Compact SMPS for LED illumination

In connection with theLED based lightsource for replacement of fluorescent tubes it became necessary to develop a special power supply from scratch, since no one matching the demands of the LED light tube, could be found on the market.

The power supply is placed inside the LED-tube and since the LED-tube has to be able to fit into a standard T5 fitting (16 mm in diameter), there can only be a cross sectional area of 13 x 13 mm for the power supply. The largest LED-tubes (T8 1500 mm) consists of 40 LEDs. Each of them need a current of 350 mA through them to produce the required illumination. To do so in the most economic and energy saving way all the LEDs must be connected in one serial connection. The power supply must therefore be able to deliver up to approximately 144 V of output voltage. The output voltage interval lays between 100 and 144 V to maintain a constant current through the LEDs, no matter if there are 32 or 40 LEDs in the LED-tube. The lifetime of the power supply must ofcause be at least as the LEDs (50.000 hours of operation). Furthermore the power supply must comply with all laws including Power Factor Correction (PFC) and CE-approval.

The power supply has now been realised on a PCB measuring 12 x 350 mm with the components placed so that the max. height is only 12 mm. The power supply is a special switch-mode type (SMPS) with a input voltage of 230 VAC from the grid, and a constant average output current of 350 mA. The outputvoltage is not DC, but instead a sawtooth. This is done to avoid large electrolytic capacitors (they have a very short lifetime). The sawtooth output voltage can not be seen on the light as long as the average current is constant. The average output current, and thereby the illumination level of the LEDs, can be adjusted between 0 and 350 mA. In the LED-tube this current is adjusted by a luxostat so that the illumination level in its surrounding can be keept om a predetermined constant level, dependent of the light contribution from other lightsources or the daylight.

For more information about this project, or a quote for a similar project, please fell free to contact Allan Krogh Jensen.

Power supply for the LED tube

Enclosed into elctrically isolating plastic case