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When we use laptops, tablets, smartphones, TVs, and in-vehicle electronic devices, we sometimes hear a “chirping” noise, which is called “howling”. The cause of “howling” may be passive components such as capacitors and inductors.
The principles of howling in capacitors and inductors are different, especially the howling of inductors, which has various and complex causes.
Today, let’s share this article to analyze the causes and effective countermeasures of howling in power inductors, the main components of power supply circuits such as DC-DC converters.
Causes of power inductor howling
First:Intermittent operation, variable frequency mode, load changes, etc. may cause vibrations at frequencies audible to the human ear
Sound waves are elastic waves propagating in the air, and human hearing can hear “sounds” in the frequency range of about 20 to 20kHz. In the power inductor of the DC-DC converter, when an AC current and a pulse wave with a frequency in the audible range of the human ear flow through it, the inductor body vibrates. This phenomenon is called “coil noise” and is sometimes heard as a howling phenomenon. The mechanism of power inductor howling is shown in the figure below.
As the functions of electronic devices continue to improve, the power inductor of the DC-DC converter has also become one of the sources of noise. The DC-DC converter generates a pulsed current by turning the switching device on/off. By controlling the length of time (pulse width) of the ON state, a stable DC current with a constant voltage can be obtained. This method is called PWM (pulse amplitude modulation), and it is widely used as the mainstream method of DC-DC converters.
However, the switching frequency of the DC-DC converter is relatively high, reaching several 100kHz to several MHz. Since the frequency vibration is beyond the audible range of the human ear, the noise is not felt. So why does the power inductor of the DC-DC converter emit a “chirping” whistle?
There are several possible reasons. First, it may be that the DC-DC converter is operated intermittently for the purpose of saving battery power, or the DC-DC converter is switched from PWM mode to PFM (pulse frequency modulation) mode and operated in variable frequency mode. The figure below shows the basic principles of PWM (pulse amplitude modulation) mode and PFM (pulse frequency modulation) mode.
Howling caused by intermittent operation of DC-DC converters such as PWM dimming
For the purpose of energy saving, intermittent operation of DC-DC converters has been introduced in the automatic dimming function of the backlight of the LCD display of mobile devices. This is a system that automatically dims the backlight brightness according to the illumination of the use environment to extend the battery life.
There are many ways to dim, among which the method of controlling the length of time the LED is on and off is called PWM dimming. The advantage of the PWM dimming system is that the color change caused by dimming is small, and it is mainly used in the backlight of notebook computers and tablets.
PWM dimming makes the DC-DC converter work intermittently at a low frequency of about 200Hz, and adjusts the brightness by repeatedly turning on/off. In the constant cycle of lighting/off, it will become brighter when the lighting time is adjusted longer, and it will become darker when it is adjusted shorter. In intermittent operation of about 200Hz, the eyes will basically not notice the backlight flicker. However, since it is in the audible frequency of human ears, when the intermittent current flows through the power inductor mounted on the substrate, the inductor body will vibrate due to the frequency, resulting in howling.
Duty cycle:
In a DC-DC converter, the ratio of the ON time relative to the switching cycle (ON time + OFF time of the switching device) is called the duty cycle. When PWM dimming is performed on an LED, the light-on time/(light-on time + light-off time) is called the duty cycle and represents the brightness.
Howling caused by variable frequency mode DC-DC converter
The characteristic of PWM DC-DC converter is that its efficiency can be as high as about 80~90% or more in normal operation. However, the efficiency will be seriously reduced under light load conditions such as standby time. The loss caused by switching is proportional to the frequency. For this reason, constant switching loss occurs under light load conditions, which reduces the efficiency.
Therefore, in order to improve this problem, a DC-DC converter that automatically replaces the PWM method with the PFM (pulse frequency modulation) method is used under light load conditions. The PFM method is a method of controlling the switching frequency under a fixed ON time in conjunction with load reduction. Since the ON time is constant, the switching frequency will gradually decrease by extending the OFF time. Since the switching loss is proportional to the frequency, high efficiency can be achieved under light load conditions by reducing the frequency. However, the frequency after the reduction will enter the range of about 20~20kHz, which is audible to the human ear, and the power inductor will whistle at this time.
Howling caused by load
In order to save battery power, various power saving technologies are used in mobile devices such as laptops, which may cause inductors to whistle. For example, in order to balance low power consumption and processing power, the CPU of a laptop computer has a mode that periodically changes the consumption current. When the cycle is in the audible frequency range of the human ear, the power inductor may whistle due to this influence.
The role of the power inductor in the DC-DC converter:
The inductor allows the DC current to flow smoothly, and for the changing current such as the AC current, it generates an electromotive force in the direction of preventing the change through self-induction, and plays the role of resistance. At this time, the inductor converts electrical energy into magnetic energy, accumulates it, and releases it after converting it into electrical energy. The amount of this energy is proportional to the inductance value of the inductor.
Power inductors, also known as power coils and power chokes, are the main components used in switching power supply circuits such as DC-DC converters. By coordinating with capacitors, they make the high-frequency pulses generated by the ON/OFF of switching devices smoother.
Since large currents flow through power inductors in power supply circuits, winding types are the mainstream products. This is because by using high-permeability magnetic materials (ferrite or soft magnetic metal) in the core, high inductance values can be achieved with fewer turns, making the product more compact.
