By: Lawrence Meares
Intusoft
Today's applications for magnetic design runs a wide spectrum, including
high power transformers for aerospace and industrial power design, subminiature
planar magnetics for communications and signal processing, 400 Hz transformers
present in aviation, and flyback design for switched-mode power supplies
used in a gamut of today's electronic products.
But designing magnetics can be time consuming using traditional paper
and lab techniques. Luckily, CAE tools take much of the computational
burden and time away from the design of magnetic devices. Classical design
automation methods often rely on home-grown software programs that are
somewhat patched together. Alternatively, lab "build and test"
iterations are still commonplace, which are exercised until the appropriate
behavior of the prototype component is achieved.
Today, a strong magnetic design application is different than former finite
element analysis (FEA) programs. It can select the core geometry then
synthesize a winding structure for you, which dramatically increases design
throughput. Tedious drawings and accountability for complex physical design
characteristics are virtually eliminated. In fact, all that needs specifying
with a good magnetic design tool is core family, core material, type of
wire (i.e., formvar, square, Litz, foil, pcb trace, etc.) and each winding's
electrical characteristics. In the end, single phase, layer and sector
wound inductors and transformers, ranging from 10 Hz to more than 5 MHz,
can be synthesized with very high degree of accuracy.
The software tool we're examining for magnetic device design accounts
for both analysis and synthesis of transformers and inductors. In short,
the user selects his or her design parameters such as wire size and type,
number of strands, inductance, insulation thickness and related aspects.
Figures 1 and 2 illustrate a collection of design criteria that are quickly
entered into the form. Then, characteristics such as peak and AC flux
density, DC and high-frequency AC resistance, core and copper losses,
leakage inductance and winding capacitance are predicted. Once finished,
a summary of cores that were optimized is listed, as are suggestions on
how to optionally improve the design if specified constraints were not
met.
Figure 1: The Bobbin Screen is used to define bobbin dimensions for
layer or sector wound devices.
Figure 2: Transformer Design Screen provides trade off of more than 20 different design parameters. The software automatically flags design constraint violations.
Flyback Magnetic Design Example
Let's put the software tool to work by examining a flyback
transformer design. Note that although called a transformer, the principal
magnetic component in a flyback regulator (Figure 3) is actually an inductor
because the energy for the output is stored in the inductor.
Our first step is to determine the appropriate specifications for the
device. When flyback regulators operate in the discontinuous conduction
mode (DCM) using peak current sensing, they have excellent power line
noise rejection and the turn-on switching stress is small. Noise rejection
comes from the fact that the power delivered is 1/2*Lp*Ip2*F, which is
not dependant on line voltage, except at start-up when the regulator transitions
through the continuous conduction mode (CCM). Each switching cycle in
DCM begins with no energy stored in the flyback inductor so that it is
not necessary to commutate the rectifier diode; hence, reverse recovery
generated losses are not present.
Figure 3 Flyback Regulator including basic V/I waveforms
During the "on" time of the switch, S1, energy is stored in
the inductor. During its "off" time, energy is released to the
output.
The required parameters for the inductive flyback design are: core type
and material, operating frequency, Edt(volt-second capacity of the primary),
primary and secondary (AC and DC) currents, required inductance, and peak
primary current.
The type of core we will select is the EI-Ferrite from TDK, chosen from
a database of more than 7,000 cores and dozens of materials from this
computer-aided engineering tool.
The solution of the equations shown in Figure 3 requires calculations
that are tedious and error prone. The tool we are using simplifies the
work by providing an SMPS wizard that converts the power supply requirements
into the data required for the flyback transformer design. The SMPS wizard
inputs, and the resultant spreadsheet entries, are shown next in Figure
4.
Figure 4: Inductor Screen with parametric entries
Setting the Percent Ripple to 100 allows independent control of Primary
and Secondary Duty Ratio, operating the converter in DCM.
When the Apply button is pressed, the data is transferred to the spreadsheet
and a design is started. The software performs thousands of calculations.
It tries different stranding arrangements, various wire sizes and so forth.
These iterations are performed in an effort to generate a design with
minimum power loss and in the smallest core possible to still meet the
fill and temperature rise specifications. A list of cores tried, along
with their respective fills and temperature rises, are provided as shown
in Figure 5.
