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    CamSemi moves into LED lighting

    CamSemi has announced a family of off-line isolated LED drivers for lighting.

    Dubbed C312x, the drivers implement a quasi-resonant boost/flyback topology, claimed to offer +/-5% current regulation, efficiency over 80% and power factors greater than 0.9 from an isolated design.

    "Three driver ICs target 4, 8 and 12W designs making them suited to solid-state lighting products from replacements for halogen spotlights and incandescent lamps, to fluorescent tubes," said the Cam Semi.

    The firm specialises in controllers in small packages, made on a low-voltage processes, that use emitter control of cheap external high-voltage NPN transistor to handle mains switching.

    The 8W C3122, for example, comes in a SOT23-6 case and typically operates at 3.5V (5V max).

    Like its C266x off-line charger chips, the LED drivers use primary side sensing to eliminate opto-coupler feedback across the isolation barrier.

    However, there are changes to the architecture to suite LEDs.

    "There is a difference with the LED drivers, the emphasis is focussed on the constant current side," Dr Aidong Xu, application director at Cam Semi, told Electronics Weekly.

    The firm will not discuss the subtleties of internal operation, but Xu would talk about a novel mode modification to improve power factor.

    "Typically in our circuits, we have flyback operation," he said. "For LED driving we added an extra inductor to keep the input current as constant as possible. We call this boosted flyback." See below

    Any particular implementation, said Xu, will maintain +/-5% current regulation at any output voltage from the maximum designed output down to 5% of the maximum, at the same time as the input varies from 85V to 265V.

    "For design, you have to make assumption on the maximum output voltage required under worst case - for example with six LEDs in series, allow 4V per LED which is 24V," he said. "When actually in use, current will be same with six, five, three or even one LED."

    The 8W C3122 is the first to be announced and has an internal emitter switch mosfet - see diagram.

    C3123 will appear later and will handle loads up to 12W using a small low-voltage external mosfet to switch the external high-voltage NPN.

    The 4W C3121 will be similar to the 3122, with simpler in-package construction to reduce its price.

    All will include protection against over-temperature, output overvoltage, and output short-circuit.

    Inductor improves power factor
    CamSemi moves into LED lighting

    Cam Semi has added an inductor to its flyback converter to improve its input power factor.

    Adding Lpfc (see diagram) adds a touch of forward conversion to the flyback, in a topology that the firm calls boosted flyback.

    When the transistor switch turns on, current builds up in inductor Lpfc at the same time as current builds up in the transformer primary.

    When the transistor switches off, the transformer flies back, delivering current to the load - typical flyback operation.

    At the moment of turn-off, current cannot stop flowing in Lpfc because of its inductance, so its current continues to be drawn from the mains - extending input conduction time to improve power factor - and, in the manner of a forward converter, is delivered to the load through the transformer.

    "The major energy is still provided through flyback," said Cam Semi application director Dr Aidong Xu. "The effect is that even during the transistor off period you still have input current which improves power factor, and the capacitors in the input filter can be much smaller."

    The additional inductor does not upset regulation, added Xu, because the combined charging current of Lpfc and the transformer is still set by the IC's current sense resistor.

    Because of its simple construction, the firm estimated the inductor will add only 5% of the cost of the more sophisticated transformer.

    It will be E10 size (10mm square) when T1 is E16, and can be as small as E7, said the firm.