APR34709

Document number: DS41354 Rev. 5 - 3

7 of 13

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June 2022

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APR34709

Synchronous Rectification Principle Description

SR MOSFET Turn On

The APR34709 determines the synchronous rectification MOSFET turning on time by monitoring the MOSFET drain-to-source voltage. When the

drain voltage is lower than the turn-on threshold voltage VTHON, the IC outputs a positive drive voltage after a turn-on delay time (tDON). The

MOSFET will turn on and the current will transfer from the body diode into the MOSFET’s channel. Because of parasitic parameter, the voltage on

MOSFET drain pin has moderate voltage ringing at this moment, which maybe impact on the VDET voltage and resulting in the turning-off fault.

To prevent fault situation from happening, a Minimum On-Time (tONMIN) blanking period is used to maintain the power MOSFET on for a minimum

amount of time.

In Figure 1, the turn on blanking time tONMIN is to prevent the MOSFET drain-to-source voltage ringing effect.

Turn Off Operation

The DCM operation of the SR is described with timing diagram shown in Figure 1.

In the process of drain current decreasing linearly toward zero, the drain-source voltage rises synchronically. When it rises over the turn off

threshold voltage VTHOFF, the APR34709 pulls the drive signal down after a turn-off delay (tDOFF).

I,V

t

t

IS

VDET

VTHON

VTHOFF

tDON

tDOFF

0

0

1V

tONMIN

0.9VDRISR

VDRISR

Figure 1. Typical Waveforms of APR34709 in DCM