Design method of flyback transformer based on boost time requirement

Abstract

The invention provides a method for designing a flyback transformer according to an energy transfer rule between an input end and an output end of a flyback converter. The method aims at a special power supply system which takes flyback topology as a main power circuit and charges an output capacitor, and transformer parameters are designed according to the following steps. Calculating energy required for charging the output capacitor to a preset value; calculating the relationship between the time required for completing the energy transmission by the circuit topology and the inductance value of the transformer; calculating the number of turns of a primary coil of the transformer; calculating the diameters of primary and secondary coils of the transformer; and rechecking the parameter design of the transformer through an AP method. The test result of the example shows that the method for designing the parameters required by the flyback transformer is feasible.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and more specifically to the field of transformer parameter design of a flyback special power supply based on high pulse discharge. Technical Background

[0002] High pulse discharge systems are now widely used in high-voltage ignition and electromagnetic weapons. For example, in the field of electromagnetic weapons, the high-voltage capacitor in the high pulse discharge system is used to discharge instantly to the solenoid or metal rail to generate a strong magnetic field, which drives the launch of the missile or the take-off of the carrier-based aircraft; in the field of high-voltage ignition, the high-voltage capacitor in the high pulse discharge system is used to discharge instantly to the discharge tube to generate high-speed flying pieces to cause electric sparks, which then trigger the detonation of explosives or the ignition of fuel.

[0003] The high pulse discharge system is a circuit system that releases the electric energy stored in the energy storage element to the load in a single pulse or a short pulse of a certain frequency. The system is generally composed of four parts, namely the primary power supply, the charging power supply, the energy storage element and the energy release network. The function of the primary power supply is to use a voltage regulator to provide the system with an initial voltage; the function of the charging power supply is to convert the low DC voltage into a DC high voltage and then store it in the energy storage element. Because of its special function, it is also called a special power supply; the common energy storage elements are high-voltage capacitors and high-voltage inductors. Among them, the use of high-voltage capacitors for energy storage is more researched and more widely used. The energy release network is generally played by a load connected to an electronic switch. The work of the high pulse discharge system is divided into two stages: energy storage and energy release. In the energy storage stage, the high-voltage special power supply is first used to charge the high-voltage capacitor to the rated voltage in a relatively short time. In the energy release stage, the stored energy is released in a very short time. Then enter the next charging and discharging process.

[0004] In high pulse discharge systems, the power converter must be able to boost the initial voltage of tens of volts to thousands of volts or even more than ten kilovolts. As the core of the entire system, it is directly related to whether the system can operate efficiently, normally and stably. In different occasions, such as high-voltage ignition, laser power supply, vacuum coating, electrostatic dust removal and electromagnetic weapons, there are different and strict requirements for the boost time of the power supply output voltage, and the boost time is closely related to the parameters of the power supply transformer, output capacitor and other power components. In this context, it is of great significance to explore the mathematical relationship between the boost time and the power supply transformer, output capacitor and other power components, and to use this as a guide to design a transformer that can achieve the boost time required by the project.

[0005] Taking the flyback special power supply based on high pulse discharge as an example, the existing methods for designing flyback transformers are mostly to calculate the various parameters of the transformer according to the input voltage, output voltage, switching frequency and power of the flyback switching power supply, and then make the flyback transformer. For example, in 2021, Gu Weikang proposed a simple and practical design method (Gu Weikang. Design of single-ended flyback switching power supply transformer [J]. Communication Power Technology, 2021, 38(01): 59-62). This method unifies the calculation formulas for the transformer working in current continuous mode and discontinuous mode, effectively solves the design of parameters such as primary inductance value, number of coil turns, wire diameter, core size, etc., reduces the design difficulty and improves the design efficiency. However, the flyback transformer parameters calculated by this method and the flyback transformer made are used in the flyback special power supply, but the boost time of its output voltage is very different from the boost time required by the project engineering.

[0006] Purpose of the Invention

[0007] In view of the above problems, the present invention proposes a method for designing a flyback transformer according to the energy transfer law between the input terminal and the output terminal of the flyback converter. Summary of the invention

[0008] For a special power supply system that uses a flyback topology as the main power circuit to charge the output capacitor, the transformer parameters are designed according to the following steps. Among them, the known conditions given by the engineering requirements include the input voltage U in , output voltage V out , the capacitance C of the output capacitor and the boost time t. The design steps of the present invention are as follows:

[0009] Step 1: Calculate the energy required to charge the output capacitor to a predetermined value

[0010] According to the requirements, the output capacitance of the flyback special power supply is known to be C, and the output voltage is predetermined to be V out The energy required to charge the output capacitor to the predetermined value is W c for:

[0011]

[0012] The power P required to charge the output capacitor to the predetermined value is out for:

[0013]

