Universal Serial Bus (USB) charging has become a common method of powering small electronics. Many new consumer Electronic devices (eg: smartphones, tablets, e-readers, etc.) have AC power adapters/battery chargers in the 5 to 25W power range and have a USB standard A port. 5V adapter output voltage has become the first choice for charging and communication with compatible PC/desktop ports. The current mainstream connection method is to use a standard (mini or Micro-B) USB cable, but in most cases non-standard connectors are used. As the focus on battery charging has grown, the old-fashioned “brick adapter” is being transformed into a “cool”, lightweight, sleek, and safe-green charger. In addition to meeting standard regulation requirements, OEMs continue to push performance limitations in adapter efficiency and no-load power consumption (standby power). For example, the major manufacturers of mobile phone chargers have consistently implemented a five-star (no-load power consumption less than 30 mW) charger power consumption rating system. This makes it easier for consumers to compare and choose those energy-efficient chargers.
Recently, there has been a lot of discussion about how to standardize mobile phone inputs and how to produce a universal charger that can charge all phones. In 2006, China issued a new regulation aimed at standardizing wall chargers and their connecting cables. Coincidentally, the GSM Association (GSMA) is now leading the development of the “Universal Charging Solution” adapter program, the goal of which is to power mobile phones with a micro USB interface. Common chargers require a voltage of 5V±5%, a minimum current of 850 mA, and a no-load power consumption of less than 150 mW. Additionally, it must comply with the USB Design Forum (USB-IF) Battery Charging Specification 1.1 (BC1.1). *In addition to being convenient for consumers, standardized chargers can reduce a lot of redundant chargers. In addition, AC adapters with multiple USB jacks allow consumers to conveniently charge a variety of electronic devices without the need for numerous dedicated chargers. Some high output current chargers also allow fast battery charging. This is a major advantage over standard USB 2.0 ports that are limited to 500mA. The increasing demand for these improved performance and the miniaturization of adapter designs make thermal management in this “black box” a huge challenge for power supply designers.
Considering the power consumption, the reverse topology shown in Figure 1 is our first choice due to its simplicity and low cost. The conduction loss of the secondary-side Schottky diode rectifier (Figure 1a) becomes a limiting factor in achieving a high-efficiency, compact-fit design. For example, in a typical 5-V/3-A adapter, the diode rectifier itself can dissipate 30% to 40% of the total system losses at full load (ignoring the combined effect of secondary losses on high primary losses ). Installing a synchronous rectifier (SR) for the output (Figure 1b) improves the overall efficiency of the converter and makes system thermal management easier because less heat is generated (critical in adapter design).
picture1 Simplified reverse topology
*USB-IF BC1.2 extends the charging current range from 1.5A to 5A.
Adding an SR to the classic reverse topology is not complicated, but it can greatly reduce the total system power consumption. This approach effectively changes the power dissipation level, which is decreasing with the rapid development of MOSFET technology. Therefore, synchronous rectification is now suitable for a wide variety of products. The low power consumption of SR allows designers to use some smaller components. These components have fewer heat-dissipating components, resulting in higher power density while reducing assembly cost, product size, and packaging weight.
Note that system power performance may be degraded if the SR MOSFET is allowed to switch in no-load/standby state. In addition to the static power dissipation required by the SR controller IC, the SR-MOSFET switching power dissipation can be the limiting factor in achieving the best possible system no-load performance.
Green output rectification: full load to no load
This article will now introduce you such as TI UCC24610TMHow ICs such as green rectifier controllers simplify USB charger design and how to achieve high system efficiency over the full load range. Figure 2 shows simplified system waveforms for a flyback converter with and without synchronous rectification. These waveforms are the result of a control scheme that directly senses the MOSFET drain-to-source voltage (VDS). Compared to other implementations, such as primary-side synchronization or synchronous control using secondary-side current transformers, this control method is widely used today. In this control scheme, the turn-off threshold of the SR controller needs to be as close to zero as possible to achieve the maximum conduction time of the MOSFET channel.
Figure 2 Simplified reverse waveform of output rectification using Schottky diode and SR-MOSFET
We can design the flyback converter to operate in different modes depending on the end application requirements. For designs operating in continuous conduction mode (CCM), the current in the transformer secondary winding does not drop to zero until the primary MOSFET is turned on, resulting in cross conduction for a period of time. After implementing synchronous rectification in this type of converter, it is extremely important that the SR MOSFET turns off as soon as the primary side switch turns on. This prevents reverse conduction and controls additional power dissipation and device stress. The synchronous function of the “green rectifier” turns off the SR MOSFET after detecting the primary conduction transition. Figure 3 depicts how the SR gate turn-off transition is now controlled by the primary-side sync signal, independent of VDSdetection control.
As mentioned earlier, implementing synchronous rectification may reduce light-load efficiency and no-load power consumption. The main cause of light load or no-load power dissipation is the SR-MOSFET switch and SR controller IC biasing. The “green rectifier” successfully solves these problems by: (1) using an automatic light load detection circuit that turns off the gate switch of the SR MOSFET when its conduction time falls below a certain threshold; (2) using the EN function, Puts the IC into sleep mode, eliminating static power consumption. The light-load detection circuit compares the SR conduction time to the set minimum “on” time (MOT) for each switching cycle. When the load decreases, the secondary conduction time is shorter than the MOT, and the next SR gate pulse fails. Further reduction of no-load power consumption can be achieved by utilizing the EN function of the controller IC. We can limit the IC’s bias current consumption to 100 µA by using a simple equalization circuit for the MOSFET drain voltage to put the IC into sleep mode at no load. Using this method, the no-load power consumption can be reduced by another 10mW. The final step to improve no-load performance is to add a low current Schottky diode in parallel with the SR MOSFET.
picture3 Typical for primary side synchronizationCCM reverse waveform
For example, we designed a 3A rated USB charger for a tablet end application using two controller chipsets (TI UCC28610 and UCC24610). A reference design for this charger (PMP4305) can be viewed by visiting the website address at the end of this article. The UCC24610 is ideal for applications that use a 5-V reverse switch mode power supply and can operate over the 4.75 to 5.25 V specified USB voltage range. Therefore, this SR controller is directly biased to the converter output, eliminating the need for an auxiliary winding on the main power transformer. This controller also allows external programming using two blanking timers, preventing turn-on and turn-off transitions from detected VDSRinging causes SR false triggering. Figure 4 shows the typical power stage waveforms of the PMP4305 at full load. IC control scheme is not affected by turn-on when VDSSevere ringing of the signal is affected because the programmable MOT timer disables V during this periodTHOFFComparators.
picture4 PMP4305 full load waveform
Figure 5 shows the comparison between the output rectification efficiency of the SR-MOSFET and Schottky diode at 115V and 230V AC line voltage. Synchronous rectification is realized, which can achieve more than 80% efficiency from full load to about 25% full load. In addition, over this load range, 3 to 5 percent efficiency improvement can be achieved with Schottky diode rectification.
Figure 5 Comparison of efficiency between Schottky diodes and synchronous rectification (SR) systems
Consumer device USB charging solutions are receiving more and more attention. Universal standard for 10W to 25W chargers with multiple USB ports to power multiple devices without needing a new wall charger for every new electronic device. We need to use some high-efficiency AC/DC converters to meet the growing demand for high-density small-scale adapters. Devices such as the UCC24610 “green rectifier” can help improve AC/DC converter efficiency and enable high-density USB charger designs.
For more details on the tablet charger reference design, please visit: www.ti.com.cn/tool/cn/PMP4305