For Schottky rectification, what are the advantages of synchronous rectification?

For PC power supplies, two structures are usually mentioned: "synchronous rectification" and "Schottky rectification". These two rectification structures are also the mainstream of PC power supplies at present, and their market shares are also described as "evenly divided". Very appropriate. So what advantages can synchronous rectification bring? The energy loss of synchronous rectification mainly comes from the conduction loss of MosFET, and the conduction loss of MosFET is basically determined by its internal resistance. Generally, MosFETs suitable for synchronous rectification have extremely low internal resistance, most of which are only about 5mΩ. Therefore, the same output current is 10A. Schottky rectification may cause a loss of 4W, while synchronous rectification is based on P=I2R. The formula can calculate the loss as 10A*10A*0.005Ω=0.5W. The loss under the same output is only one-eighth of Schottky rectification. Therefore, the conversion efficiency of synchronous rectification is much higher than that of Schottky rectification. On average It can reach 90% or even 95% level.

Most synchronous rectification MosFETs have only 4mΩ to 5mΩ internal resistance.

Compared with Schottky rectification, the difficulty of adjusting synchronous rectification lies in the adjustment of its drive circuit. Therefore, in the early development of PC power supplies, synchronous rectification was generally only used in high-end products as a means of demonstrating the strength of manufacturers. Most mainstream-level products use Schottky rectifiers with simple circuit structures. In recent years, synchronous rectification driver ICs have made great progress, making the control of synchronous rectification no longer a difficult task. Therefore, synchronous rectification has now gradually advanced from the high-end to the mainstream market.

In addition, synchronous rectification has another advantage over Schottky rectification, that is, under light load output, due to the unidirectional conduction of the diode, the output of Schottky rectification will enter the discontinuous mode, and the current waveform will be in an intermittent state, and the voltage will Ringing will occur and high-frequency harmonics will be released; the transistor used in synchronous rectification allows the current to pass in the reverse direction, so the current waveform is continuous, and the circuit can always work in a continuous state, but because the reverse current is output by the capacitor, it is lightly The synchronous rectification circuit under load does not have a significant advantage over Schottky rectification in terms of conversion efficiency, but compared to the latter's disadvantage of releasing high-frequency harmonics, the "defects" of the synchronous rectification circuit under light load are almost negligible. Not counted.

So can synchronous rectification replace Schottky rectification?

Since synchronous rectification has such advantages over Schottky rectification, will synchronous rectification dominate PC power supplies in the future? In fact, unless there is a strong enough force to promote this situation, it will not happen for a long time in the future. Here, everyone doesn’t have to expect to see it. Like the power supply topology, Schottky rectification and synchronous rectification are not technically superior or inferior. They are simply different structures derived from different needs. Each of them has its own applicable fields. Although the current synchronous rectification is mature enough, Schottky diodes still have an absolute advantage in terms of the overall cost of the circuit. After all, the latter does not require a special drive circuit.

Therefore, in some power supply products that are not sensitive to performance, but the overall cost is very tight, which is what we often call "entry-level products" or "price-sensitive products", Schottky rectification still has its place. value. However, it is very difficult for Schottky rectifier to counterattack synchronous rectification. After all, the latter also has insurmountable advantages in electrical performance. Therefore, for mid-to-high-end products with "performance priority", synchronous rectification basically It is already standard equipment, and it is naturally the first choice for players who value power supply performance.