In lamp design, how to estimate the voltage of LED is a seemingly simple knowledge that needs to pay attention to many details. First of all, the forward bias voltage of each LED produced by each factory will be more or less different even if tested under the same conditions. Secondly, the LED voltage will change with the temperature. To ensure the usability and reliability of lamps, we must understand several factors that will affect the LED voltage and properly evaluate the error value, so as to correctly select the output voltage specification of the driver. This article introduces the three biggest factors affecting led voltage, and briefly explains the consideration points and estimation. In addition, in view of the influence of temperature, the new LED power supply has the function of adapting to the environment. The benefits of this to the lamp design will also be explained below. At the end of the article, an example of complete design considerations will be provided for reference.
General lamp design will involve optics, heat and electricity. Of course, the main purpose of lamps is to meet the requirements of the application environment for light, such as illumination or color temperature. To meet these requirements, the model, type, quantity and driving current of LED must be defined first. When the above conditions are determined, the driving conditions of each single LED have been basically set, and the overall power of the lamp is also fixed. However, the overall voltage and current of lamps depend on how the designer arranges and combines the LEDs, but they are all in series, or in series or in parallel. This part is usually related to the maximum voltage required by safety regulations or the design of LED modularization. The overall approximate voltage of the lamp can be obtained from the following formula
Vforward_ total = Vforward x Num/String
The above formula is relatively simple and can provide a general direction for the designer, but it is insufficient for the final design of lamps. Complete design considerations should at least pay attention to the following three points:
1) V-I characteristics of LED
2) LED production differences
3) Temperature coefficient of LED
The following is divided into three paragraphs.
V-I characteristics of LED
The ideal led V-I curve is shown in Figure 1. The LED voltage is not affected by the flowing current. However, in fact, the driving current of the LED will affect the voltage on the LED. Taking Figure 2 as an example, when the LED is operated at 350mA, the voltage is about 3.2V. When the LED is operated at 1a, the voltage will rise to 3.8V. The small differential pressure of a single LED will become more obvious when multiple LEDs are connected in series. The designer should pay attention to whether the current conditions of the nominal voltage test on the LED specification are consistent with the lamp design. If the lamp adopts another current or the peak value of ripple current of the power supply changes greatly, the voltage value shall be re estimated according to the V-I characteristic curve.
LED production difference
The forward bias of LED will have a certain degree of production difference. A mature LED production line usually has small difference and normal distribution, as shown in Figure 3. The influence of production difference on LED voltage is usually less than 10%, which can be seen from the ratio of the maximum value of reference voltage to the standard value in the LED specification. The exact value or distribution curve must be consulted with the LED manufacturer.
Although the most extreme situation may be + / – 10%, in practice, the more LEDs in the same string, the more likely they are to be close to the middle value. If the objective conditions permit, it is generally recommended to reserve a 10% margin for the power supply, which can ensure that the driving current is not affected by the production difference of LED. Secondly, even if it is not used in this interval, a little load reduction of the power supply will also help to prolong the service life of the power supply.
LED temperature coefficient
Led voltage belongs to negative temperature coefficient, which means that when the temperature rises, the voltage will drop. On the contrary, when the temperature is low, the voltage is relatively high. The heat flow design of the lamp usually balances the self heating of the LED, so the temperature and voltage of the LED are relatively fixed in the steady state. For voltage, the most severe condition will fall when the lamp is cold started. The voltage in the LED specification table is usually set at the normal working temperature. If it is necessary to estimate the cold start voltage, it is necessary to refer to the V-T curve or use the tool software input parameter (TJ) provided by the LED manufacturer.
The biggest difference between the temperature coefficient and the previous two factors (current and production) is that the influence of temperature on voltage is temporary. When the LED is started, the voltage will gradually decrease and return to the normal level due to its own heating. In other words, the power supply does not need to unconditionally reserve the voltage margin required for cold start, but only temporarily increase the output voltage range for a short period of time after start-up.
Some advanced models on the market provide environment adaptive function, which allows the detection and automatic adjustment of output voltage and current during driving, and enter the constant current area after the lamps reach the stable operation state. Hlg-480h-c series has this function. The power supply can automatically reduce the current and increase the voltage by up to 120%, so as to ensure that the lamps can be turned on smoothly at low temperature. When the lamps are turned on and heated up, the current will automatically return to the original design value. This design can ensure the low-temperature start-up of the lamp without affecting the service life of the power supply. The designer does not need to spend more budget to select a higher power supply to ensure the short-term demand of start-up. To give an actual figure, hlg-480h-c1400 can operate at 171 ~ 343v, and can provide an instantaneous voltage of up to 412v at low temperature (such as – 40 ° C) until the lamp enters stable operation.
HVGC’s constant power series also has similar functions. Users can temporarily reduce the current and increase the maximum output voltage through dimming control. There are also many possibilities for other models. If you have questions about LED cold start, please ask Mingwei power experts everywhere.
Voltage calculation example
A lamp is driven by 100 LEDs at 1.05A, 2 in parallel and 50 in series on the lamp board, and the minimum ambient temperature of the lamp device is 0 °. How should the designer decide the appropriate LED power supply?
Method 1: ask the LED manufacturer if there is any tool software to use, input the parameters and get the answer directly.
Method 2: get the answer from the LED specification and refer to relevant materials.
Step 1: check the LED V-I curve and set the intermediate voltage.
According to figure 2, the LED is 3.8V at 1.05A
Step 2: 50 strings, so the total voltage is 50 times that of a single one.
3.8 （V） x 50 （pcs） = 190 V
Step 3: consider the production error.
a. The middle value of LED in Table 1 is 3.2V, and the maximum value is 3.48v
b.3.48 （V） / 3.2 （V） = 108.75 %
c.190 （V） x 108.75% = 206.6 （V）
The middle value of the total voltage of LED in steady state is 190V.
The maximum value of total voltage of LED in steady state is 207v * (assuming that the ripple current of LED power supply is very small and can be ignored).
Step 4: refer to the temperature coefficient.
As can be seen from Figure 4, the LED (350mA) is 3.2V at 85 ° C and 3.6V at 0 ° C
3.6 （V， Tj=0） / 3.2 （V， Tj=85） = 1.125 《 1.2
During cold start
The median value of the total led voltage is 190V x 1.2 = 228 v
The maximum total voltage of LED is 207v x 1.2 = 248.4 v
Selection of LED power supply:
The recommended model hlg-480h-c2100 is for the following reasons.
Lamps generally operate at 190V / 2.1a (399w) in steady state, and the worst condition is 207v (435W). This condition can be met by hlg-480h-c2100. At the same time, its ripple current is low and can be ignored. When the machine is cold, the maximum voltage will reach 249v, which is not within its fixed output range, but its environment adaptive function can provide the maximum 275v instantaneous voltage to ensure the stability of startup. Therefore, hlg-480h-c2100 is the best model.