There are a number of issues mentioned and I want to put some side notes here:
Interference needs a source and a victim. Sources: * The lowest frequency emitted by any of the 4 problem power conversions is the main switching frequency. * In low power solutions the lowest emitted frequency is steadily increasing to above 200 kHz, and will all be coped with by current EMC regulations. * Commercial filters are built to attenuate incoming interferences and in generally provide a suboptimal attenuation of emitted frequencies . * For medium and high power switchers the switching frequency may be as low as 5 kHz, a frequency difficult to filter out, and due to the absence of regulations in the frequency range of 2-150 kHz, the market provides many solutions for this from insufficient filtering to no filtering at all. Filter solutions at these frequencies require large costly inductors and large costly capacitors. Insufficient power filtering creates a low power quality grid, and not only interdevice problems, but also creating functional problems in the power network for distribution transformers (overheating and power capping below specs). * Virtually all filter solutions on the market are not designed for frequencies below 150 KHz, so manufacturers have to build their own; lack of EMC and filter knowledge leads to suboptimal or no filtering at all. * The current situation is that reduction of EMI from high power systems is (if regulated at all) driven by power quality regulations (utilities) only, not by EMC regulations. Victims: * Many victim immunity problems can be easily fixed by a suitable input filter, and in low power devices, very simple and cost effective filters may do the job. In high power systems often only the (low power) control part need to be effectively filtered. * I am not aware of commercial standards (but 55103-2) having immunity limits for conducted immunity in the frequency range 2-150 kHz. * Many commercial mains filters are built to attenuate incoming interferences above 150 KHz, and are asymmetric in nature. Filters are the solution to the problems, being commercial or integrated into the design: * Filter manufacturers may need to create cost effective and small footprint filters below 150 kHz, for currents well below 1 amp as seems to be the limit now. * Filter specifications should not be made in 50 ohm environments but in much lower impedance ranges (0.5 -5 Ohms), so the product will better fulfil the expectations of their users * Filter manufacturers need to be aware of the existence of emission and immunity regulations and create fully bi-directional filters, for lower input currents (due to lower power consumptions of modern devices) * Adequate input/output filters do not need high value touch current capacitors. By using T-type of filters (LCL) instead of PI-types (CLC) the values of decoupling caps can be heavily reduced (no touch current), at increased cost of dual CM-inductors. But we are used to that aren't we? * Shunting a low impedance signal with high value capacitors is NOT the way to go. Attenuation needs a high series impedance followed by a low ground path, I think we all agree about that. Gert Gremmen Ce-test, qualified testing bv - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
