Switch Mode Power Supplies (SMPS) can generate common and differential mode noise currents that conduct out to the load and back to the power source, creating both conducted and radiated emissions. Therefore, it is important to filter not only the load side, but the power lines and the SMPS itself. High-end SMPS and other electric or electronic equipment (e.g., Variable frequency drives, solar inverters) benefit greatly from EMI suppression (safety) capacitors and EMI filters. KEMET's Safety Film and EMI Filter offer reliable solutions for energy, automotive, industrial, consumer, and medical applications, and much more.
Differential mode noise can be studied and understood by looking at the schematic, PCB layout, or wiring diagram of the SMPS circuit. Common mode noise is unwanted, difficult to understand, and often involves the physics of currents flowing around parasitic capacitance or another seemingly random source that becomes more complex in high-power electronics.
Common mode noise can be introduced back into the power line when equipment is connected to the local power grid or AC mains. To prevent propagating noise to other equipment that is connected to the AC (power) line, an Electromagnetic Interference (EMI) filter is placed between the AC line and the rectifier in the SMPS. The capacitors filter the power line, decoupling it from any common-mode noise that may be generated by the SMPS, and suppress EMI.
Class-X and Class-Y capacitors are commonly designed for filtering noise from the AC power line (mains) that supplies electric and electronic equipment. They are identified as X-capacitors (CX) or Y-capacitors (CY) depending on the type of noise they help to filter. CX, located between line and neutral, inhibits the SMPS from introducing differential mode interference. CY-capacitors are connected between the power line and the main ground plane, or chassis of the SMPS, and filter out common-mode noise. X- and Y-capacitors are located in the EMI filter before the rectifying stage in the SMPS. Protecting the SMPS from the AC mains and vice-versa adds both EMI filtering and safety.
Figure 1: KEMET Film technology offers a complete family of capacitor solutions that meet each of the specifications required on each stage of an SMPS. For more details, please refer to https://www.kemet.com/en/us/applications/filtering.html
Y-capacitors are typically constructed of metallized polypropylene film, impregnated paper, or ceramic dielectric technology. Metallized film and paper capacitors have excellent self-healing property and can recover from short circuit events and avoid more critical catastrophic failures by typically failing as an open-mode circuit. On the contrary, ceramic capacitors can become unstable with temperatures and time and have no self-healing properties. Ceramic capacitors also tend to short circuit. Since EMI has been always a fundamental difficulty when converting power or integrating different systems, filtering is a primary tool in a designer’s toolbox.
Figure 2: The EMI filter (top) has film capacitors CX and CY. At the bottom is the waveform before and after filtering with EMI suppression capacitors or EMI filter.
After rectifying the AC voltage, capacitors are another key component in an SMPS. The “ideal” design has a power factor of 1.0 and, therefore, can consume all of the energy that can be delivered to it. Power factor correction (PFC) pre-regulators increase the efficiency by raising the power factor and help to reduce the harmonic content in the current input. Capacitors compensate for the loss of real power due to inductive loads. PFC circuitry compensates whenever voltage and current waveforms are out of phase, reducing the level of harmonic distortion. Since the PFC also requires the use of semiconductor switching devices, it requires, together with the input EMI Filter, the use of additional EMI filtering (safety) capacitors.
The switching or switched-mode circuit is the heart of the SMPS. Transistors are switched on and off at high frequencies, creating a clean AC waveform at the desired frequency and levels of voltage and current. An SMPS provides high efficiency with low levels of heat dissipation. However, the switching action introduces ripples, transients, and noise in general. Another filter stage on the output of the SMPS is necessary for high-quality power to the load.
A high-end SMPS will have an output stage since ripple on the output voltage of a switching circuit is inherent. Other areas can also introduce noise (e.g., Parasitic capacitance) — the effects of which can be seen on the output waveforms of the SMPS.
A capacitive filter smooths additional pulses in the output stage so that an almost constant DC voltage is supplied to the load. The output filter charges up to the peak of the input voltage as seen across CF (the positive portion of the input). As the input voltage to the output stage descends below 0V, the capacitor discharges into the load. The rate at which it discharges is influenced by the RC time constant that is formed by load resistance and the capacitor.
Some applications require accuracy and are less tolerant of noise, such as in medical, industrial, and consumer applications. Noise riding on the supply voltage rail to sensitive equipment can cause unexpected results at random moments. In some cases, it can cost a life or huge monetary loss of products in manufacturing, for instance, if noise affects equipment at a critical moment. Before purchasing all new equipment or replacing the power supply, an easy-to-install and predesigned EMI filter can help to resolve EMI issues at a much lower cost and faster design. KEMET offers a variety of EMI/RFI filters.
