TI's latest generation ACF Controller UCC28782 helps miniaturize adapters
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--South China Team, Daniel Shen
Recently, Texas Instruments (TI) launched the latest generation of ACF Controller UCC28782. TI has been committed to the development of the latest AC-DC conversion technology. In the first half of 2018, it took the lead in mass-producing the industry's first ACF Controller UCC28780 . After nearly two years of polishing, UCC28782 has significantly improved its integration and performance, which will surely further help miniaturize adapters.
1. Fast charging drives adapter miniaturization
In recent years, with the popularity of mobile phone fast charging technology and USB Type C PD, the charging power of portable electronic devices such as mobile phones and computer laptops has become increasingly larger. At the same power density, the increase in power will inevitably lead to an increase in the volume of mobile phone and computer laptop adapters (hereinafter referred to as: adapters). However, consumers always want the adapter to be as light and compact as possible. Taking a 20W mobile phone adapter with a volume of 75cm3 and a power density of 0.267W/cm3 as an example, if the power increases to 50W and the power density is maintained at the same level, the volume of the adapter will increase to 150cm3, which is difficult for consumers to accept. If the same volume is to be maintained, the power density needs to be increased to 0.667W/cm3. It can be seen that in order to make the adapter light and compact, the power density must be continuously improved. The following table is a comparison of the power density of several typical adapters.
Comparison of power density of several typical adapters
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Adapter
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Power(W)
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Size(cm3)
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Density(W/cm3)
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Lenovo 65W Lipstick Adapter
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65
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76.7
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0.848
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Xiaomi 65W GaN Adapter
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65
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52.9
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1.229
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OPPO 50W Biscuit Adapter
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50
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33.7
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1.485
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2. Ultra-high efficiency active clamp flyback topology ACF
In order to improve power density, power efficiency and switching frequency must be improved as much as possible. Soft switching topology is the main method to achieve the above goals. Most traditional adapters use quasi-resonant flyback (QR), also known as transition mode passive clamp flyback (TM PCF). Quasi-resonant flyback has the advantages of simple structure and easy control, but the power density improvement space is small and cannot meet the requirements of adapter miniaturization. For this reason, the industry has been looking for new topologies that support ultra-high power density adapters. Finally, the active clamp flyback (ACF) topology stands out among many topologies. The ACF topology realizes soft switching of power tubes by active clamping, which not only reduces switching losses and improves conversion efficiency, but also supports higher switching frequencies, fully utilizes the advantages of new power tube gallium nitride GaN, and is conducive to the miniaturization of magnetic components. However, compared with the QR topology, the ACF topology has higher requirements for the controller. Only by using a suitable ACF Controller can the advantages of ACF be fully utilized.
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Comparison of topological structures between ACF and PCF
3. Detailed analysis of the latest generation ACF Controller UCC28782
With its excellent design, UCC28782 achieves high power density in the ACF topology while maintaining high efficiency in the full voltage range and full power range. Advanced automatic adjustment technology and dead zone control can achieve zero voltage switching (ZVS) in a wide input voltage and output voltage range. Multi-mode control achieves burst mode audio noise elimination and output ripple reduction while maintaining high efficiency. Through adjustable switching frequency (up to 1.5MHz), integrated Boost self-powered circuit, built-in burst mode compensation circuit and unique EMI frequency jittering function, UCC28782 reduces the size of peripheral devices and reduces BOM cost. In addition, UCC28782 also has X-capacitor discharge and IPC intelligent power control functions, which can achieve higher efficiency when combined with the synchronous rectifier controller UCC24612 . The following highlights several special features of UCC28782:
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UCC28782 Typical Application Block Diagram
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Full load 65W full voltage range efficiency curve
Integrated Boost self-powered circuit (Integrated Biased Power) to improve the efficiency of the self-powered circuit
UCC28782 integrates Boost self-powered circuit, which saves external devices compared with traditional 1 winding + LDO and 2 windings + LDO. The most important thing is that the efficiency is significantly improved under the wide output range of USB PD 3.0 3.3V~21V.
Active X -cap Discharge to improve no-load efficiency
UCC28782 has active discharge of X capacitors, which can avoid the loss of passive X capacitor discharge resistors and effectively improve no-load efficiency. At the same time, the XCD pin also has a fast fault reset function. After detecting that the AC line is restored, UCC28782 will start the corresponding reset action.
Intelligent Power Control (IPC) , supports multi-mode control
UCC28782 has a unique intelligent power control (IPC). By configuring the IPC pin, the following benefits can be achieved: (1) Reduce light load and no-load losses, such as achieving no-load input loss of less than 75mW; (2) Improve light load efficiency at low output voltage (such as 5V, 9V); (3) Reduce output voltage ripple at low output voltage burst mode, which can effectively reduce current ripple for applications requiring constant current output; (4) Reduce the over-power limit value at low output voltage; (5) Cooperate with the S13 pin to implement PFC start-stop control (PFC Disable Control), and high-power adapters (>=75W) can disable PFC at low output voltage (such as 5V, 9V) to further improve efficiency at medium and low loads.
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