Texas Instruments Incorporated Power Management Low-cost flyback solutions for 10-mW standby power By Adnaan Lokhandwala Product Manager For low-power AC/DC conversion, flyback topology remains converter is being heavily scrutinized to minimize the the preferred choice due to its simplicity and low cost. overall power drain when it seems the converter is doing Using a small number of external components, this topology nothing. For example, a flyback power supply used in an can provide one or more outputs for a very wide input- AC wall charger may have a mass-production specification voltage range. It is used in isolated and non-isolated forms of less than 30 mW. If the actual supply consumes only to cover a broad range of applications, such as battery 10 mW of standby power, the 20-mW difference can allow a chargers in smartphones and tablets; auxiliary power higher margin for leaky circuit components such as input supplies in TVs, desktop computers, and home appliances; filters, capacitors, and bias components, reducing overall AC adapters for portable computing, set-top boxes, and solution cost. Similarly, a flyback converter with low networking; and many more. Figure 1 shows the typical standby-power consumption can allow more system func- power levels in some of these applications. The wide- tions to be active in standby mode while keeping the end spread applicability and use of the flyback topology in equipment’s total power consumption to a minimum. high-volume consumer markets (estimated 2012 world- wide shipments for the markets shown in Figure 1 alone The push towards green power exceeded a few billion units) make it a perfect candidate There is an array of initiatives and directives in the power for optimizing every possible performance specification, industry addressing efficiency and standby power that such as cost, efficiency, and standby power. vary by end equipment, power level, and governing In most applications, flyback converters are stand-alone authority. In the U.S., these include the California Energy external power supplies in wall chargers/adapters. In some Commission and the Environmental Protection Agency’s cases they are powering either a portion of larger equip- ENERGY STAR®,1 and in Europe, the European Union’s ment or providing standby power to maintain system func- Standby Initiative, to name a few. After a quick glance at tions like the user display and remote control when the many of these energy-conservation initiatives, it is clear equipment is not performing its primary function. In all that they all have a common theme—driving minimal cases, the standby-power consumption of the flyback power loss at light loads and no-load/standby. Many Figure 1. Typical power levels for AC/DC flyback-converter designs 20 15 10 Power Level (W) 5 Feature Smart- Tablet Set-Top Home TV Desktop Phone phone Box Appliance Computer Battery Chargers/Adapters Auxiliary Power 9 Analog Applications Journal 1Q, 2014 www.ti.com/aaj High-Performance Analog Products Power Management Texas Instruments Incorporated regions in the world are also introducing mandatory and flyback that contribute most to its standby-power loss are voluntary limits for standby-power consumption and light- shown along with their relative cost. Traditionally, this type load operating efficiency of external power supplies. of converter compares its output voltage with a secondary- In the U.S., the California Energy Commission adopted side reference. An optoisolator is used to transfer an error for its own state a battery-charging efficiency standard signal across the isolation barrier. that became effective in February 2013. Additionally, the There are two fundamental issues with this approach. U.S. Department of Energy is finalizing a draft that will First, a low-cost reference like the widely used TL431 affect current regulations for power-supply efficiency shunt regulator from Texas Instruments (TI) needs a mini- worldwide. Similarly, the Joint Research Centre of the mum cathode bias current (~1 mA) independent of con- European Commission (EC) published the final draft of verter loading under all conditions. Second, the standard Version 5 of its Code of Conduct on Energy Efficiency of optocoupler configuration is such that it consumes the External Power Supplies in October 2013. These new most current under no-load conditions. Note that in order voluntary specifications, which propose tightening of to achieve standby-power consumption of less than 10 mW, active-mode efficiency and no-load power consumption, a more expensive reference such as TI’s TLV431 shunt are tougher to meet than the mandatory specifications of regulator with very low bias current may need to be used the EC’s current Ecodesign Directive. for feedback control. To ensure that the external supply is efficient in the idle One way to address these issues is to use a