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Posted on 29 June 2019

Dielectric Performance of Ceramic Capacitors

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The resultant effects on the performance of a filter

Syfer Technology has considerable experience in the manufacture of MLC capacitors and low pass filters, both panel mount and surface mount, together with ceramic filtering elements such as discoidal capacitors & planar arrays supplied to the low pass filter industry.

By Steve Hopwood, Senior Applications Engineer, Syfer

 

Everybody ages, some faster than others, and some of us remain stable, but others were unstable to start with! Capacitors can be viewed in exactly the same light.

A recent letter to the EMC Journal issue 90, September 2010, highlighted how capacitor ageing resulted in 3 year old lighting ballasts failing EMC testing despite having been tested and passed when new. Investigations showed that the problem was paper film X2 capacitors ageing at 10% per decade.

Over the last few months, Syfer Technology has seen a significant increase in the number of enquiries relating to how ceramic capacitors change depending on the temperature and voltage in their application.

This has highlighted a general lack of understanding about the way capacitors age, the effects that operating conditions can have on component performance and the importance of the dielectric material and design used to make the capacitor in the first place.

Dielectric Codes, TC & VC characteristics

Most people are familiar with the ceramic dielectric designations C0G/NP0, X7R & Z5U – and some will also recognise other designations such as X5R & X8R – but this is only the tip of a whole range of dielectric types.

Each designation identifies the manner in which the dielectric changes with time, temperature and occasionally voltage. As a general rule, the higher the dielectric constant of a given material the more capacitance you can achieve per unit volume, but it becomes more unstable with temperature, voltage and over time.

Each designation indicates the performance of the dielectric over the operating temperature range.

It can be seen that for the most stable performance a C0G / NP0 dielectric should be specified, and that some dielectrics can lose as much as 90% of the quoted capacitance just by operating them within the limits of their temperature range.

However, this is only part of the story. The EIA codes shown in Figure 1 only characterise a dielectric with respect to change due to temperature – the TC characteristic. Most dielectrics also change due to the application of voltage - the VC characteristic.

Summary of ceramic characteristics

To include this it is necessary to look beyond the commonly recognised EIA codes to the CECC or MIL codes, which include VC characteristics.

Ultra stable Class I ceramic and Stable class II ceramic

So C0G/NP0 is stable and exhibits minimal change through temperature or voltage, but a standard X7R or 2R1 capacitor has no required voltage characteristic and it is not unusual for standard X7R to lose as much as 75% of the quoted capacitance when full rated voltage is applied. Codes such as 2C1 & 2X1 allow the change through applied voltage to be taken into account, but reduce the capacitance range that can be achieved. It's possible that physically larger capacitors may be required to achieve the required performance. Less stable codes such as Z5U, Y5V etc generally have no equivalent codes taking voltage into account.

Of course, one option is to increase the voltage rating of the capacitor or filter so it is operated at a lower voltage than its rating – thus inferring an inherent voltage characteristic. For example, at Syfer, we have standardised our filter range at a 500V rating wherever possible to improve performance figures, compared to the 100V (typical max) industry standard.

Actual Performance

We have seen that the dielectric codes define variation with temperature and sometimes voltage. Now let’s make things more complex!

A dielectric code, for example X7R/2C1 (BZ) defines the performance characteristics, but only for that family. Think of it as, say, defining a small family car – it must have 5 doors, seat 4 and have a minimum engine size of 1.6ltrs, but it doesn’t define that it must be petrol or diesel, manual or automatic transmission, with or without air conditioning. These additional factors come from the actual make and model selection. In our case from the actual dielectric material type. Many different dielectric powders, when used in conjunction with defined design rules, will meet the requirements of the X7R 2C1 (BZ) coding, but they may all have different actual performance characteristics, just as a Ford will differ from a VW.

If we look at the typical standard X7R family of capacitors, most manufacturers will sell a range of parts but within that range there will be a number of actual dielectric powder formulations. Each will meet the X7R specification but will be optimised for different characteristics (for example a high voltage dielectric is likely to differ from a low voltage one) and are likely to have different actual performance curves.

All 3 dielectrics comfortably meet the X7R requirements of ±15% over the temperature range, but exhibit three different performance curves.

