UC3842 is designed to control flyback 50W switching power supply

Add another thousand points and I'll give you more details.

One. The circuit uses a large energy storage filter inductor and a freewheeling diode more than the flyback transformer switching power supply. You can see it here in the basic circuit, two.

The modulation amplitude of the output voltage of the switching power supply of the forward transformer by the duty cycle is much lower than that of the switching power supply of the flyback transformer, therefore, the error signal amplitude of the switching power supply of the forward transformer requires that the duty cycle is relatively high, and the gain and dynamic range of the error signal amplifier are also relatively large. Third, in order to reduce the excitation current of the transformer and improve the work efficiency, the volt-second capacity of the transformer is generally relatively large, and in order to prevent the reverse electromotive force generated by the primary coil of the transformer from breaking down the switch tube, the transformer of the forward transformer switching power supply is more than the transformer of the flyback transformer switching power supply with one more back electromotive force absorption winding, therefore, the volume of the transformer of the forward transformer switching power supply is larger than the volume of the transformer of the flyback transformer switching power supply. Four.

Another greater disadvantage of the forward transformer switching power supply is that when the control switch is turned off, the back electromotive force voltage generated by the primary coil of the transformer is higher than that generated by the reverse electromotive force voltage generated by the flyback transformer switching power supply. Because when the switching power supply of the general forward transformer is working, the duty cycle of the control switch is taken on the left and right, and the duty cycle of the control switch of the switching power supply of the flyback transformer is relatively small. The main thing is that it's more difficult to adjust.

The difference in application is that flyback is mainly used below 150-200 watts, while forward is used between 150W and several hundred watts. The reason why flyback is more widespread is because the power supply below 100W is more common in our daily life, and the application is more common, so it is more extensive. The principle is that one transforms the voltage through energy storage and then through the conversion ratio, and the other is directly transforms the voltage through the conversion ratio.

The positive primary winding of the same name is the positive pole, so it is called positive, and the flyback is called positive because one is positive and the other is negative, so it is called flyback.

When the UC3842 was designed, it was finalized as a flyback type, and the flyback switching power supply circuit was simple in design and low in cost; The design of forward switching power supply circuit is complex and costly.

Forward specifically refers to the fact that when the switch is turned on, the output transformer acts as a medium to directly couple the magnetic field energy, and the electrical energy is converted into magnetic energy, and the magnetic energy is converted into electrical energy, and the input and output are carried out at the same time.

The structure of the forward switching power supply is slightly complex, but the output power is much larger than that of the flyback switching power supply, so it is widely used.

Advantages: The power is larger than the flyback switching power supply, the output transformer utilization rate is high, and it is suitable for 100W 300W switching power supply.

Disadvantages: It is necessary to add a back EMF winding, or a topological drive, and add 1 more rectifier inductor in the secondary stage, which is costly.

Because UC3842 has been finalized as flyback when designing, the flyback switching power supply circuit is simple in design and low in cost; The design of forward switching power supply circuit is complex and costly.

Flyback is relatively simple and convenient. Forward excitation is more complex.

Detailed explanation of the circuit UC3842 circuit that electronics beginners must understand.

The design of the transformer mainly considers whether the magnetic induction intensity is too large, and the magnetic induction intensity is too large at high frequency to increase iron consumption. On the other hand, the excitation current is too large. However, the excitation current energy in the flyback topology is directly transmitted to the secondary side, that is, to make the excitation current larger, so there is no need to consider the latter, only the former.

A pulsed transformer in a flyback topology should be considered as a mutual inductance.

Choose a magnetic material according to the frequency, your frequency of 500kHz should be 3F3 or something, check the manual to know.

Then determine the size.

First, the flux change of the primary edge is calculated, and the magnetic field in the flyback topology is used in one direction, that is, the flux change is the flux amplitude, which is used to determine the maximum magnetic flux bm. It can be known from Faraday's law of electromagnetic induction.

psimax=umax*tonmax=16*2us=32uwb

bmax=phimax/s=psimax/n1/ae

The bmax is much smaller than the saturation magnetic induction intensity BSAT to reduce the iron loss, and here the approximate N1*AE can be determined, and AE is the effective cross-sectional area.

Then calculate the excitation current energy, because the excitation current grows linearly when the switch is turned on, and its slope is affected by the primary self-induction, so the primary self-inductance can be determined. If you design with a large output, then make the rubber L1 smaller, i.e. increase the LE and reduce the AE (this has been calculated when determining the BMax). The improvement of LE is naturally the increase in the volume of the entire transformer.

When the switch tube is turned on, the rate of change of the beam Xiangxiang current di dt=u l, considering the worst-case scenario, the minimum of 10v allows your current voltage to be integrated in a cycle, that is, the power slightly exceeds your maximum output power. In this way, it is determined that l1 is the original edge self-inductance.

L1=PM*N1 2=UI*U0*AE LE, where UI is the relative permeability, U0 is the absolute permeability of the vacuum, AE is the effective cross-sectional area, and LE is the core length. In the case of an air gap, this formula is not used, the general core will give PM, and the self-inductance of random cultivation can be calculated by multiplying PM by the square of the number of turns.

After the primary edge self-sensing is determined, you notice that l1=pm*n1 2, if you choose a smaller pm, that is, le is larger, and the volume is large, the number of turns can be less. On the contrary, if the volume is small, the number of turns will be more. Balance it yourself.

My explanation is also only the simplest principle, there are definitely many loopholes, learn from each other.