Why is the flyback circuit automatically equalized when multiple outputs? Thank you! 5

When the flyback circuit is multi-output.

Flyback: It is suitable for low-power power supply below 200W, and low-power electronic products are more popular in daily applications. Switch tubes.

At cut-off, energy is delivered to the secondary stage, the circuit is simple, the number of components is small, the cost is relatively low, and although the filter inductor is used in the output circuit, the requirements are not high (the fixed value is generally used, and there is no need to calculate).

Forward type: The switch tube transmits energy when it is on, and is suitable for power supply circuits above 200W. Its high-frequency transformer transmission efficiency is higher than that of flyback, which can make the transformer smaller and output ripple.

It is smaller than the flyback type, but the parameters of the filter inductance should be calculated, and the disadvantages of the forward type are: the switching loss is greater than that of the flyback, the noise is greater than that of the flyback, and the number of components is more than that of the flyback. Electronic products above 200W are less used in daily life, and flyback is suitable for low-power power supply below 200W, while low-power electronic products are more popular in daily applications, which is the reason why the flyback type is used more than the forward type.

In fact, when the flyback power supply is multi-output, only the main feedback can ensure the voltage accuracy, and the other channels are affected by the leakage inductance of each winding on the secondary side of the transformer, and there is a difference in the cross-modulation ability, which is related to the degree of winding coil coupling;

It is necessary to ensure that the main feedback has a certain load to ensure that the power supply remains stable when the other channels are loaded.

Because it is a closed-loop control system, it can be adjusted automatically.

You can only find the main output voltage stabilization, and other voltages will have some fluctuations, such as output 5V and 12V, 5V is used to stabilize the voltage, and 12V is normal at 11--13V.

The flyback circuit circuit is simple, and the forward circuit is mostly used in high power.

The counterattack power supply is based on the energy storage of the inductor (transformer) to release energy to the output capacitor through the secondary, that is to say, the main shock power tube and the output rectifier tube are not working synchronously, if there is no feedback circuit to strictly control the pre-duty cycle, and the output is no-load, the energy of the inductor will have nowhere to be released, which will cause the voltage of the secondary and primary coils to rise a lot (the theory is that the voltage is infinitely high), and the main shock power tube will be damaged by breakdown (overheating). Many flyback power supplies now have feedback circuits and can also be no-loaded.

The power tube and the output rectifier tube of the forward power supply work synchronously, if the output terminal is unloaded, the primary inductance will increase a lot, and only a small no-load current flows through the primary coil (the no-load loss is only a few percent to ten, and the coil voltage will not rise), so it can run without load. In general, the efficiency of a forward power supply is higher than that of a flyback power supply. You can also QQ and I 844468403 learn together.

The basic efficiency of more than 100W must reach more than 85%, and the mountain wax can be regarded as up to the standard In the power supply of Kai La, the efficiency of small power is more difficult, and the efficiency of high power is relatively good to do some rotten, after all, the base is relatively large, and the efficiency of 100W is 80% Compared with 5W, the 5W circuit can only be consumed, and the 100W can be lost on the circuit 25W

Reasons for adding air gaps to flyback circuits:

1. The air gap of the magnetic core is to prevent the core saturation of the flyback converter. There are two functions of the open air gap: one is to control the inductance, the appropriate inductance can meet the design requirements, the inductance is too large and the energy cannot be charged, and the inductance is too small, the current stress of the switch tube increases; The second is to reduce the magnetic flux density b.

Assuming that the amount of inductance, current, and magnetic material have all been determined, increasing the air gap can reduce the operating magnetic flux density of the inductor and prevent saturation.

2. The first is to open the air gap to achieve the required inductance. Because the energy stored by the flyback circuit when the switch tube is turned on is related to the inductance, if the inductance is large, the energy stored in the on-time is small. In this way, in order to meet the requirements of output power, the DC point will be automatically increased, that is, the minimum primary current will be increased, so that the circuit will work in a continuous state.

Theoretically, this will reduce the peak current of the primary and secondary sides, which is beneficial to the circuit. However, this will also shift the magnetic induction intensity generated by DC upward, and the magnetic core tends to be saturated, which leads to another purpose of opening the air gap.

3. Absorb DC magnetic field to avoid core saturation. For closed magnetic circuits, a small DC current is enough to saturate it, as mentioned above, on the one hand, from the circuit level, a large inductance is beneficial to the circuit parameters, and a large inductance means that the air gap needs to be reduced (of course, the number of turns can also be increased), but at the same time, the air gap for the core should be larger to not be saturated, in fact, the difficulty of design is how to calculate the best point.

4. If the primary inductance obtained by using the closed core, the minimum primary current of the primary is still less than 0, so that there is no air gap, but the power of the circuit to meet such conditions must not be large.

The high-frequency transformer of the flyback switching power supply is in the first quadrant of the hysteresis loop, during the switch-on period, the primary coil of the transformer only stores energy, and the energy stored in the primary coil is transferred to the secondary stage during the cut-off period, so it is both a transformer and an energy storage inductor. Opening a certain air gap in its center column can reduce the residual magnetic field, increase the DC magnetic field strength of the core, so that it can withstand a large number of ampere turns, prevent the core from saturation, and can achieve the required inductance (al-value=l1 n1) by adjusting the air gap.

The output characteristics of voltage and current of flyback switching power supply are worse than those of forward switching power supply.

The flyback switching power supply does not provide power output to the load during the control switch is turned on, and only converts the stored energy into back electromotive force to provide output to the load during the shutdown of the control switch, but the duty cycle of the control switch is about half of the voltage output by the transformer secondary coil, and the current flowing through the load is exactly equal to a quarter of the maximum current of the transformer secondary coil.

The flyback circuit itself has a small output power and is not allowed to carry heavy loads. Under the same component, the output current of the flyback circuit is 1 4 of the forward circuit. Do not use flyback circuits if it is not necessary, it is inefficient and has low power.