APR347

Document number: DS41207 Rev. 5 - 2

5 of 9

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December 2020

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APR347

Synchronous Rectification Principle Description

SR MOSFET Turn on

The APR347 determines the synchronous rectification MOSFET turn-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 the parasitic parameter, the voltage on the

MOSFET drain pin may have a moderate voltage ringing at this moment, which may impact the VDET voltage and result in a turning-off fault. To

avoid this fault situation 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 the timing diagram shown in Figure 1.

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

threshold voltage VTHOFF, the APR347 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 APR347 in DCM

Minimum On/Off Time

When the controlled MOSFET gate is turned on, some ringing noise is generated. The minimum on-time timer blanks the VTHOFF comparator,

keeping the controlled MOSFET on for at least the minimum on time. During the minimum on time, the turn off threshold is totally blanked.

After the SR driver turns off, the SR control block initiates a minimum off-time timer, during which the SR will remain off to avoid the ringing from

turning on the synchronous MOSFET.