The synchronous rectification method uses synchronized MOSFETs in the DC/DC converter, instead of freewheeling diodes (Schottky barrier diode: SBD) that are used to prevent backfeeding (reverse flow) of the electric current.
The driver composition and arrangement is one element that greatly controls the performance, particularly the efficiency, of the DC/DC converter.
We explain how the synchronous rectification method, which is an essential technique for increasing the efficiency of the DC/DC converter, differs from other methods.
1. Non-synchronous type
This combines the Pch FET and SBD (Schottky barrier diode).The efficiency is inferior because of the power loss due to the forward direction voltage drop of the SBD. However, it has the merit that the DC/DC converter internal control circuit is simple to make.
The SBD is external, so the number of components increases compared to the synchronous rectification method.
2. Synchronous rectification method
This method synchronizes the MOSFETs and uses them instead of the SBD of the freewheeling diode.Since there is no diode VF power loss, this method is able to achieve high efficiency both when the output current is high and when it is low.
In the case of a step-down DC/DC converter, a Pch is used for the High Side driver FET, while an Nch FET is used for the Low Side driver FET.
3. Bootstrap method
In the synchronous rectification method, this method uses an Nch FET for the driver for both the High Side and the Low Side.This method has the merit that driver size can be reduced compared to the synchronous rectification method, because an Nch FET is used on the High Side.Due to this, it is possible to achieve a lower on resistance with the same size, or increase compactness or lower costs if the performance is equivalent.
However, to drive the Nch FET gate on the High Side, a bootstrap capacitance is required since the voltage needs to be at least as high as the VIN voltage.
| Driver structure | Main switch | Backfeeding element | Characteristics | Efficiency | Number of components |
|---|---|---|---|---|---|
| Non-synchronous type | Pch FET | SBD | Control circuit simple | Medium | Normal |
| Synchronous type | Pch FET | Nch FET | High efficiency, large current handling, multiple controls required | High | Few |
| Bootstrap type | Nch FET | Nch FET | High efficiency, large current handling, multiple controls required | Extremely high | Normal |
Joined Torex Semiconductor in 2005.
After six years engaged in the development of power ICs such as DC/DC converters, worked for three years providing technical support for offline LED drivers.
Later took charge of technical support for Torex’s power IC customers.
With expertise gained through product development and extensive know-how accumulated from years of customer support — including solutions for power-related issues and troubleshooting — currently engaged in providing technical support and product proposals for Torex products.