My opinion on the design of VCC winding of low-power flyback power supply

Regarding the design of the VCC winding of a low-power flyback power supply, the overall reliability of the circuit is very important. In my opinion, the VCC design parameters directly affect the power consumption and temperature of the PWM; affect the protection method of the power supply output short circuit; are related to the fluctuation range of VCC under load and input voltage fluctuations, and directly affect the stability and reliability of the circuit.

Let's use the most commonly used and original 3842 as an example. Usually I will remove the resistor in series with the VCC rectifier circuit. Many colleagues do this to ensure the voltage variation range of VCC under different load conditions. The disadvantage of this is that the existence of this resistor increases the failure probability of the circuit! One more component increases the failure probability.

The principle of circuit design is: let each device play its greatest role! If no one can save any device on the circuit you designed (without affecting the principle of function), your design can be basically considered successful. Secondly, the design voltage selection point should be as low as possible and not too high. For 3842, I chose the 12V calculated value (about 11V when unloaded, about 13.5-14V when fully loaded). To achieve this, it is not a problem that can be solved by simply selecting the voltage point. We need to cooperate with the selection of the starting resistor (this resistance value requires that as long as it can be started, it should be as large as possible to avoid the increase in power consumption. Of course, this parameter needs to meet the range of the PWM chip specification. Different PWMs have different parameters, but the design ideas are basically the same.) There is also the selection of the VCC filter capacitor. One is that the size of the capacity affects the startup time. The conventional solution to this problem is to follow the figure below!

It is worth reminding that: it is best to use a ground leakage capacitor for the 4.7uF capacitor, otherwise it will be difficult for the startup resistor to charge it. When the circuit is not started (the startup voltage of 3842 is above 16V, the voltage on C49 will rise rapidly until the IC starts. At this time, as long as the circuit is started (just start it like the usual hiccup protection, it will be fine, and there will be about 12V on the transformer Vcc, and the circuit will be able to work normally! When designing the winding of the transformer VCC winding, attention must be paid to: good coupling with the secondary, ensuring that the VCC voltage becomes lower instead of the usual increase when the secondary is short-circuited, so that it can be automatically protected. To achieve this, the wire diameter of the VCC winding must be relatively small, and the maximum current is generally around 50mA! Controlling this circuit is achieved by the internal resistance of the VCC winding, and it also replaces the function of the series resistor on the VCC, making it more reliable.

At the same time, the power supply needs to work in CCM state as much as possible, that is, at least under the condition of the highest voltage input, 80% of the load must enter the CCM critical state (at this time, reducing the input voltage to 220-240V is just in the critical state, at this time the switching loss is the smallest and the efficiency is the highest, so it can be said that it is relatively more reliable within our normal voltage range) Input 90V-264V, output 12V5A; operating frequency 65KHZ; transformer PQ2620; PC40&PC44 (TDK). Primary 38TS, VCC and output are 6TS, especially stating that the VCC winding wire diameter is between 0.15-0.17mm; primary inductance 560uH; winding method is primary-secondary-VCC. In this way, the no-load VCC is 10.5V-11V; the full-load VCC with 90V input is 14V-14.5V; the short-circuit VCC first surges to about 18V (in a very short time, about 15uS-30uS, which means it will drop to above the cut-off operating voltage of the IC in less than two cycles. Even if it is loaded slowly, it will be less than 20V). In this way, the circuit is very stable and reliable!