Ceramic output capacitors on MAX1771 converter?

[Moses]

Before I let the magic smoke out of half a dozen MAX1771 ICs.. has anyone ever tried using ceramic output capacitors? It wants a low ESR capacitor, so ceramics may work well.

The datasheet doesn't mention ceramics on the output side.. but then again it was written a few decades ago when the required voltage/capacitance probably was not readily available.

Regards,

-Moses

[David Forbes]

I haven't tried it, but I can make some observations. The MAX1771 isn't connected directly to the output circuit, so it's not likely to suffer from a problem. The current flows through the inductor which will accommodate a momentary short circuit caused by the capacitor. 

In short, it shouldn't be a problem.

Bear in mind that the effective capacitance of a modern ceramic capacitor is much lower with a DC bias near its rated voltage, so you would need to use either capacitors rated for 5x the output voltage, or about 5x the desired capacitance.

Some capacitor data sheets publish this reduction in capacitance as a function of bias voltage, most don't. Look for it.

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[Roger Brinkman]

Hi Moses,

What David writes below about the capacitance reduction of ceramic capacitors is correct. 

I have experienced success using polymer aluminium electrolytics (solid electrolyte) to replace conventional low-ESR electrolytic capacitors that regularly fail in similar applications. 

You might like to experiment with these. 

Best regards 

Roger Brinkman. 

On 19 May 2022, at 12:42 pm, David Forbes <nixie...@gmail.com> wrote:



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[Moses]

Thank you. Yes, I almost forgot about the DC bias voltage :(

https://passive-components.eu/dc-and-ac-bias-dependence-of-mlcc-capacitors-including-temperature-dependence/

You are right, most of the ceramic caps I was looking at don't publish any data on decrease due to DC bias. I may try it anyway just to see.. but it doesn't look economical right now.

Regards,

-Moses

[Moses]

Roger,

Yes. Looks like the common dielectrics (X7T, X7R) available in the 250v 1-3uf range suffer from a 30-80% reduction in capacitance at 180v. I hadn't realized it was that high.

Thank you, I'll look at the polymer caps. It's my last remaining electrolytic on my board and if possible I would like to use a solid one instead.

Regards,

-Moses

[Dekatron42]

A few other things that I experienced at my previous employer was that larger (sizewise) MLCC capacitors easily developed cracks, came loose from the circuit board and also needed reforming after storage.

/Martin

[SWISSNIXIE - Jonathan F.]

I've used a MAX1771 with ceramic capacitors a while ago for a pandicon-circuit and a smaller nixie project. Both consume less than 15mA@170V. Not sure how the circuit would behave at more current.

[Ian Vine]

Could someone explain what the reforming involves ?

Thanks

Ian

On 20 May 2022, at 06:12, SWISSNIXIE - Jonathan F. <jfre...@gmail.com> wrote:

I've used a MAX1771 with ceramic capacitors a while ago for a pandicon-circuit and a smaller nixie project. Both consume less than 15mA@170V. Not sure how the circuit would behave at more current.

To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/bb141fcc-15ab-45de-a7ae-c9b38a3710dbn%40googlegroups.com.

[Dekatron42]

If I remember correctly my colleagues spoke about the normal capacitor reforming by applying voltage to them after long storage (1-2years or more in storage) and also the "resetting" that they are subjected to when soldering: https://ec.kemet.com/blog/mlcc-dielectric-differences/

" Ceramic Capacitor Aging

Aging is another characteristic exhibited by ferroelectric, or Class II and III dielectrics. While manufacturing the ceramic capacitor, the dielectric is exposed to temperatures more than 1000°C. For Barium Titanate devices, the Curie temperature can be in the range of 130°C to 150°C, depending on the particular formulation. When exposed to the Curie temperature, the crystalline structure aligns to a tetragonal pattern. Once cooled, the ceramic’s crystalline structure changes to a cubic change. As this structure changes, so does the material’s dielectric constant.

Over time, the capacitance will continue to decline. It is possible to reset this aging cycle by “resetting” the material, by exposing it to its Curie temperature this usually occurs during re-flow. Typically, you can find the aging rate in the catalog for a particular part type. Below is an example of aging rates:"

Our problems started when capacitors started to break and/or fall of the circuit boards even though our designed were approved by the capacitor manufacturer! We also experienced the problems with degrading capacitance in MLCC capacitors but it was even worse with self-healing polypropylene capacitors as it affected the smart-meters power supplies - it all turned out to be manufacturing problems of the capacitors due to moisture in the materials.

https://www.researchgate.net/publication/348506751_Analysis_of_Failure_Mechanism_of_Smart_Meter_Capacitance

"In practical applications, smart meters will fail, and even have batch quality problems. In addition to the sampling inspection of the arrival quality of smart meters, the analysis of the failure mechanism of smart meters has become an important and long-term task. This article mainly analyzes the component capacitance commonly used in smart meters, and analyses the failure of chip multilayer ceramic dielectric capacitors from appearance observation, electrical parameter testing, metallographic slicing, and at the same time from electrolytic appearance observation, electrical parameter testing, X- RAY, SEM and EDS for failure analysis. The results of failure mechanism research not only help to improve the reliability of the domestic energy meter manufacturing level, but also can save energy meter maintenance and transformation costs, and have very important practical significance for the construction of smart grids."

/Martin

[Ian Vine]

Thanks Martin

Very detailed. Much appreciated. 

Ian

On 20 May 2022, at 19:00, Dekatron42 <martin....@gmail.com> wrote:



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