AN4129 datasheet(31/39 Pages) STMICROELECTRONICS

31 / 39 page

AN4129

Circuit description and design guidance

Doc ID 023314 Rev 1

31/39

The unit produces usable light in about 0.1 seconds, and nearly full output in about 0.2

seconds. The vast majority of the startup time is the charging of the output filter capacitor

(C11) to the LED threshold voltage - reduced startup time can be traded off against

increased LED ripple current if faster startup is required.

4.16

Component stress

4.16.1

Thermal

The unit was mounted above the bench in free air with the narrow end (AC input) down.

Temperatures were recorded after 45 minutes of operation.

The dimmed temperatures were taken with a Triac dimmer feeding the unit. The dimmer was

adjusted to the point where the power analyzer reported the greatest loss. Undimmed input

voltage was 121 V. Dimmed input voltage was 115 Vrms, conduction angle about 150

degrees. Efficiency measured 85.4%, loss measured 1.667 W.

4.16.2

Electrical

The scope image below was taken at 120 V input with an 18 LED load. The trigger was set

to pick up only the highest voltage, which occurs near the peak of each half cycle.

Table 1.

Temperatures after 45 minutes in free air

Measured point

Undimmed

Dimmed

Ambient

23.5

23.4

41.5

50.3

38.9

51.3

BR1

38.1

L2, L3

46.4

58.7

63.1

50.2

54.7

51.6

53.5

C11

38.7

39.3

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