Description:

EMC testing calls for surge immunity testing per IEC/EN 61000-4-5 among others. This is performed on all longer connections (> 10m) of a operational system. In order to prevent transient energy from going the wrong way, a decoupling network is used. Usually, these consist of large air-core or iron-cored inductors and a bunch of voltage clamping devices such as varistors.

For mains coupling, this is perfectly fine. The frequency is very low, so the voltage and current won't be affected by those inductors very much. Things get different at lower voltages and higher frequencies. Single inductors will affect the data signal just as much as the transients, if not more. A sort of recommended solution exists in the form of common mode inductors. These will allow for balanced communication signals such as RS485, CAN-bus and Ethernet. However, these common mode inductors won't do anything against differential mode signals. And due to the construction of balanced surge coupling networks, there is no guarantee that there won't be any differential surge energy. And even if the construction were perfectly balanced, things would get screwed anyway if one of the signals got shorted inside the EUT. Either of these issues will cause a large amount of energy to be diverted into the associated equipment!

The first solution is to provide surge suppression. However, that has the downside of dissipating surge energy that should have been directed at the EUT. Another approach is to decouple from the transient in a bit more literal way. That is the method that we choose for these decoupling networks.

Purpose:

We wanted a decoupling network to facilitate engineering and production testing of our rugged CAN.net devices. This requires power and one or more CAN busses to create an operational CAN network which must be able to deal with simultaneous application of 1.2/50µs and 10/700µs surges.

The decoupling network consists of a string of Bourns TBU devices which disconnect their terminals when the current exceeds 500mA. While disconnected, only a small holding current is present. This is way less than the typical surge energy, therefore has negligible effect on the transient pulse, particularly after being triggered.

Specifications:

• 3 or 6 channels
• Series resistance 60Ω
• Parallel resistance 180kΩ
• Maximum current 500mA
• Holding current ≤34mA (1mA + Vsrg / Rp)
• Transient voltages ±6kV
• Safe side clamping ±33V TVS
• Dimensions 160×80×55mm (3-channel)
• Dimensions 160×160×55mm (6-channel)
• Connections: banana jacks
• Each unit individually surge tested to 6.6kV