Secondary-side current calculation circuit, calculation method and flyback converter

Abstract

This application provides a secondary-side current calculation circuit, calculation method, and flyback converter. A secondary-side current calculation circuit is provided on the secondary side of the flyback converter. The output current can be calculated using information such as output voltage, secondary-side freewheeling time, freewheeling duty cycle, transformer turns ratio, and transformer magnetizing inductance. Therefore, the output current information of the secondary side can be obtained without setting a current sampling resistor on the secondary side of the flyback converter. This enables the flyback converter to achieve constant current control function with lower loss and cost in SSR (Secondary Side Regulator) applications.

Description

Technical Field

[0001] This application relates to the field of switching power supply technology, specifically to a secondary current calculation circuit, calculation method, and flyback converter. Background Technology

[0002] Power supplies are an indispensable component of all electronic devices. Their performance directly affects the technical specifications and safe, reliable operation of these devices. Currently, the mainstream application is the switching power supply (SMT). Also known as a switching converter or power converter, a switching power supply utilizes modern power electronics technology to maintain a constant output voltage / current by adjusting the conduction ratio of switching devices. In switching power supply applications, isolated converters are widely used because they protect the load from high-voltage surges and damage to the input bus. They are widely used in telecommunications wireless networks, automotive, and medical equipment. Among various isolated converter topologies, the flyback converter topology, which eliminates the need for an output filter inductor, has a simple circuit structure, provides output isolation, and is low-cost, thus holding a high proportion in end-device applications.

[0003] Figure 1 This diagram illustrates the structure of a flyback converter in the related art. (Refer to...) Figure 1 A typical structure of the flyback converter 100 includes: a transformer TR, a primary power switch Q1 coupled to the primary winding Np of the transformer TR, a rectifier diode D1 coupled to the secondary winding Ns of the transformer TR, a primary control circuit 110, a secondary control circuit 120, and a snubber network 130. The snubber network 130 is connected in parallel across the primary winding Np of the transformer TR. The primary control circuit 110 controls the switching on and off of the primary power switch Q1. The secondary control circuit 120 generates a first error amplification signal (denoted as Vcomp1) characterizing the output voltage Vo, and transmits it to the primary control circuit 110 through the isolation circuit 130, thereby controlling the primary current through the switching on and off of the primary power switch Q1.

[0004] For applications involving voltage feedback and PWM modulation on the secondary side of a flyback converter, i.e., SSR (Secondary Side Regulator) applications, when constant current control of the output needs to be achieved on the secondary side, traditional solutions typically require setting a current sampling resistor (e.g., ...) on the secondary side of the flyback converter. Figure 1The secondary-side output current Io is sampled using a resistor Rcs in the primary circuit. Simultaneously, the secondary-side control circuit 120 generates a second error amplification signal (denoted as Vcomp2) characterizing the output current Io based on the sampling result. This signal is then transmitted to the primary-side control circuit 110 via the isolation circuit 130. Constant current control is achieved by controlling the on and off times of the primary-side power switch Q1. However, this method of setting a current sampling resistor on the secondary side of the flyback converter easily increases losses and costs. Summary of the Invention

[0005] In view of the above-mentioned technical problems, the purpose of this application is to provide a secondary-side current calculation circuit, calculation method and flyback converter. In this application, the output current is calculated on the secondary side of the flyback converter using information such as output voltage and secondary-side freewheeling time. Therefore, the output current information of the secondary side can be obtained without setting a current sampling resistor on the secondary side of the flyback converter. This enables the flyback converter to achieve the function of output constant current control with lower loss and cost in SSR (Secondary Side Regulator) applications.

[0006] According to a first aspect of this application, a secondary current calculation circuit is provided for a flyback converter, the flyback converter including a transformer comprising a primary winding and a secondary winding, a power unit connected to the primary winding, and a rectifier unit connected to the secondary winding.

[0007] The secondary current calculation circuit includes:

[0008] The volt-second integration circuit performs volt-second integration based on the output voltage of the flyback converter and the secondary freewheeling time of the flyback converter, and performs averaging on the volt-second integration result to obtain a first voltage signal. The first voltage signal represents the product information of the output voltage of the flyback converter, the secondary freewheeling time, and the freewheeling duty cycle.

[0009] The arithmetic circuit performs a multiplication operation on the first voltage signal and the first proportional coefficient to obtain a current sampling signal characterizing the output current information on the secondary side of the flyback converter. The first proportional coefficient at least includes information on the quotient of the square of the transformer's turns ratio and the transformer's magnetizing inductance.

