SLUSB72D March 2013 – April 2021 UCD3138064
The UCD3138064 has built in logic for optimizing the performance of the synchronous rectifier MOSFETs. This comes in two forms:
When starting up a power supply, It is not uncommon for there to already be a voltage present on the output – this is called pre-bias. It can be very difficult to calculate the ideal synchronous rectifier MOSFET on-time for this case. If it is not calculated correctly, it may pull down the pre-bias voltage, causing the power supply to sink current. To avoid this, the synchronous rectifier MOSFETs are not turned on until after the power supply has ramped up to the nominal output voltage. The synchronous rectifier MOSFETs are then turned on slowly in order to avoid an output voltage glitch. The synchronous rectifier MOSFET ramp logic can be used to turn them on at a rate well below the bandwidth of the filter.
In discontinuous mode, the ideal on-time for the synchronous rectifier MOSFETs is a function of Vin, Vout, and the primary side duty cycle (D). The IDE logic in the UCD3138064 takes Vin and Vout data from the firmware and combines it with D data from the filter hardware. It uses this information to calculate the ideal on-time for the synchronous rectifier MOSFETs.