Application of AP3765 in ultra-low standby power consumption charger solution

　　At present, government agencies, environmental protection organizations, and industry alliances around the world are actively promoting the development of new power supply technologies with high efficiency and low consumption from various aspects such as market access and consumer behavior guidance. Among them, the standby or no-load power consumption of AC-DC adapters and chargers of consumer electronic products has received more and more attention. As a result, many new technical standards have also been generated. According to incomplete statistics, there are nearly 20 such standards that have been promulgated and implemented, applicable to different industries and regions. For example, the "Energy Star" in the United States, the "TLC Certification" in China, and the charger star logo proposed by some large mobile phone manufacturers. Most of these standards have made clear and strict requirements on no-load loss and work efficiency. When the product meets the standard, the manufacturer can affix relevant logos on its product, such as "Energy Star", "TLC", 5-star, etc. It not only intuitively displays the technical characteristics of the product, but also effectively guides and influences the purchasing behavior of consumers.

　　At present, these standards are gradually being understood and recognized by consumers around the world. At the same time, more and more industry management agencies have begun to refer to these standards to formulate mandatory technical specifications for related industries or regions. This is not only a technological innovation and challenge, but also greatly increases the threshold for market access. Therefore, more and more terminal manufacturers and related upstream and downstream supporting suppliers have begun to adopt new methods, new technologies, and new materials on a large scale to meet the challenges posed by new standards. As a professional designer and manufacturer of power management integrated circuits, BCD has timely launched the AP376X series of primary-side PFM switching power supply controllers to help power engineers easily cope with new standards and new challenges.

　　The charger introduced in this article uses BCD's AP3765 to achieve ultra-low standby power consumption and high efficiency. AP3765 is one of BCD's latest PFM mode switching power supply primary side controllers - the AP376X series. It continues BCD's best primary side control mode, while ensuring the overall design is simple and reliable, with "zero" current startup and low operating current characteristics. It is also the "zero" current startup and low operating current characteristics that make the charger designed with AP3765 meet the current five-star charger standby 30mW requirements and the latest Energy Star standards.

　　AP3765 also has very practical functions such as system open-loop protection, short-circuit protection, soft start and frequency jitter, etc. These features make AP3765 have significant advantages in applications such as low-power chargers, adapters, LED lighting, etc.

　　Figure 1 shows a charger circuit designed using AP3765. The following is a brief description of the design.

Figure 1 AP3765 typical application circuit

　　This circuit is a typical IC-controlled flyback power supply. The working mode of the whole system is controlled by IC-AP3765, and the whole full voltage load section is in DCM. The circuit can be divided into input part, startup part, feedback (current and voltage) part, power conversion part and output rectification part. The input part is composed of rectifier D1-D4, as well as EC1, EC2, L1 and L2. The input adopts "∏" type filter circuit, which can effectively attenuate differential mode noise interference. R1, R6, EC5, D6 and R18 form the startup and power supply circuit of the system, which is also a key area for adjusting standby power consumption. Generally, a 4M-20M startup resistor is selected, and a 2.2uF-10uF startup capacitor is matched. R8, R9 and R10 are voltage feedback circuits, and the reference voltage of the FB pin is 4V. R2, R4 and R11 form a current detection loop, and the current detection voltage is 500mV. Then we can get the output voltage and output current through the following calculation formula. First, define the ratio of the primary winding to the output winding as N1, the turns ratio of the output winding to the feedback winding as N2, the output voltage as Vo, the output current as Io, and the primary peak current as Ipk. Figure 2 shows the voltage and current characteristics of a 5V700mA charger made with AP3765.

　　Output voltage (ignore Schottky voltage drop)

　　Output Current

Figure 2 Voltage and current curves of a charger made with AP3765

　　The power conversion part is composed of transistors Q1 and T1 driven by AP3765. When Q1 is turned on, the IC is driven to a high level, energy is stored in transformer T1, and the primary current is detected. When Q1 is turned off, the IC is driven to a low level, energy is released to the back end through transformer T1, and the primary feedback voltage is detected. The output part is composed of D8, C5, R14, EC3, and R13. Through a brief description of each component, it can be seen that the system design using AP3765 is very simple, and even for beginners it can be completed relatively easily.

　　We will focus on the method of reducing standby power consumption. The main loss points when the system is in standby are: startup circuit part (R1, R6, R11), IC ( AP3765 ) working and driving part (Q1), energy conversion part T1, output rectification part (D8, C5, R14), output dummy load (R13). Other losses such as capacitor losses, sampling resistor losses and absorption circuit losses can be ignored in most cases. In actual design, the startup resistor is about 20M, and the output dummy load uses 10K (5V output). The working voltage of the IC is designed to be around 20V, so under the input voltage condition of 230Vac/50Hz, we can first estimate the standby loss approximately:

　　From the above, we can see that the fixed loss of the system is about 14mW, and there are also losses in the transistor and output diode. Because the operating frequency of the IC is very low in standby mode, only about tens of Hz, the switching loss of the transistor can be ignored, and only the conduction loss is considered. The output diode has almost no current passing through it, and its loss can also be ignored.

　　Theoretically, the total loss is about 17mW. Considering the transformer and diode losses, it should be about 20mW, which is within the 30mW required by the charger. The actual test result using the power meter WT210 is about 24mW.

　　The design ideas and techniques used in the above case are typical and universal, and are suitable for most conventional applications. However, for some special applications, it is necessary to fully consider the losses in each link of the circuit and make a comprehensive compromise to achieve the best effect. For example, in order to cooperate with the special application of a certain ultra-low power microcontroller, we once designed an AC-DC power supply with a no-load current of only 20A. During the design, we conducted a comprehensive analysis of various components that may cause no-load losses, tested and adjusted them one by one, and minimized the no-load power consumption.

　　"Reduce loss and improve efficiency" is the eternal theme and eternal goal of the power supply industry. This article takes the typical circuit of AP3765 as an example to introduce some experience and methods on how to effectively reduce no-load loss. Not only for the application of AP3765, these ideas, methods and techniques are also applicable to the design of other small low-power chargers.