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.

The trend in power supply designs is to adopt smaller, lighter, faster, and greener sources of energy. 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-evolving requirements and power-efficiency goals.

Known for its low circuit component count, the flyback topology depends on essential design and analysis of the coupled inductor or “flyback transformer” to match updated requirements. Because the magnetics are typically the largest component of the circuit, demands for next-generation magnetics 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 about 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.

The flyback topology in magnetics is an energy-storage medium compared to what most know as traditional energy transfer. A flyback transformer doesn’t match the classic definition of a “transformer.”

Its operation is fundamentally that of a highly coupled inductor that is used to “store” energy. In a magnetics design, this is typically accomplished by putting a small air gap somewhere in the core magnetic flux path.

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

Translating circuit operation to an actual design requires a combination of hand calculations, circuit simulators, analytical tools, and the use of finite element analysis (FEA) software. Optimizing finished products is accomplished via various core and coil design iterations as well as the building and measurement of samples to verify the calculation/simulation process.

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

The initial coupled inductor design that follows does not take high-frequency AC coil effects into consideration. The subsequent designs covered incorporate various winding and layering techniques that reduce AC coil resistance for reduced power loss.

Design specifications (Table 1) for a 60-W flyback transformer include five windings: one primary, three secondary, and one auxiliary.

To meet normal safety standards, secondary windings need reinforced insulated wire. An important consideration is that insulated wires can increase the size of the wire by up to 30% because of the wire-coating thickness. 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.

Total DC + AC copper loss is denoted Pcu:

Core loss is found using flux density ripple ΔB from Faraday’s law (Equation 1.1), where Vin is the input voltage, Ac is the core area, D is the duty cycle, T is the switching period, and Np is the number of primary turns:

The core loss Pfe is calculated where Vc is the core’s volume, f is the switching frequency, Bmax is the peak flux density, and Kc, α, and β are Steinmetz parameters derived from core material properties:

For Design 1, the total loss Ptotal equals copper loss plus core loss combined:

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

Ansys thermal simulation estimated a target total temperature rise of 40˚C for both core and coil. 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 coil AC resistance contributes most to copper losses, and core losses are negligible compared with the winding losses.

Design 2 takes fringing effects into consideration by changing the distance between the core gap and the start of the first winding. Fringing effects produce increases in AC resistance as a result of the bulging of magnetic flux around the core gap, as opposed to traveling straight across it.

Distance is increased between core gap and coil using thicker bobbin material or spacer tape before the coil winding begins. This increases the mean length turn and DC resistance as well; nonetheless, greater AC resistance reduction outweighs it.

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

Thermal simulation shows the temperature total as 78˚C, which is a reduction of 20˚C but is still too high based on the target.

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

For example, a winding has current flowing in one direction (positive) where current flowing in the next layer would be negative. The attraction of positive and negative charges alters the distribution of current so that it doesn’t travel uniformly through the conductor and bunches to one side. This influences even AC current, thereby increasing AC resistance.

Decreasing layers reduces AC resistance of the primary winding and secondary winding 1. However, AC resistance increases for secondary windings 2 and 3 because of unavoidable bunching and uneven layering of primary and secondary 1 before the last two winds. These tradeoffs produce marginally better overall loss in Design 3, but it is still too high. Thermal simulation was not performed, as total losses were only slightly improved.

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

Design 4 looks to improve the reductions by taking wire skin effects into account.

A wire carrying an AC current generates an AC field, which produces a cavitating wire effect called eddy currents. Eddy currents inhibit the even distribution of electron current flow in the entire cross-section of a piece of wire. The cavitation effect of eddy currents pushes current (density) to the outside of the wire. Because skin effects are frequency-dependent, higher-frequency AC current (density) won’t protrude as deep into 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 larger-diameter, or gauge, single wire. Multiple wire strands have the capability of reducing AC resistance loss due to the skin effect. Higher-frequency AC currents require more strands of wire.

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

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

The tests performed on the various flyback transformer designs presented demonstrate that incorporating Litz wire delivered the greatest copper loss reduction and decreased the overall temperature and temperature rise of the final design to near specification levels. 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 resulted in reduction of power loss for the magnetic design iterations and directly related to increased efficiency of the total power supply circuit. Bourns offers extensive custom transformer design capabilities that include ferrite cores and Litz wire construction.

Magnetics Components That Match 1,500-V Battery Energy Storage System Requirements

Shane O’Connor is a magnetics application design engineer at Bourns Inc.

Kyle Moldenhauer is a magnetics product applications and marketing engineer at Bourns Inc.