X7R Dielectrics – TC examples

Ageing

We have shown that the quoted nominal capacitance may not be actually what is being achieved, but there is still more to take into account.

Ceramic dielectric ages at a known linear rate, and of course C0G/NP0 is the best having negligible ageing whereas X7R ages at <2% per decade and Z5U/Y5V at typically 6% per decade, but it’s important to understand how this works in practice: C0G does not age, so the value will remain constant.

X7R ages at <2% per decade and in practice typically 1% per decade. This means that the capacitance value will decrease logarithmically after it is cooled through its curie point (about 135°C) by approximately:

a) 1% between 1 and 10 hours

b) An additional 1% between the following 10 and 100 hours

c) An additional 1% between the following 100 and 1000 hours

d) An additional 1% between the following 1,000 and 10,000 hours, etc.

 The ageing rate continues in this manner throughout the capacitor’s life, although ‘1,000hrs to 10,000hrs’ equates to ‘6 weeks to 59 weeks’ and ’10,000hrs to 100,000hrs’ equates to ’59 weeks to 11 years’ so ageing is considered flat beyond 1000hrs for most practical purposes.

The less stable dielectric categories, such as Z5U / Y5V etc., age in a similar manner to X7R, but at a faster rate – typically 6% per decade.

It has been standard practice in the ceramic capacitor industry to quote capacitance based on the 1000hr value for many years. This allows the user to have some confidence that the capacitance will not vary much from the quoted value.

However, this has not universally been transferred to the filter industry where either a nominal value or GMV (Guaranteed Minimum Value) is quoted. Again, be careful with this – GMV does not usually take ageing into account and relates to the value as supplied, not a guarantee of what the capacitance will age to and certainly not what it will be under normal working conditions.

If in doubt, ask the supplier exactly what value they are quoting or supplying and remember to take the tolerance into account. The filter industry is lagging somewhat behind the capacitor industry in still quoting very wide tolerances. If a part is quoted with a tolerance of - 20% +80%, then the value targeted in manufacture will probably be +20% with respect to nominal.

Whilst discussing GMV it is also worth noting that this can be an excuse for supplying whatever parts the supplier has in stock at that time. For example a 10nF GMV part could have 15nF or 100nF nominal capacitance and vary batch to batch, not very useful for knowing how the circuit will perform over time.

Effect on value and performance

It has been demonstrated that the capacitance of a part can actually vary considerably compared to the quoted nominal value. Consider now the typical performance of a 5,000pF filter capacitor, offered in standard dielectric classifications, operating at a voltage of 100Vdc at 85°C and at an age of 10,000 hours. The final capacitance value can fall within a range of values taking into account the ageing process, and the effects of temperature and voltage.

The final capacitance value can fall within a range of values taking into account the ageing process

It’s not easy to test at the extremes of voltage, temperature and ageing, but we can get an approximation of relative performance by testing capacitors of nominal values to match the extremes. In the above example we have a nominal 5,000pF capacitor which could have an actual value of 250pF. But let’s assume it’s not quite as bad as that and compare the attenuation curves of a 5000pF MLCC feedthrough chip and a 330pF MLCC feedthrough chip, as these are readily available standard values.

Capacitance value - performance comparison

And similarly, a comparison of panel mount ‘C’ filters with capacitance values of 10nF and 100nF can also show how problems can occur, with up to 20dB degradation in the filtering performance due to the change.

Comparison of 10nF and 100nF Filters

Potentially a problem we would suggest.

Conclusion

Dielectrics other than C0G/NP0, or the stable X5R/X7R/X8R, should not be used to make ceramic chip capacitors. Whereas, the less stable Z5U/Y5V/X7W dielectrics are still commonly found in the manufacture of feedthrough filters where high capacitance values are required from older technologies (e.g. single layer capacitors). It is certainly worth carefully checking the dielectric characteristics when specifying these components, especially if you should you try to make your own filter.

But most of all don’t rush in when specifying capacitors, and don’t assume the value is what it says on the packet. Ask as many questions as you need – a reputable manufacturer like Syfer will always be willing to help, that’s what we’re here for!

 

 

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