[0010] The flyback converter drives the power unit based on the current sampling signal and the reference signal to achieve constant current output.

[0011] Optionally, the volt-second integration circuit includes:

[0012] The voltage-to-current circuit and the integrating capacitor are provided. The voltage-to-current circuit is used to generate a current signal related to the output voltage. The volt-second integrating circuit uses the current signal to charge the integrating capacitor during the secondary side freewheeling time.

[0013] The average value acquisition unit is used to calculate the average value of the voltage on the integrating capacitor in one switching cycle to obtain the first voltage signal.

[0014] Optionally, the volt-second integration circuit determines the secondary freewheeling time by detecting the voltage of the first node, where the first node is the common connection node of the rectifier unit and the secondary winding;

[0015] Within a switching cycle, the moment when the voltage of the first node decreases to less than a preset first reference voltage indicates the start of the secondary side freewheeling time, and the moment when the voltage of the first node increases to more than a preset second reference voltage indicates the end of the secondary side freewheeling time, wherein the first reference voltage is less than the second reference voltage.

[0016] Optionally, the rectifier unit is a synchronous rectifier tube;

[0017] The volt-second integrator circuit uses the turn-on time of the synchronous rectifier tube as the secondary side freewheeling time.

[0018] Optionally, the rectifier unit is a synchronous rectifier tube;

[0019] The current signal is positively correlated with the output voltage.

[0020] Optionally, the rectifier unit is a rectifier diode;

[0021] The current signal is positively correlated with the sum of the output voltage and the forward voltage drop of the rectifier diode.

[0022] Optionally, the secondary current calculation circuit obtains the output voltage information by sampling the output signal of the flyback converter.

[0023] Optionally, the secondary current calculation circuit obtains the output voltage information by sampling the voltage of the first node and averaging the voltage of the first node, where the first node is the common connection node of the rectifier unit and the secondary winding.

[0024] Optionally, the first proportionality coefficient satisfies the following formula:

[0025] Where k represents the first proportionality coefficient, N represents the turns ratio of the primary and secondary windings in the transformer, Lm represents the magnetizing inductance of the transformer, and CVT The value of the integrating capacitor is represented by k, and k0 represents the conversion coefficient of the voltage-to-current circuit when performing voltage-to-current operation.

[0026] According to a second aspect of this application, a flyback converter is provided, comprising:

[0027] A transformer consists of a primary winding and a secondary winding;

[0028] The power unit is connected to the primary winding;

[0029] The rectifier unit is connected to the secondary winding;

[0030] The secondary-side current calculation circuit disclosed in any embodiment of this application is used to calculate the output current on the secondary side of the flyback converter and obtain a current sampling signal characterizing the output current on the secondary side.

[0031] The flyback converter drives the power unit according to the current sampling signal and the reference signal to achieve constant current output.

[0032] According to a third aspect of this application, a method for calculating secondary current is provided for a flyback converter, the flyback converter comprising a transformer including a primary winding and a secondary winding, a power unit connected to the primary winding, and a rectifier unit connected to the secondary winding.

[0033] The method for calculating the secondary current includes:

[0034] A first voltage signal is obtained by performing volt-second integration based on the output voltage of the flyback converter and the secondary freewheeling time of the flyback converter, and averaging the volt-second integration result. The first voltage signal represents the product information of the output voltage of the flyback converter, the secondary freewheeling time, and the freewheeling duty cycle.

[0035] The first voltage signal and the first proportional coefficient are multiplied to obtain a current sampling signal characterizing the output current information on the secondary side of the flyback converter. The first proportional coefficient at least includes the quotient of the square of the transformer's turns ratio and the transformer's magnetizing inductance.

[0036] The flyback converter drives the power unit based on the current sampling signal and the reference signal to achieve constant current output.

[0037] The beneficial effects of this application include at least the following:

[0038] The secondary current calculation circuit, calculation method, and flyback converter provided in this application are configured with a secondary current calculation circuit on the secondary side of the flyback converter. The output current is calculated by using information such as the output voltage of the flyback converter, the secondary freewheeling time, the freewheeling duty cycle, and the quotient of the square of the transformer turns ratio and the transformer's magnetizing inductance. Therefore, the output current information of the secondary side can be obtained without setting a current sampling resistor on the secondary side of the flyback converter. The accuracy is high, enabling the flyback converter to achieve the function of constant output current control with lower losses and costs in SSR (Secondary Side Regulator) applications.

