MOPP FOR US EQUIPMENT

[Dindin]

Hi
I'm designing a new ultrasound device (for external use only). I'm using an OEM US module which is connected to a single board computer. The whole system is powered by a certified medical grade power supply (2MOPP). My questions:
1. do I have to add an additional MOPP/MOOP between the US module and the probe (type B applied part)?
Important comment: no additional outgoing connection. So the whole system is isolated.
2. I also plan to put a kind of touchscreen on the prob. Except for 1mopp isolation gaps between the screen and the transducer, Do I add/pay attention to something else?
3. Re the single board computer (enclosed inside the cart), is there any special requirements? I guess there is no need for a medical grade PC here? (Again, no external connections)

Thanks

 

[Peter Selvey]

You will certainly need isolation between the internal electronics of the probe and "applied part", but for US probes the applied part is usually the plastic case of the probe (the main body and ultrasound window + sealing). So you won't (necessarily) need an isolation barrier in the electronics like a dc/dc converter, optocouplers and so on, the plastic case is the isolation barrier. For the working voltage of this isolation barrier, you need to keep in mind the driving voltage of the transducers which can be in the >100Vpeak region, although the rms value is usually low and pulses in the MHz region so it's not really a serious risk, but needs to be addressed somehow. Simple option is just design the plastic to meet the 2MOPP @ peak voltage, otherwise you need complicated (but entirely valid) arguments about why lower values for dielectric strength, creep and clearance are justified for this kind of waveform despite the relatively high peak voltage. Also while the waveforms to the transducer may be low rms, there may be parts in the US drive circuit that are constantly high voltage like 150Vdc (e.g. output of a dc/dc converter, capacitors) that supply the switching. So these may need some attention from an operator protection point of view (2MOOP @ "x" Vdc)

The touchscreen needs more info, like is it for the operator or the patient, can the patient contact it anyway. Or "Keep it Simple" put a plastic film over it that can handle e.g. 1000Vdc, call it 2MOPP @ 5V.

The single board computer has no special requirements, from an isolation point of view it's just part of the same block of electronics we normally refer to as "secondary". So you will have mains - secondary taken care of by the PSU, and secondary to patient by the plastic case. Plus the xVdc from the HV US drive, if applicable.

It sounds like you need to draft up an isolation diagram and then run it by someone with experience in IEC 60601-1 before you commit to the design. But it's not that complicated apart from the high peak voltages for the transducer drive, if applicable.

 

[Dindin]

Hi Peter,
thanks for your answer.

1. so, from your answer I understand that the isolation is impmented mechanicaly at the transducer point. and no MOP required at the US electronics (BTW, this is what the US module manufacturer says and I wasn't sure about...). so for next step I need to check the construction of the transducer and the handle and make sure they have the deeded isoltation "(2MOOP @ "x" Vdc)".

2. the touchscreen is intended for the operator only. no contact with the patient. it's placed on the hand peice. any special requirents needed here? I'm concered about how the touchscreen will work with a film on top..

TNX again.

 

[Peter Selvey]

So the patient related bit needs to be 2MOPP (not 2MOOP) which is a little tougher than 2MOOP, higher requirements. For simplicity you could design the whole thing for 2MOPP rather than trying to draw a boundary of where the patient bit ends and operator starts, but of course that's up to the designer. In principle the isolating parts only need to handle 2MOP at the true working voltage in the parts next to the isolation barriers, and 1MOP at the highest single fault voltage, but reality it's easier to assume that the highest voltage in the circuit and design everything for 2MOP at that voltage. So it's really up to you for the touch screen: do you want to assess the true working voltage in NC and SFC (which may require some analysis and justification if there is say 150Vdc somewhere in the circuit without an isolation barrier) or do you want to just assume 150Vdc and design for that, in which case a normal touch screen may not be designed for it. Or do you want to run the US related stuff off an isolated circuit so you don't need to worry about the touch screen, buttons, screws near secondary etc. There's a lot of options here so you need to start with knowing what the max volts in the system are, then decide a strategy, and then you can start looking at specific design requirements like dielectric strength, creepage, clearance, sealed joints etc.

 

[Dindin]

OK,
I checked the max operating voltage of the trransducer, it's 140 Vp-p(50V RMS)\ ~400Hz.
accoriong to table 12 of IEC60606-1, I need 4.6mm of creepage distance & 2.5mm of air clearance. that means the housing (hand piece) polymer has to be of a thickness of at least 4.6mm?

Re. the touchscreen, it's 5Vdc operated and connected to the SBC. so I need to keep the same values of creepage distance & air clearance from the screen and the transducer itself?

 

[Peter Selvey]

The rms sounds high, and the frequency sounds wrong. Also creepage is different to insulation thickness. It sounds like you need to find someone with a basic knowledge of IEC 60601-1 on your design team (not a consultant, but an actual member of the team), it's not possible to learn the basics in a forum like this, it would take too long to explain and wouldn't be effective anyway. A forum is good for questions about specific points, or interpretation, not building an isolation plan from scratch.

 

[Dindin]

ok. Thank you Peter

re. the values , yes it is. it's not a conventional US device..

 

[mr9000]

OK,
I checked the max operating voltage of the trransducer, it's 140 Vp-p(50V RMS)\ ~400Hz.
accoriong to table 12 of IEC60606-1, I need 4.6mm of creepage distance & 2.5mm of air clearance. that means the housing (hand piece) polymer has to be of a thickness of at least 4.6mm?

Re. the touchscreen, it's 5Vdc operated and connected to the SBC. so I need to keep the same values of creepage distance & air clearance from the screen and the transducer itself?

Creepage distances and air clearances are not the same as isolation thickness. For isolation barriers they must withstand a certain voltage, not having a set distance (although there are minimum thickness requirements).

 

[Dindin]

Creepage distances and air clearances are not the same as isolation thickness. For isolation barriers they must withstand a certain voltage, not having a set distance (although there are minimum thickness requirements).

Understand, Thank you very much!