[0014] Step 2: Calculate the relationship between the time required for the circuit topology to complete the energy transfer and the transformer inductance

[0015] Set the switching frequency of the switch tube in the flyback special power supply to f, the duty cycle to D, and the conduction time to t onThese three parameters are customized according to the needs in power supply design, among which t on =D / f. When the switch is working and in the conduction stage, the primary coil of the transformer begins to charge, and the current of the primary coil of the transformer rises linearly from zero. If the inductance of the primary coil of the transformer is set to L1, then the inductance voltage u of the primary coil of the transformer is L1 for:

[0016]

[0017] The transformer primary current i1 is

[0018] i1=K·t on (4) Where K is the rising rate of the primary inductance current of the transformer, which is approximately:

[0019]

[0020] The inductance of the primary coil of the transformer is on The instantaneous power P during L1 for:

[0021] P L1 =u L1 ·i1 (6)

[0022] The inductance of the primary coil of the transformer is on The energy stored during the period W L for:

[0023]

[0024] According to equations (5) and (7), the energy W stored in the transformer in one switching cycle can be obtained as L for:

[0025]

[0026] After n cycles, the capacitor is fully charged. Assuming the capacitor is fully charged, the total power transmitted by the primary side of the transformer is P in ,but:

[0027]

[0028] Assume the transformer transmission efficiency is η and the transformer output power is P out ,but:

[0029] P out =P in ·η (10)

[0030] The time required for the output capacitor to charge to a predetermined value is:

[0031]

[0032] According to the above equations (9), (10) and (11), the mathematical relationship between the boost time t and the transformer primary coil inductance L1 can be obtained as follows:

[0033]

[0034] Therefore, according to formula (12), the inductance of the primary coil of the transformer can be obtained by the boost time and other known conditions. And by substituting the result of formula (12) into formula (9), the total power P transmitted by the primary side of the transformer can be obtained. in size.

[0035] Step 3: Calculate the number of turns of the transformer primary coil

[0036] Select a suitable transformer core. Among the transformer core parameters, the equivalent inductance when the core has no air gap is A. L Therefore, the number of turns N1 of the transformer primary coil is:

[0037]

[0038] In order to prevent the transformer from magnetic saturation during operation, the number of N1 windings can be appropriately increased and the air gap can be opened to keep the inductance of the primary coil of the transformer at L1.

[0039] Step 4: Calculate the number of turns of the transformer secondary coil

[0040] When the flyback special power supply is working, the withstand voltage of the switch tube is V ds , the reflected voltage of the switch tube V or for:

[0041]

[0042] Transformer primary and secondary coil ratio for:

[0043]

[0044] Therefore, the number of turns N2 of the transformer secondary coil should be:

[0045]

[0046] Step 5: Calculate the wire diameter of the transformer primary and secondary coils

[0047] The maximum duty cycle is D max , the average current of the primary switch tube I ave for:

[0048]

[0049] The primary switch current pulsation ΔI is:

[0050]

[0051] Primary switch peak current I P for:

[0052]

[0053] Primary current RMS I RMS for:

[0054]

[0055] Where K RP is the current ripple factor.

[0056] Minimum primary coil diameter d min for:

[0057]

[0058] Where ρ is the current density.

[0059] Secondary current peak I s for:

[0060]

[0061] Secondary current RMS value I s-RMS for:

[0062]

[0063] Minimum secondary coil diameter d s-min for:

[0064]

[0065] Step 6: Review transformer parameter design using AP method

[0066] Calculate A by design parameters p ′ value and core A p Value comparison, where A p 'for:

[0067]

[0068] Among them, B m is the transformer flux density, K o is the transformer window filling factor, K c is the transformer core filling factor. p ' p The transformer parameters are designed reasonably. ​

[0069] After the above calculation, the parameters required for making the transformer can be obtained: Transformer output power P out , the number of turns of the primary coil N1, the minimum wire diameter d required for the primary coil min , the inductance of the primary coil is L1, the number of turns of the secondary coil is N2, and the minimum wire diameter required for the secondary coil is d s-min .

[0070] Effects of the Invention

[0071] The implementation of the present invention is verified by the following examples. A flyback transformer is manufactured according to the calculated transformer parameters, and the manufactured flyback transformer is connected to other components of the flyback special power supply circuit. The circuit schematic diagram is shown in FIG. Figure 1 shown.

[0072] After the circuit connection is completed, the output voltage and boost time of the flyback special power supply are tested at the output end through a high-voltage differential probe and an oscilloscope. After the system is powered on, the flyback transformer and power supply circuit work normally and stably. The output waveform is observed and the test results are recorded. The test results are as follows Figure 2 shown.

[0073] The test results show that the flyback special power supply circuit system achieves an output voltage of 1.97KV, with a boost time of 147ms and an error within 5%. The test results prove that it is feasible to calculate the mathematical relationship between the boost time and the inductance of the primary coil of the transformer based on the energy transfer between the input and output ends of the converter, and then calculate the parameters required for designing the flyback transformer.

