APR345 datasheet(5/11 Pages) DIODES

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APR345

Document number: DS38010 Rev. 5 - 2

5 of 11

www.diodes.com

June 2016

APR345

Synchronous Rectification Principle Description

SR MOSFET Turn on

APR345 determines the synchronous rectification MOSFET turning on time by monitoring the MOSFET drain-to-source voltage. For both of DCM

and CCM operation, the turning on principle is same. 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. Since of parasitic parameter, the voltage on MOSFET drain pin has moderate voltage ringing at this moment, which maybe

impact on SR controller VDET voltage sense and bring about turning off fault. To avoid fault situation happening, a Minimum On Time (tONMIN)

blanking period is used that will 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 affect. During this time, the VDRISR is pulled

up to VCC; after tONMIN, the drive voltage stops be pulled up by the driver, and begins to drop; when VDRISR drops to VDRI_HOLD, it will be held at this

voltage until be pulled down.

DCM 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, APR345 pulls the drive signal down after a turn-off delay (tDOFF).

I,V

VDET

VDRI_HIGH

VTHON

VTHOFF

tDON

tDOFF

tONMIN

0.9VDRI_HOLD

VDRI_HOLD

Figure 1. Typical Waveforms of APR345 in DCM

CCM Turn off Operation

The CCM pin is used to sense trigger signal for turning off the SR MOSFET before primary switch turning on in Continuous Conduction Mode

(CCM) system. After tONMIN, if the CCM pin voltage rises over the threshold voltage VTHCCM, the drive voltage will be pulled down after a short delay

time tDCCM to turn off SR MOSFET. The CCM pin senses trigger signal coming from primary switch turn on signal through a RC networks circuit, a

–type isolating capacitor C

CCM, two resistors R2 and R3. Note variations of these resistors, of CCCM, and of the dV/dt across CCCM require that

worst-case tolerances be taken into account when determining the minimum value of CCCM. For example, the value of this resistor will impact the

rise time of CCM voltage. The bigger resistor, the slower the CCM voltage rises.

The zener diode ZD1 is used for ESD test.

The value of CY should be much higher than that of CCCM. If necessary, increase the value of CY to ensure that CY >> CCCM; do not decrease CCCM.

CY is the main common-mode capacitance between the primary and the secondary sides of the system. This is usually a discrete component,

whose value ranges from 47pF to 2200pF. Aside from any EMI-control purposes, it also serves as the return path for the CCM signal charging and

discharging current pulses.

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