[0039] It should be noted that the above general description and the following detailed description are merely exemplary and explanatory, and do not limit this application. Attached Figure Description

[0040] Figure 1 This diagram illustrates the structure of a flyback converter in the related technology.

[0041] Figure 2 This diagram shows a structural schematic of a flyback converter according to a first embodiment of this application;

[0042] Figure 3 A schematic diagram of the structure of a flyback converter according to a second embodiment of this application is shown;

[0043] Figure 4 Show Figure 2 and Figure 3 A schematic diagram of an embodiment of the secondary current calculation circuit in the circuit;

[0044] Figure 5 Show Figure 2 and Figure 3 A schematic diagram of another embodiment of the secondary current calculation circuit in the diagram;

[0045] Figure 6 A schematic diagram of the timing waveforms of some signals in the flyback converter provided in an embodiment of this application is shown;

[0046] Figure 7 A flowchart illustrating the method for calculating the secondary current of a flyback converter according to an embodiment of this application is shown. Detailed Implementation

[0047] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application may be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0048] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0049] In the description of this application, words such as "exemplary" or "for example" are used to indicate that they are examples, illustrations, or descriptions. Any embodiment described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments. The term "and / or" in this document describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone. "Multiple" refers to two or more. Furthermore, to facilitate a clear description of the technical solutions of the embodiments of this application, the terms "first," "second," etc., are used to distinguish identical or similar items with substantially the same function and effect. Those skilled in the art will understand that the terms "first," "second," etc., do not limit the quantity or execution order, and that "first," "second," etc., do not necessarily imply differences.

[0050] In addition, the same reference numerals in the figures indicate the same or similar structures, so repeated descriptions of them will be omitted. That is, the various parts in this specification are described in a combination of parallel and progressive manner. Each part focuses on the differences from other parts, and the same or similar parts between the various parts can be referred to each other.

[0051] Figure 2 A schematic diagram of the flyback converter provided in the first embodiment of this application is shown. Figure 3 A schematic diagram of the structure of the flyback converter provided in the second embodiment of this application is shown.

[0052] like Figure 2 and Figure 3 As shown, the flyback converter 200 provided in this application includes: a primary winding N P The transformer TR of the secondary winding Ns, the voltage input circuit connected to the primary winding Np, the voltage output circuit connected to the secondary winding Ns, as well as the power unit, the rectifier unit, the primary control circuit 210 and the secondary control circuit 220.

[0053] In some examples, the voltage input circuit includes an input capacitor Ci connected between the input voltage terminal and a reference ground, and a snubber network 230. The input voltage terminal receives the input voltage Vin, and the snubber network 230 is connected in parallel across the primary winding Np to absorb the leakage inductance current of the primary winding Np, thereby improving transformer performance. The snubber network 230 includes, for example, a resistor, a capacitor, and a diode. In some examples, the voltage input circuit also includes at least one of a filter circuit, a rectifier circuit, and a power factor correction circuit connected between the power input port of the flyback converter 200 and the input voltage terminal.

[0054] In some examples, the voltage output circuit includes an output capacitor Co connected to the output voltage terminal, which is used to connect a load that receives electrical energy (e.g., output voltage Vo and output current Io) converted by the flyback converter through the output voltage terminal.

[0055] The power unit is coupled between the primary winding Np and the reference ground, and the power unit includes, for example, a power transistor Q1. In one possible embodiment, the power transistor Q1 is an NMOS field-effect transistor.

[0056] The rectifier unit is connected between the secondary winding Ns and the voltage output circuit. Optionally, the rectifier unit can be implemented as a synchronous rectifier diode Q2, such as an NMOS field-effect transistor. Figure 2 As shown; it can also be implemented as a rectifier diode D1, such as Figure 3 As shown.

[0057] The primary-side control circuit 210 is connected to the control terminal of the power unit (such as the gate of the power transistor Q1) and is used to control the switching state of the power unit. The primary-side control circuit 210 drives the power unit to periodically conduct to realize the conversion of the input voltage Vin, obtain the output voltage Vo and the output current Io, thereby realizing the energy transfer from the primary side to the secondary side of the flyback converter 200.

[0058] The secondary-side control circuit 220 includes a secondary-side current calculation circuit 221. This circuit calculates the output current Io on the secondary side of the flyback converter 200 to obtain a current sampling signal (denoted as Vcs_s) characterizing the secondary-side output current Io. The flyback converter 200 drives the power unit based on the current sampling signal Vcs_s output by the secondary-side current calculation circuit 221 and a reference signal to achieve constant current output.

