Using Synchronous Rectifier in Boost Converter


Thread Starter


I know that buck converters take well to synchronous rectification with high load applications. And occasionally low Vout.

However, is there any use in a synchronous BOOST converter?

Your thoughts would be much appreciated.
Synchronous diodes are good where the output voltage is low and the current is high. If you are bucking to make 1.8V and the diode has a loss of 0.7V a synchronous diode really helps. example; You need to make 2.5V and loose 0.7V to get 1.8V so about 1/3 of the power is lost in a diode. A buck can leave the diode on for a high percentage of the time.

In a boost, say 24V to 200V the 0.7V loss in a diode is of little importance. (0.35% of the power is lost in the diode) In a boost the output current is usually small. The diode is on for a very small percent of the time.
In my experience, you could see a much higher efficiency. However, the drawback tends to be that the high output voltage outweighs the potentially high efficiency.
Something you should definitely think about .... is there enough dead time between each MOSFET drive to avoid cross-conduction?

And, would the synchronous MOSFET have to have a diode emulation mode?
Do you have any specific needs when it comes to package type? switching speed? Rds-ON?
The Rds on needs be low enough that the voltage drop across the MOSFET is much lower than one diode drop. (0.7V) If the voltage drop (current resistance) approaches what a diode does then it is a waste of time.
In addition, if we already have a schottky diode as the freewheeling diode (not the normal PN junction diode).

What would we usually need to take into consideration if we tried to replace the SBD with a MOSFET?


It would be most important to consider —

Switching speeds
Specified currents and voltages
Also, cost, packaging, what's available