MOOPs in Medical Device Charger

[WalkerNelson678]

Hi everyone,

I have a question which I have been considering over the past few days regarding a medical device project I'm working on. I'm feeling a bit stuck so I figured I'd call in the big guns and see what you all have to say. Thanks in advance for the time!

The Device:

I am developing a battery powered wireless medical device which is charged using a docking station containing pogo pins. The device will snap into the docking station using magnets, and the pogo pins will contact some pads on the wireless device to charge the battery. The device cannot be charged while in use because of it's geometry, so I'm not expecting the charger to be either contacted by the patient, or present in the patient environment.

The charger requires a 5VDC input to a USB-C port.

Design Context:

This question has to do with methods of operator protection which are required in the design of the charger to protect against touch leakage current. My understanding is that we must provide 2 MOOPs to ensure that the touch leakage current remains within acceptable levels.

Our plan to do this is to rely on a 62368-1 compliant AC/DC adapter to provide one MOOP. We will not supply an AC/DC adapter with the device; the customer will be responsible for supplying their own. It will be written into the IFU that any adapter used with the charger must be compliant with IEC 62368-1.

Right now we are thinking that the second MOOP will be provided by some element in our secondary circuit (on the charger PCB) in-between the exposed pogo pins and the USB-C inlet of the device. However, we're not sure whether or not this strategy in general is necessary (maybe it is overkill), and what the best type of protection to install would be.

The Question(s):

I am looking for some guidance on a strategy for this secondary MOOP. Here are my questions:

1. My understanding of 62368-1 leads me to believe that we could use the AC/DC adapter as 2 MOOPs since components connecting to mains power must be double-insulated according to 62368-1. Is this a correct understanding of the interaction between 60601 and 62381 regarding MOOPs?
2. Assuming we do need 1 internal MOOP, will a PTC fuse between the pogo pins and the USB inlet be acceptable considering that it will react to an inrush of current by becoming extremely high impedance?
3. If a PTC is not an acceptable MOOP, how about a DC-DC isolator? Any advice on finding a cheap version of this component; we have looked into this option and are having difficulty committing to it due to cost.

Please let me know what you think. Here is a diagram to help explain my question as well.

 

[mr9000]

If you provide only 1 MOOP in your device and RELY on the user using a certified AC/DC adapter, my first guess for "foreseeable misuse" would be the user not using a certified adapter and just plugging it into a cable from his computer. Why not provide an extra AC/DC adapter and stating that only your adapter is to be used? But then again, when providing an USB-C Inlet, why not plug in just anything.
If you happen to stumble over any component providing isolation which isn't a DC-DC (or AC/DC or AC/AC) isolator, happy to hear! But if you go that road and implement an isolator anyways, why not directly a mains-5VDC from-the-shelf isolator, which can be bought directly having up to 2MOPP? You can find them at mouser.

If I recall my training the 62368-1 is mostly a problem if using it to achieve MOPPs.

 

[Souheil]

Hi, as testing lab, in this case we require an MOPP for MAINS between USB adapter and pogo pins. The reason is the "foreseeable misuse" of a non certified adaptor.
Unfortunately, we are facing this case very often, and we recommend the user either to implement a DC/DC isolator behind the USB connector or to replace the standard USB connector by a "custom" connector, that forces the user to use a provided certified adaptor.

Hope this helps.