The grounding of the same secondary reference stabilized voltage IC has the same requirements as the grounding of the primary IC, that is, they cannot be directly connected with the cold ground and hot ground of the transformer. If connected together, the result is that the carrying capacity decreases and the howling is proportional to the output power. When the output load is large and close to the power limit of the power supply, the switch transformer may enter an unstable state: the duty cycle of the switch tube in the previous cycle is too large, the conduction time is too long, and too much energy is transmitted through the high-frequency transformer; The energy storage inductor of DC rectifier is not fully released in this cycle, and PWM determines that there is no driving signal to make the switch on in the next cycle or the duty cycle is too small; The switch tube is in the cut-off state in the following whole cycle, or the conduction time is too short; After more than one full cycle of energy release, the output voltage of the energy storage inductor drops, and the duty cycle of the switch tube in the next cycle will increase again... This cycle will cause the transformer to vibrate at a lower frequency (regular intermittent full cut-off cycle or frequency with drastic change in duty cycle), and produce a lower frequency sound that can be heard by the human ear. At the same time, the output voltage fluctuation will also increase compared with normal operation. When the number of intermittent full cut-off cycles in a unit time reaches a considerable proportion of the total number of cycles, the vibration frequency of the transformer that originally worked in the ultrasonic frequency band will even be reduced, enter the audible frequency range, and emit a sharp high-frequency "whistle". At this time, the switch transformer works in a serious overload state, and it is always possible to burn out - this is the origin of many "screams" before the power supply burns out. I believe that some users have had similar experiences.

    The switch tube may also have an intermittent full cut-off cycle when it is no-load or the load is very light. The switch transformer also works in an overload state, which is also very dangerous. To solve this problem, we can preset the false load at the output end, but it still happens occasionally in some "saving" or high-power power supplies. When the load is not loaded or the load is too light, the back EMF generated by the transformer during operation cannot be well absorbed. In this way, the transformer will couple many clutter signals to your 1.2 winding. This clutter signal includes many AC components of different spectrum. There are also many low-frequency waves. When the low-frequency wave is consistent with the natural oscillation frequency of your transformer, the circuit will form low-frequency self-excitation. The magnetic core of the transformer will not make a sound. We know that the range of human hearing is 20-20KHZ. So when we design the circuit, we usually add a frequency selection circuit. To filter out low-frequency components. From your schematic diagram, you‘d better add a band-pass circuit to the feedback circuit to prevent low-frequency self-excitation, or make your switching power supply a fixed frequency.