Minimizing Noise Generated by Switched-Mode Power Supplies

By Ashok Bindra

Contributed By Electronic Products

From servers in data centers to telecom equipment and industrial systems, switched-mode power supplies (SMPS) are used in a wide range of applications because of benefits such as high efficiency, power density, and faster transient response at low cost. However, while offering many advantages, SMPS supplies such as switching buck and step-up DC/DC converters and point-of-load (POL) regulators are known to generate noise. This noise is undesirable in many applications seeking to maintain data integrity and high performance. In addition, to pass stricter new regulatory standards EMI generated by power supplies must be kept lower than ever before.

In reality, there are many different types of noise generated by the switching frequencies of these power supplies. In a December 2015 article entitled "Reducing Noise Generated by Switching Regulators"[1] Analog Devices power business technical manager Frederik Dostal identifies them as switching noise from the switching-frequency, high-frequency noise resulting from the switching transitions, ringing after switching transitions, and beat frequencies caused by multiple switching regulators running in one system.

Here we will examine these different types of noise generated by switching regulators and DC/DC converters, and discuss solutions, including filtering techniques, to reduce and minimize this noise in switching SMPS supplies.

SMPS noise

According to Dostal, the dominant noise type is the switching noise generated by the switching frequency of the supply. Typically, he says, the frequency band for this noise is between 500 kHz and 3 MHz for non-isolated DC/DC converters. However, because it is dependent on the switching frequency, it can be easily controlled and filtered out using a low-pass filter. The switching noise results in output ripple voltage, which is shown in Figure 1. It can be easily filtered out using a passive LC low-pass filter or an active low-pass filter.

Output ripple voltage due to switching frequency of the switching regulator (top). Attenuated ripple voltage using LC filter is shown at the bottom.

Figure 1: Output ripple voltage due to switching frequency of the switching regulator (top). Attenuated ripple voltage using LC filter is shown at the bottom.

Before we get into filter design, however, let's examine output ripple voltage in some detail.

As shown in equation 1, the output ripple voltage of the switching regulator can be accurately calculated by the inductor current ripple, which is based on the actual inductance value of the inductor, the switching converter's input and output voltages, switching frequency (fSW), and the output capacitor (COUT) including its equivalent series resistor (ESR) and equivalent series inductor (ESL).

As shown in equation 1, the output ripple voltage of the switching regulator can be accurately calculated by the inductor current ripple, which is based on the actual inductance value of the inductor, the switching converter’s input and output voltages, switching frequency (fsw), and the output capacitor (Cout) including its equivalent series resistor (ESR) and equivalent series inductor (ESL).

 

As per ADI's switching converter data sheet, there are some trade-offs with respect to inductor selection. For example, a small inductor gives better transient response at the expense of larger inductor current ripple while a large inductor leads to a smaller inductor current ripple at the expense of slower transient response capability. Using a low-ESR capacitor minimizes the output ripple of the switching regulator. A ceramic capacitor with dielectric X5R or X7R is a good choice. Although large capacitance is often used to lower the output ripple, the size and number of output capacitors can come at the expense of cost and board space.

While some semiconductor suppliers like Analog Devices and Texas Instruments have created tools to assist designers in selecting the right switching converter or regulator with external components based on input- and output-voltage specifications, including switching frequency, passive-component suppliers have crafted LC design tools for power supplies. For example, in partnership with Nuhertz Technologies, Coilcraft offers a design tool that allows one to create elliptic low-pass filters using actual Coilcraft inductance values, not just ideal components. It ensures that the simulated filter comes close to the performance of the real-world filter. Since the frequency of the output voltage ripple is based on the switching frequency of the converter, the corner or cut-off frequency of the low-pass LC filter must be lower than the switching frequency of the converter to attenuate the output voltage ripple.

