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How to maximise efficiency in flyback power supplies

Posted: 04 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power-supply? rectifier? phone charger? ideal diode emulators? AC/DC?

Above is a schematic for a two-switch flyback. During the on time, both switches turn on simultaneously to store energy in the primary winding. During the demagnetisation time, the leakage energy is recycled back to the input through D2 and D6, while the magnetizing energy is delivered to the output through the secondary winding. With a two-switch flyback, the reflected output voltage must be less than the minimum input voltage, and the duty cycle is limited to 50%. This imposes some restrictions on the transformer turns ratio and inductance, and may result in higher voltage stress on the output rectifier.

Figure 3: You can easily implement a two-switch flyback with SR using conventional controllers. (Click on image to enlarge)

The above image describes how a 10W phone charger might look if implemented with a two-switch flyback using an SR with an IDE driver. A second high-voltage switch replaces the leakage clamp. This switch requires an isolated gate drive. Recovering the leakage energy is offset in part by the increased conduction losses of two switches conducting at the same time. However, the Vds of both switches are clamped to the input voltage, so they only need to be rated for 400V.

Below is the resulting efficiency improvements of these techniques. The largest gain comes from implementing the SR. This reduces the losses at maximum load by about 750 mW and improves efficiency from just below 84% to over 88%. Adding a two-switch flyback to this design picks up another 1.5% and pushes the maximum load efficiency to over 90%.

Figure 4: Implementing a two-switch flyback with SR can achieve up to 90% efficiency. (Click on image to enlarge)

The AC/DC power adapter continues to evolve, largely due to government energy-conservation programs. Smartphone manufacturers and IC suppliers are constantly searching for simple, cost-effective techniques to make modest gains in efficiency. Implementing SR with IDE control is an easy and low-cost way to significantly reduce overall power loss. Recovering the leakage energy is another way to gain another 1 C 2% in efficiency, but requires added complexity and cost.

About the author
Brian King is an applications engineer in the Texas Instruments Power group and a senior member if its technical staff. King is a member of IEEE and holds bachelor's and master's degrees in electrical engineering from the University of Arkansas.

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