Flyback converter using current mode control

　　In a flyback converter, the primary inductance of the transformer (to be precise, a multi-winding inductor) is usually much smaller than the inductance of similar components in the forward converter. Therefore, during the primary conduction period (conduction period), the primary current change rate is large, and its pulse is a large triangle. The triangular wave is an ideal waveform for current-type control applications because it reduces the effect of noise and sends a well-defined switching level to the current comparator.

　　Current control has two control loops in operation. One is the fast-acting inner loop, which controls the peak current of the primary side and the other is the much slower outer loop, which adjusts the current control loop to make the output voltage constant. The overall function of these two control loops makes the power adapter work like a voltage-controlled current source.

　　The current control mode also has many advantages. First, the primary side of the system is like a high-impedance current source. The effective inductance of the transformer transformer for small signal changes is eliminated from the output filter equivalent circuit, making its transfer function a simple first-order. Therefore, the control circuit has better high frequency response, improved input transient response performance, and power adapter ripple suppression and loop stability are also improved. The second main advantage is the automatic primary side current limiting function without additional components.

　　5V1A power adapter is used for power limitation and current mode control of self-excited oscillation flyback converter

　　The self-oscillation full energy transfer flyback converter is particularly suitable for current-type control. This can be illustrated from the circuit shown in Figure 2.6.4.

　　The voltage across R4 (setting the maximum collector current) cannot exceed 0.6V under any conditions, because at this point Q: will be turned on and the power element Q1 will be turned off. Because the control circuit cannot make the base voltage of Q2 negative, this will be the case regardless of the conditions of the voltage control circuit. As a result, the input current peak value is determined by the VL of R4 and Q2 and cannot be exceeded. Therefore R4 and Q2 provide automatic primary power limitation. The other parts of the control circuit will only lower the limit.

　　Although the circuit is simple, the power limiting effect is very good. Under pulse control, the conduction period ends when the primary current reaches a limited peak value. The current limiting loop also limits the maximum value of the transmitted power, namely 1/2LpIp2ƒ.

　　It should be noted that since the limitation is a constant power limitation, when the output voltage changes to zero under overload conditions, the output current will increase. If this situation is not desired, an additional circuit must be introduced to reduce the power limit when the load changes to a short circuit or cut off the power adapter when the load is overloaded.

　　Also, the slower voltage control loop (R1, R1, V1, OC, and OC1) adjusts the bias of Q2 according to the change in output voltage to reduce the peak current of the primary side required for Q2 to turn on. The voltage control circuit adjusts this value to maintain a constant output voltage.

　　Self-oscillation direct offline flyback converter

　　Because of their simplicity and low cost, these converters provide some of the most economical solutions for low-power, multiple-output requirements. If it is designed well, extremely effective switching action and reliable performance can be obtained. A large number of problems that plague the drive converter, especially cross-conduction and transformer saturation, have been overcome due to the self-oscillation structure. Since it always works in the full energy transfer mode, current-type control can be applied very easily, so a fast and stable single-pole closed-loop response can be obtained.

　　Since the self-oscillating converter requires fewer driving and control components, the cost is extremely low. Note that good filter design and transformer shielding make this converter suitable for computers, video display terminals and similar applications.https://powerstation-gf.com/prodcuts/SPS-001-wall-mounted-home-energy-storage_283.html

　　There is a tendency to think that these simple devices are not really competitive in professional power adapter applications. This misunderstanding may be caused by the poor performance of some early self-oscillation designs. There are also some applications that do not want to consider the variation of the operating frequency with the load and input voltage. However, since the output is DC, in most applications, as long as efficient input and output filtering and magnetic screen wear are provided, the operating frequency is not a problem.

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The power transistor should be used like this in the power adapter