Power supply - The reflected voltage of the flyback power supply has another determining factor
The reflected voltage of the flyback power supply is also related to a parameter, that is, the output voltage. The lower the output voltage, the larger the transformer turns ratio, the larger the transformer leakage inductance, and the higher the switching tube withstand voltage, which may break down the switching tube and absorb The greater the power consumption of the circuit, the permanent failure of the snubber power device may occur (especially the circuit using the transient voltage suppression diode). Care must be taken in the optimization process of designing a low-voltage output low-power flyback power supply. There are several ways to deal with it:
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 supply, reduce loss, reduce output ripple, and improve the cross adjustment rate of multi-channel output power supply. It is generally common in switches for household appliances Power supply, such as CD player, DVB set-top box, etc.
2. If the condition does not allow to increase the magnetic core, the only way to reduce the reflected voltage is to reduce the duty cycle. Reducing the reflected voltage can reduce the leakage inductance, but it may reduce the power conversion efficiency. The two are a contradiction. There must be a substitution process to find a suitable point. During the transformer substitution experiment, the primary side of the transformer can be detected. Inverting the peak voltage, reducing the width and amplitude of the anti-peak voltage pulse as much as possible can increase the operating safety margin of the converter. Generally, the reflected voltage is more suitable at 110V.
3. Enhance coupling, reduce loss, adopt new technology, and winding process. In order to meet safety regulations, the transformer 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 performance 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, and the insulator between the primary and secondary is removed to enhance the coupling, and even wide copper can be used for winding.
The low-voltage output in this article refers to the output less than or equal to 5V. Like this kind of low-power power supply, my experience is that if the power output is greater than 20W, the forward type can be used to obtain the best cost performance. Of course, this is not absolute. Personal habits are related to the application environment. Next time, I will talk about the magnetic core for the flyback power supply and some understanding of the air gap in the magnetic circuit. I hope you can give me some advice.
The magnetic core of the 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 the air gap. I understand the principle of why the air gap is open: since the power ferrite also has a working characteristic curve (hysteresis loop) that is similar to a rectangle, the Y axis on the working characteristic curve represents the magnetic induction intensity (B), and the current production process is generally The saturation point is above 400mT. Generally, this value should be 200-300mT in the design. The X-axis indicates the magnetic field strength (H). This value is proportional to the magnetizing current strength. Opening the air gap in the magnetic circuit is equivalent to tilting the hysteresis loop of the magnet 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 stored in the switch tube. When it is discharged to the load circuit through the secondary of the transformer, the air gap of the flyback power core has two functions. One is to transfer more energy, and the other is to prevent the core from going into saturation.
The transformer of the flyback power supply works in a unidirectional magnetization state, not only to transfer energy through magnetic coupling, but also to undertake multiple functions of voltage conversion input and output isolation. Therefore, the treatment of the air gap needs to be very careful. If the air gap is too large, the leakage inductance will increase, the hysteresis loss will increase, and the iron loss and copper loss will increase, which will affect the overall performance of the power supply. Too small an air gap may saturate the transformer core, causing damage to the power supply
The so-called continuous and discontinuous mode of the flyback power supply refers to the working state of the transformer. In the full load state, the transformer works in the working mode of complete energy transfer or incomplete transfer. Generally, it should be designed according to the working environment. The conventional flyback power supply should work in continuous mode, so that the loss of the switch tube and the line is relatively small, and the working stress of the input and output capacitors can be reduced, but there are some exceptions. It needs to be pointed out here: due to the characteristics of the flyback power supply, it is more suitable to be designed as a high-voltage power supply, and the high-voltage power supply transformer generally works in discontinuous mode. I understand that because the output of the high-voltage power supply needs to use a high-voltage rectifier diode. Due to the characteristics of the manufacturing process, the high reverse voltage diode has a long reverse recovery time and low speed. In the continuous current state, the diode recovers when there is forward bias, and the energy loss during reverse recovery is very large, which is not conducive to the performance of the converter. The improvement will reduce the conversion efficiency at least, the rectifier tube will heat up seriously, and even burn the rectifier tube at worst. Since the diode is reverse biased at zero bias in discontinuous mode, losses can be reduced to a relatively low level. Therefore, the high-voltage power supply works in discontinuous mode, and the operating frequency cannot be too high. 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 frequency modulation and width modulation dual mode. Some low-cost self-excited power supplies (RCC) often use this form. In order to ensure output stability, the transformer The working frequency changes with the output current or input voltage. When the transformer is close to full load, the transformer is always kept between continuous and intermittent. This kind of power supply is only suitable for low power output, otherwise the processing of electromagnetic compatibility characteristics will be very troublesome.
The 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, and there are many The number of turns of the coil, along with the large leakage inductance and distributed capacitance, may not be worth the candle. So how to determine this parameter, through many times of practice and analysis of the design of the peers, I think that when the nominal voltage is input, the output reaches 50%~60%, and it is more appropriate for the transformer to transition from intermittent to continuous state. Or in the state of the highest input voltage, when the output is fully loaded, the transformer can transition to a continuous state.
