Solutions to improve Sound Current Noise Caused by Ripples in Switching Power Adapters
power adapters
Compared with linear power adapters, the most prominent advantage of switching power adapters
(including AC/DC converters, DC/DC converters, AC/DC modules, and DC/DC modules) is their high conversion efficiency, which can generally reach 80% to 85%, and up to 90% to 97%; Secondly, the switch power adapter uses high-frequency electronic transformers instead of bulky power frequency transformers, which not only reduces weight but also reduces size, and is increasingly used as a power supply for audio and visual equipment. However, due to its MOSFET switch operating in a high-frequency switching state, the output ripple and noise voltage are relatively high, generally around 1% of the output voltage (the low is about 0.5% of the output voltage), and the best products also have ripple and noise voltage of tens of millivolts (mV). Although the operating frequency of switching power adapters far exceeds the human hearing range, they still generate audio noise under specific load conditions. The excessive current and sound caused by this has always been a headache for many electronic engineers in the design process.
Switching power adapter noise comes from three aspects: PCB design, circuit oscillation, and magnetic components:
1) The circuit oscillates and the output of the power adapter has large low-frequency ripple. It is often caused by insufficient circuit stability redundancy. In theory, the frequency domain method/time domain method or the Routh criterion in system control theory can be used for theoretical analysis. Now? Computer simulation methods can be used to conveniently verify circuit stability to avoid self oscillation, and there are multiple software options available. For circuits that have already been completed, methods such as increasing output filtering capacitors or inductors, changing signal feedback positions, increasing integral capacitors for PI regulation, and reducing open-loop amplification can be used to improve.
2) PCB design
A) Mainly caused by EMI noise, RF noise adjusts the PI regulator to include disturbances in the output error signal. Mainly check whether the high-frequency capacitor is too far away from the related components, whether there is a large C-shaped surrounding wiring, and so on
B) At least two or more PCB lines of the control circuit are shared with the power circuit. PCB copper clad wire is not an ideal conductor, it can always be equivalent to an inductor or resistor. When power current flows through the PCB wire shared with the control circuit, voltage drop occurs on the PCB. When the nodes of the control circuit are dispersed in different positions, the voltage drop caused by power current disturbs the control network and causes noise in the circuit. This phenomenon often occurs on the power ground wire, and attention should be paid to single point grounding to improve it.
3) Magnetic element
Magnetic materials have the characteristic of magnetic to strain, and enameled wires are also subjected to electrodynamic forces in leakage magnetic fields. Under the combined action of these factors, local overtones or 1/N frequency resonance may occur. Changing the switching frequency and immersing the magnetic components in paint can improve.
Below, I will share my insights on this aspect in the product application process with everyone.
Problem analysis:
The speaker is powered by a switch power adapter, and in the absence of signal input, a clear current sound can be heard at the output end. However, when using a DC stabilized power supply or battery power supply, the current sound is minimal and it is difficult to feel when the ear is close to the speaker. After testing and analysis, there is a significant relationship between the current sound and the power supply adapter it is caused by the output ripple of the switch power adapter.
Solution:
Add an LC filter circuit to the front end of the audio power amplifier IC power supply (as shown in the following figure)
After verification, adding the LC filtering circuit to the front end of the power amplifier IC power supply and using a switching power adapter for power supply, the current noise heard from the speaker is minimal, which has greatly improved.
Note: The inductance we use is a wound inductance, and the capacitance is a 1000uF/16V electrolytic capacitor (small volume). If any value of L and C increases, the current sound will be more improved.
Why is there current noise when using a power adapter for audio systems?
Switching power supply is much better than transformer rectifier regulator circuit, with an oscillation frequency of over 1000KHZ, which is impossible for the human ear to hear, and a higher harmonic frequency. However, its biggest problem is the ability of switching power supply to replace the load. High power switching power supply is still a research topic so far. In my opinion, the reason for the noise in audio power supply is as follows: 1. Due to the insufficient ability of switching power supply to replace the load, The power amplifier output stage OCL circuit has a Crossover distortion II. Because the switching power supply has insufficient capacity to replace the load, the overload makes the power supply itself in the state of vibration stop, the interference generated when the oscillation frequency drops to 10-100KHZ, and the output voltage drops greatly, which has an impact on all parts of the power amplifier circuit. It is recommended that you use a low-power butt tube to try the power amplifier output stage OCL circuit.
Post time: Apr-12-2023