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How to Simplify Synchronous Rectification

By Richard Lin, AC-DC Business Unit Manager

 

With tens of billions of new devices expected to come online soon, including all the billions more that won't be connected, the need for small and efficient external power conversion continues to increase. Offline converters power everything from charger adapters for portable 'smart' devices, to routers, computers, games consoles, and telecom and networking equipment. What all these devices share is their need to convert high voltage AC into a much lower DC voltage.

If a power supply is plugged in, even if it is not actively powering or charging a device, it may still consume energy. That's why, fifteen years ago, the California Energy Commission announced its intent to restrict the sale of non-efficient external power supplies. Since then, stringent legislation has come into force to regulate waste energy reduction based on two parameters: the maximum allowable power consumption in sleep mode and the minimum energy efficiency level in a steady state. The 'eco-design' regulations—that is, conforming to DoE Level VI and EU Tier 2 standards—mean that external power supplies now need to hit efficiencies of 89% or preferably more. These regulations came into effect in 2016 (US) and April 2020 (EU).

Modifying power supply design is by no means easy. A design change to the primary side would require the product to be resubmitted to update the safety certifications, which is why many OEMs have focused on modifying the secondary side.

Synchronous rectification to the rescue

Traditionally, on the secondary side, diodes were used to work with the primary side. These are now being replaced by transistors that behave in much the same way as the diodes, but they deliver the additional benefit of higher switching efficiency. The associated controller implements synchronous rectification to enable better management under variable load conditions. However, the percentage gain in efficiency is dependent on how synchronous rectification is implemented. So, there are still things to consider, such as the topology used.

Synchronous rectification is normally implemented in either continuous or discontinuous conduction mode (CCM/DCM) or quasi-resonant (QR) flyback mode, all of which are popular in offline conversion. CCM generally occurs at heavier loads and, in this mode, the current through the inductor never falls to zero. At lighter loads, DCM is more efficient, and here the inductor current can fall to zero. Quasi-resonant flyback is like DCM, as the device does not have a fixed switching frequency. Instead, the controller switches on when there's a valley in the drain voltage.

The APR348 secondary-side MOSFET driver supports all these modes for high-side and low-side flyback converters up to 20V. By including this level of configurability and functionality, design engineers can better optimize their design based on the type of load expected. As the APR348 is highly integrated, it requires very few additional external components, meaning OEMs can still realize small and efficient power supply designs.

Key features of the APR348 are its fast turn-on and turn-off times (30ns and 25ns, respectively), which reduce power loss and ensure safe operation when in CCM mode. It also features a blanking period to set the minimum turn-on time for the MOSFET, improving overall performance by limiting the impact of voltage ringing. In light load (LL) mode, its internal timer skips cycles when there is only a light load or no load. This feature also lowers the standby power under no load. In addition, high-side switching doesn't require an additional transformer winding.

Using a component such as the APR348 brings new levels of efficiency to offline conversion, while removing the need for many additional external components. This reduces overall BOM and keeps the design compact. It also improves the overall efficiency and sleep-mode power consumption of the end product, enabling engineers to hit the required DoE Level VI and EU Tier 2 standards with headroom.