How to improve the noise of flyback switching power supply
Audio noise generally refers to the audio signal with the frequency of 20-20kHz generated by the switching power supply itself in the process of working, which can be heard by people's ears, especially when it is 2-40kHz, as shown in the following equal-response curve 1. When the oscillation frequency of electronic and magnetic components is within the hearing range of human ears, they will produce audible signals. This phenomenon has been known in the early stage of power conversion research. Transformers operating at 50 and 60Hz power frequency often produce annoying AC noise. If the load is modulated by audio components, the switching power converter operating at a constant ultrasonic frequency will also produce audio noise. This paper first introduces the causes of switching power supply noise, and then expounds the solutions to improve the noise of flyback switching power supply.
How to improve the noise of flyback switching power supply
1. Good layout design suppresses noise.
In the design process, engineers will add absorption circuits at both ends of the field effect transistor DS to reduce the peak, which can effectively reduce the output noise of the power module.
In practical application, the output ripple and noise of the module can be further reduced by adding capacitors to the input and output terminals of the module and cooperating with a good PCB layout. The layout of PCB board, the up-put capacitor according to the current flow direction, and the ripple noise of power module are no longer a problem. The following figure lists two layout methods.
2, transformer audio noise solution
Firstly, the transformer should be soaked uniformly, so as to effectively fill the inherent gaps between coils, between coils and skeleton, and between skeleton and magnetic core, and reduce the possibility of displacement of moving parts. If necessary, the contact surface between magnetic elements and circuit board can be filled with white glue or sprayed with three-proof paint, further reducing the space of mechanical vibration and effectively reducing noise.
If conditions permit, the peak magnetic flux density should be reduced as much as possible, the saturation magnetic flux density at high temperature should be fully considered, and enough margin should be left to prevent the working curve from entering the nonlinear region, which can effectively reduce the audio noise of the transformer. Experiments have proved that the noise emitted can be reduced by 5dB to 15dB when the peak magnetic flux density is reduced from 3,000 Gauss to 2,000 Gauss.
Conditions permit the use of soft magnetic materials such as amorphous and ultra-microcrystalline alloys. Their magnetic uniformity is much better than that of ordinary ferrites, and their magnetostrictive effect tends to zero, so they are insensitive to stress.
3. General solutions to capacitive noise
The solution is to replace the high-voltage ceramic capacitor used in the absorption circuit with a polyester film capacitor with little electrostrictive effect, which can basically eliminate the noise generated by the capacitor.
To determine whether ceramic capacitance is the main noise source, it can be replaced by capacitors of different insulators. Thin film capacitor is a cost-effective substitute. However, attention should be paid to whether the replacement can withstand repeated peak current and voltage stress.
Another option with competitive price is to use Zener clamp circuit instead of RCD clamp circuit. The price of Zener clamp is comparable to that of RCD clamp, but it takes up much less space and is more efficient.