[0059] Optionally, the secondary-side control circuit 220 can directly transmit the current sampling signal Vcs_s output by the secondary-side current calculation circuit 221 to the primary-side control circuit 210 via an isolation device. The primary-side control circuit 210 then generates a drive signal for the power unit based on the current sampling signal Vcs_s and a reference signal, thereby achieving constant current output. Alternatively, the secondary-side control circuit 220 can first generate an error amplification signal characterizing the output current Io of the flyback converter 200 based on the current sampling signal Vcs_s and a reference signal, and then transmit this error amplification signal to the primary-side control circuit 210 via an isolation device. The primary-side control circuit 210 then generates a drive signal for the power unit based on this error amplification signal, thereby achieving constant current output. Alternatively, the secondary-side control circuit 220 can directly generate a control signal for the power unit on the primary side based on the current sampling signal Vcs_s and a reference signal, and then transmit this control signal to the primary-side control circuit 210 via an isolation device. The primary-side control circuit 210 then drives the power unit based on this control signal, thereby achieving constant current output.

[0060] Alternatively, the isolation device may be any of the following: an isolation transformer (such as a transformer TR or a transformer other than a transformer TR), an optocoupler, an isolation capacitor, and an isolation chip.

[0061] It should be noted that this application Figure 2 and Figure 3 The above description is merely an example of the solution provided in this application using a conventional flyback converter. The solution provided in this application can also be applied to other types of isolated flyback converters, such as active clamp flyback converters.

[0062] Figure 4 It shows Figure 2 and Figure 3 A schematic diagram of an embodiment of the secondary current calculation circuit in [the circuit]. Figure 5 It shows Figure 2 and Figure 3 A schematic diagram of another embodiment of the secondary current calculation circuit in the diagram. Figure 6 A schematic diagram of the timing waveforms of some signals of the flyback converter provided in this application is shown.

[0063] like Figure 4 and Figure 5As shown in the figure, the secondary current calculation circuit 221 in this application specifically includes a volt-second integrator circuit 222 and an arithmetic circuit 223. The volt-second integrator circuit 222 performs volt-second integration based on the output voltage Vo of the flyback converter 200 and the secondary freewheeling time Tsec, and averages the volt-second integration result to obtain a first voltage signal VTavg. The first voltage signal VTavg represents the product information of the output voltage Vo of the flyback converter 200, the secondary freewheeling time Tsec, and the freewheeling duty cycle Ds. The arithmetic circuit 223 performs a multiplication operation on the first voltage signal VTavg and a first proportional coefficient k to obtain a current sampling signal Vcs_s representing the output current Io of the secondary side of the flyback converter 200. The first proportional coefficient k at least includes the quotient information of the square of the turns ratio of the transformer TR and the magnetizing inductance of the transformer TR.

[0064] When the flyback converter 200 operates in BCM mode (critical conduction mode) or DCM mode (discontinuous conduction mode), the peak current (denoted as Is_peak) on the secondary side of the flyback converter 200 satisfies the following formula: (1),

[0065] Where Vo represents the output voltage of the flyback converter 200, Tsec represents the secondary freewheeling time of the flyback converter 200, N represents the turns ratio between the primary winding Np and the secondary winding Ns in the transformer TR, and Lm represents the magnetizing inductance of the transformer TR.

[0066] Based on the peak current Is_peak on the secondary side, the output current Io of the flyback converter 200 satisfies the following formula: (2),

[0067] Where Ds represents the freewheeling duty cycle on the secondary side of the flyback converter 200. When the rectifier unit in the flyback converter 200 is implemented as a synchronous rectifier, such as... Figure 2 As shown, Ds also represents the duty cycle of the drive signal for the synchronous rectifier Q2.

[0068] Combining the above formulas (1) and (2), the output current Io of the flyback converter 200 will satisfy the following formula: (3).

[0069] Assume k1= ,Right now Then the above formula (3) can be transformed into: (4).

[0070] Assume k2= Then the above formula (4) can be transformed into: (5).

[0071] Based on the above principle, this embodiment of the application utilizes a volt-second integrator circuit 222 to integrate the output voltage Vo and the secondary freewheeling time Tsec, and then averages the results to obtain a first voltage signal VTavg representing the product of the output voltage Vo, the secondary freewheeling time Tsec, and the freewheeling duty cycle Ds. The arithmetic circuit 223 then multiplies the first voltage signal VTavg with a first proportional coefficient k. The output signal of the arithmetic circuit 223 (i.e., the current sampling signal Vcs_s) will then represent the output voltage Vo, the secondary freewheeling time Tsec, the freewheeling duty cycle Ds, and the quotient of the square of the turns ratio of the transformer TR and the magnetizing inductance of the transformer TR (i.e.,...). The total product of the two signals, combined with the formula above (4), shows that the current sampling signal Vcs_s output by the operational circuit 223 can characterize the output current Io information on the secondary side of the flyback converter 200, which is equivalent to realizing the calculation of the designed output current Io. In the entire process of acquiring the output current Io information, there is no need to set a corresponding current sampling resistor on the secondary side of the flyback converter 200.

