current value, the branch current limiting circuit 4 corresponding to the primary current Ipi and the total current limiting module 3 are turned on. The auxiliary power supply Vaux is connected to the circuit, which increases the sampling voltage output by the voltage output terminal Vcs. This triggers the PWM control chip to initiate current limiting protection before the primary current Ipi reaches the actual current limit value, allowing sufficient time for the current limiting protection to operate.

[0076] Optionally, the branch current limiting module 4 of this embodiment includes a third resistor R3i, a fourth resistor R4i, a fifth resistor R5i and a first switch tube Q1i. The value of i can be 1, 2, 3...n. Figure 3 , R31, R32, R33 ... R3n represent a third resistor, R41, R42, R43 ... R4n represent a fourth resistor, R51, R52, R53 ... R5n represent a third resistor, and Q11, Q12, Q13 ... Q1n represent a first transistor.

[0077] The first end of the third resistor R3i is connected to the first end of the fourth resistor R4i and the second end of its corresponding absorption unit 1i; the second end of the third resistor R3i is connected to the first end of the fifth resistor R5i, the emitter of the first switch tube Q1i, the first end of its corresponding first resistor R1i and the anode of its corresponding first diode D1i; the second end of the fourth resistor R4i is connected to the second end of the fifth resistor R5i and the base of the first switch tube Q1i; the collector of the first switch tube Q1i is connected to the first end of the total current limiting module 3.

[0078] In this embodiment, the first switch tube Q1i may be an NPN transistor, and the voltage across the third resistor R3i may be used to control whether the branch current limiting module 4 and the total current limiting module 3 are turned on or off.

[0079] When the load is light, the primary current Ipi is small, the voltage across the third resistor R3i is small, the first switch tube Q1i is not conducting, and accordingly, the branch current limiting module 4 and the total current limiting module 3 do not work. The voltage output end can output the voltage corresponding to the maximum current in the collected primary current Ipi.

[0080] When one of the primary currents Ipi reaches a preset current value, the voltage across the third resistor R3i through which the current passes can turn on the corresponding first switch tube Q1i, and further turn on the total current limiting module 3. The auxiliary power supply is connected to the circuit, thereby increasing the sampling voltage output by the voltage output terminal Vcs, triggering the current limiting protection of the subsequent PWM control chip.

[0081] Optionally, the total current limiting module in this embodiment includes a sixth resistor R6, a seventh resistor R7, an eighth resistor R8 and a second switch tube Q2.

[0082] The junction where the first end of the sixth resistor R6 and the first end of the seventh resistor R7 are connected serves as the first end of the total current limiting module 3; the junction where the second end of the sixth resistor R6 and the emitter of the second switch tube Q2 are connected serves as the second end of the total current limiting module 3; the second end of the seventh resistor R7 is connected to the base of the second switch tube Q2; and the collector of the second switch tube Q2 is connected to the voltage output terminal Vcs through the eighth resistor R8.

[0083] In this embodiment, the junction where the first end of the sixth resistor R6 and the first end of the seventh resistor R7 are connected is connected to the collector of the first switch Q1i in each branch current limiting module 4. The junction where the second end of the sixth resistor R6 and the emitter of the second switch Q2 are connected is connected to the auxiliary power supply Vaux.

[0084] When the second switch tube Q2 is turned on, the auxiliary power supply Vaux is connected to the circuit for power supply, and can be connected to the voltage output terminal Vcs through the second switch tube Q2 and the eighth resistor R8, thereby increasing the output sampling voltage.

[0085] Optionally, the resistance of the first resistor R1i is determined according to the turns ratio of the current transformer, the voltage drop of the diode, and the voltage of the auxiliary power supply Vaux.

[0086] In this embodiment, in order to ensure that the total current limiting module 3 and the branch current limiting module 4 can be turned on when the primary current Ipi reaches a preset current value, it is necessary to ensure that the resistance of the first resistor R1i is not too large, so that after the maximum current flows through the first resistor R1i, the voltage across the first resistor R1i will not exceed the voltage of the auxiliary power supply Vaux.

[0087] At the same time, since the first diode D1i needs to be used to screen out the maximum voltage and the third diode D3i needs to be used to remove the negative current, the resistance value of the first resistor R1i also needs to be as large as possible to ensure that the diode can be turned on.

[0088] Furthermore, in the maximum current sampling circuit, the primary current is collected using current transformer Ti. The magnitude of the secondary current of current transformer Ti is related not only to the magnitude of the primary current but also to the turns ratio of current transformer Ti. For a given primary current, a larger turns ratio of current transformer Ti results in a smaller secondary current of current transformer Ti, which in turn affects the voltage across first resistor R1i.

