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EDNE MARCH 2013

By Mark Fortunato • Maxim Integrated Why your 4.7-μF ceramic cap becomes a 0.33-μF cap A few years ago, after more than 25 years of AN INVESTIGATION INTO TEMPERATURE AND VOLTAGE VARIATIONS IN X7R CAPACITORS UNDERSCORES THE IMPORTANCE OF DATA SHEETS. working with ceramic capacitors, I learned something new about them. I was working on an LED-light-bulb driver, and the time constant of an RC circuit in my project simply did not seem to be right. I immediately assumed that there was an incorrect component value installed on the board, so I measured the two resistors serving as a voltage divider. They were just fine. I desoldered the capacitor from the board and measured that component; the cap, too, was fine. Just to be sure, I measured and installed new resistors and a new capacitor, fired up the circuit, checked that the basic operation was proper, and then went to see whether the component swap had resolved my RC time-constant problem. It had not. A TEMPERATURE PROBLEM? I was testing the circuit in its natural environment: in its housing, which itself was in an enclosure to mimic a “can” for ceiling lighting. The component temperatures in some instances reached well over +100°C. Even in the short time it had taken me to retest the RC behavior, things had become quite hot. My next conclusion, of course, was that the temperature variation of the capacitor was the issue. I was skeptical of that conclusion even as I drew it, however, because I was using X7R capacitors, which to my recollection varied only ±15% up to +125°C. I trusted my memory, but to be sure, I reviewed the data sheet for the capacitor that I was using. That is when my ceramic-capacitor reeducation began. backgrounder Table 1 shows the letters and numbers used for various ceramic capacitor types and what each means. The table describes Class II and Class III ceramics. Without getting too deep into details, Class I capacitors include the common COG (NPO) type; these are not as volumetrically efficient as the ones listed in the table, but they are far more stable over varying environmental conditions, and they do not exhibit piezo effects. The capacitors listed in the table, by contrast, can have widely varying characteristics; they will expand and contract with applied voltage, sometimes causing audible (buzzing or ringing) piezo effects. Of the many capacitor types shown, the most common, in my experience, are X5R, X7R, and Y5V. I never use the Y5Vs, because they exhibit extremely large capacitance variation over the range of environmental conditions. When capacitor companies develop products, they choose materials with characteristics that will enable the capacitors to operate within the specified variation (third character; Table 1) over the specified temperature range (first and second characters). The X7R capacitors that I was using should not have varied more than ±15% over a temperature range of −55°C to +125°C, so either I had a bad batch of capacitors or something else was happening in my circuit. TA BLE 1 COMMON CERAMIC-CAPACITOR TYPES First character: low temp Second character: high temp Third character: Change over temp (max) Char Temp (°C) Num Temp (°C) Char Change (%) Z 10 2 45 A ±1 Y 30 4 65 B ±1.5 X 55 5 85 C ±2.2 — — 6 105 D ±3.3 — — 7 125 E ±4.7 — — 8 150 F ±7.5 — — 9 200 P ±10 — — — — R ±15 — — — — S ±22 — — — — T 22, −33 — — — — U 22, −56 — — — — V 22, −82 w ww.edn-europe.com MARCH 2013 | EDN Europe 23


EDNE MARCH 2013
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