Informational How to consider the Working Voltage for splitting 2MOPP

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Roland chung

Trusted Information Resource
#12
Hi Peter,

The original discussion was based on the linear transformer. But what working voltage for barrier after 60950 switching power supply should be?

60950 power supply can provide one MOPP only, the insulation, after such SPS, between secondary and applied part should therefore meet one MOPP at least. For SPS, the measured working voltage between pri and sec is 350Vrms/ 670Vpeak, in this case. For barrier after the SPS, I think 240V reference voltage is appropriate. Because if pri shorts to sec, the switching circuit will be destroyed. That means 350Vrms/ 670Vpeak will die.

If I got something wrong, please correct me. Thanks.
 

Peter Selvey

Staff member
Super Moderator
#13
Yes, basically correct.

The standard always requires 2 MOP.

For the purpose of the 350Vrms / 670Vp:

1st MOP is the isolation barrier
2nd MOP is the self desctruction of the 350Vrms in case of fault.

Of course both must be verified by inspection or test, and confirmed to be reliable MOP for regular production.
 

Peter Selvey

Staff member
Super Moderator
#15
It's kind of like the use of earthing.

1st MOP = basic insulation
2nd MOP = earthing + fuse (or circuit breaker)

Although we tend to think of just earthing being the MOP, actually the fuse is also important. In the case of 1st MOP fails, the fuse link is destroyed (or circuit breaker opens) isolating the supply.

So for your SMPS it is similar, in case of 1st MOP fails, switching MOSFET drive circuit is destroyed, stopping the creation of 350Vrms. It is not as neat as fuse/circuit breaker, but it is still the same effect.
 

Roland chung

Trusted Information Resource
#16
Peter, you did remind me of the self destruction of SMPS. But this seems that additional barrier after SMPS to protection against the stress of mains voltage is not needed anymore, because of the shutdown.
 

Peter Selvey

Staff member
Super Moderator
#17
Whether you need another MOPP (or MOOP) for mains voltage after the SMPS stops will depend on the test result and design.

I think it goes back to the original point: if you can design for double insulation at the highest voltage (e.g. 350Vrms), then life is simple.

The alternate is possible but messy, complicated and easy to make mistakes in the rationale.
 
R

raymond02302

#19
Hello peter,

In the post you gave as follow, there seems to be an inappropriate method for analysing the working voltage(W.V) that you use the SFC to determine the W.V.

Consider a transformer with input terminals P1 and P2, and output is S1 and S2, where P2 and S2 have the same polarity, and P1 is 0V (earthed), and the secondary is floating. The SFC analysis shows:

Short P1 to S1: S1 = 0V, S2 = 240V
Short P1 to S2: S1 = 240V, S1 = 0V
Short P2 to S1: S1 = 240V, S2 = 480V
Short P2 to S2: S1 = 0V, S2 = 240V
Working voltage is highest voltage to which the insulation or the component under consideration is, or can be, subjected when the equipment is operating under conditions of normal use (from 60950),
sometimes we should consider the open circuit condition to measure W.V.(from IEC 60664-1).
Therefore, When we measure the W.V, we should not consider the fault condition.

Actually, the key point to affect the W.V is the input and output referencing.


,
 
Last edited by a moderator:

Peter Selvey

Staff member
Super Moderator
#20
I agree, it's not per the definition in the standard.

Mainly I was just trying to show that 480V in SFC was possible. The original post indicates that summation of voltage cannot occur in SFC, but this is not true.

There may be other (rare) SFC where voltage increase well beyond NC, so that using the value measured in NC makes no sense (e.g. failures in the feedback loop of high voltage step up regulator).

We might see future standards modified to consider this.
 
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