Optimizing Adapter Power Supply Design

Jul 1, 2010 12:00 PM
Jason Sun, Fairchild Semiconductor, China

Today, ac adapters are used to supply dc power to a variety of systems, from laptop computers to many computer peripherals. These power supply designs must consider not just functionality requirements, but also their impact on the environment. For example, they must deliver to their target electronic appliance a standby power consumption and an average efficiency that meets the compulsory requirements specified by regulations such as ENERGYSTAR^®, as well as offering safety guarantees. To design an effective ac adapter power supply, it must comply with strict regulations and also survive in harsh application environments.

The first step in the design is to select a circuit topology for the design (typically flyback, forward, dual switch forward, half bridge resonant or a full bridge). Then, determine whether the selected topology meets the cost requirements. The topology should be as cost-effective as possible, while also meeting the specifications to ensure that the final product has a sharp competitive advantage.

A designer can select from the most commonly used circuit topologies, according to the power range of the design that they are working on. For example, for designs less than 100W in power: flyback is the most common; half-bridge resonant or forward for those less than 300W; dual switch forward for those less than 500W, and full-bridge for those greater than 500W. Once the topology has been decided, the designer must then select a controller and a transformer to match.

Controller selection is an important step in power supply design. Most makers categorized their solutions in terms of the different applications that they target. The designer can select the desired solutions according to the product guide provided by the controller maker. In order to meet time-to-market pressures, most designers like to choose solutions that they are familiar with. This does shorten the development cycle, but it can place limitations on the final design. By ignoring new solutions that are significantly improved in terms of performance, functionality, integration and reliability, designers could well ignore more competitive controllers in favor of earlier iterations. Therefore, controller selection can be the key factor in designing for higher performance, lower cost and a sharper competitive advantage.

CORE SELECTION

To select a transformer for the design, we first determine the type of transformer the design needs using the AP (area product) method. There are many types of transformers available. Figure 1 shows the shapes of the magnetic cores commonly used in transformers.

A number of factors may need to be considered when selecting a transformer. For instance, designer needs to find a transformer bobbin category, and try to select a type that matches the design requirements most, in terms of its dimensions, nodes and node pitch.

The selection of magnetic core material depends on the working frequency and the inductance current mode that the power supply uses. For example, if the frequency is around 50KHz and discontinuous current mode (DCM) is used, a ferrite core is usually the best choice; for designs using continuous current mode (CCM) and requiring an inductor with small current ripples and a small hysteretic lag, the magnetic core should be made of material with higher magnetic saturation (such as Kool-mu and powdered iron), as ferrite is limited in magnetic saturation.

The more widely used transformer bobbins are usually less expensive. On the other hand, there are different types of transformer windings. Transformers using triple insulated wire (TIW) are more expensive than those using enameled wires. For example, PQ, RM and POT have smaller winding space than ER and EE. They must deploy TIW wires in the interests of safe spacing.

It should be noted that a larger couple area between the magnetic core and the winding creates low magnetic leakage. For example, PQ, RM and POT bobbins deliver a better performance in magnetic leakage and EMI than either EE or ER bobbins. However, if the design uses resonant topology, it needs to leverage the magnetic leakage effect. This is why ER bobbins are often used in resonant topology. In the example given in the following article, we will discuss how to select a quasi-resonant DC/DC transformer and DCM PFC inductance for a design.

Once the controller is selected, attention can then be turned to circuit design, taking into account such considerations about electromagnetic interference (EMI), protection capability, and regulation factor and sequence if multiple outputs are required. Once the circuit diagram is completed, the designer then moves to the PCB layout. When creating the PCB layout, great attention should be paid to factors such as the spacing between components, the consequent EMI, component heat dissipation, controller feedback loop, large current loop and grounding route. The PCB layout has a direct influence on both design testing and product performance.

A designer needs to determine what specifications (voltage, current, etc.) the devices to be used in the design will require, first by theoretical calculations and then by experimentation, for each of the function block and then for the whole system.

The test items include voltage regulation factor, voltage ripple, protection functions, inductor saturation status, maximum stress on main switch transistors and the temperature endurance margin of these transistors.

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