Hi Joe:
What you describe is very similar to the flash tube circuits in a digital camera. Since you have measured the 150 V and the 300 Hz and determined they are not limited current circuits, I will offer comments based on these circuits being hazardous circuits. (You did not specify the voltage for the 300 Hz.) First comment: Can the circuit be altered such that the maximum current from the 150 V DC is less than 2 mA? If so, both the 150 V DC and the 300 Hz will be limited current circuits, and no safeguards are necessary. (If one circuit is deemed a limited current circuit, then all circuits derived from that circuit are taken as limited current circuits.) First question: In a cell phone that is fully isolated >from earth and everything else, what is the current path through the body? There must be a "goesinta" the body and a "goesouta" the body electrical connections. Safeguards would be applied between the circuits and these two points. Second question: What is the insulation between the 300 Hz (voltage?) and the outside or the screen that is touched? This may not be physically discrete insulation, but obviously it does comprise insulation. Since the 150 V DC and 300 Hz circuits are derived from a battery, the electric strength of the insulation between the circuits and the accessible parts of the screen need not include consideration of transient overvoltages. Table 5B does not apply. (I suggest an insulation resistance test as part of your evaluation.) Second comment: A single fault between the 3.7 V and the 150 V DC is likely to shut down the inverter. This proves that no isolation is required between 3.7 V and 150 V. This is application of 2.2.4. Third comment: Study both the circuit and the construction to determine a current path through the body, for both normal operating conditions, and single- fault conditions. If you want to talk further about this, give me a call. Best wishes for a Merry Christmas, Rich 858-592-2620 On 12/14/2010 18:18, Joe Randolph wrote: > Hello All: > > I have been asked to suggest a UL/EN 60950 compliance method for a low > power, high voltage circuit that resides inside a cell phone. The > circuit activates a piezo-electric transducer that physically vibrates > the LCD display at about 300 Hz in 30 mS bursts to provide the sensation > of a key "click" when the user touches a virtual key on the touch > screen. Note that what the user contacts is the physical vibration, not > the actual electrical signal that activates the piezo transducer. > Following are some general characteristics of the driver circuit for the > piezo transducer: > > * A non-isolated DC/DC converter powered from the 3.7V cell phone > battery generates a 150 VDC driver supply that will deliver about 5 mA > into a 2K ohm load. > * The 300 Hz AC output of the driver will deliver about 4 mA RMS into a > 2K ohm load. > * Both the 150 VDC supply and the 300 Hz AC output share the same > circuit reference node with the rest of the phone circuits. > * This circuit reference node is normally floating with respect to earth > ground, but it can become grounded through the USB port. > > > I'm pretty familiar with 60950 and the standard compliance methods for > the various circuits that appear in ITE equipment. However, this > particular circuit does not fit neatly into any of the standard categories. > > A brute-force compliance analysis would classify this circuit as a > hazardous circuit, and would require an isolation barrier between this > circuit and any SELV circuits, such as the USB and headset ports on the > phone. The voltage is too high to meet the definition of TNV-2, and the > current output, while very small, is too high to qualify as a Limited > Current Circuit. > > My sense is that it may be possible to show compliance by using > single-fault testing to demonstrate that under fault conditions, all > user-accessible points remain within SELV limits. In particular, I'm > looking at the wording of clause 2.2.4, "Connection of SELV Circuits to > Other Circuits." > > I have never attempted to apply the method of 2.2.4 to a product, so I'm > looking for feedback on whether the method is appropriate for this > application. Any other suggestions or insights would be most welcome. > > It seems to me that this type of situation may come up in other > applications that use high voltage at very low currents, such as CCFL > backlights, EL backlights, and possibly camera flashes. > > > > Thanks, > > Joe Randolph > Telecom Design Consultant > Randolph Telecom, Inc. > 781-721-2848 (USA) > j...@randolph-telecom.com > http://www.randolph-telecom.com <http://www.randolph-telecom.com/> - ---------------------------------------------------------------- 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 <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. 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