Key points for designing green switching power supply

Earlier, it was also called "green switching power supply". There have been multiple versions of "Energy Star" for switching power supply. With the upgrade of the version, the efficiency of switching power supply is getting higher and higher, and the standby power consumption is getting smaller and smaller, but the design difficulty will also increase. The efficiency of switching power supply is generally tested at full load output power, but the average efficiency of the design in CEC (California, USA) and "Energy Star" specifications is actually measured at four test points of the load, which is a common concern of the industry.

With the development of modern science and technology, the performance of devices has improved, especially energy-saving power supply chips have sprung up like mushrooms after rain. With the maturity of circuit design, it is not difficult to design energy-saving switching power supplies with high efficiency and low standby power consumption. Designing energy-saving switching power supplies is just in line with the needs of energy conservation and emission reduction. In recent years, the implementation of the US "Energy Star" has played a role in promoting the design and manufacture of high-performance energy-saving switching power supplies.

Switching power supplies that meet the "Energy Star" standard are also called "green switching power supplies". There are already multiple versions of "Energy Star" for switching power supplies. With the upgrade of the version, the efficiency of switching power supplies is getting higher and higher, and the standby power consumption is getting lower and lower, but the design difficulty will also increase. The efficiency of switching power supplies is generally tested at full load output power, but the average efficiency of the design in the CEC (California, USA) and "Energy Star" specifications is actually measured at four test points under load. These four points are: 25%, 50%, 75% and 100%, so the design is somewhat difficult.

I will now use the circuit diagram in Figure 1 as an example and Table U-2 (implemented in January 2008) of the California Energy Star Act (Level IV) as the design basis to explain the key points of designing a green switching power supply. The requirements of Table U-2 of the California Energy Star Act (Level IV) are as follows:

Note 1: Pn in the table is the standard output power of the external switching power supply, and Ln is the natural logarithm.

Note 2: California's "Energy Star" is mandatory and is a local regulation in California. Commercial switching power supplies entering California must comply with this regulation. Many manufacturers in my country design and produce green switching power supplies in accordance with California's "Energy Star" Act.

Figure 1 shows a parallel-type flyback switching power supply with an output of 12V3A, which is used as a power adapter for 14-inch to 17-inch LCD monitors.

The main content of green switching power supply is high efficiency and low standby power consumption. The efficiency requirement of 36W is greater than 83%, and the standby power consumption is less than 0.5W. The current power chip suppliers generally provide green power chips, and the standby power consumption is generally very small. However, high efficiency is not only related to the chip, but also to other devices. Reverse leakage current + reverse leakage current Idss × reverse voltage, 6N60 plus heat sink can meet the requirements.

The secondary rectifier tube works in a high-frequency pulse state. When the output is 12V3A, the working voltage resistance is required to be greater than 4 times the output voltage, and the working current is greater than 3 times the output current. The SB1660 Schottky diode with a voltage resistance of 60V and a working current of 16A can meet the requirements, but a heat sink must be added.

The lower the power consumption of the power chip, the better, because it not only affects the efficiency and standby power consumption of the whole machine, but also involves the working stability of the whole machine. The greater the power consumption, the higher the temperature rise and the more unstable the work. For example, the green switching power chip 0B2269AP of Angbao Company has a normal working power consumption of less than 30mW and works stably.

