Introduction to the Magnetics Designer

Michael Seeman, Ph.D., February 23, 2018

Many power converters use custom-designed inductors and transformers to meet specific power supply requirements. Eta Designer offers a new tool to help engineers design and optimize custom magnetics in the context of their power converter. Once a core geometry and material are chosen, windings can be defined and arranged within the window, then the core, DC resistance, and AC losses are calculated based on the actual current waveforms of the converter.

Introduction

Simple power converters, like low-power buck and boost converters, can typically utilize off-the-shelf inductors. However, when higher-power custom supplies are considered, often no satisfactory commercially-available solutions exist. Custom magnetics must be utilized to achieve the desired power levels, turns ratios, frequency, power density and efficiency. Analyzing custom magnetics can be difficult; besides the DC resistance and core losses, proximity and fringe-field losses are difficult to calculate and can significantly impact the efficiency of a power converter.

Eta Designer offers a unique tool to easily create and analyze custom magnetics designs for your power converter. This article will discuss the procedures for creating and analyzing custom magnetics in the context of an off-line flyback converter.

Figure 1: Set design variables to worst-case values

We'll start by creating a new flyback converter using the New Converter Wizard. We will simplify the converter by considering a DC input and neglecting the filter, rectifier and bulk capacitor elements of the converter. We'll use an input voltage of 90 to 350 volts, and output voltage of 12 volts at up to 3 amps. To keep things simple, we'll use a switching frequency of 100 kHz.

The Magnetics Designer considers losses at the default operating condition. We'll start by changing the default variable values by choosing Edit Design Variables... from the Simulate menu. We'll now choose the worst-case conditions for the transformer design: minimum input voltage and maximum load, shown in figure 1.

Figure 2: Connect custom transformer to circuit

Next, we'll convert the main transformer to a custom magnetic part. We'll select the transformer in the schematic, then choose Realize as Custom Magnetic from the Library menu. The symbol changes as a function of the transformer's turns ratio. We'll start by creating space around the transformer on the schematic and then re-connect the transformer to the circuit. The circuit is wired correctly when there's a green check mark on the lower-right-hand side of the window, indicating the background simulation has completed successfully. A red exclamation point indicates there's an error with the circuit.

Next, we'll edit the custom transformer by double-clicking it in the schematic. The Magnetics Designer window opens. We'll start by defining the core components. Start by choosing an RM8/I core by clicking Select next to the Geometry section. We'll then choose the 3C92 ferrite material from Ferroxcube. Finally, we'll choose one of the bobbins from the menu to hold our windings. Now, we'll examine the components in the Magnetics Designer window.

Figure 3: Magnetics designer window
The Magnetics Designer Window

The top of the Magnetics Designer window allows you to specify the schematic part designator and provide a name for the transformer. You can also import and export the design from a file. Using the Load and Save As buttons allows you to share transformers between designs and include pre-made models in your power converters.

Next, the left-hand side specifies the construction of the transformer. Start by choosing the core geometry and material for the transformer. If a bobbin is used for a wirewound design, you may select that as well to contain the wires.

The next section specifies the gap and inductance for the transformer design. The gap length, magnetizing inductance, and turns count of a transformer or inductor are interrelated. Specify two out of three of these parameters, and Eta Designer will calculate the remaining value. Finally, you can specify the leakage of the design, or if desired, Eta Designer can estimate the leakage inductance for you based on the winding geometry.

The last section on the left describes the windings on the transformer. While a transformer can have only one primary winding, it can have any number of secondary or auxiliary windings. You can add or delete windings in this section, or edit any winding by double-clicking it in the list.

The right-half of the window has three panels: Basics, Winding Layout and Losses. The Basics panel shows a figure of the core geometry and the schematic symbol of the transformer including the windings.

Figure 4: Winding Layout panel

The next panel, Winding Layout shows a cross-section view of the transformer. The windings are shown in their respective groups, and color-coded based on the calculated AC to DC resistance losses. In this view, the windings can be dragged to re-arrange them in the core window. Hover your mouse over each turn to see the AC to DC winding loss ratio specific to each turn. The lower-half of this panel shows each winding group. You can double-click a group to edit which windings are in that group and the desired turns layout. Finally, you can create tape objects as spacers to separate windings.

Figure 5: Loss panel

The third panel provides the results of the magnetic analysis. The top part of the panel shows the estimated saturation current (or magnetizing current for transformers), and the volt-second capability of the transformer (referred to the primary winding). Next, the panel shows the estimated magnetizing inductance at the given operating current.

The bottom half of the panel analyzes the losses of the magnetic. The core loss is calculated based on the simulated current waveforms of the converter. The DC resistance of each winding is calculated as well as the DC loss. Eta Designer also calculates the skin-depth loss for each winding and simulates the fields within the magnetic structure to determine proximity and fringe-field losses. These AC losses are all grouped together for each winding. Finally, Eta Designer adds these losses together to find the total magnetic loss and also estimates the free-air temperature rise of the magnetic based on its power dissipation and surface area.

