Analysis of the Flyback Synchronous Rectification Control Circuit of the Adapter

With the development of consumer electronics, the proportion of electricity consumed by external power supplies (adapters) in global energy consumption has increased dramatically, becoming a major energy consumer that cannot be ignored. Take the United States as an example, adapters consume 300 billion kWh of electricity per year, accounting for 11% of the country's total annual electricity consumption.

As energy conservation and emission reduction are becoming more and more popular, the requirements for external power supplies in the current laws and regulations of various governments are becoming more and more stringent. The US Energy Star 5.0 also makes more stringent regulations on the average efficiency of external power supplies.

Table 1: Power supply efficiency when output voltage Vout>6V.

Table 2: Power supply efficiency when output voltage Vout < 6V.

High power density and high integration have undoubtedly become the development direction of electronic technology. Improving power efficiency can not only reduce the size of the power supply but also greatly improve the reliability of the power supply.

As a low-power consumer product, the design cost of the adapter has become the primary key factor that design engineers consider. The Flyback structure has become a commonly used circuit architecture for designing adapters below 150W due to its simple circuit.

In the traditional design that uses Schottky as the rectifier output, due to the existence of Schottky's barrier voltage VF, the loss consumed on the Schottky is very large when the current output is large, which not only causes low power efficiency, but also reduces the reliability of the power supply due to excessive temperature. In order to solve this problem, synchronous rectification technology came into being. Synchronous rectification is a new technology that uses a dedicated power MOSFET with extremely low on-resistance to replace the rectifier Schottky diode to reduce rectification loss.

Provide customers with more cost-effective and efficient synchronous rectification solutions to meet their design needs for high efficiency and high power density.

[page] Quasi-resonance can effectively reduce the switching loss of Flyback and improve efficiency because of valley-turning. However, it should be noted that the conduction loss of QR critical current mode is larger than that of continuous current mode (CCM). Therefore, in the case of 115Vac voltage input without PFC, the QR control method is not as efficient as CCM. Moreover, QR is variable frequency control. The switching frequency is very low under low input voltage and full load, which requires a larger transformer and limits the size of the power supply. Therefore, in order to improve efficiency, many manufacturers currently adopt multi-mode control, that is, working in QR mode under high voltage input and working in CCM mode under low voltage input. As we all know, the synchronous rectification controllers of all manufacturers can only work in intermittent or critical mode. The synchronous rectification control IC launched by Qingli Technology adopts dv/dt and pre-detection patented technology, which can perfectly and freely perform CCM/DCM synchronous control and improve the efficiency of the power supply to the extreme.

Figure 3: Flyback synchronous rectification control circuit commonly used in adapters.

Figure 3 is a flyback synchronous rectification control circuit commonly used in adapters. The secondary side uses SP6018/SP6019 from Qingli Technology as the rectification control, combined with the dedicated synchronous Trench Mosfet from IXYS of the United States as the output rectification Mosfet. Because the Trench Mosfet from IXYS has extremely low Rds(on) and Qg, it can minimize the conduction loss and improve the reliability of the power supply.

Figure 4: CCM operating waveform

Figure 5: QR operating waveform

[page] Figures 4 and 5 are the synchronous working waveforms under CCM and DCM respectively, where the light green is the Vds waveform of the synchronous MOS and the purple is the output waveform of SP6018/SP6019. Because of the dead zone programming control function, it can ensure that the power supply works safely in CCM mode. This is also the only synchronous rectification control IC in the industry that can cope with continuous mode excellently.

The excellent synchronous detection control of Qingli Technology's synchronous rectification allows customers to not only adopt the low-cost CCM+SR control method when designing power supplies without PFC below 75W, but also adopt the (CCM+QR)+SR multi-mode control method, which greatly improves the efficiency of the power supply and reduces the design cost.

In the design of power supply with PFC, we provide customers with QR+SR control mode, which maximizes the use of the conduction time of synchronous MOS and is more efficient.

As a professional agent of electronic parts for new energy products, Pengyuan Electronics is committed to providing customers with one-stop professional services. The company has a group of experienced engineers and has good cooperative relations with many research institutions. It can provide customers with value-added services such as component selection and complete solution provision, focusing on providing customers with efficient and green solutions.