KEMET’s F862-V054 capacitor is ideal for both the EMI input filtering and the PFC stages of SMPS or any designs with similar requirements. The F862-V054 also fits the criteria for applications that require a higher level of safety and long-term stability under harsh environments. This capacitor has excellent thermal advantages due to remarkably high self-healing performance and resistance to ionization due to its special construction against high temperature and humidity bias conditions.
Excellent temperature-humidity-bias (THB) testing results are critical for identifying adequate performance in real life on harsh environmental conditions. Film capacitors such as KEMET F862-V054 are ideal for withstanding the kind of variable harsh conditions exon boarded in automotive hybrid/EV onboard charging systems, solar-energy microinverters, and smart power meters.
Figure 3: F862-V054 X2 Internal Construction
F862-V054 Class X2 capacitors are constructed with metallized polypropylene film encapsulated in self-extinguishing resin (see figure 3 above). They meet the Automotive Electronics Council’s AEC–Q200 and have a grade of IIB (under THB test with 85°C, 85% R.H. 310 VAC, 500 hrs.) in accordance with the latest IEC standard. They are rated to 310VAC/630 VDC, and have an operating temperature range of -40°C to +110°C.
The SMP253 is the only available SMD, certified safety class-Y2, paper capacitor in the industry. The Y-capacitors in the input filter stage attenuates the common mode noise emitted by the device to the grid/power line or vice versa.
The SMP253 provides the highest performance and reliability of existing capacitor technologies in a surface-mounted device (SMD) package and is ideal for mass assembling small, portable products. Y–capacitors connected from one power line phase leg to earth ground (connected to a chassis) must handle transients without failures that may cause a short circuit or a high leakage current. Lightning strikes are a perfect example of why rechargeable consumer electronics need filters that protect them from dirty power, lightning strikes, or whatever is carried on a public AC power line.
Figure 4: SMP253 Y2 Internal Construction
SMP253 capacitors are constructed with epoxy-impregnated paper as the dielectric material (see figure 4 above). This construction minimizes the risk of formation of internal air pockets. Such pockets may start ionization that over time will oxidize the metallization in the capacitor resulting in loss of capacitance. In humid conditions, the water vapor may enhance the oxidation process and accelerate the loss of capacitance. However, this type of capacitor technology does not exhibit this type of phenomenon. On the contrary, the absorption of water will result in an increase in capacitance due to the contribution of the higher dielectric constant of water.
Figure 5: Accelerated Life, Temperature Humidity Bias test (85° C/ 85% R.H.) on different EMI Suppression Film Technologies. Example: Paper: SMP253 Y2; Heavy Duty: Polypropylene F862-V054 X2; Standard: Polypropylene R46 X2.
During Temperature Humidity Bias, accelerated life test (figure 5), the SMP253 paper dielectric absorbs water vapor with a higher dielectric constant than the others capacitor technologies, resulting in an increase in capacitance value. Conversely, the heavy-duty design (blue trend in figure 5) is a clear representation of the performance of the F862-V054 X2 PP technology, maintaining a very stable low capacitance value drop under similar harsh test conditions for extended hours.
The KEMET SMP253 Y2 maintains safety, filtering, and transient handling capabilities for reducing EMI in a low-profile package. The SMP253 is the best in safety performance for Y-capacitors due to excellent self-healing properties that can prevent catastrophic failures. For critical and demanding applications such as military and medical, which require high-performance as well, the capacitance value must remain constant regardless of how long it must continuously operate. The SMP253 is highly stable if hit with impulse voltage transients, designed for maximum reliability and safety, and it only begins to lose a minimum capacitance value after more than 11 years of constant operation.
The KEMET FLLE2 five series (P, Q, R, S, U) are external EMI filters that were designed specifically for decoupling the common-mode and differential mode noises from equipment that is contaminating shared connections. The external FLLE2 filters clean SMPS-generated noise from influencing other equipment via the AC mains power line. FLLE2-(P,..,U) series are an all-in-one external single-phase filters with high attenuation, which improves the margin for manufacturing variations between systems. The characteristics of the FLLE2’s make them ideal for industrial or medical applications.
Figure 6: Example of circuit configuration for FLLE2-P and FLLE2-U series (lowest and highest attenuations)
The FLLE2’s series are rated 300 VAC/VDC with standard current ratings from 1A to 32A (at 40°C). A range of five filter series offers increased insertion loss performance, a choice of mid-, high-, and extra high-attenuation levels, and medical-grade versions to ensure zero leakage current. There is also an option with flexible wire connections.
Filters are like pre-paid insurance against troubleshooting equipment problems later. However, the quality of filter components can make or break actual results. High-performance filtering can be addressed with quality components from KEMET.
The FLLE2-(P,Q,R,S,U) filter, SMP253, and F862-V054 capacitors offer industry-defining quality for a host of applications that require safe filtering options.
