High isolation voltage flyback transformer - Part 2

Edgar C. Taculog, TT Electronics


Transformer design and performance

Part 1 of this article provided an overview of TT Electronics’ AEC Q200 qualified and certified transformers, which are optimized for use with a gate drive optocoupler to improve and simplify the design of isolated power supplies. This introduction looked at the theory of operation and how this impacts on key design parameters such as primary, secondary and leakage inductance, and considerations of turns ratios and winding techniques.

This part aims to understand how this theory translates into practice, covering various aspects of actual transformer design and performance. It also provides an insight into the AEC-Q200 certification requirements for stress testing passive components.

Transformer design and performance

HA00-10043ALFTR and HA00-14013LFTR are flyback, step-up transformers for DC-DC conversion with a high isolation voltage between primary and secondary. One of the basic design criteria is selecting a core material with the right characteristics, each of which impacts on cost, size, frequency performance and efficiency.

Other constraints relate to the volume occupied by the transformers, and weight, since weight minimization is an important goal in today's electronics. Finally, overall cost effectiveness is always an important consideration.

Perhaps not surprisingly, transformer efficiency, regulation, and heat dissipation are all interrelated. The minimum size of a transformer is basically determined either by a temperature rise limit, or by allowable voltage regulation, assuming that size and weight are to be minimized. Accordingly, the power handling capability of the core is related to WaAc, the product of the window area and core area.

The following criteria are also important parameters when designing magnetic components.

1-Inductance – Amount of magnetic field for a given current.

2- Inductance with DC Bias – Amount of inductance with DC current injected into the supply voltage applied to the magnetic component.

3-Leakage Inductance – Amount of inductance lost due to poor coupling, which is considered a circuit loss as good coupling is required for this application.

4-Saturation or Rated Current – The current level at which the inductance is reduced by 30%.

5-Heating Current – The current level at which the temperature of the magnetic component                   increases by 40° Celsius (50° Celsius for some products).

6-Direct Current Resistance – The resistance of the wire in an inductor coil.

Experience comes into play when dealing with problems related to losses. Transformer losses are limited by the temperature rise of the core surface at the center of the windings at the point when the temperature rise (°C) equals thermal resistance (°C/Watt) times power loss (Watts).

As stated above, the appropriate core size for the application is the smallest core that will handle the required power with losses that are acceptable in terms of transformer temperature rise or power supply efficiency. In consumer or industrial applications, a transformer temperature rise of 40-50°C may be acceptable, resulting in a maximum internal temperature of 100°C.

Temperature rise depends not only upon transformer losses, but also upon the thermal resistance (°C/Watt) from the external ambient to the central hot spot. Thermal resistance is a key parameter, but very difficult to define with a reasonable degree of accuracy. It has two main components, internal thermal resistance between the heat sources (core and windings) and the transformer surface, and the external thermal resistance from the surface to the external ambient. Internal thermal resistance depends greatly upon the physical construction.

It is difficult to quantify because the heat sources are distributed throughout the transformer. Internal resistance from surface to internal hot spot is not relevant because very little heat is actually generated at that point. Heat generated within the winding is distributed from the surface to the internal core. Although copper has very low thermal resistance, electrical insulation and voids raises the thermal resistance within the winding.

This is a design area where expertise and experience is very helpful, especially in minimizing and crudely quantifying thermal resistance. In the final analysis, an operational check must be conducted with a thermocouple at the hot spot near the middle of the transformer, with the transformer mounted in a power supply prototype or evaluation board. Transformer losses should be examined under the worst-case conditions that the power supply is expected to operate at over long periods of time, not under transient conditions.

Ferrites inherently have a higher resistivity, which is conducive to their use in high frequency applications that result in lower eddy current losses. However, their permeability is generally lower, resulting in a greater magnetizing current, which must be dealt with snubbers and clamps in the end customer’s board design. Core size can be determined by a number of widely used methods, most are variations on the ‘area product’ obtained by multiplying the core’s magnetic cross-section area by the window area available for the winding.

However, there are many variables involved in estimating the appropriate core size. Also, a core’s power handling capability does not scale linearly with area product or with core volume. The thermal environment is equally difficult to evaluate accurately, whether with forced air or natural convection.

Some core manufacturers no longer provide area product information on their data sheets, often substituting their own methodology to make an initial core size choice for various applications.

What is AEC-Q200?

AEC-Q200 qualification is the global standard for stress resistance that all passive electronic components must meet if they are intended for use within the automotive industry. The standard covers a range of applications that require automotive qualified components as shown in Table 1. Parts are deemed to be "AEC-Q200 qualified" if they have passed a stringent suite of stress tests defined by the standard. 

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Table 1: Stress Qualification for Passive Components

AEC-Q200-D splits the level of qualification required for different parts of the industry into five grades, numbered 0 – 4 as tabled above. Grade 0 is the most stringent, requiring testing throughout the -50°C to +150°C temperature range. Components graded to this level can be used in any application throughout the automotive industry, regardless of location within the vehicle.

The level of testing required then decreases through the grades, grade 1 parts that are suitable for most under-the-hood uses are required to be tested through the -40 to +125°C temperature range, grade 2 parts are less stringently tested and are suitable for use in hot spots within the passenger compartments, grade 3 parts are for use within most of the passenger compartment, while finally grade 4 is the qualification grade used for non automotive parts.

Looking forward
TT Electronics’ AEC-Q200 certified HA00-10043ALFTR and HA00-14013LFTR high voltage isolation transformers offer a 10% higher saturation capability and 22% improvement in leakage inductance compared to competitive solutions (see Figure 1). And through its field application engineers, combining a better leakage signature with a higher saturation capability, applications using these transformers will have a reduced time to market with a faster turn-around time for design and development.

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Figure 8: Bias Inductance