Diodes AL8807 - Manual (Page 16)

AL8807
Document number: DS35281 Rev. 5 - 2
16 of 20
www.diodes.com
March 2013
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AL8807
Application Information
(cont.)
The associated EMI measurements for this board using the AL8807 is shown in figure 40.
Figure 40 AL8807EV2 Radiated EMI Performance
The EMI performance was measured at 12V
DC
driving two white LEDs (V
F
= 3.1V at 660mA) on the AL8807EV2. The red bold line is for EN55022
class B used for domestic equipment including lighting. The bottom magenta line is the noise floor of the test chamber. The middle purple line is the
EMI emitted radiation of the AL8807 over 30MHz to 1000MHz. This shows that the AL8807 passes the standard with at least 16dB margin.
MR16 lamps typically operate from 12V
DC
or 12V
AC
, using conventional electromagnetic transformers or electronic transformers.
In enclosed lamps such MR16 the ability for the device to operate at high ambient temperatures is critical and figure 41 shows the surface
temperature of the AL8807 on AL8807EV2 in operation under the same conditions as the EMI tests at an free air temperature of 25°C. It is
anticipated that the internal junction temperature is approximately 6°C hotter than the surface temperature.
Figure 41 Thermal picture of AL8807EV2 at 12V
DC
2 white LEDS at 660mA
The thermal image shows that components increasing the board temperature are the inductor, Schottky diodes and the AL8807.
An inductor choice of 33µH with saturation current higher than 1.1A, will limit the frequency variation between 180kHz and 400kHz over the whole
input voltage variation (8V to 18V), and therefore represent the best choice for an MR16 solution also taking into account the size constraint of the
lamp.
The AL8807 guarantees high performance levels with both 12V
AC
and 12V
DC
power supplies.
The efficiency is generally higher than 81% and current regulation is better than 0.1mA/V in for a DC input voltage in the range from 8V to 18V.