ESD protection for floating circuits

[likephysics]

How do you protect a power supply that is floating and not connected to Earth ground.
Do you still connect the ESD diodes to ground and Vcc?
Are there different techniques.

 

[donpacino]

you connect the ESD diodes to whatever the negative and positive terminals of the power supply are attached too.

 

[berkeman]

How do you protect a power supply that is floating and not connected to Earth ground.
Do you still connect the ESD diodes to ground and Vcc?
Are there different techniques.

Could you describe your setup in more detail? Is this power supply plugged into a wall socket, or battery powered? What is the HiPot rating of the power supply from output to AC Mains input? What is it powering? Have any pics?

 

[skeptic2]

Also following what Berkeman said, what are the inputs and outputs of the circuit the power supply is connected to, connected to.

 

[Mike_In_Plano]

Actually, this is a really common problem, and it's easy to cure.
You commonly see this problem in the following areas:
- Patient isolation for medical products
- Isolated AC power supplies
- Chassis isolated instruments (typically to prevent ground fault currents)
- Battery powered (floating) equipment.

Firstly, you package your item to protect against unwanted arcs reaching internal circuits. Typically a minimum creepage distance of 1mm per kv stand off is applied.

Then, you ensure that any exposed pins of the isolated circuit are clamped to the ground plane near their entry onto the board.

Next, you need an RC circuit between the primary and isolated circuits to ensure that the voltage does not become high enough to arc over. Typically 2.7 nF, X class capacitor between the isolated circuit and primary circuit is large enough to absorb the ESD hit without failing due to voltage. A combined resistance of typically 1 meg-ohm is commonly placed across the capacitor and serves to drain off the accumulated charge across it. Usually, the resistor is comprised of three resistors in series to handle the peak voltage across the capacitor.

In medical equipment, two capacitors in series are required to ensure low leakage current should one of the capacitors fail in a single-point failure. (reference IEC 60601).

In battery operated equipment, there may not be a path to discharge the current, so the test technician must use a conductive brush to remove the remaining charge after every zap.

I hope this is helpful.

Best Regards,

Mike_In_Plano