CN115347794A - An asymmetrical half-bridge flyback converter and its design method - Google Patents

Abstract

The invention discloses an asymmetrical half-bridge flyback converter and a design method thereof, including the design of a main power circuit and a control circuit, and the derivation of voltage fluctuation on a resonant capacitor and resonant time. The design of the main power circuit includes a planar transformer: the winding method of PPPSSS uses one turn of the primary side as a shielding layer to greatly reduce the coupling capacitance of the primary side and the secondary side, and does not require additional series resonant inductors to reduce losses. According to the derivation of the voltage fluctuation of the resonant capacitor, a reasonable resonant capacitor value can be designed. According to the derivation of the resonant time, an appropriate resonant time can be obtained to realize the zero-current shutdown of the synchronous rectifier tube on the secondary side, and at the same time reduce the current circulation in the resonant cavity to improve efficiency. The invention can realize soft switching of all switching tubes in wide ranges of input voltage, output voltage and power to achieve high efficiency.

Description

An asymmetrical half-bridge flyback converter and its design method

technical field

The invention relates to the DC-DC converter technology of an electric energy conversion device, in particular to an asymmetrical half-bridge flyback converter and a design method thereof.

Background technique

With the widespread use of portable electronic devices, electronic device charging adapters have become a necessity. With the development of mobile phone fast charging and other fields in recent years, higher requirements have been put forward for the power and volume of the power supply system, and the charging speed and small size of the charging equipment are required. At the same time, with the increase in the power of charging equipment, the EMI requirements of the power grid for the converter are also increasing, and it is required to meet the national setting standards.

For the small and medium power switching power supplies required for fast charging of mobile phones and mobile electronic devices, the loss is dissipated in the form of heat energy. Therefore, the requirements for converters can be summarized as the requirements for high efficiency and high power density of converters. Two simple and practical switching power supply topologies commonly used at present can both meet the required power requirements, one is a forward converter and the other is a flyback converter. The energy of the forward converter can be directly transferred to the transformer without using the transformer to store energy, so the transformer has a higher magnetizing inductance and no air gap, which can improve the utilization of the transformer; the secondary output of the transformer is filtered by LC, which can ensure Reduces the output voltage ripple while reducing the peak current on the transistor, thereby reducing voltage stress. Compared with the forward converter, the flyback transformer is mainly used to store energy in the circuit operation, and its application in high-power occasions is limited. At the same time, the secondary side of the converter does not require a filter inductor. The voltage range output and high gain requirements have absolute advantages, so the flyback converter is the best choice in low power applications.

There are also many problems in the practical application of the flyback converter. First, because the conventional flyback circuit works in the hard switching mode, it cannot achieve high efficiency. Second, due to the consideration of reducing cost and volume, the transformer needs to leave an air gap to prevent The magnetic flux saturation is achieved, but this action leads to an increase in the leakage inductance of the transformer, which increases the power loss and the voltage stress of the MOS tube, and reduces the efficiency of the converter and the stability of the circuit. In addition, the resonant peak voltage caused by the secondary winding of the converter will affect the working efficiency of the switching power supply transformer ^[3] . In hard switching mode, the EMI performance of the converter will deteriorate, which also limits the use of flyback converters in some applications. Aiming at the above three major problems of the flyback converter, scholars have conducted a lot of research on the flyback converter.

In order to reduce the voltage stress and voltage spikes of the flyback converter, the quasi-resonant circuit (QR) often uses a passive RCD snubber circuit, which consumes the remaining energy in the leakage inductance and limits the efficiency of the flyback converter. The active clamp flyback circuit (ACF) can effectively use the energy in the leakage inductance through the clamp capacitor, but the problem of high voltage stress of the switching tube still restricts the development and application of the flyback converter.

Contents of the invention

The purpose of the invention of the present invention is to provide a kind of asymmetrical half-bridge flyback converter and design method thereof for the deficiency of above-mentioned background technology, make converter can satisfy wide input wide output (input 90VAC-220VAC, output 5V-15V , 5A) requirements, efficiency as high as possible, power density as large as possible.