[0072] In specific implementations, in some embodiments, the volt-second integrating circuit 222 further includes: a voltage-to-current circuit 224 and an integrating capacitor C. TV The switching transistor Q3 and the average value acquisition unit 225, wherein the voltage-to-current circuit 224 is used to generate a current signal I1 related to the output voltage Vo based on the output voltage Vo, and the integrating capacitor C TV The switching transistor Q3 is connected in parallel with the integrating capacitor C between the output terminal of the voltage-to-current converter and the reference ground. TV At both ends. During operation, the switching transistor Q3 is turned off during the secondary freewheeling time Tsec. The volt-second integrator circuit 222 uses the current signal I1 to influence the integrating capacitor C during the period when the switching transistor Q3 is turned off (i.e., during the secondary freewheeling time Tsec). TV Charge is performed, and the integrating capacitor C is used for charging. TV The voltage VT is obtained from the input voltage, enabling volt-second integration of the output voltage Vo and the secondary freewheeling time Tsec. After the secondary freewheeling time Tsec ends, the switching transistor Q3 turns on, the volt-second integration circuit 222 stops volt-second integration, and controls the integrating capacitor C. TV The voltage is reset to achieve zeroing of the volt-second integral. The average acquisition unit 225 receives the integration capacitor C. TV The voltage VT is used to obtain the volt-second integral result, and the integrating capacitor C is calculated. TV The first voltage signal VTavg is obtained by averaging the voltage VT on the device over one switching cycle.

[0073] For example, a voltage-controlled current source can be used in the voltage-to-current conversion circuit 224.

[0074] Optionally, when the rectifier unit is implemented as a synchronous rectifier tube Q2, such as Figure 2 As shown, the voltage-to-current circuit 224 is configured to perform voltage-to-current conversion on the output voltage Vo to generate a current signal I1. In this case, the current signal I1 is positively correlated with the output voltage Vo, and can be denoted as I1 = k0 * Vo, where k0 represents the conversion coefficient of the voltage-to-current circuit 224 during the voltage-to-current operation. When the rectifier unit is implemented as a rectifier diode D1, as... Figure 3 As shown, the voltage-to-current circuit 224 is configured to perform voltage-to-current processing on the sum of the output voltage Vo and the forward voltage drop of the rectifier diode D1 (denoted as Vf) to generate a current signal I1. At this time, the current signal I1 is positively correlated with the sum of the output voltage Vo and the forward voltage drop of the rectifier diode D1 Vf (i.e., Vo+Vf), which can be denoted as I1=k0*(Vo+Vf).

[0075] The mean acquisition unit 225 can be implemented, for example, by a filtering unit, such as... Figure 4 and Figure 5 As shown, the mean acquisition unit 225 uses resistor R1 and capacitor C1 to measure the integral capacitor C TV The voltage VT is filtered to obtain the integrating capacitor C. TV The voltage VT on the circuit is the average value over one switching cycle, i.e., the first voltage signal VTavg. Combined with... Figure 6 The waveforms of voltage VT and the first voltage signal VTavg shown in the figure during one switching cycle can be understood from the integrating capacitor C. TV The average value of the voltage VT over one switching cycle, i.e., the first voltage signal VTavg, can be used to characterize the output voltage Vo, the product of the secondary freewheeling time Tsec and 1 / 2 and the freewheeling duty cycle Ds.

[0076] Optionally, the secondary current calculation circuit 221 can obtain the output voltage Vo information by directly sampling the output signal of the flyback converter 200. Alternatively, the secondary current calculation circuit 221 can obtain the output voltage Vo information by sampling the voltage of the first node A (denoted as Vsw) and averaging the voltage Vsw of the first node A, where the first node A is the common connection node of the rectifier unit and the secondary winding Ns.

