Design of a High Efficiency AC / DC Power Supply

Due to the increase of power density, the density of energy loss is more concentrated. Higher efficiency means lower heat loss. Improving power efficiency is rapidly becoming the only feasible measure to improve power density. In the AC / DC power supply discussed in this paper, more than 80% efficiency can be regarded as high efficiency. Now, some of the commercially available power supplies have achieved 90% efficiency, but these products are aimed at the high-end market.

Efficiency under light load

Previously, efficiency was not a key factor in many designs. The working load is less than 60% for most of the power supply life. The power supply rarely works for a long time under full load (100%). However, the specifications received at the beginning of the design are only given for the full load, so the design is also optimized for the efficiency at full load. Now, manufacturers take the efficiency under light load as the selling point of their design, because it can better reflect the real performance of the power supply. CECP, EPA and other organizations are also studying new regulations on efficiency under light load conditions. New technologies (such as digital control) are being used to improve efficiency over the full load range. Under light load conditions, the switching loss accounts for the main position, while under larger load, the conduction loss accounts for the main part (see Figure 1).

Topology of converter

The topology of the converter is the main factor affecting the overall efficiency of the system. The choice of topology is often inseparable  

Open trade-offs between cost, power consumption, size, switching frequency and efficiency. In the low efficiency design with low power (up to 200W), cost is the biggest factor, and flyback and forward converters are more common. These designs are inefficient because they can only transfer power in half of the switching cycle. In the other half of the switching cycle, the transformer needs to dissipate any energy it stores (leakage inductance). Because this part of energy is wasted, the overall system efficiency is reduced. Because the voltage and current on the switching element are too large, it can not be used in higher power applications.

Half bridge rectifier is an improvement on the method of forward converter (and flyback converter), because it only allows the switch to withstand a voltage stress equal to the DC input voltage, which is half of the stress on the forward converter. The lower voltage on the switch means the reduction of switching loss. It has the advantage of recycling any leakage inductance current (rather than allowing it to dissipate in a buffer circuit), so it improves the efficiency. Full bridge rectifier goes further and can turn on / off more power. From the perspective of efficiency, it is the preferred method because it minimizes the loss of primary coil and maximizes the use of transformer. Compared with half bridge structure, the switching current of full bridge structure is only half of the former. This also means less loss.

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