I'd like to get some general feedback on an EMI issue we're struggling with. We've got an electronics board which uses Cincon CFM1001S SMPS to convert ~200-240VAC to 5VDC for all the low-voltage systems. We've recently discovered that this system is creating unacceptable levels of EMI on the mains supply that feeds it, failing to stay within EN55014 (nearly identical to EN55022) limits. We've tried a variety of EMI filters (including Corcom 5VK1, 3EZ1, 3EQ1, 6VN1, & 6EU1) which still weren't sufficient to reduce noise to acceptable levels.
The power supply supposedly meets the EN55022 noise requirements, but is failing miserably for us. We've done variety of tests to confirm that the noise is coming from the SMPS and I'm quite confident that it's the source. The SMPS supplier tells us that the reason we're having trouble is because we connect the DC negative output to ground (one point only, star grounding.) Here's what they said.
If it is true, are we just out of luck? Part of our electronics system requires the common frame ground to also be the DC negative as part of a water level sensing scheme. We'd have to do *MAJOR* mechanical and electronics re-designs in order to isolate the DC system. Besides, I'm not sure I'd want to - I've heard that with an isolated DC system, it's possible to have fault modes which result in lethal high voltages piggybacking on ostensibly "safe" low voltage lines (like, instead of a floating pair of wires with 5VDC between them, you've got a pair of wires with 240VAC on them, and a 5VDC offset between them.)
My current thinking is that this particular SMPS is especially noisy and not a good fit for us, and that we just need to try some alternatives. This is not a pleasant thought, since the manufacturer specs don't seem to mean anything and we'd basically be trying supplies at random while paying top dollar to spend time in an EMI test lab.
I'd love to know if tying DC common to frame ground is really a big no-no in terms of noise. If so, how do other manufacturers handle this? I feel like frame ground/DC common bonding is pretty common.
The power supply supposedly meets the EN55022 noise requirements, but is failing miserably for us. We've done variety of tests to confirm that the noise is coming from the SMPS and I'm quite confident that it's the source. The SMPS supplier tells us that the reason we're having trouble is because we connect the DC negative output to ground (one point only, star grounding.) Here's what they said.
Does this make sense to you guys? It seems strange to me.Engineering replied the main reason the customer is seeing high emissions is because the CFM1001S is designed as a Class II (double insulated) product but being used in a Class I application.
If it is true, are we just out of luck? Part of our electronics system requires the common frame ground to also be the DC negative as part of a water level sensing scheme. We'd have to do *MAJOR* mechanical and electronics re-designs in order to isolate the DC system. Besides, I'm not sure I'd want to - I've heard that with an isolated DC system, it's possible to have fault modes which result in lethal high voltages piggybacking on ostensibly "safe" low voltage lines (like, instead of a floating pair of wires with 5VDC between them, you've got a pair of wires with 240VAC on them, and a 5VDC offset between them.)
My current thinking is that this particular SMPS is especially noisy and not a good fit for us, and that we just need to try some alternatives. This is not a pleasant thought, since the manufacturer specs don't seem to mean anything and we'd basically be trying supplies at random while paying top dollar to spend time in an EMI test lab.
I'd love to know if tying DC common to frame ground is really a big no-no in terms of noise. If so, how do other manufacturers handle this? I feel like frame ground/DC common bonding is pretty common.