[0089] Therefore, when selecting the resistance value of the first resistor R1i, it is necessary to consider the turns ratio of the current transformer, the voltage drop of the diode, and the voltage of the auxiliary power supply Vaux.

[0090] For example, if the maximum primary current is 2A, the voltage of the auxiliary power supply Vaux is 5V, the voltage drop across the diode is 0.7V, and the turns ratio of the current transformer Ti is 50:1, then the secondary current of the current transformer Ti is 0.04A. To ensure that the voltage across the first resistor R1i is as large as possible, ignoring the diode voltage drop, and less than the voltage of the auxiliary power supply Vaux, the voltage across the first resistor R1i can be selected to be 4V at the maximum primary current. Accordingly, the resistance of the first resistor R1i can be 100Ω. It should be noted that the above values are merely examples of the process for selecting the resistance value of the first resistor R1i and are not intended to be limiting.

[0091] In some embodiments, a multi-transistor isolation circuit having four primary windings is used as an example for explanation. Figure 4 As shown, the corresponding maximum current sampling circuit also has four branch sampling modules 1 for maximum current sampling. For example, the amplitudes of the primary currents Ip1, Ip2, Ip3, and Ip4 of the four primary windings in the multi-transistor isolation circuit are 1A, 2A, 3A, and 5A, respectively. These currents are sampled by current transformers and then converted into voltages Vp1, Vp2, Vp3, and Vp4 via first resistors R11, R12, R13, and R14. The maximum voltage Vsap is filtered out by four cathode-connected first diodes D11, D12, D13, and D14.

[0092] like Figure 5 As shown, the amplitudes of voltages Vp1, Vp2, Vp3, and Vp4 are approximately 2V, 4V, 6V, and 7.7V, respectively, corresponding to the primary currents Ip1, Ip2, Ip3, and Ip4. The amplitude of the maximum voltage Vsap screened out is 7V, and the difference between it and the maximum voltage Vp4 is the forward voltage drop of a diode. It can be seen that the maximum current sampling circuit provided in this embodiment samples the voltage corresponding to the maximum primary current.

[0093] On this basis, the total current limiting module 3 and the four branch current limiting modules 4 are added, and the maximum current sampling is performed. The results are as follows Figure 6 As shown, when the current of the primary current Ip4 becomes larger, that is, the voltage VR34 across the third resistor R34 in the fourth branch current limiting module 4 is higher than a certain value, the voltage Vcs is directly pulled up and is no longer a normal sampling triangle waveform, which can achieve the current limiting protection of the PWM control chip triggered in advance.

[0094] The embodiment of the present application sets up multiple branch sampling modules, and each branch sampling module can be used to separately collect the current of the primary winding in the multi-tube isolation circuit; through the absorption unit, the first resistor and the first diode set on the secondary side of the current transformer in the branch sampling module, the current collected by the current transformer can be first converted into a voltage using the first resistor; because the cathodes of all the first diodes are connected to the same contact, the maximum voltage can be screened out by each first diode and output using the voltage output terminal, thereby outputting the sampling voltage corresponding to the maximum current of each primary winding in the multi-tube isolation circuit, so that the subsequent control chip can accurately control the current limiting of the multi-tube isolation circuit according to the sampling voltage corresponding to the maximum current, avoiding the situation where the current of one primary winding is too large and causing overcurrent damage to the circuit. By setting up a total current limiting module and setting a branch current limiting module in each branch sampling module, the sampling voltage can be raised when the primary current reaches a preset current value, triggering the current limiting protection of the subsequent PWM control chip, so that the subsequent current limiting protection has sufficient action time.

[0095] The present application also provides a multi-transistor isolation circuit and a switching power supply. For details not described in detail, reference can be made to the corresponding circuit embodiments described above.

[0096] In some embodiments, the multi-transistor isolation circuit may include the maximum current sampling circuit in any of the above embodiments.

[0097] In this embodiment, the primary winding of the current transformer Ti in the maximum current sampling circuit can be connected in series with the primary winding of the multi-transistor isolation circuit to sample the primary current. The voltage output terminal Vcs of the maximum current sampling circuit is connected to the PWM control chip of the multi-transistor isolation circuit to input the sampled voltage corresponding to the sampled maximum current into the PWM control chip, which then controls the PWM control chip based on the sampled voltage.