[0074] Figures and their descriptions

[0075] Figure 1 It is the schematic diagram of the flyback special power supply circuit.

[0076] Figure 2 It is the test result of the boost time of the flyback special power supply circuit.

[0077] Figure 3 It is a schematic diagram of the overall process of a design method of a flyback transformer based on boost time requirements proposed by the present invention. Specific embodiments

[0078] In this embodiment, the known conditions given by the engineering requirements include: a special power supply circuit with a flyback topology as the main power circuit charges the output capacitor, and the power input voltage U in is 20V, the output voltage V outThe output voltage is 2KV, the capacitance C of the output capacitor is 0.4μF / 2KV, and the boost time t is controlled at about 150mS. According to the above transformer parameter design method, a transformer with PC40EE16 as the magnetic core is designed. The specific process of this embodiment is as follows:

[0079] Step 1: Calculate the energy required to charge the output capacitor to a predetermined value

[0080] The output capacitor value of the flyback special power supply is C = 0.4μF / 2KV, and the output voltage is V out =2KV. Then the energy required to charge the output capacitor to the predetermined value is W c for:

[0081]

[0082] The power P required to charge the output capacitor to the predetermined value is out for:

[0083]

[0084] Step 2: Calculate the relationship between the time required for the circuit topology to complete the energy transfer and the transformer inductance

[0085] In the flyback special power supply, the switching frequency of the switch tube is set to f = 100KHz, the duty cycle D is equal to 0.75, and the conduction time is t on =7.5μS. When the switch is working and in the on stage, the primary coil of the transformer starts to charge, and the current of the primary coil of the transformer rises linearly from zero. The inductance of the primary coil of the transformer is set to L1, then the inductance voltage of the primary coil of the transformer is u L1 for:

[0086]

[0087] The primary current i1 of the transformer is

[0088] i1=K·t on =7.5K (29)

[0089] Where K is the current rise rate of the primary inductor of the transformer, which is approximately:

[0090]

[0091] The inductance of the primary coil of the transformer is on The instantaneous power P during L1 for:

[0092] P L1 =u L1 ·i1 (31)

[0093] The inductance of the primary coil of the transformer is on The energy stored during the period W L for:

[0094]

[0095] According to equations (29) and (32), the energy W stored in the transformer in one switching cycle can be obtained as L Approximately:

[0096]

[0097] After n cycles, the capacitor is fully charged. Assume that the total power transmitted by the primary side of the transformer is P in ,but:

[0098]

[0099] Assume the transformer transmission efficiency is η equal to 90%, and the transformer output power is P out ,but

[0100] P out =P in ·η=5.3W (35)

[0101] The time required for the output capacitor to charge to a predetermined value is:

[0102]

[0103] The conditions given by the engineering indicators are known input voltage U in =20V, output voltage V out =2KV, boost time t=150mS. According to the above equations (34), (35) and (36), the transformer primary coil inductance L1 can be obtained as:

[0104]

[0105] At the same time, according to the result obtained from formula (37) and substituted into formula (34), the total power P transmitted by the primary side of the transformer can be obtained: in for:

[0106]

[0107] Step 3: Calculate the number of turns of the transformer primary coil

[0108] The transformer core selected in this design is PC40EE16. Among the transformer core parameters, the equivalent inductance when the core has no air gap is A L =1140nH / N 2 Therefore, the number of turns N1 of the transformer primary coil is:

[0109]

[0110] In order to prevent the transformer from magnetic saturation during operation, the number of N1 windings is appropriately increased to 40 turns, and an air gap is opened to keep the inductance of the primary coil of the transformer at 190 μH.

[0111] Step 4: Calculate the number of turns of the transformer secondary coil

[0112] The switch tube of the flyback special power supply uses a MOS tube, and its withstand voltage value is V ds =150V, the reflected voltage of MOS tube V or for:

[0113]

[0114] Transformer primary and secondary coil ratio for:

[0115]

[0116] Therefore, the number of turns N2 of the transformer secondary coil should be:

[0117]

[0118] Step 5: Calculate the wire diameter of the transformer primary and secondary coils

[0119] Maximum duty cycle D max =0.75, combined with formula (38), we can get P in =5.9W, the average current of the primary switch tube is I ave for:

[0120]

[0121] The primary switch current pulsation ΔI is:

[0122]

[0123] Primary switch peak current I P for:

[0124]

[0125] Primary current RMS I RMS for:

[0126]

[0127] Where K RP is the current ripple coefficient, which takes a value of 1.

