Flyback Transformer Specification Made Easy with Power Stage Designer

It is widely accepted that the popular isolated topology is the flyback. Technology has developed vast amounts in the last few years, meaning that today's modern flyback controllers are able to modulate switching frequency and primary peak current. Historically, this kind of modulation technique needed a custom-made transformer, which often complicates design and development. In this article, Texas Instruments’ outlines the key methods that can help speed up and simplify the process.

 

Flux

Let’s begin with Flux. The current that flows through the windings of a transformer produces a magnetomotive force (MMF) that generates a magnetic flux inside the core, which is invariably limited by the reluctance of the core. Unfortunately, low core reluctance leads to a high magnetic flux, which can push the transformer into saturation. A minimum number of primary windings is needed to limit the flux, while an air gap can be used to further reduce the flux.

 

Skin and Proximity Effect

When AC current flows through a wire, the current flow in the outer regions of the wire’s cross-section is larger compared to the center, because eddy currents reduce the current flow in the center. As the frequency increases, the current is pushed towards the surface of the wire — a phenomenon is called “skin effect.” Another similar anomaly is called the proximity effect, which occurs when there are windings side by side. The skin and proximity effects increase the AC resistance of the winding, reducing the AC resistance but increasing the DC resistance.

 

Figure 1: The Power Stage Designer (PSD) tool from Texas Instruments

 

Leakage Inductance

Leakage inductance is caused by a magnetic flux that is unable to couple with other windings. Though there are ways to avoid this, it complicates the design process.

Typically, a power supply designer works in collaboration with a transformer manufacturer, meaning the designer doesn’t usually need to consider skin, proximity or the parasitic effects of the transformer. The transformer manufacturer has to remedy these concerns with solutions including fill factor, current density, windows-area product, air gap to provide the best transformer structure, core, and bobbin for the application.

Before anything else, the design engineer must provide the manufacturer with a light version of the specification, an outline of which can be found below:

• Turns ratio
• Primary inductance
• Leakage inductance
• Volt second product
• Frequency range
• RMS and peak currents
• Safety, isolation requirements
• Preferred winding structure.

The Power Stage Designer (PSD) tool from Texas Instruments can be used to create mini specs like the one above. The main creation window of the PSD tool displays different topologies. After clicking on the Quasi-resonant flyback picture, a new window displays the schematic with a set of input fields and various output values. The parameters (e.g. input voltage, output voltage, load current) of the power supply specification can be entered here.

 

Turns Ratio

The turns ratio from the primary winding to the secondary winding defines the reflected voltage. It influences the valley of the switchnode resonant ringing, the maximum Mosfet drain-source voltage, and the RMS and peak currents.

Choosing the optimal turns-ratio for a specific application is an iterative process. The PSD tool is useful here, as it displays all values when changing the turns ratio. A good turns ratio starting point is to choose a value that results in a flyback voltage that is equal to the minimum input voltage.

 

Primary inductance

There are two kinds of flyback modulating techniques with variable frequency. The first, a valley switching technique, modulates the switching frequency while simultaneously keeping the primary peak current constant. The controller always operates in discontinuous conduction mode (DCM).

The second, a quasi-resonant technique, modulates the switching frequency and the primary peak current simultaneously in order to switch on the first valley of the resonant ringing, which occurs just after the demagnetizing time. The controller operates at the boundary between CCM and DCM, which is sometimes called transition mode. You can choose both techniques in the PSD Tool (Figure 1).

A good starting point when specifying the primary inductance is to choose a primary inductance that allows the controller to operate in transition mode for a full load and minimum input voltage.

Therefore, you should choose the “Quasi-Resonant” operation and select minimum input voltage Vin_min. Typically, the datasheet of a controller shows the Control Law Profile. Figure 2 shows an example of the UCC28742 datasheet.

 

Figure 2: Example of the UCC28742 datasheet

 

As seen above, the controller is operating with a valley switching technique (from medium to full output power, Vcl > 3.2V), because it varies the switching frequency while keeping the primary peak current (I_pp) constant.

Now select “Frequency Modulation” in the PSD tool and then enter the primary peak current Ipp. Take the value “Ipp_QR” (from Quasi-Resonant operation at minimum input voltage), which will typically provide a solid starting point. For fine-tuning, the primary inductance and primary peak current can be changed iteratively. Always check that all voltages, currents and the switching frequency are within the limits of the controller and power stage (for Vin_min and Vin_max).

The next important check is to ensure that the minimum ON-time of the controller won’t be violated. Take the primary peak current Ipp_min for light load from the datasheet or the control law (note it’s typically much smaller than Ipp_max) and enter this value in the PSD, checking the minimum on-time for Vin_max. The on-time must be larger than the minimum ONt-time of the controller. If this isn’t the case, it means the primary inductance must be increased.

 

Maximum Leakage Inductance

The energy stored in the leakage inductance is typically dissipated in a snubber network and, therefore, decreases efficiency. Hence, it’s vital the leakage inductance be as small as possible.

 

Volt-second Product and Switching Frequency Range 

The next point is the volt-second product. Based on equation 1, the minimum number of primary windings to prevent saturation of the core can be identified. The volt-second product is simply the maximum primary voltage times the maximum on-time of the controller. The maximum primary voltage, on-time, and the maximum switching frequency can easily be calculated by the PDS tool.

 

$$Primary\_Windings_{min} = \frac{VoltSeconds\_max}{A_e \cdot B_s}$$

 

A typical value for the flux Bs = 300mT if a ferrite core is used. This value can be reduced to 200mT for applications that need a higher margin. The effective core area is defined by the given core.

The switching frequency range shows the PSD tool. Note the values for minimum and maximum input voltage.

 

RMS and Peak Currents

The PDS tool provides the maximum peak currents, as well as the root, mean square currents of the primary and secondary side.

 

Winding Structure

Typically, modern flyback controllers need a good coupling of the auxiliary winding, mainly because the controller gathers information from this winding that’s needed for stable operation. A recommended winding structure is called the “sandwich” technique. Here, the primary is split with the auxiliary bias and the second layer is sandwiched in between, resulting in an effective coupling of the windings.

 

The Power Stage Designer

The transformer is the most important component besides the controller and plays a critical role in driving performance. Because the complex specification must be carried out so carefully, the PSD tool is extremely helpful in avoiding mistakes early on in the design stage. It speeds up the process and serves as a quick and effective sanity check.

Power Stage Designer calculates the voltages and currents of 20 other topologies, and it also contains a Bode plotting tool and toolbox with various functions to make a power supply design easier. Texas Instruments provides this tool for free.

 

About the Author

Florian Müller was born in Rosenheim, Germany, in 1976. He received his degree in electrical engineering from the University of Haag. After working for several years as a freelancer in the field of electrical engineering, he joined TI in 2011 and is working in the European Power Design Services Group, based in Freising, Germany. His design activity includes isolated and non-isolated DC/DC and AC/DC converters for all application segments

 

This article originally appeared in the Bodo’s Power Systems magazine.