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What is Synchronous rectification?

A technique, used in DC-DC converters and elsewhere, that replaces a rectifying or freewheeling diode with a MOSFET turned on at the moments the diode would conduct. The loss changes from the diode's forward voltage × current to the MOSFET's current² × on-resistance; at 10 A, for example, a diode with a 0.5 V forward voltage dissipates 5 W, while a MOSFET with 5 mΩ on-resistance dissipates 0.5 W. The lower the output voltage, the greater the benefit. A dead time is needed so that the two switches are never on at the same time, and during it the body diode carries the current.

Read the beginner explainer: How AC Adapters and Chargers Work: From Wall Outlet to 5 V

Japanese name: 同期整流

Synchronous rectification appears in 4 practice questions across 2 topics, including Power Electronics Circuits, Power semiconductors.

Field: Power Electronics Circuits

Try it: questions about Synchronous rectification

In a DC-DC converter with a low output voltage, which of the following is the main purpose of synchronous rectification, which uses a MOSFET in place of the freewheeling diode?

  1. ATo make the output voltage higher than the input voltage
  2. BTo keep the switching frequency constant automatically even when the load current changes
  3. CTo reduce the conduction loss caused by the diode forward voltage and raise efficiency
Show answer and explanation

Answer: C. To reduce the conduction loss caused by the diode forward voltage and raise efficiency

A diode has a forward voltage of about 0.3 to 0.5 V even if it is a Schottky barrier diode, producing a loss of current × forward voltage. Turning on a MOSFET with a low on-resistance in place of the diode changes the loss to current² × on-resistance, which can reduce it greatly. The lower the output voltage, such as around 1 V, the larger the diode drop relative to the output, so synchronous rectification helps more. In exchange, the switches must be driven with a dead time so that the two are never on at the same time.

Synchronous rectification in the explanations

Questions that mention Synchronous rectification (4)

  1. In a DC-DC converter with a low output voltage, which of the following is the main purpose of synchronous rectification, which uses a MOSFET in place of the freewheeling diode?
  2. What is the parasitic diode that forms structurally inside a power MOSFET called?
  3. In an ideal step-down chopper (buck converter) with an input voltage of 12 V and a switch duty cycle of 0.25, what is the output voltage?
  4. In a step-down chopper (buck converter), which of the following correctly describes the path of the inductor current while the switch is off?

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Related terms

Buck converter DC-DC converter Schottky barrier diode On-resistance Dead time Power MOSFET Body diode Inverter Duty cycle Back EMF

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