5

Operation description

SRK1001 (see Figure 1. Internal block diagram) controller is specifically designed for synchronous rectification in

flyback converters operating in QR and mixed CCM/DCM fixed frequency. This IC turns on the SR MOSFET with

a minimum delay when the rectified current starts flowing through the body diode, and turns off the SR MOSFET

when current approaches zero, using an adaptive mechanism that leads the body diode residual conduction time

after turn-off to the target value Tdiode_off (refer to Table 4 for the various parameter values).

The adaptive turn-off method presents certain advantages compared to a standard approach based on a

comparator with fixed threshold.

The first advantage is that the adaptive method automatically compensates stray inductances LS in series to

rectified current path: this parasitic (mainly the SR MOSFET package inductances in series to drain and source

terminals) normally produces an offset on the sensed voltage across MOSFET RDS_ON that anticipates by TX =

LS/RDS_ON the turn-off in case of standard comparator.

The second aspect to consider is that a standard comparator with fixed threshold turns off at a current level IOFF

that depends on RDS_ON of the chosen SR MOSFET. Referring to Figure 5. Comparison between adaptive turn-

off and comparator based turn-off and considering a fixed comparator threshold VTH = -5 mV, the turn-off current

in the rectifier can be calculated.

In an application with a SR MOSFET having channel resistance RDS_ON = 2.5 mΩ and package stray inductance

LS = 2.5 nH, where the current slope is di/dt = -3 A/µs (for example starting from a peak current of 15 A with 5 µs

transformer demagnetization), the turn off current is:

•

IOFF=– VTHRDS_ON–TXdidt=5Aforcomparatorturn-off

•

IOFF_AD=−Tdiode_off*didt=0.9Aforadaptiveturn-off

Furthermore, the adaptive turn-off method also performs better in applications with CC regulation, where the

standard comparator with fixed threshold increasingly anticipates the turn-off during load impedance drops (since

current slope also decreases), while the adaptive method fixes Tdiode_off.

The SRK1001 controller starts operation when the VCC pin voltage exceeds the turn-on threshold VCC_On; then it

stops operation when the VCC voltage drops below the turn-off threshold VCC_Off.

In order to guarantee SR switching even with low VCC supply voltage (in the case of chargers operating in CC

regulation), the device is provided with the VAUX pin. When the VCC voltage decreases below the threshold

VCC_SO_On (> VCC_Off), an internal switch is turned on, allowing VCC pin capacitor to be charged up to the turn-on

threshold VCC_On by a current drawn through VAUX pin connected, for example, to the rectified SR MOSFET

drain voltage or to another auxiliary voltage of the flyback transformer.

For maximum flexibility across all kinds of applications and to overcome noise and ringing problems that may

arise after turn-on and turn-off events, the SRK1001 allows users to program the blanking time after turn-on and

after turn-off by means of two resistors connected between TON and TOFF pins to GND, respectively.

SRK1001

Operation description

DS13155 - Rev 1

page 9/27