UCD3138128 datasheet(56/80 Pages) TI

56 / 80 page

UCD3138128, UCD3138A64

SLUSBZ8A – JUNE 2014 – REVISED SEPTEMBER 2014

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9.6 Synchronous Rectifier MOSFET Ramp And IDE Calculation

The device has built in logic for optimizing the performance of the synchronous rectifier MOSFETs. This comes in

two forms:

•

Synchronous Rectifier MOSFET ramp for softly turning on and off the MOSFETs

•

Ideal Diode Emulation (IDE) calculation

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 UCD3138x 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.

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UCD3138128 Datasheet(PDF) 56 Page - Texas Instruments