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Synchronous Rectifier

Synchronous RectifierDesigns with synchronous rectifiers have become wide-spread nowadays. Synchronous rectifier circuits were developed long time ago. Regular n-channel field-effect transistors are used in their designs and substitute rectifier diodes. Channel resistance of field-effect transistors is really small. This allows to significantly reduce voltage drop and correspondingly generation of heat. However, there is one peculiar feature when using field-effect transistors in rectifiers. Field-effect transistors for power electronics are made with a built-in bypass diode. That is why the field-effect transistors have to be turned on in the inverse mode in a synchronous rectifier. So how does the synchronous rectifier work? We shall explain that by the following example.Let's look at the circuit. The operation of a synchronous switch depends on the operation of other controlled switches, specifically on the operation of a switching transistor Q1. The process of connection of a synchronous switch starts from the moment of turning off the Q1 commutating switch. When control voltage across the gate of a switching transistor becomes low then voltage across its drain moves from the level of input voltage to zero. The current continues flowing through the switching transistor till the output capacitor discharges and the parasite diode becomes forward-biased. At this moment the synchronous switch receives all current, and the switching transistor turns off. After that control voltage to the Q1 transistor gate is set, determined by the controller delay. The transistor's opened channel straps the parasite diode. The process of turning off the synchronous switch starts from switching the driver output into a low state. However, the current still continues flowing through the parasite diode in the same direction. When the commuting switch receives a control signal then the current starts switching over to it. Voltage across the synchronous switch drain increases from the ground to supply level only after the current starts flowing through the switching transistor and the parasite diode. The synchronous rectifier capacitor charges quickly. Voltage across the synchronous switch increases at the same rate.