It is recommended to create some voltage head room, a margin of 10% of the typical voltage is considered safe. Production data, such as actual forward voltage distribution, will most likely need to be requested from the LED manufacturer directly.Īlthough the absolute maximum/minimum is +/- 10%, statistically, the more LEDs connected in series, the more likely the combined forward voltage will settle around the typical voltage value. The typical voltage tolerance due to production variation is less than 10%, which can be indirectly derived from the ratio between the typical and maximum voltages published in the LED’s datasheet, such as shown in the following table: The forward voltages on each LED die have variations due to process drift.Ī mature production process should provide a tighter tolerance resulting in a normal distribution (Fig.
Led driver controller constant current power high voltage driver#
The situation could get even worse if the LED driver has a high ripple current, which would result in a peak current higher than 1A and thus the peak voltage would exceed 3.8V (Fig 2.). This 0.6V difference could lead to a very different result when a high number of LEDs are put in series.
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If LED is not used at 350mA but 1A, then instead of 3.2V/LED, the typical LED voltage becomes 3.8V/LED. In the example below, the specification shows the typical voltage of the LED is 3.2V. However, the forward voltage does change with current, and it is important to check the LED voltage based on the actual current drawn during use, instead of referring to the specification data obtained under standard test conditions. In an ideal world, the LED’s forward voltage does not change when current increases (Fig 1.). To accurately determine the required output voltage of the LED driver, the following attributes should be considered: However, this number is not an absolute value and will not result in a high-quality electrical design. This calculation gives a rough estimate of the operating voltage range, and together with the determined driving current can be used to work out the power output requirement of the LED driver. Once these factors have been defined, the first estimation of the LED working voltage can be made by multiplying the number of LEDs in one string by the typical forward voltage (Vforward) of a single LED. Depending on certain safety considerations and/or a modular design approach, a certain number of LEDs are placed in the same string. To achieve the specified optical requirements, the type and the quantity of LEDs and the required driving current are decided first. The design of a LED luminaire requires multi-dimensional engineering skills, considering optical, thermal and electrical design concerns. Therefore, it is worth looking into the factors which affect the LED voltage. Secondly, the LED voltage varies when the junction temperature increases or decreases.Ĭorrect operation of the LED driver is crucial to the functionality and reliability of the LED luminaire. Of course, this then means the remaining 3 LEDs are now driven with 333mA, which means it won’t be long before they too fail. This will most likely result in another of the LEDs failing soon. This means all 4 LEDs are now being overdriven. Because there are now only four LEDs connected to the LED driver, they will all be driven with a current of 250mA. As the first LED is being over driven, it is quite likely that its lifespan will be reduced, and it could fail prematurely.
![led driver controller constant current power high voltage led driver controller constant current power high voltage](https://media.monolithicpower.com/catalog/product/cache/ccb9ba928e4f463ae145b55d0127b66f/m/p/mp2480_led_lighting_illumination_typical_application_1.jpg)
The other LEDs will be slightly under powered. Due to the permissible manufacturing tolerance one of the LEDs will come on once the output voltage of the LED driver reaches 9V, another will come on at 9.3V, and the others at 9.5V, 9.6V and 9.7V.īecause the first LED is on before the others it will draw a slightly higher current than it needs. Let’s say you have a constant current LED driver with a 1A output powering 5 LEDs with an input current rating of 200mA. However, this is not always as straight forward as it seems.įirstly, LED forward voltages are not the same from die to die, there is a tolerance that is usually specified by the manufacturer. This is why ADM recommends that you do not connect LEDs to a constant current LED driver in parallel. Selecting a LED driver with the correct forward voltage range is critical for LED luminaire designs, and when matching a constant current LED driver to a light fitting.