[0128] Minimum primary coil diameter d min for:

[0129]

[0130] Where ρ is the current density, which is

[0131] Secondary current peak I s for:

[0132]

[0133] Secondary current RMS value I s-RMS for:

[0134]

[0135] Minimum secondary coil diameter d s-min for:

[0136]

[0137] Step 6: Review transformer parameter design using AP method

[0138] Calculate A by design parameters p ′ value and core A p Value comparison, where A p 'for:

[0139]

[0140] Among them, B m is the transformer magnetic flux density, which is 3000G; K o is the transformer window filling factor, which is 0.4; K c is the transformer core filling factor, which is 1. According to the PC40EE16 core parameters, its core A p The value is 76.5×10 - 3 cm 4 The calculation results according to formula (51) show that A p ' p , the transformer parameters are designed reasonably.

[0141] After the above calculations, the parameters of the transformer designed with PC40EE16 as the core are: the transformer output power is 5.3W, the primary coil has 40 turns, the primary coil wire diameter is greater than or equal to 0.4mm, the primary coil inductance is 190μH, the secondary coil has 1667 turns, and the secondary coil wire diameter is greater than or equal to 0.05mm.​

Claims

1. A design method for a flyback transformer based on boost time requirements, characterized in that: The steps include: Step 1: Calculate the energy required to charge the output capacitor to a predetermined value According to the requirements, the output capacitance of the flyback special power supply is known to be C, and the output voltage is predetermined to be V out ; The energy required to charge the output capacitor to the predetermined value is W c for: The power P required to charge the output capacitor to the predetermined value is out for: Step 2: Calculate the relationship between the time required for the circuit topology to complete the energy transfer and the transformer inductance Set the switching frequency of the switch tube in the flyback special power supply to f, the duty cycle to D, and the conduction time to t on These three parameters are customized according to the needs in power supply design, among which t on =D / f; When the switch is working and in the conduction stage, the primary coil of the transformer begins to charge, and the current of the primary coil of the transformer rises linearly from zero; the inductance of the primary coil of the transformer is set to L1, then the inductance voltage of the primary coil of the transformer u L1 for: The primary current i1 of the transformer is i1 = K·t on (4) Where K is the current rise rate of the primary inductor of the transformer, which is approximately: The inductance of the primary coil of the transformer is on The instantaneous power P during L1 for: P L1 =in L1 ·i1 (6) The inductance of the primary coil of the transformer is on The energy stored during the period W L for: According to equations (5) and (7), the energy W stored in the transformer in one switching cycle can be obtained as L for: After n cycles, the capacitor is fully charged. Assuming that the capacitor is fully charged, the total power transmitted by the primary side of the transformer is P in ,but: Assume the transformer transmission efficiency is η and the transformer output power is P out ,but: P.S out JP in ·η (10) The time required for the output capacitor to charge to a predetermined value is: According to the above equations (9), (10) and (11), the mathematical relationship between the boost time t and the transformer primary coil inductance L1 can be obtained as follows: Therefore, according to formula (12), the inductance of the primary coil of the transformer can be obtained by the boost time and other known conditions; and by substituting the result obtained by formula (12) into formula (9), the total power P transmitted by the primary side of the transformer can be obtained. in size; Step 3: Calculate the number of turns of the transformer primary coil Select a suitable transformer core; among the transformer core parameters, the equivalent inductance when the core has no air gap is A L ; Therefore, the number of turns N1 of the transformer primary coil is: In order to prevent the transformer from magnetic saturation during operation, the number of N1 windings can be appropriately increased and the air gap can be opened to keep the transformer primary coil inductance at L1. Step 4: Calculate the number of turns of the secondary coil of the transformer When the flyback special power supply is working, the withstand voltage of the switch tube is V ds , the reflected voltage of the switch tube V or for: Transformer primary and secondary coil ratio for: Therefore, the number of turns N2 of the transformer secondary coil should be: Step 5: Calculate the wire diameter of the transformer primary and secondary coils The maximum duty cycle is D max , the average current of the primary switch tube I ave for: The primary switch current pulsation ΔI is: Primary switch peak current I P for: Primary current RMS I RMS for: Where K RP is the current ripple factor; Minimum primary coil diameter d min for: Where ρ is the current density; Secondary current peak I s for: Secondary current RMS value I s-RMS for: Minimum secondary coil diameter d s-min for: Step 6: Review transformer parameter design using AP method Calculate A by design parameters p ′ value and core A p Value comparison, where A p 'for: Among them, B m is the transformer flux density, K o is the transformer window filling factor, K c is the transformer core filling factor; when A p ' p When the transformer parameters are designed reasonably;​ After the calculations in the above steps, the parameters required for making the transformer can be obtained: Transformer output power P out , the number of turns of the primary coil N1, the minimum wire diameter d required for the primary coil min , the inductance of the primary coil is L1, the number of turns of the secondary coil is N2, and the minimum wire diameter required for the secondary coil is d s-min .

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