STKNX circuit, high power loss at single fault condition: CFH (capacitor) shorted.

[SBart.5]

We based our circuit on the "STKNX Reference Design" (V1.0, 14. Nov. 2019)).

In the case of the single fault condition of a shorted CFH capacitor during operation almost the whole KNX bus voltage (21-30V) is permanently over the 68R resistor (RTX) and generates a big power loss of over 5-12 W (depends on the current KNX voltage).

In the case of  a single fault condition at 30 V KNX bus voltage, the overload of RTX leads to an broken RTX with high impedance, so the overload ends automatically --> no danger.

However in the case of a single fault condition at 21 V KNX bus voltage (where the power loss is lower) the lower overload do not lead to an high impedance RTX (maybe the impedance increase a bit, but not enough). So that a power dissipation of 4-5 W are permanentely available, wich heats our case illegally.

For RTX we are using the following type: YAGEO RC2512JK-0768RL

Can you help us to fix this problem?

[AScha.3]

>single fault condition of a shorted CFH capacitor

1. This should never happen ! So choose a CFH cap with min. 50V rating, it will get no short. Problem away.

2. if you want the RTX be "safe" , choose a PTC (or R + PTC) for this; the PTC has to "switch" (get hi resistamce) in case of permanent too hi current here. But no."1" is better solution.

[SBart.5]

Hi AScha.3,

thank you for your quick response.

To 1):

We are using an MLCC, 50 V type. But for these kind of capacitors, as far as i know, a short circuit is a common failure mode ("Low Insulation resistance (IR) or short, along with low or no capacitance are the two most common failure modes for MLCCs.", Source: https://www.kemet.com/en/us/technical-resources/failure-analysis-of-capacitors-and-inductors.html).

So in my mind a shorted CFH cap. is a possible single fault which i have to consider.

To 2):

To put some PTC in this reference design circuit for KNX communication may lead to other problems i don't know, because of the different behavoirs of the resistance on different temperatures and communication load conditions.

My suggestion

For the shorted CFH, my suggestion is to put 2 capacitors in series with a total capacity of CFH according to data sheet. So if one capacitor breaks (shorted), the other capacitor is still there and prevents the problem described above.

Further error case:

The problem of permanentely overload of RTX exists also if i consider a single fault at the STKNX (for example a short circuit between pin R68  and ground).
(My opinion regarding to singel fault condition consideration and ICs (integrated circuits): In the single fault consideration theoretically i have to short every pin with every pin. And Pin R68 shorted to ground is problematically in my mind). 

Please correct my opinion if am wrong.

[AScha.3]

1. If you so scared about a mlcc /short , (happens almost only on mechanical stress, bending the board) then dont use it: take film cap , through hole, this cannot short by itself.

MKS2-63 47N MKS2 PET-Kondensator, 47 nF, 5 %, 63 VDC, RM 5

2. What are the allowed tolerances here ? (To calculate /select a matching PTC , we have to know.)

[SBart.5]

to 2): What are the allowed tolerances here? (To calculate /select a matching PTC , we have to know.)

We use the given circuits according to the "STKNX Reference Design" (V1.0, 14. Nov. 2019)).

With RTX = 68 Ohm  (>= 1 W power dissipation, 5% tolerance).

Our max. environment temperature in the case is +85 °C

Our minimal environemnt temperature is -20 °C

In the case of a single fault its ok, if the environment temperature increase to +115 °C.

I don't know the max tolerated deviation of the RTX for proper functionality (there is no information in the STKNX datasheet or STKNX Reference Design).

[AScha.3]

>2> I don't know the max tolerated deviation of the RTX for proper functionality.

Me too. :)   So we cannot design a proper replacement for RTX . It is 68 r.

Maybe you could use a "fuse" resistor...(i dont know, whether you want+ can get this kind of resistor.)

660-TPR1CTA680J (mouser)  1 W type :