Input filter caps need to be able to supply a quick burst of energy and to suppress noise generated in the switch circuit. Important considerations for the input filter cap are ESR, ESL, and ripple current. High CV density is preferred in the input filter caps to reduce board space, although it is more critical for the output filter caps.

Output filter caps must allow charging and discharging in concert with the rise and fall of the ripple current at the output. Both ESR and ESL are important considerations for the output filter capacitor. High CV density is preferred in the output filter caps in order to reduce board space as capacitance demands of output filters are typically high.

Electrolytic Capacitors

Historically, electrolytic capacitors have been the most popular choice for SMPS filters, especially input filters. They offer very high CV density per package size typically at comparably low cost. The problem is that the high CV density comes at a rather high price in terms of technical disadvantages. There are two popular types of electrolytic capacitors: so-called aluminum electrolytics and tantalums.

Aluminum electrolytics (AE) employ an ultra-thin dielectric composed of aluminum oxide deposited on a thin, etched aluminum foil. The etched surface increases the dielectric’s surface area, increasing CV density significantly.

AE caps are often the popular choice for use as SMPS filters due to their very high capacitance density and relative low cost. Offsetting these advantages are several significant disadvantages that the circuit designer must consider:

  • Due to their construction, AE caps exhibit inherently high ESR (Equivalent Series Resistance). As frequencies increase, this ESR disadvantage becomes more of a problem for circuit designers. To make up for this high ESR, designers will often have to parallel many AE caps to reduce the ESR to meet the application requirements. This paralleling may require from 10 to 100X the theoretical cap value in order to achieve the required ESR.
  • AE caps are polar devices, and failure to maintain polarity can have catastrophic consequences.
  • High temperature usage and even high temperature storage can cause instability including increasing leakage current (reduced Insulation Resistance), loss of capacitance, and reduced usable life.
  • AE caps have limited life due to possible evaporation of the electrolyte fluid over time. Most manufacturers quote lifetimes of 5000 or 10,000 hours due to this evaporation issue.
  • AE caps can explode in an over-voltage condition and may release a toxic fluid.
  • AE caps contain potentially toxic ingredients that may be harmful to the environment.

Tantalum capacitors (TA) employ an extremely porous anode material which offers a large dielectric surface area. This allows for a very high CV density.

TA caps generally have more favorable characteristics for SMPS filtering than AE caps, however, raw material availability has driven up their prices and lead-times. In addition to that, Tantalum capacitors also have several disadvantages that circuit designers must consider:

  • TA caps are polar devices, and failure to maintain polarity can have catastrophic consequences.
  • TA caps exhibit very high ESR, typically higher than their AE cap cousins. The ESR significantly increases at frequencies higher than 100 Hz.
  • TA caps typically exhibit significant capacitance loss at higher frequencies.
  • TA caps degrade when exposed to multiple charge/discharge cycles.
  • TA caps are not typically available in higher voltage ratings. Normally, the maximum voltage rating that can be achieved is 50 VDC, and many TA manufacturers recommend that the TA devices not be used at greater than 50% of the rated voltage, making the effective maximum voltage 25 VDC, even at room temperature.
  • TA caps are not usable at temperatures above 125°C, and their voltage ratings typically apply at 85°C. Between 85°C and 125°C they must be derated.
  • Higher leakage currents of TA caps make them less suitable in many applications.
  • Due to their construction, TA caps often fail by means of a runaway exothermic reaction which sometimes results in fire or the release of toxic/acidic contents onto other components on the PC board.
  • Tantalum capacitors cannot handle over-voltage spikes as well as ceramic capacitors, so more consideration needs to be given to inductive loads.
  • Tantalum capacitors contain potentially toxic ingredients that may be harmful to the environment.

Film Capacitors

Film capacitors (MLP) offer advantages that make them a good choice for high current applications and applications where transients are likely, such as snubber circuits. In the case of polypropylene dielectric film caps, the low dissipation factor makes them ideal for AC applications, especially at higher frequencies such as 400Hz.

MLP caps are constructed by metalizing polymer films and either winding or stacking the film into layers. They are available in a wide variety of dielectrics and are uniquely able to self-heal under certain failure conditions. Film caps also possess inherent characteristics that may challenge circuit designers and must be given due consideration:

  • Although MLP caps offer better ESR/ESL performance than AE or TA caps, they typically do not match the ESR/ESL of NP0 MLCC ceramic designs.
  • Film caps are typically limited to 105°C temperature rating. 125°C operation is typically not possible. Although some polyester dielectric film caps can be rated at 125°C, their inherent lossiness limits their use in high frequency AC applications.
  • MLP caps can be rated at high voltages, but at temperatures >85°C, the voltages must be derated by as much as 50%.
  • When used in AC applications, corona can cause the film to carbonize and fail short circuit if the voltage rating is exceeded.
  • Temperature rise is limited to +15°C and cannot be allowed to exceed the maximum rated temperature of the MLP device.
  • Recent trends in the availability of films have resulted in extraordinarily long lead-times for some MLP caps.
  • Some film caps contain potentially toxic ingredients that may be harmful to the environment.

Ceramic Capacitors

Multilayer Ceramic Capacitors
Ceramic capacitors offer properties that work well in SMPS applications and in some cases offer a good compromise between cost/availability issues and the technical properties required for SMPS filtering.

Single Layer Ceramic Capacitors (SLCC), or ceramic disk capacitors, are constructed of a ceramic slug or disk that is metalized on the two sides. SLCC caps are typically through hole (radial lead) capacitors that are popular in many legacy circuit designs. SLCC devices offer high voltage ratings, >10KV, and stable performance over the entire temperature range.

