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Tool/software:
Many TI ICs support TINA simulations, including the LM5180 Primary Side Regulated (PSR) flyback device and LM5160 Fly-buck device. It is well known that a flyback or fly-back transformer can be easily modeled in Pspice with individual inductors to represent each winding, and a K-factor model instance to couple them. However, it is not so straightforward in TINA, especially when the flyback transformer has more than two windings.
TINA-TI does include a basic coupled inductor model in its part library, but it just has two windings. For many beginners and occasional TINA users, it is not very clear about how to model a multiple winding flyback or fly-buck transformer. In this post, I present a simple TINA behavioral model of a three winding flyback transformer, which can easily be expanded to include more output rails, and an example of updating the model from three windings to four windings is also given. The same approach can be used to model multi-winding fly-buck transformers.
For convenience, let me use the LM5180EVM-DUAL’s transformer as an example. It is a YA8916-BL by Coilcraft, and the transformer drawing is shown in Figure 1. The key parameters are listed in Table 1.
Figure 1. A Three-Winding Flyback Transformer
Table 1. Key Parameters of the Example Transformer
Flyback Transformer Windings |
Pri |
Sec1 |
Sec2 |
Normalized Number of Turns (N---) |
1 |
1 |
0.52 |
Inductance (Lmag) |
30 µH |
|
|
Leakage (Llk) |
565 nH |
|
|
Winding Resistance (R---) |
360 mΩ |
695 mΩ |
392 mΩ |
Since the flyback transformer windings are basically coupled inductors, we can use an inductor to model the primary inductor Lmag, and use the TINA ideal transformers to represent each secondary winding. Specifically,
N2 : N1 = Nsec1 : Npri = 1 : 1 = 1
The winding resistance Rsec1 is placed in series connection with the winding output.
N2 : N1 = Nsec2 : Npri = 0.52 : 1 = 520m
The winding resistance Rsec2 is placed in series connection with the winding output.
Figure 3 is the TINA model of the LM5180EVM-DUAL circuit, modified from LM5180-Q1 TINA-TI Dual Startup 15Vout1 and -8Vout2 Transient Model with the transformer model replaced inside the dashed-line box. Special attention should be paid to the winding polarity configurations in the flyback schematic. In this example, you can see that the primary “winding” is cross-connected to the primary side circuit for the right polarity use.
Figure 2. TINA Behavioral Model of the Example Transformer
Figure 3. TINA Behavioral Model of the LM5180EVM-DUAL Circuit
To add more windings, you can simply parallel more ideal transformers. Let’s show this in an example of adding a 5V output rail to the previous transformer, as shown in Figure 4. For 5Vout, TR3’s turns ratio should be
N2 : N1 = Nsec3 : Npri = 0.33 : 1 = 330m
The additional winding to the model in Figure 2 is highlighted in red as shown in Figure 5, where the winding resistor of Nsec3 is assumed to be Rsec3 = 200 mΩ, and all other parameters are assumed to be the same as listed in Table 1. Following the same approach you can make the behavioral flyback transformer model of any number of windings in TINA.
Figure 4. An Example of 4-winding Flyback Transformer
Figure 5. Updating the Flyback Transformer Model with More Windings
The same approach can also be used to model the multi winding fly-buck transformers. Assign Lmag as the primary inductor, and use the ideal transformer to represent each of the secondary windings. In this way you can expand the fly-buck TINA test bench, like the LM5160 TINA-TI Flyback Reference Design, to simulate designs with more output rails. Again, please make sure the winding polarity is used correctly.