ST has introduced a pair of autonomous secondary-side synchronous-rectification controllers for fly-back converters, suitable for quasi-resonant and continuous-discontinuous conduction fixed frequency circuits up to 300kHz.

Called SRK1000A and SRK1000B, they include adaptive switching logic “such that the synchronous rectifier mosfet is switched on as soon as current starts flowing through its body diode and it is then switched off as current approaches zero,” according to the company. “Fast turn-on, with minimum delay, and the adaptive turn-off allow maximising the mosfet conduction time and eliminate the effect of parasitic inductance in the circuit.”

STSRK1000A-SRK1000BThe devices are also capable of detecting when the primary controller enters burst-mode, when the device will switch to a 160μA low power mode. The same thing happens when synchronous mosfet conduction drops below a programmed minimum on-time. In these ways “converter efficiency improves at light load where synchronous rectification is no longer beneficial”, said ST.

STSRK1000A-SRK1000BNormal operation is resumed after the primary converter re-starts switching or current conduction in the rectifiers increases 20% above the programmed minimum on-time value.

Both devices have resistor-programmable blanking time after turn-on to prevent noise from inducing spurious behaviour.

SRK1000A has a 2μs fixed blanking time after turn-off, while SRK1000B has a 3μs blanking time.

Operation is with outputs up to 19V, and regulation can be maintained down to 2V output in conrinuous condiction mode. Chip supply range is 4.5 to 32V, with a drain-source voltage rating of 100V.

The output can sink up to 1A and source up to 0.6A to control the external (n-channel) synchronous-mosfet.

Packaging is 2.9 x 2.8mm 6pin SOT23-6L.

Applications are expected in battery chargers, quick chargers, adapters and USB power-delivery outlets.

THe product page is here, and the data sheet is worth a browse as there is a lot of automation in this little chip.