Power Electronics Circuits
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?
- ATo make the output voltage higher than the input voltage
- BTo keep the switching frequency constant automatically even when the load current changes
- 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.
- A. Whether the converter steps up depends on the circuit topology, not on synchronous rectification.
- B. The switching frequency is set by the control circuit and has nothing to do with synchronous rectification.
- C. Correct. It replaces the loss from the forward voltage with the small loss from the MOSFET on-resistance.