Increasing the Efficiency of Multi-Output Flyback Transformers in Switching Power Supplies

The trend in power supply design is to adopt smaller, lighter, faster and greener power sources. This evolution has already taken place with switch-mode power supplies (SMPS), which today operate at faster switching speeds and with higher efficiencies. As these designs continue to advance for increasing use in vehicles to charge batteries, the magnetic components that are key elements used in SMPS circuit topologies must also advance to meet ever-changing requirements and efficiency goals. energy.

Known for its low circuit component count, the flyback topology depends on the critical design and analysis of the coupled inductor or “flyback transformer” to meet updated requirements. Since the magnetic elements are usually the most important component in the circuit, demands for next-generation magnetic elements focus on reducing their size while ensuring operation at higher frequencies and efficiency.

This article will cover several flyback transformer designs, providing associated measurements and analysis of each type. It will also highlight designs using Litz wire that have been shown to reduce AC power loss in the total coil design.

Designers will also learn the overall efficiency increases that can be achieved with Litz wire in terms specifically of total core and coil losses from the final component design.

Fundamentals of flyback transformers

The flyback topology in magnetism is an energy storage medium compared to what most would call traditional energy transfer. A flyback transformer does not fit the classic definition of a “transformer”.

Its operation is basically that of a strongly coupled inductor which is used to “store” energy. In a magnetic design, this is usually accomplished by placing a small air gap somewhere in the core’s magnetic flux path.

When the primary switch (transistor) turns on in the flyback transformer, the current energy in the primary winding is stored in the core and space via E = L × I2 (where E is energy, L is the primary inductance, and I is the primary current). When the switch opens, the polarity of the primary winding changes and the reverse-biased diode on the secondary side allows current to flow through the secondary winding. The stored primary energy moves to the secondary winding and decays.

Figure 1: Flyback transformer circuit diagram

Translating circuit operation into an actual design requires a combination of hand calculations, circuit simulators, analytical tools, and the use of finite element analysis (FEA) software. Finished product optimization is achieved through various core and coil design iterations as well as building and measuring samples to verify the calculation/simulation process.

Using this methodology, solving AC coil loss must go from a simple equation to matrix and serial analysis. Therefore, advanced computational tools such as Ansys PEXPRT and Ansys PEMAG can provide good representations of AC losses, which include eddy current skin, fringe and proximity effects.

The following initial coupled inductor design does not take into account the effects of the high frequency AC coil. The following designs covered incorporate various winding and layering techniques that reduce AC coil resistance for reduced power loss.

60 W flyback transformer — Design 1

The design specifications (Table 1) for a 60 W flyback transformer include five windings: one primary, three secondaries and one auxiliary.

4 winding loss design
Table 1: Design Winding Loss 4

To meet normal safety standards, the secondary windings require reinforced insulated wire. An important consideration is that insulated wires can increase wire size by up to 30% due to the thickness of the wire coating. A Ferroxcube EE30 3C94 Mn-Zn ferrite core is used.

AC and DC resistance values ​​were obtained from FEA simulation. AC resistance was multiplied by RMS current squared with DC loss found using DC current.

The total DC + AC copper loss is shown Pcu:

The core loss is found using the flux density ripple ΔB of Faraday’s law (equation 1.1), where Vin is the input voltage, Avs is the central zone, D is the duty cycle, J is the switching period, and NOTp is the number
of primary towers:

The basic loss Pdo is calculated where Vvs is the volume of the nucleus, F is the switching frequency, Bmaximum is the peak flux density, and Kvs, αand β are Steinmetz parameters derived from the properties of the base material:

For Design 1, the total loss Ptotal is equal to the copper loss plus the combined core loss:

Core and coil losses generate heat, which affects the core and coil at the highest level at the center of the component and radiates to the surface, where convection cooling occurs.

Ansys thermal simulation estimated a target total temperature rise of 40°C for the core and coil. The ambient temperature used was 22°C. Design 1 had a total core + coil temperature of 98˚C and a temperature rise of 76˚C, which far exceeds the specified temperature rise. Note that the AC resistance of the coil contributes the most to the copper losses and the core losses are negligible compared to the winding losses.

Fringing and proximity effects — Drawings 2 and 3

Design 2 accounts for fringe effects by changing the distance between the core air gap and the start of the first winding. Fringing effects produce increases in AC resistance due to magnetic flux bulging around the core air gap, instead of moving directly across it.

The distance is increased between the core space and the coil using thicker coil material or spacer tape before coil winding begins. It also increases the average length turn and DC resistance; however, a greater reduction in AC resistance prevails.

This technique results in an overall power loss of 4.15W and a 30% reduction in copper loss. Core loss is unchanged.

The thermal simulation shows that the total temperature is 78˚C, which represents a reduction of 20˚C but remains too high compared to the target.

Proximity effects are accounted for in Design 3 by reducing the total number of coil wire layers. Proximity effect has a direct relationship with AC resistance and occurs when the current distribution in one winding layer influences the distribution in another.

For example, one winding has current flowing in one (positive) direction where the current flowing in the next layer would be negative. The attraction of positive and negative charges changes the distribution of current so that it does not travel evenly through the conductor and cluster to one side. It even influences the alternating current, thus increasing the resistance of the alternating current.

The decrease in layers reduces the AC resistance of the primary winding and the secondary winding 1. However, the AC resistance increases for the secondary windings 2 and 3 due to the inevitable bunching and uneven overlapping of the primary and secondary 1 before the last two winds. These trade-offs produce a slightly better overall loss in Design 3, but it is still too high. Thermal simulation was not performed, as the total losses were only slightly improved.

Litz’s thread explains the skin effect — Design 4

While proximity and fringe effects were reduced in previous designs, the reduction in power loss was not sufficient and the AC resistance remained high.

Design 4 seeks to improve the reductions by taking into account the skin effects of the threads.

A wire carrying alternating current generates an alternating field, which produces a cavitating wire effect called eddy currents. Eddy currents inhibit the uniform distribution of electron current flow throughout the cross section of a piece of wire. The cavitation effect of eddy currents pushes the current (density) out of the wire. Since skin effects are frequency dependent, high frequency alternating current (density) will not protrude so deep in the middle of the conductor, also known as skin depth.

To combat this, several to hundreds of strands of smaller wire are twisted together to create a single larger diameter or gauge wire. Multiple strands of wire have the ability to reduce AC resistance loss due to skin effect. High frequency alternating currents require more strands of wire.

To reduce AC resistance in Design 4, Litz wire was used on all secondaries. Thicker wire on the windings reduced copper losses. Additionally, this design uses a standard spool with a thinner barrel wall.

The core loss remains unchanged and the total loss is less than the maximum power dissipation. This improved design resulted in an acceptable overall temperature rise level. Litz’s wire flyback transformer design was tested in an energy storage battery charger application development board.

Conclusion

Tests performed on the various flyback transformer designs shown demonstrate that the incorporation of Litz wire provided the greatest reduction in copper losses and reduced the overall temperature and temperature rise of the final design to near levels. specifications. Bourns’ tests and measurements also showed that the use of Litz wire produced lower AC resistance than conventional wire and was superior to any other method applied to reduce AC resistance. This has resulted in reduced power loss for magnetic design iterations and is directly related to increased efficiency of the total power circuit. Bourns offers extensive custom transformer design capabilities that include ferrite cores and Litz wire construction.


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Rosemary C. Kearney