Sending simple signals from one PCB to another ...connection surge current problems?

[treez]

Hello,
We have two PCBs, and we wish to connect low frequency signals (0V, 5V type) from one to the other. Each of these PCBs is fed by a different, isolated 24V offline SMPS.
Obviously we will have to connect the grounds of the two PCBs in order to send signals from one to the other. However, we are concerned that being isolated offline SMPS’s, their secondary 0V tracks may “float” up to high or low voltages. –Then when we connect them together there will be surge currents flowing temporarily when the grounds of each PCB is initially connected (due to the possible difference in potential between their grounds).
Do you think that this is likely.?

 

[esp1]

hi,
As both SMPS are isolated, connecting their 0V commons will not be a problem, its common practice.
E

 

[betwixt]

Good practice would be to ensure the grounds connect before the signal lines though. Some kind of protection against the signal lines connecting first would be advisable as a defense mechanism but at low frequencies and both supplies being able to 'float' you can probably use normal diodes to dump excessive voltage back to the supply and ground lines. That would ensure that no matter which connected first, the signal lines could never go out of range and the PSU grounds would be pulled together until actually linked.

Brian.

 

[treez]

Thanks, now I am worried, as you appear to be confirming that secondaries of isolated offline SMPS's can float up to "whatever" voltage......the resistors in this diagram are only 0603, and could be overvoltaged by a floated up voltage? (0603'S can only handle up to 50V, and so if one of the offline smps grounds floats up to more than this, then we are in for a bang!)?

 

[betwixt]

I doubt you would get a bang!

The voltage they can float to will depend on the PSU design but could potentially be up to the peak mains voltage or even higher. Understand that the primary reason why the isolated output may float is because of capacitive coupling between input and output sides through the board and transformer. It should be a very tiny amount and in most SMPSU there will be some deliberate filtering and maybe resistive leakage path to 'discharge' the output. You should check with the PSU specifications to find out.

Solutions may be to add a resistor between each PSU output and ground (~100K to 1M) to drain the leakage away or to add diodes rated to at least the maximum expected voltage between the signal wires and ground/supply on your board. Adding lots of diodes is obviously more invasive on board design than a few external resitors. Whichever way, the amount of current that should flow is only enough to discharge a small capacitance so it is unlikely to damage the existing resistors.

Brian.

 

[zsolt1]

hi,
it will work , the two supply units will never have different gnd potentials since you connect them together (they will be aquipotential points, so the diference is 0 )

 

[betwixt]

I think the issue is what damage could result before the PSU grounds are connected together. The scenario is two isolated PSUs are being used and there is a danger that before being joined, their outputs have assumed different potentials. As they join there may be some kind of discharge between them and that's why the circuit may need protection. Obviously discharging through a signal wire is not the best way to do it but if the plug/socket cannot be made to join the grounds before other connections the possibility of a signal wire becoming the discharge path has to be taken into account.

Brian.

 

[FvM]

I think, the shown circuit has sufficient protection, even if the signal pins are connected first. What you can expect at the maximum is a few nF SMPS Y capacitor charged to the mains peak voltage which will discharge through the 6.2 k resistor. Stored energy is below 1 mJ, no problem for a 0603 resistor. The Z-diode will safely protect the MOSFET gate. Possible ESD events are a bit more problematic because they bypass the resistor and have to be absorbed by the Z-diode alone. Circuit damage is still unlikely. A rugged design would use larger resistors, however.

Another question is if you actually need to design the circuit for hot-plugging the connector?

 

[treez]

..yes it will get hot plugged in use...but not often....once every few months say...so do you mean that the 62K resistor should be a 1206 instead of a 0603?..(because 1206 has a 400v overload pulse rating, above the 150v overload rating of 0603)

 

[betwixt]

1206 would buy you a greater safety margin. It isn't the power rating that matters, the energy to be discharged is tiny but the maximum voltage could well exceed the rating of an 0603.

As FvM tells you, the risk is very low even with 0603 resistors. The voltage would have to be high which is unlikely and the resistor would have to flash over which is also unlikely. Even then, the Zener would probably provide enough protection. You could do statistical analysis but it wouldn't be worth the effort. I would guess the predicted breakdown time would be longer than the life expectance of many of the components anyway.

Brian.

 

[treez]

thanks, and surely if the secondary of an isolated offline SMPS floats up to a high voltage then it is actually faulty?...I mean, Y capacitors are deliberately designed to have a certain amount of leakage current so that secondaries do not float up, surely?

 

[FvM]

Within the scope if this thread, Y-capacitors can be considered as ideal capacitors. Capacitive leakage current and stored energy according to capacitance and applied voltage. The secondary voltage of an SMPS without earth connection can "float high" as long as it's floating. That's just normal operation.

 

[treez]

is the reason that secondaries of offline smps's get connected to earth (eg chassis) done to stop them floating up as described?

 

[betwixt]

Exactly. The 'floating' is to give you the versatility to do whatever you want with SMPS, for example stack the outputs one on top of another or provide isolation for safety reasons such as in some medical equipment. You wouldn't normally leave it floating, you would provide a leakage path externally to whatever was deemed to be a safe potential within the scope of the application.

Brian.