Electronic – Getting signal from high voltage side of PCB to low voltage side

pcb-design

schematic

^{simulate this circuit – Schematic created using CircuitLab}

I am designing a PCB that has both a high voltage side and a low voltage side.

They are separated by roughly 8mm.

There are I/O lines on the high side that, most of the time, input a very low voltage/low current signal connected to an op amp and then to an ADC in an MCU.

These same input lines are also used to output a high voltage signal for a very brief period. Obviously the low voltage side has to be protected from these high voltage output.

Typically I would use a couple resistors and zener diodes, but in this case, I still need to get a signal across to the low voltage side.

How is this done?

In case you're wondering, this is for an AED with a novel delivery mechanism that I've tested.

Low voltage is the ECG sense while High Voltage is the shock.

The PCB is for a somewhat production prototype.

edit: simplified schematic (ignore component names) and simplified PCB layout added

Best Answer

Analog Devices and Burr-Brown (now a part of Texas Inst.) are big in isolation amplifiers. Probably more expansive than home-brew, but you get tested, documented, and repeatable performance complete with safety certifications.

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DDR Bus Design – Reviewing DDR Bus Design for High-Speed Signal Integrity

Looks good and you may just get lucky with that layout.

Being an engineer, luck is usually not a method I rely on :-) So let me show you what I would do:

1) Define the PCB stackup. Looks like you are on a 4-layer stackup, but we need to know material and thickness of laminate/prepreg etc.

2) Calculate trace widths to give you 50R on all layers. Your traces looks wide, but you didn't give your stackup so they may be okay. I would worry a bit about crosstalk though if those traces really are 50R (because I then know that they are far from your reference plane, which increases crosstalk).

3) Engineer a great low impedance power delivery network (PDN). I read between the lines that you have two planes for power and ground - which is a really good idea. I would use my tool at pdntool.com to select the right capacitor combination. And use the knowledge that bypass capacitor location is fairly unimportant. So the caps would be placed last so the don't interfere with the routing.

4) Repeat this for your Vtt supply. The termination voltage is being constantly pulled in both directions, so it needs a low impedance as well. With DDR1 on a low layer count board, Vtt ripple is a common problem (and make sure Vref is not connected to Vtt!!!). This would usually require a Vtt island with sufficient bypass. Remember about half the ripple on Vtt will be present as noise on top of any input signal terminated to Vtt.

5) Do some quick IBIS simulations to find a trace separation that gives you acceptable crosstalk. Use Hyperlynx, SigXplorer or some such tool for this. Or get someone to do it for you.

6) Do your timing analysis to find the acceptable tolerance on trace length matching (don't overdo length matching - just keep within your calculated tolerance).

7) Document the above in a nice document and call a peer review - this is a great time to find errors. You could also post that here and ask for problems in your reasoning.

8) Enter everything as routing rules in your CAD tool and do that layout. Remember with a well engineered PDN and 50R on all layers your via count is irrelevant. Also if you just route your differential clock as two 50R traces of same length (within half a rise-time or so), you need not treat them special.

For inspiration you can also look at the layout examples on the JEDEC website.

Hope this helps - feel free to ask more questions.

Electronic – How to cover up exposed high voltage pcb traces

You may want to consider finding a relay that has better construction such that the AC load switch lead does not go over in between the coil leads. This will lead to better isolation from low voltage control side to the AC side.

If you have arranged all the connections on the bottom side of the board and then mounted the board in the enclosure with the bottom of the board facing to the base of the enclosure then this leaves things about as safe as you can get without excessive other measures. With this configuration I think you are good to go after providing appropriate safety labeling on the outside.

The AC connection looks like you are planning a screw terminal quick connect type of affair. Does this mean that users are going to be opening the box to attach the AC wires? There could be safety concerns with this because there is no good way to regulate how the attachment is done and how the wire in-feed is protected (i.e. how is strain relief and wire abrasion protection provided).

### Update after Comments Discussion ###

I have previously designed and deployed a similar AC load switching type device. I happened to use an SSR (solid state relay) in place of the relay but that need not be done. To greatly simplify the AC power wiring for the user I supplied the enclosure of my product with an AC input plug and an AC output plug. For the AC input the following IEC type connector was used:

enter image description here

For the switched AC load side the following IEC C13 type connector was used:

enter image description here

This scheme lets the power be supplied to the switching box with any one of a number of readily available AC power cords that have an appropriate country specific plug on one end and the IEC style end that plugs into the input side of the switching box.

The output side connector can be supported by readily available load side plugs that have IEC C13 and C14 type plugs at each end such as this cable:

enter image description here

This scheme also has several additional advantages:

The safety ground can be connected inside the enclosure.
The safety ground can be connected all the way out to the switched load device.
The enclosure could contain an additional internal load on the AC input to power internal electronics from a small PC mounted power supply.

The product design that I developed and deployed actually had two switching circuits that supported two AC input plugs and two AC output plugs. The switching circuits were also wired through thermal circuit breakers rated at 15A. The internal microprocessor electronics was powered from the first AC input. Here is a picture of the rear panel of the unit.

enter image description here

Best Answer

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Electronic – How to cover up exposed high voltage pcb traces

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