Feature: Power
Figure 1: QuarEgg sub-70W flyback with ZVS MOSFET in secondary side
ZVS operation eliminates COSS discharge losses as well as reduces peak circuit currents, thereby enabling designers to use smaller, lower-cost, lower-resistance filters and components and simplified transformer designs. In addition, the gate drive is also simplified, since there are no ‘Miller’ effects. QuarEgg controllers ensure ZVS-
forcing pulses on the secondary side that can discharge the primary side node without requiring expensive ACF-style cost. Additionally, the controllers can do synchronous rectification, to further increase efficiency in comparison with ideal diode controllers.
QuarEgg Eggtronic created its QuarEgg reference designs to demonstrate forced ZVS for USB-PD flyback converters rated below 70W. Using the company’s EPIC1AFQ01 controller located on the transformer’s secondary side allows a low-cost signal transformer to replace the isolating optocoupler; see Figure 1. Te design is optimised for low-power and no-load efficiency, and achieves at least 40% lower standby power consumption than typical QR designs whilst maintaining consistently high efficiencies (> 94.5% at 230VAC and > 93.6% at 90VAC) across the load range and with any input voltage. Te controller integrates the ZVS gate driver and additional features, including USB PD 3.1 support and circuit protection.
Figure 2: QuarEgg PD flyback converter with forced ZVS ensures > 91% efficiency at sub-10% load
Secondary and primary side regulation Regulating the flyback converter output voltage is challenging, complicated by the presence of the transformer that provides the isolation required for safety; any chosen regulation scheme can’t compromise this isolation. Primary-side (PSR) and secondary-side regulation (SSR) approaches can be used to control the main MOSFET on the primary side; however, each comes with its own advantages and tradeoffs. In SSR, continuous monitoring of
the output voltage permits fast transient response. Also, tight regulation is possible for all outputs in a multi-output design.
However, communicating the sensed output voltage value non-galvanically across the isolation barrier to the flyback controller on the primary side typically calls for an optocoupler in the feedback loop. Tis increases costs, and its ageing effects mean the optocoupler is usually the least reliable component in the setup. In PSR, the controller sits on the primary
side and uses the transformer’s magnetic field as feedback to infer the output voltage. Monitoring from the primary side removes the need for an optocoupler, reducing costs but improving reliability. However, the output voltage can only be sampled when the output diode is conducting. If the output voltage changes between samples,
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