Figure 5, History of trials
The last geometry is the one that the program finally settled on. The total time to calculate full design specifications is under 1 second, thus encouraging the user to explore many different options.
Using the Litz wire recommendation could reduce the increased AC resistance in the secondary; however, Litz wire is costly in manufacturing. The program default is to choose up to 2 strands per winding. These strands are considered to be planar; that is, they aren't twisted. Using multiple strands gives an effect somewhat like using Litz wire, but without incurring the added manufacturing cost. Changing the Max Strands constraint from 2 to 4 resulted in an improved design. Before accepting the design, a field solution will provide greater accuracy and will account for gap field induced eddy currents. To see these losses, the "apply using fields button" is pressed. The temperatures rise changes very little (about 0.5 Deg C.) and the winding stack shown in Figure 6 appears to be acceptable.
Figure 6, The winding stack
The primary is next to the gap and the field configuration shown is for
the secondary conducting with no primary current. The highest intensity
field is black so you can see the region just above the gap where the
gap-induced eddy current losses are highest. The field simulation used
here is not an FEA, but a special solution of Maxwell's equations first
described by Bennet and Larson[1] in 1940. Later on Dowel[2] and others
produced similar results, but with more complex formulations. The trick
is to mathematically replace each round wire turn with a rectangular wire
that is rotated in the field so that the boundary conditions of the solution
are satisfied. Once that is accomplished the Fr or ratio of AC/DC resistance
can be calculated for each turn. Removing the effective surface winding
that surrounds the core reveals a gap field winding that simulates gap-induced
fields. The whole process takes about a second to compute for this design.
That's too long for the thousands of trials that lead up to the design
creation; however, it's much faster than an FEA and the accuracy is acceptable.
The finished design summary is shown in Figure 7.
Figure 7, the final design
Accuracy in magnetic models is overwhelmingly determined by manufacturing process controls. The wire can be stretched by as much as ½ a gauge from tension in the winding apparatus. The Insulation can be compressed considerably, affecting electrical and thermal properties and variations in the thermal path to the ambient environment. These factors can cause the predicted temperature rise to vary by an order of magnitude. While each of these parameters can be controlled in the design spreadsheet, it's up to the user to input the correct data for the manufacturing process. The need for a prototype cannot be eliminated if this data is inaccurate.
One of the characteristics of high frequency designs using regular magnet wire (Heavy Formvar in this case) is that the window will be under filled to achieve the lowest overall power dissipation. That's because increasing the size of the wire becomes counter productive due to rising AC resistance. However, the requirement to keep the windings away from the air-gap mitigates this effect.
At this point, one could explore the design further by splitting windings
or trying different materials. A winding sheet and design summary report
can be printed out or copied to Microsoft Excel.
Figure 8 illustrates a SPICE model and schematic symbol for the design.
This enables simulation of the device in order to validate the input data.
The SPICE model can be used with any SPICE program.
Figure 8: SPICE Model with Schematic Symbol
Magnetics Combine with Power Supply Templates for SMPS Design
Besides the SMPS Inductor wizard, there is also an SMPS Transformer
wizard for Forward, Push-Pull and Bridge Topologies. Each of these topologies
has a corresponding SPICE simulation template. These templates include
both average and switching models. The SPICE model can simply be pasted
into the switching subcircuit and the template can be scaled to the input/output
voltage, current and frequency. Having done that, a switching simulation
can be run and the key magnetic design parameters can then be extracted
and compared with the values calculated by the SMPS design wizard. This
Spreadsheet vs. simulation check provides the confidence required to commit
to ordering prototype parts. Figure 9 shows the template schematic and
Figure 10 the waveforms and data extracted form the simulation.
Figure 9, the flyback template schematic
Figure 10, Simulation results, Wizard predictions in parenthesis
[1] Bennett and Larson, "Effective Resistance to Alternating Currents
of Multilayer Windings", AIEE Transactions 1940, vol. 59, pages 1010-1017.
[2] P.L. Dowell, "Effects of eddy currents in transformer windings",
Proceedings of the IEEE, col. 113, pp1387-1394, Aug 1966
Backgrounder:
Lawrence Meares is the founder and president of Intusoft, established
in 1985. Intusoft produces a complete line of analog and mixed-signal
design tools, including the Magnetic Designer and ICAP/4 SPICE simulator
products featured in this article. For more information contact Intusoft:
1.310.329.3295: www.intusoft.com.