Recent trends toward higher CV density multilayer ceramic capacitor (MLCC) designs have impacted the availability of SLCC products, as manufacturers have decreased capacity and announced end-of- life for numerous part numbers. In addition to the availability issue, SLCC designs have the following disadvantages that circuit designers should consider:

  • SLCC caps are typically only available in radial leaded format, narrowing board design choices.
  • Lead spacing and size are comparatively large, especially as voltage ratings increase.
  • SLCC CV density is very limited due to the “single layer” design.

Multilayer Ceramic Capacitors (MLCC) are constructed of multiple layers of thin ceramic materials that are metalized and alternately stacked. The device is sintered into a monolithic block and then the exposed electrodes are metalized, forming end caps. MLCC design allows multiple layers of very thin ceramic dielectric to be connected in parallel to achieve relatively high CV density. In recent years, the high cost of precious metals utilized in the electrode layers of previous MLCC designs has been successfully replaced with lower cost base metals such as copper and nickel. This evolution has not reached all types of MLCC design, and some of the larger MLCC devices still utilize precious metals.

MLCC devices can be manufactured from a wide variety of dielectric ceramics including both Class I (Ultra-stable) and Class II (Stable) materials. The most common ceramic dielectric for SMPS applications is X7R, an EIA standard for Class II dielectrics. This is because Class II dielectrics including X7R offer a relatively high dielectric constant (K) whereas Class I dielectrics have a very low K. With the higher K of the Class II dielectrics, a much higher CV density can be achieved.

MLCC devices do not have any significant wear-out mechanism other than their inherent predicted failure rate (FITs). Broadly speaking, MLCC reliability is at least 10X better than TA or AE.

MLCC designs offer extremely low ESR. Especially at higher frequencies, this low ESR allows the circuit designer to use lower capacitance values in MLCC as compared to AE, TA, and MLP devices. The low ESR reduces the power loss (heating) of the capacitor when handling high inrush current (di/dt) to support increased power requirements. In addition, when used as an output filter, the lower ESR of the MLCC device decreases output ripple voltage.

MLCC designs typically are also lower in ESL than AE, TA, and MLP, but product format needs to be considered for ESL. Radial lead capacitors, for instance, have higher ESL than surface mount capacitors due to the inductance added by the leads. MLCC offers better ripple current capability than other technologies. MLCCs are non-polar, and their voltage rating is good over the entire range of temperature ratings. MLCCs are available in environmentally friendly RoHS compliant designs. Additionally, MLCCs come in many physical formats, ranging from surface mount chips to leaded stacked capacitors.

Stacked capacitor designs of MLCC are especially useful for SMPS filters because these applications typically require high capacitance or high CV density. The “stacked” design allows the capacitor manufacturer to build multiple surface mount caps into stacks and achieve up to 5X the CV product for a given footprint. In addition, the lead frames used in stacked capacitor designs offer excellent protection against both thermal and mechanical stresses that might be introduced during soldering or board handling after assembly. The stacked MLCC design may reduce microphonic noise, which typically affects audio circuits that may exist in surface mount circuit designs.

Some designers shy away from stacked capacitors because of worries about shock and vibration stresses introduced in harsh environments. Although the higher center of gravity and larger mass of the stacked caps do make them more susceptible to shock and vibration, they have been and are being successfully utilized in very harsh environments, including aerospace, military hardware, and down-hole drilling applications.

MLCC devices are also available in high temperature ratings, up to 250°C. These are ideal for automotive applications, engine controllers, down-hole drilling, and a host of other high temperature applications. Most MLCC devices are delivered as RoHS compliant, but many are available with Pb solders on request

In addition to the possible harsh environment susceptibility concerns, MLCC devices have some disadvantages that circuit designers should take into consideration:

  • The CV density of MLCCs cannot match that of AE, TA, and some MLP devices, but given the significantly improved ESR and ESL of the MLCCs, the circuit design may not require the same amount of capacitance as a similar circuit using AE, TA, or MLP caps.
  • Ceramic materials utilized in the MLCC design are weak in tension and need to be handled carefully. In addition, the ceramic materials are poor in thermal conductivity, so precautions must be taken during soldering. Larger sizes are more susceptible than smaller surface mount devices.
  • Class II dielectrics used in MLCC design are piezoelectric materials, and MLCC devices do exhibit some piezoelectric characteristics.
  • Class II devices exhibit aging and temperature characteristics. Aging is a logarithmic decay in capacitance value over time. Typical X7R aging runs around 1%/decade hour for MLCCs. Aging can be reversed by heating the device to a temperature exceeding the Curie point of the ceramic (typically ~120°C). The temperature characteristics of X7R dielectrics must be within +15% of the room temperature cap reading over the temperature range of 55°C to 125°C to comply with the EIA standard. Different vendors’ X7R materials’ TCCs vary, so the exact TCC of the device being purchased should be researched.
  • Class II dielectrics decrease in capacitance when a DC bias voltage is applied. The effect of DC bias on the net capacitance is a direct function of the chosen dielectric and the dielectric thickness chosen for the design, and worst case, can be as much as 80% as the RV of the capacitor is app - roached.
  • Class II(X7R) dielectrics exhibit a Dissipation Factor (DF) of 1.5 to 2.5% typically. This loss factor is fine for DC applications but may cause heating to occur in AC applications, especially when frequencies exceed 60 Hz.

Summary

Each technology has its own strengths and weaknesses for SMPS filtering. Given the extremely low ESR and ESL of MLCC devices, CV density may not be a disadvantage, depending on the circuit. With very good di/dt, lack of polarity, high temperature performance, long life, environmentally friendly construction, among other positive attributes, MLCCs make good choices for SMPS filtering needs.

This article was written by Daniel Jordan, Consultant. For more information, Click Here .


Defense Tech Briefs Magazine

This article first appeared in the December, 2011 issue of Defense Tech Briefs Magazine.

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