Recently, there has been a heated discussion on how to standardize mobile phone input and how to produce a universal charger that can charge all mobile phones. In 2006, China issued a new regulation aimed at standardizing wall chargers and their connecting wires. Coincidentally, the GSM Association (GSMA) is also leading the development of the "universal charging solution" adapter plan, which aims to power mobile phones with micro USB interface. The ordinary charger is required to provide 5V 5% voltage, the minimum current is 850 Ma, and the no-load power consumption is less than 150 MW. In addition, it must comply with USB Design Forum (usb-if) battery charging specification 1.1 (bc1.1)* In addition to being convenient for consumers, standardized chargers can also reduce a large number of redundant chargers. In addition, the AC adapter with multiple USB jacks allows consumers to easily charge a variety of electronic devices without using many special chargers. Some high output current chargers also allow fast battery charging. This is an important advantage over the standard USB 2.0 port that limits the current of 500mA. People's demand for these improved performance is increasing, and the adapter design is becoming more and more miniaturized, which makes the thermal management in this "black box" a huge problem in front of the majority of power supply designers.
Universal serial bus (USB) charging has become a common power supply method for small electronic products. Many new consumer electronic devices (e.g. smart phones, tablets, e-readers, etc.) have AC power adapters / battery chargers with a power range of 5 to 25W and have a USB standard a interface. 5V adapter output voltage has become the first choice for charging and communication of compatible PC / desktop port. At present, the mainstream connection method is to use a standard (mini or micro-b) USB cable, while in most cases, non-standard connectors are used. As people pay more attention to battery charging, the old "block brick adapter" is becoming a cool, light, fashionable, safe and green charger. In addition to meeting standard conditioning requirements, OEMs continue to break the performance limitations of adapter efficiency and no-load power consumption (standby power consumption). For example, major mobile phone charger manufacturers have consistently implemented a five-star (no-load power consumption less than 30 MW) charger power rating system. This makes it easier for consumers to compare and choose those energy-efficient chargers.
Considering the power consumption, the reverse topology shown in Figure 1 has become our first choice because of its simplicity and low cost. The conduction loss of the Schottky diode rectifier on the secondary side (Fig. 1a) has become a limiting factor for realizing high-efficiency and compact adaptation design. For example, in a typical 5-V / 3-A adapter, the power loss of diode rectifier itself can reach 30% to 40% of the total system loss under full load (ignoring the comprehensive impact of secondary loss on high primary side loss). Installing a synchronous rectifier (SR) for the output (Fig. 1b) can improve the overall efficiency of the converter, and the system thermal management is easier due to the less heat generated (which is crucial in the adapter design).
Figure 1 Simplified reverse topologyUsb-if bc1.2 extends the charging current range from 1.5A to 5A.Adding an SR to the classical reverse topology is not complicated, but it can greatly reduce the total system power consumption. This method can effectively change the power level, and the power consumption decreases with the rapid development of MOSFET technology. Therefore, synchronous rectification is now applicable to a wide variety of products. The low power consumption of SR allows designers to use some smaller components. These components have fewer heat dissipation components, which can reduce assembly cost, product size and packaging weight while improving power density.
Please note that if SR MOSFET is allowed to switch in no-load / standby mode, the power consumption performance of the system may be reduced. In addition to the static power consumption required by the SR controller IC, the switching power consumption of sr-mosfet will be a limiting factor to achieve the best feasible system no-load performance.Green output rectifier: full load to no load
This paper will now introduce how ICs such as Ti ucc24610tm green Rectifier Controller simplify the design of USB charger and how to achieve high system efficiency in full load range. Fig. 2 shows a simplified system waveform of a reverse converter with and without synchronous rectification. These waveforms are the result of a control scheme that directly detects the MOSFET drain to source voltage (VDS). Compared with other implementation methods, such as primary side synchronization or synchronous control using secondary side current transformer, this control method has been widely used today. In this control scheme, the closing threshold of SR controller should be as close to zero as possible, so as to realize the maximum conduction time of MOSFET channel.
Fig. 2 simplified reverse waveform using Schottky diode and sr-mosfet output rectification
We can design the inverter so that it can work in different modes according to the requirements of terminal applications. For the design working in continuous conduction mode (CCM), the current of transformer secondary winding will not drop to zero before the primary side MOSFET is turned on, resulting in cross conduction for a certain time. After realizing synchronous rectification in this kind of converter, once the primary switch is turned on, the SR MOSFET will be turned off immediately, which is very important. This prevents reverse conduction and controls additional power consumption and device stress. After the synchronization function of "green rectifier" detects the conduction transition on the primary side, turn off the SR MOSFET. Fig. 3 depicts how the SR gate off transition is now controlled by the primary side synchronization signal rather than VDS detection.
As mentioned earlier, realizing synchronous rectification may reduce light load efficiency and no-load power consumption. The main reason for light load or no-load power consumption is the bias of sr-mosfet switch and Sr controller IC. "Green rectifier" successfully solves these problems by: (1) using an automatic light load detection circuit, turn off the gate switch of SR MOSFET when its conduction time falls below a certain threshold; (2) Use the en function to put the IC into sleep mode and eliminate static power consumption. The light load detection circuit compares the SR conduction time of each switching cycle with the set minimum "on" time (MOT). When the load decreases, the secondary conduction time is shorter than mot, and the next SR gate pulse fails. The no-load power consumption can be further reduced by using the en function of the controller IC. We can use a simple MOSFET drain voltage equalization circuit to put the IC into sleep mode under no-load state, so as to limit the bias current consumption of the IC to 100 A With this method, the no-load power consumption of 10MW can be reduced. The last step to improve the no-load performance is to add a low current Schottky diode in parallel with SR MOSFET.