EMC Problems in Switching Power Supplies and How to Solve Them

Efficiency and Noise

A switch-mode power supply chops the input voltage by switching a transistor on and off rapidly and filters these pulses with an inductor and capacitor to obtain the required output voltage. Very little energy is lost while the transistor is fully on or fully off, which is why switch-mode supplies are much more efficient than linear regulators.

But at the moments the transistor switches on and off, voltage and current change very fast. These rapid changes produce harmonics extending far above the switching frequency. The faster the switching, the higher the efficiency, but the wider the frequency content of the noise produced. This requires a constant balance between efficiency and EMC in power supply design.

Noise Paths

The noise generated by the power supply spreads along three main paths. The first is noise conducted into the mains through the input cable. This is measured in the conducted emission test and is usually suppressed with the input filter. Differential-mode noise flowing between phase and neutral dominates at low frequencies, and common-mode noise returning through earth at high frequencies.

The second path is noise radiated directly into the air from fast current loops and the switching node on the board. The third is the transfer of the switching node's fast voltage changes through parasitic capacitances to the housing, heatsink and cables. This third path is the main source of common-mode noise and is responsible for a significant part of radiated emission problems. We explain conducted emission measurement in our article on conducted emission.

• Noise conducted into the mains through the input cable
• Radiation into the air from fast current loops
• Common-mode noise from the switching node through parasitic capacitances

Input Filter Design

The input filter is the basic tool for suppressing the noise a power supply feeds into the mains. X capacitors and differential chokes are used for differential-mode noise, and common-mode chokes and Y capacitors for common-mode noise. The size of the filter is set by the power supply's power, switching frequency and target emission class.

The value of Y capacitors is limited by leakage current limits; these limits are defined in electrical safety standards and are very low, particularly in medical devices. The common-mode choke therefore plays a large role in suppressing common-mode noise. The placement of the filter is as important as its values: input and output sides should be kept apart and filter components placed with short connections.

Board Layout Measures

One of the most important factors in a power supply's EMC performance is board layout. The loops where current changes rapidly during switching, the hot loop formed by the input capacitor, switch and diode or synchronous switch, must be kept in as small an area as possible. The area of this loop directly determines the size of the radiated magnetic field.

The switching node, the copper area where voltage changes rapidly, should not be larger than necessary, because it radiates common-mode noise through parasitic capacitances. The switching node should not run near sensitive signals, connectors or areas close to the housing. A continuous ground plane under the power supply area also helps keep loops short. We explain these rules in detail in our article on EMC rules in PCB design.

Switching Speed and Damping

The speed at which the transistor switches on and off determines the high-frequency content of the noise. Slowing switching slightly, for example by increasing the gate resistance, noticeably reduces high-frequency noise, but switching losses and heating increase. This balance should be optimised with pre-test measurements.

Oscillations at switching moments, known as ringing, can cause sharp emission peaks at specific frequencies. These oscillations arise from parasitic inductances and capacitances and can be suppressed with suitable damping circuits, or snubbers. Comparing the ringing frequency with peaks in the emission spectrum makes finding the source easier.

Does Spread Spectrum Help?

Some power supply controllers offer spread spectrum, which constantly varies the switching frequency over a certain range. This technique spreads the noise energy over a wider frequency range instead of concentrating it at a single frequency. Because emission measurements are made with a specific bandwidth, the measured peak values can drop noticeably.

However, spread spectrum does not reduce total noise energy; it only distributes it. So in some sensitive applications, for example where a nearby radio receiver listens to a wide band, it may not give the expected benefit. It can also cause low-frequency ripple on the output voltage. Spread spectrum should be seen as a complement to, not a replacement for, good filter and layout design.

Ready-Made Module or Custom Design?

Ready-made power supply modules are usually EMC-tested and documented products. But a module passing tests on its own does not mean the whole product will pass. The cables connected to the module's output, the board layout and the product housing can change total emissions. It is important to follow the input filter and layout rules recommended by the module manufacturer.

A custom design offers the solution best suited to the product, but requires more design effort and pre-testing for EMC. When choosing, the product's power level, production volume, cost target and development time should be evaluated together. Whichever option is chosen, measurements in a pre-compliance room are the most effective way to find problems early. For compact test infrastructure, see our SAC-3 page.