Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

Changes in power supply design are rarely revolutionary. Rather, they are evolutionary and dependent upon a host of component and manufacturing technologies that, for the most part, develop at a modest pace. Moore’s law simply doesn’t apply to power supply design; if it did, a 200 W switcher the size of a thumbnail would have arrived some time ago. However, even marginally improved versions of some critical omponents can have a dramatic affect on power supply size. In this article we take a look at the technologies that impact the design of 50 W to 200 W AC/DC switchers and the contribution each makes to the overall size of the end product. In doing so, I aim to give some guidance to engineers who design power supplies in-house but also to help engineers who specify power supplies from external sources to better understand the solutions they are offered and the potential specification trade-offs that they may face.

AC/DC basic topologies

Broadly speaking, switchers up to 150 Watts tend to use flyback topologies in which the energy is only transferred to the load during the off-time of the switching element. Above this rating, typically forward converters are adopted. In a flyback, the converter can operate in two states: continuous, where the input inductor current does not start at zero at the beginning of the cycle, and discontinuous, where the inductor current starts at zero at the beginning of each cycle. Both topologies use a transformer to provide isolation. In the case of the flyback converter there is only one main energy storage magnetic device, the transformer.  In the forward converter there are two, the main transformer and output inductor. Figure 1 shows isolated flyback and forward converter topologies. The largest individual components in such power supplies will be the energy storage capacitors and magnetic components.

Switching frequency

Higher switching frequencies enable smaller inductors and capacitors to be used. At relatively high frequencies, less energy per cycle needs to be stored in inductors and capacitors, resulting in lower inductance and lower capacitance values. The trade-off is that switching losses increase with frequency, leading to decreased efficiency. So the practical limit for flyback converters, where energy is stored in the primary of the transformer every cycle, is 100 kHz. In forward converters energy is not stored in the main transformer but in the output inductor and practical switching frequencies are up to 200 kHz. Of course, low voltage DC/DC converters often operate at 2-4 times these frequencies simply because the relatively close turns ratio of transformers means smaller windings, lower leakage and lower losses, so it’s easier to achieve efficient power conversion.

Magnetic components show future promise

From a power supply transformer perspective, the objective is to find a low-cost core material with enough inductance to store energy, one that does its job with acceptable temperature rise and does not create excessive electromagnetic interference. Powdered iron cores offer the best flux density and magnetic coupling, but are relatively lossy due to the inherent distributed air gaps. Ferrites are efficient, but they are usually too easily saturated to be useful where high energy levels are involved. Gapping the core, with either EI or EE construction, reduces the saturation problem to some degree.

Recent developments in composite cores can help to overcome the disadvantages of both traditional types. For example, in a standard 2425-size core, when one half of the magnetic path length of an iron powder core set is replaced with soft ferrite, losses can be reduced by around 50%. By replacing even more of the iron, reductions of over 60% are possible. Figure 2 shows the core loss vs. flux density for such a core using iron powder, soft ferrite, and a composite material. Clearly, composites can deliver low losses and high flux density and, by varying their composition, the optimum performance characteristics for a given application can be attained.

Changes in power supply design are rarely revolutionary. Rather, they are evolutionary and dependent upon a host of component and manufacturing technologies that, for the most part, develop at a modest pace. Moore’s law simply doesn’t apply to power supply design; if it did, a 200 W switcher the size of a thumbnail would have arrived some time ago. However, even marginally improved versions of some critical components can have a dramatic affect on power supply size. In this article we take a look at the technologies that impact the design of 50 W to 200 W AC/DC switchers and the contribution each makes to the overall size of the end product. In doing so, I aim to give some guidance to engineers who design power supplies in-house but also to help engineers who specify power supplies from external sources to better understand the solutions they are offered and the potential specification trade-offs that they may face.

Developments in composite magnetic materials will have significant impact on power supply design over the next few years, their take-up being limited only by availability and price. As with most new technologies, availability of composite cores is restricted at present to relatively few suppliers.