Datasheet SRK1001 (STMicroelectronics) - 9

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SRK1001 Operation description 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: V • I TH di OFF = – R – T DS_ON Xdt = 5A for comparator turn-off • IOFF_AD = − Tdiode_off*di dt = 0.9A for adaptive turn-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.
DS13155
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Rev 1 page 9/27
Document Outline 1 Block diagram and pin connection 2 Maximum ratings 3 Typical application schematic 4 Electrical characteristics 5 Operation description 5.1 Drain voltage sensing 5.2 Turn-on 5.3 Minimum TON programming 5.4 Adaptive turn-off and TIMER 5.4.1 Fixed frequency mixed DCM/CCM operation 5.4.2 QR operation (with valley skipping) 5.4.3 Fixed frequency DCM operation 5.4.4 Variable frequency CCM operation 5.5 Minimum TOFF programming 5.6 Start-up phase 5.7 Low consumption mode operation: sleep-mode, burst mode, disable state 5.8 DIS/SYNC pin functions 5.9 VAUX pin operation in CC regulation 5.9.1 Example for parameter calculations 5.10 Operation in CC regulation and short circuit 5.11 Adaptive gate drive 6 Layout guidelines 7 Package mechanical data Revision history