Peter (and Treg) The tube circuit for a backlight LCD is usually a simple inverter circuit with a high voltage output. The high voltage output is fed to the tube via a series resistor which then limits the current when the tube strikes over. Some devices may have the output rectified.
The inverter transformer will either have: 1. a single input winding (which also forms part of an oscillator circuit) with a combined oscillator phase splitting transistor (which is also the driver) or, 2. sometimes a dual driver with common centre point is used (i.e. using a centre tapped dual input winding). The second variety will require more testing as the driver stage (either push pull n type + p type devices, or alternate phasing method i.e. both devices being the same type) is seperated from the oscillator (clock source). The first variety almost always fails safe as any single fault applied usually stops the oscillator from running. The second type may use an astable type ciruit with a dual phase output. This type of ciruit usually generates two square wave outputs, one phase shifted 180 deg to provide "Q" and "not Q" outputs to drive the output stage. This second type may continue to generate a high voltage if a single fault removes drive from one phase only but the output will be reduced by at least 50 %. In both ciruit types, the oscillator / driver is typ running at between 50 and 200 kHz. In terms of proving compliance, you should also consider the requirements for limited current devices under single fault conditions. As mentioned, the open circuit voltage may be high 1500v to 2000v but when the tube strikes and current begins to flow, voltage drops down to typ 200 - 300v. The low current and high frequency of operation fit well with these requirements. Hope you find this information useful Bill Ellingford, Approvals Manager, Motion Media Technology ---------- From: Peter Merguerian[SMTP:pe...@itl.co.il] Sent: 18 May 1999 20:30 To: 'emc-pstc'; treg@world.std.com Cc: pmerguer...@itl.co.il; al...@itl.co.il; ico...@itl.co.il; plei...@itl.co.il; zoh...@itl.co.il Subject: Back-Light Circuit Dear All, I am investigating an ITE product with a backlight-circuit (used to provide the background lighting for an LCD display located in SELV). However, manufacturer of product cannot get hold of the backlight circuit schematics. I intend to use Clause 2.3.5 of the UL1950/EN 60 950 standard. This allows the SELV to be connected to hazardous voltages (approx 1500 V) in the backlight circuit as long as the SELV limits are maintained even after single faults. Missing the schematics, I wonder if if anyone can explain how a backlight circuit works and what components in such a circuit are candidates for shorting and/or open-circuiting? Thanks and Best Regards, PETER S. MERGUERIAN MANAGING DIRECTOR PRODUCT TESTING DIVISION I.T.L. (PRODUCT TESTING) LTD. HACHAROSHET 26, P.O.B. 211 OR YEHUDA 60251, ISRAEL TEL: 972-3-5339022 FAX: 972-3-5339019 E-MAIL: pe...@itl.co.il Visit our Website: http://www.itl.co.il
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