Power Figure 3: Piezo effect as demonstrated in materials such as quarz
is known as the ‘reverse piezo effect‘. The reverse piezoelectric effect causes a length change in these materials when electrical voltage is applied. This actuator effect converts electrical energy into mechanical energy. Changes in voltage also alter the geometric mass of ceramic capacitors, resulting in them acting like tiny speakers that emit pressure waves into the vicinity.
Switching topologies and feedback loops
The drive to ever more efficient power conversion means that switching topologies are being integrated into even the simplest power supply products. The primary switching frequency chosen in such designs will often be selected to lie above the limit of human perception (>20kHz). However, in switching solutions that rely on changing their switching frequency to adapt to changing load and input voltage, this may drop into the audible range in order to maintain optimal conversion efficiency. In fixed frequency solutions, features such as cycle skipping or burst mode operation can result in a switching pattern that falls into the audible range, despite the switching
frequency itself lying above 20kHz. If the solution displays regular switching pulses broken irregularly by periods of two or more skipped pulses, this may indicate issues with the feedback circuit (figure 4). Here it is worthwhile reviewing the feedback circuit components and the operating region of any optocouplers.
Determining and resolving audible noise issues
With SMPSs becoming increasingly more compact thanks to the push for ever higher power densities, it can be challenging to even determine which component exactly is the audible noise source. Assuming that the design is operating correctly from an electrical standpoint, one approach is to use a non-conductive object, such as a chopstick, to apply light pressure to individual components on the circuit board while the device is operating. Changes or reductions in noise, especially amongst prime candidate components, such as ceramic or magnetic devices, may provide a good starting point. If there is no safe non-conductive probing device to hand, a rudimentary ear trumpet can be created from a sheet of paper. Rolled
Figure 4: Issues in the feedback circuit can result in irregular pulseless periods (bottom graph) in fixed-frequency switching designs.
up into a cone, the small end’s aperture can be directed toward suspect components to evaluate noise generating sources. Ceramic capacitors that undergo high dv/ dt swings often prove to be audibly noisy and tend to be found in clamp and snubber circuits, as well as in the output stages. To test whether they are the noise source, they can be replaced by capacitors with alternative dielectrics such as metal film, or their series resistor could be increased. Should the audible noise be reduced, a permanent change in component should be evaluated. Changing
clamp circuits to use Zener diodes can also help. Problematic output stage capacitors could be swapped out for a different dielectric or replaced with equivalent value parallel ceramic capacitors if space allows. If magnetic components are the noise source, first ensure that the input voltage and output load are always within the specified range. Increasing capacitance on the input side can help if the input voltage is sometimes dropping too low. Dip varnishing of transformers, and dip varnished and potted inductors, are one approach to reducing noise. Long core length transformers also tend to resonate more audibly than those of short core length. Where possible, consider changing to an alternative shorter core that can still accommodate the required number of windings. It should be borne in mind that, for all of the possible approaches highlighted, the repetition of verification and production testing will be highly likely.
Summary
Both the force impact of current-carrying conductors in magnetic fields and the reverse piezo effect of capacitors are primarily
Figure 5: The capacitor in the snubber circuit can be swapped out for a metal film type, or a larger resistance can be tried.
responsible for the audible noises emitted by power supply units. And, despite advances in simulation, audible noise typically only becomes apparent once a design has been physically built, and sometimes only once a quantity of power supplies has been prepared for pre-production.
Although most audible noise in power supplies should raise little cause for concern from a perspective of functionality or safety, it can be annoying and even be perceived as a quality issue by customers. By following some of the simple tips provided here, components acting as noise sources can be quickly identified and, using the suggested approaches, be replaced, affixed or changed to minimise or eradicate the errant sounds being generated.
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www.cieonline.co.uk. Components in Electronics November 2023 43
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