The present invention adopts following technical scheme for realizing above-mentioned purpose of the invention:

An asymmetrical half-bridge flyback converter, including a main power circuit, the main power circuit includes V _B is the bus voltage of the AC input voltage passing through the rectifier bridge and the input filter capacitor, S ₁ and S ₂ are the primary switch tubes, Using a GaN device, since the GaN device has no body diode but has a third quadrant, the diode connected in parallel with S ₁ and S ₂ in the schematic diagram represents its third quadrant to describe the freewheeling process. The transformer model can be equivalent to an ideal transformer with leakage inductance L _r , excitation inductance L _m and an n:1 turn ratio, C _r is the resonant capacitor, Q ₁ is the secondary synchronous rectifier tube, C _o is the output capacitor, R _o is the load.

A design method for an asymmetrical half-bridge flyback converter, which is used for the above-mentioned asymmetrical half-bridge flyback converter, comprising the following steps:

Step A: Analyze the equation of the leakage inductance current i _Lr and the voltage across the resonant capacitor v _Cr in each mode according to the working mode of the converter;

Step B: According to the volt-second balance of the excitation inductance, the duty cycle _of the upper tube S1 is obtained as

其中I_Lmmax与I_Lmmin分别为励磁电感的正负峰值电流。其具体计算为

其中t_off为上管关断时间；Step C: According to the integral of input power, the output current can be obtained

Among them, I _Lmmax and I _Lmmin are the positive and negative peak currents of the exciting inductance respectively. Its specific calculation is

Where t _off is the turn-off time of the upper tube;

Step D: The secondary current can be approximated as a half sine wave, and the average value of the secondary current in one cycle is equal to the output current I _o to obtain the secondary peak current

Step E: The size of the input capacitor C _in depends on the minimum effective value V _{inrms_min} of the AC voltage and the minimum capacitor voltage V _{B_min} , and the value of the input filter capacitor is

根据副边同步整流管Q₁耐压可得最小匝比

由此求得变压器匝比n；Step F: Set the maximum duty cycle according to the upper tube to get the maximum turn ratio

According to the withstand voltage _of the secondary synchronous rectifier tube Q1, the minimum turn ratio can be obtained

From this, the transformer turns ratio n is obtained;

由此求得变压器原边匝数N_p以及磁芯气隙大小δ；Step G: Carry out transformer design, according to the effective area A _e of the magnetic core and the magnetic flux density variable ΔB, the number of turns of the secondary side of the transformer can be obtained

From this, the number of turns N _p on the primary side of the transformer and the size δ of the air gap of the magnetic core are obtained;

Step H: Since the secondary current is approximately half a sine wave, the intersection points of its waveform and I _o on the waveform are t _A1 and t _A2 , and the output capacitance is obtained according to the requirements of the output voltage ripple amplitude ΔU

Preferably, the valley value of the voltage fluctuation on the resonant capacitor is that the voltage value of the resonant capacitor at time t1 in Fig. ₁ is

In the formula

The peak value of the voltage fluctuation on the resonant capacitor is the voltage value of the resonant capacitor at time t5 in Figure ₁

In the formula

Preferably, the upper tube is turned off, that is, the turn-on time t _off of the lower tube (neglecting the dead time) needs to match the resonance time as much as possible, so as to reduce the circulating current in the resonance cavity and make the secondary synchronous rectifier zero-current shutdown, and the resonance time is

In the formula

Preferably, a planar transformer design is adopted, and the PPPSSS winding method is used to design it as a layer of shielding layer with a single turn of the primary coil, which is at the same potential as the positive pole of the resonant capacitor C _r , so that the coupling capacitance between the primary and secondary sides is coupled to a On a potential with little fluctuation, it can effectively reduce common mode noise, and at the same time, no additional series resonant inductor is needed to reduce loss.

Preferably, the upper tube of the primary switching tube is GS-065-011-1-L from GaN Systems, the lower tube is GS66508B from GaN Systems, the secondary synchronous rectifier is BSC093N15NS5 from Infineon, and the magnetic core is from TDK. PC95EL25X8.6-Z; the control chip uses Infineon's XDPS2201 chip.

Compared with the prior art by adopting the above technical scheme, the present invention has the following beneficial effects:

1. The present invention provides an asymmetrical half-bridge flyback converter and its design method, by selecting a random frequency within the range of two switching frequency changes in each control cycle to randomize the switching frequency, which can be controlled within the switching frequency range In a narrower case, the harmonic dispersion effect of the random SVPWM method is improved, and electromagnetic interference is further suppressed.

2. The present invention provides a kind of asymmetrical half-bridge flyback converter and design method thereof, in the scope of wide input voltage, output voltage and output power, can realize the zero-voltage switch (ZVS) of primary side switch tube, secondary Zero-current switching (ZCS) of side synchronous rectifiers.

