Design points of high-frequency transformer for single-chip switching power supply

2019-06-03 11:57:41 JUKE CHINA ODM OEM Transformer factory Read

Design points of high-frequency transformer for single-chip switching power supply

The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, minimal peripheral circuit, and best performance index, and can form an isolated switching power supply with high efficiency and no power frequency transformer. In 1994-2001, TOPSwitch, TOPSwitch-II, TOPSwitch-FX, TOPSwitch-GX, TinySwitch, TinySwitch-II and other series of single-chip switching power supply products were introduced internationally, and now it has become a medium and small power switch. Preferred integrated circuits for power supplies, precision switching power supplies, and switching power supply modules.

High-frequency transformer is an important component of energy storage and transmission in switching power supply. The performance of high-frequency transformer in single-chip switching power supply not only has a great impact on power efficiency, but also directly relates to other technical specifications and electromagnetic compatibility of power supply. Sex (EMC). To this end, a high-efficiency high-frequency transformer should have conditions such as low DC loss and low AC loss, low leakage inductance, distributed capacitance of the winding itself, and small coupling capacitance between the windings. The design points are described below.

2 Reduce High frequency transformer losses
2.1 DC loss
The DC loss of a high frequency transformer is caused by the copper loss of the coil. In order to improve efficiency, we should try to choose a thicker wire and take the current density J=4~10A/mm2.

2.2 AC loss
The AC loss of a high-frequency transformer is caused by the skin effect of high-frequency current and the loss of the core. When a high-frequency current passes through a wire, it tends to flow from the surface, which reduces the effective flow area of the wire and makes the wire's AC equivalent impedance much higher than the copper resistance. The penetration capability of the high-frequency current to the conductor is inversely proportional to the square root of the switching frequency. To reduce the impedance of the AC copper, the radius of the wire must not exceed twice the depth of the high-frequency current. The relationship between the available wire diameter and the switching frequency is shown in Figure 1. For example, when f = 100 kHz, the wire diameter can theoretically be φ 0.4 mm. However, in order to reduce the skin effect, it is practical to use a thinner wire to be wound in multiple strands instead of a thick wire.
Design points of high-frequency transformer for single-chip switching power supply.jpg

The core loss of the high frequency transformer also reduces the power supply efficiency. The AC flux density can be estimated by the following formula:

BAC=(0.4πNPIPKRP)/2δ

Where: BAC is the AC flux density, the unit is T;

NP and IP are primary turns and primary peak currents, respectively;

KRP is the ratio of primary pulsating current to peak current;

δ is the air gap width of the magnetic core, and the unit is cm.

To design a high frequency transformer operating in continuous mode, the typical value of BAC is approximately 0.04 to 0.075T. The loss of the ferrite core at 100 kHz should be less than 50 mW/cm3.

3 Reduce the leakage inductance of high frequency transformer

The leakage inductance must be minimized when designing high frequency transformers. Because the larger the leakage inductance, the higher the peak voltage amplitude generated, the greater the loss of the drain clamp circuit, which inevitably leads to a decrease in power supply efficiency. For a high-frequency transformer that meets the insulation and safety standards, the leakage inductance should be 1% to 3% of the primary inductance at the secondary open circuit. In order to achieve the index below 1%, it will be difficult to achieve in the manufacturing process. The following measures can be taken to reduce leakage inductance:

1) reducing the number of turns NP of the primary winding;

2) increase the width of the winding (for example, select the EE core to increase the width b of the skeleton);

3) increase the height to width ratio of the winding;

4) reducing the insulation between the windings;

5) Increase the degree of coupling between the windings.

3.1 Reduce the number of primary winding turns and increase the ratio of height to width
Selecting a suitable core shape and reducing the number of primary turns and increasing the ratio of height to width can effectively reduce the leakage inductance. The amount of leakage inductance is proportional to the square of the primary turns. The size of the selected core should be large enough to allow the primary winding to be wound into 2 or even less layers, which minimizes primary leakage inductance and distributed capacitance. Do not use the core of the short window, because it is large in size, small in ratio of height to width, and large in leakage inductance, it is not suitable. It corresponds to POT, RM, PQ and some E-cores. It is recommended to use a slim core with a large height to width ratio, which corresponds to EE, ETD, EI, EC cores.
Triple Insulated Wire is a high-performance insulated wire newly developed in recent years in the world. This wire has three insulating layers with a core wire in the middle. The insulating layer is a golden yellow polyamide film, which is called "golden film" in foreign countries. The total thickness of the insulating layer is only 20-100μm, but it can withstand a pulse high voltage of several kV. The triple insulated wire is suitable for cutting-edge technology and defense. In the field, high-frequency transformer windings for micro-motor windings and miniaturized switching power supplies are produced. The advantage is that the dielectric strength is high (the safety voltage of AC 3000V can be withstand between any two layers), no barrier layer is needed to ensure the safety margin, and there is no need to wrap the insulating tape layer between the stages; the current density is large. The high-frequency transformer wound with it can be reduced by half compared to the volume wound with an enameled wire. An optimized design of the high-frequency transformer is to wind the primary and feedback stages with a common high-strength enameled wire, and to wind the secondary with a triple insulated wire. In this way, the amount of leakage inductance is greatly reduced, and the volume of the high-frequency transformer can be reduced by 1/2 to 1/3.

