If you have ever wondered why SMPS makes noise, you’re not alone. The feedback loop, fan blowing hot air out of the case, and LC filter all contribute to the noise. The shunt capacitor is also responsible for preventing AC currents from induced in the loop. But what causes SMPS hums? Luckily, there are some passive solutions to minimise noise production. The key is to find out the specific source of the noise.
SMPS’s feedback loop causes noise
The feedback loop in an SMPS circuit generates noise. In most applications, the switching frequency of SMPS circuits is above 20 kHz, which is outside the audible range. However, some topologies automatically vary the switching frequency, resulting in audible switching patterns. Nevertheless, if you’re using an SMPS to power an electronic device, you should avoid it.
SMPS’s audible noise is typically caused by the reverse piezoelectric effect of capacitors and the magnetic field of the current-carrying conductors. While it is unlikely to pose any functional issues, this noise can be annoying. To reduce this noise, you should first identify the causes of the hum. Listed below are some common noise causes and how to reduce them. If you can’t find the source, you can use a non-conductive object to apply light pressure to the individual components of the circuit board. If you find the noise is coming from one of these two components, you can try to find the source.
The noise generated by an SMPS is generated by its feedback loop, and it is necessary to identify its proper layout to ensure that it achieves optimal performance. SMPSs create an alternating magnetic field, which can have adverse effects. Therefore, the design of an SMPS should be such that the output voltage is regulated, not distorted. This noise is also harmful for other circuits. You should place SMPSs in an area away from sensitive circuits, as noise from power supply circuits can affect other signals.
SMPS’s fan blows out hot air from the case
Most SMPS are positioned at the bottom of the case to allow cold air to pass through while the back opening allows hot air to exit. To maximize cooling, the SMPS should be positioned in a position where the fan can blow air outward. You can use a ventilation slot on the back of the case to mount the SMPS. This is the best option for cases with a PSU shroud.
The fan on the back of the SMPS should be large enough to push air out of the case while preventing the junction temperature from going over a certain limit. The fan should also be installed above the heat sink of the CPU. SMPS should be installed with a CPU Stand to prevent hot air from entering the case while the PSU is on. A CPU Stand is also required if the fan on the back is placed at the front.
SMPS’s LC filter prevents large voltage spikes from transmitting into the rest of the circuit
An SMPS has several advantages over other power supplies. The most significant benefit is its high efficiency. It is able to achieve up to 20 times the power density of conventional power supplies. However, it does have a few drawbacks. First, it is prone to large voltage spikes, and second, its LC filter must be highly effective at preventing large spikes from transmitting into the rest of the circuit.
The LC filter’s first notch is approximately 20% lower than the switching frequency of the converter. It is connected to a converter operating with a square-wave current transient of 1A. Therefore, high-frequency oscillations are largely a result of the filter’s first notch frequency. The active damping circuit reduces the 60-Hz component of the filter current.
Second, SMPS’s LC filter reduces ripple voltage. Resonant frequency is twice as high as the fundamental frequency. A typical plug-in-wall power supply runs on 60 Hz AC power, but it is never at resonant frequency. Therefore, it is vital to select a power supply with a low ripple frequency and an LC filter.
LC filter’s 10 pF shunt capacitor prevents AC currents from induced in the loop
The LC filter’s 10 pF inductor is connected to a DC source and the load is coupled to a common LC circuit. The LC filter’s inductor has the same polarity as the AC source, allowing the two to have identical DC bias voltages. The inductor’s inductance can be changed by shorting or tap-changing it. DC bias source details are omitted for simplicity.
An LC filter’s 10 pF inductor can be replaced with a resistor or other DC-dissipating component. This configuration does not have the same inductor characteristics as the LC filter’s shunt capacitor. This means that the capacitor should have a voltage rating at least twice as high as the DC-to-DC converter’s power rating.
An LC filter’s 10 pF inductor is made of electrolytic copper. During normal operation, it will prevent AC currents from entering the loop. The capacitor inductor’s shunt channel is designed to limit the DC-to-DC conversion ratio, which prevents induced AC currents from propagating in the loop.
LC filter’s 9 MO resistor in the tip
The LC filter is an electro-mechanical device with a series-connected inductor and a nine-MO resistor in the tip. The inductor acts as a choke at higher frequencies, blocking the passage of AC components. The capacitor is an effective path for high-frequency components to flow, but the 9 MO resistor in the tip makes noise. The combination of the two components makes a highly effective LC filter.
The LC filter’s 9-MO resistor in the tip makes noise, but this is not as critical as it might sound. The resistance of this LC filter is not as large as the resistors in the first stage, which is the main reason why the second stage is necessary. The 9-MO resistor in the tip makes noise, and it must be placed inside the feedback loop. This avoids regulation penalties, but it requires more engineering involvement and design validation. Its low-noise profile is another plus point. It also requires less BOM and little additional power loss.
A common-mode filter also damps the resonant frequency. A differential-mode filter dampens its resonant frequency with a speaker, but an amplifier without a speaker will have underdamped filter response. A common-mode filter with damping resistors in the tip minimizes the attenuation of high frequencies by damping the response at its resonant frequency.
10x probe’s 9 MO resistor in the tip
When you insert a 9 MO resistor in the tip of a 10x probe, the resulting voltage divider is ten times the normal value. This probe is often called a “low capacitance” probe, but the 9 MO resistor makes noise when the resulting voltage is higher than the signal impedance of the oscilloscope. Unlike a low capacitance probe, which makes noise when the signal is higher than the oscilloscope’s sensitivity, a high-impedance probe is capable of recording very high-frequency signals at a lower frequency.
10x probe
If you use a 10x probe to measure the voltage on an SMPS power rail, you’re likely to experience a lot of noise. These regulators are notorious for generating large amounts of noise, largely due to their high current switching capability. These regulators are also prone to EMI pick-up noise, which can result from the large switching currents radiating from the board. Additionally, the coax cable connecting the low impedance of the power rail to the high-impedance 1 Meg input of the scope will also produce noise.
Fortunately, there are a number of ways to reduce noise from digital circuitry. One way is to install short ground clips at the input terminals of audio equipment. To do this, simply remove the spring clip on the probe’s end. You can then use a small springy ground point to ground the probe. This way, the probe will pick up less noise when you’re measuring digital circuitry.
Another way to reduce noise is to measure the voltage directly on the power rail. The noise generated by this process is the same as that of common-mode noise, but it’s less severe. The probe’s loop area acts as an antenna for common-mode noise currents flowing along the output cables. This noise translates into RF signals that are induced on the ground lead. By detecting this noise, you can eliminate it from your digital circuits and reduce noise and improve its quality.
