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I'm designing a board which has -48V input. For the power module, I'm considering using KSTW010A0A Barracuda from GE(DOSA compliant 1/16 brick, Isolated DC-DC Converter)

In the datasheet, there is this figure which shows suggested configuration for EMC filtering:

Suggested Configuration for EMC filtering in KSTW010A0A

The problem is:this configuration is intended for three-wire system where there is Vin- & Vin+. But my system is a two-wire one. So my question is how to use this configuration for my design.

I guess the Y-caps will be removed and the choke will be replaced by an inductor? How much caps values will affect by this change? what is the value of new inductor?

Thank you

PS1: In the first place I draw my schematic just like the configuration and obviously it's wrong! my wrong! schematic

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  • \$\begingroup\$ No, this is a two wire system plus ground/chassis for removal of EMI artefacts. \$\endgroup\$
    – Andy aka
    Commented Nov 4, 2015 at 15:05
  • \$\begingroup\$ really? then my system is an one wire system? because all I have is a -48V plus ground/chassis. \$\endgroup\$
    – Babak
    Commented Nov 4, 2015 at 15:18
  • \$\begingroup\$ You can justify that by showing a circuit. I expect that your DC to DC converter will be bolted to a chassis with one of the input power wires also directly connected to the chassis. \$\endgroup\$
    – Andy aka
    Commented Nov 4, 2015 at 15:19
  • \$\begingroup\$ @Andyaka I added my schematic to the post for more clarification. \$\endgroup\$
    – Babak
    Commented Nov 4, 2015 at 15:37
  • \$\begingroup\$ Just because you have used an OrCAD symbol that represents ground/chassis doesn't mean that your connection to actual ground/chassis isn't via some wire that could create a loop antenna and emit RFI. So, physically how far are your components from the ground/chassis of the equipment? To answer one of your questions, the choke shouldn't be replaced by an inductor. \$\endgroup\$
    – Andy aka
    Commented Nov 4, 2015 at 15:42

1 Answer 1

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You need to be VERY sure you truly only have a 2wire system (CHASSIS + PWR) and not a 3 connection system (CHASSIS + P_PWR + N_PWR) and some system engineer hasn't treated a chassis reference as the P_PWR reference/return.

Consider the setup below.

schematic

simulate this circuit – Schematic created using CircuitLab

You as a consumer have been provided with a +-PWR and Chassis but it can be seen that the SOURCE has tied the -PWR to chassis... Why not just use chassis as the return and thus a 2+Chassis becomes a 2wire system?

This is wrong because of a phenomenon called Electromigration. The +PWR and -PWR will consist of some wire gauge with a level of impedance. If a consumer utilises chassis as the PWR return there will be a DC current flowing along chassis, as the prefered route.

NOTE: the use of isolated DCDC converters that are then referenced to chassis is fine as the current loop will not be via chassis

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  • \$\begingroup\$ Excellent point! Actually this was just unregarded and -PWR was connected to chassis ground. So back to 2+Chassis system, I am now confused about VO- ; According to Suggested Configuration it should connect to chassis ground? but that means my whole board's ground will connect to chassis? \$\endgroup\$
    – Babak
    Commented Nov 4, 2015 at 16:32

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