Feature: Power
Flyback converter innovations for efficiency-conscious, cost-sensitive applications
T
By Igor Spinella, CEO and Founder, Eggtronic drain-to-source voltage (VDS
he flyback converter is a widely-used power architecture in home and office electrical products, as well as adapters and chargers for consumer systems. Light-
load efficiency and no-load or standby power are coming under increasing scrutiny as many appliances and chargers operate predominantly in standby or energy-saving modes with optimised power consumption. And, whilst pressure on costs is ever present, ecodesign regulations continue to intensify, particularly at low loads, like the EU Code of Conduct (CoC) Tier 2, which introduced a minimum efficiency target of 10% of full load, in addition to values at 25%, 50% and 75%.
Flyback switching principles Te flyback converter’s biggest asset is its simplicity, allowing compact dimensions, lower costs, fast assembly and good reliability. Basic flyback converters operate with
hard switching, turning on the MOSFET in the primary side with a high value of
30 June 2024
www.electronicsworld.co.uk ), which results
in high switching energy losses. Quasi- resonant (QR) flyback operation reduces losses and lowers EMI by turning the MOSFET on when VDS
load efficiency because of conduction losses; i.e., QR is not good enough for modern flyback adapters.
is low. Tis is also
known as “valley switching”, where “valleys” are generated by the ringing (quasi- resonance) between the leakage inductance of the transformer and the primary-side node capacitance. In a small power adapter at full load, QR
flyback can achieve close-to-zero voltage switching (ZVS) at nearly no extra cost, although this is dependent on the input voltage. At European line voltages, which are the highest in the world, the QR circuit can’t discharge the parasitic FET at low load such that the valleys are too far from zero, and hard switching occurs causing increased losses. On the other hand, in the US and Japan, where the mains voltages are relatively low, the efficiency has typically been good enough for past regulations. However, the need for a compromise between node capacitance, leakage inductance and EMI, requires large and highly resistive input filters that affect full-
Valley switching and ZVS Active clamp flyback (ACF) is oſten used to resolve this problem. Although effective, the circuit requires a high-side FET with high voltage and current ratings, which does drive up costs. Despite this, ACF offers good performance above 50% load. At low load, the circuit operates in burst mode, which degrades its efficiency. Tis combination of higher cost and weak performance at low load means that ACF is also unattractive for future flyback converters. To overcome the disadvantages of QR
and ACF approaches, Eggtronic developed QuarEgg – a series of proprietary controllers that force ZVS operation, with the same cost efficiency of traditional QR techniques and flat efficiency that is higher than ACF at any input voltage and output load condition. Ensuring switching occurs only when = 0V eliminates
the primary MOSFET VDS
turn-on transition losses, significantly improving efficiency. In addition, forced
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