The loss of the switching transformer is the largest part of the whole machine loss, which includes copper loss and magnetic loss. It is difficult to make a low-loss switching transformer by simply copying other people's high-quality switching transformers, because the loss of the switching transformer is a complex relationship, which is related to the quality, design, winding process, etc. of the magnetic core. Take the design of the flyback switching transformer as an example, it is related to the operating frequency, conduction time, winding method, magnetic gap (grinding magnetic core), etc. The design of the switching transformer is the core of the design of the switching power supply. It requires profound theoretical knowledge and years of experience. Excellent switching power supply design engineers can design a switching transformer with an efficiency of more than 97% by selecting high-quality magnetic cores and combining advanced winding processes. The magnetic core of the switching transformer in Figure 1 is selected from Zhejiang Tiantong's EE-30L, the switching operating frequency is 65KHz, the conduction time is 0.42T (T is the period), the primary is 78 turns, the secondary is 8 turns, the chip power supply winding is 9 turns, the primary inductance is 1.2mH, the leakage inductance is less than 35uH, the efficiency of the transformer can reach 96%, and the temperature rise is less than 60℃ during operation. The damping absorption network is composed of R2, C3, and D1. D1 is UF4007, C3 is a 1000V4700PF high-voltage ceramic capacitor, and R2 is a 120K2W resistor. R2 and C3 match well, and the pulse absorption effect is obvious. If R2 and C3 are mismatched too much, not only the loss will increase, but also electromagnetic waves will be radiated, which will make EMC fail. After testing, the loss of the network is less than 0.4W. It should be noted that the design of this network is related to the design of the power chip, operating frequency, and transformer, and cannot be applied to all switching power supply circuits.

Since the secondary rectifier tube will generate sharp pulses when working, noise will be superimposed on the output, and it will also radiate electromagnetic waves, making it difficult to pass the EMC test. Therefore, the sharp pulse absorption network R8 and C8 should be connected in parallel at both ends of the diode. In Figure 1, C8 is a 22000PF100V ceramic capacitor and R8 is a 56 ohm 0.5W carbon film resistor. This absorption network also has a certain power consumption. The larger C8 and the smaller R8, the greater the loss.

The power loss of the spike pulse absorption network in Figure 1 is less than 0.25 W. It should also be noted that the design of the absorption network is also related to the power chip, switching frequency, characteristics of the Schottky diode, and the design of the transformer.

The wave-blocking inductor L2 can reduce the output ripple. It is usually made of enameled wire wound 8 to 16 times on a 6mm thick magnetic rod. It should be noted that since a large current needs to pass through, the enameled wire should be thick and the number of turns should not be too many, so as to avoid excessive voltage drop and increase loss. The inductance of L2 in Figure 1 is 12uH, and it is wound with 0.8mm thick enameled wire. As for the number of turns, it depends on the magnetic material parameters of the magnetic rod.

The design of L2 is related to the operating frequency and output current of the power supply. We would like to remind you that the DC output line (not shown in the figure) must reach or exceed 3A, otherwise, due to the large current passing through, the power loss that may be generated on the DC output line is the biggest victim.

The DC output line has a large price share in the power adapter. Many manufacturers will reduce its current carrying capacity to reduce costs, which results in efficiency not being guaranteed.

The flyback switching power supply I designed according to the circuit parameters in Figure 1, under the domestic standard input voltage (220V50Hz sinusoidal AC), when tested at 25% of the output power, the output ripple is 45mVpp, and the efficiency is 83.4%; when tested at 50% of the output power, the output ripple is 48mVpp, and the efficiency is 83.8%; when tested at 75% of the output power, the output ripple is 53mVpp, and the efficiency is 84.5%; when tested at full load output power, the output ripple is 58mVpp, and the efficiency is 84.8%. The standby power consumption is less than 0.4W. It meets the requirements of the green switch in Table U-2 (implemented in January 2008) of the CEC (California, USA) "Energy Star" Act.

Switching power supply design engineers must have a deep theoretical foundation and rich design experience and pay attention to design details to meet the design requirements of Level IV of the California Energy Star Act. However, a green switching power supply that meets the "Energy Star" is not necessarily a high-quality power supply, and other parameters must also be met. The green switching power supply only improves the parameter standards related to energy efficiency on the original switching power supply parameters.

With the demand for low-carbon life, the energy efficiency requirements of switching power supplies will become higher and higher, and the design difficulty of switching power supplies will become greater and greater. This requires switching power supply design engineers to constantly update their knowledge, familiarize themselves with new materials, learn new technologies, master new processes, and strive for excellence in technology, so as to design high-quality green switching power supplies.