When operating power supplies that use synchronous rectification in the secondary-side rectifier circuit in series, special caution is required.

1. Synchronous Rectification

Synchronous Rectification is one of the circuit topology to improve efficiency and generally its topology is used for rectification circuit at secondary side. MOSFETs is used for rectification instead of diode.

2. Behavior in Series Operation When One Power Supply Shuts Down

When one side of power supply shut down in series operation, synchronous rectification MOSFET turns off. Operating power supply continues to generate the output current and output current flows via body-diode of MOSFET in the power supply which is shut down.
The forwarding voltage (Vf) of body-diode in MOSFET is approx. 0.7-1.3V and power loss of MOSFET is higher than normal operation.
To prevent this issue, schottky barrier diode which has low Vf characteristics is recommended to put between output voltage.

For details of the product's circuit topology, please contact us.