Primary-to-primary creepage distance

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What creepage distances are required between nodes on the primary circuit side, to meet IEC 60950 safety requirements? My circuit uses an LNK606 running from 240Va.c. (nominal) with a peak voltage on the drain pin measured at about 530V. I'd like to know the gap to leave between PCB tracks and in particular between adjacent pins on the transformer (primary side). I'm using a FR4 PCB with solder resist, in a domestic environment.

 

Does Table 2Q apply? (Minimum separation distances for coated printed boards) Otherwise is it Table 2N? (Minimum creepage distances). If Table 2Q then presumably the voltage to use is the peak volateg on the drain pin. If table 2N then what voltage should be used?

 

(I'm sure there must be a thread that covers this topic - perhaps someone can point me there). 

 

Thanks - Charles

Hi,

I just saw that table 2Q is applied for coated board, which require to cover the board with conformal coating. This will decrease the required creepage or clearance a lot. If you do not cover the two conductive parts on the board, you can not use them as a reference.

To be clear, there is a difference between the clerarance and creepage, you can find the difference in the standard. Usually the peak voltage level will determine the required clearance , and the permanent or equal RMS voltage level will determine the required creepage.

If you find that the distance between drain and source does not reach to the required creepage, you could put a slot between these two pins to increase the creepage.

what is primary-primary means in this circuit, is this the distance between Line Neutral ,Drain-Source or this means two different hazordous voltages like Phase 1 or Phase 2.. I thing Primary to primary means the second one..

 

For the distance there is a website www.creepage.com which calculates minimum distances with respect to 60950, 

Hi,

To be clear, you will need to define different voltage blocks if there is a voltage between these two blocks rated  higher than 51Vdc or 36Vac. You can find these information the standard.

So you will need to define the creepage or clearance requirement between line to Neutral, Drain to source, drain to line, drain to netrual, line to secondary, drain to secondary...ect since the voltage differenct between these blocks is over 36Vac or 51Vdc. It is a not simple matrix if you want to make sure all of them reach to the requiremen from the standard. Then you will need to apply this rule to your PCB design either by manual checking or automatical setting into the PCB layout software.

When you ask what is primary to primary, it is how you define it. You brought this name up in your first post, I thought you were saying drain to source voltage.

Hope this will help your design.  

I am still keen to have an answer to this question: what rules apply to distances between nodes on the primary side? The highest voltage I measure on the drain pin is 530V. What distance must I preserve to the source pin and feedback pins? I doubt I can get get more than 1mm between the drain pin on the transformer and adjacent pins on the primary side of the transformer. Is this enough?

 

This must be an issue with every flyback design. What are the rules? Someone must know.

Hi,

The required distance of a 530V (peak voltage you get) is really depends on your applicaiton. There is a big difference of this requirement if your power supply design for applications with different pollution degree level. If you refer to the standard, you can get the different results for polution degress 1, 2, or 3.

For the creepage, you can refer to the tableN as you pointed out. If 1 mm is not enough, you will have to choose different transformer bobbins to reach this requirement. Or you could use bobbins with more pins and aviod using adjacent pin from the drain to the bias voltage.

I visited a test house yesterday, to discuss approval of my product to IEC 60950. Their view was that the necessary creepage distanec would be 2.x mm, but if that could not be met (it can't) then they would assume a short circuit fault condition between the two circuit nodes, and check that the product failed safe if the two nodes were shorted together. So: short Drain to Source or Drain to the feedback pins.

 

Do other readers have the same interpretation?

Hi,

I will think it is a very professional way to consult the test house. The comments from them is more focusing o the safety aspect. For my point of view, to design a reliable design, it is best to make the design respecting the required rules, this will make sure the quality of the design.

To set an upper limit of voltages between adjacent pins on the primary side of a flyback transformer, one can do this:

 

Put the drain pin on one corner - say pin 1. Put the rectified a.c. on pin 2. Put the feedback pins and source pins on pins 3,4, 5 etc.

 

That way, the maxiumum voltage between any two adjacent transformer pins will never be more than the rectified a.c.voltage. 

 

Obvious when you think about it.....

Hi,

We use LNK304 in the circuit in off line application. I have a doubt about creepage distance. We powered hte circuit with 230Vac+20%=276Vac after the rectifier diode we have 390Vdc. For a double insulation, where I have 276Vac creepage distance is 6.4mm, after the diode (390Vdc) 8mm. that's right or I have to use 8mm everywhere. Thanks.

Hi Franco,

Double insulation applies only between primary and secondary circuits. Or between primary and secondary pins/traces of isolating component like IC. While, functional insulation applies in primary circuits after the fuse.

Functional insulation is necessary only for the correct functioning of the equipment.

Functional creepage is the same as the basic creepage requirement, if required. But if the functional creepage cannot be met, you can apply single fault test by shorting the two terminals(of a certain component) to evaluate if there no any safety hazard when they are shorted. Compliance criteria is equipment should remain safe (no fire or flame, shall not emit molten metal, no opening of any trace, temperature is within the limits, no insulation breakdown, etc).

I hope that helps.