3.2 Winding arrangement
To reduce leakage inductance, the windings should be arranged in a concentric manner, as shown in Figure 2. In Figure 2(a), the secondary is wound with triple insulated wire; in Figure 2(b), all are wrapped with enameled wire, but with a safety margin and a reinforced insulation layer between the secondary winding and the feedback winding. . For multi-output switching power supplies, the secondary winding with the highest output power should be close to the primary to increase coupling and reduce magnetic field leakage. When the number of secondary turns is small, in order to increase the coupling with the primary, multiple strands should be evenly distributed in parallel and around the entire skeleton to increase the coverage area. The use of foil windings as a secondary is also a good way to increase coupling, if conditions permit.
Design points of high-frequency transformer for single-chip switching power supply.jpg

Design points of high-frequency transformer for single-chip switching power supply.jpg

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Design points of high-frequency transformer for single-chip switching power supply
Source: Cheng Pin Technology Release time:2017-09-23 Hits:1336 times

The single-chip switching power supply IC has the advantages of high integration, high cost performance, simplest peripheral circuits, and best performance indicators, and can form an isolated switching power supply with high efficiency and no power frequency transformer. In 1994-2001, TOPSwitch, TOPSwitch-II, TOPSwitch-FX, TOPSwitch-GX, TinySwitch, TinySwitch-II and other series of single-chip switching power supply products were introduced internationally, and now it has become a medium and small power switch. Preferred integrated circuits for power supplies, precision switching power supplies, and switching power supply modules.

High-frequency transformer is an important component of energy storage and transmission in switching power supply. The performance of high-frequency transformer in single-chip switching power supply not only has a great impact on power efficiency, but also directly relates to other technical specifications and electromagnetic compatibility of power supply. Sex (EMC). To this end, a high-efficiency high-frequency transformer should have conditions such as low DC loss and low AC loss, low leakage inductance, distributed capacitance of the winding itself, and small coupling capacitance between the windings. The design points are described below.

2 Reduce high frequency transformer losses
2.1 DC loss
The DC loss of a high frequency transformer is caused by the copper loss of the coil. In order to improve efficiency, we should try to choose a thicker wire and take the current density J=4~10A/mm2.

2.2 AC loss
The AC loss of a high-frequency transformer is caused by the skin effect of high-frequency current and the loss of the core. When a high-frequency current passes through a wire, it tends to flow from the surface, which reduces the effective flow area of the wire and makes the wire's AC equivalent impedance much higher than the copper resistance. The penetration capability of the high-frequency current to the conductor is inversely proportional to the square root of the switching frequency. To reduce the impedance of the AC copper, the radius of the wire must not exceed twice the depth of the high-frequency current. The relationship between the available wire diameter and the switching frequency is shown in Figure 1. For example, when f = 100 kHz, the wire diameter can theoretically be φ 0.4 mm. However, in order to reduce the skin effect, it is practical to use a thinner wire to be wound in multiple strands instead of a thick wire.

The core loss of the high frequency transformer also reduces the power supply efficiency. The AC flux density can be estimated by the following formula:

BAC=(0.4πNPIPKRP)/2δ

Where: BAC is the AC flux density, the unit is T;

NP and IP are primary turns and primary peak currents, respectively;

KRP is the ratio of primary pulsating current to peak current;

δ is the air gap width of the magnetic core, and the unit is cm.

To design a high frequency transformer operating in continuous mode, the typical value of BAC is approximately 0.04 to 0.075T. The loss of the ferrite core at 100 kHz should be less than 50 mW/cm3.

3 Reduce the leakage inductance of high frequency transformer

The leakage inductance must be minimized when designing high frequency transformers. Because the larger the leakage inductance, the higher the peak voltage amplitude generated, the greater the loss of the drain clamp circuit, which inevitably leads to a decrease in power supply efficiency. For a high-frequency transformer that meets the insulation and safety standards, the leakage inductance should be 1% to 3% of the primary inductance at the secondary open circuit. In order to achieve the index below 1%, it will be difficult to achieve in the manufacturing process. The following measures can be taken to reduce leakage inductance:

1) reducing the number of turns NP of the primary winding;

2) increase the width of the winding (for example, select the EE core to increase the width b of the skeleton);

3) increase the height to width ratio of the winding;

4) reducing the insulation between the windings;

5) Increase the degree of coupling between the windings.

