Meeting charger efficiency regulations through primary-side regulation

Although most home appliances and office equipment are powered by high-voltage AC power that plugs directly into a wall outlet, their internal circuits actually require low-voltage DC power. Therefore, the power supply must convert the AC voltage to a low DC voltage. According to research by Ecos consulting, there are approximately 3 billion AC/DC power supplies in use in the United States and approximately 10 billion worldwide. As these power supplies become more popular and have a greater impact on the environment, there is a growing concern about power efficiency around the world. The California Energy Commission (CEC) has proposed mandatory efficiency standards for external power supplies, and regions that currently use voluntary regulation programs are also considering developing mandatory standards to promote further improvements in power efficiency.

More than half of external power supplies are used in portable electronic products such as laptops, mobile phones and MP3 players, so they have output voltage and output current regulation capabilities for battery charging, as shown in Figure 1. For applications that require accurate output current regulation, current sensing is necessary, but this causes additional losses. In an environment where energy-saving regulations are increasing, output current sensing has always been a difficult challenge for power supply designers.

Figure 1. Conventional secondary-side regulated flyback converter

In charger design, primary-side regulation of the power supply may be the best solution to easily meet CEC efficiency requirements. This is because primary-side regulation only uses information from the primary side of the power supply to accurately control the output voltage and current, which not only avoids output current detection losses, but also eliminates all secondary feedback circuits, which is conducive to achieving a more efficient power supply design without incurring huge costs.

Basic Concepts of Primary-Side Regulation

Figure 2 shows the basic circuit of a primary-side regulated flyback converter and its typical waveforms. Generally speaking, discontinuous conduction mode (DCM) is the preferred operating mode for primary-side regulation due to its better output regulation performance. The key to primary-side regulation is how to obtain information about the output voltage and current without direct detection. Once these values ​​are obtained, they can be easily controlled using traditional PI control methods.

Figure 2 Primary-side regulated flyback converter and its typical waveform

During the MOSFET on-time TON, the primary-side inductor Lm is loaded with the input voltage VIN. As a result, the MOSFET current Ids increases linearly from 0 to the peak value Ipk. During this period, energy is transferred from the input terminal and stored in the inductor. When the MOSFET is turned off, the energy stored in the inductor causes the rectifier diode D to conduct. During the diode on-time TD, the output voltage Vo is loaded on the secondary-side inductor, which is Lm×Ns2/Np2, and the diode current ID decreases linearly from the peak value (Ipk×Np/Ns) to 0. At the end of TD, all the energy stored in the inductor is released to the output terminal. During this period, the sum of the output voltage and the diode forward voltage drop is reflected to the auxiliary winding terminal, which is expressed as (Vo+VF)×Na/Ns. Since the diode forward voltage drop decreases as the current decreases, at the end of the on-time, the diode current decreases to 0, so the auxiliary winding voltage at this time can best reflect the output voltage. Therefore, by simply sampling the winding voltage at the end of the diode conduction, information about the output voltage can be obtained. The diode conduction time can be obtained by monitoring the auxiliary winding voltage.

Figure 3 Schematic diagram of the internal modules of the integrated power switch (FSCQ series)

Integrated primary-side regulation controller

初级端调节 PWM 控制器如飞兆半导体公司的FAN102，是一种专门处理初级端调节电源设计的技术。这种技术可显著简化以满足更严苛效率要求的设计难题，并省去增加成本和可靠性较差的外部组件。FAN102还具有用于待机模式的绿色工作模式，并满足国际能源署(IEA)1W倡议要求，1W倡议旨在把待机功耗降至1W以下。图3给出了FAN102的内部结构。该器件带有一个集成式输出电缆压降补偿和外部组件温度变化补偿电路。内部振荡器的抖频则可减小EMI。FAN102的另一个重要特性是VDD范围很宽，为5～28V。当电源工作在恒定输出流模式时，控制IC的供电电压VDD随输出电压而变化。因此，VDD范围决定了恒定电流控制的范围，而且FAN102即使在输出电压低于四分之一额定值时也能够实现稳定的恒定电流调节。