AN2711 datasheet(7/36 Pages) STMICROELECTRONICS

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AN2711

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Doc ID 14425 Rev 3

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Figure 2.

Non-dimmable power converter LED driver

Many fluorescent, compact fluorescent, and LED drivers have warnings stating “not for use

with dimmers” because they are unable to cope with the sudden rise of voltage late in the

half cycle, or the reduced average voltage. Many low-cost drivers directly rectify the

incoming AC and apply the resulting pulsed DC to capacitive filters. Current is drawn from

the source during a very short period at the peak of the incoming voltage waveform, and no

current is drawn during the remainder of each half cycle.

The Triac requires a minimum holding current, typically 30 to 50 mA, during the entire half

cycle. If the current falls below that level, or if it reverses, the Triac turns off. It may or may

not fire again during that half cycle, and an unknown voltage is left on the dimmer's timing

capacitor for the next half cycle. Chaotic operation results if the holding current is not

maintained.

Switchmode lamp drivers of any kind must include EMI filters at their AC inputs. In addition,

the Triac dimmer may (and should) contain its own L-C filter, and the AC line is slightly

inductive. All the filters typically contain only inductive and capacitive components - the

sharply increasing input voltage, when the Triac fires each half cycle, causes current ringing

which reverses several times during the half cycle. The Triac may continue to conduct during

brief reversals (microseconds to tens of microseconds), or it may turn off. The variable

voltage step and variable timing can cause the gating circuit to apply triggers to the Triac at

random times, leading to flickering in the lamp.

Current drawn by a capacitive EMI filter varies considerably, even without the Triac dimmer.

At low power levels (around 15 W) the current drawn by the EMI filter is a significant portion

of the total current. It must be taken into account in the design. The incoming AC voltage

waveform is distorted by other loads - its slope varies from the smooth sine wave one would

expect. Typically, there is a notch at the top of the waveform, caused by rectifiers on other

equipment which draws current, followed by a flat top caused by capacitive loads drawing

current through rectifiers, followed by a step downward as the rectifiers go through reverse

recovery (abruptly ceasing to draw reverse current). The rapid downward slope can cause a

capacitive load such as the EMI filter input to supply reverse current through the Triac to the

source. Input current (Triac holding current) is sharply reduced or reversed. The current is

out of phase with the voltage between the voltage peaks and the voltage zero crossings.

The filter supplies some of the load current during these intervals, stealing holding current

from the Triac.

AN2711 Datasheet(PDF) 7 Page - STMicroelectronics