[0077] Optionally, in some embodiments, the volt-second integrator circuit 222 is configured to determine the secondary freewheeling time Tsec by detecting the voltage Vsw of the first node A, wherein, within one switching cycle, the voltage Vsw of the first node A decreases to less than a preset first reference voltage V.SW_REF1 (For example, the first reference voltage V) SW_REF1 The moment when the voltage is negative indicates the start of the secondary freewheeling time Tswec, when the voltage Vsw at the first node A increases to a level greater than the preset second reference voltage V. SW_REF2 (For example, the second reference voltage V) SW_REF2 The moment when the voltage is positive indicates the end of the secondary freewheeling time Tsec. Wherein, the first reference voltage V... SW_REF1 Less than the second reference voltage V SW_REF2 .

[0078] In specific implementation, such as Figure 4 As shown, in these embodiments, the volt-second integration circuit 222 further includes: comparator 410, comparator 420, and RS flip-flop 430, wherein the positive input terminal of comparator 410 receives a first reference voltage V. SW_REF1 The negative input of comparator 410 receives the voltage Vsw of the first node A, and the output of comparator 410 is connected to the set input S of RS flip-flop 430. The positive input of comparator 420 receives the voltage Vsw of the first node A, and the negative input of comparator 420 receives the second reference voltage V. SW_REF2 The output of comparator 420 is connected to the reset terminal R of RS flip-flop 430, and the inverting output of RS flip-flop 430 is connected to the control terminal of switch Q3. Alternatively, the output of comparator 410 can also be connected to the reset terminal R of RS flip-flop 430, while the output of comparator 420 is connected to the set terminal S of RS flip-flop 430, and the non-inverting output of RS flip-flop 430 is connected to the control terminal of switch Q3. During operation, the output signal of RS flip-flop 430 decreases the voltage Vsw at the first node A to less than the preset first reference voltage V. SW_REF1 Time (e.g.) Figure 6 At time t1, the control switch Q3 is switched to the off state. At this time, the volt-second integrator 222 starts to use the current signal I1 to control the integrating capacitor C. TV Charging begins, i.e., volt-second integration of the output voltage Vo and the secondary freewheeling time Tsec; the output signal of RS flip-flop 430 increases the voltage Vsw at the first node A to be greater than the preset second reference voltage V. SW_REF2 Time (e.g.) Figure 6 At time t3, the control switch Q3 is switched to the on state, and at this time the integrating capacitor C TV Discharge to ground begins via the conducting switch Q3, and the volt-second integration circuit 222 stops integrating the output voltage Vo and the secondary freewheeling time Tsec, thus achieving zero volt-second integration.

[0079] Optionally, in other embodiments, when the rectifier unit is implemented as a synchronous rectifier Q2, the volt-second integration circuit 222 can also be configured to determine the secondary freewheeling time Tsec by detecting the drive signal Vgs2 of the synchronous rectifier Q2. In this case, the volt-second integration circuit 222 uses the turn-on time of the synchronous rectifier Q2 (i.e., the time when the drive signal Vgs2 of the synchronous rectifier Q2 is in an active state, such as the time when it is in a high-level state) as the secondary freewheeling time Tsec.

[0080] In specific implementation, such as Figure 5 As shown, in these embodiments, the volt-second integrating circuit 222 further includes an inverter 510, wherein the input terminal of the inverter 510 receives the drive signal Vgs2 of the synchronous rectifier Q2, and the output terminal of the inverter 510 is connected to the control terminal of the switching transistor Q3. During operation, the output signal of the inverter 510 changes from an invalid state to an active state when the drive signal Vgs2 of the synchronous rectifier Q2 changes from an invalid state (e.g., ...). Figure 6 At time t2, the control switch Q3 is switched to the off state. At this time, the volt-second integrator circuit 222 starts to use the current signal I1 to control the integrating capacitor C. TV Charging begins, i.e., volt-second integration of the output voltage Vo and the secondary freewheeling time Tsec; the output signal of inverter 510 changes from an active state to an inactive state when the drive signal Vgs2 of synchronous rectifier Q2 changes (e.g.) Figure 6 At time t3, the control switch Q3 is switched to the on state, and at this time the integrating capacitor C TV Discharge to ground begins via the conducting switch Q3, and the volt-second integration circuit 222 stops integrating the output voltage Vo and the secondary freewheeling time Tsec, thus achieving zero volt-second integration.

