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AN10952 Datasheet(PDF) 10 Page - NXP Semiconductors |
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AN10952 Datasheet(HTML) 10 Page - NXP Semiconductors |
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10 / 40 page  AN10952 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved. Application note Rev. 1 — 24 December 2010 10 of 40 NXP Semiconductors AN10952 SSL2103 flexible, mains-dimmable LED driver Dimmers can be divided into two operational categories: positive angle/forward phase operation, also known leading edge dimming, and negative angle/reverse phase operation, or trailing edge dimming. Forward phase dimmers work by varying the switch ON point of the supply current to a lighting circuit. They detect the start (zero-crossing point) of each half-cycle of the supply voltage, and then wait for a predetermined period before switching on the current (see Figure 4 and Figure 5). Reverse phase dimmers vary the switch OFF point, cutting off the supply current to the lighting circuit at predetermined intervals (see Figure 6 and Figure 7). Two switching methods can be distinguished: triac dimming and transistor dimming. Triac switching is always used with forward phase dimmers. Transistor switching can be used with forward and reverse phase dimmers. Transistor dimmers have the advantage of being able to switch capacitive loads, but most existing dimmers are still triac based. 4.2 Triac dimmer A triac is a bidirectional gated switching device with distinct latching properties. A number of conditions must be met to ensure reliable latching and to maintain current flow once the device has been latched: • When the triac is triggered, the voltage across the device must be sufficient to enable the minimum latch current to flow. IL is the minimum current needed to hold the component in the conducting state after the trigger (or gate) current has been removed. IL must flow for long enough (the firing time) to completely latch the device. • Once the device has been latched, a continuous current must flow through the device in one direction. This is the hold current, IH. If the polarity of IH changes (i.e. at a zero cross point), the triac will switch off. • A triac is not a fully symmetrical device - the values of the above parameters depend on the direction of current flow and on temperature. A timing circuit inside the dimmer, usually consisting of a resistor/capacitor combination, must be reset at zero crossing (the capacitor must be fully discharged). If the above stable dimmer operation conditions are not met, problems may arise that can lead to unstable operation. LED systems without oversized buffering are unforgiving, because they respond a lot faster to changes in power dissipation than incandescent lamps do. Even small variations in light output in the response frequency of the human eye (200 Hz to 120 Hz) can cause flickering and be disturbing. So the switching frequency should not be allowed to fall below 200 Hz. It should be noted that the human eye is more susceptible to variations in color than brightness. 4.3 Transistor dimmer A transistor dimmer uses a rectifier bridge in combination with a switching device, such as a MOSFET or bipolar transistor, to switch the main current. It contains additional circuitry to drive these devices, and this circuitry needs to be powered. The power is tapped from the switch while it is open, and stored in a capacitor. Though the energy dissipated in the dimmer electronics is not substantial, the current drawn over the switch can be a lot greater than the current required for a passive timing circuit. |
Similar Part No. - AN10952_15 |
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Similar Description - AN10952_15 |
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