[0098] Optionally, the multi-transistor isolation circuit may be a multi-transistor flyback circuit or a multi-transistor forward circuit.

[0099] In some embodiments, the switching power supply may include the multi-transistor isolation circuit as described above.

[0100] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant description of other embodiments.

[0101] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A maximum current sampling circuit, characterized in that: It includes a voltage output terminal and multiple branch sampling modules; wherein the number of the branch sampling modules is the same as the number of primary windings in the multi-transistor isolation circuit to be sampled, and the branch sampling modules are used to collect the current of the corresponding primary windings in the multi-transistor isolation circuit; The branch sampling module includes a current transformer, an absorption unit, a first resistor and a first diode; The primary winding of the current transformer is connected in series with the corresponding primary winding in the multi-transistor isolation circuit; The same-name end of the secondary winding of the current transformer is connected to the first end of the absorption unit; the second end of the absorption unit is connected to the first end of the first resistor and the anode of the first diode; the opposite-name end of the secondary winding of the current transformer, the third end of the absorption unit and the second end of the first resistor are connected and grounded; The cathode of each first diode is connected to the voltage output end, and the voltage output end is used to output a sampling voltage corresponding to the maximum current collected by the multiple branch sampling modules.

2. The maximum current sampling circuit according to claim 1, characterized in that: The absorption unit includes a second resistor, a second diode and a third diode; The first end of the second resistor is connected to the anode of the third diode to serve as the first end of the absorption unit; The cathode of the third diode serves as the second end of the absorption unit; The second end of the second resistor is connected to the cathode of the second diode; the anode of the second diode serves as the third end of the absorption unit.

3. The maximum current sampling circuit according to claim 1, characterized in that: The maximum current sampling circuit further includes a total current limiting module and a plurality of branch current limiting modules; wherein the number of the branch current limiting modules is the same as the number of the branch sampling modules, and one branch current limiting module is provided in one branch sampling module; The second end of the absorption unit in each branch sampling module is connected to the first end of the first resistor and the anode of the first diode through the branch current limiting module provided therein; The third end of each branch current limiting circuit is connected to the first end of the total current limiting module, the second end of the total current limiting module is connected to the auxiliary power supply, and the third end of the total current limiting module is connected to the voltage output end.

4. The maximum current sampling circuit according to claim 3, characterized in that: The branch current limiting module includes a third resistor, a fourth resistor, a fifth resistor and a first switch tube; The first end of the third resistor is connected to the first end of the fourth resistor and the second end of the corresponding absorption unit; the second end of the third resistor is connected to the first end of the fifth resistor, the emitter of the first switching tube, the first end of the corresponding first resistor and the anode of the corresponding first diode; The second end of the fourth resistor is connected to the second end of the fifth resistor and the base of the first switch tube; the collector of the first switch tube is connected to the first end of the total current limiting module.

5. The maximum current sampling circuit according to claim 3, characterized in that: The total current limiting module includes a sixth resistor, a seventh resistor, an eighth resistor and a second switch tube; A junction point where the first end of the sixth resistor and the first end of the seventh resistor are connected serves as the first end of the total current limiting module; A junction point where the second end of the sixth resistor and the emitter of the second switch tube are connected serves as the second end of the total current limiting module; The second end of the seventh resistor is connected to the base of the second switching tube; The collector of the second switching tube is connected to the voltage output end through the eighth resistor.

6. The maximum current sampling circuit according to any one of claims 1 to 5, characterized in that: The maximum current sampling circuit further includes a first voltage-dividing resistor, a second voltage-dividing resistor and a first capacitor; The cathode of the first diode in each branch sampling module is connected to the first end of the second voltage-dividing resistor, the first end of the first capacitor and the voltage output end through the first voltage-dividing resistor; The second end of the second voltage-dividing resistor and the second end of the first capacitor are grounded.

7. The maximum current sampling circuit according to claim 6, characterized in that: The ratio of the sum of the resistances of the first voltage-dividing resistor and the second voltage-dividing resistor to the resistance of one first resistor is a preset multiple.

8. The maximum current sampling circuit according to claim 3, characterized in that: The resistance of the first resistor is determined according to the turns ratio of the current transformer, the voltage drop of the diode and the voltage of the auxiliary power supply.

9. A multi-tube isolation circuit, characterized in that: The maximum current sampling circuit comprises the maximum current sampling circuit according to any one of claims 1 to 8.

10. A switching power supply, characterized in that: The multi-tube isolation circuit according to claim 9 is included.

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