constant- and standby modes of some applications, the EC has added voltage, constant-current (CVCC) controller with primary- an additional efficiency requirement at 10% load beyond side regulation, such as TI’s UCC28710. This type of con- the four-point active-mode average-efficiency requirement. troller can simplify and improve performance in AC/DC The EC also has added an additional classification for designs. The UCC28710 regulates the flyback output mobile handheld battery-driven external supplies of less voltage and output current within 5% accuracy without than 8 W that must limit no-load power consumption to less optocoupler feedback. It also processes information from than 75 mW starting in 2014. Finally, the EC’s Ecodesign the primary power switch and transformer auxiliary wind- Directive for energy-related products, Lot 6, Tier 2 took ing for precise output CVCC control. effect in January 2013. This part of the directive limits To reduce its no-load consumption, the controller enters total system standby-power consumption of household smart sleep modes as the converter load decreases and the and office equipment to less than 0.5 W. controller reduces its average current consumption down to 95 µA. The control algorithm modulates the converter’s Less than 10 mW of standby-power consumption switching frequency and the primary current’s peak ampli- The typical architecture for an isolated flyback converter tude while maintaining MOSFET valley switching for high that consumes less than 10 mW of standby power is shown conversion efficiency across line and load. Finally, thanks in Figure 2. Four key elements (labeled A through D) in a to high-voltage IC technology, the external HV start-up Figure 2. Conventional AC/DC flyback consumes less than 10 mW of standby power + VOUT A ($) B ($) – VAC D ($$$) R R A: Controller HV start-up SU F4 F1 B: Primary snubber circuit C: Flyback switching device DRV D: Feedback components for Green-Mode ( ) C $$$$ RF3 output-voltage regulation Flyback CS Controller Auxiliary VDD VS Winding RCS GND RF2 10 High-Performance Analog Products www.ti.com/aaj 1Q, 2014 Analog Applications Journal Texas Instruments Incorporated Power Management MOSFET is also integrated into the con- troller to further reduce component count Figure 3. Simplified flyback schematics and simplify the solution (Figure 3a). The choice of a flyback converter switch is very application-specific and + V performance-driven. In some situations, a OUT bipolar junction transistor (BJT) can be a better choice than a MOSFET. Funda men- B ($) – tally, BJTs cost less than power MOSFETs VAC because their fabrication involves a sim- HV pler process with fewer layers, particularly UCC28710 for high-voltage (≥700-V) and low-power applications. BJTs with very high voltage DRV (>900 V) are economical options today, Auxiliary ( ) making BJT-based designs attractive in Winding VDD C $$$$ off-line power supplies for the industrial CS market and in regions with widely varying VS R AC utility voltages. GND CS Converters with BJTs can have lower manufacturing cost because they normally have less di/dt and dV/dt switching stress, EMI compliance is easier with no Y capaci- (a) With UCC28710 controller tor, no common-mode choke is required, and transformer construction is simpler. + Also, due to slow di/dt at turn-off, some VOUT energy in the transformer’s leakage induc- tance can be dissipated at the BJT turn- – off transition, potentially eliminating snub- VAC ber circuits in some designs. On the flip HV side, BJTs suffer from higher switching UCC28720 losses, are limited to designs with lower switching frequencies, and require a com- C ($$) plex drive scheme. DRV Auxiliary A highly integrated solution for driving a Winding VDD BJT is shown in Figure 3b. The UCC28720 CS controller incorporates a driver that dynam- VS ically adjusts the base-current amplitude GND RCS based on the converter load. This ensures that the BJT always operates in optimal switching conditions with minimal switch- ing and conduction losses even for higher- (b) With UCC28720 controller power AC/DC designs. 11 Analog Applications Journal 1Q, 2014 www.ti.com/aaj High-Performance Analog Products Power Management Texas Instruments Incorporated Figure 4. Summary of test data for AC/DC flyback designs with UCC287xx controllers 43 x 28 x 16 mm Standby Power Average Efficiency 20 88 22 x 21 x 20 mm 35.7 x 25.7 x 14 mm ) ) % ( mW ( 15 84 10 80 verage Efficiency Standby Power A 5 76 0 72 115 V 230 V 115 V 230 V 115 V 230 V MOSFET BJT BJT (5 V/1 A) (5 V/1 A) (5 V/1.5 A) Two 5-V/1-A USB chargers were designed to illustrate References some of the preceding points. Their test data are summa- 1. Adnaan Lokhandwala. (May 20, 2013). “The no- rized in Figure 4. Note that the controllers enable ultralow load power crunch: 30 mW and beyond.” Power standby-power consumption of less than 10 mW.