3.1 Reduce the number of primary winding turns and increase the ratio of height to width
Selecting a suitable core shape and reducing the number of primary turns and increasing the ratio of height to width can effectively reduce the leakage inductance. The amount of leakage inductance is proportional to the square of the primary turns. The size of the selected core should be large enough to allow the primary winding to be wound into 2 or even less layers, which minimizes primary leakage inductance and distributed capacitance. Do not use the core of the short window, because it is large in size, small in ratio of height to width, and large in leakage inductance, it is not suitable. It corresponds to POT, RM, PQ and some E-cores. It is recommended to use a slim core with a large height to width ratio, which corresponds to EE, ETD, EI, EC cores.
Triple Insulated Wire is a high-performance insulated wire newly developed in recent years in the world. This wire has three insulating layers with a core wire in the middle. The insulating layer is a golden yellow polyamide film, which is called "golden film" in foreign countries. The total thickness of the insulating layer is only 20-100μm, but it can withstand a pulse high voltage of several kV. The triple insulated wire is suitable for cutting-edge technology and defense. In the field, high-frequency transformer windings for micro-motor windings and miniaturized switching power supplies are produced. The advantage is that the dielectric strength is high (the safety voltage of AC 3000V can be withstand between any two layers), no barrier layer is needed to ensure the safety margin, and there is no need to wrap the insulating tape layer between the stages; the current density is large. The high-frequency transformer wound with it can be reduced by half compared to the volume wound with an enameled wire. An optimized design of the high-frequency transformer is to wind the primary and feedback stages with a common high-strength enameled wire, and to wind the secondary with a triple insulated wire. In this way, the amount of leakage inductance is greatly reduced, and the volume of the high-frequency transformer can be reduced by 1/2 to 1/3.

3.2 Winding arrangement
To reduce leakage inductance, the windings should be arranged in a concentric manner, as shown in Figure 2. In Figure 2(a), the secondary is wound with triple insulated wire; in Figure 2(b), all are wrapped with enameled wire, but with a safety margin and a reinforced insulation layer between the secondary winding and the feedback winding. . For multi-output switching power supplies, the secondary winding with the highest output power should be close to the primary to increase coupling and reduce magnetic field leakage. When the number of secondary turns is small, in order to increase the coupling with the primary, multiple strands should be evenly distributed in parallel and around the entire skeleton to increase the coverage area. The use of foil windings as a secondary is also a good way to increase coupling, if conditions permit.

 During the operation of the switching power supply, the distributed capacitance of the winding is repeatedly charged and discharged, and the energy on it is absorbed. The distributed capacitance not only reduces the power supply efficiency, but also forms an LC oscillator with the distributed inductance of the winding, which generates ringing noise. The effect of the primary winding distributed capacitance is particularly significant. In order to reduce the distributed capacitance, the length of each lead wire should be minimized, and the beginning of the primary winding should be connected to the drain, and a part of the primary winding can be used as a shielding function to reduce the coupling degree of the adjacent winding.

4 suppress high frequency transformer audio noise
4.1 Suppressing the audible noise of high frequency transformers
The attractive force between the high-frequency transformer EE or the EI core can cause the two cores to be displaced; the gravitational or repulsive force of the winding currents can also cause the coil to shift. In addition, it can cause periodic changes when subjected to mechanical vibration. These factors can cause high frequency transformers to emit audible noise during operation. The audio noise frequency of a single-chip switching power supply below 10 W is approximately 10 kHz to 20 kHz.
In order to prevent relative displacement between the magnetic cores, epoxy resin is usually used as a glue to bond the three contact faces (including the center column) of the two magnetic cores. However, the effect of this rigid connection is not ideal. Because this does not minimize audible noise, and the glue is too much, the core is easily broken when subjected to mechanical stress. Recently, a special "glass beads" adhesive has been used abroad to bond EI, EI and other ferrite cores, which is very effective. The glue is a mixture of glass beads and a binder in a ratio of 1:9, which is cured by being placed in a temperature environment of 100 ° C or more for 1 hour. Its role is similar to that of ball bearings. After curing, each core can still be deformed or displaced independently in a small range, and the overall position is unchanged, which inhibits the deformation. The internal structure of the high-frequency transformer bonded by the glass bead glue is shown in Fig. 3. This process reduces audible noise by 5dB.
Design points of high-frequency transformer for single-chip switching power supply.jpg

4.2 Shielding of high frequency transformers
In order to prevent the leakage magnetic field of the high-frequency transformer from causing interference to adjacent circuits, a copper piece may be wrapped around the outside of the transformer to form a shielding tape as shown in FIG. The shielding tape is equivalent to a short-circuit ring, which can suppress the leakage magnetic field, and the shielding tape should be connected to the ground.

5 Conclusion

The design points of the single-chip switching power supply high-frequency transformer are divided into three aspects:

1) Minimize the loss of high frequency transformers;

2) Minimize the leakage inductance of high frequency transformers;

3) Try to suppress the audible noise of the high frequency transformer.