[0081] Figure 6 In the diagram, the time period Ton corresponding to t0-t1 represents the conduction time of the power unit on the primary side of the flyback converter 200, and the time period Tsec corresponding to t1-t3 represents the conduction time of the flyback converter 200 based on... Figure 4 The secondary side freewheeling time determined by the scheme, t2-t3, represents the time period corresponding to the flyback converter 200 based on Figure 5 The secondary-side freewheeling time is determined by the scheme. The time intervals t1-t3 and t2-t3 are very similar, almost equal, and can both be used as parameters for calculating the output current Io, only with slight differences in calculation accuracy. Figure 4 Taking the scheme shown as an example, the integrating capacitor C TV The voltage VT on the secondary side begins to rise at the beginning of the secondary side freewheeling time Tsec, i.e., at time t1, and is quickly cleared to zero at the end of the secondary side freewheeling time Tsec, i.e., at time t3.

[0082] Combination Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6 It can be deduced that the aforementioned k2 also satisfies the following formula: (6),

[0083] Where VTavg represents the voltage value of the first voltage signal, C VT The value of the integrating capacitor is represented by k, and k0 represents the conversion coefficient of the voltage-to-current circuit 224 when performing voltage-to-current operation.

[0084] Combining formulas (5) and (6), the output current Io of the flyback converter 200 will satisfy the following formula: (7).

[0085] Therefore, by setting an appropriate first proportional coefficient k, the current sampling signal Vcs_s, representing the output current Io, can be accurately obtained by the multiplication operation of the first proportional coefficient k and the first voltage signal VTavg by the arithmetic circuit 223, without the need to set a current sampling resistor on the secondary side of the flyback converter. The first proportional coefficient k satisfies the following formula: (8).

[0086] Furthermore, in Figure 2 In the illustrated embodiment, the secondary-side control circuit 220 includes at least: an SW pin, a GT pin, a GND pin, a VT pin, and a VCC pin. The secondary-side control circuit 220 is connected to the first node A via the SW pin to receive the voltage signal Vsw from the first node A. The secondary-side control circuit 220 is connected to the control terminal of the synchronous rectifier Q2 via the GT pin to provide a corresponding drive signal to the synchronous rectifier Q2. The secondary-side control circuit 220 is connected to the reference ground on the secondary side of the flyback converter 200 via the GND pin. The secondary-side control circuit 220 is connected to the external integrating capacitor C via the VT pin. VT Connection, based on integrating capacitor C VT The voltage on the circuit is used to obtain the volt-second integral result; the secondary control circuit 220 is connected to the output of the flyback converter 200 through the VCC pin to obtain the supply voltage.

[0087] exist Figure 3 In the embodiment shown, the pin portion of the secondary control circuit 220 is connected to... Figure 2 The examples shown are basically the same, the difference being: Figure 3 In the middle, the secondary control circuit 220 does not require setting the GT pin.

[0088] Furthermore, this application also provides a method for calculating secondary-side current, which can be used in the flyback converter and its secondary-side current calculation circuit as described in any of the foregoing embodiments. Specifically, as... Figure 7 As shown, the method for calculating the secondary current includes the following steps:

[0089] Step 710: Perform volt-second integration based on the output voltage of the flyback converter and the secondary freewheeling time of the flyback converter, and average the volt-second integration result to obtain a first voltage signal. The first voltage signal represents the product information of the output voltage, secondary freewheeling time and freewheeling duty cycle of the flyback converter.

[0090] Step 720: Perform a multiplication operation on the first voltage signal and the first proportional coefficient to obtain a current sampling signal characterizing the output current information of the secondary side of the flyback converter. The first proportional coefficient includes at least the quotient information of the square of the transformer turns ratio and the excitation inductance of the transformer. The flyback converter drives the power unit based on the current sampling signal and the reference signal to achieve constant current output.

[0091] It should be noted that the specific implementation of each step in the flyback converter secondary current calculation method described above and the corresponding technical effects that can be achieved can be found in the foregoing descriptions of the various embodiments of the flyback converter 200 and its secondary current calculation circuit 221, and will not be repeated here.

[0092] In summary, the secondary current calculation circuit, calculation method, and flyback converter provided in this application include a secondary current calculation circuit on the secondary side of the flyback converter. This circuit calculates the output current using information such as the output voltage of the flyback converter, the secondary freewheeling time, the freewheeling duty cycle, and the quotient of the square of the transformer turns ratio and the transformer's magnetizing inductance. Therefore, it eliminates the need for a current sampling resistor on the secondary side of the flyback converter to obtain the secondary output current information, resulting in high accuracy. This enables the flyback converter to achieve constant current control with lower losses and costs in SSR (Secondary Side Regulator) applications.

