Hi all, Historically, we have made products with CCFT (Cold Cathode Flourescent Tube) Backlight for the Liquid Crystal Displays.
These backlight circuits run from SELV, usually 12VDC; and then they create 1000VAC @ 60Khz (typical). These circuits typically use a small transformer with really small gauge secondary wiring (around 40AWG). The voltage is high; but the current capability is about 10mA. For safety testing, I have always submitted specifications for parts within this circuit. I have also always provided color coded artworks of the circuitboard showing the high voltage traces. We have never had a problem with these circuits during safety testing. In a way, that's bad. Why? Because no failures means that I don't have anything to fix and I don't learn how to get the circuit to pass. So, in essence, I don't know why we're passing. We throw the circuit on the board with almost no regard for creepage and clearance. Maybe we've just been getting lucky? That's nice; but it doesn't give me a warm fuzzy about how we design our next CCFT circuit. Specific questions: 1. Are these circuits considered as energy limited? For instance, I know that the lab performs short circuit testing on this circuit; but, since its current capability is so low, nothing ever happens. 2. Should we design to the Creepage and Clearance distances for Double Insulation (our chassis is ungrounded); or do we just need to meet functional insulation requirements. 3. How are the startup transients considered? For instance, our CCFTs run at about 900VAC steady state; but during startup the voltage can climb above 2KV while the tube is warming up. Do we just consider the 900VAC and ignore the transients? Sorry for the rambling nature of this email; but such "rambliness" typifies my thoughts on this subject; hence my appeal to the group for some insight. Any thoughts are appreciated. Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
