Feature: Power supplies
be maintained to avoid the noise peaking. Ten as the output power falls below 1W, the peak current can be held constant, and the switching frequency can linearly decrease from 11kHz to lower frequencies.
Figure 3: Switch node waveform
Solution space Te control strategy detailed here can be modified to skip certain switching frequencies that fall too close to the magnetic components’ mechanical SRF. Magnetic components can also be designed so that the resonant frequency exceeds 20kHz, or is much lower than the minimum switching frequency required, to remain within regulation across the full output power range. Te human ear is most sensitive between 2kHz and 5kHz, so changing the stiffness constant of the winding can also change the resonant frequency. Another way to change the resonant
frequency is by reducing the tension of the magnet wire to reduce the resonant frequency. Creating a service loop to release the strain in the magnet wire as it connects to the bobbin pin can also help reduce the stiffness constant. Replacing the switching regulator
Figure 4: Audio spectrum
with a higher peak-current variant may offer a different range of switching frequencies that excludes the self-resonant frequency, which could solve the audible noise problem. Changing the form of the magnetic assembly or its mechanical structure can also help reduce audible noise, since the SRF may be outside the switching frequency range for one type of mechanical structure. One such example may be a shielded vs. an unshielded core, or even a different manufacturer for the same inductance value.
Figure 5: Noise (in dB) and power (in W) vs. frequency
Addressing audible noise Several solutions address audible noise in a power supply. Changing the control strategy to avoid certain frequencies, or changing the peak current, can reduce audible noise. Changing the magnetic design to change the stiffness constant or the weight of the board or the construction of the coil can also mitigate noise. One or several of these methods can be combined to eliminate or minimise audible noise in a power supply.
www.electronicsworld.co.uk November/December 2020 35
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
