Comparison and Precautions of Film Capacitors, Ceramic Capacitors and Electrolytic Capacitors

Film capacitors, ceramic capacitors, tantalum capacitors, and electrolytic capacitors—what are the differences between these types of capacitors, and what precautions should be taken when using them? This article compares several different types of capacitors and shares the experience of several American engineers in using capacitors...

Don't assume that simple, passive components in your design won't cause problems. Simple components like capacitors can lead to significant issues. Analog whiz Bob Pease wrote an article in 1982 about the static charge on capacitor dielectrics, and the principles he discussed remain valid 39 years later.

Pease described the problem of dielectric static charge as similar to the memory effect of a capacitor. If a high-voltage capacitor is discharged, the voltage across it bounces back when the short circuit is removed. Another name for dielectric static charge is dielectric absorption, which indicates that the insulating dielectric in the capacitor has absorbed charge, and the charge is "trapped" in the dielectric. It cannot discharge, so the voltage reappears. This is not surprising, as electret microphones work by intentionally retaining charge in the dielectric layer. Wetting can pose a risk of electric shock to capacitors in high-voltage motor applications and can severely disrupt signal processing applications, as the charge is stored in the capacitor, which is part of an analog-to-digital converter. Residual charge is a direct factor in introducing errors, giving incorrect measurement readings.

I have a friend who works for an ADSL company, where their line cards are used at the telephone company's central office. These cards are part of the telephone company's hardware, and the DSL modem communicates with them. DSL uses a large number of frequency bins during operation. The line card measures the impedance of the twisted-pair cable entering the house and then adapts to the specific impedance profile of that particular twisted-pair cable. The prototype line card worked perfectly. Then the engineer applied a ringing signal to the line, just like a telephone ringing. The DSL line card and modem would unlock and then renegotiate the connection, even though nothing had changed in the wiring between them. A telephone ringing signal is approximately 90V AC. This high voltage caused dielectric absorption in some capacitors in the line card's circuitry, and the line card sensed the impedance change, thus unlocking and renegotiating. When my friend realized this and replaced the capacitors with low-wetting capacitors, the problem disappeared.

Although the schematic symbol for a capacitor is simple, capacitors actually have many different structures. To achieve maximum capacitance, capacitor manufacturers produce aluminum electrolytic capacitors with foil and a liquid dielectric. This structure presents two problems: electrolytic capacitors become polarized, and they are unreliable. If a negative voltage is applied to the positive terminal, the capacitor will explode with great force. This is why safety glasses should be worn when powering on a prototype and troubleshooting, even if the technician can ensure that all capacitors are soldered correctly.

Tantalum capacitors also have high capacitance values ​​and are polarized. Like electrolytic capacitors, tantalum capacitors are several orders of magnitude less reliable than any other component, with the exception of potentiometers. For high reliability, wet-plug military-grade tantalum capacitors can be used as an alternative, although these are more expensive. It's also important to note that heat will worsen the situation; even at room temperature, the current flowing into these large-capacity capacitors can make them very hot.

Many engineers today consider ceramic capacitors to be ideal capacitors, but this is not the case. While their maximum capacitance is currently several hundred µF, these large-capacitance capacitors have poor temperature coefficients, meaning their capacitance varies significantly with temperature fluctuations. To achieve temperature stability, NPO or COG dielectrics must be used, which have much smaller capacitance values, ranging from a few pF to several thousand pF. Since higher voltage-rated capacitors require greater spacing between their plates, size, voltage, and capacitance are directly related. If a high-voltage, high-capacitance capacitor is needed, its larger physical size must be accepted.

Another problem with ceramic capacitors is that they produce noise. A friend of mine designed a 60W LED bulb. He knew that to make the power supply as reliable as the LED emitter, all the electrolytic capacitors had to be removed. He found some large-capacitance ceramic capacitors, but discovered that they "screamed" when the dimming circuit was switched to the audio range switching frequency. Ceramic capacitors also have a chattering effect. If you tap a ceramic capacitor in the signal path, you'll see a sharp jump in the signal on the oscilloscope.

I used to work for military contractors, and they always had a full range of mica capacitors on hand. These were low-loss capacitors, near perfect in terms of dielectric absorption and temperature, but this came at the cost of large size and small capacitance. Another option was wound or stacked film capacitors. Film dielectrics have almost no wetting and a moderate temperature coefficient. Another advantage they had over their associated polypropylene capacitors was their self-healing ability, much like the reliable old-fashioned Orange Drop capacitors. If high voltage broke through the insulation, the breached film would partially melt and protect the rest of the capacitor. The capacitance was almost exactly the same, and the fault wasn't a short circuit or an open circuit.

The key parameter of a capacitor is its internal impedance, which is both equivalent series inductance (ESL) and equivalent series resistance (ESR). A perfect capacitor simply doesn't exist, and perfection can actually cause problems. Many switching power supply architectures rely on the ESR of their output capacitors. ESR provides a small signal proportional to the current flowing into the capacitor, which the control chip uses in the feedback circuit. Therefore, replacing an electrolytic capacitor with a more reliable ceramic capacitor might only cause the power supply to oscillate under partial or full load. Most power supply chip datasheets clearly state whether ceramic output capacitors can be used. Make sure to model the ESL and ESR of the capacitors used using Spice simulation. The most extreme capacitors are supercapacitors, with capacitance values ​​reaching several farads, but typically operating at lower voltages. You can think of supercapacitors as batteries; they have three important advantages. First, their lifespan is not shortened; even after 10,000 charge-discharge cycles, their capacitance and performance remain unchanged. The second advantage of supercapacitors is their low internal impedance, which is what we referred to as ESL and ESR. This means that it charges and discharges very quickly, for example, it can drive a camera flash or the initial boost charge of a power circuit; the third advantage is that it can discharge supercapacitors all the way to zero volts, releasing all energy, while lithium-ion batteries can only discharge from 4.2V to 3.3V.

Wikipedia describes capacitors and their types very well. It's essential to consult experienced engineers and application engineers to understand capacitors and their differences. I saw a discussion about PCB factories where a Chinese assembly plant convinced its engineers to replace all their passive components with those from Chinese manufacturers. This decision was incredibly reckless unless you take the time to identify dozens of these parts and evaluate how their performance changes over time and temperature. When a capacitor fails or cannot maintain its capacitance, the circuit breaks, much like some ICs burn out. Always buy capacitors from reputable distributors, as their inventory consists of high-quality products from well-known manufacturers.