Reference Voltage for Switching Power Supply

Roland chung

Trusted Information Resource
#1
Hi cove.

Regarding the switching power supply, the voltage after the MAINS rectifier is about 400V DC if input is 240V AC. My question is whether the reference voltage should be 400V DC rather than the 240V AC when determining the insulation between the MAINS parts and accessible parts.

I know many test labs use the reference voltage of 240V AC as always they did. Reason from one German NB is that the test values for the barrier based on primary transients up to 6kV. Compared to that difference between 400V DC and 240V AC is negligible. This explanation seems strange and differents from IEC 60664 a little bit.

Please kindly share your experience. Thanks and regards.
Roland
 
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Stijloor

Staff member
Super Moderator
#2
Hi cove.

Regarding the switching power supply, the voltage after the MAINS rectifier is about 400V DC if input is 240V AC. My question is whether the reference voltage should be 400V DC rather than the 240V AC when determining the insulation between the MAINS parts and accessible parts.

I know many test labs use the reference voltage of 240V AC as always they did. Reason from one German NB is that the test values for the barrier based on primary transients up to 6kV. Compared to that difference between 400V DC and 240V AC is negligible. This explanation seems strange and different from IEC 60664 a little bit.

Please kindly share your experience. Thanks and regards.
Roland
Can someone help Roland?

Thank you!

Stijloor.
 
R

raymand

#3
Hi, Roland:

I think you are right. In my experience, the referece voltage should be 400VDC not 240VAC. And, according to 3rd IEC 60601-1, the WV should be meausred between Primary to Secondary parts. The value is approximatively equal to 560Vp.
 

Peter Selvey

Staff member
Super Moderator
#4
Sorry for slow reply.

There are a couple of considerations:

Usually, there is only a limited areas that are above 240Vac.

A good test lab will apply extra requirements for spacing around those parts only, not the whole primary circuit. This is against the 2nd edition, but it makes sense, and I have seen this approach many times including in CB reports without problems.

So for example, the feedback optocoupler is normally <240Vac working voltage, so normal 5/8mm apply (cl/cr). But pin 1 of the transformer could be 380Vrms working voltage, for that point 7/12mm might be applied to the secondary side.

The second consideration is that limits in 601-1 2nd ed can be overkill depending on the location. For example, IEC 60664-1 says 8mm is OK for up to 800Vrms on a PCB for double insulation. So, if the test lab measures the working voltage and inspects IEC 60664-1 and confirms it is appropriate, then it is also reasonable.

However, just ignoring the working voltage is wrong. It's a kind of lazy way, but unfortunately popular.

Under the 3rd edition, MOOP requirements are now reduced. So, if the test lab ignores real working voltage, and applies reduced 3rd edition limits, it can be unsafe.
 

Roland chung

Trusted Information Resource
#5
Hi peter,

Good said. Pertaining to the working voltage, do you think higher voltage in SFC would stress the barrier shall be taken into account?

In addition, the 3rd edition standard allows interpolation when we comfirm the creepage value. If the INPUT is rated at 230V or 240V, do you think interpolation shall be used? For example, creepage of 8mm is for 250V, if we use the interpolation, the value for 230V will therefore be reduced and less than 8mm.

Thanks and regards,
Roland
 

Peter Selvey

Staff member
Super Moderator
#6
In principle, limits for creepage distance in SFC should be for basic insulation only. So it is possible for a part which is in NC 230Vac to be 380Vdc in SFC; but the limit of 8mm would probably be OK for both cases.

For interpolation, yes it is reasonable in concept, but in practice I would not recommend. Many test labs feel comfortable to apply special rationales as long as 8/5mm cr/cl exists. If you want to start going below 8/5mm, there is a good chance someone won't like it at some point.
 

Roland chung

Trusted Information Resource
#7
Hi, Peter.

The test house did measure the working voltage between the primary pins of transformer. The max. measured voltage (320Vrms) is higher than MAINS (240Vrms) and also higher than the working voltage between the primary pin and secondary pin (290Vrms).

Do you think it is reasonable to use the primary voltage (320Vrms) as a basis for the testing of barrier (primary to enclosure/ secondary)?

Regards,
Roland
 

Peter Selvey

Staff member
Super Moderator
#8
Using the working voltage is reasonable, not the primary voltage.

However, there is a trap: the working voltage is usually higher when the mains input is lower. So if your switching power supply is rated for 100-240V, you will find measurements at 100Vac to be higher than 240Vac. The measurement should be true rms (ac + dc), not just ac.

The measurement of 320Vrms seems a bit low so I'm guessing they may be measured at 240Vac input.

The voltage at the "hot" pin of the transformer with respect to earth is normally 380Vdc minus (-) a half wave rectified mains supply. So, if the mains supply is higher the voltage goes down.
 

Roland chung

Trusted Information Resource
#9
The voltage at the "hot" pin of the transformer with respect to earth is normally 380Vdc minus (-) a half wave rectified mains supply. So, if the mains supply is higher the voltage goes down.
This point seems unpopular. The logic is that the higher the INPUT, the greater the rectified voltage. Anyway, I will do a measurement to verify it.

Did you mention the working voltage between primary pins, not the primary pin to secondary pin?
 
R

raymond02302

#10
hello peter,
The working voltages measured between the pri winding and sec winding of transformer and opto-coupler may be less than the mains voltage.Generally, the working voltage between pri and sec of opto-coupler is less than the mains voltage. If it is this case, should i use the working voltage or mains voltage to determine the CL?
Simplely speaking, it is permit that reference voltage is less than mains voltage in primary circuit?
I think it is correct to evaluate the barrier using the actual voltage which stresses the barrier.
How about your opinion?

thanks in advance.
 
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