I am designing a dual supply powered system, where certain loads, depending on some conditions will be powered by one or other power rail, in order to achieve some safety levels.
The problem arises when trying to design an EMI/RFI filter for the system. First and foremost, adding the filter after the switch is not possible. This is because there are certain loads powered by the redundant power, whereas others not. Apart from the filter not being nearest to the device input, it would imply replicating filters where loads exist, which may bring more issues than benefits.
The first approach is this
But common mode filtering will never work due to the current splitting.
I have been struggling with this, and the only solution I have come up with is to use FETs as switches to open the return power path only if voltage is present in the respective positive rail.
This way, the current will flow through where is supposed to.
However, I find the solution too appealing. I have the feeling that I am missing something.
- Is my solution to the problem valid? Any details/quirks to be considered, apart from the thermal ones?
- Do you have any other solution to my problem?
ADDED: One of the answers suggests a 3P filter. This solution is to be applied to small scale electronics, so that would be my last resort.
Best Answer
It seems to me that a three-phase filter might do the job for your example case. These usually have all the windings on the same core.
Figure 1. A random three-phase filter. Image source: RS.
Connect Va via L1, Vb via L2 and connect G via L3 or N. I'm assuming that the N winding is the same as the other three. This leaves you with one spare feed path.
Just to be clear, you would then have the two supply commons connected at NIN in and have all the load commons connected at NOUT.