[0093] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating this application and are not intended to limit the implementation. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A secondary current calculation circuit for a flyback converter, the flyback converter comprising a transformer including a primary winding and a secondary winding, a power unit connected to the primary winding, and a rectifier unit connected to the secondary winding. The secondary current calculation circuit includes: The volt-second integration circuit performs volt-second integration based on the output voltage of the flyback converter and the secondary freewheeling time of the flyback converter, and performs averaging on the volt-second integration result to obtain a first voltage signal. The first voltage signal represents the product information of the output voltage of the flyback converter, the secondary freewheeling time, and the freewheeling duty cycle. The arithmetic circuit performs a multiplication operation on the first voltage signal and the first proportional coefficient to obtain a current sampling signal characterizing the output current information on the secondary side of the flyback converter. The first proportional coefficient at least includes information on the quotient of the square of the transformer's turns ratio and the transformer's magnetizing inductance. The flyback converter drives the power unit based on the current sampling signal and the reference signal to achieve constant current output.

2. The secondary current calculation circuit according to claim 1, wherein, The volt-second integration circuit includes: The voltage-to-current circuit and the integrating capacitor are provided. The voltage-to-current circuit is used to generate a current signal related to the output voltage. The volt-second integrating circuit uses the current signal to charge the integrating capacitor during the secondary side freewheeling time. The average value acquisition unit is used to calculate the average value of the voltage on the integrating capacitor in one switching cycle to obtain the first voltage signal.

3. The secondary current calculation circuit according to claim 2, wherein, The volt-second integration circuit determines the secondary freewheeling time by detecting the voltage of the first node, where the first node is the common connection node of the rectifier unit and the secondary winding. Within a switching cycle, the moment when the voltage of the first node decreases to less than a preset first reference voltage indicates the start of the secondary side freewheeling time, and the moment when the voltage of the first node increases to more than a preset second reference voltage indicates the end of the secondary side freewheeling time, wherein the first reference voltage is less than the second reference voltage.

4. The secondary current calculation circuit according to claim 2, wherein, The rectifier unit is a synchronous rectifier tube; The volt-second integrator circuit uses the turn-on time of the synchronous rectifier tube as the secondary side freewheeling time.

5. The secondary-side current calculation circuit according to claim 2, wherein, The rectifier unit is a synchronous rectifier tube; The current signal is positively correlated with the output voltage.

6. The secondary-side current calculation circuit according to claim 2, wherein, The rectifier unit is a rectifier diode; The current signal is positively correlated with the sum of the output voltage and the forward voltage drop of the rectifier diode.

7. The secondary-side current calculation circuit according to claim 1, wherein, The secondary current calculation circuit obtains the output voltage information by sampling the output signal of the flyback converter.

8. The secondary current calculation circuit according to claim 1, wherein, The secondary current calculation circuit obtains the output voltage information by sampling the voltage of the first node and averaging the voltage of the first node. The first node is the common connection node of the rectifier unit and the secondary winding.

9. The secondary-side current calculation circuit according to claim 2, wherein, The first proportionality coefficient satisfies the following formula: Where k represents the first proportionality coefficient, N represents the turns ratio of the primary and secondary windings in the transformer, Lm represents the magnetizing inductance of the transformer, and C VT The value of the integrating capacitor is represented by k, and k0 represents the conversion coefficient of the voltage-to-current circuit when performing voltage-to-current operation.

10. A flyback converter, comprising: A transformer consists of a primary winding and a secondary winding; The power unit is connected to the primary winding; The rectifier unit is connected to the secondary winding; The secondary-side current calculation circuit as described in any one of claims 1-9 is used to calculate the output current on the secondary side of the flyback converter and obtain a current sampling signal characterizing the output current on the secondary side. The flyback converter drives the power unit according to the current sampling signal and the reference signal to achieve constant current output.

11. A method for calculating secondary current in a flyback converter, the flyback converter comprising a transformer having a primary winding and a secondary winding, a power unit connected to the primary winding, and a rectifier unit connected to the secondary winding. The method for calculating the secondary current includes: A first voltage signal is obtained by performing volt-second integration based on the output voltage of the flyback converter and the secondary freewheeling time of the flyback converter, and averaging the volt-second integration result. The first voltage signal represents the product information of the output voltage of the flyback converter, the secondary freewheeling time, and the freewheeling duty cycle. The first voltage signal and the first proportional coefficient are multiplied to obtain a current sampling signal characterizing the output current information on the secondary side of the flyback converter. The first proportional coefficient at least includes the quotient of the square of the transformer's turns ratio and the transformer's magnetizing inductance. The flyback converter drives the power unit based on the current sampling signal and the reference signal to achieve constant current output.

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