Synchronous Rectification vs. Asynchronous Rectification: What's the Difference?

Synchronous Rectification: It utilizes power MOSFETs with low on-resistance to replace the freewheeling diode in asynchronous rectification, reducing the freewheeling loss.

Asynchronous Rectification: It consists of a high-side MOSFET and a freewheeling diode.

So, what are the differences between the two?

Here's how we can differentiate them:

For DC-DC converters with both top and bottom MOSFETs, it's synchronous, while if there's only a top MOSFET and a Schottky diode at the bottom, it's asynchronous.

Asynchronous Rectification: When the step-down ratio is high, the conduction time of the freewheeling diode is long. If Vout is low, the overall loss ratio will increase due to the forward voltage of the freewheeling diode.

For instance, when the input voltage is 5V, the output voltage is 1V, and the oscillation frequency is 1MHz, the current through the diode flows in one direction only, becoming discontinuous operation, followed by ringing.

Synchronous Rectification: During light loads, the inductor current may become 0A, and the current flows back through the MOSFET, maintaining and stabilizing continuous operation.

When the input voltage of the switching power supply is below 5V, and the output current is large, using a Schottky diode for rectification will result in significant power consumption, leading to a substantial decrease in power supply efficiency.

So, what are the advantages and disadvantages of each?

Asynchronous Rectification:

Since the voltage drop across the diode is constant, when the current flowing through it is large, the proportion of power loss on the diode will decrease. This also leads to lower efficiency when the DC-DC operates at high currents, but it's acceptable when the input voltage is high.

This is because, with a high output voltage, the proportion of the forward conduction voltage drop of the diode is small, resulting in a lower impact on efficiency.

Asynchronous rectification belongs to natural rectification. Compared to synchronous rectification, because there's no bottom MOSFET, there's no need for special control circuits for synchronization, making the design simpler.

Synchronous Rectification: The on-resistance (Rdson) of the MOSFET is very small, meaning the voltage drop across the MOSFET after conduction will be very low.

The I-V characteristics difference between MOSFET and diode:

Under the same conditions, the voltage drop during conduction is much smaller for MOSFETs compared to Schottky diodes. Therefore, the power loss of MOSFETs is much smaller than that of diodes.

However, in synchronous rectification, additional drive circuits are needed for both top and bottom MOSFETs to synchronize them. In contrast, asynchronous diodes naturally rectify, requiring no additional drive control circuits. Hence, synchronous circuits are somewhat more complex.

It's important to note that since MOSFETs have switching times, there should be a dead time between the top and bottom MOSFETs. If the turn-off time of the top MOSFET overlaps with the turn-on time of the bottom MOSFET, simultaneous conduction of both MOSFETs will occur, leading to MOSFET damage due to excessive current.