Synchronous rectification controller for affordable high efficiency power adapters

Synchronous rectification controller for affordable high efficiency power adapters

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By Nick Flaherty

STMicroelectronics has developed a secondary-side synchronous rectification controller with adaptive logic that can run up to 300kHz to reduce the size and cost of the power adapter designs.

The SRK1001 controller introduces adaptive features that reduces the bill-of-materials costs, minimize circuit size, and simplify the design of power adapters, chargers, USB Power Delivery outlets, and lighting power supplies based on flyback topologies. It is designed for fast turn-on with minimum delay and uses adaptive turn-off logic to maximise the synchronous-rectification MOSFET conduction time. The design requires no external compensation circuitry to eliminate parasitic inductance effects.

The design of the SRK1001 also aims to maintain efficiency at light loads, where synchronous rectification is no longer beneficial. The device automatically enters low-consumption mode where the quiescent current is 160µA. This occurs when the MOSFET conduction period reaches the programmed minimum on time or when burst-mode operation of the primary controller is detected.

The controller can also be used in quasi-resonant (QR) converters and mixed continuous/discontinuous current mode (CCM/DCM) fixed-frequency operation at up to 300kHz. Robustness is assured, with programmable blanking time after both turn-on and turn-off to prevent noise from inducing spurious behaviour. It can be used for applications with an output voltage up to 24V, and is capable of maintaining regulation down to 2V output in CCM. It has a supply voltage range of 4.5V to 32V and can sink up to 1A and source up to 0.6A to control the gate of the external N-channel synchronous-rectification MOSFET.

A dedicated Disable/Synchronization pin is provided to turn off the synchronous MOSFET and remotely trigger low-consumption mode, or to synchronize MOSFET turn-off during CCM operation. There is also drain-voltage sensing input capable of monitoring the external MOSFET drain-source voltage up to 185V.

The SRK1001 is in production now, packaged as an 8-pin SO8 device, from $0.33 for orders of 1000 pieces.

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