Choosing a Capacitor for Use as a Switch-Mode Power Supply Filter

Two key functions of switch-mode power supply (SMPS) filter caps are input filtering and output filtering.

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.