3. The present invention provides a kind of asymmetrical half-bridge flyback converter and design method thereof, proposes a kind of magnetic shielding technical method of novel asymmetrical half-bridge flyback converter planar transformer design, utilizes primary side one-turn winding as The method of greatly reducing the coupling capacitance of the primary and secondary sides by the shielding layer makes the converter meet the requirements of EN55022B.

4. The present invention provides an asymmetrical half-bridge flyback converter and a design method thereof, which reduces the AC impedance of the transformer by adopting mixed copper thickness, and improves the efficiency and power density of the converter.

Description of drawings

In order to more clearly illustrate the specific implementation of the present invention or the technical solutions in the prior art, the following will briefly introduce the accompanying drawings that need to be used in the specific implementation or description of the prior art. Obviously, the accompanying drawings in the following description The drawings show some implementations of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative work.

Figure 1 is the key waveform of the converter in one switching cycle;

Figure 2 is the main circuit of the asymmetrical half-bridge flyback converter;

Fig. 3 is a working mode diagram of the converter;

Figure 4 is a schematic diagram of the PPPSSS winding method;

Fig. 5 is a cross-sectional view of a transformer winding;

Figure 6 is a spatial structure diagram of the transformer winding.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

1. Working principle of asymmetrical half-bridge flyback converter:

Figure 1 shows the key waveforms of the converter in a switching cycle, from top to bottom are the upper tube drive signal, the lower tube drive signal, the resonant current and excitation current, the secondary side current and the half-bridge midpoint voltage, Figure 2 It is the main circuit of the asymmetrical half-bridge flyback converter. Figure 3 shows the working mode diagram of the converter.

Mode 1: stage t ₁ ~ t ₂ , S ₁ is open and S ₂ is closed. The current in the transformer increases, the voltage in the resonant capacitor _Cr increases, and both devices store energy. _The secondary diode D1 is reverse biased, so no energy is transferred to the secondary side.

Mode 2: During the period from t ₂ to t ₃ , the switching tubes S ₁ and S ₂ are both closed. The primary current charges the parasitic capacitance of S ₁ and discharges the parasitic capacitance of S ₂ at the same time, at time t ₃ the charging and discharging process of the parasitic capacitance of S ₁ and S ₂ ends, and the voltage drop across S ₂ is zero, which is S ₂ provides the conditions for the body diode to turn on.

Mode ₃ : During the period from _t3 to t4, the switches S1 and S2 are _both closed, and the body diode _of S2 _clamps the voltage. The primary side of the transformer has the same voltage as the capacitor _Cr . _The voltage across S2 is the conduction voltage of the body diode.

Mode 4: From t ₄ to t ₅ , in the ZVS state, S ₁ is closed, S ₂ is open, and the voltage on the secondary side is equal to the voltage of C _r on the capacitor divided by the turns ratio. Current begins to flow through _Q1 , and the energy in the capacitor, transformer, is transferred to the output. Since the LC tank consists of _Lr (transformer leakage inductance) and the resonant capacitor, the secondary current is a sine wave determined by the resonant period of these two devices. The primary current is the sum of the excitation current plus the secondary reflected current. The current in the resonant cavity is still positive, mainly driven by the excitation inductance _of the transformer T1, and flows into the resonant capacitor C _r for further charging.

Mode 5: stage t ₅ ~ t ₆ is the continuation of the previous stage. S ₁ is turned off, S ₂ is turned on, the energy is still transferred to the secondary side but the resonant tank current and voltage reverse direction, and the voltage is provided by the resonant capacitor C _r . The energy of the resonant capacitor is not only transferred to the secondary side, but also helps to reduce the magnetizing current T1 of the transformer to a negative value at the turn _{- on} of S2.

Mode 6: t ₆ ~ t ₇ stage, in this stage, the secondary side current finally drops to zero, and the primary side current continues to drop after resonating to zero, providing enough negative current for S ₁ body diode to turn on, so as to realize S ₁ zero The voltage is turned on.

Mode 7: During the period from t ₇ to t ₈ , the switching tubes S ₁ and S ₂ are both closed. The primary current charges the parasitic capacitance of S ₂ and discharges the parasitic capacitance of S ₁ at the same time. At the time t ₈ , the charging and discharging process of the parasitic capacitance of S ₂ and S ₁ ends, and the voltage drop across S ₁ is zero, which is S ₁ provides the conditions for the body diode to turn on. .

