Electronic Design - The duty cycle of flyback power supply in PCB design

Regarding the duty cycle of the flyback power supply in the switching power supply PCB design, in principle, the maximum duty cycle of the flyback power supply should be less than 0.5, otherwise the loop is not easy to compensate and may be unstable, but there are some exceptions. The duty cycle is determined by the turns ratio of the primary and secondary sides of the transformer. My opinion on flyback is to first determine the reflected voltage (the output voltage is reflected to the primary side through the transformer coupling), and the reflected voltage increases within a certain voltage range. The duty cycle increases, and the switching tube loss decreases. The reflected voltage decreases, the duty cycle decreases, and the switching tube loss increases. Of course, this is also a prerequisite. When the duty cycle increases, it means that the output diode conduction time is shortened. In order to keep the output stable, more time will be guaranteed by the output capacitor discharge current, and the output capacitor will withstand greater high frequency. Ripple current washes away and heats up, which is not allowed under many conditions. Generally, the reflected voltage of the PCB evaluation board should be lower than this value at about 110V. Each of these two types has advantages and disadvantages:

The first category: Disadvantages: Weak anti-overvoltage capability, small duty cycle, and large primary pulse current of the transformer. Advantages: transformer leakage inductance is small, electromagnetic radiation is low, ripple index is high, switching tube loss is small, and the conversion efficiency is not necessarily lower than the second type.

The second category: Disadvantages The loss of the switch tube is larger, the leakage inductance of the transformer is larger, and the ripple is worse. Advantages: stronger resistance to overvoltage, larger duty cycle, lower transformer loss, and higher efficiency.

There is another determinant factor for the reflected voltage of the PCB flyback power supply. The reflected voltage of the flyback power supply is also related to a parameter, which is the output voltage. The lower the output voltage, the greater the transformer turns ratio, the greater the leakage inductance of the transformer, and the switching tube bears. The higher the voltage, the greater the possibility of breakdown of the switch tube and the greater the power consumption of the absorption circuit, which may cause permanent failure of the absorption loop power device (especially the circuit using transient voltage suppression diodes). Care must be taken in the optimization process of designing low-voltage output and low-power flyback power supplies. There are several treatment methods:

1. Use a magnetic core with a higher power level to reduce leakage inductance, which can improve the conversion efficiency of low-voltage flyback power supplies, reduce losses, reduce output ripple, and improve the crossover adjustment rate of multiple output power supplies. It is generally common in household appliances switches. Power supply, such as CD player, DVB set-top box, etc.

2. If conditions do not allow to increase the magnetic core, only the reflected voltage can be reduced and the duty cycle can be reduced. Reducing the reflected voltage can reduce the leakage inductance but may reduce the power conversion efficiency. The two are a contradiction. There must be a replacement process to find a suitable point. During the transformer replacement experiment, the primary side of the transformer can be detected. Anti-peak voltage, try to reduce the width and amplitude of the anti-peak voltage pulse, which can increase the working safety margin of the converter. Generally, the reflected voltage is more appropriate at 110V.

3. Enhance coupling, reduce loss, adopt new technology, and winding process. In order to meet safety regulations, transformers will take insulation measures between the primary side and the secondary side, such as insulating tape and insulating tape. These will affect the leakage inductance of the transformer. In actual production, the primary winding can be used to wrap the secondary winding. Or the secondary is wound with triple insulated wire, eliminating the insulator between the primary and secondary, which can enhance the coupling, and even can be wound with wide copper skin.

The low-voltage output in the article refers to the output less than or equal to 5V. Like this type of low-power power supply, my experience is that the power output is greater than 20W, the output can be forward-excited, and the best cost performance can be obtained. Of course, this is not absolute. Personal habits are related to the application environment. Next time, I will talk about some understanding of the magnetic core for flyback power supply and the open air gap in the magnetic circuit. I hope you will give some advice.

The core of the PCB flyback power transformer is working in a unidirectional magnetization state, so the magnetic circuit needs to open an air gap, similar to a pulsating DC inductor. Part of the magnetic circuit is coupled through an air gap. Why the principle of opening the air gap I understand as: because the power ferrite also has an approximate rectangular operating characteristic curve (hysteresis loop), the Y axis on the operating characteristic curve represents the magnetic induction intensity (B), the current PCB production process Generally, the saturation point is above 400mT. Generally, this value should be between 200-300mT in the design. The X-axis represents the magnetic field intensity (H), which is proportional to the magnetizing current intensity. Opening the air gap in the magnetic circuit is equivalent to tilting the magnet's hysteresis loop to the X axis. Under the same magnetic induction intensity, it can withstand a larger magnetizing current, which is equivalent to storing more energy in the magnetic core. This energy is cut off at the switch tube. When it is discharged to the load circuit through the secondary of the transformer, the open air gap of the PCB flyback power supply magnetic core has two functions. One is to transfer more energy, and the other is to prevent the magnetic core from entering a saturated state.

There is also a type of flyback power supply that works in a critical state. Generally, this type of power supply works in frequency modulation mode or dual frequency and width modulation mode. Some low-cost self-excited power supplies (RCC) often use this form. In order to ensure stable output, transformers The operating frequency changes with the output current or input voltage. The transformer always remains between continuous and intermittent when it is close to full load. This kind of power supply is only suitable for low-power output, otherwise the treatment of electromagnetic compatibility characteristics will be a headache.

The PCB flyback switching power supply transformer should work in continuous mode, which requires a relatively large winding inductance. Of course, there is a certain degree of continuity. It is unrealistic to pursue absolute continuity too much. It may require a large magnetic core, which is very much The number of turns of the coil, accompanied by large leakage inductance and distributed capacitance, may outweigh the gain. So how to determine this parameter? After many times of practice and analysis of the PCB design of the peers, when the nominal voltage is input, the output reaches 50%~60% and the transformer transitions from intermittent to continuous state. Or in the highest input voltage state, when the full load output, the transformer can transition to a continuous state.