In the DCDC step-down circuit, there are two working modes of synchronous rectification and asynchronous rectification. The working principles of these two modes are as follows
As can be seen from the above figure, the difference between asynchronous rectification and synchronous rectification is that synchronous rectification uses MOSFET tubes with extremely low on-state resistance instead of diodes. Compared with the diodes used in asynchronous rectification, synchronous rectification greatly reduces losses! But the price is relatively expensive!
The benefits of using synchronous rectification and asynchronous rectification!
When synchronous rectification is used, the loss is reduced and the work efficiency is improved. Correspondingly, the chip is expensive.
Asynchronous diodes have large losses at light loads
For example, the rectifier circuit will pass the switch current, so it will generate loss at the switch, so that the efficiency will be affected. During asynchronous rectification, the positive voltage conduction voltage drop Vf of the diode will pass the current, especially at light load, the diode will be in this state most of the time. Even if it is a Schottky diode, the loss generated on VF when the switching current is 1A will be as high as about 0.3 to 0.5W. Correspondingly, the resistance of a MOSFET with extremely low on-state resistance can be as low as about 50mΩ, and if the loss generated is calculated, it is only 0.05W.
At light load, synchronous rectification works better, and asynchronous mode will generate a lot of noise because it works in discontinuous mode!
The status of synchronous rectification and asynchronous rectification at light load
When the load is light, the load current will be very small and even drop to zero crossing level. In this state, in the asynchronous rectification mode, the diode will work intermittently, and the current can only flow in one direction. At this time, the loss of the diode is reduced instead. However, due to the discontinuous mode of operation, the switching voltage will ring and generate high harmonic noise. In the synchronous rectification mode, when the MOSFET is turned on, the current will flow in two directions. At this time, the synchronous rectification mode works in a continuous mode, and the loss will not be reduced. But the switching voltage does not produce ringing.
The article is reproduced from: He Xuetao Hardware Design