IR11672AS datasheet(13/25 Pages) IRF

13 / 25 page

IR11672AS

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General Description

The IR11672A Smart Rectifier IC can emulate the operation of diode rectifier by properly driving a

Synchronous Rectifier (SR) MOSFET. The direction of the rectified current is sensed by the input comparator

using the power MOSFET RDSon as a shunt resistance and the GATE pin of the MOSFET is driven

accordingly. Internal blanking logic is used to prevent spurious transitions and guarantee operation in

continuous (CCM), discontinuous (DCM) and critical (CrCM) conduction mode.

IR11672A is suitable for Flyback and Resonant Half-Bridge topologies.

VGate

VTH1

VTH2

VTH3

VDS

Figure 1: Input comparator thresholds

Flyback Application

The modes of operation for a Flyback circuit differ mainly for the turn-off phase of the SR switch, while the

turn-on phase of the secondary switch (which corresponds to the turn off of the primary side switch) is

identical.

Turn-on phase

When the conduction phase of the SR FET is initiated, current will start flowing through its body diode,

generating a negative VDS voltage across it. The body diode has generally a much higher voltage drop than the

one caused by the MOSFET on resistance and therefore will trigger the turn-on threshold VTH2.

At that point the IR11672A will drive the gate of MOSFET on which will in turn cause the conduction voltage

VDS to drop down. This drop is usually accompanied by some amount of ringing, that can trigger the input

comparator to turn off; hence,

a Minimum On Time (MOT) blanking period is used that will maintain the

power MOSFET on for a minimum amount of time.

The programmed MOT will limit also the minimum duty cycle of the SR MOSFET and, as a consequence, the

max duty cycle of the primary side switch.

DCM/CrCM Turn-off phase

Once the SR MOSFET has been turned on, it will remain on until the rectified current will decay to the level

where VDS will cross the turn-off threshold VTH1. This will happen differently depending on the mode of

operation.

In DCM the current will cross the threshold with a relatively low dI/dt. Once the threshold is crossed, the

current will start flowing again thru the body diode, causing the VDS voltage to jump negative. Depending on

the amount of residual current, VDS may trigger once again the turn on threshold: for this reason VTH2 is

blanked for a certain amount of time (TBLANK) after VTH1 has been triggered.

The blanking time is internally set. As soon as VDS crosses the positive threshold VTH3 also the blanking time is

terminated and the IC is ready for next conduction cycle.

IR11672AS Datasheet(PDF) 13 Page - International Rectifier