Mode 8: stage t ₈ ～ t ₉ , in this stage, both switches S ₁ and S ₂ are turned off. The body diode of S ₁ conducts, which provides conditions for the zero-voltage turn-on of S ₁ .

Mode 9: t ₉ ~ t ₁₀ stage, similar to the first stage, under the ZVS condition, S ₁ is turned on and S ₂ is turned off, but the current in the transformer resonant cavity is still negative, which means that there is redundant energy will be sent back to the input.

2. Circuit parameter design:

Step A: Analyze the equation of the leakage inductance current i _Lr and the voltage v _Cr at both ends of the resonant capacitor in each mode according to the working mode of the converter.

Step B: According to the volt-second balance of the excitation inductance, the duty cycle _of the upper tube S1 is obtained as

其中I_Lmmax与I_Lmmin分别为励磁电感的正负峰值电流。其具体计算为

其中t_off为上管关断时间。Step C: According to the integral of input power, the output current can be obtained

Among them, I _Lmmax and I _Lmmin are the positive and negative peak currents of the exciting inductance respectively. Its specific calculation is

Among them, t _off is the turn-off time of the upper tube.

Step D: The secondary current can be approximated as a half sine wave, and the average value of the secondary current in one cycle is equal to the output current I _o to obtain the secondary peak current

Step E: The size of the input capacitor C _in depends on the minimum effective value V _{inrms_min} of the AC voltage and the minimum capacitor voltage V _{B_min} . The input filter capacitor value is

根据副边同步整流管Q₁耐压可得最小匝比

由此求得变压器匝比n。Step F: Set the maximum duty cycle according to the upper tube to get the maximum turn ratio

According to the withstand voltage _of the secondary synchronous rectifier tube Q1, the minimum turn ratio can be obtained

From this, the transformer turns ratio n is obtained.

由此求得变压器原边匝数N_p以及磁芯气隙大小δ。Step G: Carry out transformer design, according to the effective area A _e of the magnetic core and the magnetic flux density variable ΔB, the number of turns of the secondary side of the transformer can be obtained

From this, the number of turns N _p on the primary side of the transformer and the size δ of the air gap of the magnetic core are obtained.

Step H: Since the secondary current is approximately half a sine wave, the intersection points of its waveform and I _o on the waveform are t _A1 and t _A2 , and the output capacitance is obtained according to the requirements of the output voltage ripple amplitude ΔU

3. Reference for the voltage fluctuation value on the resonant capacitor:

The valley value of the voltage fluctuation on the resonant capacitor is the voltage value of the resonant capacitor at time t1 in Figure ₁

In the formula

The peak value of the voltage fluctuation on the resonant capacitor is the voltage value of the resonant capacitor at time t5 in Figure ₁

In the formula

4. The opening time design of the lower tube:

The turn-off of the upper tube, that is, the turn-on time t _off of the lower tube (neglecting the dead time), needs to match the resonance time as much as possible, so as to reduce the circulating current in the resonance cavity and make the secondary synchronous rectifier turn off with zero current. The resonance time is

In the formula

5. Planar transformer design:

In order to improve the power density, a planar transformer design is adopted. The winding methods of the planar transformer of the six-layer board mainly include PPPSSS winding method and PSPSPS winding method. PSPSPS winding transformer winding cross-sectional view and space structure diagram shown in Figure 5 (a) and Figure 6 (a) respectively. This winding method will cause multiple coupling capacitors C _PS between the primary and secondary sides, and the common mode noise is transmitted through C _PS , so the common mode noise of this winding method is relatively large. The schematic diagram of the PPPSSS winding method is shown in Figure 4, and the cross-sectional view and spatial structure diagram of the transformer winding are shown in Figure 5(b) and Figure 6(b) respectively. This winding method uses a single turn of the primary coil to design it as a layer of shielding layer, which has the same potential as the positive electrode of the resonant capacitor C _r , so that the coupling capacitance between the primary and secondary sides is coupled to a potential with little fluctuation, which can Effectively reduce common mode noise.

When the excitation inductance is 55μH, the parameters of the two winding methods under the same inductance value are measured by LCR, as shown in Table 1. Through measurement, the primary and secondary side coupling capacitance of the PPPSSS winding method is much smaller than that of the PSPSPS winding method, and the inductance impedance and AC impedance do not change much. Therefore, the common mode noise can be effectively reduced without affecting the efficiency. At the same time, the leakage inductance is increased to avoid additional series resonant inductance, so as to improve efficiency and increase power density.

