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Comments on How do I calculate and manipulate the inductance and capacitance of outputs to a connector on a PCB?

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How do I calculate and manipulate the inductance and capacitance of outputs to a connector on a PCB?

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I am designing a PCB that has to have class 1 div 1 associated apparatus (only the outputs to the sensor) certification. Essentially, I am using a boost converter to convert the voltage from a lithium battery of 3.9V to 18V to power a 4-20mA sensor in a hazardous zone, while the main product will be in a safer zone with a class 1 div 2 cert.

I know there could be extra components, such as fuses/diode return; however, it isn't the focus of my question and have simplified the schematic a little bit to focus on the concept I am trying to understand: Image alt text

The rules for total effective inductance and capacitance can be seen here: Image alt text

I can get the inductance and capacitance of the sensor + cable (Li+Lc/Ci+Cc) from the manufacturer of the intrinsically safe sensor, but how do I calculate the inductance and capacitance out from the barrier to make sure it is less than the sensor values? The Co and Lo also have to abide by rules to limit the maximum energy allowed in the hazardous area.

The examples I have seen with a similar schematic end up with less than 1uF of capacitance and 2-4mH of inductance for Co and Lo. This seems odd to me when there is only a 10uH inductor and an over abundance of capacitance at 4.7uH on the output of the boost converter. From my understanding, the L and C from the traces will be very minimal in comparison to these numbers.

I do not have a PCB laid out, because I am trying to understand the methodologies and standards first. I understand that means these things can't be completely calculated yet. I'm not asking for someone to design something for me or do calculations for me; I'm just wanting to understand the methodologies behind this question to better understand the concept to use in all of my engineering moving forward.

How do I calculate the theoretical inductance and capacitance at an output? Are there tricks to increasing inductance and decreasing capacitance through the way the board is laid out, or adding components for the specific purpose of manipulating those values?

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General comments (5 comments)
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How do I calculate the inductance and capacitance out from the barrier to make sure it is less than the sensor values?

The output capacitance and inductance from your zener barrier is irrelevant - you design the barrier components (fuse, zeners and output resistor) to limit the short circuit current and limit the open circuit voltage. Once designed, the barrier is a zener limited voltage source in series with an infallible resistor. It does not store energy and it does not have any capacitance or inductance.

Then, using graphs for the gas group your unit is to be installed in, you see if there might be a spark energy that can ignite a gas should the sensor have too much inductance or capacitance (when connected to the barrier).

In other words, the barrier buffers (fuse, zeners and resistor) your switching converter to intrinsically safe output levels. But, it's only intrinsically safe if what you hang on the output (the sensor) is permitted i.e. less than the maximum allowable capacitance and inductance.

I'm just wanting to understand the methodologies behind this question to better understand the concept to use in all of my engineering moving forward.

It seems you my be a little inexperienced in this area but it should all become clear when you look in the appropriate standards for intrinsically safety. Your zener barrier maximum output voltage dictates the maximum capacitance that your sensor (and its wiring) can have AND, your zener barrier maximum short circuit current dictates the maximum inductance that your sensor (and its wiring) can have.

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General comments (5 comments)
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Hawk2020‭ wrote over 4 years ago

Andy,

That makes a lot more sense. My confusion came from a conversation with an engineer at a cert lab:

"For large capacitance/inductance from the“main” circuit, isolate/limit it from your barrier circuit. Output entity parameters on the barrier will be based on how you limit I,V,C,L to the barrier. " After reading your answer, my assumption is that the C and L from the main system is important in relation to the amount of energy that can be transferred into the barrier. Is that correct?

Andy aka‭ wrote over 4 years ago · edited over 4 years ago

@Hawk2000 The barrier doesn't care about energy transferred to it; rather it's the other way round. The barrier is situated in a less onerous gas environment so clearly, it is protecting down stream (into the more onerous gas environment) from excessive energy. The barrier "buffers" anything on its input side so that the output can be regarded as an infallible open circuit voltage in series with an infallible current limiting resistor.

Lundin‭ wrote over 4 years ago

Also from what I recall, you are supposed to increase the number of parallel zener diodes depending on safety class. From what I remember, EX class 0 with intrinsically safe system requires 3 zeners. Supposedly to even out the heat between them.

Hawk2020‭ wrote over 4 years ago

@Lundin I have seen some designs that said to do that for the power dissipation requirements, but the "ia" part of the standard allows for the lab to cause 2 "countable faults", so the third zener is so that there is still protection if they fault 2 of them in the test.

Andy aka‭ wrote over 4 years ago

Safe after two faults is the reason. They all, individually, have to be able to survive the input current that might pass through the fuse indefinitely.