What is secondary-side synchronous rectification?

In a switching power supply, when the output current becomes large, heat generation due to power loss in the secondary side rectifier diode becomes a problem.
Therefore, by changing the rectifier diode to rectification using MOSFETs, heat generation due to power loss in the secondary-side rectifier circuit can be reduced.
Secondary-side synchronous rectification solves the problem of heat generation in the secondary-side rectifier circuit, increasing the efficiency of switching power supplies.

Advantages
Power loss in the secondary rectifier circuit is reduced.
Heat generation in the secondary rectifier circuit is reduced.
The efficiency of the switching power supply is increased.

Disadvantages
The circuit becomes more complex, increasing costs.
Increased noise. (Low side is less advantageous than high side.)
May not operate normally in continuous current mode.
Synchronous rectification control circuits are sensitive and prone to malfunction, so experience and know-how are required for design.

MOSFET current direction
In a synchronous rectifier circuit, the direction of current flowing through the MOSFET is from source to drain, which is the opposite of how it is normally used.
This is because the body diode is used to allow current to flow even when the MOSFET is not turned on.
Placing the MOSFET on the high side requires an auxiliary winding in the transformer, complicating the circuit, so it is more common to place the MOSFET on the low side.

Basic operation of secondary side synchronous rectification

①	When the primary side is turned off, the MOSFET gate is turned on at the maximum negative Vds voltage.
②	When the negative Vds voltage is at the minimum, the MOSFET gate is turned off (about 0V). There is a short period of time when current flows through the body diode.(The gate is not turned on until Vds becomes positive)
③	When the primary side is turned on, Vds becomes positive → Return to ①


Secondary side synchronous rectification malfunction