Feature: Power supplies
switching frequencies were outside the audible range (> 20kHz). Figure 3 shows the waveform captured at one of the load currents. T e 13.16kHz frequency was matched with the spectrum generated by the app that captured the peak of the sound coming from the drum core inductor at 13.242kHz (< 1% error). T e audible noise was recorded by the
phone’s microphone. T en a spectrum analysis app installed on the phone was used to plot all the noise frequencies and their respective decibel levels, to measure the volume of the sound.
Figure 1: Application of Hooke’s law to PCB assembly and below as a a differential equation
from the inductor. T e load current on the converter was varied to sweep the diff erent switching frequencies, and the resulting sound was measured by the phone. T e oscilloscope measured the switching
with Equation 1: (1) Figure 2: Experimental setup
where k is the spring’s stiff ness constant, and m is the mass.
Experimental setup T e power converter used for this experiment was MP174A from MPS, a constant peak current regulator with adjustable frequency. With this device, the switching frequency changes proportionately with changes in load current and output power to maintain regulation. Figure 2 shows the lab setup to measure
the audible noise generated by a drum core inductor and to fi nd the mechanical SRF. T e spectrum analyzer app and phone microphone were used to measure the sound. T e phone was always placed 5cm
34 November/December 2020
www.electronicsworld.co.uk
frequency at diff erent load currents, and then the sound was measured at each load current. T e oscilloscope waveforms matched the frequency peak measured by the phone at each load current. T e load current varied from 10% to
80%. Measurements above 80% of the full load were not taken because their
Graphical analysis T e audible noise frequency is the same as the switching frequency due to the correlation between the switching waveform and the audible noise measured by the spectrum analysis app; see Figure 4. However, a noise peak is observed at the 12.4kHz switching frequency. T is peak shows an increase of 10dB
in the noise amplitude at 1W of output power. T is is due to the mechanical self- resonant frequency of the inductor, and can be mitigated by avoiding switching frequencies between 11kHz and 13kHz. Instead of keeping the peak current constant, as is the case for the MP174A, it can be reduces linearly, and a minimum constant switching frequency of 13kHz can
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