Designing the Flyback Transformer
Figure 6: Steady-state waveforms

Now we'll start designing our flyback transformer. Besides choosing the core and material, which we did earlier, the first step is to choose the turns count and magnetizing inductance. We'll exit the Magnetics Designer window and simulate the longest pulse width which occurs at minimum input voltage and maximum load. We'll look at the startup transient to find the peak primary-side current during startup as well as judge the appropriate magnetizing inductance. It appears that the magnetizing inductance is roughly correct: at maximum load, the magnetizing and discharge times add up to the period. The peak current during startup is about 2.3 A.

Figure 7: Secondary Winding

Open the Magnetics Designer again by double-clicking on the transformer. First, we'll set the turns ratio and configure the secondary winding. Edit the secondary winding by double-clicking it in the list on the left. We'll start by setting a turns ratio of 10:1 by first choosing Turns Ratio instead of Turns Count and then entering 0.1 as the value. While we're here, we'll edit the wire configuration.

High voltage power supplies often require reinforced isolation between the primary and secondary for safety. There are two common ways of achieving this: tape can be used to separate the primary and secondary sides, or wire with three layers of high-quality insulation can be used. In this design, we'll use the latter to simplify the construction. The former can be achieved by using the Add Tape button in the Winding Layout window. We'll select Triple PFA from the Insulation Type pop-up menu. As the wire size is set to Auto, the copper area will change automatically to fit into the winding window.

Now we'll adjust the primary turns count to acheive the desired saturation current. Open the Loss panel to view the saturation current. We see that the default value of 30 turns yields an insufficient saturation current. We'll choose a value of 50 turns to yield a 4.64 amp saturation current, giving us sufficient margin and a suitably-low core loss. However, the AC loss looks extremely high; we'll address that next.

Figure 8: Layout arrangment

Now, let's examine the winding layout by clicking on the Winding Layout tab. The Magnetics Designer calculates the AC loss of the transformer based on the winding layout. The windings are initially side-by-side. This yields very high leakage inductance as well as high proximity effect losses. Start by dragging one of the windings next to the other to layer them on top of each other, as shown in figure 8. The leakage inductance is reduced and the power loss goes down significantly. Each wire cross-section is color-coded based on the ratio of AC to DC loss in that turn: yellow is best while magenta is worst. In this design, the gap is fairly large and contributes a signficant fringing-field loss to the adjacent windings. Increasing the magnetizing inductance and using parallel strands or Litz wire can help reduce this loss.

Figure 9: Adding an auxiliary winding

Finally, we'll add an auxiliary winding to power the primary-side IC. A transformer design can have multiple secondary or auxiliary windings. An auxiliary winding is an output winding, but located on the primary side of the isolation barrier. We'll click the + button in the windings section to add a new winding. We'll choose an auxiliary winding with 5 turns, and select Flip Dot to reverse the polarity of the winding on the schematic, as desired for this flyback design. Since this winding handles little power, we'll choose a small wire size, e.g. AWG 26.

The next step is to position the winding in the layout. Since the auxiliary is small, we'll drag it to the side of the main windings to take up little space. In an actual design, this auxiliary can be wound next to the primary winding.

Analyzing the Converter
Figure 10: Auxiliary winding schematic

Exit the Magnetics Designer. The transformer symbol is updated to reflect the changed turns ratio and the added auxiliary winding. Update the schematic to reconnect the terminals and to add a dummy load to the auxiliary winding, as shown in figure 10. A transient simulation can be run to ensure the auxiliary winding works as expected; the lightly-loaded auxiliary output voltage will be slightly higher than the main output as it is unregulated.

Figure 11: Converter efficiency breakdown including custom magnetic

Finally, we can examine the efficiency of the converter. Select Efficiency Breakdown from the Simulate menu. The transformer losses shown include the core loss, DC resistance loss and AC losses. By varying the input voltage, output current, and any other variable, one can see how the efficiency of the transformer design varies with operating condition. All the powerful analysis tools within Eta Designer can be used to analyze and optimize power converters using custom magnetics.

Conclusion

Eta Designer offers a new Magnetics Designer allowing quick and easy evaluation of custom magnetic structures. Windings can be laid out within a core window in a flexible manner using solid wire, Litz or foil windings. The losses of the inductor or transformer are evaluated based on actual waveforms in the power converter. Finally, an electrostatic simulation is performed to predict the AC losses in the converter windings including proximity and fringe-field effects. Eta Designer provides engineers with the tools to quickly and easily design and analyze custom magnetic structures for their power converters to improve efficiency and power density.

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