cupoftea
Advanced Member level 6
Imagine you have a rack of mains connected PCBs, each having an isolated offline SMPS on it.
The secondaries of all these offline SMPS's is connected to Earth.
Each PCB is fed by a cable containing mains line and neutral wires. No Earth connection at the mains connector.
The easiest way to connect all the secondaries to Earth is just to have a big, metal, earthed backplane,
and then the heatsink bracket of each PCB screws directly to the earthed backplane, and then a track on the
PCB takes its earth connection from the screw "restring" on the PCB.
..And voila, job done, all PCBs now have an earth connection to their secondary...And its nice and quick for the assembly staff to do.
But what a disaster this is from a common mode emissions point of view. Common mode emissions can literally trot off down this connection to the earthed backplane.
The loop of these CM emissions from the mains line and neutral, and then back down this backplane earth, will be a very large loop. Very Bad,
The common mode loop is now vary large in area, and this will mean high common mode emissions.
It would be better to also have an earth wire connection to the mains connector of each PCB. Then you can Y cap
connect to that earth on the PCB near to the mains connector, and do this right by the common mode choke. You can also use a
short bit of insulated wire to connect the secondary to this earth_at_the_mains_connector. This would be far better from a common mode
EMC viewpoint.
However its much more costly and lengthy to assemble.
So the only answer then, would you agree, would be to relent, and accept the backplane_earth_connection_method,
but then also have a large
common mode choke and Y capacitor filter board upstream of the whole system?
The secondaries of all these offline SMPS's is connected to Earth.
Each PCB is fed by a cable containing mains line and neutral wires. No Earth connection at the mains connector.
The easiest way to connect all the secondaries to Earth is just to have a big, metal, earthed backplane,
and then the heatsink bracket of each PCB screws directly to the earthed backplane, and then a track on the
PCB takes its earth connection from the screw "restring" on the PCB.
..And voila, job done, all PCBs now have an earth connection to their secondary...And its nice and quick for the assembly staff to do.
But what a disaster this is from a common mode emissions point of view. Common mode emissions can literally trot off down this connection to the earthed backplane.
The loop of these CM emissions from the mains line and neutral, and then back down this backplane earth, will be a very large loop. Very Bad,
The common mode loop is now vary large in area, and this will mean high common mode emissions.
It would be better to also have an earth wire connection to the mains connector of each PCB. Then you can Y cap
connect to that earth on the PCB near to the mains connector, and do this right by the common mode choke. You can also use a
short bit of insulated wire to connect the secondary to this earth_at_the_mains_connector. This would be far better from a common mode
EMC viewpoint.
However its much more costly and lengthy to assemble.
So the only answer then, would you agree, would be to relent, and accept the backplane_earth_connection_method,
but then also have a large
common mode choke and Y capacitor filter board upstream of the whole system?