Adaptive Synchronous Rectification Controller for Flyback Converter Demo Board

August 28, 2019 by Paul Shepard

The EVLSRK1001-PF from STMicroelectronics is a demonstration board, designed for evaluation of the SRK1001 synchronous rectification controller. The SRK1001 implements a control scheme specific for secondary-side synchronous rectification in flyback converters and provides high-current gate-drive outputs for driving N-channel power MOSFET.

The device can operate both in quasi-resonant (QR) applications and in fixed frequency (FF) mixed DCM-CCM applications. The board is provided with a setting suitable for QR application. To use in FF applications, a 100pF capacitor needs to be added in C4. A 120kΩ resistor (R3) is provided on the TON pin that fixes the blanking after the turn-on to about 1.44µs.

The blanking time after turn-off is set to 3μs through a 100kΩ resistor (R4) on TOFF pin. In order to use the DIS/SYNC pin functionality, the user has to remove the zero ohm resistor R9: the PCB already provides a NPN transistor connected to this pin for remote ON/OFF.

The board includes the SR MOSFET (PowFLAT 5 x 6 package) and can be easily implemented into an existing converter to substitute rectifier diodes.

Key Features

  • Synchronous rectifier for flyback converter, with fixed turn-on and adaptive turn-off
  • VCC range: 3.5V to 32V
  • DVS sensing voltage: 185V AMR
  • Max frequency: 500kHz
  • Internal gate drive for N-channel MOSFET
  • SR MOSFET type: BSC110N15NS5 standard level (150V - 11mΩ) PowerFLAT 5 x 6

SRK1001 synchronous rectification controller

The SRK1001 is a controller intended for secondary side synchronous rectification (SR) in flyback converters, suitable for operation in QR and mixed CCM/DCM fixed frequency circuits.

It provides a high-current gate-drive output, capable of driving N-channel power MOSFETs.

The control scheme of this IC is such that the SR MOSFET is switched on as soon as current starts flowing through its body diode and it is then switched off as current approaches zero.

The fast turn-on, with minimum delay, and the innovative adaptive turn-off logic allow maximizing the conduction time of the SR MOSFET and eliminating the effect of parasitic inductance in the circuit.

The device enters low consumption mode when it detects primary controller burst-mode operation, or when the SR MOSFET conduction becomes lower than the programmed minimum TON. In this way, converter efficiency improves at light load where synchronous rectification is no longer beneficial.

After the converter restarts switching or the IC detects that the current conduction in the rectifiers has increased 20% above the min TON programmed value, the IC exits low consumption mode and resumes switching operation.