Table 1

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the various embodiments of the present invention. scope.

Claims (6)

1. An asymmetric half-bridge flyback converter, comprising: includes a main power circuit including V _B For the bus voltage of the AC input voltage via a rectifier bridge and an input filter capacitor, S ₁ 、S ₂ For the primary side switching tube, a GaN device is adopted, and since the GaN device does not have a body diode but has a third quadrant, the GaN device uses the same S in a schematic diagram ₁ 、S ₂ The parallel diode represents its third quadrant in order to describe the freewheeling process. The transformer model can be equivalent to leakage inductance L _r Excitation inductance of L _m With an ideal transformer having a turn ratio of n:1, C _r Is a resonant capacitor, Q ₁ Is a secondary side synchronous rectifier tube, C _o As an output capacitance, R _o Is a load.

2. A design method of an asymmetric half-bridge flyback converter is used for the asymmetric half-bridge flyback converter and is characterized by comprising the following steps:

step A: analyzing leakage inductance current i under each mode according to working modes of the converter _Lr And the voltage v at two ends of the resonant capacitor _Cr An equation;

and B, step B: obtaining an upper pipe S according to the volt-second balance of the excitation inductance ₁ Duty ratio of

And C: the output current is obtained according to the integral of the input power

Wherein I _Lmmax And I _Lmmin Respectively positive and negative peak currents of the exciting inductor. It is specifically calculated as

Wherein t is _off The upper tube turn-off time;

step D: the secondary side current can be approximately regarded as a half sine wave, and the secondary side current in one periodThe average value of the current being equal to the output current I _o The secondary side peak current can be obtained

Step E: input capacitance C _in Is dependent on the minimum effective value V of the alternating voltage _{inrms_min} And a minimum capacitor voltage V _{B_min} Input filter capacitance value of

Step F: maximum turn ratio obtained by setting maximum duty ratio according to upper tube

According to the secondary side synchronous rectifier Q ₁ Minimum turn ratio for withstand voltage

Thus, obtaining the turn ratio n of the transformer;

g: designing a transformer according to the effective area A of the magnetic core _e And the number of turns of the secondary side of the transformer can be obtained by the variable Delta B of the magnetic flux density

Thus, the number of turns N of the primary side of the transformer is obtained _p And a core air gap size δ;

step H: since the secondary current is approximately half a sine wave, the waveform is equal to I _o Has an intersection point t on the waveform _A1 And t _A2 Obtaining the output capacitor according to the requirement of the ripple amplitude delta U of the output voltage

3. The design method of an asymmetric half-bridge flyback converter according to claim 2, wherein: the valley of the voltage fluctuation on the resonant capacitor, i.e. t in FIG. 1 ₁ The voltage value of the resonance capacitor at the moment is

In the formula

The peak value of the voltage fluctuation on the resonant capacitor, i.e. t in FIG. 1 ₅ The voltage value of the resonance capacitor at the moment is

In the formula

4. The design method of the asymmetric half-bridge flyback converter according to claim 2, wherein: upper tube turn-off, i.e. lower tube turn-on time t _off The dead time is ignored, the dead time is matched with the resonance time to the greatest extent, so that the circulation current in the resonance cavity is reduced, meanwhile, the secondary side synchronous rectifier tube is turned off at zero current, and the resonance time is

In the formula

5. The method of claim 2The design method of the asymmetric half-bridge flyback converter is characterized by comprising the following steps: the planar transformer is designed by PPPSSS winding method, and the primary coil is designed into a shielding layer and a resonant capacitor C by using a single turn of the primary coil _r The positive pole is at the same potential, so that the coupling capacitors between the original secondary sides are coupled to a potential with small fluctuation, common mode noise can be effectively reduced, and meanwhile, additional series resonance inductors are not needed to reduce loss.

6. The design method of the asymmetric half-bridge flyback converter according to claim 2, wherein: the upper tube of the primary side switch tube is selected from GS-065-011-1-L of GaN Systems, the lower tube is selected from GS66508B of GaN Systems, the secondary side synchronous rectifier tube is selected from BSC093N15NS5 of England flying company, and the magnetic core is selected from PC95EL25X8.6-Z of TDK; the control chip adopts XDPS2201 chip of the England flying company.

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