Similarly, another type of noise generated by the switching frequency is the switching transition noise, which ADI's Dostal calls the most challenging noise to tackle. It is associated with the parasitic inductances in the current path. It includes parasitic inductances resulting from the printed circuit board (PCB) traces, IC package leads, and multilayer PCB vias, and die-bonding wires. As a rule of thumb, Dostal says, an inch of PCB trace has about 20 nH of parasitic inductance. This parasitic inductance generates offset voltage which can be easily calculated using the formula V = L*di/dt. The following example shows the amount of noise generated by the parasitic inductance in a typical modern switching converter or regulator.

Since today's switching regulators employ faster switching MOSFETs in the output stage, the ADI article assumes a switching transition speed of 30 ns in a typical switching regulator design with a 5 A of output current. Using the above formula, the voltage offset generated by the 20 nH parasitic inductance is 3.3 V. This generated offset voltage will appear as unwanted noise in the output of the switching regulator as shown in Figure 2.

The parasitic inductance in a fast-switching buck regulator results in high-offset voltage that appears as undesired noise at the output.

Figure 2: The parasitic inductance in a fast-switching buck regulator results in high-offset voltage that appears as undesired noise at the output.

Obviously, parasitic inductance is a key contributor to this noise. Therefore, it must be minimized by proper PCB layout which, in turn, reduces the offset voltage. Toward that goal one should keep the interconnect traces on the PCB as short as possible and use surface-mount components to eliminate package leads. Because this noise is between 10 MHz and 300 MHz, Dostal recommends using ferrite beads to attenuate this noise.

Voltage ringing, Beat frequencies

Parasitics also generate another type of noise called voltage ringing[2]. It occurs during switching transition on the switch-node and is superimposed on the output ripple voltage as shown in Figure 3.

Switch-node voltage ringing is superimposed on the output ripple voltage. (Courtesy of Analog Devices)

Figure 3: Switch-node voltage ringing is superimposed on the output ripple voltage. (Courtesy of Analog Devices)

Fortunately, it can be reduced by using snubbers or active clamp circuitry, Dostal explains. A passive snubber comprising a resistor and capacitor will dissipate the energy of this ringing into the resistor and generate heat. On the other hand, active clamp circuitry feeds the energy of the ringing partially back into the circuit, increasing the overall efficiency of the power supply.

In systems where multiple switching converters or regulators are deployed to generate multiple voltages to drive a variety of loads like processor cores, I/O interfaces, FPGAs, ASICs, and analog circuits, beat frequencies are a common problem if the switching frequencies of these converters or POL regulators are not synchronized. This problem also occurs in applications when two or more switching DC/DC converters are paralleled either for higher output power capability or higher reliability using an N+1 redundant solution. Now, if the converters are nonsynchronous with fixed-frequency switching around 1 MHz and the source is a common bus, they tend to produce low-frequency noise at the input called beat frequency. It produces undesired input AC ripple current at audio frequencies causing undesired audible sounds along with the ripple current.

An easy way to mitigate this problem is to use integrated-synchronous-switching DC/DC converters like the Analog Devices ADP5135 that offers multiple-switching buck regulators from a single package as illustrated in Figure 4. Because the switching frequency of such a regulator is synchronized to a common source, it eliminates the beat-frequency problem found in nonsynchronous solutions.

ADI's ADP5135 combines three high-performance buck regulators in a single 24-lead, 4 mm × 4 mm-LFCSP package. (Courtesy of Analog Devices)

Figure 4: The ADP5135 combines three high-performance buck regulators in a single 24-lead, 4 mm × 4 mm-LFCSP package. (Courtesy of Analog Devices)

Although using multiple-switching DC/DC converters on a single system board can generate a variety of switching noises at the input and output of such power supplies, suppliers like Analog Devices and Texas Instruments, among others, have generated simple solutions to tackle such problems.

For more information on the products discussed in this article, use the links provided to access product pages on the Digi-Key website.

References:

  1. "Reducing Noise Generated by Switching Regulators" by Frederick Dostal, Analog Devices, Munich, Germany, How2PowerToday, December 2015.
  2. Application Note AN-1144, "Measuring Output Ripple and Switching Transients in Switching Regulators" by Aldrick S. Limjoco, Analog Devices, Norwood, MA

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Ashok Bindra

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Electronic Products

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