Achieving Low Standby Power and High Efficiency in a Flyback Converter

    Ever wonder how chargers keep increasing in power (for example, leveraging the USB Type-C standard) while remaining small? We can only dissipate so much power inside a sealed plastic box before the charger doubles as a hand warmer and becomes unreliable. You have to reach higher efficiencies.

    In addition to the demand for high efficiency, stricter standards will make meeting efficiency requirements more challenging. A new standard for the European Code of Conduct (COC) will take effect in January 2016. As shown in Table 1, a 15W low-voltage power supply must achieve an average efficiency of better than 81.8% and a 10% efficiency rating of better than 72.5%.

    Table 1: Energy efficiency standards for low-voltage external power supplies in active mode 

    In a charger, the component that dissipates the most power is typically the output rectifier diode. One way to improve the efficiency of a 5V mobile phone charger by 5% or more, or a 19.2V laptop or Ultrabook adapter by 2% or more, is to replace the output diode with a synchronous rectifier (SR) MOSFET and controller. 

    SR controllers can improve the efficiency of chargers. TI recently released the UCC24630 SR controller with ultra-low standby current.

    The UCC24630SR controller is a high-performance controller and driver for N-channel MOSFET power devices used for secondary-side synchronous rectification.
The controller and MOSFET combination form a near-ideal diode rectifier. This solution not only directly reduces rectifier power dissipation but also reduces primary-side losses due to improved efficiency.
The UCC24630 utilizes a volt-second balance control method and is not directly connected to the MOSFET drain, making it an ideal choice for flyback power supplies over a wide output voltage range. The SR driver turn-off threshold is independent of the MOSFET RDS(on), optimizing for maximum on-time. Furthermore, secondary-side current ringing caused by device and wiring inductance does not affect the SR turn-off threshold.

    The UCC24630 SR controller maximizes efficiency with minimal impact on standby power consumption. Figure 1 shows a typical application of the UCC24630 chip. 

    Figure 1: Typical flyback converter application using the UCC24630

    Synchronous rectification significantly improves efficiency by replacing the diode forward voltage drop with an IR voltage drop. To maximize the benefits of the controller, the MOSFET must be driven with optimal drive levels and timing. Currently, most SR controllers use VDS level sensing to determine when to turn the SR MOSFET on and off. Two popular VDS sensing controllers affect the SR conduction time, or the voltage between the MOSFET drain and source during secondary current conduction. The first type, fixed threshold detection, causes the MOSFET to turn off earlier, especially at low RDSON. MOSFETs are designed to improve conduction losses. The second type, proportional drive, improves the on-time but at the expense of a higher voltage drop across the MOSFET with a variable gate drive voltage. Both are sensitive to the MOSFET's RDSON.

    The UCC24630's driver timing is based on the volt-second balance principle, enabling accurate SR timing at all drive voltages. The turn-off timing is insensitive to the MOSFET's R DSON. We recognize the benefits of using a MOSFET with lower R DSON, thereby reducing conduction losses in the SR MOSFET. Using a 5V, 15W charger with a 3.5mΩ SR MOSFET R DSON as an example, the UCC24630 SR MOSFET losses are approximately 10% lower than fixed-threshold VDS sensing and 35% lower than a ratiometric VDS-sensing SR controller.

    For designs with stringent no-load power requirements, the auto-detection of low-power operating modes and the low standby mode current of 110µA make the load impact minimal: <1mW on a typical 5V charger. Many SR controllers consume up to 1-2mA, which can be a significant amount for the standby power budget. In a 19.4V notebook or Ultrabook adapter, this 1-2mA to 110µA difference translates to an additional 17 to 34mW of power consumed by the SR controller.

    For higher-power adapters, many designs operate the flyback converter in continuous conduction mode (CCM) to improve efficiency. CCM operation presents challenges for the SR controller because the secondary current does not reach zero before the primary switch turns on. Incorrect timing can lead to cross-conduction of the primary and secondary switches. The UCC24630 includes CCM dead-time control to ensure that the SR MOSFET turns off before the primary turns on, eliminating cross-conduction.

    The new standards make it more difficult to achieve efficiency standards in chargers, but using SR controllers such as the UCC24630 can meet these standards. How does the growing demand for high efficiency change the way we design?